- Email: [email protected]
Accuracy of 3 conceptually different die systems used for implant casts
Accuracy of 3 conceptually different die systems used for implant casts Alvin G. Wee, BDS, MS,a Ansgar C. Cheng, BDS, MS,b and Ryan N. Eskridge, BSc College of Dentistry, The Ohio State University, Columbus, Ohio, and Faculty of Dentistry, University of Toronto, Ontario, Canada Statement of problem. Given that meticulous implant prosthodontic procedures are recommended to obtain the best possible intraoral fit, the die systems used for multi-implant casts warrant further investigation. Purpose. The purpose of this in vitro study was to compare the accuracy of implant casts fabricated from 3 conceptually different die systems at the solid, sectioned, and repeated stages. Material and methods. Thirty direct transfer implant impressions were made of the master cast with a polyether impression material. Ten experimental implant casts were fabricated for each of the 3 different die systems tested: double-pour (Pindex), plastic base (DVA), and die tray (KO Tray). The solid experimental casts were sectioned and then removed from the die system 30 times. Linear distances between steel balls placed on each abutment replica were measured with a traveling microscope to determine the accuracy of the experimental casts at different stages. Data were analyzed with repeated-measures analysis of variance (α=.05) and the post hoc Ryan-Einot-Gabriel-Welsch multiple-range test (REGWQ). Results. Repeated-measures analysis of variance revealed a significant interaction between the die systems and different stages (P=.0432). REGWQ showed the die tray system to be significantly more accurate at the solid than at the sectioned and repeated stages. The die tray system was significantly less accurate than the double-pour and plastic base systems at the sectioned stage. Conclusion. Within the limitations of this study, the use of a double-pour or plastic base die system is recommended when sectioned dies are needed for a multi-implant–retained prosthesis. (J Prosthet Dent 2002;87:23-9.)
CLINICAL IMPLICATIONS Accuracy of the implant cast facilitates clinical efficiency during fabrication of the implant prosthesis. The results of this study support the use of the double-pour or plastic base die system when removable dies are preferred to facilitate the waxing procedure for a multi-implant–retained prosthesis.
A
lthough true passive fit of multi-implant–supported prostheses to their intraoral implant abutments does not seem attainable,1-6 the degree of implant prosthesis misfit that will lead to complications is unclear. For this reason, meticulous and accurate implant prosthodontic procedures are recommended to obtain the best possible fit.5 The implant cast is the foundation on which the
This project was supported in part by The Ohio State University College of Dentistry Student Summer Research Program (NIH DE 07155-13 grant) and presented in part at the International Association of Dental Research annual session, April 2000, Washington, DC. aAssistant Professor, Section of Restorative Dentistry, Prosthodontics and Endodontics, The Ohio State University. bAssistant Professor, Department of Prosthodontics, University of Toronto; Head of Maxillofacial Prosthetics, University Health Network–Princess Margaret Hospital. cThird-year dental student, The Ohio State University. JANUARY 2002
prosthesis is indirectly fabricated. The use of the implant cast as a reference for extraoral implant framework fit facilitates the clinical evaluation of fit. Implant prosthesis fit strategies5,7-11 may be completed on the master cast prior to the patient’s appointment. The accuracy of the implant cast depends on the type of impression material used,12,13 the implant impression technique,12,14-21 die material accuracy,22 and the implant master cast technique.19,23 Die systems for fixed prosthodontics range from a solid cast with individual dies to the more conventional pour and base, or “double-pour,” system.24 As die systems have evolved, the second pour of latter system has been eliminated, which in turn has eliminated the effect of the expanding gypsum base on the accuracy of inter-die position. These evolved systems, which require the use of a preformed plastic base (often called a die tray25,26) improve the efficiency of the die fabrication procedure. Although no difference between different die tray systems has been found,26 a lack of THE JOURNAL OF PROSTHETIC DENTISTRY 23
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Fig. 1. Linear measurements made on solid experimental implant cast. Abutment replicas were spaced 12 mm apart and labeled A through E.
stability in vertical shift was noted with die trays compared to the double-pour system.25 Another type of die system, termed the “plastic base” die system for the purposes of this article, was introduced to the United States in 1990.27,28 This system requires use of a plastic base that allows an individualized, predetermined die-pin position for a particular impression. Theoretically, predetermination of the die-pin position will correct for the expansion of the die material.29 To date, a limited number of studies have evaluated this die system for implant casts.19,23,30 Although the use of a solid implant cast is the most convenient and common method for fullarch, implant-supported, fixed detachable complete dentures, the use of die systems may be convenient for the dental technician when a natural tooth/implantsupported prosthesis is required. In vitro studies have shown the use of a plastic base system for an implant cast to be slightly more accurate than a solid implant cast19,23 or an implant cast made with a double-pour system.23 Hsu et al19 evaluated 4 implant impression techniques with 2 different master cast systems: solid cast and plastic base (Zeiser System; Girrbach Dental, Santa Rosa, Calif.). With a profile projector and an electronic digital micrometer with 1-µm accuracy, 2 horizontal and 4 vertical measurements were made on a 4-implant model. Only half of the measurements (2 vertical and the posterior horizontal) on the Zeiser system were found to be significantly more accurate than those on the solid cast. Another study compared implant solid casts with casts made from a plastic base system (Zeiser; Girrbach Dental) and a double-pour system (Pindex; Coltene/ Whaledent, Mahwah, N.J.).23 Visual assessment of the fit of the master template showed that 14 of the 15 casts fabricated with the plastic base system, but only 1 fabricated with the double-pour system, displayed 24
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close alignment of the template on all 6 abutments. Only horizontal cross-arch measurements of the 6implant model were more accurate when implant casts were fabricated with the plastic base die system (0.09%) than with a solid cast (0.39%) or double-pour die system (0.49%). Another study evaluated 4 removable die systems (3 double-pour and 1 plastic base) with a 3-dimensional measuring technique.30 The authors attempted to postulate the results for both fixed and implant prosthodontics, leading to questionable conclusions since the calculation of error differed between fixed and implant prosthodontics. The study also used fixed prosthodontic impression techniques, which are different from implant impressions. Contrary to the manufacturer’s claims29 and the results of previous studies,19,23 the plastic base system was not found to compensate for expansion of the solid cast. Teraoka and Takahashi31 evaluated the expansion of gypsum in a silicone rubber impression. An impression of a master cast that represented the alveolar ridge was made with a silicone impression material, and dimensional changes in the stone cast were measured in 3 different directions (x-, y-, and z-axes). Two types of impression trays were used: one with an open surface and the other with a closed surface. A significant difference was found in expansion in the vertical (z-axis) and horizontal directions (x- and y-axes) of stone casts fabricated with the open tray. No such differences were found for the stone casts fabricated with the closed tray. The results of this study suggest that the setting environment can affect the amount and uniformity of expansion. The use of conceptually different die systems therefore may influence the accuracy of solid casts with reference to the arch from which the impression was made. No study has evaluated the accuracy of the 3 conceptually different die systems for multi-implant–retained prosthesis. The purpose of this in vitro investigation was to compare the accuracy of implant casts fabricated from these systems at the solid, sectioned, and repeated stages.
