COMPARATIVE EVALUATION OF MARGINAL DISCREPANCY AND

SURFACE ROUGHNESS OF CAST COPINGS MADE BY CONVENTIONAL AND

ACCELERATED CASTING TECHNIQUES USING TWO DIFFERENT PATTERN

MATERIALS - AN INVITRO STUDY.

DISSERTATION SUBMITTED IN PARTIAL FULFILLMENT

FOR THE DEGREE OF MASTER OF DENTAL SURGERY (M.D.S)

TO

Dr. N.T.R UNIVERSITY OF HEALTH SCIENCES

NOVEMBER 2009 Dr.A.PREMALATHA

DEPARTMENT OF PROSTHODONTICS INCLUDING CROWN

AND BRIDGE AND IMPLANTOLOGY

NARAYANA DENTAL COLLEGE & HOSPITAL

BATCH: 2007-2010

CERTIFICATE

This is to certify that the dissertation entitled “COMPARATIVE EVALUATION

OF MARGINAL DISCREPANCY AND SURFACE ROUGHNESS OF CAST COPINGS MADE BY

CONVENTIONAL AND ACCELERATED CASTING TECHNIQUES USING TWO DIFFERENT

PATTERN MATERIALS - AN INVITRO STUDY” submitted for the degree of Master of Dental

Surgery (M.D.S) is a bonafide work carried out by DR. A.PREMALATHA, during the period

from 2007-2010 under our guidance and that this work has not formed the award of any other

degree, diploma, associateship, fellowship or other similar titles.

DR.P.MAHESH. MDS

Guide

Professor & HOD, Dept of Prosthodontics

Principal, Narayana Dental College

DR.P.SRINIVAS RAO. MDS

Co-Guide

Professor, Dept of Prosthodontics

Narayana Dental College.

Place – Nellore

Date – November 2009

ENDORSEMENT BY

THE PRINCIPAL OF THE INSTITUTION

This is to certify that the dissertation entitled “COMPARATIVE EVALUATION OF

MARGINAL DISCREPANCY AND SURFACE ROUGHNESS OF CAST COPINGS MADE

BY CONVENTIONAL AND ACCELERATED CASTING TECHNIQUES USING TWO

DIFFERENT PATTERN MATERIALS - AN INVITRO STUDY” is a bonafide research work

done by Dr. A. Premalatha under the guidance of Dr. P. Mahesh, Professor & HOD, Department

of Prosthodontics including Crown and Bridge and Implantology.

Date: November 2009

Place: Nellore

Dr. P. MAHESH Principal

Narayana Dental College & Hospital, Nellore.

ACKNOWLEDGEMENTS

“A journey is easier when you travel together”. This dissertation work is the result of many accompanied, supported and guided people. So it is a pleasant aspect that I have the opportunity to express my gratitude for all of them.

I express my sincere indebtedness and heartfelt thanks to my guide, role model teacher and excellent academician Dr.P.Mahesh MDS, Professor and Head, Department of Prosthodontics, Narayana Dental College and Hospital, for his constant support, encouragement and valuable guidance that enabled me comprehend this dissertation and research and its successful culmination.

I sincerely express my deepest gratitude and humble thanks to my esteemed teacher, Dr.P.Srinivas Rao MDS, Professor, Department of Prosthodontics, Narayana Dental College and Hospital, for his immeasurable encouragement, unwavering guidance and constructive suggestions.

I would like to express my deepest gratitude to Dr.Dwarakananda Naik MDS, Reader, Department of Prosthodontics, Narayana Dental College and Hospital, for his constant support and extreme help.

I would like to specially express my appreciations and sincere gratitude to Dr.T.Pavan Kumar MDS, Sr. lecturer for his extreme support, help and valuable suggestions. Iam also highly indebted to him for his help during the study in the fabrication of die and preparing the samples.

I would like to express my heartfelt thanks to Dr.V.H.C.Kumar MDS, Sr. lecturer for his constructive suggestions and precious ideas.

I express my heartfelt thanks to Dr. A. Sudheer MDS, Sr.Lecturer for his appreciation, guidance and precious ideas.

Iam also thankful to Dr. Vijay Shankar Yadav & Dr.D.V.V. Vamsikrishna MDS, senior lecturers for their valuble support and guidance during my study period.

I sincerely express my gratitude to Dr.V.Vamsi Krishna Reddy MDS, Sr.Lecturer for his extreme help, support and timely suggestions.

I sincerely express my thanks to Mr. Ravi Chandra, Department of Nuclear Physics, Madras University, Chennai, for doing the vertical marginal discrepancy measurements in the study.

I express my deepest gratitude to Dr.Omkumar and Mr.Balan, Manufacturing Engineering Department, Anna University, Chennai, for doing the surface roughness measurements in the study.

I thank Mr.R. Ravanan, Bio-statistician for help to carryout the analysis of the study.

I extend my gratitude to my batch mates, Dr. K. Shalini, Dr. K. Kiran Kumar Reddy, my seniors Dr.V. Vamsi Krishna Reddy, Dr. M. Reddi Narasimha Rao, Dr. P. Gautham and my loving juniors, Dr.Y.ManojKumar, Dr.M.Dhanalakshmi, Dr.K.Surya Narayana Murthy, Dr. B. Swapna, Dr. C. Sameer Kumar Reddy, Dr. M.Mahesh Babu and without whom I would not have completed this work.

My sincere special thanks to Dr.V.Vamsikrishna Reddy, my senior for his cooperation during the study, guidance and precious ideas.

I thank all non-teaching staff of Department Of Prosthodontics for their assistance through the study.

Last but not least, I express my heartfelt gratitude to all my family members especially my husband Mr. M.Bharadwaja, who supported through out my life, my loving daughter M.Kusuma Chowdary, my dearest son M.Venkata Ramarao Chowdary, my parents, mother in-law, father in-law, sister, brother, sister in laws, who deserve the credit for what I am, and for all what I have achieved.

Dr. A.Prema latha.

CONTENTS

S .NO: CONTENTS PAGE

1 INTRODUCTION 1-4

2 REVIEW OF LITERATURE 5-25

3 MATERIALS & METHODS 26-47

4 RESULTS 48-59

5 DISCUSSION 60-69

6 SUMMARY & CONCLUSION 70-72

7 BIBLIOGRAPHY 73-78

LIST OF TABLES

TABLE TITLE PAGE

Table 1

The results obtained in this study for the vertical marginal discrepancy along with the mean estimated for each sample in the G-I technique calculated in microns (µ)

48

Table 2

The results obtained in this study for the vertical marginal discrepancy along with the mean estimated for each sample in the G-II technique calculated in microns (µ)

48

Table 3

The results obtained in this study for the vertical marginal discrepancy along with the mean estimated for each sample in the G-III technique calculated in microns (µ)

49

Table 4

The results obtained in this study for the vertical marginal discrepancy along with the mean estimated for each sample in the G-IV technique calculated in microns (µ)

49

Table 5The mean vertical marginal discrepancy obtained from basic mean values of four techniques (G-I, G-II,G-III & G-IV)

52

Table 6Test of significance for the mean vertical marginal discrepancy obtained from four techniques (G-I, G-II,G-III &G-IV)

53

Table 7

The results obtained in this study for the surface roughness along with the mean estimated for each sample in the G-I technique calculated in microns (µ).

54

Table 8

The results obtained in this study for the surface roughness along with the mean estimated for each sample in the G-II technique calculated in microns (µ).

54

Table 9

The results obtained in this study for the surface roughness along with the mean estimated for each sample in the G-III technique calculated in microns (µ).

55

Table 10

The results obtained in this study for the surface roughness along with the mean estimated for each sample in the G-IV technique calculated in microns (µ).

55

Table 11The mean surface roughness obtained from basic mean values of four techniques (G-I,G-II,G-III & G-IV)

58

LIST OF GRAPHS

GRAPH TITLE PAGE

GRAPH 1The basic data of vertical marginal discrepancy results obtained by G-I technique.

50

GRAPH 2The basic data of vertical marginal discrepancy results obtained by G-II technique.

50

GRAPH 3The basic data of vertical marginal discrepancy results obtained by G-III technique.

51

GRAPH 4The basic data of vertical marginal discrepancy results obtained by G-IV technique.

51

GRAPH 5

Comparison of the mean vertical marginal discrepancy obtained from basic mean values of four techniques (G-I,G-II,G-III &G-IV)

52

GRAPH 6The basic data of surface roughness results obtained by G-I technique.

56

GRAPH 7The basic data of surface roughness results obtained by G-II technique.

56

GRAPH 8The basic data of surface roughness results obtained by G-III technique.

57

GRAPH 9The basic data of surface roughness results obtained by G-IV technique.

57

GRAPH 10

Comparison of the mean surface roughness obtained from basic mean values of four techniques (G-I,G-II,G-III &G-IV)

58

LIST OF FIGURES

FIGURE TITLE PAGE

FIG.1 Axial view of custom made stainless steel former assembly (A) and stainless steel master die (B)

36

FIG.2 Occlusal View of Custom made stainless steel former assembly (A) and stainless steel master die (B)

36

FIG.3 Line Diagram of custom- made stainless steel master die and custom-made stainless steel former

37

FIG.4 Line diagram of custom-made stainless steel master die & stainless steel former in place. a. Custom – made stainless steel die. b. Custom- made stainless steel former.

37

FIG.5 A. Pattern Resin and B. Inlay wax 38

FIG.6 Investment powder & special Liquid 38

FIG.7 Nickel – Chromium alloy 38

FIG.8 Photo Microscope 39

FIG.9 Surface Roughness analyzer 39

FIG.10 Preparation of Inlay Wax Pattern 40

FIG.11 Preparation of Pattern Resin 40

FIG.12 Wax coping showing 0.5mm thickness 40

FIG.13 Pattern Resin coping showing 0.5mm thickness 40

FIG.14 Wax Pattern Attached to crucible former 41

FIG.15 Ring Liner Placed 3mm short of Ring Margin 41

FIG.16 Wax pattern in position in the casting ring 41

FIG.17 Divested casting 41

FIG.18 Cast coping seated on the die 42

FIG.19 Metal Coping showing 0.5mm thickness 42

FIG.20 Marginal gap of 34.02 at 80x magnification by using photo microscope of a cast coping obtained by conventional casting technique employing three stage wax elimination

43

FIG.21 Marginal gap of 44.39 at 80x magnification by using photo microscope of a cast coping obtained by Accelerated casting technique employing single stage wax elimination

43

FIG.22 Marginal gap of 37.06 at 80x magnification by using photo microscope of a cast coping obtained by conventional casting technique employing three stage Resin elimination

44

FIG.23 Marginal gap of 46.86 at 80x magnification by using photo microscope of a cast coping obtained by Accelerated casting technique employing single stage Resin elimination

44

FIG.24 line diagram of custom made stainless steel coping holder 45

FIG.25 custom made stainless steel coping holder with coping 45

FIG.26 custom made stainless steel coping holder with coping on surface roughness Analyzer

45

FIG.27 surface roughness graph obtained by conventional casting technique with three stage wax elimination

46

FIG.28 surface roughness graph obtained by Accelerated casting technique with single stage wax elimination

46

FIG.29 surface roughness graph obtained by conventional casting technique with three stage Resin elimination

47

FIG.30 surface roughness graph obtained by Accelerated casting technique with single stage Resin elimination

47

Dedicated to my husbandMr. BHARADWAJA

Introduction The success of any cast restoration depends upon its fit on to the underlying tooth

structure.1 The accuracy of the restoration is essential for its longevity, as it allows less plaque

accumulation at the marginal area provides better mechanical properties, less cement space and

improves the esthetic result.2,3 A deficient margin leads to plaque retention resulting in gingival

inflammation, marginal leakage which can lead to secondary caries, sensitivity, gingival

recession, cement dissolution and debonding of the restoration.1,4,5

The marginal discrepancies of cast restorations are inevitable, inspite of careful

attention to waxing, investing and casting procedures. Even though published data on clinically

acceptable gaps varies from 30µm to 200µm, standard reference6 on cast restorations mention a

marginal gap of up to 74µm is clinically acceptable. It is one of the tasks of luting cements to

close these discrepancies. However, cement will dissolve rapidly under the margins if the

discrepancy is too large.4The rate of luting cement dissolution has been related empirically to the

degree of marginal opening. Thus, the larger the marginal discrepancy and subsequent exposure

of the dental luting cement to oral fluids the more rapid is the rate of cement dissolution. Saliva

increasingly influences the dissolution of the cement if the marginal discrepancies are wider than

