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RESEARCH PAPER Accepted for publication in International Journal of Medical & dental Sciences on 14-04-2009. The accuracy, design and uses of custom impression trays in prosthodontics: - A Clinical guide. Manish Kinra 1 , Vijay Kumar 2 , Dr Monica Kinra 3 1. Genesis Institute of Dental Sciences and Research Ferozeur, Punjab, India. 2. Genesis Institute of Dental Sciences and Research Ferozeur, Punjab, India. 3. Jaipur Dental College & Hospital Jaipur. Rajasthan, India. Corresponding Author: - Dr Manish Kinra Flat No: 42, Staff Quarters, Genesis Institute of Dental Sciences and Research Ferozepur-Moga Road, Ferozepur, Punjab, India. Acknowledgement: - We would like to thank those academicians, clinicians and lab technicians who participated in discussion related to use of stock and custom trays thus help us encourage for the formulation of this paper. INTRO he need to make accurate impression is fundamental to the practice of prosthodontics. This necessitates the dental clinicians to make careful assessment of the tissues to be recorded in the impression, type of impression trays, impression material and the technique to be used. After many informal discussions with academicians, leading practitioners and lab technicians, we felt that stock metal trays are the most commonly used impression trays. Few among them know about the modification of stock trays to make them more precise. Very few clinicians use custom trays for making impression of partially edentulous arches. DUCTION T IJMDS – www.ijmds.org - 2009;1(1) 29-39

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RESEARCH PAPER

Accepted for publication in International Journal of Medical & dental Sciences on 14-04-2009.

The accuracy, design and uses of custom impression trays in prosthodontics: - A Clinical guide. Manish Kinra1, Vijay Kumar2, Dr Monica Kinra3

1. Genesis Institute of Dental Sciences and Research Ferozeur, Punjab, India. 2. Genesis Institute of Dental Sciences and Research Ferozeur, Punjab, India. 3. Jaipur Dental College & Hospital Jaipur. Rajasthan, India. Corresponding Author: - Dr Manish Kinra Flat No: 42, Staff Quarters, Genesis Institute of Dental Sciences and Research Ferozepur-Moga Road, Ferozepur, Punjab, India.

Acknowledgement: -

We would like to thank those academicians, clinicians and lab technicians who participated in discussion related to use of stock and custom trays thus help us encourage for the formulation of this paper.

INTROhe need to make accurate impression is fundamental to the practice of prosthodontics. This necessitates the dental clinicians to make careful assessment of the tissues to be recorded in the impression, type of impression

trays, impression material and the technique to be used. After many informal discussions with academicians, leading practitioners and lab technicians, we felt that stock metal trays are the most commonly used impression trays. Few among them know about the modification of stock trays to make them more precise. Very few clinicians use custom trays for making impression of partially edentulous arches.

DUCTION

T

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Conventional practice in removable prosthodontics is to use stock trays for making primary impressions. The selected stock tray must cover all denture bearing area in over-extended manner. A primary cast with good coverage of proposed denture bearing area gives freedom of using any spacer design with tissue stops within the custom tray, thus dictating impression technique and allowing better final impressions. Though, custom trays are used for making complete denture final impression but there is inadequate knowledge of custom tray design among clinicians and most of the clinicians depend upon lab technicians for their designing. Many lab technicians also felt the need to modify the primary cast prior to construction of custom trays which may be because of inadequacy of many primary impressions to record the desired area of mouth. Thus we have decided to enumerate design specifications of custom trays for removable and fixed prosthodontics. Custom trays can be used to record dental arches in following situations:

1. When stock trays do not cover proposed denture bearing area. 2. When the stock trays are poorly fitting on the arches leading to sub-optimum

thickness of impression material, thus producing the potential for inaccuracy. 3. When there is impingement of tray borders on the tissues. 4. When there is an unusual distribution of missing teeth. 5. When the last tooth in the arch is to be included in the impression. In these

circumstances it is not common to produce ‘drag’ in the impression when using stock trays.

6. When mobile teeth are to be recorded with low viscosity impression material, which is most unlikely to be confined within the stock trays.

