JPOR-267; No. of Pages 8 journal of prosthodontic research xxx (2015) xxx–xxx
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Original article
Fabrication of crown restoration retrofitting to existing clasps using CAD/CAM: Fitness accuracy and retentive force Daisuke Ozawa DMD a,*, Yasunori Suzuki DMD, PhDa, Noboru Kawamura CDTb, Chikahiro Ohkubo DMD, PhDa a b
Department of Removable Prosthodontics, Tsurumi University School of Dental Medicine, Yokohama, Japan Dental Technician Institute, Tsurumi University School of Dental Medicine, Yokohama, Japan
article info
abstract
Article history:
Purpose: A crown restoration engaged by a clasp as an abutment tooth for a removable
Received 16 September 2014
partial denture (RPD) occasionally might be removed and eliminated due to secondary caries
Received in revised form
or apical lesions. However, if the RPD is clinically acceptable without any problems and
9 January 2015
refabricating the RPD is not recommended, the new crown must be made to retrofit to the
Accepted 13 January 2015
existing clasp of the RPD.
Available online xxx
This in vitro study evaluated the conventional and CAD/CAM procedures for retrofitting crown restorations to the existing clasps by measuring the fitness accuracy and the
Keywords:
retentive forces.
Digital dentistry
Methods: The crown restoration on #44 was fabricated with CP titanium and zirconium on
Abutment teeth
the plaster model with #45 and #46 teeth missing to retrofit to the existing clasp using
CAD/CAM
conventional thin coping and CAD/CAM procedures. The gap distance between the clasp
Removable prosthodontics
(tip, shoulder, and rest regions) and the fabricated crown was measured using silicone impression material. The retentive force of the clasp was also measured, using an autograph at a crosshead speed of 50 mm/min. The obtained data were analyzed by one-way ANOVA/ Tukey’s multiple comparison test (a = 0.05). Results: The CAD/CAM procedure caused significantly smaller gap distances in all of the clasp regions, as compared to the conventional procedure ( p < 0.05). The retentive force of the CAD/CAM crown was significantly higher than for the conventional one ( p < 0.05). Conclusion: When a crown restoration must be remade to retrofit an existing clasp, CAD/ CAM fabrication can be recommended so that both appropriate fitness and retentive force are obtained. # 2015 Japan Prosthodontic Society. Published by Elsevier Ireland. All rights reserved.
* Corresponding author at: 2-1-3 Tsurumi Tusurumi-ku, Yokohama 230-8501, Japan. Tel.: +81 45 580 8421; fax: +81 45 573 9599. E-mail address: [email protected] (D. Ozawa). http://dx.doi.org/10.1016/j.jpor.2015.01.002 1883-1958/# 2015 Japan Prosthodontic Society. Published by Elsevier Ireland. All rights reserved.
Please cite this article in press as: Ozawa D, et al. Fabrication of crown restoration retrofitting to existing clasps using CAD/CAM: Fitness accuracy and retentive force. J Prosthodont Res (2015), http://dx.doi.org/10.1016/j.jpor.2015.01.002
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1.
