Introduction

When one considers critically and realistically, all the

advancements achieved in the overall area of restorative dentistry, a

gearing exception comes to the fore; that exception is the technique

for the preparation of teeth and the philosophy behind it. The

approach to preparations has not kept pace with other advances in

dentistry. Almost all dental literature, dental school, teaching,

essayists, programs and philosophies of dentists demonstrate this

traditional holdover.

Teeth require preparation to receive restorations, and these

preparations must be based on fundamental principles from which

basic criteria can be developed that help predict the success of

prosthodontic treatment.

The principles of tooth preparation may be divided into three

broad categories:

1) Biologic considerations.

2) Mechanical considerations.

3) Esthetic considerations.

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Last week one of my colleague spoke on two mechanical

factors i.e. the retention and resistance form. Continuing the

principles of tooth preparation today I will be discussing one more

mechanical principle of tooth preparation i.e. structural durability

and one biologic principle i.e. marginal integrity.

So today’s topic of this seminar is structural durability and

marginal integrity.

Structure is any constructed thing here it refers the crown or

bridge.

Durability means “likely to last longer”.

Structures durability is the durability of the restoration so as

to how longer a crown or bridge lasts.

Features that contribute to the durability of the restoration are:

1) Alloy selection.

2) Adequate tooth reduction.

Occlusal reduction.

Axial reduction.

Provision for reinforcing struts.

3) Poor metal-ceramic framework design.

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1) Alloy Selection:

The Type I and Type II gold alloys are satisfactory for

intracoronal cast restorations, they are too soft for crowns and fixed

partial dentures, for which Type III or Type IV gold alloys are

choosen. There are harder and their strength and hardness can be

increased by heat treatment.

High-noble metal content metal-ceramic alloys have a

hardness equivalent to Type IV gold whereas nickel-chromium

alloys are considerably harder. These may be indicated when large

forces are anticipated. Such as long span FPD, although their use

presents some problems.

2) Adequate Tooth Reduction:

Occlusal Reduction:

Enough tooth structure must be removed from the

occlusal surface of the preparation so that when the restoration

is built back to ideal occlusion it must be thick enough to

prevent wearing through or distorting.

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Even the stronger alloys need sufficient bulk if they are

to withstand occlusal forces.

The occlusal thickness will vary with the restorative

material used. A gold crown requires approximately 1.5mm

clearance over the functional cusps and 1.0mm over the non-

functional cusps. Harder metals require slightly less. If a

porcelain veneer is extended onto the occlusal surface an

additional 0.5mm of space is needed.

The amount of occlusal reduction is not always the

same as the clearance needed. Often part of a tipped tooth is

already short of the occlusal plane and will require less

reduction than would a tooth in ideal occlusion.

Occlusal reductions of the posterior teeth can be

performed in three ways:

1) The occlusal surface of a tooth can be reduced entirely in a

flat plane.

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2) It can be reduced uniformly following the geometric outline of

cusps or fossae.

3) Either a partial reduction in the form of a cavity or a cusp

reduction with a cavity can be made not including all the

surfaces.

The second one is a uniform occlusal reduction,

provides for an adequate thickness of metal without

jeopardizing the help, resists shearing stresses, and is more

rigid because of its “CORRUGATED EFFECT”.

The third type is the occlusal cavity reduction which

obliterates the occlusal groove for its outline. Its depth usually

extends into the dentin. The cavosurface may be a margin

unless adjacent cusps are also reduced.

Creating steep planes with sharp angles should be

avoided since these can increase the stress and hinder

complete seating of the casting. To diminish stress round the

angles and deep grooves in the center of the occlusal surface

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keeping is avoided and the angulation of the occlusal plane

will be shallow.

Any necessary equilibration of the opposing teeth

should be done before the restorative procedure is begun.

Opposing cusps that are missing or short of their ideal

position should be replaced in a diagnostic wax up on a cast so

that the required amount of occlusal reduction can be

determined.

Functional Cusp Bevel

A wide bevel should be placed on the functional cusps

of posterior teeth to provide structural durability in this

critical area. It also rounds over the occlusal line angle, which

is the area of high stress concentrations. The angle on the non-

functional cusp is rounded over lightly.

Failure to place a functional bevel can result in thin,

weak areas in the restoration.

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In crossbite occlusal relationship the functional cusps

are revered.

Bevel is placed on the facial cusps of the maxillary teeth

and the lingual cusps of mandibular teeth.

Axial Reduction

A second prerequisite for structural durability is

adequate axial reduction.