Design characteristics of custom impression trays for removable (complete and partial dentures) and fixed partial denture situations: 1. Material of choice for making custom trays: a) Type II chemical cure poly-methyl methacrylate (pikka-tray material) contains more filler-particles which reduces polymerization shrinkage and improves linear dimensional accuracy. Thus clinicians should encourage the use of this material as custom impression trays to have accurate master casts instead of conventional chemical cure PMMA resin. b) Visible light cure (VLC) dimethacrylate resins offer superior mechanical properties to the currently available alternatives. Although the material is relatively expensive, require special light-curing unit for processing and difficult to trim when cured, trays made from this material has sufficient rigidity to be used in fairly thin section and excellent dimensional stability. VLC resins can be disinfected by immersing into commonly used disinfectant solution such as 1000 ppm sodium

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hypochlorite, without any significant changes in their physical or mechanical properties1. This can also be used in patients who are allergic to PMMA resins, because of no residual polymers in set material. 1. Optimum extension of custom trays: The periphery of the tray should incorporate all denture bearing area without distorting the tissue of the vestibules through over extension. Marking primary cast with pencil 2mm short of the vestibule , guides the lab technician to make optimum extensions of custom impression trays which saves clinician’s time on adjusting the tray. In partially edentulous situations, the custom impression trays should be kept 3 to 5 mm away from the gingival margin and about 3 mm beyond the most distal tooth. 2. Spacer design and thickness used: Thickness of wax spacer for complete and partially edentulous situations is 1 mm and 2-3 mm respectively. Wax spacer thickness may vary according to load bearing capacity of the tissue and attachment of soft tissue with periosteum. Presence of flabby and mobile tissue over the ridges demands for extra thickness of spacer to allow their undistorted recording in the impressions. The design of custom trays for complete dentures depends upon choice of impression material and technique to be used. Impression techniques such as mucocompressive, selective-pressure and mucostatic demands for specific spacer design and impression material. Mucocompressive technique is used for making primary impression of edentulous arches and thus does not require any spacer. Mucostatic technique require full spacer with 2 to 4 tissue stops with in custom trays, thus allowing wash impression material (ZOE paste) to record tissue details under minimum pressure as recording of tissues under “no-pressure” is not practically possible. This technique does not demand for border extension into vestibules, thus border molding was not suggested. Thus custom trays with borders app. 2 mm short of vestibules are recommended here2. A variant of mucostatic technique can be used in cases with very prominent mid palatine raphae, excessively flabby and knife edge ridges by making recess within custom trays in appropriate areas and recording them with very light viscosity impression materials such as impression plaster, ZOE and light body addition silicone3. The spacer design for the selective pressure is directly governed by the knowledge of the stress bearing and relief areas. The stress bearing areas in the maxillary arch are the horizontal plates of the palatine bone and the relieving areas are mid-palatine raphae and the incisive papilla. For mandible, the primary stress bearing area is buccal-shelf area and relieving area is sharp mylohyoid ridge and crest of alveolar ridge. But views of different authors on how to achieve selective pressure impression are different4 (Table-I)

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(Table -1) Guidelines for spacer design and tissue stops for selective pressure technique .

Author Spacer Tissue stops

RoyMac Gregor

Metal foil spacer in incisive papilla and mid palatine raphe (Fig -1).

Neill 0.9mm casting wax full spacer/relief except PPS.

Sharry Base plate wax spacer all area including PPS ( Fig -2)

4 tissue stops, 2 mm wide in molar and cuspid region, extended from Palatal aspect to mucobuccal fold.

Bouchers 1 mm base plate wax spacer except PPS in maxilla, (Fig no 3) . In mandible buccal shelf area and retro molar pad (Fig no 4) .

Morrow, Rudd, rhoads

Full wax spacer 2mm short of borders ( Fig no 5).

3 tissue stops, 4x4mm equidistance from each other

Barnard Levin

1 layer of pink base plate wax about 2mm thick all over the ridges except PPS and buccal shelf area.

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Fig 1 – Roy Mac Gregor’s design of spacer

Fig no-2 Sharry’s design of wax spacer

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Fig no 3- Boucher’s design of wax spacer in maxillary complete denture.

Fig no 4 -Boucher design of wax spacer in mandibular complete denture situation.

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Fig no 5 – Morrow, Rudd, Rhoad’s design of wax spacer.

Considering the views given by different authors it has been established that wax spacer is not placed in primary stress bearing areas allowing positive contact of custom trays with these areas. This permits selective placement of more pressure in primary stress bearing areas and spacer design reduces pressure in other areas thus called selective pressure technique. 4) Tissue Stops: Strategically placed tissue stops provide even thickness of impression material in custom impression trays. Placement of four tissue stops of 2mm width in cuspid and molar regions which extends from palatal aspect of ridge to the muco buccal fold are usually recommended in completely edentulous cases4 ( Table I). In situations requiring fixed partial dentures, tissue stops are placed on widely separated three or

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four non-functional cusps of teeth which do not require preparation (buccal of maxillary and lingual of mandibular). If all teeth are involved a large soft tissue stop can be placed on the crest of the alveolar ridge or in the centre of the hard palate. Tissue stops are made by removing wax at an angle of 450 to the occlusal surface of three or four teeth that have a tripod or quadrangular arrangement in the arch. This provide stability to the tray and the 450 angulated stops will help centre the tray during insertion5 (.Fig-6)

Fig no 6 - Wax spacer design and tissue stops in partially edentulous

situation.