Introduction
A cast clasp engaging an abutment tooth has been used for retention of most removable partial dentures (RPDs). Some complications of the clasp, such as clasp-arm breakage, ill fit, decrease of retention, and abutment tooth disease, are frequently observed with long-term use of the RPD [1,2]. If the crown used as the abutment tooth is removed or eliminated because of secondary caries or occurrence of periapical periodontitis, the RPD will be repaired or remade. However, if the RPD is clinically acceptable for use and remaking the prosthesis is not indicated, fabrication of a new crown to retrofit the clasp of the existing RPD should be selected. By continuing to use the existing RPD with a retrofitted crown, the economic burden for the patient and the number of visits to the hospital can be reduced. Various direct and indirect techniques for simplifying the process and improving accuracy have been reported for fabricating crowns to retrofit to existing clasps [3–8]. In one technique, a lost wax procedure is carried out for metal restorations, but there are still many problems. Since all laboratory processes are manually completed, the quality of the prosthesis is affected by the skill of the dental technicians [9–11]. It is difficult to give appropriate ideal contours, undercuts, rest seats, and guiding planes to the new retrofitted crowns using the conventional lost wax technique. Recently, nonmetal restorations have been attracting attention, due to esthetics demands and increased disease from suspected dental metal allergies. In many ways, dental ceramic for crown restorations is an excellent material – it has color similar to that of natural teeth, is fluorescent and permeable, and discoloration or coloration is unlikely to occur; it also has high biocompatibility [12,13]. The all-ceramic crown has been suggested to be a predictable and successful alternative to conventional metal and metal-fused ceramic crowns. In recent years, Computer-Aided Design/Computer-Aided Manufacturing (CAD/CAM) has been developed in industrial circles. It may enter the mainstream of restoration processing in the near future because the machining efficiency and accuracy are rapidly improving with growing research and the development of new products in the field of dentistry [14–17]. Particularly, a zirconia and lithium disilicate restoration demonstrates high intensity, high toughness, and resistance to compression [18–20]. Also, duplicating an existing crown and fabricating a surveyed crown can be performed with great accuracy using CAD/CAM. Although several fabrication methods have been introduced for restoring crowns retrofitted to the clasp, little is known regarding the fitting accuracy between the clasp and the crown, especially with CAD/CAM fabrication. In this in vitro study, remade crowns were fabricated to retrofit to the existing clasps using both conventional lost wax casting and CAD/CAM under two clinical conditions in cases of lost and present existing crowns. The four fabricating ways were quantitatively evaluated by measuring the fitness to the existing clasp and the retentive force. The aim of this research was to evaluate the fitness accuracy and retentive force of existing clasp and remade crowns in four fabricating technique.
2.
Materials and methods
2.1.
Fabrication of the specimens
2.1.1.
Experimental model
A commercial simulation model (Lower model with missing teeth; Nissin Dental Products Inc., Tokyo, Japan) with the right second premolar and the first molar missing in the lower jaw was used. The occlusal rests and guide planes were prepared on the adjacent teeth (the right first premolar and a second molar) as abutment teeth using a parallel milling machine (Frasgerat F1; Degussa, Essen, Germany). A working cast of the simulation model was fabricated using a silicone impression material (Duplicone, Shofu Inc., Kyoto, Japan) and extra-hard plaster (NEW FUJIROCK IMP; GC Corp., Tokyo, Japan).
2.1.2.
Fabrication of the clasp
An Akers clasp and a ring clasp were designed for the first premolar and the second molar, respectively, so that half of the clasp arm’s length was engaged on the undercut area of the abutment, and the tip was engaged at the 0.25 mm and 0.75 mm undercuts, respectively. After the blockout was done, the duplicate cast was made conventionally using silicone rubber impression material (Duplicone, Shofu Inc., Tokyo, Japan) and investment material (Heravest Speed, Heraeus Kulzer Japan, Tokyo, Japan). The framework pattern that connected both clasp tangs of the Akers clasp (tips: 2.0 mm width and l.0 mm height; shoulders: 2.4 mm width and l.3 mm height; lengths: 12.0 mm) and the ring clasp (tips: 2.0 mm width and l.0 mm height; shoulders: 2.4 mm width and l.3 mm height; lengths: 30.0 mm) was conventionally cast with a Co–Cr alloy (MC alloy; DeguDent, Hanau, Germany). After casting, the framework was conventionally finished and polished (Fig. 1). The fitness between the original crown and the clasp was measured using the method mentioned below.
2.1.3.