MATERIAL AND METHODS An implant master cast (Anderson Precision Machining, Inc, Iowa City, Iowa) was milled from a solid aluminum block. Five stainless steel abutment replicas (DCA 174; Nobel Biocare USA Inc, Chicago, Ill.) were cemented symmetrically in an arch with an adhesive resin cement (Panavia 21; J. Morita USA, Irvine, Calif.). The abutment replicas were spaced 12 mm apart and labeled A through E (Fig. 1). Ten similar impression trays were fabricated from light-polymerized resin (Triad; Dentsply, York, Pa.), and 10 sets of 5 direct square impression copings (DCB 026; Nobel Biocare USA Inc) were distributed randomly. The direct impression copings were handVOLUME 87 NUMBER 1
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Fig. 2. Double-pour die system (Pindex) for sectioned experimental implant cast.
tightened to the master cast with guide pins. Thirty direct implant polyether impressions13 (Impregum F; Premier Dental Products Co, Norristown, Pa.) were made of the aluminum master model with use of the 10 randomly assigned impression trays. The polyether was dispensed from an electric mixing and dispensing machine (Pentamix; ESPE Premier, Norristown, Pa.). The impressions were made on the master cast and allowed to set for twice as long as recommended by the manufacturer.32 Stainless steel abutment replicas (DCA 174; Nobel Biocare USA Inc) were hand-tightened carefully with 15-mm guide pins to the 5 direct impression copings in each impression. To simulate the clinical situation, impressions were poured 30 minutes after removal from the master cast. As recommended in a previous study,22 impressions were poured in Resin Rock (Whip Mix Corp, Louisville, Ky.) that was vacuum-mixed with distilled water, per the manufacturer’s instructions. Three conceptually different die systems were evaluated: a double-pour system (Pindex; Coltene/Whaledent) (Fig. 2); a die tray system (KO trays; Vident Inc, Brea, Calif.) (Fig. 3); and a plastic base system (DVA; Dental Ventures of America, Corona, Calif.) (Fig. 4). Ten experimental implant casts were fabricated with each of these die systems, resulting in a total of 30 experimental casts. The experimental implant casts were allowed to set for 1 hour before the guide pins were unscrewed and the direct implant impression was removed. Any debris remaining on the abutment replicas was removed. All experimental implant casts were numbered and stored in ambient conditions for at least 24 hours.33 All measurements of cast accuracy with respect to the master cast were performed by a single examiner in accordance with the method used in a previous study.13 Five 1.98-mm steel balls (McMaster-Carr, Aurora, Ohio) were placed on the central screw access channel JANUARY 2002
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Fig. 3. Die tray die system (KO Tray) for sectioned experimental implant cast. Arrow points to “spine” structure.
Fig. 4. Plastic base die system (DVA) for sectioned experimental implant cast.
of the abutments to provide a reliable central reflective reference. Linear distances between the steel balls were measured with a traveling microscope (Model MM-11; Nikon Corporation, Tokyo, Japan) capable of recording the x-, y-, and z-axes to an accuracy of 0.5 µm. On each cast, the middle abutment replica (C) was used as a reference for the distortion measurements of the remaining steel balls/abutment replicas. Thus, measurements were made of CA, CB, CD, and CE (Fig. 1) for all experimental casts in their solid stage. Each measurement was made on 3 separate occasions, and the data were averaged. A 45-minute time limit was observed for each measurement to prevent eye fatigue.34 An experimental set-up measurement error (SD) of 3 µm was computed by measuring the distance between abutment replicas A and E on the master cast 10 times. Each experimental cast was sectioned between the 5 abutment replicas, leaving their bases intact (the plastic base for the KO trays and DVA system and the second pour of the Pindex system) (Figs. 2 through 4). The sectioned portions then were replaced on their 25
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Fig. 5. Experimental design of study.
Fig. 6. Group mean absolute cast error associated with die systems at different stages. Groups with same letters were not significantly different (P >.05).
respective bases. Each individual segment was removed and replaced 30 times to evaluate stability of the segments in a simulated laboratory scenario.25,35 Linear measurements were made again to determine CA, CB, CD, and CE distances on each cast after it was sectioned (sectioned stage) and after the sections were repeatedly removed and returned to the base (repeated stage). The experimental design is illustrated in Figure 5. The data were entered into a spreadsheet program on a personal computer. The corresponding linear distances on the master model (CA, CB, and so forth) were subtracted from each mean linear distance on the experimental cast, with the absolute value taken as the result: CAexperimental – CAmaster = [A] 26
With this formula, the absolute micron deviation of the experimental cast abutment from the master model was determined. The micron deviation for each of the 4 linear measurements ([A], [B], [D], and [E]) was recorded for each cast, and these values were averaged to obtain the mean absolute cast error. This error was calculated for the 3 different stages of each experimental implant cast. The absolute micron deviations of all linear measurements were compared with repeated-measures analysis of variance (α=.05) to evaluate within-group (stages within the die system) and between-group (stages between the die systems) differences as well as the interaction between these variables (die systems and stages). Thereafter, a post hoc Ryan-EinotGabriel-Welsch multiple range test (REGWQ) was used to rank the groups. All statistical analyses were VOLUME 87 NUMBER 1
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performed at the 95% confidence level (SAS Institute, Cary, N.C.).
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Table I. Repeated-measures analysis of variance for mean absolute cast error for die systems at different stages df
Sum of squares
F value
P value
Between-subjects effect Model 4 Error 27
8068.03392 36630.5948
2.97 —
.0681 —
Within-subjects effects Stages 2 Stages × materials 4 Error 54
25859.1596 8168.8725 41679.1962
16.75 2.65
.0001 .0432
RESULTS The results of the study are provided in Figure 6. Repeated-measures analysis of variance showed a significant interaction between the die systems and measurement stages (P=.0432; Table I). The REGWQ test (Table II) revealed that, for the die tray system, casts were significantly different and more accurate at the solid stage than at the sectioned and repeated stages. No significant differences among the 3 stages were found for either the double-pour or plastic base system. Within the sectioned stage, casts produced with the die tray system were significantly different and less accurate than those produced with either the double-pour or plastic base system. Measurements made at the solid and repeated stages were not significantly different among the die systems tested.