150µm.4, 7

The objective of the casting process is to accurately reproduce a wax pattern. A

defective casting results in a considerable loss of time and effort.8 Types of defects in castings

have been classified into 4 broad categories:

1. Distortion

2. Surface roughness and irregularities

3. Porosities

4. Incomplete or missing castings.6, 8

Surface roughness refers to finely spaced imperfections, of which the height,

width and direction establish the predominant surface pattern. The average surface roughness of

castings made from noble alloy according to Pomes et al was in the order of 0.76µm.8 Various

studies demonstrate that the surface roughness of as-cast gold alloy specimens ranged from

approximately 2 to 30µm on average.9 The surface roughness of dental castings is often greater

than that of the wax patterns from which they are cast. Surface roughness can be lessened by

abrading and polishing procedures, ensuring in this way a good tissue response to the alloy.8 A

smooth surface not only prevents plaque and calculus accumulation, but it also improves the

corrosion resistance of the alloy. The surface roughness on the intaglio surface of the cast

restoration affects the fit of the restoration. Therefore, the smaller these flaws, the better the fit of

a restoration to the surface of the prepared tooth.8

Several implications are related to surface roughness of metal copings in PFM’s

especially in relation to adhesion. If the ceramic penetrates well and locks with the metal, it

provides more room for chemical bonds to form there by increasing the ceramic –metal interface

bond. If the ceramic does not penetrate into the surface and voids present on the interface it may

lead to bond failure and also compromise esthetics.10

There are a variety of factors that have an important role in controlling surface

roughness and irregularities. These factors include the liquid-powder ratio of the investment, air-

bubbles in the investment, water films, rapid heating rates, under heating, prolonged heating, the

temperature of the melting alloy, casting pressure, composition of the investment ,investment

technique, foreign bodies, impact of molten alloy, carbon inclusions, and mixing and melting

different alloys together.8

Keeping this in mind, many materials and methods have been suggested by various

authors to improve the fit, marginal accuracy and surface roughness of the casting. The marginal

fit and surface roughness is affected by the quality of the preparation, the impression, the

working cast, the type and quality of the wax used for the lost wax technique, and by the

accuracy of the casting.2The accuracy of the casting is subjected to volumetric changes occurring

due to shrinkage of wax, resin and alloys. This shrinkage can be compensated by setting

expansion, hygroscopic and thermal expansion of the investment.3

There have been numerous reports, on attempts to perfect the casting procedures

by improving investment materials and techniques.2,3,9,11,12 The majority of these efforts deal with

so-called “conventional” investing and casting techniques, which usually require at least 1 hour

bench set for the investment, followed by a one, two or three stage Inlay Wax and Pattern Resin

elimination cycle as recommended by the manufacturer before the casting procedure.6,9,12,13 The

whole process is time consuming and requires approximately 2 to 4 hours for completion.9,12

The accelerated casting techniques have been reported in an effort to achieve

similar quality results in significantly less time.9, 12 The pattern is invested, cast and delivered in a

cost effective, time saving manner. This combination offers many advantages to the patient,

dentist, and dental laboratory technician, and has received increased attention as a method of

improving productivity.12 Though studies9,12 have reported that marginal discrepancies and

surface roughness by accelerated casting technique are with in the clinically acceptable limits,

some studies9,11,12 have reported that this procedure is technique sensitive. Most of these studies

have reported the effect of accelerated casting procedures on the fit of noble alloy castings.

However, the effect of accelerated procedures on the marginal discrepancy and surface

roughness of base metal alloy restorations has not been adequately studied.

Hence this invitro study was conducted to measure and compare the vertical

marginal discrepancy and surface roughness of base metal alloy cast copings made by

conventional and accelerated casting techniques using Inlay Wax and Pattern Resin.

The objectives of this study included the following:-

1. To evaluate the vertical marginal discrepancy of cast copings obtained by conventional casting

technique using Inlay Wax and Pattern Resin.

2. To evaluate the vertical marginal discrepancy of cast copings obtained by accelerated casting

technique using Inlay Wax and Pattern Resin.

3. To compare the vertical marginal discrepancy of cast copings obtained by conventional and

accelerated casting technique using Inlay Wax and Pattern Resin.

4. To evaluate the surface roughness of cast copings obtained by conventional casting technique

using Inlay Wax and Pattern Resin.

5. To evaluate the surface roughness of cast copings obtained by accelerated casting technique

using Inlay Wax and Pattern Resin.

6. To compare the surface roughness of cast copings obtained by conventional and accelerated

casting technique using Inlay Wax and Pattern Resin.

Review of literature

Cooney JP et al (1979)14 evaluated two phosphate-bonded investments and one

calcium sulfate investment for the surface smoothness and marginal fit they impart to gold

castings. A modified technique was also evaluated for each phosphate-bonded investment, where

the silica sol was not diluted and the spatulation time was reduced. The results of this study lead

to the following conclusions: (1) The marginal fits obtained with all four phosphate-bonded

methods were comparable to each other and superior to that obtained with the calcium sulfate

investment. (2) The presence of nodules on the surface of the castings was more prevalent with

the phosphate-bonded investments. However, this effect was not statistically significant. (3)

Clinical assessment of the roughness of the castings revealed that all the methods tested

produced clinically acceptable castings. (4) Visual observation by five dentists revealed that both

the recommended and modified techniques for one of the phosphate-bonded investments

(Ceramigold) produced a smoother surface than any other investment tested. Rating of scanning

electron microscope photographs (X 600) revealed no difference in the surface roughness

between any of the castings .Consequently, no definitive relation between investment type or

technique and surface roughness was established. (5) No correlation was demonstrated between

surface roughness, as evaluated by either clinical observation or scanning electron microscope

photography, and marginal fit of the castings.

Neiman R, Sarma AC (1980) 15 studied the setting reactions and thermal degradation of

the phosphate binders are interpreted from the DTA and X-ray diffraction data. The simple

chemical reaction has generally been shown to be: MgO + NH4H2PO4 = NH4MgPO4.6H2O.

However, the setting reaction is in reality a more complex system of multi-molecular structure as

described herein. On heating, the set product (NH4MgPO4.6H2O)n dehydrates to

(NH4MgPO4.H2O)n and subsequently degrades into polymeric (Mg2 P2 07 )n', crystalline Mg2 P2

07 ; then the latter reacts with excess MgO present to form the final product, Mg3(PO4)2.

Duncan JD (1980)16 studied nickel-chromium alloys as substitutes for gold alloys in

casting crown and bridge prostheses and found out more definitive research is needed on the

casting accuracy, finishing characteristics, porcelain-to-alloy bonding, and corrosion sensitivities

of these materials.

Gavelis J.R. et al (1981)17 conducted his study to correlate margin design with the

seating and sealing of cemented full cast crowns under standardized, simulated clinical

conditions. The crowns were waxed on the steel dies, invested, and cast. The crowns were

cemented onto the Duralay dies and tested in an Instron testing machine. Measurements were

made of the cement thickness at the margin, shoulder, axial wall, and occlusal surface. The

cement thickness at the margin and occlusal surface were analyzed to find the amount of seal and

seat afforded by the various preparations. The 90-degree shoulder had a cement space of 67 µ at

the margins. The 45-degree shoulder, shoulder with 30-degree bevel, and shoulder with 45-

degree bevel followed, with spaces of 95, 99, and 105 µ, respectively. With regard to seating of

the restoration, the 90-degree full shoulder demonstrated the best seat, followed in order by the

45-degree shoulder, 90- degree shoulder with 45-degree bevel, featheredge, 90-degree shoulder

with 30-degree bevel, chamfer with parallel bevel, and finally go-degree shoulder with parallel

bevel.

Ogura H et al (1981)18 investigated six variables that could affect the surface roughness

of a casting. The variables were (1) type of alloy, (2) mold temperature, (3) metal casting

temperature, (4) casting machine, (5) sandblasting, and (6) location of each section. It was

determined that the trailing portion of a complete cast crown had rougher surfaces than the

leading portion. Higher mold and casting temperatures produced rougher castings, and this effect

was more pronounced in the case of the base metal alloy. Sandblasting reduced the roughness,

but produced scratched surfaces. Sandblasting had a more pronounced affect on the surface

roughness of the base metal alloy cast either at a higher mold temperature or metal casting

temperature.

Duncan JD (1982)19 investigated the casting accuracy of four nickel-chromium alloys

(Ultratek, Omega, Microbond N/P2, and Nobil-Ceram) was compared to that of a precious alloy,

Jelenko “0”. The experiment indicated that Jelenko “0” had the greatest casting accuracy,

followed successively by Ultratek, Nobil-Ceram, Microbond N/P2, and Omega. The following

conclusions may be drawn from this experiment: (1) The nickel-chromium alloys tested did not

cast as consistently or as accurately as precious alloy Jelenko“0”. (2) The casting accuracy may

be related to the amount of casting shrinkage that occurs in each alloy type, and further research

is necessary to determine if expansion compensation of the selected phosphate-bonded

investment recommended by the alloy manufacturers is adequate for the casting shrinkage of the

test alloys. (3) Alloy composition and technique parameters may also influence the accuracy of

the casting, but further research is necessary to determine their singular effects.

Lacy AM et al (1983)20 investigated the related effects of (1) mixing rate, (2) ring liner

position, and (3) storage conditions on the setting expansion of both gypsum-bonded and

phosphate-bonded investment molds; and subsequently to correlate casting size with measured

expansion data. The results of these studies indicate the need for careful standardization of

investing and casting techniques if consistent results are to be expected. Results also reveal that

the position and extent of ring liners, rates of mixing, and conditions of storage may be even

more significant in determining ultimate casting size than classically accepted factors such as

liquid/powder ratios or numbers of ring liners. The dynamic nature of setting expansion within

the first 60 minutes after mixing suggests that consistent results demand waiting at least that long

prior to burnout. If molds are to be stored overnight, maximum dimensional stability is probably

ensured by keeping them in 100% relative humidity, particularly if CaSO4. 2H2O-containing

gypsum-bonded investments are used.

Vermilyea SG et al (1983)21 assessed the influence of three such investment

materials, Ceramigold 2 and Hi-Temp Casting Investment (Whip-Mix Corp., Louisville, Ky.)

and Neoloy Hi-Heat Crown and Bridge Investment (Neoloy Products, Inc., Posen, III.), on the fit

of copings cast from five base metal alloys (Biobond, Dentsply International, York, Pa.;

Ceramalloy II, Ceramco, Inc., East Windsor, N. J.; Unibond, Unitek, Inc., Monrovia, Calif.;

Biocast, Jeneric Gold Co., Wallingford, Conn.; and Neobond II, Neoloy Products, Inc.). Overall,

the fit of the test castings was poor. Individual alloy-investment interaction appears to be

significant. Although marketed for use with base metal alloys, it appears that investment

manufacturers’ recommended techniques require alteration to enhance the fit of base metal

restorations.

Marsaw FA et al (1984) 22 developed a technique to evaluate setting expansion in

the pattern region of an investment mold. The volumetric system provides a method to acquire

information on setting expansion at the location of the wax pattern. Internally determined setting

expansion values do not agree with values derived from external measurements (ADA trough

method). Measurements obtained by the volumetric method show less variability than those

obtained by the trough method. No significant difference in volume was seen between a

restricted metal ring and an unrestricted split rubber ring, which suggests a semisolid behavior of

the investment during the period of setting. The findings indicate a need to reevaluate the

methods by which setting expansion is measured, as well as the mechanism by which the

expansion takes place. The need for further study by means of multiple VRSs or strain gauges

embedded in the investment is indicated, and investigation is currently in progress. The influence

of variously shaped casting rings and pattern position on resulting accuracy of the casting could

be determined by this method of investigation.

Dedmon HW (1985)23 studied the marginal fit of full cast crowns made by

commercial dental laboratories with the design of the margin. When cast restorations were made

by commercial dental laboratories, margins prepared with unbeveled heavy chamfers and

shoulders were most likely to have openings that exceeded 39 µm on the dies. Unbeveled heavy

chamfers and shoulders were also most likely to have metal flash left on the margins. Knife-

edged and beveled margins were least likely to have metal flash or openings that exceeded 39

µm on the dies.

Smith CD et al (1985)24 in this study developed a method for measuring the changes in

size and in marginal length of a complete crown wax pattern that occur during the casting

process. These values, combined with initial wax margin discrepancy, are used to calculate

marginal discrepancy values that are as accurate as the direct measurement of the same

discrepancy. This method is capable of measuring the effect of individual variables on casting

size, marginal reproduction, and marginal discrepancy. The method is capable of determining

these values for oversized as well as undersized castings.