Fabrication of the crown
An abutment preparation of the right first premolar was performed on the working cast (Fig. 2). The crowns were fabricated to retrofit to the existing Akers clasp in a total of 4 different ways using the conventional casting method and the CAD/CAM system under two clinical conditions, namely, the loss and presence of the previous abutment tooth’s crown form (Fig. 3). All crowns were fabricated by one dentist with four years of clinical experience.
2.1.3.1. Loss of the previous abutment tooth crown form. After a thin plastic coping was fabricated on the abutment tooth with autopolymerized acrylic resin (pattern resin; GC Dental Industrial Corp., Tokyo, Japan), the same resin was applied on the coping to record the inner surface of the clasp using the brush-on technique. After the clasp was held for 3 min on the coping under a constant load of 9.8 N to prevent deformation, it was removed from the coping. The conventional crown was waxed up (Inlay Wax; GC) to the anatomical form while keeping the clasp’s inner surface on the coping (CO1) and contour to add a 0.2-mm undercut. The wax pattern of the crown (CO1) and the prepared die forms were scanned using a
Please cite this article in press as: Ozawa D, et al. Fabrication of crown restoration retrofitting to existing clasps using CAD/CAM: Fitness accuracy and retentive force. J Prosthodont Res (2015), http://dx.doi.org/10.1016/j.jpor.2015.01.002
JPOR-267; No. of Pages 8 journal of prosthodontic research xxx (2015) xxx–xxx
Fig. 1 – Experimental model and flame work.
zirconia, including yttrium discs (Aadva zirconia disc; GC) using the Aadva system (D810, Dental Designer, GM-1000; GC). Milled zirconia crowns were sintered in a furnace (SuperBurn; Motoyama Co., Osaka, Japan) based on the schedules shown in Table 1. Using the polishing kit (CeroPol Set, Hatho, Germany), the CAD/CAM crowns were conventionally finished and polished to similar surface roughnesses (Ra: approximately 0.85 0.07; NH-4H, Mitaka Koki, Tokyo). All crowns were placed on the prepared abutment tooth, and the fitting was confirmed on the working cast. Each of the 5 crowns was fabricated in four ways (CO1, CO2, CAD1, and CAD2), titanium and zirconia crowns were fabricated for CAD1 and CAD2, and four different undercuts were fabricated for CAD1, for a total of 45 crowns prepared for this experiment.
2.2.
Fig. 2 – Tooth preparation of the mandibular first premolar on the working cast.
digital scanner (D810; GC), and the design of the crown form was modified using computer-aided design (CAD). CAD crowns were designed to four forms with 0.2-, 0.3-, 0.4-, and 0.5-mm undercut values using design software (Dental Designer; GC) (CAD1).
2.1.3.2. Presence of the previous abutment tooth crown form. The silicone core of the previous abutment tooth crown form of the simulation model was made using pate-type silicone impression material (Exafine; GC), and it was overlaid on the prepared abutment crown on the working cast. The crown was waxed up similarly to the original crown form using the silicone core (CO2). For the CAD crown, the previous abutment tooth crown and the prepared die forms were scanned using the digital scanner. The scanned data were precisely overlapped with the bases of the remaining teeth without a divergence, both horizontally and vertically. The crown restoration data were morphed to the scanned previous crown form using design software (CAD2). Before the casting of the conventional crowns (CO1, CO2), cyanoacrylate adhesives (Startrite; Nissin Giken Co., Ltd., Tokyo, Japan) were applied to add one layer to the wax patterns as a margin for polishing. CO1 and CO2 crowns were casted with a 12% Ag–Pd–Au alloy (Castwell M.C. 12; GC) and were conventionally polished. For CAD/CAM crowns (CAD1 and CAD2), the completed design data were transferred to a milling machine (GM-1000; GC). CAD1 and CAD2 crowns were milled from titanium alloy (Aadva titanium disc; GC) and
3
Measuring accuracy
White high viscosity silicone impression material (Fit Checker; GC) was mixed (base: catalyst = 6:1) and amply applied on the internal surface of the clasp; the clasp was then placed on the remade crown. After the clasp was held for 3 min under a constant load of 9.8 N, it was removed from the crown. The black silicone material (Bite-Checker; GC) was then mixed (base: catalyst = 6:1) and poured inside the white silicone material on each clasp. The two combined silicone materials were removed from the clasp after 3 min. Each silicone block was sectioned using a razor blade buccolingually 1.0 mm from the end of the clasp tip, clasp shoulders, and rest regions. To measure the gap distance between the crown and the clasp as a fitness check, the thickness of the white silicone layer at the sections was observed using a profile projector (V-16E; Nikon, Tokyo, Japan) at a magnification of 50.
