Dental Materials Journal 2011; 30(3): 281–285 The influence of zirconia coping designs on the fracture load of all-ceramic molar crowns Yuji KOKUBO1,2, Mitsuyoshi TSUMITA1, Takamitsu KANO1 and Shunji FUKUSHIMA1 1Department of Fixed Prosthodontics, Tsurumi University School of Dental Medicine, 2-1-3 Tsurumi Tsurumi-ku, Yokohama 230-0063 Japan 2Division of Oral and Maxillofacial Implantology, Tsurumi University School of Dental Medicine, 2-1-3 Tsurumi Tsurumi-ku, Yokohama 230-0063 Japan Corresponding author, Yuji KOKUBO; E-mail: kokubo-y@tsurumi-u.ac.jp This study investigated the influence of zirconia coping designs on the fracture load of all-ceramic crown. Four kinds of zirconia copings were designed (a: Conventional zirconia coping with flat occlusal surface: thickness of the each coping is 0.6 mm evenly, and at the cervical margin area, the coping is adjusted sharply so as to fit preparation margin, b: Conventional zirconia coping with shoulder collar of 1 mm: thickness of the each coping is 0.6 mm evenly, and there is a collar of 0.6 mm from the margin, c: Zirconia coping with following original cuspal configuration (concave): two inclined cusp planes, and at the cervical margin area, the coping is adjusted sharply so as to fit preparation margin, and d: Zirconia coping with supporting configuration on the occlusal area: supporting configuration against the occlusal force, and at the cervical margin area, the coping is adjusted sharply so as to fit preparation margin) and porcelain was fired. Vertical and lateral load were conducted until fracture. Coping design affected the fracture load; conventional uniform thickness coping design showed the lowest load (a), whereas cuspal configuration to perform even thickness of porcelain showed the highest fracture load both load directions (c). Keywords: Framework design, Coping design, Zirconia, All-ceramic crown, Fracture load INTRODUCTION showed from in vitro study that the design of the yittria stabilized zirconia (Y-TZP) framework with a The alumina- or zirconia-supported ceramic crown support-type is important for all-ceramic restorations. instead of metal-supported ones has been widely used At the beginning of introduction of high strength because of their high esthetics, biocompatibility, and structural ceramic Y-TZP, the coping was generally chemical durability. At the beginning, glass or alumina fabricated uniform thickness to the prepared tooth. have used for restorations of anterior teeth. The clinicians Then, porcelain was fired on the coping to be completed have been attempted for restoring molar teeth, however, as all-ceramic crown. However, when fabricating the the fracture problems often occurred not only from cusp, veneered porcelain has been tended to be thick, porcelain but also from the coping, which showed global depending on the prepared tooth configuration. As the fracture, and needed to replace1,2). The introduction of porcelain is inherently brittle material, in case of lateral

zirconia copings, which were fabricated by computer- load was applied to the cusps, it will be easily fractured, aided design/computer-aided manufacturing (CAD/ or chipping, because porcelain is weak material against CAM) or CAM technologies, has been developed precisely tensile load. The modified coping design, which might and shown their high mechanical strength, as a result, fabricate porcelain thickness evenly, or support the reduced fracture problems. occlusal and lateral load as compressive load could avoid Until now, clinical results have shown that porcelain fracture. Molin et al.15) reported that one all-ceramic crowns fabricated by using zirconia coping technical approach to improve the clinical behavior is a showed high success rate after long-term function, only strict anatomically oriented framework design that minor chipping was detected particularly for molar reinforce the ceramic veneer. Rosentritt et al. 16) concluded region3). There may be some reasons to fail of veneered that the change in the design of the coping should be porcelain; flexural strength4), bond strength between effectively reduced the number of chipping. Marchack et coping and porcelain5), excessive load6), porosities and al.17) reported that zirconia coping should be designed as surface conditions of the porcelain7), improper coping adequate coping thickness, adequate even porcelain design8), and thermal stress when firing porcelain9). In thickness, and butt joints at the porcelain-to-coping clinical case, there might have sever contact under some junctions. However, they do not have long-term clinical situations, if there are no coping support of veneered results to conclude. Furthermore, Bonfante et al.8) porcelain, chipping problems will increase. showed the effectiveness of core-design modification of Also, there might be some factors that will influence In-Ceram crown on the fracture load when applying the fracture evolution of all-ceramic crowns. Rekow et vertical load at the central fossa, whereas Lorenzoni et al.10) revealed that the height of axial wall increases, al.18) reported that there are no effect of core-design loads to cause failure increase. Other reports mentioned modification of Y-TZP coping crown applying cyclic that fracture load will depend on coping thickness, loading. marginal design, and applied luting agent11-13). Mori14) The purpose of this study is to investigate the