DISCUSSION The data from this study reveal that casts fabricated with the die tray system were significantly different at the 3 stages and, when sectioned, were significantly less accurate than the experimental implant casts fabricated with the other systems. The internal jagged edge of the gypsum cast produced by the die tray system is meant to stabilize the individual sectioned dies. These jagged edges were insufficient to prevent mobility of the sectioned die components, which resulted in instability and decreased accuracy. Covo et al25 compared the Accu-Trac die system (Duroc, Ransom and Randolph Co, Toledo, Ohio) to a double-pour die system and found inaccuracies in the vertical dimension of the Accu-Trac casts. The KO tray used in this study is an improvement over previous die tray models such as Accu-Trac.25,26 The addition of a plastic “spine” structure is intended to help prevent instability in the vertical dimension (Fig. 3). However, the measurement methodology employed in the present study assessed the “global” accuracy of the implant cast rather than the accuracy of the individual dies in 3-dimensional distortion (x-, y- and z-axes). In the solid stage, casts fabricated with the die tray system were more accurate (16 µm) than those fabricated with the other systems tested (21 µm). The results of a study by Teraoka and Takahashi,31 who evaluated the expansion of gypsum in open and closed impression trays, suggest that the rigidity of the plastic die tray used in the present study may have restrained the expansion of the setting gypsum, thus resulting in a lower mean absolute cast error. With respect to the use of the plastic base die system for implant casts, the results of this study differ from the manufacturer’s claim.29 Theoretically, it is presumed that the predetermined position of die pins JANUARY 2002
Table II. Ryan-Einot-Gabriel-Welsch multiple range test (REGWQ) groupings for mean absolute cast error Mean absolute cast error (µm)
83.68 69.63 58.09 42.61 41.01 40.19 21.49 21.36 16.88
Die system
Stage
Die tray Die tray Plastic base Plastic base Double-pour Double-pour Plastic base Double-pour Die tray
Sectioned Repeated Repeated Sectioned Sectioned Repeated Solid Solid Solid
REGWQ groupings
B B B B B
A A A D D D D D D
C C C C C C
Groups with the same letters were not significantly different (P>.05).
on the plastic base with respect to the impression (be it the tooth preparation or an implant abutment) will correct for the expansion of the die material. In the present study, no differences were found between presectioning and postsectioning of the die. This differs from other studies19,23 that showed a slight postsectioning improvement in casts fabricated with a plastic base die system. The methodology used in this study assessed only the resultant cast’s mean absolute error. This “global” error is clinically relevant in implant prosthodontics, given that the framework is connected and secured to all abutment replicas on the same cast. The final stress produced in the framework/abutment system is a reflection of a more global distortion rather than of the individual distortion components, namely translation distortion (x-, y-, z-axes) and rotation distortion (δθx, δθy, and δθz).5 The methodology used in this study would not have detected the more differential nature of accuracy reported in previous studies.19,23 These studies evaluated the vertical and horizontal dimensional changes of the experimental cast’s abutment replicas. The use of a low-expansion stone may have influenced the results of this study. Vigolo and Millstein23 evaluated Die-Keen, an ADA type V high-expansion stone. It is possible that the theoretic model of how 27
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the plastic base die system compensates for expansion of the die material is more relevant to high-expansion stones. Because a low-expansion stone was used in the present study (as previously recommended22), the methodology may not have been sensitive enough to detect dimensional changes. As in a previous study,13 the accuracy of the relative position between implant abutments was evaluated with the use of steel balls on the abutment replicas. It should be noted that the purpose of the present study was not to evaluate the implant abutment-to-framework relationship, which has been characterized in other investigations of die systems.19,23 The disadvantage of using a repositioning matrix and measuring the gap between the analogous sections is the possibility of causing flexure of the bases and/or tilting the individual dies, which would cause erroneous conclusions. This study evaluated only the resultant translation distortion (x-, y-, and z-axes) of the abutment replicas to one another, not the more complex rotational distortion that occurs in implant prosthodontics.2-6,20,21 The significant difference detected among the groups would indicate that the methodology used in this study was appropriately sensitive to achieve the established objectives. Because the degree of implant prosthesis fit needed to prevent clinical complications has yet to be determined,5 the clinical significance of the results of this study cannot be stated conclusively.
CONCLUSIONS Within the limitations of this study, no significant improvement in the accuracy of the multi-implant experimental casts, assessed from the solid to the sectioned and repeated stages, was found for any of the die systems tested. The use of a solid implant cast is still recommended for multi-implant prostheses. No significant differences between the sectioned and repeated stages were found for casts fabricated with the double-pour or plastic base die systems. Within the sectioned stage, the die tray system produced less accurate casts than the other systems tested. Thus, when sectioned dies are needed, the use of a doublepour or plastic base die system is recommended. We would like to thank Dr William Johnston for his statistical assistance. The work done by Iaen Lee (former research associate, College of Dentistry, The Ohio State University) is much appreciated. We would also like to thank the Dental Ventures of North America and Vident Inc for the loan and use of their die systems.
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11. 12.
13. 14.
