Mohamed Salah Gomaa*, Rabab Mohamed Ibrahiem** and Rasha Nabil Mohamed Sami***.

Transcription

1 THE EFFECT OF STIFFNESS OF IMPRESSION MATERIALS AND FLEXIBILITY OF TRAYS ON THE CASTING ACCURACY (An In-Vitro Study): Mohamed Salah Gomaa*, Rabab Mohamed Ibrahiem** and Rasha Nabil Mohamed Sami***. *MSc. of Fixed Prosthodontics, Faculty of Oral and Dental Medicine, Cairo University. ** Professor of Fixed Prosthodontics, Faculty of Oral and Dental Medicine, Cairo University. *** Lecturer of Fixed Prosthodontics, Faculty of Oral and Dental Medicine, Cairo University. ABSTRACT Purpose: Study the effect of using two impression materials of different stiffness with three impression trays of different flexibility on the casting accuracy of cast metal copings. Materials and Methods: The mandibular right first molar of a typodont model mounted to a semiadjustable articulator was prepared for a full coverage cast metal coping. Thirty impressions were made with metal dual-arch impression trays, plastic dual-arch impression trays and a custom made acrylic resin impression tray using polyvinyl siloxane and polyether impression materials. Each impression was cast in type IV improved dental stone and the occlusogingival, mesiodistal and buccolingual dimensions of the obtained dies were measured using a traveling microscope and the prepared tooth served as a control. Wax patterns were fabricated on each stone die then cast into metal copings. Each metal coping was fitted on the prepared tooth and examined for vertical marginal gap accuracy using the travelling microscope. One way ANOVA followed by pair-wise tukey's post-hoc tests were performed to detect the significance between groups, then separate student t-tests to detect the significance between subgroups. Results: The removable stone dies and cast metal copings obtained from the plastic dual-arch impression trays showed the greatest distortion in the occlusogingival, mesiodistal, buccolingual and vertical marginal gap measurements and the metal-dual arch impression trays recorded the least changes in all measurements. Conclusion: The increased rigidity of the impression trays as well as the increased stiffness of the impression materials produce more accurate dies and cast metal copings. Key Words: Elastomers, Dual-arch trays, Marginal gap.

2 Introduction INTRODUCTION T he success of any fixed prosthodontic treatment plan depends on multiple phases and the impression making procedure is considered one of the key steps for reaching a successful final restoration. Its overall goal is to produce an exact three dimensional negative replica of the hard and soft tissues of the oral cavity and its quality significantly affects the accurate fit of the final restoration. Despite of the contemporary technical improvements in the field of computer-aided design / computer-aided manufacturing (CAD/CAM) systems and the three dimensional (3D) imaging procedures, the conventional dental impression process still plays an important role in transferring the relevant information from the patient to the dental laboratory. The quality of dental impressions is influenced by many factors such as the impression technique and its correct application, the impression tray / impression material combination, local conditions in the oral cavity and finally the properties of the impression material used. Since the introduction of the dual-arch impression technique, its popularity has steadily grown due to its wide range of advantages which includes patient comfort, efficiency, economy of materials and the convenience of capturing the prepared abutment, the inter-occlusal record and the opposing arch simultaneously. The technique has been referred to as the dual-arch impression technique, double-arch impression technique, closed-mouth impression technique or triple-tray impression technique. 1

3 Introduction The accuracy of the castings generated from the dual-arch impression technique using impression materials of different stiffness along with impression trays of different flexibility however remains in question. 2

4 REVIEW OF LITERATURE E ach step of any prosthodontic rehabilitation process must be meticulously executed to yield satisfactory final results. In order to obtain accurate and precise models and restorations with no distortion, it is important to acquire accurate impressions and to use stable and precise impression and die materials. A variety of impression trays and impression techniques have been made available for the use in the fabrication of fixed indirect restorations; each with advantages, disadvantages and indications for use. Prefabricated metal or plastic stock trays as well as custom made trays may be used to record a full or a partial arch. Detail reproduction in impression making has improved with the evolution from reversible hydrocolloid to polysulfide, condensation silicone, polyether and finally polyvinyl siloxane. Accuracy of elastomeric impression materials: Making an impression represents a crucial step in the processing and fitting of a fixed dental prosthesis. For that reason the impression quality is decisive for the accurate fitting and success of the final restoration. The impression material should be accurate to fabricate restorations with ideal marginal fit, internal fit, inter-proximal and occlusal contacts. However, among all impression materials available; non aqueous elastomeric impression materials are considered to be highly accurate in surface detail reproduction and dimensional stability. 3