Schwartz IS (1986)25 reviewed the methods and techniques to improve the fit of cast

restoration because the marginal fit of castings is one factor that leads directly or indirectly to

secondary caries, adverse pulpal reactions and periodontal disease. He found several factors that

were necessary for good fit of the castings and some of them were perceptive tooth preparation,

accurate impressions, precision castings and careful finishing procedures, but in addition to these

factors he reported that internal relief was basic for accurate marginal fit of the cemented

restorations.

Asgar K (1988)26 reviewed casting metals in dentistry. In the literature, credit is given to

Dr. Swasey (1890), who introduced a technique where a solid gold inlay could be prepared. Wax

was used for making gold inlays for the first time by Martin (1891). A few years later, Dr.

Philbrook (1896) introduced a pressure-casting method of producing gold inlays. About 10 years

later, Dr.Taggart (1907) presented a paper before the New York Odontological Group, in which

he discussed his casting technique and machine. Castings made using Taggart's casting machine

and his investment were generally too small and did not fit the cavities properly. Van Horn

(1910) suggested and promoted the idea of thermally expanding wax patterns prior to investing.

The development of cristobalite investment by Coleman and Weinstein in 1929, who obtained a

U.S. patent a few years later (1933), as well as the introduction of the hygroscopic technique

(Scheu, 1932), were responsible for the greatest improvement in the fit of dental castings. From

the time that Dr. Scheu introduced his hygroscopic technique, various aspects of hygroscopic

investments were studied by many individuals, and some theories were postulated. Finally,

Mahler and Ady (1960), in their classic paper, showed that the hygroscopic expansion of dental

investments is a continuation of setting expansion and proposed the theory which is accepted

today. The quality of dental castings would not be where it is today if a better understanding of

the basic nature of dental castings and improvement in investments and alloys had not been

accomplished. Much work in the various aspects of dental casting techniques — such as the

effect of mold and metal temperature on the fit of castings, castability of the various alloys,

choice of sprue as well as its size and location, and the effects of various types of dental wax on

the resultant casting — has been reported.

Holmes JR et al (1989)5 inferred that the measurements of misfit at different

locations are geometrically related to each other and defined as internal gap, marginal gap,

vertical marginal discrepancy, horizontal marginal discrepancy, overextended margin, under

extended margin, absolute marginal discrepancy, and seating discrepancy. The significance and

difference in magnitude of different locations are presented. The best alternative is perhaps the

absolute marginal discrepancy, which would always be the largest measurement of error at the

margin and would reflect the total misfit at that point.

Hunter AJ, Hunter AR (1990)27 in this review stated that there is variation regarding

the maximum acceptable marginal discrepancy, there is little argument that poorly fitting

margins are a frequent finding. Large discrepancies are clinically significant, since they facilitate

plaque retention. Margins incorporating slip joint geometry have usually been favored as a

method of minimizing seating and sealing discrepancies. However, many of these discussions

largely ignored the effects of the cementing medium and the clinical applicability of slip joint

geometry is based on questionable assumptions with regard to casting accuracy and seating.

Greater understanding of the role of restorative margins and gingival health indicates the need

for shallow margin placement within the crevice, which requires a reassessment of the use of

long bevels. Horizontal margins can be made accurately and, when combined with procedures to

maximize crown seating, may provide the best method of minimizing seating discrepancies and

maximizing gingival health.

Felton DA et al (1991) 28 evaluated the relationship between marginal adaptation

of dental castings and periodontal tissue health quantitatively. Forty-two crown restorations in 29

randomly selected patients were selected for this study using three criteria. (1) The crowns were

placed at the University Of North Carolina School Of Dentistry; (2) the crowns were in service

for a minimum of 4 years; and (3) the crown margins were within the intracrevicular crevice

(subgingival). Replica impressions of the facial margins of specific crowns were made with a

vinyl polysiloxane impression material, and poured casts were prepared for scanning electron

micrograph evaluation. Marginal discrepancy measurements were identified on each micrograph

at 10 equally spaced locations along the margin and averaged for each specimen. Periodontal

indices of pocket depths, crevicular fluid volume, and gingival index were accumulated for

clinical measurements. Pearson correlation and Bonferroni adjusted probability tests were

performed, but no significant correlation was found between marginal discrepancy (0.16 ± 0.13

mm) and pocket depth (2.4 ± 0.9 mm). However, a strong correlation (p < 0.001) existed

between marginal discrepancy and gingival index (2 ± 0.8) and between marginal discrepancies

and crevicular fluid volume (49.9 ± 31.1). These results established that a significant quantitative

relationship existed between the marginal discrepancy and periodontal tissue inflammation for

subgingivally located crown margins.

Jacobs MS et al (1991) 29 investigated the rate of type I zinc phosphate cement

solubility as it relates to the degree of marginal opening. Standardized test samples were

constructed that would simulate clinically relevant marginal gaps of 25, 50, 75, and 150 microns

and their subsequent cement lines. The study was divided into two phases. Phase 1 evaluated the

effects of simple diffusion on cement solubility in a static environment, whereas phase 2

investigated the effects of convective forces on cement dissolution in a dynamic environment.

Both the phase 1 and phase 2 studies demonstrated no significant difference in the rate of cement

dissolution for the 25-, 50-, and 75-micron test groups. The 150-micron test groups for both and

phase 2 studies should not be compared because different methodologies were used.

Campagni WV et al (1993)30 study compares an accelerated technique for the casting

of post-and-core restorations with four traditional techniques. The accelerated technique uses two

phosphate-bonded investments and the traditional techniques use gypsum- and a phosphate-

bonded investment. The study measures and compares the differences between the seating of the

casting and the seating of the acrylic resin pattern. The seating of the patterns after 3 months of

storage was consistently worse than the 2-week measurements of fit. The ferrule and nonferrule

patterns were not statistically different in seating. Measurement of the castings showed that the

ferruled castings seated significantly worse than the nonferrule castings. The difference in the

seating of the castings as compared with the patterns was considered clinically unacceptable,

showing a range of 0.301 mm t o 0.528 mm. The nonferrule castings showed a significant

difference in seating among groups. The difference ranged from -0.099 mm to 0.322 mm. The

effects of the techniques on the fit of castings with and without a ferrule are also compared. The

castings of the ferrule subgroups were considered clinically unacceptable and were not analyzed

for significance. Among the nonferrule castings, the group using a gypsum investment and

conventional technique for investing and burnout but no ring liner showed the best seating. The

accelerated technique was intermediate in seating with a difference of 0.148 mm from the seating

of the patterns. This group was significantly different from the two best groups but not from the

remaining three groups.

Hutton JE Marshall GW (1993) 31 in this study determined whether three

different phosphate bonded investments could provide adequate expansion to compensate for the

casting shrinkage of AgPd. The investments were mixed with distilled water or their unique

special liquids provided by the manufacturers and allowed setting times of 1 hour or 24 hours.

Setting expansions were measured with a vertical dilatometer. When mixed with special liquid,

material I had a mean setting (1 hour) expansion of 0.19% ± 0.01%; material II, 0.14% ± 0.03%;

and material III, 1.17% ± 0.08%. Twenty-four hours of setting did not significantly increase the

setting expansion (p > 0.05). Mixing the three investments with distilled water drastically

reduced setting expansions. A three-way analysis of variance was computed to evaluate the data

and investigate significant interactive and main effects. The two-way interaction (material x

liquid) was significant. The results were consistent with the concept of a higher silica-containing

special liquid for material III compared with the other materials.

Bailey JH, Sherrard DJ (1994) 32 study determined the mean time interval from

start of mixing to the maximum exothermic setting reaction temperature for each investment. A

chromel/alumel thermocouple was placed at the heat center of a methylcellulose lined casting

ring, using wet or dry ring liner. Investments were vacuum mixed at the recommended ratio for

the accelerated technique. Colloidal silica solution and ddH2O were combined at a 50:50 ratio to

meet the manufacturer's recommended liquid volume. Part two determined the dimensional

reproduction of a standardized pattern and its casting using both casting techniques. Mixing

ratios were the same as in part one for the accelerated technique and 75% colloidal silica to 25%

double-distilled water (ddH20) for the conventional technique. The accelerated technique used

the mean setting time established in part one followed by a 15-minute furnace holding time at

725°C (1350°F). The conventional technique used a I-hour bench setting time, followed by

placing the mold into a cold furnace. A controlled rate of climb to a maximum temperature of

725°C (1350°F) was used with a I-hour soak time. Each pattern and its casting were measured at

four sites: (1) Length of the post-and-core assembly, (2) maximum core diameter, (3) post

diameter at the core base, and (4) post diameter at its apex. A significant difference was found

between the time interval to maximum exothermic setting reaction temperature for all the

investments (P < .01). The accelerated technique produced castings with a relative dimensional

increase of 0.11% to 4.80%. The conventional technique ranged from a 0.04% decrease in size to

an increase of 3.65%. Castings made with the accelerated technique were significantly different

than those made with the conventional technique (P < .01) Differences in the time interval to

maximum exothermic setting reaction temperature indicate that each phosphate investment

should have a recommended setting time before introduction into the furnace. The carbon-

containing investment showed the least relative change of the three investments evaluated for

both casting techniques.

Schneider RL (1994) 33 investigated an accelerated method of using a light-cured

acrylic resin and rapid burnout for casting a direct-pattern post and core restoration. Light-cured

acrylic resins are an alternative to chemically cured acrylic resins or indirect patterns formed

from an elastomeric impression. The procedure can eliminate an appointment for the patient in

the fabrication of the post and core restoration and can be completed in most dental offices with

minimal laboratory facilities. Chair-side time is saved because of the elimination of one

provisional restoration when two are usually required. Laboratory time is also saved because of

the decrease in investment setting and burnout time.

Iglesias A et al (1996) 34 compared the marginal fit of MOD inlay and full-crown

patterns fabricated from wax, autopolymerized acrylic resin, and two light-polymerized,

diacrylate resin pattern materials on standardized dies. For the MOD inlay patterns, marginal

gaps ranged from 7 to 23 µm, and the light-polymerized, diacrylate resins and autopolymerized

acrylic resin material had statistically smaller gaps than the inlay wax. For the full-crown

patterns, marginal gaps ranged from 10 to 23 µm, with the exception of the autopolymerized

acrylic resin prepared by the bulk technique (40 to 46 µm). With the incremental technique, the

light-polymerized, diacrylate resins and inlay wax had statistically smaller gaps than the

autopolymerized acrylic resin material. Overall, the incremental technique produced equal or

smaller marginal gaps than the bulk technique for full-crown patterns. Generally, the patterns

measured at 1 hour had smaller marginal gaps than at 24 hours. When measured on intra- and

extracoronal master dies, the light-polymerized, diacrylate resins had equal or better marginal fit,

compared with wax or autopolymerized acrylic resin, and were less affected by placement

technique and storage. The marginal gaps of all four pattern materials ranged from 7 to 46 µm

and are within the range of clinical acceptability.

Ito M et al (1996)35 evaluated the relationship between flow characteristics,

bending strength, and softening temperature of paraffin and dental inlay waxes to casting

shrinkage when patterns were invested with a phosphate-bonded investment. This study found

that the casting shrinkage decreased as the flow of the wax pattern increased. The flow of the

wax pattern increased as the exothermic reaction increased. A larger casting ring is suggested for

castings when a relatively thick wax pattern or an inlay wax that has a high strength, softening

temperature, and low flow percentage is used. When wax patterns are formed for cast

restorations, it is important to select the type of wax with the most desirable properties for the

margin and the occlusal portions. Moreover, to accurately fabricate castings, it is necessary to

understand the physical properties of the chosen waxes.

Earnshaw R et al (1997) 36 In an earlier investigation, it was shown that when

full crowns are cast in gypsum-bonded investments, their relative inaccuracy is affected by both

the investment's potential expansion and its hot strength. This study repeated the earlier one, but

used a high-melting gold alloy and two phosphate-bonded investments. The investments were

used under conditions which gave a range of potential expansions and hot strengths. Casting

inaccuracies were determined both diametrally and axially. All castings showed distortion, which

varied under the different conditions. All were oversized axially, by amounts varying from

+0.8% to +2.3%. Diametral inaccuracies ranged from -0.2% to + 0.7%. Investment expansion

had a strong effect on axial inaccuracy, but a negligible effect on diametral inaccuracy.