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REFERENCES 1. Jemt T, Carlsson L, Boss A, Jorneus L. In vivo load measurements on osseointegrated implants supporting fixed or removable prostheses: a comparative pilot study. Int J Oral Maxillofac Implants 1991; 6:413-7. 2. Jemt T. Three-dimensional distortion of gold alloy castings and welded
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27. 28. 29. 30.
titanium frameworks. Measurement of the precision of fit between completed implant prostheses and the master casts in routine edentulous situations. J Oral Rehabil 1995;22:557-64. Jemt T. In vivo measurements of precision of fit involving implant-supported prostheses in the edentulous jaw. Int J Oral Maxillofac Implants 1996;11:151-8. Tan KB. The clinical significance of distortion in implant prosthodontics: is there such a thing as passive fit? Ann Acad Med Singapore 1995;24:138-57. Wee AG, Aquilino SA, Schneider RL. Strategies to achieve fit in implant prosthodontics: a review of the literature. Int J Prosthodont 1999;12:16778. Jemt T, Rubenstein JE, Carlsson L, Lang BR. Measuring fit at the implant prosthodontic interface. J Prosthet Dent 1996;75:314-25. Riedy SJ, Lang BR, Lang BE. Fit of implant frameworks fabricated by different techniques. J Prosthet Dent 1997;78:596-604. Jemt T, Linden B. Fixed implant-supported prostheses with welded titanium frameworks. Int J Periodontics Restorative Dent 1992; 12:177-84. Linehan AD, Windeler AS. Passive fit of implant-retained prosthetic superstructures improved by electric discharge machining. J Prosthodont 1994;3:88-95. Schmitt SM, Chance DA. Fabrication of titanium implant-retained restorations with nontraditional machining techniques. Int J Prosthodont 1995;8:332-6. LaBarge KW. Electrical discharge machining. J Dent Technol 1997;14:19-22. Barrett MG, de Rijk WG, Burgess JO. The accuracy of six impression techniques for osseointegrated implants. J Prosthodont 1993;2:7582. Wee AG. Comparison of impression materials for direct multi-implant impressions. J Prosthet Dent 2000;83:323-31. Humphries RM, Yaman P, Bloem TJ. The accuracy of implant master cast constructed from transfer impression. Int J Oral Maxillofac Implants 1990;5:331-6. Spector MR, Donovan TE, Nicholls JI. An evaluation of impression techniques for osseointegrated implants. J Prosthet Dent 1990;63:4447. Carr AB. Comparison of impression techniques for a five-implant mandibular model. Int J Oral Maxillofac Implants 1991;6:448-55. Carr AB. Comparison of impression techniques for a two-implant 15degree divergent model. Int J Oral Maxillofac Implants 1992; 7:468-75. Assif D, Fenton A, Zarb G, Schmitt A. Comparative accuracy of implant impression procedures. Int J Periodontics Restorative Dent 1992;12:11221. Hsu CC, Millstein PL, Stein RS. A comparative analysis of the accuracy of implant transfer techniques. J Prosthet Dent 1993;69:588-93. Phillips KM, Nicholls JI, Ma T, Rubenstein JE. The accuracy of three implant impression techniques: a 3-dimensional analysis. Int J Oral Maxillofac Implants 1994;9:533-40. Assif D, Marshak B, Schmidt A. Accuracy of implant impression techniques. Int J Oral Maxillofac Implants 1996;11:216-22. Wee AG, Schneider RL, Aquilino SA, Huff TL, Linquist TJ, Williamson DL. Evaluation of the accuracy of solid implant casts. J Prosthodont 1998;7:161-9. Vigolo P, Millstein PL. Evaluation of master cast technique for multiple abutment implant prostheses. Int J Oral Maxillofac Implants 1993;8:43946. Rosenstiel SF, Land MF, Fujimoto J. Working casts and dies. In: Rosenstiel SF, Land MF, Fujimoto J, editors. Contemporary fixed prosthodontics. 3rd ed. St Louis (MO): Mosby; 2001. p. 434-41. Covo LM, Ziebert GJ, Balthazar Y, Christensen LV. Accuracy and comparative stability of three removable die systems. J Prosthet Dent 1988;59:314-8. Richardson DW, Sanchez RA, Baker PS, Haug SP. Positional accuracy of four die tray systems. J Prosthet Dent 1991;66:39-45. Millstein P, Filapancic J. The Zeiser system: a method for accurate die placement. Quintessence Dent Technol 1990;14:188-92. Barbaro V, Battistelli A, Massironi D. A simplified system to develop precision master models. J Dent Technol 1996;13:19-23. America DV. Technical assistance video for the DVA Model System [Video]. Anaheim Hills (CA): Dental Ventures of America; 1992. Serrano JG, Lepe X, Townsend JD, Johnson GH, Thielke S. An accuracy
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evaluation of four removable die systems. J Prosthet Dent 1998;80:57586. Teraoka F, Takahashi J. Dimensional changes and pressure of dental stones set in silicone rubber impressions. Dent Mater 2000;16:1459. Revised American Dental Association Specification no. 19 for Nonaqueous, Elastomeric Dental Impression Materials. J Am Dent Assoc 1977;94:733-41. Arlo K. Evaluation of the Vident die and model system. San Antonio (TX): USAF Dental Investigation Services; 1991. Report No: Project 90-63. Nicholls JL. The measurement of distortion: concluding remarks. J Prosthet Dent 1980;43:218-23. Myers M, Hembree JH Jr. Relative accuracy of four removable die systems. J Prosthet Dent 1982;48:163-5.
Noteworthy Abstracts of the Current Literature
Reprint requests to: DR ALVIN G. WEE SECTION OF RESTORATIVE DENTISTRY, PROSTHODONTICS AND ENDODONTICS THE OHIO STATE UNIVERSITY POSTLE HALL, 305 W 12TH AVE COLUMBUS, OH 43210-1241 FAX: (614)292-9422 E-MAIL: [email protected] Copyright © 2002 by The Editorial Council of The Journal of Prosthetic Dentistry. 0022-3913/2002/$35.00 + 0. 10/1/121110 doi:10.1067/mpr.2002.121110
The performance of bonded vs. pin-retained complex amalgam restorations: A five-year clinical evaluation. Summitt JB, Burgess JO, Berry TG, Robbins JW, Osborne JW, Haveman CW. J Am Dent Assoc 2001;132:923-31.