5 Pamenius and Ohlson (1995) (1) evaluated the accuracy of rubber impression materials by taking impressions of a steel model then pouring it with stone and comparing certain measurements between steel and stone models. They assumed that acceptance is achieved when the difference in measurements is within permissible limits defined by clinical criteria. Idris et al. (1995) (2) compared the putty wash one step and two step techniques for making addition silicone impressions. For each technique fifteen impressions were made of a stainless steel base to which three tapered posts were attached. Stone models were made for all impressions. Results indicated that the inter-abutment distances increased slightly when compared with the stainless steel model for both techniques but the differences were considered to be clinically insignificant. Boulton et al. (1996) (3) determined the accuracy of impressions taken from experimental stainless steel models representing a premolar and a molar bridge preparation using three elastomeric impression materials; polysulphide, polyether and polyvinyl siloxane in custom and stock impression trays. Horizontal and vertical individual abutment and inter-abutment dimensions were measured on the obtained stone dies then measurements were compared with those obtained from the stainless steel master model. Results demonstrated that polysulphide is the least accurate impression material for both vertical and horizontal individual abutment dimensions. However, for inter-abutment horizontal dimensions no statistical differences were noted between impression material types when using a custom tray. Stock trays produced less accurate results for all the materials tested when compared to custom trays. 4

6 Nissan et al. (2000) (4) assessed the accuracy of three putty wash impression techniques using the same impression material (polyvinyl siloxane) in a laboratory model. The three putty wash impression techniques used were one step, two step with two mm relief and two step with a polyethylene spacer. For each technique fifteen impressions were made of a stainless steel master model that contained three complete crown abutment preparations which were used as the positive control. Accuracy was assessed by measuring six dimensions on the stone dies poured from impressions of the master model. Statistically significant differences among the three putty wash impression techniques were found for all intra-abutment and inter-abutment measurements. The polyvinyl siloxane two step two mm relief putty wash impression technique was the most accurate for fabricating stone dies. Chen et al. (2004) (5) evaluated the effects of various impression materials, different storage times and the proportion of inorganic filler on the accuracy and stability of elastomeric impression materials. The impression materials studied included three alginate impression materials, five commercial silicone impression materials and two experimental silicone impression materials. Impressions were made of ten metal dies that simulated prepared crowns. After the impressions were taken, dental stone was immediately poured into the alginate impressions while the silicone impressions were poured thirty minutes later and left for one hour to set. The second and third stone dies were made one and twenty four hours later respectively. The diameters of the occlusal surfaces of the metal and stone dies were determined using photographs of the surfaces taken with a digital camera. The pictures were then measured using a photomicrograph digitized integration system to calculate any 5

7 discrepancies. Results showed that there was a significant interaction effect for materials and storage times on the accuracy of the impressions. The two addition type silicone materials had the greatest accuracy and stability. Lu et al. (2004) (6) compared the mechanical properties including; elastic recovery, strain in compression, tear energy and tensile strength of two addition silicone impression materials and a polyether impression material. Two consistencies of each material (light body and heavy body) were investigated. Five specimens were made for each group for a total of twenty four groups and one hundred and twenty specimens. The new soft polyether impression material had higher strain in compression and lower tensile strength when compared to the new hydrophilic addition silicone impression materials. Heavy body materials had higher tear properties and tensile strength than the light body materials. Shah et al. (2004) (7) compared the dimensional accuracy of a polyether impression material and a polyvinyl siloxane impression material using a laser scanner with (3D) superimposition software. Twenty impressions; ten with polyether and ten with polyvinyl siloxane for a stone master model that resembled a dental arch containing three acrylic posterior teeth prepared for full metal crowns were cast in orthodontic stone. The (3D) surface viewer software superimposed the master model to the stone replica and the differences between the images were analyzed. Both impression materials provided an accurate replica of the prepared teeth supporting the view that these materials are highly accurate. 6