Conversely, hot strength had a strong effect on diametral inaccuracy, but only a very weak effect

on axial inaccuracy. With phosphate-bonded investments, both potential expansion and hot

strength are important parameters of relative casting inaccuracy. In combination, these properties

showed very strong correlations with both diametral and axial inaccuracies. The observed

distortions were the result of anisotropic mould expansion and anisotropic alloy shrinkage. The

best fit, and least distortion, occurred with an investment setting under dry conditions.

Konstantoulakis E et al (1998) 9 evaluated the marginal fit and surface roughness of

complete crowns made with a conventional and an accelerated casting technique, and found out

no statistical difference in the marginal discrepancy of cast crowns made by using accelerated

technique as compared with conventional technique. There was no difference in the average

surface roughness of cast crowns between the accelerated and the conventional techniques.

Clinically acceptable complete castings can be obtained with the accelerated technique if

optimum heating conditions are selected for each investment. Therefore they concluded that the

accelerated casting technique described in this study could be a vital alternative to the time-

consuming conventional technique.

Schilling ER et al (1999)12 measured the marginal gap and determined the clinical

acceptability of single castings invested in a phosphate-bonded investment with the use of

conventional and accelerated methods. The following conclusions were drawn from this study:

(1) Marginal gaps for castings made with an accelerated technique showed no statistical

difference when compared with a conventional casting group. (2) The accelerated casting

technique offers a cost effective and time-saving method by which single-unit castings for

metal/ceramic crowns can be fabricated. (3) The methods used for accelerating the casting

process are technique sensitive. Minor variations in the procedures can cause casting defects

such as nodules, fins, and porosity. Repeated use of the accelerated technique can provide the

dental laboratory technician with predictable, clinically acceptable castings for metal/ceramic

crowns.

Blackman RB (2000)11 This pilot study investigated the effect of 2 rapid mold

preparation schedules on full crown castings by comparing size, margin sharpness, and surface

roughness. Three groups of 10 crowns were cast with a type III gold alloy. All crowns were

nominally identical, only their mold preparation schedules differed. Two groups used accelerated

schedules; the third group was cast using a conventional schedule. Group comparisons were

based on direct microscopic measurements of crown diameters (×50 magnification), and surface

roughness was measured. Margin sharpness was judged by amount of marginal length lost in the

axial direction as a result of the casting process. Crowns were successfully cast using accelerated

mold preparation techniques and considerable time was saved, but a small loss of margin length

or fineness was observed.

Castillo RD et al (2000)37 evaluated the influence of: (1) a cellulose ring liner,

and (2) a lower casting temperature of the metal ring, on the dimensions of a cast post.

Experimental posts were measured before and after casting to determine the effect of ring liner

and casting ring temperature on the dimensional behavior of a phosphate-bonded investment

material. Within the limits of this study, it was found that decreasing the casting ring temperature

from 815°C to 600°C, along with the absence of a ring liner, produced undersized cast posts. A

slightly undersized cast post may be easier to fit and cement in the prepared root canal, and thus

decrease chairside time.

Groten M et al (2000) 38 in this study estimated the minimum number of gap

measurements on margins of single crowns to produce relevant results for gap analysis. Ten all-

ceramic crowns were fabricated on a master steel die. Gaps along crown margins were

investigated in a scanning electron microscope on the master steel die without cementation and

on replica dies after conventional cementation. Measurements were made in 100 µm steps

according to 3 gap definitions. The initial number of measurements per crown (n = 230) was

reduced to smaller subsets using both systematic and random approaches to determine the impact

on the quality of results. On the data of gap definition 1, reduction from 230 to about 50

measurements caused less than ±5 µm variability for arithmetic means. Analysis of standard

errors showed slowly increasing values smaller than 3 µm, both indicating no relevant impact on

the quality of results. Smaller data sizes yielded accelerated increase of standard errors and

divergent variabilities of mean. The minimum of 50 measurements did not depend on gap

definition or on cementation condition. Fifty measurements are required for clinically relevant

information about gap size regardless of whether the measurement sites are selected in a

systematic or random manner, which is far more than what current in vitro studies use.

Lombardas P et al (2000) 2 compared the vertical margin accuracy of lost wax

castings produced with the conventional casting technique using a metal ring and a technique

that uses a ringless system. The following conclusions were drawn: (1) The vertical margin

discrepancy of the ringless group for the buccal, the lingual, and the distal sites were

significantly less than that of the 2 ring groups (P<.001). (2) There was no significant difference

of the vertical margin discrepancy between the 2 metal ring groups. (3) There was no significant

difference in the vertical margin discrepancy at the buccal, lingual, mesial, and distal surfaces

within the same group. (4) The ringless technique was clinically acceptable and can be used for

the fabrication of fixed prosthodontic restorations.

Ushiwata O et al (2000)39 evaluated the technique of internal adjustment of

castings with use of duplicated stone dies and a disclosing agent to improve marginal fit

discrepancy. Marginal fit discrepancies of copings were significantly reduced with an internal

adjustment technique (mean > 52%) for all experimental groups. Tooth preparations with greater

convergence and internally relieved castings recorded a better marginal fit. The casting internal

adjustment technique with use of duplicated stone dies and a disclosing agent substantially

reduced marginal fit discrepancy.

Ayad MF (2002) 40 characterized the elemental compositional stability of as-

received and recast type III gold alloy. The effect of combining these alloys on the marginal

accuracy of complete cast crowns also was evaluated. Elemental composition was significantly

different among the casting groups (P<.001). The mean weight percentage values were 72.4% to

75.7% Au, 4.5% to 7.0% Pd, 10.7% to 11.1% Ag, 7.8% to 8.4% Cu, and 1.0% to 1.4% Zn.

Statistically but not clinically significant differences also were found for marginal accuracy. The

marginal discrepancy was less than 25 µm for all casting conditions, with the lowest value

recorded for Group A (7 µm), the highest for Group C (12 µm), and an intermediate value for

Group B (9 µm) specimens. Recasting type III gold alloys may adversely affect the marginal

accuracy of complete cast crowns.

Ito M et al (2002)41 investigated the relationship between wax characteristics and the

casting accuracy of a three-quarter crown. Dental casting accuracy is influenced by the setting

expansion of investment materials. Although setting expansion can help compensate for casting

shrinkage, it cannot be fully realized under a confined wax pattern. Exactly how soft a wax

pattern should be to ensure optimum setting expansion has not been determined. Within the

limitations of this study, casting shrinkage was affected by the type of wax used and was

sensitive to the site at which dimensional measurements were performed. The higher the

softening temperature, the larger the casting shrinkage. At gingival measurement sites, less

casting shrinkage was found when 100% special liquid investment was used with all waxes

except P38. At facial measurement sites, only S42 exhibited a significant difference between

100% and 75% special liquid investments.

Bezzon OL et al (2004)42 evaluated the surface roughness of 2 base metal alloys,

submitted to different casting techniques, to determine the influence of surface roughness on loss

of mass after polishing compared to commercially pure titanium castings. Within the limitations

of this study, the following conclusions were drawn: (1) Vacuum casting provided significantly

smoother alloy specimens compared to flame casting. (2) Vacuum casting of base metal alloys

provided specimens with a surface smoothness that was not significantly different from those of

commercially pure titanium. (3) There were no significant differences in loss of mass after

polishing for all tested specimens.

Gassino G et al (2004) 43 evaluated the marginal fit of experimental and custom-made

fixed prosthetic restorations through a new 360-degree external examination. The minimum

number of gap measurements required to produce relevant results for gap analysis was also

investigated. The marginal fit of six experimental and eight custom-made crowns was observed

microscopically by means of a mechanical device, and software was employed to measure the

gap. Two crowns, chosen from among the 14 previously evaluated, were reanalyzed to determine

the minimum number of gap measurements required to produce significant results for gap

analysis. Differences in fit between experimental specimens and custom-made ones showed that

experimental results might not always be obtained in clinical practice. Within the limitations of

the protocol of this study, the minimum number of measurements required to ensure relevant

results for gap analysis was 18 for experimental and 90 for custom-made crowns.

Milan FM et al (2004)44 evaluated the relationship between the application of die-

spacer prior to wax pattern fabrication and metal removal from the inner surface of the casting on

marginal and internal discrepancies of complete cast crowns. One hundred and twenty complete

crowns were cast with palladium-silver alloy melted by gas-oxygen torch or electrical resistance

and cast with a centrifuge casting machine. After casting, the crowns were seated on each type of

different marginal configuration dies (90-degree shoulder, 20-degree beveled shoulder, and 45-

degree chamfered shoulder) with a static load of 90 N during 1 min. Evaluation of the marginal

fit of the specimens was made using a digital micrometer. The crowns were embedded in acrylic

resin and longitudinally sectioned to verify the internal discrepancy that occurred in lateral and

occlusal interfaces with a digital micrometer. The data were submitted to ANOVA and Tukey’s

test with a significance level of 5%. The best marginal and inner fits were obtained with the gas-

oxygen torch source. The 45-degree chamfered shoulder showed the best marginal and inner fit,

and better internal relief was obtained in the crowns abraded with 50 µm Al2O3 particles.

Abhyankar V, Nagda S et al (2005)45 investigated the effect of ring liner and casting

ring temperature on the dimensional changes in morphologic cast posts. Prosthodontic treatment

of an endodontically treated tooth poses a challenge to the practitioner. Endodontic therapy has

provided a solution to retain mutilated teeth. Coronoradicular reconstruction in the form of cast

post and core is used as a method to provide retention and resistance form to the restoration. To

prevent fracture and support crown and bridge, reinforcement in the form of intraradicular

devices is being used. A cast post is one such method. A cast post and core should fit passively

in the canal. Even a minimally oversized post can lead to transfer of stresses to the canal walls

and increase the risk of root fracture. Therefore it is necessary to ensure that there is passive fit

of the post and core. Shrinkage of the mould cavity is desired during the casting process to allow

a passive fit. The effect of lined and unlined rings in the dimensional behavior of the investment

during setting and subsequent heating has been investigated and it is shown that casting made of

unlined rings are undersized.

Boeckler AF et al (2005)4 study describes the correlation between objective

marginal fit and its subjective evaluation by dentists and dental technicians. All crowns showed

marginal gaps as well as marginal overextensions. All marginal gaps and overextensions were in

a clinically acceptable range. Objective measurement of marginal gaps and overextended

margins correlated significantly with their subjective evaluation by dentists and technicians. The

findings regarding the marginal gap and the overextended margin correlated significantly with

the subjective evaluation of the clinical acceptability of dentists and technicians. Evaluations of

dentists and technicians showed a significant correlation. The marginal gap had no significant

influence on the decision among dentists and technicians regarding the marginal fit and the

perceived clinical acceptability of the tested crowns. Overextended margins had significant

effects on the decision of dentists and technicians regarding marginal fit and clinical

acceptability of the crowns.

Brosnon MR et al (2005) 1 studied to examine margin acceptability using an

explorer versus the actual marginal gap widths at four locations on uncemented crowns on three

extracted teeth using both predoctoral students and prosthodontists as evaluators. Upon casting,

marginal gaps ranged from 40µm to 615µm. The data provided evidence that those surfaces

associated with greater marginal gaps tended to have a greater proportion of ratings of “clinically

unacceptable.’’ The proportion of prosthodontists and predoctoral students rating a margin

“clinically unacceptable’’ were highly correlated.

Singh GP, Datta K (2005) 3 evaluated the marginal gap of complete crowns made

by using wet and dry ceramic ring liners using a scanning electron microscope. Two groups of

thirty castings each were prepared with dry and wet ceramic ring liners respectively and assessed

for marginal fit. Results showed that crowns made by using dry ceramic ring liners had

significantly less marginal gap as compared to the crowns made by using wet ceramic ring liners.

Yang CC et al (2007) 46 evaluated the characteristics of commercial quick-heating

phosphate-bonded investments. Two different heating methods – the Quick Heating Method

(QHM) and Conventional Heating Method (CHM) – were used with the investments. The

dimensional accuracy and surface roughness of the nickel-chromium alloy castings obtained

from the investments were also examined. The setting expansion (1.1% to 2.2%) was obtained

after a 30-minute setting time; the fired strength of both investments was greater with QHM

(21.2 to 27.7 MPa) than with CHM (13.8 to 17.9 MPa); the thermal expansion of the investments

was higher with QHM (1.4% TO 1.7%) than with CHM (1.2% to 1.4%). In addition, the surface

roughness of the Ni-Cr castings obtained from the investment was not significantly dependent on

the heating method and the dimensional accuracy of the castings using the investments, are

acceptable.