When axial reduction is sufficient, restoration walls can

have satisfactorily thick.

Therefore the more common result of inadequate axial

reduction is an overcontoured restoration.

3) Reinforcing Struts

Structural durability at the margins of full veneer crown is

relatively easy to achieve because of high tensile strength of

the metal.

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The circumferential axial walls of a restoration hold

themselves together much as a barrel is held together by its

hoops.

When one or more surfaces of a tooth are left uncovered in a

partial veneer crown, the circumferential binding is lost. The

restoration margins can distort during fabrication, cementation

unless features are added to reinforce the structure.

In a three-quarter crown it is a connecting rib of metal that

joins the grooves to provide a “trussing effect”. This

reinforcing rib takes the form of an offset on a maxillary

preparation and an occlusal shoulder on a mandibular one.

Marginal Integrity

The restoration can survive in the biological environment of

the oral cavity only if the margins are closely adapted to the

cavosurface finish line of the preparation. Marginal integrity

includes the location, configuration and marginal fit of the crown.

The configuration dictates the shape and bulk of the restoration. If

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can also affect both the marginal adaptation and the degree of

seating of the restoration.

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Margin Placement:

The placement of finish lines has a direct bearing on the ease

of fabricating a restoration and on the ultimate success of the

restoration. The best results can be expected from margins that are

as smooth as possible and are fully exposed to a cleansing action.

Finish lines should be placed in enamel when it is possible to do so.

There are three locations in which to prepare crown margin:

1. Supragingival.

2. At the crest of the gingival or equigingival.

3. Subgingival.

In the past, the traditional concept has been to place the

finishlines as far subgingivally as possible based on the mistaken

concept given by G.V. Black, that the subgingival sulcus is caries

free. The preponderance of recently published literature, however,

strongly supports and have advocated the use of the supragingival

margin. In early 1941 Orban proposed supragingival margin for

improved periodontal health.

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Larato showed that the most crowns with subgingival margins

exhibited gingival inflammation compared with supragingival.

Silness used plaque index, an index of gingival health, and pocket

depth to compare crowns that had subgingival and supragingival

margins. He showed an increased accumulation of plaque when

compared with crowns with supragingival margins.

In a study on dogs, MARCUM found that crown margins

located at the crest of the gingival caused less inflammation than

either those below or above the gingival crest. HARRISON

supported this view and pointed out that possibly the reason for

successful crown margins located at the gingival crest was the

presence of keratinized epithelium in this region, unlike the

epithelium deeper in the gingival sulcus.

Ritcher and Veno reported that there was no difference

between subgingival and supragingival margins in a 3 year clinical

study. ESSMANN et al and KOTH made a similar recommendation.

These studies merely demonstrate that margin location is not as

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crucial when placed properly. Crown fit and finish may be more

significant to gingival health than the location of the margin.

However whenever possible margins are placed

supragingivally on the enamel of the anatomic crown.

Advantages of Supragingival Placed Margins

- Favourable reaction of gingiva.

- Wider shoulder tooth preparation can accommodate an

adequate bulk of porcelain without-pulpal injury.

- Metal margin finishing is easy.

There are some situations which require intracrevicular

margin placement they are:

- Esthetic demands.

- Caries removal.

- Subgingival tooth fracture.

- Cover existing subgingival restorations.

- To gain needed crown length.

- To provide a more favourable crown contour that is

furcation involvement.

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Berman has given a method of placing the margins

subgingivally with a collar of metal.

First step is to prepare the tooth to the crest of the gingiva.

Gingival retraction is obtained with a chord or electrosurgery. A

diamond point with an angled tip of calibrated length is introduced

to prepare the bevel. This instrument eliminates the sharp edge of

the shoulder and the undercut which extends apically from the

shoulder.

Margin Configurations:

There are four basic types of finish lines:

- Shoulder – PARDO in 1982 horizontal.

- Beveled shoulder.

- Chamfer – PARDO in 1982 Inclined vertical.

- Knife edge or feather edge.

The question seems to be whether to use shoulders or

chamfers with or without bevels or whether which of these

acceptable forms should be used in which situation.

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Miller and Belsky in 1965 advocated a full-shoulder

preparation.

Stein in 1977 recommended a uniform chamfer with bevel.

Preston in 1977 advocated a multiple approach using chamfers

with or without bevels in non-porcelain bearing areas shoulders with

bevels in porcelain bearing areas.

Other authors:

Rosner D., Sozio R.B., Berman and Preston strongly

advocated a beveled finishline, pointing out as superior marginal

closing through the “SLIP-JOINT” effect.