2.3.
Measurement of retentive forces
The tensile force (N) of the Akers clasp from each remade crown was measured using the tensile test apparatus (EZ-S; Shimadzu, Kyoto, Japan) at a crosshead speed of 50 mm/min. The maximum tensile force obtained when the clasp was separated from the crown was evaluated as the retentive force.
2.4.
Statistical analysis
The data of the fitness accuracy and the retentive force were analyzed using the SPSS statistical package (version 12.0; SPSS, Inc., Tokyo, Japan) by one-way ANOVA and Tukey’s multiple comparison test at a significance level of a = 0.05.
3.
Results
3.1.
Fitness accuracy
The fitness accuracy between Akers clasp and previous abutment tooth crown and retrofitted crowns in each fabrication method (CO1, CO2, CAD1 Ti, CAD2 Ti) is shown in Fig. 4. CAD1 showed the thinnest silicone film layer in all regions (clasp tip, 55 8 mm; clasp shoulder, 70.8 7 mm; and rest, 39.5 7 mm) as compared to the other fabrication
Please cite this article in press as: Ozawa D, et al. Fabrication of crown restoration retrofitting to existing clasps using CAD/CAM: Fitness accuracy and retentive force. J Prosthodont Res (2015), http://dx.doi.org/10.1016/j.jpor.2015.01.002
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Fig. 3 – Flowchart for the fabrication of crown restorations to the existing clasp in four different ways (CO1, CAD1, CO2, and CAD2).
Table 1 – Sintering schedules for zirconia crowns in the furnace.
Temperature (8C) Time (h)
Warming
Warming
Insulation
Cooling
1000 2
1450 4.5
1450 2
1000 1
Fig. 4 – Differences in the fitness accuracy of each crown with four fabrication methods. (a) The tip region, (b) the shoulder region, and (c) the rest region (*p < 0.05).
methods including original crown (claspt tip, 70 4.5 mm; clasp shoulder, 85 11 mm; and rests, 49 5 mm). Conversely, CO1 showed the thickest film layer in all regions (clasp tip, 117.8 15.8 mm; clasp shoulder, 146 16.9 mm; and rest, 208.7 34.6 mm). There were significant differences between the CAD/CAM (CAD1 and CAD2) and conventional fabrications (CO1 and CO2) (P > 0.05). However, no significant difference was found between CAD1 and CAD2 or between CO1 and CO2 (P > 0.05). Of the regions, the clasp tip tended to show the best fitness accuracy. As for the rest, CO1 and CO2 showed the worst accuracy with CAD1, while CAD2 demonstrated the best accuracy. Fig. 5 shows the differences of the fitness accuracy between Akers clasp and the CAD/CAM crowns with titanium and zirconia. When comparing each region, similar thickness was shown in both materials titanium and zirconia measured tips,
55 9 mm and 52 8 mm; shoulders, 75 7 mm and 74 7 mm; and rests, 43 6 mm and 42 7 mm, respectively. In both CAD1 and CAD2, there was no significant difference between the two materials (P > 0.05).
3.2.