Received Aug 18, 2010: Accepted Dec 3, 2010 doi:10.4012/dmj.2010-130 JOI JST.JSTAGE/dmj/2010-130

282 Dent Mater J 2011; 30(3): 281–285 influence of zirconia coping designs on the fracture load copied using same resin and casted for abutment teeth of all-ceramic crown. The hypothesis is that the design of using cobalt based casting alloy (Wisil, Elephant Dental zirconia coping would influence the fracture load of B.V., Hoorn, Netherlands ). molar all-ceramic crown. On the casted model, all-ceramic model was completed by wax, and silicone core was recorded. Then, MATERIALS AND METHODS four kinds of models were fabricated by using autoplymerizing resin (Pattern resin) as follows (Fig. 2); The right mandibular first molar was prepared on (a) Conventional zirconia coping with flat occlusal autoplymerizing resin (Pattern resin, GC Corp., Tokyo, surface: thickness of the each coping is 0.6 mm evenly,

Japan) as replacement by all-ceramic crown according to and at the cervical margin area, the coping is adjusted the manufacturer’s recommendation, which included sharply so as to fit preparation margin. that mesial and distal, buccal, and lingual height were (b) Conventional zirconia coping with shoulder collar of 1 3.0 mm, 5.5 mm, and 4.0 mm, respectively. Rounded mm: thickness of the each coping is 0.6 mm evenly, shoulder was conducted with 1.5 mm uniform width, and and there is a collar of 0.6 mm from the margin. convergence angle was 6.5 degree evenly. Then, all sharp (c) Zirconia coping with following original cuspal line angles were rounded (Fig. 1). This master model was configuration (concave): two inclined cusp planes, and at the cervical margin area, the coping is adjusted sharply so as to fit preparation margin. (d) Zirconia coping with supporting configuration on the occlusal area: supporting configuration against the occlusal force, and at the cervical margin area, the coping is adjusted sharply so as to fit preparation margin. All copings were double scanned by Procera Forte (Nobel Biocare, Zurich, Switzerland), then Nobel Procera crown zirconia copings (Nobel Biocare) were fabricated as shown in Fig. 2. Each coping was checked the marginal fit by using light scope, and completed by porcelain (Nobel Rondo zirconia, Nobel Biocare) in a commercial Fig. 1 Plastic model of prepared tooth (mandibuar first laboratory (KS Dental Laboratory, Yokohama, Japan), molar). following firing schedule; body porcelain, start temp.: Fig. 2 Completed designed copings. (a) Conventional zirconia coping with flat occlusal surface: thickness of the each coping is 0.6 mm, and at the cervical margin area, the coping is adjusted sharply so as to fit preparation margin. (b) Conventional zirconia coping with shoulder collar: thickness of the each coping is 0.6 mm, and there is a collar of 0.6 mm from the margin. (c) Zirconia coping with following original cuspal configuration (concave): two inclined cusp planes, and at the cervical margin area, the coping is adjusted sharply so as to fit preparation margin.

(d) Zirconia coping with supporting configuration on the occlusal area: supporting configuration against the occlusal force, and at the cervical margin area, the coping is adjusted sharply so as to fit preparation margin.

Dent Mater J 2011; 30(3): 281–285 283 Fig. 3 Schematic drawing of completed all-ceramic crown. 650°C, drying time: 5 min, temp. rising speed: 45°C/min, vacuum start (+), sintering temp.: 910°C, holding time: 1 min, cooling time: 5 min, Glaze, start temp.: 650°C, drying time: 5 min, temp rising speed: 45°C/min, vacuum start (−), sintering temp.: 880°C, holding time: 1 min, cooling time: 0 min. Configuration of final crown was adjusted by using silicone core. Ten all-ceramic crowns were fabricated for each model (Fig. 3). Before cementing, casted model and inside of completed all-ceramic crowns were blasted by 50 µm alumina particle for 20 seconds, and applied AZ primer (Shofu Inc. Kyoto, Japan). After that, crowns were luted onto model by adhesive resin cement (ResiCem, Shofu