8 Samet et al. (2005) (8) evaluated the quality of impressions sent to commercial laboratories for the fabrication of fixed partial dentures by describing the frequency of clinically detectable errors and by analyzing correlations between the various factors involved. A total of one hundred and ninety three fixed partial denture impressions were evaluated immediately after their arrival at eleven dental laboratories by three calibrated examiners. The impression technique, material used, tray type and number of prepared units were recorded for each impression. Significant correlations were found between material types, voids or tears at the finish lines and polymerization problems. Walker et al. (2005) (9) evaluated and compared the dimensional accuracy and surface detail reproduction of two hydrophilic polyvinyl siloxane and two polyether impression materials when used under dry and moist conditions. Impressions were made of stainless steel dies with two vertical and three horizontal lines inscribed on the dies superior surface. Dimensional accuracy was measured by comparing the average length of the middle horizontal line in each impression with the same line on the metal die using a measuring microscope. The surface detail evaluation indicated that moisture had a significant effect on the detail reproduction of the PVS materials. Under dry conditions, all materials produced satisfactory detail reproduction in 100% of the time. However, under moist conditions only 29% of the PVS impressions produced satisfactory detail reproduction while 100% of the PE impressions still met the surface detail criteria. 7

9 Beier et al. (2007) (10) evaluated a hydrophilic polyvinyl siloxane impression material for fixed dental restorations under various clinical conditions. A total of one thousand four hundred and sixty six preparations for fixed restorations including inlay, onlay, crown, veneer, post, adhesive wing preparations and implants for gold, porcelain fused to metal and ceramic restorations were evaluated. The preparation finish line relative to the crest of the gingival margin, type of restoration and position of the teeth were recorded. These categories were established to rate the impression quality; perfect impressions with absence of any voids were rated (criteria 1), acceptable impressions with minimal defects were rated (criteria 2) and unacceptable impressions with larger voids were rated (criteria 3) % of the final impressions were clinically acceptable. Grundke et al. (2008) (11) reported the surface properties of unset and set polymer impression materials by performing surface tension and contact angle measurements. Three types of impression materials were investigated; polyether, addition curing and condensation curing silicones. The high surface tension of about 50 mj/m 2 for the base and 38 mj/m 2 for the accelerator of the polyether system was caused by the polar carbonyl and ether groups respectively. The surface tension of the silicones was in the range of 20 up to 23 mj/m 2 for both the base and the accelerator regardless of the internal surfactants that were incorporated to the addition curing silicone. These different surface tension values at the liquid / air interface resulted in the hydrophilic surface properties of the unset and the set polyether surfaces and in the hydrophobic surface properties of the silicones in contact with the aqueous media. 8

10 Finger et al. (2008) (12) investigated the depth reproduction of differently wide sulci with elastomeric impression materials by single and double mix techniques using a tooth and sulcus model simulating clinical conditions. Impressions with one polyvinyl siloxane, two polyethers and one hybrid PVS/PE elastomer were taken from a truncated steel cone with a circumferential two mm deep sulcus; 50, 100 or 200 μm wide. The root surface was in steel and the periodontal tissues were in reversible hydrocolloid. Single mix impressions were taken with light body or monophase pastes and double mix impressions were taken with light body as a syringe material and a medium or a heavy body as tray materials. Sulcus reproduction was determined by (3D) laser topography of impressions at eight locations 45 apart. All tested materials and techniques reproduced 200 μm wide sulci. Raigrodski et al. (2009) (13) compared polyvinyl siloxane impression materials using the one step impression technique for single and two unit crowns. Stock trays were used to make a total of forty master impressions. The order of impression making was randomized as related to different groups. Impressions were rated visually by two evaluators. The rating resulted in data for the outcome variables (alpha: excellent, no defects; bravo: acceptable, small defects; charlie: defects requiring the impression to be remade; delta: unacceptable, defects at the finish line). 92% of the experimental and 96% of the control impressions were rated as alpha or bravo by the evaluator. Dies of both groups were rated alpha or bravo by the dental technician. There was no difference in the clinical performance of the experimental and the control PVS impression materials. 9

