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Impressions in fixed partial denture 2009

INTRODUCTION:

Many Impression materials are suitable for use in dentistry. Impression materials are used to record intraoral structures for the fabrication of definitive restorations. Accurate impressions are necessary for construction of any dental prosthesis. The relationship between static and mobile oral structures must be reproduced accurately for an optimum cast. Making a cast in gypsum materials from an impression of dental anatomy aids dentists in designing and constructing fixed prosthesis. The accuracy of these final restorations depends greatly on the impression materials and techniques. The more common types of impressions are used for fabricating diagnostic and master casts. Diagnostic casts are used to aid in treatment planning while master casts are used for producing fixed partial dentures. Accurate impressions depend on identifying the applications that do or do not fit each materials characteristics. Materials used without adequate knowledge of their characteristics can impair a successful outcome. Often, the choice of impression material depends on the subjective choice of the operator based on personal preferences and past experience with particular materials. The dentist has a host of excellent impression materials available for making impressions in fixed prosthodontics. With proper material selection and manipulation, accurate impressions can be obtained for fabrication of tooth supported restorations. An acceptable impression must be an exact record of all aspects of the prepared tooth. This means it must include sufficient unprepared tooth structure immediately adjacent to the margins for the dentist and laboratory technician to identify the contour of the tooth and all prepared surfaces. The contour of the unprepared tooth structure cervical to the preparation margin is critical information that must be available when the restoration is fabricated in the dental laboratory.1

All teeth in the arch and the soft tissues immediately surrounding the tooth preparation must be reproduced in the impression. They will allow the cast to be accurately articulated and will contribute to proper contouring of the planned restoration. Particular attention is given toPage 1

Impressions in fixed partial denture 2009

reproducing the lingual surfaces of anterior teeth because they influence anterior guidance, which determines the occlusal morphology of the posterior teeth. The impression must be free of air bubbles, tears, thin spots, and other imperfections that might produce inaccuracies. The patient's mouth is a challenging environment to make an accurate impression. Moisture control is probably one of the most important aspects of successful impression making. Except for the polyethers, all elastomeric impression materials are hydrophobic i.e. they do not tolerate or displace moisture. Any moisture will result in voids. Consequently, saliva flow into the area must be reduced and diverted to obtain the necessary dry field of operation.1

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Impressions in fixed partial denture 2009

REVIEW OF LITERATURE:

Fusayama T, Iwaku M, Daito K, Kurosaki N. and Takatsu T. in 1974 studied about the accuracy of the laminated single impression technique with silicone materials. In this technique, the heavy and wash types were mixed at the same time and the wash type was laminated in a thin layer on the surface of the heavy type, and this is loaded in a tray and immediately impressed upon the preparation. The accuracy of this technique was compared with single mix, double mix without spacing and double mix with spacing. They found that the double impression technique without spacing showed the greatest distortion with considerably decreased outer and increased inner dimensions. The new laminated single impression technique produced the least distortion though the difference in results between this technique and the double impression technique with spacing was not statistically significant.2 Herfort T.W., Gerberich W.W., Macosko C.W., and Goodkind R.J. in 1978 compared the tear strength of various elastomeric impression materials those included polysulfide, silicone and polyethers. They found a direct relationship between shear modulus and difficulty in removing impression from the mouth and the ease of removing an impression from the mouth was in the order: light bodied silicones, light bodied polysulfides, heavy bodied silicones, heavy bodied polysulfides and polyethers. On testing the maximum tear strength in 10-15 minutes, the polysulfide materials showed three to six times the tear strength of silicones. The polyether material displayed a tear resistance slightly higher than that of the silicones but one third to one fifth as high as that of the polysulfides.3 Eames W B, Sieweke J C, Wallace S W and Rogers L B in 1979 studied the effect of bulk on accuracy of various elastomeric impression materials. Trays were constructed using 2 mm plastic on an Omnivac machine to provide 2, 4 and 6 mm spaces for impression materials. Their study showed that the interface space of 2 mm produced the most accurate impression than the 4 or 6 mm tray spaces.4

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Impressions in fixed partial denture 2009