There are studies either to support or reject the commonly

held opinions on marginal configurations. Two areas have been

tested.

El-Ebrashi et al studied experimental stress analysis on

photoelastic models as it relates to differing marginal geometry.

They found that the shoulder with a rounded internal line angle and

the chamfer showed the least stress concentration and that shoulders

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with bevels and feather edges showed the most stress concentration.

In general they showed that margins with relatively large bulk and

no sharp line angles were superior to margins with either sharp or

acute line angles. FARAH and CRAIG found the similar results.

They showed that the chamfer was the optimum marginal

configuration.

Other area of research tested has been the effect of porcelain

firing distortion on different marginal shapes. SHILLINGBURG et

al found that under routine firing cycles used to condition and apply

porcelain to metals, the shoulder configuration with or without bevel

showed significantly less distortion than did a shamfer with or

without a bevel. This is because of metal found at the internal line

angle of a shoulder when compared with a chamfer.

The consensus on the matter of marginal configurations seem

to be that feather edge or knife edge margins are usually not

desirable. But this does not imply that we should throw reason and

experience to the wind and just stick to the shoulder and chamfer.

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There can be a situation where knife edge margin can have a distinct

advantage.

Chamfer margin: Jacobsen and Robinson defined chamfer as width

greater than 0.3mm at its cervical termination precluded any margin

being called a chamfer. A chamfer margin is particularly suitable for

cast metal veneers. It is an obtuse angled gingival termination. It is a

concave extracoronal finishline that possesses greater angulation

than a knife edge with less width than a shoulder. This finish line

has been shown experimentally to exhibits the least stresses so that

the cement underlying it will have less likelihood of failure.

The most suitable instrument for making a chamfer margin is

the tapered diamond with a rounded tip, the margin formed is the

exact image of the instrument.

Tilting the bur away from the tooth will create an undercut,

angling it toward the tooth will lead to overreduction.

A heavy chamfer is used to provide a 90° cavosurface angle

with a large radius rounded intraoral angle.

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Sometimes it may create an undesirable fragile “lip” of

enamel at the cavosurface. The heavy chamfer provides better

support for a ceramic crown.

Shoulder Margin: Defined using marginal geometry, where the

discriminating features are an external cavosurface angle of 90° and

a corresponding butt joint of restoration / tooth at the margin.

The shoulder has long been the finish line of choice for the

all-ceramic crown. The wide ledge provides resistance to

occlusal forces and minimizes stresses that might lead to

fracture of porcelain.

It produces the space for healthy restoration contours and

maximum esthetics.

It requires the destruction of more tooth structure than any

other finish line.

The sharp 90° line angle associated with shoulder concentrates

stress in the tooth and is conducive to coronal fracture.

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To overcome this a modified shoulder line is used with the

shoulder width being slightly lessened by the rounded internal

angle. So that the stress concentration is less. This type is also

called as RADIAL SHOULDER.

Full shoulder usage was increased after the introduction of

injectable ceramics such as dicor and cerestore, hiceram, IPS

empress.

The shoulder with a beveled margin is often recommended for

the facial surface of a metal-ceramic restoration where a metal

collar is to be used.

It is also utilized as the gingival finish line on the proximal

box of inlays and onlays, and for the occlusal shoulder of

onlays and mandibular three quarter crowns.

It can be used in those situations where a shoulder is already

present, either because of destruction by caries or the presence

of previous restorations.

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The beveling removes unsupported enamel and may allow

finishing of the metal.

Beveled shoulder is referred to as a biologic and esthetic

finish line. Esthetic, because the metal margin can be thinned

to a knife edge and hidden in the sulcus without the need to

position the margin closer to the epithelial attachment.

It is a good finish line for preparations with extremely short

walls, since it facilitates axial walls that are nearly parallel.

According to Giboe and Thayer an advantage of beveled

shoulder preparation were that they allowed the incorporation

of physiologic contours in tooth the temporary and final

crown.

KNIFE EDGE MARGIN

The ultimate in finish lines that permits an acute margin of

metal is the KNIFE EDGE.

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Unfortunately its use can create problems unless it is cut

carefully, the axial reduction may slide out instead of

terminating in a definite finish line.

The thin margin of the restoration that fits this finish line may

be difficult to accurately wax and cast resulting in

overcontoured restorations when an attempt is made to obtain

adequate bulk.

Inspite of its drawbacks in some situations knife edge margins

has a distinct advantage.

- Used on lingual surfaces of mandibular posterior teeth.