Bedi A et al (2008)8 evaluated the surface roughness and irregularities of gold

palladium alloy castings obtained using different investment techniques. Within the limitations

of this study, the following conclusions were drawn: (1) The surface roughness values of

castings obtained by 4 investment techniques tested using carbon-free phosphate-bonded

investment material were not significantly different. (2) Specimens allowed to set under

atmospheric pressure are more likely to present surface irregularities than specimens set under

positive pressure. As a result, adjustment and finishing of the crown can be easier for both the

technician and the clinician, while the fit of the restorations can be improved as well.

Materials & Methods This study was conducted to measure the vertical marginal discrepancy and

surface roughness of base metal alloy cast copings made by Inlay Wax and Pattern Resin with

two different methods of casting techniques (conventional casting technique with three stage wax

elimination and accelerated casting technique).

The following materials were used for the study:

1. Inlay wax (GC Corporation, TOKYO, JAPAN) (Fig-5B).

2. Pattern Resin (Acrylic resin for patterns, GC corporation,TOKYO, JAPAN) (Fig-5A).

3. Die lubricant (DIE LUBE WAX SEP, Dentecon, LosAngeles, USA).

4. Sprue wax, 2.5mm diameter (YETI DENTAL, DURON, GERMANY).

5. Ring liner (Flexvest liner, Ivoclar Vivadent,GERMANY).

6. Surfactant spray (Aurofilm, Bego, GERMANY).

7. Phosphate bonded investment powder (PCT Flexvest ivoclar vivadent technical, Italy)

(Fig-6).

8. PBI liquid (PCT Flexvest liquid, Ivoclar vivadent technical,Italy) (Fig-6).

9. Distilled water.

10. Base metal Nikel Chromium Alloy (CB80, DENTSPLY SANKIN, JAPAN) (Fig-7).

11. Aluminium oxide powder for sand blasting (110 micron) (Delta, INDIA).

12. Separating discs (0.25 to 0.7 mm thickness) (Dentorium, New York, USA).

The following equipments were used for the study:

1. Stainless steel master die and stainless steel former assembly (custom-made)

(Fig-1 & 2).

2. Crucible former (Whip mix, USA).

3. Alloy casing rings of 4 cm diameter and 5 cm length (Whip mix, USA).

4. Vacuum power mixer (Tornado product).

5. Muffle furnace (Technico, Technico laboratory products PVT.LTD, Chennai, INDIA).

6. Induction casting machine (LC-cast60, Belgium).

7. Sand blaster (Ideal blaster, Delta, Delta labs, INDIA).

8. High speed alloy grinder (RAY FOSTER DENTAL Equipment, CA).

9. Reichert Polyvar 2 met photo microscope (Reichert AUSTRIA) (Fig-8).

10. Stainless steel coping holder for making the cast coping parallel to the ground for

measuring surface roughness (custom-made) (Fig – 25).

11. Taly Surf computer controlled surface roughness analyzer (Kosaka lab.) (Fig-9).

Description of custom made stainless steel master die and stainless steel former assembly:-

The stainless steel master die and stainless steel former (Fig-1 & 2)employed in this

study was custom made, based on the model employed by Konstantoulakis et al, Schilling et

al for their studies. This assembly essentially is of 2 parts namely, the stainless steel master

die and the stainless steel former which fits over the die. The base has a height of 30mm and

a diameter of 20mm.The base is sectioned along its circumference which divides it into an

upper one third part and a lower two third part. The upper one third can be moved up and

down from the lower two third of the base. This aids in easy removal of wax pattern. The

stainless steel master die simulated a crown preparation with a 10-degree total axial wall

taper. The height of the die and its occlusal diameter is 6mm. The occlusal surface had

occlusal cross hairs (or grooves) to aid in repositioning of the pattern and casting. Four

markings present on the base of the die, separated by 90-degree, each serve as standard

reference points for measurement of the vertical marginal discrepancy of all the cast copings.

A custom-made stainless steel former was fabricated, such that it can be accurately

positioned over the stainless steel master die. The internal surface of the stainless steel former

assembly was larger than the die in all dimensions by 0.5mm uniformly. This was done to

maintain a space of 0.5mm throughout between the master die and former. This space helped to

obtain the wax patterns with a uniform thickness of 0.5mm and a 90-degree shoulder margin.

METHODOLOGY

The following methodology was adopted for the study:

a) Inlay Wax and Pattern Resin fabrication with attachment of sprue and removal of

pattern from master die.

b) Investing the Inlay Wax and Pattern Resin separately.

c) Inlay Wax and Pattern Resin burn out procedure with two different techniques.

d) Casting procedure.

e) Devesting, sprue cutting and finishing the cast coping.

f) Evaluation of vertical marginal discrepancy.

g) Evaluation of surface roughness.

This study was conducted to evaluate the vertical marginal discrepancy and surface

roughness of 40 cast copings obtained by 4 techniques (G-I, G-II, G-III and G-IV) as given

below:

G-I: Conventional casting technique of Inlay Wax copings with 3 stage burnout

procedure (10 samples).

G-II: Accelerated casting technique of Inlay Wax copings with single stage burnout procedure

(10 samples).

G-III: Conventional casting technique of Pattern Resin copings with 3 stage burnout procedure

(10 samples).

G-IV: Accelerated casting technique of Pattern Resin copings with single stage burnout

procedure (10 samples).

a) INLAY WAX & RESIN PATTERN FABRICATION WITH ATTACHMENT OF SPRUE

AND REMOVAL OF PATTERN FROM MASTER DIE:

The custom made stainless steel master die and former assembly as described previously was

used to fabricate standardized wax and resin pattern. A fine coating of die lubricant (Die Lube

Wax sep, Dentecon, Los Angeles, USA) was applied on to the die and the fitting surface of the

stainless steel former using small paint brush for easy removal of Inlay Wax and Pattern Resin

from the die and prevents the pattern from adhering to the stainless steel former. The stainless

steel former was filled with molten Inlay Wax (GC Corporation, TOKYO, JAPAN) and pressed

on the stainless steel die. The die former assembly was held together for 1 minute with finger

pressure. The die separated from the former and the Wax pattern obtained. The excess Inlay Wax

was trimmed using a PKT no.4 carver/BP blade. For easy removal of the Wax pattern and to

minimize distortion, the Inlay Wax pattern was sprued with preformed wax sprue (YETI

DENTAL, DURON, GERMANY) of 2.5 mm diameter and 2.5cm length and was attached to the

Inlay Wax and Pattern Resin with a reservoir 3mm from the end of the pattern. One end of the

sprue was attached to the pattern at an angle of 450. The Inlay Wax and Pattern Resin was

removed from the die with a gentle pressure and the other end of the sprue was attached to the

crucible former. The Inlay Wax and Pattern Resin was cleaned to obtain clean pattern with

surfactant spray (Aurofilm, Bego, GERMANY) to reduce surface tension of all Inlay Wax and

Pattern Resin there by improving wettability with the investment. Then pattern was checked with

wax caliper to verify the even distribution of 0.5mm thickness. A total of 20 Inlay Wax patterns

were fabricated. Similarly 20 Pattern Resin copings were made. They were divided into 4

groups, namely G-I, G-II, G-III and G-IV and ten specimens were prepared for each group of the

study.

b) INVESTING THE INLAY WAX & PATTERN RESIN SEPARATELY:

After spruing each pattern, they were invested immediately to minimize distortion. Casting

rings were lined with one non overlapping layer of wet ceramic ring liner (Flexvest liner, Ivoclar

vivadent, GERMANY) by leaving 3mm of space below and top. Inlay Wax and Pattern Resin

were invested individually with carbon free, phosphate bonded investment material (PCT

Flexvest, Ivoclar vivadent technical, Italy). The liquid is prepared by mixing the PCT Flexvest

liquid with distilled water at a ratio of 80:20 respectively to achieve optimum expansion;

therefore 800 ml of Flexvest PCT liquid (Ivoclar, vivadent technical, ITALY) was mixed

with200 ml of distilled water to obtain the above mentioned ratio. Weight of 60 gm of phosphate

bonded investment (PCT Flexvest, Ivoclar vivadent technical, Italy) was mixed with 13ml of

premixed liquid. Initially the powder and liquid mixed normally with spatula to wet the powder

particles thoroughly and then mechanical mixing was done under vacuum using vacuum mixer

(Tornado products) for 90 seconds. Once the investment was mixed the pattern was painted with

a thin layer of investment using small paint brush to avoid air bubble entrapment. The casting

ring (whip mix, USA) was positioned on the crucible former, and the remainder of the

investment was poured slowly in to the ring under vibrations. Excessive vibration is avoided to

prevent formation of the bubbles and separation of the pattern from the sprue. The investment

pattern was allowed to bench set for 30 minutes. All the 40 patterns in four groups are invested

in the same procedure.

c) INLAY WAX & PATTERN RESIN BURN OUT PROCEDURE WITH TWO DIFFERENT

TECHNIQUES:

The Inlay Wax and Pattern Resin burnout procedure is different for each of the four test

groups (G-I, G-II, G-III and G-IV) as described below:

1) G-I: Conventional casting technique of Inlay Wax copings with 3 stage burnout procedure

(10 samples)

After a 30 minute bench set time, the set investment mold ring was placed in burnout

furnace (Technico, Technico laboratory products PVT LTD, Chennai, INDIA). The wax burnout

was done using a programmed preheating schedule, i.e. the ring was kept in the furnace at room

temperature and was heated till 2700 c rise of temp at a rate of 80 c / min and was held at this

temperature for 30 min. Then ring was heated from 2700 c to 5600 c rise of temp at a rate of 80 c /

min and was held at this temperature for 30 min. Terminal burnout was carried out from 5600 c

to 8500 c rise of temp at a rate of 80 c / min and was held at this temperature for 30 min. The

investment mold was placed initially into the furnace such that it allows for the escape of molten

wax and vapours. The investment mold was later averted near the end of the burnout cycle with

the sprue hole facing upward to enable escape of the entrapped gases and allow oxygen to

contact to ensure complete burnout of the wax pattern and mold expansion.

2) G-II: Accelerated casting technique of Inlay Wax copings with single stage burnout

procedure (10 samples)

After a 30 minute bench set time, the set investment mold ring was placed directly in a

preheated burnout furnace (Technico, Technico laboratory products PVT LTD, Chennai, INDIA)

at 8500 c, and held for 30 min to ensure complete burnout of the wax pattern. The investment

mold was placed into the furnace such that it allows for the escape of molten wax and vapours.

The investment mold was later averted near the end of the burnout cycle with the sprue hole

facing upward to enable escape of the entrapped gases & allow oxygen to contact to ensure

complete burnout of the wax pattern & mold expansion.

3) G-III: Conventional casting technique of Pattern Resin copings with 3 stage burnout

procedure (10 samples)

After a 30 minute bench set time, the set investment mold ring was placed in burnout

furnace (Technico, Technico laboratory products PVT LTD, Chennai, INDIA). The resin

burnout was done using a programmed preheating schedule, i.e. the ring was kept in the furnace

at room temperature and was heated till 2700 c rise of temp at a rate of 80 c / min and was held at

this temperature for 40 min. Then ring was heated from 2700 c to 5600 c rise of temp at a rate of

80 c / min and was held at this temperature for 40 min. Terminal burnout was conducted from

5600 c to 8500 c rise of temp at a rate of 80 c / min and was held at this temperature for 40 min.

The investment mold was placed initially into the furnace such that it allows for the escape of

molten Pattern Resin and vapours. The investment mold was later averted near the end of the

burnout cycle with the sprue hole facing upward to enable escape of the entrapped gases and

allow oxygen to contact to ensure complete burnout of the Pattern Resin and mold expansion.

4) G-IV: Accelerated casting technique of Pattern Resin copings with single stage burnout

procedure (10 samples)

After a 30 minute bench set time, the set investment mold ring was placed directly in a

preheated burnout furnace (Technico, Technico laboratory products PVT LTD, Chennai, INDIA)

at 8500 c, and held for 40 min to ensure complete burnout of the Pattern Resin. The investment

mold was placed into the furnace such that it allows for the escape of molten Pattern Resin and

vapours. The investment mold was later averted near the end of the burnout cycle with the sprue

hole facing upward to enable escape of the entrapped gases & allow oxygen to contact to ensure

complete burnout of the Pattern Resin & mold expansion.

d) CASTING PROCEDURE:

The casting procedure was performed quickly to prevent heat loss from the ring resulting in

the thermal contraction of the mold. The preheated casting crucible and the investment mold

were taken out of the furnace and were placed in the casting machine. The casting was done in

an induction casting machine (LC-Cast 60). The Nickel-Chromium alloy (CB 80, non precious

alloy, DENTSPLY) was heated sufficiently till the alloy ingot turned to molten state and the

lever was released and centrifugal force ensures the completion of the casting procedure.