Purpose. This 5-year clinical study compared the failure rates, marginal discoloration, recurrent caries rates, marginal adaptation, sensitivity, and tooth vitality associated with bonded and pinretained complex amalgam restorations. Material and methods. Sixty teeth were restored by 5 clinicians. Twenty-eight of the teeth were restored with pin-retained amalgam (group 1), and 32 were restored with a bonded amalgam restoration (group 2). Rubber dam isolation was used in conjunction with restoration placement. All restorations encompassed at least 1 proximal surface and replaced at least 1 cusp. All restored teeth were deemed vital before restoration placement and were opposed by an antagonist tooth. Within group 1, a vertical TMS Minim pin (Thread-Mate System; Coltene-Whaledent) was used for each missing cusp. Horizontal TMS Minikin pins were used at the discretion of the clinician. Teeth in Group 2 received no additional mechanical retention but were bonded with AmalgamBond Plus (Parkell) combined with high-performance additive powder. Patients returned for a baseline evaluation 1 week after restoration placement. Recall evaluations were completed 6 months and 1, 2, 3, 4, and 5 years later. At each recall appointment, the clinicians confirmed tooth vitality. Tooth sensitivity was assessed thermally, and marginal integrity, marginal discoloration, and recurrent caries were evaluated with the Cvar/Ryge criteria. The Fisher exact test was used to analyze the data. Results. At the 4-year recall evaluation, 6 restorations had failed. Of the 40 restorations available for assessment at the 5-year recall evaluation, 3 had failed. A total of 7 pin-retained and 2 bonded amalgam restorations failed over the study period. Failures included restorations that had to be replaced, restorations that required considerable repair, and teeth that required endodontic therapy or extraction. No significant difference in failure rate was observed between the 2 groups at 5 years. At the 6-month evaluation appointment, a significant difference in sensitivity was noted between the groups; no significant differences in the other assessment criteria were found over the 5-year evaluation period. Conclusion. Amalgam restorations that replace at least one cusp may be effectively retained with a 4-META-based bonding system over a 5-year period. 32 References.—DL Dixon
JANUARY 2002
29
CLINICAL IMPLICATIONS Accuracy of the implant cast facilitates clinical efficiency during fabrication of the implant prosthesis. The results of this study support the use of the double-pour or plastic base die system when removable dies are preferred to facilitate the waxing procedure for a multi-implant–retained prosthesis.
A
lthough true passive fit of multi-implant–supported prostheses to their intraoral implant abutments does not seem attainable,1-6 the degree of implant prosthesis misfit that will lead to complications is unclear. For this reason, meticulous and accurate implant prosthodontic procedures are recommended to obtain the best possible fit.5 The implant cast is the foundation on which the
This project was supported in part by The Ohio State University College of Dentistry Student Summer Research Program (NIH DE 07155-13 grant) and presented in part at the International Association of Dental Research annual session, April 2000, Washington, DC. aAssistant Professor, Section of Restorative Dentistry, Prosthodontics and Endodontics, The Ohio State University. bAssistant Professor, Department of Prosthodontics, University of Toronto; Head of Maxillofacial Prosthetics, University Health Network–Princess Margaret Hospital. cThird-year dental student, The Ohio State University. JANUARY 2002
prosthesis is indirectly fabricated. The use of the implant cast as a reference for extraoral implant framework fit facilitates the clinical evaluation of fit. Implant prosthesis fit strategies5,7-11 may be completed on the master cast prior to the patient’s appointment. The accuracy of the implant cast depends on the type of impression material used,12,13 the implant impression technique,12,14-21 die material accuracy,22 and the implant master cast technique.19,23 Die systems for fixed prosthodontics range from a solid cast with individual dies to the more conventional pour and base, or “double-pour,” system.24 As die systems have evolved, the second pour of latter system has been eliminated, which in turn has eliminated the effect of the expanding gypsum base on the accuracy of inter-die position. These evolved systems, which require the use of a preformed plastic base (often called a die tray25,26) improve the efficiency of the die fabrication procedure. Although no difference between different die tray systems has been found,26 a lack of THE JOURNAL OF PROSTHETIC DENTISTRY 23
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Fig. 1. Linear measurements made on solid experimental implant cast. Abutment replicas were spaced 12 mm apart and labeled A through E.
stability in vertical shift was noted with die trays compared to the double-pour system.25 Another type of die system, termed the “plastic base” die system for the purposes of this article, was introduced to the United States in 1990.27,28 This system requires use of a plastic base that allows an individualized, predetermined die-pin position for a particular impression. Theoretically, predetermination of the die-pin position will correct for the expansion of the die material.29 To date, a limited number of studies have evaluated this die system for implant casts.19,23,30 Although the use of a solid implant cast is the most convenient and common method for fullarch, implant-supported, fixed detachable complete dentures, the use of die systems may be convenient for the dental technician when a natural tooth/implantsupported prosthesis is required. In vitro studies have shown the use of a plastic base system for an implant cast to be slightly more accurate than a solid implant cast19,23 or an implant cast made with a double-pour system.23 Hsu et al19 evaluated 4 implant impression techniques with 2 different master cast systems: solid cast and plastic base (Zeiser System; Girrbach Dental, Santa Rosa, Calif.). With a profile projector and an electronic digital micrometer with 1-µm accuracy, 2 horizontal and 4 vertical measurements were made on a 4-implant model. Only half of the measurements (2 vertical and the posterior horizontal) on the Zeiser system were found to be significantly more accurate than those on the solid cast. Another study compared implant solid casts with casts made from a plastic base system (Zeiser; Girrbach Dental) and a double-pour system (Pindex; Coltene/ Whaledent, Mahwah, N.J.).23 Visual assessment of the fit of the master template showed that 14 of the 15 casts fabricated with the plastic base system, but only 1 fabricated with the double-pour system, displayed 24
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close alignment of the template on all 6 abutments. Only horizontal cross-arch measurements of the 6implant model were more accurate when implant casts were fabricated with the plastic base die system (0.09%) than with a solid cast (0.39%) or double-pour die system (0.49%). Another study evaluated 4 removable die systems (3 double-pour and 1 plastic base) with a 3-dimensional measuring technique.30 The authors attempted to postulate the results for both fixed and implant prosthodontics, leading to questionable conclusions since the calculation of error differed between fixed and implant prosthodontics. The study also used fixed prosthodontic impression techniques, which are different from implant impressions. Contrary to the manufacturer’s claims29 and the results of previous studies,19,23 the plastic base system was not found to compensate for expansion of the solid cast. Teraoka and Takahashi31 evaluated the expansion of gypsum in a silicone rubber impression. An impression of a master cast that represented the alveolar ridge was made with a silicone impression material, and dimensional changes in the stone cast were measured in 3 different directions (x-, y-, and z-axes). Two types of impression trays were used: one with an open surface and the other with a closed surface. A significant difference was found in expansion in the vertical (z-axis) and horizontal directions (x- and y-axes) of stone casts fabricated with the open tray. No such differences were found for the stone casts fabricated with the closed tray. The results of this study suggest that the setting environment can affect the amount and uniformity of expansion. The use of conceptually different die systems therefore may influence the accuracy of solid casts with reference to the arch from which the impression was made. No study has evaluated the accuracy of the 3 conceptually different die systems for multi-implant–retained prosthesis. The purpose of this in vitro investigation was to compare the accuracy of implant casts fabricated from these systems at the solid, sectioned, and repeated stages.