- On teeth with very convex axial surfaces and on the

surfaces toward which a tooth may have tilted.

- In younger patients.

- On cementum.

- Privilege preparations and outline of partial veneer

crowns.

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Historically their main advantage was that they facilitated the

making of impressions with rigid modeling compound in

copper bands (a technique which is severely used today),

because there was no ledge on which band covered catch.

MARGINAL FIT

According to ADA specification No. 8 the marginal adaptation

of cemented castings should be in the range of 25µm. This

range is below the range of visual acuity.

Recently Gavelis et al used an experimental design intended to

eliminate all casting error and measure only the marginal

opening.

They found that feather edges and shoulders and chamfers

with parallel bevels had the least opening. This showed that

‘slip joint’ margins provide the least marginal discrepancy.

Rosner showed a mathematical trigonometric analysis of

marginal discrepancy. He said that bevels have been

advocated as a means of diminishing marginal discrepancy.

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If the vertical discrepancy in fit is designated as D, the closest

distance between the margin and the surface of the preparation

is a line d that is perpendicular to the surface of the tooth.

Figure - Refer Shillingburg

Now it can be stated as a function of D

d = D sin µ -------i

d = D cos ------ii

As µ becomes smaller (more acute). The sine of µ becomes

smaller or becomes larger (more obtuse) cosine becomes

smaller.

By either computation d diminishes by the same amount.

The more acute the angle of margin or more obtuse the angle

of finish line. The shorter the distance between finishline and

margin.

However this is true only if there is no cement between the

restoration and the preparation.

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The presence of cement changes the scenario completely as

suggested by osteoid.

The film thickness of the cement will prevent the complete

seating of a casting with bevels that are nearly parallel with

the path of insertion.

Film thickness imposes a limit on the reduction of the

perpendicular distance from the margin to the tooth d. The

distance d, therefore becomes a constant and the previous

equation is solved for D instead of d.

D = d/sinµ -----iii D = d/cos ------- iv

As angle of margin bevel more acute since –smaller on as

angle of finish line become more obtuse –eosine-smaller and

D become larger.

The more nearly the bevel pallels the path of insertion, the

greater the distance by which the restoration fails to seat.

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If a bevel of 45° is added to a shoulder, the crown will be

prevented from seating by a factor of 1.4.

30° - the crown is displaced twice.

15° - factor 3.9.

5° - factor 11.5

Mclean and Wilson have disputed the use of bevels for metal-

ceramic crowns because the bevel margin must be 10-20° to

noticeably improve adaptation.

Panno and associates reported no better adaptability of crowns

with highly acute 80° bevels than those with less acute 45°

bevels.

Pascoe in 1978 showed the trigonometric analysis of

relationship between internal casting discrepancy and

marginal opening.

Taking Rosner’s equation as a basic theorem Pasco applied it

to the internal discrepancy.

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As Rosner showed the marginal opening of a vertically

displaced casting (d) is related to the displacement (D) by the

formula:

d = D cos -------(i)

Where = angle of the bevel

From the figure it can be seen that this theorem is valid only

when there is:

i) Initial adaptation of the

casting is exact.

ii) There is a purely vertical

displacement of the casting such that point Y 2 is directly

above Y1.

Now this situation can arise as a result of 1) Cement film

thickness, 2) defect on the internal surface of the casting.

Distance between the axial wall of the tooth and casting is

designated as f.

Now we can derive the equations for internal discrepancy.

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D = f/cos ------------(ii)

Equating (i) and (ii)

D = d/cos = f/cos

= f cos = d cos

d = f. cos /cos -----------(iii)

Thus a direct relationship is shown between the internal

discrepancy and marginal opening.

Rosner said that a 5 degrees is the minimum acceptable taper.

Most recommended marginal bevel is 45°.

According to ADA specification No. 8 the cement should meet

the thickness of 25µm.

By substituting these values in equation (iii).

f = 25µm

= 85°

= 45°

d = f.cos /cos = 25.cos 45/cos 85°

= 25 x 0.707/0.087

= 203µm

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So the marginal opening would be 203µ which is considerably

more than the 40µ that has been advocated as acceptable

value.

Similarly by compacting for undersized cemented casting –

277µ.

Oversized cemented casting = 25µ.

So Pascoe demonstrated that slightly oversized casting with

shoulder exhibit the least marginal discrepancy.

Functions of a bevel:- Bulk of material.

- Protection of enamel rods at the margin.

- Allowance for burnishing and cementation.

- Development of circumferential retention.