11 Mishra and Chowdhary (2010) (14) evaluated the linear dimensional accuracy of some commercially available polyvinyl siloxanes of varying viscosities. Different impression techniques were used, putty wash with a two step technique with polyethylene spacer using a stock tray, putty wash with a one step technique using a stock tray, single mix technique utilizing medium viscosity in a custom tray and multiple mix technique utilizing a heavy and a low viscosity combination in a custom tray. For each technique ten impressions were made of a metallic maxillary dentulous master model. The results of this study revealed that a heavy and a low viscosity combination double mix technique in a custom tray produced accurate results in all dimensions followed by the single mix technique utilizing medium viscosity in a custom tray then the putty wash two step technique with polyethylene spacer using a stock tray. Putty wash one step simultaneous technique produced the least accurate results in all dimensions. The putty wash two step technique was found to be as accurate as the multiple mix technique utilizing a heavy and a low viscosity combination in a custom tray and the single mix technique utilizing medium viscosity in a custom tray. The dual-arch impression technique in fixed prosthodontics: The dual-arch impression technique is a concept that has been used effectively for the fabrication of cast restorations. From one impression, dentists can make casts of the prepared teeth, opposing dentition and register the inter-occlusal relationship. The technique is uncomplicated and it reduces the chair time, operating costs and the possibility of error. Dual-arch impression trays can be used with any type of impression material; Polysulfide, condensation reaction silicone, addition reaction 10

12 silicone or polyether. The decision to use the dual-arch impression technique must be based on the proper selection of the dual-arch impression tray for each specific clinical situation. Parker et al. (1997) (15) investigated and compared the accuracy of inter-occlusal relationships in maximum intercuspation for mounted casts obtained from a quadrant dual-arch impression technique and conventional full arch impression technique. Comparisons were made between an intraoral inter-occlusal record and inter-occlusal records made on mounted casts. Irreversible hydrocolloid, reversible hydrocolloid, polysulfide, polyether and polyvinyl siloxane were used for the full arch impression technique. For the quadrant dual-arch impression technique, heavy body and light body polyvinyl siloxane injection materials were used. Comparison of the records of the full arch impressions did not reveal any significant differences. The quadrant dual-arch impression technique produced mounted casts with significantly more accurate maximum intercuspal relationships than mounted casts from full arch impressions. Burke and Crisp (2001) (16) assessed an impression material used in conjunction with the dual-arch impression technique. Ten members were selected at random. Explanatory letters, questionnaire, packs of the impression material and the dual-arch trays were distributed among the evaluators. The practitioners were asked to use the materials and return the questionnaires after using them for a minimum of ten impressions. Evaluators reported that the quality of fit of the single unit restoration constructed with the dual-arch tray was the same as that obtained when a conventional tray technique was used. 11

13 Larson et al. (2002) (17) determined the accuracy of addition silicone impressions made with custom trays or made with either passive or stressed dual-arch trays. Complete crown preparations of a mandibular molar, premolar and incisor were made on a typodont. These preparations received flat parallel indices on the facial and lingual axial walls for accurate and reproducible positioning of a digital caliper. Gypsum dies were produced for the addition silicone impression material in either custom trays or dual-arch trays seated passively or with induced flexure. The facio-lingual dimensions of the dies were measured with a digital caliper accurate to ± 5 μm and compared to the dimensions of the original preparations. The flexed tray group was significantly different than the other two groups which did not differ from each other. Dual-arch impressions were comparable in accuracy to the impressions made using the custom trays. Lane et al. (2003) (18) studied whether a dual-arch impression technique could produce restorations comparable with those produced by the use of the complete arch technique and investigated the reported time and the material savings. Five practitioners each recruited ten adult patients requiring a single complete veneer crown into the study. Two sets of impressions; one complete arch in a stock metal tray and one dualarch in a plastic dual-arch tray were made for each patient using addition polymerized silicones in a random order. The time taken for each impression was recorded along with the patient s assessment of comfort, taste and overall acceptability. Equal numbers of crowns were made from the complete and the dual-arch impressions by the same laboratory technician. At the time of the crown placement, the accuracy of the fit, occlusal harmony and the time taken during the try in procedure were 12