Inc.). After mixing, the cement was poured inside the crowns using auto-mixing, and delivered on the model. Then excess cement was removed, and was hold at 1 kg Fig. 4 Mechanical strength test. weight from the top of the crown for 5 minutes. During A loading rod was positioned at the top of buccal this procedure, no irradiation was conducted. All cusp of crown or inclined lingual cusp plane, and cemented specimens were immersed in distilled water loaded until fracture occurred. for 24 hours before loading. Cemented specimens were mounted in the testing jig of a mechanical testing machine (Auto Graph AGS-5kND, Shimadzu, Kyoto, Japan). Each group was analyses were conducted at a 95 % confidence level. subdivided into two groups with five specimens in each group, which included vertical loading group and 45 degree loading group. For the former group, a loading RESULTS rod with a 2.0-mm radius tip was positioned at the top of The mean fracture load is shown in Fig. 5 (a and b). buccal cusp so as to be loaded to the axial direction of When applying vertical load, Type c showed the highest abutment. For the later group, a loading rod was value, and there was a significant differences between positioned so as to be loaded to inclined buccal cusp type a and c. On the other hand, when applying lateral plane. A load was applied at a crosshead speed of 0.5 load, Type c showed the highest value. Type c showed mm/min until fracture (Fig. 4). significantly higher load compared to other groups. The initial fracture load was recorded, and mean In this study, all fractures were occurred from fracture load was calculated (SPSS 15.0.J, SPSS, loading point to the buccal margin and proximal area Chicago, IL, USA). The mean values of each group were (Fig. 6), and cohesive failure with the veneered porcelain statistically analyzed using one-way analysis of variance were seen, which meant that failure modes were not (ANOVA) to identify the significant differences among different form coping design. There were no coping coping designs. If differences were found, Tukey HSD fractures during testing. analysis was used to evaluate significant differences. All

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Dent Mater J 2011; 30(3): 281–285 (a) (b) Fig. 5 (a) Fracture load when applying vertical load. (b) Fracture load when applying lateral load. (control), b: uniform thickness of the coping which has been conventionally used, and marginal area of the coping is strong enough compared to design a, and c: two inclined cusp planes, which might change the tensile loads to compressive loads (lateral load), and at the cervical margin area is designed the same with design a, and d: supporting configuration against the vertical load, and at the cervical margin area is designed the same with design a. When vertical load was applied, if the loading point was within the framework of occlusal area, fracture load might be high. In this study, Type b had supported- design on marginal area like Marchack et al.17) reported. Further modification of marginal design will be needed such as the height of collar needed for molar crown. If the height increase, the width of it will increase, this would support higher occlusal load. This means that after full-contour waxing was conducted, it should cut back at the top of the contour. This would be easily applied for Fig. 6 SEM image after vertical loading. molar legion, where there will be not much esthetic concern from patients. Type c showed the highest fracture load, however, there were no significant DISCUSSION differences between Type c and d. From this result, for vertical load, modification of marginal or occlusal design Recently, review report by Al-Amleh et al.19) showed that would improve the fracture load. chipping of the veneering porcelain is confirmed to be an When applying lateral load, Type c also showed the ongoing problem with zirconia all-ceramic-based highest value. This may be from a reason that lateral restorations. At the beginning of introduction of zirconia load to veneered porcelain more or less changed from restorations, uniform thickness coping was used. This shear to compressive load against the coping surface. means that so called ‘supported area’ for veneered Type c showed the highest fracture load, and coping porcelain was not designed. The combination of this design that can obtain high fracture load of all-ceramic design and thick porcelain might increase the chance of

crown may offer predictable restoration. Ideally, after chipping. To reduce this problem, zirconia coping design fabricating working model, first wax-up the complete was modified. One of the suggestion of coping design crown, making silicone core, and then cut back the modification that might reduce the chipping troubles are wax-up so as to make the space for the porcelain veneer. shown some researchers2,8,13,17), changing tensile loads to As a result, the coping will be fabricated with special compressive loads on the occlusal porcelain were design depending on abutment configuration and important when designing the copings. From these contralateral jaw. points of view, in this study, four kinds of models were Also, load position is very important like this in vitro designed; a: uniform thickness of the coping with sharp study, the load tended to tip the crown off the porcelain marginal area which has been conventionally used from supporting coping. Rekow et al.10) discussed in his Dent Mater J 2011; 30(3): 281–285 285 reports that load position played a significant role in patients: results after 5 years. J Oral Rehabil 2009; 36: stress concentration. When the load was applied in the 786-791. area beyond the bulk of the coping, tensile stress will be 2) Wassermann A, Kaiser M, Strub JR. Clinical long-term concentrated at the coping-porcelain interface. Clinician results of VITA In-Ceram Classic crowns and fixed partial dentures: A systematic literature review. Int J Prosthodont should keep in mind that there are many factors to be 2006; 19: 355-363. affected the results, such as loading point, loading 3) Örtorp A, Kihl ML, Carlsson GE. A 3-year retrospective and direction, porcelain used, the conditioning of opposing clinical follow-up study of zirconia single crowns performed in tooth, cuspal inner incline, biting force, and contact area. a private practice. J Dent 2009; 37: 731-736. From SEM observation, when loading, maximum 4) Christel P, Meunier A, Heller M, Torre JP, Peille CN. stress will concentrate at the tip of zirconia coping. Mechanical properties and short-term in vivo evaluation of yttrium-oxide-partially-stabilized zirconia. J Biomed Mater Initial failure was occurred from loading point, and cone Res 1989; 23: 45-61. cracking within the veneered porcelain (cohesive failure) 5) Isgrò G, Pallav P, van der Zel JM, Feilzer AJ. The influence of