Investment was allowed to cool down to room temperature. The casting procedure followed was

same for all the test samples. A total of 40 castings were made to obtain cast copings for the

evaluation in this study. Among the 40 castings 10 were obtained for G-I technique, 10 for G-II

technique, 10 for G-III technique and 10 for G-IV technique.

e) DEVESTING, SPRUE CUTTING AND FINISHING THE CASTING:

Following casting, the hot casting ring was bench cooled to room temperature, and

devesting was carried out carefully by tapping the button of the casting with mallet. Adherent

investment was removed from the casting initially with hand instrument and then by sandblasting

with 110 micron alumina at 80 psi pressure. The sprue was removed at the junction of the coping

with an ultra thin abrasive disc (Dentorium, New York, USA) and the copings were subjected to

ultrasonic cleaning & checked visually. The internal surface was inspected & relieved of all

nodules with a round carbide bur. This procedure was followed for each of the ten samples of the

four test groups.

f) EVALUATION OF VERTICAL MARGINAL DISCREPANCY:

Each casting was seated on the stainless steel die with finger pressure until

resistance was met. Microscopic measurements were recorded at 80 X magnification

perpendicular to the axial wall with a photomicroscope (Reichert Polyvar 2 met

photomicroscope, Reichert, AUSTRIA) at the department of Nuclear Physics, Madras

University, Chennai, INDIA. Measurements were recorded from coping margin to the stainless

steel die margin for vertical marginal discrepancy. Marginal discrepancies were measured to the

nearest micron on each casting at the 4 predetermined sites on the base of the stainless steel die

separated by 900 each. The same procedure was followed to record the vertical marginal

discrepancy for each of the ten test samples belonging to the four test groups. The measurements

thus obtained were tabulated and statistically analyzed.

g) EVALUATION OF SURFACE ROUGHNESS:

Description of custom- made stainless steel coping holder for measuring surface roughness:

For measuring surface roughness the cast metal coping should be exactly perpendicular

to the diamond indentor. The stainless steel coping holder employed for measuring surface

roughness was custom-made, with dimensions of 5cm in length, 5cm in breadth and 1.5cm in

height. The coping holder has a depression horizontally in the center to accommodate the coping

precisely in to it. The dimensions of the depression were 6.5mm in length, 3.5mm in depth at the

base of the coping, 3.25mm in depth at occlusal surface of the coping with a taper of 100 from

base to top of the coping. Only half of the coping will be inside the depression, when it is placed

horizontally in the custom-made stainless steel coping holder.

Each metal coping was placed horizontally on the coping holder for making the coping

perpendicular to the diamond indentor. The surface roughness was measured by passing the

diamond indentor over the coping surface for a distance of 1.6mm.The readings were obtained

graphically as crest and troughs with surface roughness analyzer (Taly Surf Computer Guided

Surface Roughness Analyzer, Kosaka Lab) at the Manufacturing Engineering Department, Anna

University, Chennai, INDIA. Surface roughness was measured to the nearest micron at 3

surfaces on the coping. The same procedure was followed to record the surface roughness for

each of the ten test samples belonging to the four test groups. The measurements thus obtained

were tabulated and statistically analyzed.

Fig-1 Axial view of custom made stainless steel former assembly (A) and stainless steel master die (B)

BA

Fig-2 Occlusal View of Custom made stainless steel former assembly (A) and stainless steel master die (B)

BA

20 MM

22 MM

2 MM

6 MM

50

6.5 MM

6 MM

3a

8 MM

6.5 MM

Fig -3a. Line Diagram of custom- made stainless steel master die.

3b. Line diagram of custom-made stainless former

20 MM

20 MM

6.5 MM

7 MM

3b

Fig – 4 Line diagram of custom-made stainless steel master die & stainless steel former in place. Colored space indicates the space for wax pattern.

a . Custom – made stainless steel die. b. Custom- made stainless steel former.

b

a

Fig – 5A Pattern Resin, 5B Inlay wax

A B

Fig – 6 Investment powder & special Liquid

Fig – 7 Nickel – Chromium alloy

Fig – 8 Photo Microscope

Fig – 9 Surface Roughness analyzer

Fig – 10 Preparation of Inlay Wax Pattern

Fig – 11 Preparation of Pattern Resin

Fig – 12 Wax coping showing 0.5mm thickness

Fig – 13 Pattern Resin coping showing 0.5mm thickness

Fig – 14 Wax Pattern attached to crucible former

Fig – 15 Ring Liner Placed 3mm short of Ring Margin

Fig – 17 Divested casting Fig – 16 Wax pattern in position in the casting ring

Fig – 18 Cast coping seated on the die

Fig – 19 Metal Coping showing 0.5mm thickness

Fig – 20 Marginal gap of 34.02 at 80x magnification by using photo microscope of a cast coping obtained by conventional casting technique

employing three stage wax elimination. a- Margin of custom made stainless steel die, b-Marginal gap, c- Margin of cast coping

Fig – 21 Marginal gap of 44.39 at 80x magnification by using photo microscope of a cast coping obtained by Accelerated casting technique employing single stage wax elimination

a cb

a c

b

Fig – 22 Marginal gap of 37.06 at 80x magnification by using photo microscope of a cast coping obtained by conventional casting

technique employing three stage Resin elimination

Fig – 23 Marginal gap of 46.86 at 80x magnification by using photo microscope of a cast coping obtained by Accelerated casting

technique employing single stage Resin elimination

a cb

a cb

6.5mm mm

5 cm

1.5 cm

Fig – 25 Custom made stainless steel coping holder with coping

Fig – 24 line diagram of custom made stainless steel coping holder

Fig – 26 Custom made stainless steel coping holder with coping on

surface roughness Analyzer

Cutoff 0.8mm E.length 2.371mm S.length 1.185mm

Ra 4.9µm Ry 29.6µm Rz 21.3µm

Cutoff 0.8mmE.length 1.014mmS.length 1.014mm

Ra 4.4µm Ry 25.5µm Rz 21.2µm

Fig – 27 surface roughness graph obtained by conventional casting technique with three stage wax elimination

Ra - Roughness AverageRy – Maximum height of the profile

Rz – Average Maximum height of the Profile

Fig – 28 surface roughness graph obtained by Accelerated casting technique with single stage wax elimination

S.Length – Sampling Length E. Length – Evaluation Length

Cutoff 0.8mmE.length 1.134mmS.length 1.134mm

Ra 10.7µm Ry 69.3µm Rz 36.6µm*

Cutoff 0.8mm

E.length 1.600mm

S.length 0.800mm

Ra 7.1µm

Ry 48.5µm

Rz 32.4µm*

Fig – 29 surface roughness graph obtained by conventional casting technique with three stage Resin elimination

Fig – 30 surface roughness graph obtained by Accelerated casting technique with single stage Resin elimination

Results

Table 1 shows the basic data of the results and mean obtained in G-I (Conventional casting

technique of Inlay Wax copings with 3 stage burnout procedure) to evaluate the vertical marginal

discrepancy in microns (μ).

Table 1

S.No. Point 1(µ) Point 2(µ) Point 3(µ) Point 4(µ) Mean(µ)1 28.14 37.45 28.66 29.02 30.822 35.16 29.96 32.41 40.51 34.513 36.26 37.54 31.05 30.29 33.784 39.06 41.52 31.31 33.46 36.345 29.10 32.87 36.72 29.64 32.086 46.15 32.67 31.01 28.16 34.507 34.26 38.14 31.35 45.18 37.238 35.59 42.33 29.14 30.23 34.329 30.09 31.11 29.81 38.75 32.4410 36.60 33.54 30.64 36.05 34.21

Table 2 shows the basic data of the results and mean obtained in G-II (Accelerated casting

technique of Inlay Wax copings with single stage burnout procedure) to evaluate the vertical

marginal discrepancy in microns (μ).

Table 2

S.No. Point 1(µ) Point 2(µ) Point 3(µ) Point 4(µ) Mean(µ)1 45.77 45.67 39.37 45.08 43.972 43.45 38.06 40.88 44.17 41.643 40.59 35.86 43.22 42.45 40.534 42.57 46.71 45.62 45.81 45.185 46.77 41.08 49.28 46.87 46.006 46.08 46.11 45.25 46.14 45.907 44.58 43.29 45.01 47.87 45.198 45.74 42.05 41.52 45.97 43.829 46.22 48.46 43.09 47.25 46.2610 47.32 42.79 44.91 46.76 45.44

Table 3 shows the basic data of the results and mean obtained in G-III (Conventional casting

technique of Pattern Resin copings with 3 stage burnout procedure) to evaluate the vertical

marginal discrepancy in microns (μ).

Table 3

S.No. Point 1(µ) Point 2(µ) Point 3(µ) Point 4(µ) Mean(µ)1 32.76 37.43 35.68 40.52 36.602 41.52 35.78 31.86 37.49 36.663 38.24 36.72 42.05 39.61 39.164 43.72 34.65 32.43 32.45 35.815 36.29 30.24 39.67 41.15 36.846 42.24 40.15 38.09 35.78 39.077 41.22 37.62 30.15 34.62 35.908 39.82 44.59 32.18 35.36 37.999 33.42 36.78 29.21 34.62 33.5110 35.69 38.57 39.86 42.09 39.05

Table 4 shows the basic data of the results and mean obtained in G-IV (Accelerated casting

technique of Pattern Resin copings with single stage burnout procedure) to evaluate the vertical

marginal discrepancy in microns (μ).

Table 4

S.No. Point 1(µ) Point 2(µ) Point 3(µ) Point 4(µ) Mean(µ)1 51.25 49.62 45.87 46.75 48.372 49.62 40.71 39.85 43.62 43.453 42.67 38.75 42.23 39.12 40.694 44.13 50.22 46.15 49.72 47.565 46.86 43.12 49.37 51.16 47.636 47.12 46.85 51.32 50.17 48.867 45.52 43.71 46.02 47.69 45.748 46.64 39.22 51.29 49.56 46.689 55.62 50.35 47.92 47.92 50.6510 45.11 47.93 53.19 49.76 49.00

Graph 1 shows the basic data of the results and mean obtained in G-I (Conventional casting

technique of Inlay Wax copings with 3 stage burnout procedure) to evaluate the vertical marginal

discrepancy in microns (μ).

Graph 1

Graph 2 shows the basic data of the results and mean obtained in G-II (Accelerated casting

technique of Inlay Wax copings with single stage burnout procedure) to evaluate the vertical

marginal discrepancy in microns (μ).

Graph 2

Graph 3 shows the basic data of the results and mean obtained in G-III (Conventional casting

technique of Pattern Resin copings with 3 stage burnout procedure) to evaluate the vertical

marginal discrepancy in microns (μ).

Graph 3

Graph 4 shows the basic data of the results and mean obtained in G-IV (Accelerated casting

technique of Pattern Resin copings with single stage burnout procedure) to evaluate the vertical

marginal discrepancy in microns (μ).

Graph 4

Table 5 shows the mean vertical marginal discrepancy obtained from basic mean values of four

techniques (G-I, G-II, G-III and G-IV) calculated in microns (μ).

Table 5

G-I(µ) G-II(µ) G-III(µ) G-IV(µ)

Mean (μ) 34.02 44.39 37.06 46.86

Graph 5 shows the comparison of the mean vertical marginal discrepancy obtained from basic

mean values of four techniques.

Graph 5

The obtained results were statistically analysed, mean and standard deviations were

estimated for each study group. The data were analyzed by use of t-test. In the present study,

p<0.001 was considered as the level of significance.

Table 6 shows the test of significance for the mean obtained from four techniques (G-I, G-II, G-

III and G-IV). t-test was used to calculate the P value.

Table 6

Technique Mean(μ) S.D. p-value G-I 34.02 1.91

<0.001 ** G-II 44.39 1.93 G-III 37.06 1.80 G-IV 46.86 2.93

Note: - ** denotes significant at 1% level.

* denotes significant at 5% level.

Inference:

The table 6 shows the comparison of the mean value of the vertical marginal discrepancy

obtained for each of the four techniques. Since the p-value is less than 0.001, there is highly

significant difference between the four techniques with regard to vertical marginal discrepancy.