14 also recorded. The dual-arch technique was faster, more comfortable and preferred by 80% of the patients. No significant differences were found between the two groups for the accuracy of the fit, occlusion or time taken during the try in procedure of the restorations. Kang et al. (2009) (19) assessed the accuracy of reformulated polyvinyl siloxane impression materials using the single step dual-arch impression technique. Dual-arch impressions were made on a typodont containing a master stainless steel standard crown preparation. All impressions were disinfected for ten minutes prior to pouring with type IV gypsum. Buccolingual, mesiodistal and occlusogingival dimensions were measured and compared to the master die using an optical measuring microscope. There were statistical differences among the four impression systems for the different dimensions of the master preparation. Johnson et al. (2010) (20) compared the first impression success rates for two types of impression materials and two impression tray systems. Dual viscosity impressions were made with a polyvinyl siloxane and a polyether impression material. The first impression made was evaluated for success or failure using developed criteria. A full arch perforated plastic or a plastic dual-arch impression tray were used based on clinical guidelines. One hundred ninety one impressions were evaluated and the overall success rate was 61% for polyvinyl siloxane and 54% for polyether. There was a little difference in success rates between the PVS and the PE when full arch impression trays were used but there was a greater success rate when the PVS was used with dual-arch trays. 13

15 Assessment of the stone dies dimensional accuracy: Although the cast crown is a common restorative procedure in the daily dental practice, its accurate fit is still questionable as it is not only dependent on the used impression material and the impression tray type, but it is also highly dependent on the type and expansion capacity of the dental stone used for obtaining the stone casts and removable dies. Millstein et al. (1998) (21) evaluated the accuracy of casts made from stock tray and custom tray impressions using polyvinyl siloxane impression material. The method was one in which an impression was made of a test model cast in dental stone and the differences between the stone casts and the model were compared using a precision fitted template and a dial gauge. The results indicated that all casts distort and that impressions made from custom trays were more accurate and consistent in reproduction than those made using stock trays. Martinez and Frauenhofer (1998) (22) investigated the accuracy of master cast reproduction by a polyvinyl siloxane impression material using two visible light curing resin and an auto polymerizing polymethyl methacrylate resin custom made tray materials. Custom trays were fabricated from a master cast that had three index points marked on both the inner and outer vestibules. Impressions were made of the master cast and then poured in stone. The distances between the reproduced index points were measured with a travelling microscope. No differences were found in the dimensions of the inner index points while the separations of the outer index points indicated that there were differences in the accuracy of reproduction by the three tray materials. 14

16 Breeding and Dixon (2000) (23) compared the accuracy of stone dies of a prepared tooth generated using two types of dual-arch impression tray / impression material combinations. The maxillary left first molar of a typodont mounted on an articulator was prepared for a full coverage gold crown. Ten impressions were made with both a plastic or a metal tray and a polyether or a polyvinyl siloxane impression material. Each impression was cast in improved dental stone and the buccolingual dimension of the die was measured at the midpoint of the buccal and the lingual gingival margins. The prepared tooth served as the control. The plastic / polyvinyl siloxane combination produced the largest die followed by plastic / polyether, metal / polyether and metal / polyvinyl siloxane. Thongthammachat et al. (2002) (24) evaluated the influence on the dimensional accuracy of dental casts made with different types of trays and impression materials which were poured at different and multiple times. Two types of stock trays and four types of custom tray materials were used with two types of impression materials to make impressions of a metal master model. Each tray and impression material was used to make five impressions. Using a measuring microscope, twelve distances were calculated based on the measurements of eight reference points. They concluded that the deviations in the casts made with silicone impression materials were within a clinically acceptable range and accurate casts could be made with either stock or custom trays. Cox et al. (2002) (25) compared the dimensions of dies fabricated with three types of dual-arch impressions with dies fabricated with the conventional complete arch custom tray method. Thirty five addition 15

17 silicone impressions were made of cast metal copings cemented to natural teeth prepared as complete crown abutments. Four combinations of tray types and impression material viscosities were used: complete arch custom acrylic trays loaded with heavy body material, dual-arch disposable plastic trays loaded with heavy body material, dual-arch disposable plastic trays loaded with putty material and dual-arch brass metal trays loaded with heavy body material. Immediately prior to tray insertion, light body impression material was syringed over all copings as a wash. The impressions were poured in type IV dental stone. Buccolingual and inter-abutment dimensions were measured. The plastic dual-arch tray loaded with the heavy viscosity addition silicone and the low viscosity wash produced the least accurate combination in inter and intra-abutment dimensions. Ceyhan et al. (2003) (26) compared the accuracy of working dies made from impressions with metal and plastic dual-arch trays using two different viscosities of impression materials and by altering which side of the impression was poured first. Impressions were made of a typodont mandibular arch containing a circular stainless steel crown preparation. The obtained dies were measured in three dimensions with a measuring microscope. Significant differences were found with viscosity selection for the buccolingual and occlusogingival dimensions of the working dies. Regarding tray selection, metal trays were slightly more accurate in the mesiodistal dimension and when the monophase material was used in plastic trays, gypsum dies were smaller in the mesiodistal dimension. Ceyhan et al. (2003) (27) conducted a clinical trial to compare the accuracy of gypsum working dies made from impressions with metal 16