MATERIAL AND METHODS An implant master cast (Anderson Precision Machining, Inc, Iowa City, Iowa) was milled from a solid aluminum block. Five stainless steel abutment replicas (DCA 174; Nobel Biocare USA Inc, Chicago, Ill.) were cemented symmetrically in an arch with an adhesive resin cement (Panavia 21; J. Morita USA, Irvine, Calif.). The abutment replicas were spaced 12 mm apart and labeled A through E (Fig. 1). Ten similar impression trays were fabricated from light-polymerized resin (Triad; Dentsply, York, Pa.), and 10 sets of 5 direct square impression copings (DCB 026; Nobel Biocare USA Inc) were distributed randomly. The direct impression copings were handVOLUME 87 NUMBER 1
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Fig. 2. Double-pour die system (Pindex) for sectioned experimental implant cast.
tightened to the master cast with guide pins. Thirty direct implant polyether impressions13 (Impregum F; Premier Dental Products Co, Norristown, Pa.) were made of the aluminum master model with use of the 10 randomly assigned impression trays. The polyether was dispensed from an electric mixing and dispensing machine (Pentamix; ESPE Premier, Norristown, Pa.). The impressions were made on the master cast and allowed to set for twice as long as recommended by the manufacturer.32 Stainless steel abutment replicas (DCA 174; Nobel Biocare USA Inc) were hand-tightened carefully with 15-mm guide pins to the 5 direct impression copings in each impression. To simulate the clinical situation, impressions were poured 30 minutes after removal from the master cast. As recommended in a previous study,22 impressions were poured in Resin Rock (Whip Mix Corp, Louisville, Ky.) that was vacuum-mixed with distilled water, per the manufacturer’s instructions. Three conceptually different die systems were evaluated: a double-pour system (Pindex; Coltene/Whaledent) (Fig. 2); a die tray system (KO trays; Vident Inc, Brea, Calif.) (Fig. 3); and a plastic base system (DVA; Dental Ventures of America, Corona, Calif.) (Fig. 4). Ten experimental implant casts were fabricated with each of these die systems, resulting in a total of 30 experimental casts. The experimental implant casts were allowed to set for 1 hour before the guide pins were unscrewed and the direct implant impression was removed. Any debris remaining on the abutment replicas was removed. All experimental implant casts were numbered and stored in ambient conditions for at least 24 hours.33 All measurements of cast accuracy with respect to the master cast were performed by a single examiner in accordance with the method used in a previous study.13 Five 1.98-mm steel balls (McMaster-Carr, Aurora, Ohio) were placed on the central screw access channel JANUARY 2002
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Fig. 3. Die tray die system (KO Tray) for sectioned experimental implant cast. Arrow points to “spine” structure.
Fig. 4. Plastic base die system (DVA) for sectioned experimental implant cast.
of the abutments to provide a reliable central reflective reference. Linear distances between the steel balls were measured with a traveling microscope (Model MM-11; Nikon Corporation, Tokyo, Japan) capable of recording the x-, y-, and z-axes to an accuracy of 0.5 µm. On each cast, the middle abutment replica (C) was used as a reference for the distortion measurements of the remaining steel balls/abutment replicas. Thus, measurements were made of CA, CB, CD, and CE (Fig. 1) for all experimental casts in their solid stage. Each measurement was made on 3 separate occasions, and the data were averaged. A 45-minute time limit was observed for each measurement to prevent eye fatigue.34 An experimental set-up measurement error (SD) of 3 µm was computed by measuring the distance between abutment replicas A and E on the master cast 10 times. Each experimental cast was sectioned between the 5 abutment replicas, leaving their bases intact (the plastic base for the KO trays and DVA system and the second pour of the Pindex system) (Figs. 2 through 4). The sectioned portions then were replaced on their 25
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Fig. 5. Experimental design of study.
Fig. 6. Group mean absolute cast error associated with die systems at different stages. Groups with same letters were not significantly different (P >.05).
respective bases. Each individual segment was removed and replaced 30 times to evaluate stability of the segments in a simulated laboratory scenario.25,35 Linear measurements were made again to determine CA, CB, CD, and CE distances on each cast after it was sectioned (sectioned stage) and after the sections were repeatedly removed and returned to the base (repeated stage). The experimental design is illustrated in Figure 5. The data were entered into a spreadsheet program on a personal computer. The corresponding linear distances on the master model (CA, CB, and so forth) were subtracted from each mean linear distance on the experimental cast, with the absolute value taken as the result: CAexperimental – CAmaster = [A] 26
With this formula, the absolute micron deviation of the experimental cast abutment from the master model was determined. The micron deviation for each of the 4 linear measurements ([A], [B], [D], and [E]) was recorded for each cast, and these values were averaged to obtain the mean absolute cast error. This error was calculated for the 3 different stages of each experimental implant cast. The absolute micron deviations of all linear measurements were compared with repeated-measures analysis of variance (α=.05) to evaluate within-group (stages within the die system) and between-group (stages between the die systems) differences as well as the interaction between these variables (die systems and stages). Thereafter, a post hoc Ryan-EinotGabriel-Welsch multiple range test (REGWQ) was used to rank the groups. All statistical analyses were VOLUME 87 NUMBER 1
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performed at the 95% confidence level (SAS Institute, Cary, N.C.).
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Table I. Repeated-measures analysis of variance for mean absolute cast error for die systems at different stages df
Sum of squares
F value
P value
Between-subjects effect Model 4 Error 27
8068.03392 36630.5948
2.97 —
.0681 —
Within-subjects effects Stages 2 Stages × materials 4 Error 54
25859.1596 8168.8725 41679.1962
16.75 2.65
.0001 .0432
RESULTS The results of the study are provided in Figure 6. Repeated-measures analysis of variance showed a significant interaction between the die systems and measurement stages (P=.0432; Table I). The REGWQ test (Table II) revealed that, for the die tray system, casts were significantly different and more accurate at the solid stage than at the sectioned and repeated stages. No significant differences among the 3 stages were found for either the double-pour or plastic base system. Within the sectioned stage, casts produced with the die tray system were significantly different and less accurate than those produced with either the double-pour or plastic base system. Measurements made at the solid and repeated stages were not significantly different among the die systems tested.