The mean vertical marginal discrepancy values obtained from the two conventional casting

techniques G-I (34.02µ) and G-III (37.06µ) has minimal statistical difference. The mean vertical

discrepancy values obtained from the two accelerated casting techniques G-II (44.39µ) and G-IV

(46.86µ) has minimal statistical difference. However higher values of vertical marginal

discrepancy were found with accelerated casting techniques G-II (44.39µ) and G-IV (46.86µ)

when compared to the conventional casting techniques G-I (34.02µ) and G-III (37.06µ) and this

difference was statistically significant.

Table 7 shows the basic data of the results and mean obtained in G-I (Conventional casting

technique of Inlay Wax copings with 3 stage burnout procedure) to evaluate the surface

roughness in microns (μ).

Table 7

S.No. Area 1(µ) Area 2(µ) Area 3(µ) Mean(µ)1 4.90 5.40 5.90 5.402 4.60 5.50 6.20 5.433 3.70 4.10 4.30 4.034 4.20 3.90 4.10 4.075 3.80 4.30 4.20 4.106 4.40 3.70 4.10 4.077 4.20 3.80 4.00 4.008 4.80 4.10 4.40 4.439 5.00 4.20 4.30 4.5010 3.70 4.10 3.60 3.80

Table 8 shows the basic data of the results and mean obtained in G-II (Accelerated casting

technique of Inlay Wax copings with single stage burnout procedure) to evaluate the surface

roughness in microns (μ).

Table 8

S.No. Area 1(µ) Area 2(µ) Area 3(µ) Mean(µ)1 4.90 4.40 5.60 4.972 4.20 7.30 4.50 5.333 4.60 6.90 4.50 5.334 6.30 5.80 6.10 6.075 6.20 6.00 5.90 6.036 4.80 5.50 5.70 5.337 5.20 4.90 4.70 4.938 4.70 5.30 5.10 5.039 6.10 5.70 5.40 5.7310 5.80 5.50 4.80 5.37

Table 9 shows the basic data of the results and mean obtained in G-III (Conventional casting

technique of Pattern Resin copings with 3 stage burnout procedure) to evaluate the surface

roughness in microns (μ).

Table 9

S.No. Area 1(µ) Area 2(µ) Area 3(µ) Mean(µ)1 6.40 10.70 6.30 7.802 10.20 7.00 6.40 7.873 3.40 4.00 5.80 4.404 8.70 9.20 7.10 8.335 9.30 7.60 8.10 8.336 10.10 8.70 6.30 8.377 9.70 6.50 8.20 7.138 4.90 8.30 10.70 7.979 7.30 10.60 9.40 9.1010 6.70 7.50 8.80 7.67

Table 10 shows the basic data of the results and mean obtained in G-IV (Accelerated casting

technique of Pattern Resin copings with single stage burnout procedure) to evaluate the surface

roughness in microns (μ).

Table 10

S.No. Area 1(µ) Area 2(µ) Area 3(µ) Mean(µ)1 3.20 12.10 12.20 9.172 5.12 10.50 7.10 7.573 5.70 6.20 8.30 6.734 11.60 5.70 8.90 8.735 8.50 12.60 9.40 10.176 9.70 10.90 11.70 10.777 10.80 11.60 8.70 10.378 6.90 10.50 8.60 8.679 8.70 11.80 9.60 10.0310 12.50 9.70 8.40 10.20

Graph 6 shows the basic data of the results and mean obtained in G-I (Conventional casting

technique of Inlay Wax copings with 3 stage burnout procedure) to evaluate the surface

roughness in microns (μ).

Graph 6

Graph 7 shows the basic data of the results and mean obtained in G-II (Accelerated casting

technique of Inlay Wax copings with single stage burnout procedure) to evaluate the surface

roughness in microns (μ).

Graph 7

Graph 8 shows the basic data of the results and mean obtained in G-III (Conventional casting

technique of Pattern Resin copings with 3 stage burnout procedure) to evaluate the surface

roughness in microns (μ).

Graph 8

Graph 9 shows the basic data of the results and mean obtained in G-IV (Accelerated casting

technique of Pattern Resin copings with single stage burnout procedure) to evaluate the surface

roughness in microns (μ).

Graph 9

Table 11 shows the mean surface roughness obtained from basic mean values of four techniques

(G-I, G-II, G-III and G-IV) calculated in microns (μ).

Table 11

G-I(µ) G-II(µ) G-III(µ) G-IV(µ)

Mean (μ) 4.38 5.41 7.8 9.24

Graph 10 shows the comparison of mean surface roughness obtained from basic mean values of

4 techniques.

Graph 10

The obtained results were statistically analysed, mean and standard deviations

were estimated for each study group. The data were analyzed by use of t-test. In the present

study, p<0.001 was considered as the level of significance.

Table 12 shows the test of significance for the mean obtained from four techniques (G-I, G-II, G-

III and G-IV). t-test was used to calculate the p-value.

Table 12

Technique Mean(μ) S.D. p-value G-I 4.38 0.58

<0.001 ** G-II 5.41 0.41 G-III 7.8 1.26 G-IV 9.24 1.32

Inference:

The table 12 shows the comparison of the mean value of the surface roughness obtained

for each of the four techniques. Since the p-value is less than 0.001, there is highly significant

difference between the four techniques with regard to surface roughness. The mean surface

roughness values obtained from castings done with Inlay Wax in G-I (4.38µ) and G-II (5.41µ)

techniques has minimal statistical difference. However higher values of surface roughness were

found from castings done with Pattern Resin in both G-III (7.8µ) and G-IV (9.24µ) techniques

and this difference was statistically significant.

Discussion Casting metals by lost-wax process has been recognized in industry and in arts for many

years. No record exists when and where this type of casting procedure was first developed.26In

dentistry lost wax process of casting metals became common practice after it was introduced by

Taggart in 1907.9 Castings made by Taggart were generally too small and did not fit the cavities

properly.2, 26

The fit of a casting can be defined best in terms of “misfit” measured at various points

between the casting surface and the tooth. Measurements between the casting and the tooth can

be made from points along the internal surface of the margin or on the external surface of the

casting.4,5Clinically important measurements are the marginal gap, which is the distance from the

internal surface of the casting to the axial wall of the preparation at the margin.4

The accuracy of fit of casting is essential for longevity and clinical success of the cast

restoration in the oral cavity.3 Lack of adequate fit is potentially detrimental to both the tooth and

the periodontal tissues. Clinically, defective margins act as a niche for plaque. Insufficient

marginal fit can cause secondary caries below the margins of the crown.4, 5, 11, 38 Precise marginal

sealing is important in dental restorations to fulfill biologic, physical and cosmetic requirements

lest the restoration will fail.

Marginal discrepancies are inevitable, despite careful attention to waxing, investing and

casting. It is one of the tasks of the luting cement to close these discrepancies. However, cement

can be washed out under the margins if gap is too large.4 Because of their solubility, luting

cements in general, have been described as weak link in restoring teeth with cast restorations.29

The rate of cement dissolution has been related empirically to the degree of marginal opening,

thus larger the marginal gap and subsequent exposure of the dental luting cement to oral fluids,

the more rapid is the rate of cement dissolution.29 Thus for the success of cast restoration,

marginal gap must be as minimal as possible.

From the time that dental castings were first introduced, at about the turn of the century,

efforts have been made to produce more accurate and better fitting casting with minimal

marginal discrepancy.24 The accuracy of fit is affected by the quality of tooth preparation, the

impression, the working cast, the quality of the wax that is used, and the accuracy of the casting

procedures. The accuracy of casting is subjected to the volumetric changes occurring due to

shrinkage of wax and alloys. This shrinkage can be compensated by normal setting expansion,

hygroscopic expansion and/or thermal expansion of the investment.2, 3

The casting process used in dentistry based on the lost wax technique has been receiving

continuous investigations. The majority of the efforts deal with the conventional casting

technique. The conventional investing and casting techniques require atleast 1 hour bench set for

the investment. The usual burnout temperatures for phosphate-bonded investments range from

7500 to 10300C. The highest temperatures are required for base metal alloys, especially those that

are used for ceramometal restorations. Initially one stage wax burnout procedure was followed

traditionally to achieve complete burnout as well as thermal expansion. Later on manufacturers

added a two or three stage burnout procedures to conventional techniques to achieve maximum

thermal expansion by maximum conversion of the refractory used in the investment. The entire

process involving phosphate-bonded investments takes a long time; the demand for time saving

is more. Investment manufacturers have attempted to answer this demand and accelerated casting

techniques have been reported in an effort to achieve similar quality results in significantly less

time. These techniques have the ability to shorten the investing and casting process, there by

improving productivity. This accelerated technique uses a typical method of setting and burnout

of the phosphate-bonded investment.

Usually, a bench setting time of 12 to 15 minutes and a mold burnout time of 12 to 15

minutes are employed before the casting process. The first published attempt to accelerate the

lost wax technique with the use of phosphate-bonded investment for complete crown was made

in 1988 by Marzouk and Kerby who recognized the importance of investment temperature. Their

study revealed no statistical circumferential difference between investment groups introduced in

a 13500C preheated oven after 15 minute bench set and the conventional technique.12 Campagni

et al30 tested the fit of dowel and cores made of noble alloy by an accelerated casting technique,

and similar studies were subsequently conducted by Bailey and Sherrard and Schneider.32,33 All

these investigations concluded that the use of a predetermined bench set time reduced investment

weakness and that standardized accelerated procedures for all types of investments were

inadvisable.12

Blackman11 measured marginal sharpness and diameter changes for crowns cast with type III

gold alloy by using phosphate-bonded investment and rapid burnout techniques, and concluded

that rapid mold preparation resulted in loss of marginal fineness. Konstantoulakis9 evaluated the

marginal fit and surface roughness of complete cast crowns made with a conventional and

accelerated casting technique and reported that crowns fabricated with the accelerated casting

technique were not significantly different from those fabricated with conventional technique.

Schilling et al12 evaluated marginal gap of crowns made with a phosphate-bonded investment and

accelerated technique and reported that the marginal gaps for castings made with an accelerated

technique showed no statistical difference when compared with conventional casting technique.

The accuracy of base metal alloy castings obtained by different investing and burnout procedures

with phosphate-bonded investments were not adequately studied. Though studies9,12 have

reported that marginal discrepancies by accelerated casting technique are within the clinically

acceptable limits, some studies9,11,12 have reported that this procedure is technique sensitive.

Surface roughness of dental castings is an important aspect of their quality and can potentially

affect their marginal fit and the time required for finishing and polishing. It is preferable that the

surface of as-cast crowns be smoother to obtain better marginal fit and curtail finishing or

polishing time. Surface roughness of castings is believed to be affected by several factors such as

type of alloy, mold material, mold temperature, wax pattern, and casting machine.9 Bedi et al

reported that specimens set under atmospheric pressure are much more likely to present surface

irregularities than specimens set under positive pressure. The use of pressure can help produce

castings with fewer surface irregularities.8

Thus, the fit of the casting is a critical issue in determining the longevity of the restoration

and investing and burnout procedures are an important parameter in determining the quality of

the fit of the casting. The surface roughness of the casting affects the ceramic-metal interface

bond. Hence, this study was conducted to investigate the differences in the marginal discrepancy

and surface roughness of base metal alloy cast copings employing two different techniques with

two different materials to achieve thermal expansion of the investment. The introduction of

ceramometal technology required the use of higher melting range alloys to with stand the firing

cycle of porcelain with out noticeable distortion. Base metal alloys are one of the alloys that are

routinely used for obtaining ceramometal restorations. The alloy used in this study was a nickel-

chromium alloy used for ceramometal restorations. An investment that can resist higher

temperatures and higher stress during casting2 is required. A phosphate-bonded investment

fulfills these requirements and hence used in this study.

A custom made stainless steel master die was used to fabricate the standardized Inlay Wax and

Pattern Resin. The master die was based on the models employed in similar previous studies.9,12

This standardized stainless steel dies facilitated in standardizing the dimensions of the test Inlay

Wax and Pattern Resin. Four markings present on the base of the die, separated by 90-degree,

each serve as standard reference points for measurement of the vertical marginal discrepancy of

all the cast copings. Another custom made stainless steel coping holder was used for

measurement of the surface roughness. This coping holder was fabricated for making the coping

perpendicular to the surface roughness analyzer. Each pattern was immediately invested to

minimize distortion.2,3,12 Different ratios of special liquid to distilled water have been

recommended to obtain the required mold expansion. In this study, the ratio of special liquid to

distilled water of approximately 80:20 in volume was employed. This special liquid to distilled

water ratio has been shown to offer adequate expansion for complete crown castings, as

recommended by the manufacturer and hence employed in this study. The liquid to powder ratio

was as recommended by the manufacturer i.e., 60g powder-13ml liquid.