as described previous reports3,19). From the failure mode, the veneering porcelain and different surface treatments on the interface between porcelain and zirconia coping was the biaxial flexural strength of a heat-pressed ceramic. J strong, whereas porcelain should be strong to decrease Prosthet Dent 2003; 90: 465-473. the porcelain chipping. Also, Kelly20) revealed that 6) Drummoud JL, King TJ, Bapna MS, Koperski RD. Mechanical failures took place at repeated stresses lower than property evaluation of pressable restorative ceramics. Dent Mater 2000; 16: 226-233. material’s bulk strength, further studies will be needed 7) Ohlmann B, Rammelsberg P, Schmitter M, Schwarz S, after cyclic loading which simulate mastication. Gabbert O. All-ceramic inlay-retained fixed partial dentures: When firing veneering porcelain to zirconia coping, preliminary results from a clinical study. J Dent 2008; 36: cooling protocol are key factors to avoid stresses within 692-696. the porcelain4,21). In this study cooling time was for 5 8) Bonfante EA, da Silva NR, Coelho PG, Bayardo-Gonzalez DE, minutes to minimize the stress within the porcelain. Thompson VP, Bonfante G. Effect of framework design on crown failure. Eur J Oral Sci 2009; 117: 194-199. Bonfante et al. 8) showed the effect of modified coping 9) Bagby M, Marshall SJ, Marshall GW Jr. Metal ceramic design on the fracture load. On the other hand, Lorenzoni compatibility: a review of the literature. J Prosthet Dent et al.18) reported that framework modification did not 1990; 63: 21-25. improve the fatigue life of the crown. These coping 10) Rekow ED, Zhang G, Thompson V, Kim JW, Coehlo P, Zhang designs were similar to Type b in this study. The Y. Effects of geometry on fracture initiation and propagation differences of these two articles were loading condition, in all-ceramic crowns. J Biomed Mater Res B Appl Biomater 2009; 88: 436-446. and materials used. Further studies will be needed to 11) Potiket N, Chiche G, Finger IM. In vitro fracture strength of investigate the fracture load after cyclic loading, because teeth restored with different all-ceramic crown systems. J this static loading might not be always simulate clinical Prosthet Dent 2004; 92: 491-495. condition. 12) Bindl A, Lüthy H, Mörmann WH. Thin-wall ceramic CAD/ The hypothesis was confirmed. In this study, copings

CAM crown copings: strength and fracture pattern. J Oral were designed to support porcelain against the vertical Rehabil 2006; 33: 520-528. and lateral load. This means that modifying marginal 13) Rekow ED, Harsono M, Janal M, Thompson VP, Zhang G. Factorial analysis of variables influencing stress in all-ceramic and occlusal areas were important to be completed this crowns. Dent Mater 2006; 22: 125-132. design. There are no long-term clinical studies for molar 14) Mori K. Influence of the design of zirconia framework on the zirconia coping crowns to determine the influence of fracture strength of veneering porcelain. J Stomatol Soc Jpn coping designs. 2010; 77: 67-70. 15) Molin MK, Karlsson SL. Five-year clinical prospective evaluation of zirconia-based Denzir 3-unit FPDs. Int J CONCLUSION Prosthodont 2008; 21: 223-227. 16) Rosentritt M, Steiger D, Behr M, Handel G, Kolbeck C. Within the limitation of in this model study, the following Influence of substructure design and spacer settings on the in conclusions may be drawn; vitro performance of molar zirconia crown. J Dent 2009; 37: 1. Coping design affected the fracture load of zirconia 978-983. all-ceramic crown. 17) Marchack BW, Futatsuki Y, Marchack CB, White SN. 2. Cuspal configuration to perform even thickness of Customization of milled zirconia copings for all-ceramic crowns: a clinical report. J Prosthet Dent 2008; 99: 169-173. porcelain showed the highest fracture load. 18) Lorenzoni FC, Martins LM, Silva NR, Coelho PG, Guess PC, Bonfante EA, Thompson VP, Bonfante G. Fatigue life and ACKNOWLEDGMENT failure modes of crowns systems with a modified framework design. J Dent 2010; 38: 626-634. We would like to thank Nobel Biocare for providing the 19) Al-Amleh B, Lyons K, Swain M. Clinical trials in zirconia: a zirconia copings. systematic review. J Oral Rehabil 2010; 37: 641-652. 20) Kelly JR. Clinically relevant approach to failure testing of all-ceramic restorations. J Prosthet Dent 1999; 81: 652-661. REFERENCES 21) Scherrer SS, de Rijk WG. The effect of crown length on the fracture resistance of posterior porcelain and glass-ceramic 1) Kokubo Y, Sakurai S, Tsumita M, Ogawa T, Fukushima S.

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