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SUMMARY & CONCLUSION

Principles of tooth preparation can be categorized into

biologic, mechanical and esthetic consideration. If too much

emphasis is given for any one of the principle then the success of the

procedure may be limited by a lack of considerations of other

factors.

The structure of a restoration must be sound and have

sufficient strength to prevent it from being permanently deformed

during function.

The margin is one of the components of the cast restoration

must susceptible to failure, both biologically and mechanically. If all

the principles of tooth preparation are achieved in a restoration

excluding marginal integrity the prosthesis may become detrimental

to the dental tissues, finally leading to the failure of the prosthesis.

The quality of margin may be of as much importance to gingival

health as location.

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As long as fixed prosthodontics must rely on the cemented

castings, the search for more knowledge about an innocuous,

esthetic, indestructible margin must continue.

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REFERENCES:

1. Abrahams E.J. : Combination of shoulder-feather edge veneer

crown preparation. JPD, 13: 901, 1963.

2. Becker M.C. et al : Current theories of crown contours margin

placement and pontic design. JPD, 45: 268, 1981.

3. Behrand D. : Cerammometal restoration with supragingival

margins. JPD, 47: 625, 1982.

4. Bridger D.V. : Distortion of ceramometal FPD during firing

cycle. JPD, 45: 507, 1979.

5. Bryant R.A. : Measurement of distortions in FPD resulting

from degassing. JPD, 42: 515, 1979.

6. Donovan T. : An analysis of margin configuration for metal-

ceramic crowns. JPD, 53: 153, 1985.

7. Faucher R.R. : Distortion related to margin design in porcelain

fused to metal restoration.

8. Gardiner F.M. Margins of complete crowns – A review. JPD,

48: 396, 1982.

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9. Grajower R. : A mathematical treatise on the fit of crown.

JPD, 49: 663, 1980.

10.Hunter A.J. :Gingival crown margins configurations – A

review. JPD, 64: 548, 1990.

11.Jacobson P.H. et al : Basic techniques and materials for

conservative dentistry. J Dent, 9: 101, 1981.

12.Kashani H.G. et al : The effect of bevel angulation on

marginal integrity. JADA, 103: 882, 1981.

13.Panno: Evaluation of the 45° labial bevel with a shoulder

preparation. JPD, 56: 655, 1986.

14.Pardo G.I. : A full cast restoration design of firing superior

maginal characteristics. JPD, 48: 539, 1982.

15.Pascoe D.F. : Analysis of the geometry of finishing liner for

full crown restorations. JPD, 40: 151, 1978.

16.Perel M.L. : Axial crown contours. JPD, 25: 642, 1971.

17.Prince J. : The all porcelain labial margins JPD, 50: 185,

1983.

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18.Prince J. : The all porcelain labial margin for ceramo-metal

restoration – A new concept. JPD, 50: 793, 1983.

19.Rosensteil : Contemporary fixed prosthodontics, 2 nd edition.

20.Rosner D. : Function, placement and reproduction of bevels

for gold castings. JPD, 13: 1161, 1963.

21.Samuel E.G. : Multiple preparations for fixed prosthodontics.

JPD, 23: 529, 1970.

22.Schweikert E.O. : Feather edged or knife edged and

impression techniques. JPD, 52: 243, 1984.

23.Schwartz I.S. : A review of methods and techniques to

improve the fit of cast restorations. JPD, 56: 219, 1986.

24.Shillingberg H.T. :Preparation design and margin distortion in

PFM restoration. JPD, 29: 276, 1973.

25.Shillingburg H.T. : Fundamentals of tooth preparations.

26.Shillingburg H.T. : Fundamentals of FPD 3 rd edition.

27.Smyd E.S. : The role of torque, torsion, and bending in

prosthetic failures. JPD, 11: 95, 1961.

28.Tylman: Theory and practice of FPD 8 th Edition.

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29.Watson: Margin placement of esthetic veneers crowns. JPD,

45: 499, 1981.

30.Willis C.M. :Distortion in dental soldering as affected by gap

distance. JPD, 43: 272, 1980.

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STRUCTURAL DURABILITY AND MARGINAL INTEGRITY

CONTENTS

Introduction

Structural Durability

- Alloy Selection

- Adequate Tooth Reduction

o Occlusal Reduction

o Axial Reduction

o Reinforcing Struts

- Poor Metal-Ceramic Framework Design

Marginal Integrity

- Margin Placement

- Margin Configurations

- Margin Fit

Summary & Conclusion

Bibliography

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