18 dual-arch, plastic dual-arch and complete arch custom trays. Eight patients requiring a posterior single tooth implant restoration were selected. A customized abutment was measured in three dimensions using a measuring microscope. Three polyvinyl siloxane impressions were made of the abutment and each impression was poured with type IV improved dental stone. The buccolingual, mesiodistal and occlusogingival dimensions were measured several times to obtain the mean values. These mean values served as the control and were compared with the same measurements of the gypsum dies generated by the three different impression techniques. There were no significant differences in the die accuracy among the three trays for the mesiodistal and the occlusogingival dimensions but they were significantly larger in the buccolingual dimension. Cayouette et al. (2003) (28) measured and compared the dimensions of casts made using four types of impression trays; metal dual-arch tray, plastic dual-arch tray, plastic stock tray, custom made acrylic tray and two types of impression materials to the dimensions of an original master model. Polyvinyl siloxane and polyether impressions were made of two crown preparations of ivorine teeth cemented to an acrylic master model. Two grooves were placed on each preparation. In addition, six points were placed on each occlusal surface and six were placed on each finish line. Impressions were poured with type IV dental stone. A (3D) measuring system was used to determine the coordinates of thirty two points on the master model and the resulting casts. Inter and intraabutment dimensions were calculated from the measured coordinates. Results indicated that only the custom tray did not detect any inaccuracies and was as accurate as the master model. 17

19 Kanehira et al. (2005) (29) evaluated the accuracy of stone dies produced from impressions with polyether impression materials and a polyvinyl siloxane reference material after prolonged storage at 0, 50, or 100% relative humidity (RH). Impressions were taken with light and heavy body types of the polyether and polyvinyl siloxane impression materials for a truncated steel master cone in cylindrical trays. Impressions were taken at 23 C and stone dies were poured after 1, 2, 3, 4, or 5 days storage at 23 C and 0, 50, or 100% (RH). Accuracy was determined as discrepancy between a steel ring with accurate fit on the master cone and the stone die, and was expressed as base diameter deviation. Results showed that all dies were 25 to 120 μm smaller than the master model. Reddy et al. (2009) (30) evaluated the accuracy of the inter-abutment distance in dies obtained from full dual-arch trays with those obtained from full arch stock metal trays. The dual-arch trays showed better accuracy followed by the stock dentulous trays. The pouring sequence did not have any effect on the inter-abutment distance. Assessment of the cast metal copings marginal accuracy: A common reason for the failure of a cast crown is the inadequacy of its margin; being over extended, under extended, poorly adapted or a combination of those defects. Since the impression making and casting procedures are necessary steps to make a well fitting restoration, other aids for assessing the marginal accuracy of castings have been recently developed. 18

20 Yeo, Yang and Lee (2003) (31) compared the marginal adaptation of single anterior restorations made using different systems. Marginal discrepancies of a metal restoration and three different all ceramic restorations were evaluated and compared by measuring the gap dimension between the restorations and the prepared tooth at the marginal opening. The restorations were made for one extracted maxillary central incisor prepared by milling. Thirty crowns per system were fabricated and crown measurements were recorded with an optical microscope. The marginal discrepancies were all within the clinically acceptable standard set at 120 μm. Gassino et al. (2004) (32) evaluated the marginal fit of experimental and custom made fixed prosthetic restorations through a new 360 external examination. The marginal fit of six experimental and eight custom made crowns was observed microscopically by the means of a mechanical device and software employed to measure the marginal gap. Differences in the fit between experimental specimens and custom made ones showed that experimental results might not always be obtained in clinical practice. Cox (2005) (33) evaluated the marginal fit and occlusion of posterior full crowns made from dual-arch impressions and compared them to control crowns fabricated from conventional full arch impressions. Ten patients requiring single posterior full crowns had both dual-arch and full arch impressions taken for the prepared teeth. Two crowns were fabricated for each tooth using the casts made from both impressions. Both crowns were tried in and the occlusion and margins were evaluated. Crowns fabricated from the dual-arch impressions were equivalent in 19