Lacy A.M, Fukui H, Bellman T and Jendresen M.D. in 1981 conducted a study to determine a time dependent accuracy of elastomer impression materials. The materials included were one polyether, four polysulfide rubbers, and four polyvinyl siloxanes. The modes of impression involved both putty/ wash systems and wash/ adhesive custom tray systems. The result of the study showed that the polyether which was adhesively bonded to the custom trays showed improved stability over condensation silicone and polysulfides used in the same fashion. They concluded their study stating that: (1) polyvinyl siloxane (addition polymerization) silicones are the most stable of elastomers currently available. (2) accuracy and consistency are best maintained by use of custom tray and adhesives to retain polyvinylsiloxanes. (3) polysulfide rubbers bonded to custom trays show a progressive increase in die diameter with time. (4) polyether rubber is intermediate in stability to polysulfide or silicone systems and polyvinyl siloxane when impression techniques involve adhesive bonding to custom-formed trays. (5) polyvinyl siloxane putty/ wash systems may reveal some loss of accuracy of dies produced by retrieval from multiple pours after 2-4 days, and (6) there appears to be no pronounced differences between single mix and double mix techniques for polysulfides when both are both are bonded to an acrylic resin custom tray.5 Wilson E G and Werrin S R in 1983 described the dual arch impression method with closed mouth technique to obtain cast and die placement in centric occlusion in a single step. Complete closure of the teeth was mandatory for registering the interocclusal relationship correctly. They stated that the dual-arch or double-arch impression technique is extremely accurate and a viable alternative to full-arch impressions. The accuracy of the interocclusal relationship was supposed to be more likely because of the decreased number of steps in the technique.6 Vald erhaug J. and Floyst rand F. in 1984 st udied t he dimens io nal st abilit y o f elast o mer ic impr essio n mat er ials in cust om made and st ock t rays. T he

impressio n mat er ials used for t he st udy were polyet her and s ilicone. Cust o m t rays were made o f acr ylic resin t o obt ain a t hickness o f 3 mm and no n per fo rat ed chro mium plat ed brass t rays of appropr iat e size were used as st o ck

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Impressions in fixed partial denture 2009

t rays. T he dist ances bet ween t he abut ment t eet h were measured immediat ely o n remo val o f t he impressio n and aft er 1 ho ur and 24 hours. Result s indicat ed t hat alt ho ugh a mple amount of impressio n mat erial was allo wed (2-9 mm) t he linear d imensio nal st abilit y o f t he impressio ns made in stock t rays was not infer io r t o cust o m t rays. 7 Vitre, Galburt and Maness in 1985 studied the biometric comparison of bur and electrosurgical retraction methods. They recorded the free gingival tissue height after 4, 8, and 12 weeks. They concluded that (1) the electrosurgical method showed more tissue loss at each time interval than the bur method, (2) electrosurgical retraction showed more subject variability than the bur method, (3) clinically, the tissue appearance of both group was indiscernable at the time intervals tested, and (4) the electrosurgical method provided for a greater bulk of impression material at the margin than the bur method.8 Tjan A H L, Whang S B and Sarkissian R in 1986 performed the clinically oriented evaluation of the accuracy of commonly used impression materials. They used a simulated clinical model and technique to compare the accuracy of stone casts produced from reversible hydrocolloid, polysulfide rubber, polyether rubber, condensation curing silicone rubber and addition curing silicone based rubber and to determine the effect of delay in pouring the impressions. They found the impressions made in the elastomers were significantly better than reversible hydrocolloid even though all impressions made were clinically acceptable. The data for addition curing silicones indicated that equally accurate impressions were produced with putty wash and one step impression technique.9 Johnson G.H. and Craig R.G. in 1986 eva luat ed t he accur acy o f addit io n silico nes as a funct ion o f t echnique. They invest igat ed four addit io n silico ne impressio n mat er ials and one condensat ion silico ne impr essio n mat er ial wit h t hree t echniques i.e. put t y-wash, sing le mix and double mix. The y co nclu ded t hat t he same accuracy was achieved wit h all t echniques ( i.e. put t y-wash, sing le

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