DISCUSSION The data from this study reveal that casts fabricated with the die tray system were significantly different at the 3 stages and, when sectioned, were significantly less accurate than the experimental implant casts fabricated with the other systems. The internal jagged edge of the gypsum cast produced by the die tray system is meant to stabilize the individual sectioned dies. These jagged edges were insufficient to prevent mobility of the sectioned die components, which resulted in instability and decreased accuracy. Covo et al25 compared the Accu-Trac die system (Duroc, Ransom and Randolph Co, Toledo, Ohio) to a double-pour die system and found inaccuracies in the vertical dimension of the Accu-Trac casts. The KO tray used in this study is an improvement over previous die tray models such as Accu-Trac.25,26 The addition of a plastic “spine” structure is intended to help prevent instability in the vertical dimension (Fig. 3). However, the measurement methodology employed in the present study assessed the “global” accuracy of the implant cast rather than the accuracy of the individual dies in 3-dimensional distortion (x-, y- and z-axes). In the solid stage, casts fabricated with the die tray system were more accurate (16 µm) than those fabricated with the other systems tested (21 µm). The results of a study by Teraoka and Takahashi,31 who evaluated the expansion of gypsum in open and closed impression trays, suggest that the rigidity of the plastic die tray used in the present study may have restrained the expansion of the setting gypsum, thus resulting in a lower mean absolute cast error. With respect to the use of the plastic base die system for implant casts, the results of this study differ from the manufacturer’s claim.29 Theoretically, it is presumed that the predetermined position of die pins JANUARY 2002
Table II. Ryan-Einot-Gabriel-Welsch multiple range test (REGWQ) groupings for mean absolute cast error Mean absolute cast error (µm)
83.68 69.63 58.09 42.61 41.01 40.19 21.49 21.36 16.88
Die system
Stage
Die tray Die tray Plastic base Plastic base Double-pour Double-pour Plastic base Double-pour Die tray
Sectioned Repeated Repeated Sectioned Sectioned Repeated Solid Solid Solid
REGWQ groupings
B B B B B
A A A D D D D D D
C C C C C C
Groups with the same letters were not significantly different (P>.05).
on the plastic base with respect to the impression (be it the tooth preparation or an implant abutment) will correct for the expansion of the die material. In the present study, no differences were found between presectioning and postsectioning of the die. This differs from other studies19,23 that showed a slight postsectioning improvement in casts fabricated with a plastic base die system. The methodology used in this study assessed only the resultant cast’s mean absolute error. This “global” error is clinically relevant in implant prosthodontics, given that the framework is connected and secured to all abutment replicas on the same cast. The final stress produced in the framework/abutment system is a reflection of a more global distortion rather than of the individual distortion components, namely translation distortion (x-, y-, z-axes) and rotation distortion (δθx, δθy, and δθz).5 The methodology used in this study would not have detected the more differential nature of accuracy reported in previous studies.19,23 These studies evaluated the vertical and horizontal dimensional changes of the experimental cast’s abutment replicas. The use of a low-expansion stone may have influenced the results of this study. Vigolo and Millstein23 evaluated Die-Keen, an ADA type V high-expansion stone. It is possible that the theoretic model of how 27
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the plastic base die system compensates for expansion of the die material is more relevant to high-expansion stones. Because a low-expansion stone was used in the present study (as previously recommended22), the methodology may not have been sensitive enough to detect dimensional changes. As in a previous study,13 the accuracy of the relative position between implant abutments was evaluated with the use of steel balls on the abutment replicas. It should be noted that the purpose of the present study was not to evaluate the implant abutment-to-framework relationship, which has been characterized in other investigations of die systems.19,23 The disadvantage of using a repositioning matrix and measuring the gap between the analogous sections is the possibility of causing flexure of the bases and/or tilting the individual dies, which would cause erroneous conclusions. This study evaluated only the resultant translation distortion (x-, y-, and z-axes) of the abutment replicas to one another, not the more complex rotational distortion that occurs in implant prosthodontics.2-6,20,21 The significant difference detected among the groups would indicate that the methodology used in this study was appropriately sensitive to achieve the established objectives. Because the degree of implant prosthesis fit needed to prevent clinical complications has yet to be determined,5 the clinical significance of the results of this study cannot be stated conclusively.
CONCLUSIONS Within the limitations of this study, no significant improvement in the accuracy of the multi-implant experimental casts, assessed from the solid to the sectioned and repeated stages, was found for any of the die systems tested. The use of a solid implant cast is still recommended for multi-implant prostheses. No significant differences between the sectioned and repeated stages were found for casts fabricated with the double-pour or plastic base die systems. Within the sectioned stage, the die tray system produced less accurate casts than the other systems tested. Thus, when sectioned dies are needed, the use of a doublepour or plastic base die system is recommended. We would like to thank Dr William Johnston for his statistical assistance. The work done by Iaen Lee (former research associate, College of Dentistry, The Ohio State University) is much appreciated. We would also like to thank the Dental Ventures of North America and Vident Inc for the loan and use of their die systems.
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3.
4.
5.
6. 7. 8.
9.
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11. 12.
13. 14.
15.
16. 17.
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19. 20.
21. 22.
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26.