Retentive forces
Fig. 6 compares the retentive forces of the Akers clasps from retrofitted crowns in 4 fabrication methods (CO1, CO2, CAD1 Ti, CAD2 Ti). The CAD/CAM fabrications with a 0.2-mm undercut showed approximately 1.5–2 times higher retentive forces than those of conventional fabrications. In the conventional crowns, CO2 tended to show a higher retentive force than did CO1 without significant difference (P > 0.05). CAD1 and CAD2 demonstrated similar retentive forces, and there was no significant difference between them (P < 0.05).
Please cite this article in press as: Ozawa D, et al. Fabrication of crown restoration retrofitting to existing clasps using CAD/CAM: Fitness accuracy and retentive force. J Prosthodont Res (2015), http://dx.doi.org/10.1016/j.jpor.2015.01.002
JPOR-267; No. of Pages 8 journal of prosthodontic research xxx (2015) xxx–xxx
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Fig. 5 – Differences in the fitness between two crown restoration materials.
Fig. 6 – Differences in the retentive forces of clasp from crowns in 4 fabrication methods (undercut 0.2 mm) (*p < 0.05).
Fig. 7 shows the retentive force of the Akers clasp from each crown material and the influences of the undercut values on the retentive forces. Although the zirconia crown using CAD1 tended to indicate the greatest retentive force among all of the crowns, there were no significant differences between them (P > 0.05). As the undercut value was increased, the retentive forces became significantly greater (P < 0.05). In each undercut value, similar retentive forces were obtained for titanium and zirconia (P > 0.05).
4.
Discussion
4.1.
Experimental conditions
To focus on a comparative investigation of the fitness and the retentive forces under the same condition, an in vitro study was performed because it has the advantage of providing a standardized condition with respect to the preparation design, technique, and experimental performance, which results in a more repeatable assessment. For
the first premolar and the second molar, an Akers clasp and a ring clasp were selected, respectively, since the Akers and ring clasps are basic designs of the cast clasp and have three elements—support, bracing, and retention [21]. Generally, the simulation models or working casts were made of plaster, metal, and epoxy resin, or extracted teeth were used for the in vitro study. In the present study, the working cast was made of extra-hard plaster, since it is little influenced by irregular reflections during laser scanning. Regarding the crown alloys in this study, the CAD/CAM crowns were milled from titanium alloy, but an Ag–Pd–Au alloy was chosen for the conventional method since the accuracy of cast titanium could not be enough due to the low castability of the thin area [22–24]. Many measurement methods have been reported for evaluating the accuracy of the clasp fitness [25–30]. In the present study, fitness accuracy was evaluated by measuring the film thickness using high viscosity white silicone impression material in accordance with the methods of Osada [27] and Shimpo [28]. Limitations of this study were that the pathway of insertion/removal, the fitness, and the
Please cite this article in press as: Ozawa D, et al. Fabrication of crown restoration retrofitting to existing clasps using CAD/CAM: Fitness accuracy and retentive force. J Prosthodont Res (2015), http://dx.doi.org/10.1016/j.jpor.2015.01.002
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Fig. 7 – Differences of the retentive forces of clasp from two crown materials and four undercut values (*p < 0.05).
designs of the denture were not considered. Thus, the fitness accuracy and the retentive force could not be confirmed definitively as in the in vivo study.
4.2.