Vacuum mixing was done and investing of all the samples was done as recommended by the

manufacturer. Two different techniques with two different materials were used for achieving

complete burnout: conventional casting technique with three stage Inlay Wax and Pattern Resin

elimination, accelerated casting technique with single stage Inlay Wax and Pattern Resin

elimination.

The conventional burnout procedures usually recommend burn out temperatures for

phosphate-bonded investments in a range of 7500c to 10300c. The highest temperatures are

required for base metal alloys, especially those that are used for ceramometal restorations.6 Some

manufacturers recommend additional holding stages (either 2-stage/3-stage) during burn out for

their investments. This could be attributed to maximum conversion of the refractories used in the

investment there by achieving maximum expansion of the investment. A three stage burn out

procedure was employed for obtaining the cast copings for the conventional technique as it was

recommended by the manufacturer (PCT Flexvest, Ivoclar) of the investment employed in this

study.

Accelerated techniques are time saving and hence offer advantage to commercial laboratories.

The pattern is invested, casted, and delivered in a cost-effective, time-saving manner.

Accelerated casting technique was employed for obtaining the cast copings for the second and

fourth test groups in this study. The manufacturer of the phosphate-bonded investment material

(PCT Flexvest) employed in this study recommends that the investment can be used for an

accelerated as well as for the conventional casting technique with three stage wax/resin

elimination. Hence, this material was chosen for investing the wax/resin for all the test groups in

this study. Also using a single investment material for all the test groups helps to eliminate any

variability in the test results.

The casting procedure was performed by using an induction casting machine. All castings,

one at a time were seated on the stainless steel die with finger pressure till the resistance is

obtained and the vertical marginal discrepancy was measured on four predetermined areas that

were marked on the metal die using a photomicroscope (Reichert Polyvar 2 met photo

microscope, Reichert, AUSTRIA) at a magnification of 80X. The results of this study have been

tabulated as a basic data and interpretation of this data was done by statistical analysis.

All castings, one at a time were seated on the stainless steel die and the surface roughness

was measured on three surfaces using a Taly Surf computer controlled surface roughness

analyzer (Kosaka lab.)

The basic data for vertical marginal discrepancy shows a mean value of 34.02 µm

for conventional casting technique of Inlay Wax copings with three stage burn out procedure (G-

I), 44.39 µm for accelerated casting technique of Inlay Wax copings with single stage burn out

procedure (G-II), 37.06 µm for conventional casting technique of Pattern Resin copings with 3

stage burn out procedure (G-III) and 46.86 µm for accelerated casting technique of Pattern Resin

copings with single stage burn out procedure (G-IV).

The basic data for surface roughness shows a mean value of 4.38 µm for conventional

casting technique of Inlay Wax copings with three stage burn out procedure (G-I), 5.41 µm for

accelerated casting technique of Inlay Wax copings with single stage burn out procedure (G-II),

7.80 µm for conventional casting technique of Pattern Resin copings with 3 stage burn out

procedure (G-III) and 9.24 µm for accelerated casting technique of Pattern Resin copings with

single stage burn out procedure (G-IV).

The statistical analysis by t-test indicated that the difference in the vertical marginal

discrepancy and surface roughness measurements for the four techniques showed the p-value of

< 0.001. This denotes significance at 1% level. The mean vertical marginal discrepancy values

obtained with the two conventional casting techniques has minimal statistical difference between

copings made with Inlay Wax and Pattern Resin. However higher values of vertical marginal

discrepancy were found with accelerated casting technique copings made with Inlay Wax and

Pattern Resin when compared with the conventional casting technique.

The mean surface roughness values obtained with the copings made with

Inlay Wax has minimal statistical difference in both G-I and G-II techniques. However higher

values of surface roughness were found with copings made with Pattern Resin in both G-III and

G-IV techniques.

The results indicate that, with in the limitations of the study, the castings produced by

conventional casting technique showed a lesser vertical marginal discrepancy and surface

roughness values than the castings produced using accelerated casting technique. The castings

made with Inlay Wax have lesser marginal discrepancy and lower surface roughness values

when compared with the Pattern Resin.

Papadopoulos and Axelsson reported a superior fit of crowns on dies if phosphate-bonded

investment moulds were prepared with longer burn out schedules; marginal gaps were 5 times

greater with shorter burn out schedules.11 Longer burn out cycles as recommended by

manufacturer were used in this study and was probably the reason for lesser vertical marginal

discrepancy and surface roughness values of castings made by conventional casting technique as

compared to the castings produced using accelerated casting technique. Though the marginal

discrepancy and surface roughness due to accelerated casting technique is significantly larger

than the conventional casting technique in the study, the mean marginal discrepancy of 44.39 µm

& 46.86 µm obtained by accelerated casting technique with Inlay Wax and Pattern Resin in this

study is with in the clinically acceptable limits. Accelerated techniques may take advantage of

characteristic exothermal setting reaction of phosphate-bonded investments. Heat-enhanced

setting expansion continues uninterrupted as the mold is transferred into a preheated furnace for

thermal expansion.12 This may probably be the reason for the marginal discrepancy and surface

roughness of cast copings by accelerated technique to be with in the clinically acceptable limit.

In this study two types of pattern materials, Inlay Wax and Pattern Resin were used. Most

of the studies by Schneider,33 Bailey and Sherrard32 deal pattern resin with intra coronal

restorations (post & core). Alan Iglesias MS34 compared the marginal discrepancy of both intra

coronal and extra coronal restorations using inlay wax, auto polymerized acrylic resin and two

types of light polymerized resin pattern materials. The results showed that light polymerized

resin pattern was superior with less marginal discrepancy than other two materials. The author

compares the incremental and bulk addition techniques, in which incremental technique

produced smaller marginal discrepancy than the bulk techniques.

The result of this study is correlated with Alan Iglesias study. 34 The results showed that

marginal discrepancy was minimal for Inlay Wax when compared with the auto polymerized

resin pattern with the bulk technique for extra coronal restorations. The auto polymerized resin

pattern due to high polymerization shrinkage has greater marginal discrepancy when compared

with the wax.

Accelerated casting technique has greater marginal discrepancy when compared with

the conventional casting technique. This may be due to bulk addition of the material. Most of the

literature on accelerated casting technique shows better marginal fit with intra-coronal

restorations. How ever further research to be done on the extra-coronal restorations.

In this study the surface roughness of resin pattern is greater than the Inlay Wax. This

may be due to the greater size of the polymer beads (approximately 150µm)47 and also due to

evaporation of the monomer which could not be controlled.

The specimens are fabricated with a bulk technique and smoothening of the surfaces was

not possible as it would increase the desired standardized thickness of 0.5mm. The surfactant

which is used to decrease the surface tension and contact angle probably has the little effect in

improving the contact of the investment with the pattern.

Due to rapid heating in the accelerated casting technique there is sudden production of

steam with in the investment that carries some of the salts and modifiers on to the inner wall of

the mold space. After evaporation of the steam, these modifiers settle on the inner surface of the

mold and are responsible for increase in surface roughness in accelerated casting technique.

Due to greater surface roughness on Pattern Resin it can be mostly used for

ceramometal restorations. The greater the surface roughness; better will be the mechanical

bonding.

Accelerated casting techniques using phosphate-bonded investments are considered to

have some desirable advantages. Unquestionably time is saved. Past investigations make it

apparent that, within commercially available phosphate-bonded investments, dissimilar

performance characters are expected. This knowledge suggests that casting techniques should be

studied with broad sample of alloys and phosphate-bonded investments, considering the

possibility that there may be an optimum combination in each situation.

The results of this study encourage further research with accelerated technique and

reinforce the need to identify the factors that facilitate better marginal fit and surface roughness

of cast restorations. Only one single wax pattern is investigated per casting in this study. The

performance of the described accelerated casting technique when more than one wax pattern or

fixed partial dentures invested in the same ring requires further investigation. Four

predetermined points were used to record the marginal gaps in this study. More number of

reference points for marginal gap measurements for each coping may yield a better confirmative

result. An 80% special liquid was used in this study. A further evaluation of different special

liquid to distilled water ratios on the marginal gaps and surface roughness of cast restorations

help to improve the outcome of this study. A further investigation on the influence of surface

roughness of dental casting affecting the marginal fit and the time required to finish and polish

the base metal cast restoration will enhance the outcome of these procedures. Further studies

which incorporate the above considerations are required to enhance the results obtained with this

study.

Summary & Conclusion

This study has been done to evaluate and compare the vertical marginal

discrepancy and surface roughness of cast copings made by two different casting

techniques with two different materials. The four techniques used in the study were,

conventional casting technique of Inlay Wax copings with 3 stage burnout procedure (G-

I), accelerated casting technique of Inlay Wax copings with single stage burnout

procedure (G-II), conventional casting technique of Pattern Resin copings with 3 stage

burnout procedure (G-III) and accelerated casting technique of Pattern Resin copings

with single stage burnout procedure (G-IV).

A total of 20 Inlay Wax copings, 20 Pattern Resin copings were fabricated with

stainless steel master die and former assembly, and divided into 4 groups with 10

specimens for each technique. A phosphate bonded investment was used to invest all

Inlay Wax and Pattern Resin copings with 80% special liquid in a metal ring with

ceramic ring liner. Specific burn out cycles were followed according to each technique

and all the samples were cast in nickel chromium alloy in induction casting machine. The

cast copings obtained were sand blasted. The internal surface was inspected and finishing

procedures done. The copings were seated on the stainless steel die with finger pressure

till the resistance obtained and vertical marginal discrepancy measurements were

recorded using a photo microscope. The results obtained were statistically analyzed.

A custom made stainless steel coping holder was used for measurement of

surface roughness to make the coping perpendicular to the surface roughness analyzer.

The surface roughness measurements were recorded using computer controlled surface

roughness analyzer. The results obtained were statistically analyzed.

The vertical marginal discrepancy of cast copings obtained by accelerated casting

technique showed a significantly higher value as compared to those obtained by

conventional casting technique; cast copings obtained by Pattern Resin showed a

significantly higher value as compared to those obtained by Inlay Wax. The mean vertical

marginal discrepancies of all the copings obtained by the four techniques were within

clinically acceptable limit.6

The surface roughness of cast copings obtained by accelerated casting technique

showed a significantly higher value as compared to those obtained by conventional

casting technique; cast copings obtained by Pattern Resin showed a significantly higher

value as compared to those obtained by Inlay Wax. The mean surface roughness of all the

copings obtained by the four techniques was within clinically acceptable limit.9

The following conclusions were drawn from the data obtained in this study of

comparative evaluation of the vertical marginal discrepancy and surface roughness of

cast copings obtained by conventional and accelerated casting techniques using two

different pattern materials – an invitro study.

1. The order of discrepancy values of vertical marginal discrepancy of the cast copings in

this study is as follows:

a. Least marginal discrepancy - conventional casting technique of Inlay Wax copings

with 3 stage burn out procedure (G-I) - 34.02 µm.

b. Moderate marginal discrepancy - conventional casting technique of Pattern Resin

copings with 3 stage burn out procedure (G-III) - 37.06 µm.

c. Maximum marginal discrepancy - accelerated casting technique of Inlay Wax

copings with single stage burn out procedure (G-II) - 44.39 µm and accelerated casting

technique of Pattern Resin copings with single stage burn out procedure (G-IV) - 46.86

µm.

2. The order of roughness values of surface roughness of the cast copings in this study is as

follows:

a. Least surface roughness - conventional casting technique of Inlay Wax copings with 3 stage

burn out procedure (G-I) - 4.38µm

b. Moderate surface roughness - accelerated casting technique of Inlay Wax copings with

single stage burn out procedure (G-II) - 5.41 µm

c. Maximum surface roughness - conventional casting technique of Pattern Resin copings

with 3 stage burn out procedure (G-III) -7.80 µm and accelerated casting technique of Pattern

Resin copings with single stage burn out procedure (G-IV) - 9.24 µm.

3. The mean vertical marginal discrepancy and surface roughness of all the cast copings obtained

by the four techniques (G-I, G-II, G-III and G-IV) were within the clinically acceptable limits.

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