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titanium frameworks. Measurement of the precision of fit between completed implant prostheses and the master casts in routine edentulous situations. J Oral Rehabil 1995;22:557-64. Jemt T. In vivo measurements of precision of fit involving implant-supported prostheses in the edentulous jaw. Int J Oral Maxillofac Implants 1996;11:151-8. Tan KB. The clinical significance of distortion in implant prosthodontics: is there such a thing as passive fit? Ann Acad Med Singapore 1995;24:138-57. Wee AG, Aquilino SA, Schneider RL. Strategies to achieve fit in implant prosthodontics: a review of the literature. Int J Prosthodont 1999;12:16778. Jemt T, Rubenstein JE, Carlsson L, Lang BR. Measuring fit at the implant prosthodontic interface. J Prosthet Dent 1996;75:314-25. Riedy SJ, Lang BR, Lang BE. Fit of implant frameworks fabricated by different techniques. J Prosthet Dent 1997;78:596-604. Jemt T, Linden B. Fixed implant-supported prostheses with welded titanium frameworks. Int J Periodontics Restorative Dent 1992; 12:177-84. Linehan AD, Windeler AS. Passive fit of implant-retained prosthetic superstructures improved by electric discharge machining. J Prosthodont 1994;3:88-95. Schmitt SM, Chance DA. Fabrication of titanium implant-retained restorations with nontraditional machining techniques. Int J Prosthodont 1995;8:332-6. LaBarge KW. Electrical discharge machining. J Dent Technol 1997;14:19-22. Barrett MG, de Rijk WG, Burgess JO. The accuracy of six impression techniques for osseointegrated implants. J Prosthodont 1993;2:7582. Wee AG. Comparison of impression materials for direct multi-implant impressions. J Prosthet Dent 2000;83:323-31. Humphries RM, Yaman P, Bloem TJ. The accuracy of implant master cast constructed from transfer impression. Int J Oral Maxillofac Implants 1990;5:331-6. Spector MR, Donovan TE, Nicholls JI. An evaluation of impression techniques for osseointegrated implants. J Prosthet Dent 1990;63:4447. Carr AB. Comparison of impression techniques for a five-implant mandibular model. Int J Oral Maxillofac Implants 1991;6:448-55. Carr AB. Comparison of impression techniques for a two-implant 15degree divergent model. Int J Oral Maxillofac Implants 1992; 7:468-75. Assif D, Fenton A, Zarb G, Schmitt A. Comparative accuracy of implant impression procedures. Int J Periodontics Restorative Dent 1992;12:11221. Hsu CC, Millstein PL, Stein RS. A comparative analysis of the accuracy of implant transfer techniques. J Prosthet Dent 1993;69:588-93. Phillips KM, Nicholls JI, Ma T, Rubenstein JE. The accuracy of three implant impression techniques: a 3-dimensional analysis. Int J Oral Maxillofac Implants 1994;9:533-40. Assif D, Marshak B, Schmidt A. Accuracy of implant impression techniques. Int J Oral Maxillofac Implants 1996;11:216-22. Wee AG, Schneider RL, Aquilino SA, Huff TL, Linquist TJ, Williamson DL. Evaluation of the accuracy of solid implant casts. J Prosthodont 1998;7:161-9. Vigolo P, Millstein PL. Evaluation of master cast technique for multiple abutment implant prostheses. Int J Oral Maxillofac Implants 1993;8:43946. Rosenstiel SF, Land MF, Fujimoto J. Working casts and dies. In: Rosenstiel SF, Land MF, Fujimoto J, editors. Contemporary fixed prosthodontics. 3rd ed. St Louis (MO): Mosby; 2001. p. 434-41. Covo LM, Ziebert GJ, Balthazar Y, Christensen LV. Accuracy and comparative stability of three removable die systems. J Prosthet Dent 1988;59:314-8. Richardson DW, Sanchez RA, Baker PS, Haug SP. Positional accuracy of four die tray systems. J Prosthet Dent 1991;66:39-45. Millstein P, Filapancic J. The Zeiser system: a method for accurate die placement. Quintessence Dent Technol 1990;14:188-92. Barbaro V, Battistelli A, Massironi D. A simplified system to develop precision master models. J Dent Technol 1996;13:19-23. America DV. Technical assistance video for the DVA Model System [Video]. Anaheim Hills (CA): Dental Ventures of America; 1992. Serrano JG, Lepe X, Townsend JD, Johnson GH, Thielke S. An accuracy
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evaluation of four removable die systems. J Prosthet Dent 1998;80:57586. Teraoka F, Takahashi J. Dimensional changes and pressure of dental stones set in silicone rubber impressions. Dent Mater 2000;16:1459. Revised American Dental Association Specification no. 19 for Nonaqueous, Elastomeric Dental Impression Materials. J Am Dent Assoc 1977;94:733-41. Arlo K. Evaluation of the Vident die and model system. San Antonio (TX): USAF Dental Investigation Services; 1991. Report No: Project 90-63. Nicholls JL. The measurement of distortion: concluding remarks. J Prosthet Dent 1980;43:218-23. Myers M, Hembree JH Jr. Relative accuracy of four removable die systems. J Prosthet Dent 1982;48:163-5.
Noteworthy Abstracts of the Current Literature
Reprint requests to: DR ALVIN G. WEE SECTION OF RESTORATIVE DENTISTRY, PROSTHODONTICS AND ENDODONTICS THE OHIO STATE UNIVERSITY POSTLE HALL, 305 W 12TH AVE COLUMBUS, OH 43210-1241 FAX: (614)292-9422 E-MAIL: [email protected] Copyright © 2002 by The Editorial Council of The Journal of Prosthetic Dentistry. 0022-3913/2002/$35.00 + 0. 10/1/121110 doi:10.1067/mpr.2002.121110
The performance of bonded vs. pin-retained complex amalgam restorations: A five-year clinical evaluation. Summitt JB, Burgess JO, Berry TG, Robbins JW, Osborne JW, Haveman CW. J Am Dent Assoc 2001;132:923-31.
Purpose. This 5-year clinical study compared the failure rates, marginal discoloration, recurrent caries rates, marginal adaptation, sensitivity, and tooth vitality associated with bonded and pinretained complex amalgam restorations. Material and methods. Sixty teeth were restored by 5 clinicians. Twenty-eight of the teeth were restored with pin-retained amalgam (group 1), and 32 were restored with a bonded amalgam restoration (group 2). Rubber dam isolation was used in conjunction with restoration placement. All restorations encompassed at least 1 proximal surface and replaced at least 1 cusp. All restored teeth were deemed vital before restoration placement and were opposed by an antagonist tooth. Within group 1, a vertical TMS Minim pin (Thread-Mate System; Coltene-Whaledent) was used for each missing cusp. Horizontal TMS Minikin pins were used at the discretion of the clinician. Teeth in Group 2 received no additional mechanical retention but were bonded with AmalgamBond Plus (Parkell) combined with high-performance additive powder. Patients returned for a baseline evaluation 1 week after restoration placement. Recall evaluations were completed 6 months and 1, 2, 3, 4, and 5 years later. At each recall appointment, the clinicians confirmed tooth vitality. Tooth sensitivity was assessed thermally, and marginal integrity, marginal discoloration, and recurrent caries were evaluated with the Cvar/Ryge criteria. The Fisher exact test was used to analyze the data. Results. At the 4-year recall evaluation, 6 restorations had failed. Of the 40 restorations available for assessment at the 5-year recall evaluation, 3 had failed. A total of 7 pin-retained and 2 bonded amalgam restorations failed over the study period. Failures included restorations that had to be replaced, restorations that required considerable repair, and teeth that required endodontic therapy or extraction. No significant difference in failure rate was observed between the 2 groups at 5 years. At the 6-month evaluation appointment, a significant difference in sensitivity was noted between the groups; no significant differences in the other assessment criteria were found over the 5-year evaluation period. Conclusion. Amalgam restorations that replace at least one cusp may be effectively retained with a 4-META-based bonding system over a 5-year period. 32 References.—DL Dixon
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