5) Relief holes: After removing wax spacer from inside of the tray, a series of holes are prepared, about 12.5 mm apart in the center of alveolar groove and the retro molar fossa with a no. 6 round bur. The relief holes provide escape way for the final wash impression material and relieve pressure over crest of the residual ridge and the retro molar pads when the final impression is made6. Relief holes are of no importance in partially edentulous situations as neither relief nor adhesion between impression material and custom tray is provided. For good adhesion between impression material and custom trays, use of tray adhesives should be encouraged. 6) Tray handles5: Tray handles are useful in loading, orienting and placing custom impression trays in the patient mouth. Tray handles if not properly made or placed can cause potential inaccuracy in complete denture as they distort the lip form and hence the functional sulcus resulting into overextended borders1. The handle should be 25 mm long

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from the edge of the labial border of tray. The handle is positioned in the approximate position of the upper anterior teeth so that it doesn’t distort the upper lip when the tray is in position. For mandibular custom trays, the anterior handle should be 25 mm long from the edge of the labial border to the top and 12 mm wide7 . A handle made this way enables the clinician to securely grasp the tray without any interference with the tongue and lips. Two additional handles, one on each side are placed in the first molar region. These handles are centered over the crest of the residual ridge at its lowest point and are approximately 19 mm in height. The posterior handles are used as finger rests to complete the placement of the tray on the residual ridges and to stabilize the tray on the correct position with minimal distortion of soft tissue while the final impression material sets6 (Fig-7). One anterior handle and one or two posterior handles are required for partially edentulous situation with unilateral and bilateral distal extension bases respectively (Fig-8).

Fig no 7 - Custom tray for complete denture.

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Fig no. 8 – Custom impression trays for partially edentulous situation.

Maturation time: Maturation time is the time interval between fabrication of custom trays and using it for making final impression. This is characterized by polymerization of residual monomer resulting into polymerization shrinkage which exerts significant effect upon the linear dimensional accuracy of master cast8. All custom tray materials show linear dimensional changes up to 9 hours, but maximum shrinkage occurred up to 30 minutes after tray fabrication. Thus custom trays should be used after 9 hr of fabrication. If clinical situation demand early use, than custom tray seated over the casts should be placed in boiling water for 5 minutes and then cooled to room temperature9. In complete dentures there is no significance of maturation time, thus clinicians can use the custom impression tray immediately after fabrication. 7) Tin Foil: Tin foil should be placed over wax to prevent conduction of heat from resin to wax spacer preventing wax spacer from melting. It also allows easy and clean removal of wax spacer from tray5.

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CONCLUSION Pica tray material and Visible light cure (VLC) dimethacrylate resins material, custom impression trays with specific spacer design, tissue stops, handles and extensions with tin foil must be used for various complete, partial removable and fixed prosthodontics situations. This results into accurate reproduction of impression details made from different impression techniques. We hope that through this article dental clinicians must be encouraged to make valuable changes in their daily practice by using optimal design of custom trays. As a result they may begin to leap the reward of reduced chair side time and good adaptation of final prosthesis and receiving patient’s appreciation. BIBLIOGRAPHY 1. Smith P.W, Richard .R , Mc Cord. The design and use of special trays in prosthodontics: guidelines to improve clinical effectiveness. BDJ 1999;187:423-6. 2. Lee Robert .E. Mucostatics. D.C.N.A, 1980; V:24: p. 88 . 3. Tilton Glene .E . A minimum pressure complete denture impression technique . J Prosthet Dent 1956 ; 6: 6-23 4. Shetty Sanath, Nag P. Venkat Ratna, Shenoy Kamalakanth. K. Shenoy . A review of the techniques and presentation of an alternate custom tray design. J.I.P.S 2007; 7:8-11. 5. Rosenstein S F, Land MF, Fujimoto J. Contemporary fixed Prosthodontics, 3 rd ed Mosby : St. Louis. 2001. p. 364- 5. 6. Hickey Judson C, Zarb George A, Bolender Charles L. Bouchers Prosthodontic Treatment for Edentulous Patients, 9th ed U.S.A. Mosby. 1990. p. 210-214. 7. Levin Bernard, Richardson Glenn D. Complete denture prosthodontics-A Manual for clinical procedures. 17th ed. 2002. P. 36-7. 8. Goldfogel M, Harvey WL, Winter D. Dimensional change of acrylic resin tray materials. J Prosthet Dent 1985;54:284-6. 9. Pagniano P .Roland et al. Linear dimensional change of acrylic resins used in the fabrication of custom impression trays. J Prosthet Dent 1982 ; 47 :79-283.

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