Measurement of fitness
It is conceivable that the clasp functions appropriately to retain RPDs by offering suitable fitness, retention, and supporting-tissue health. The fitness of the CAD/CAM crowns indicated a similar tendency to the previous crown. The CAD/ CAM crowns showed better fitness results in all regions as compared to conventional crowns. The fitness of CO1 and CO2 at rest was worse than that of the original crown because casting shrinkage would cause the clasp to float. Conversely, CAD1 and CAD2 demonstrated better fitness at rest. This occurs because the clasp settles correctly on the crown as there is no shrinkage factor during CAD/CAM procedures. The margin and internal fitness between the ceramic and metal crown restorations and the abutment teeth have been measured [29–33], and most of the CAD/CAM crowns showed better fitness as compared to conventional crowns. In addition, the conventional crowns demonstrated greater standard deviation values than those of the CAD/CAM crowns. Conventional crowns might have greater measurement errors caused by technicians’ inexperience, the type of casting alloys used, corrections of casting shrinkage, and treatment of the oxide layer on the surface of the castings [9,26,34,35]. Conversely, CAD/CAM crowns are fabricated by milling; thus, few machining errors might occur at every laboratory step. This should be considered a huge advantage of using CAD/CAM. There was no significant difference in the fitness accuracy between titanium and zirconia crowns. As for the contraction tolerance during the calcination of zirconia, the shrinkage correction value and the calcination temperature against the zirconia framework are effective. Bu¨chi et al. reviewed the accuracy produced by the zirconia fixed prostheses and mentioned few worse influences on clinical applications [32]. According to the results of this study and those reports, it was supposed that isotropic shrinkage and machining errors caused by contraction during the calcination of zirconia did not affect the accuracy of fitting the crowns to the existing clasps.
4.3.
Retentive forces
Although the same undercut value (0.2 mm) was used in all crown restorations in this study, CAD/CAM crowns showed retentive forces approximately twice as high as for conventional crowns. The main reason was that the CAD/CAM crowns showed superior fitness to conventional crowns. The appropriate retentive force of the clasp should be determined by many factors, namely, the partially edentulous pattern, tooth conditions, loss of periodontal tissue retention required for one RPD, number of retainers, etc. Many clinicians have reported that approximately 20 N per one RPD would be adequate to hold it in place when sticky foods are chewed and still allow the patient to easily remove the RPD. For the clasps, retention would be approximately 5–10 N, so that harmful forces are not applied to the abutment teeth [36,37]. In this study, as the undercut values were increased, the retentive force was also proportionally increased. From viewpoint of suitable retention, the CAD/CAM crowns should ideally be given 0.2–0.3-mm undercut values to maintain appropriate retention. The decrease of clasp retention was caused by daily use of the RPD. Tokue [37] also reported that Co–Cr clasps lost their retention force after 10,000 insertion-removal cycles. It may be derived from some other factors, such as microstructural phases or mechanical properties as well as wear and fatigue of the clasp materials. To recover retention for the clasp in the case of lost retentive force, fabrication of a new crown to retrofit the clasp using CAD/CAM might be a possible procedure instead of clasp adjustment and denture reproduction. In the future, dental treatments might only be possible with computer assistance. With innovations in optical impression technology, the technical work of taking impressions could be omitted in the future. Further studies are of interest for fabricating new crowns retrofit to the existing clasps using only digital data from the process of impression taking to that of milling the crowns.
5.
Conclusions
This study has evaluated various fabrication methods for crown restorations retrofit to the existing clasp using conventional and CAD/CAM procedures by measuring the
Please cite this article in press as: Ozawa D, et al. Fabrication of crown restoration retrofitting to existing clasps using CAD/CAM: Fitness accuracy and retentive force. J Prosthodont Res (2015), http://dx.doi.org/10.1016/j.jpor.2015.01.002
JPOR-267; No. of Pages 8 journal of prosthodontic research xxx (2015) xxx–xxx
fitness accuracy and the retentive forces. Within the limitations of this study, the following conclusions can be drawn: 1. For fitness and retentive forces, the CAD/CAM method showed more favorable results than did the conventional method. 2. As the undercut value was increased using CAD software, the retentive forces increased significantly. In each undercut value, similar retentive forces were obtained for titanium and zirconia crowns. 3. When a crown restoration must be remade to retrofit the existing clasp, CAD/CAM fabrication can be recommended so that both appropriate fitness and retentive force are obtained.
Conflict of interest This study was funded by GC Corp (Tokyo, Japan). The sponsor of this study had no role in the study design, conduct of the study, data collection, data interpretation or preparation of the report.
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Please cite this article in press as: Ozawa D, et al. Fabrication of crown restoration retrofitting to existing clasps using CAD/CAM: Fitness accuracy and retentive force. J Prosthodont Res (2015), http://dx.doi.org/10.1016/j.jpor.2015.01.002
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Original article
Fabrication of crown restoration retrofitting to existing clasps using CAD/CAM: Fitness accuracy and retentive force Daisuke Ozawa DMD a,*, Yasunori Suzuki DMD, PhDa, Noboru Kawamura CDTb, Chikahiro Ohkubo DMD, PhDa a b
Department of Removable Prosthodontics, Tsurumi University School of Dental Medicine, Yokohama, Japan Dental Technician Institute, Tsurumi University School of Dental Medicine, Yokohama, Japan
article info
abstract
Article history:
Purpose: A crown restoration engaged by a clasp as an abutment tooth for a removable
Received 16 September 2014
partial denture (RPD) occasionally might be removed and eliminated due to secondary caries
Received in revised form
or apical lesions. However, if the RPD is clinically acceptable without any problems and
9 January 2015
refabricating the RPD is not recommended, the new crown must be made to retrofit to the
Accepted 13 January 2015
existing clasp of the RPD.
Available online xxx
This in vitro study evaluated the conventional and CAD/CAM procedures for retrofitting crown restorations to the existing clasps by measuring the fitness accuracy and the
Keywords:
retentive forces.
Digital dentistry
Methods: The crown restoration on #44 was fabricated with CP titanium and zirconium on
Abutment teeth
the plaster model with #45 and #46 teeth missing to retrofit to the existing clasp using
CAD/CAM
conventional thin coping and CAD/CAM procedures. The gap distance between the clasp
Removable prosthodontics
(tip, shoulder, and rest regions) and the fabricated crown was measured using silicone impression material. The retentive force of the clasp was also measured, using an autograph at a crosshead speed of 50 mm/min. The obtained data were analyzed by one-way ANOVA/ Tukey’s multiple comparison test (a = 0.05). Results: The CAD/CAM procedure caused significantly smaller gap distances in all of the clasp regions, as compared to the conventional procedure ( p < 0.05). The retentive force of the CAD/CAM crown was significantly higher than for the conventional one ( p < 0.05). Conclusion: When a crown restoration must be remade to retrofit an existing clasp, CAD/ CAM fabrication can be recommended so that both appropriate fitness and retentive force are obtained. # 2015 Japan Prosthodontic Society. Published by Elsevier Ireland. All rights reserved.
* Corresponding author at: 2-1-3 Tsurumi Tusurumi-ku, Yokohama 230-8501, Japan. Tel.: +81 45 580 8421; fax: +81 45 573 9599. E-mail address: [email protected] (D. Ozawa). http://dx.doi.org/10.1016/j.jpor.2015.01.002 1883-1958/# 2015 Japan Prosthodontic Society. Published by Elsevier Ireland. All rights reserved.
Please cite this article in press as: Ozawa D, et al. Fabrication of crown restoration retrofitting to existing clasps using CAD/CAM: Fitness accuracy and retentive force. J Prosthodont Res (2015), http://dx.doi.org/10.1016/j.jpor.2015.01.002
JPOR-267; No. of Pages 8
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journal of prosthodontic research xxx (2015) xxx–xxx
1.
Introduction
A cast clasp engaging an abutment tooth has been used for retention of most removable partial dentures (RPDs). Some complications of the clasp, such as clasp-arm breakage, ill fit, decrease of retention, and abutment tooth disease, are frequently observed with long-term use of the RPD [1,2]. If the crown used as the abutment tooth is removed or eliminated because of secondary caries or occurrence of periapical periodontitis, the RPD will be repaired or remade. However, if the RPD is clinically acceptable for use and remaking the prosthesis is not indicated, fabrication of a new crown to retrofit the clasp of the existing RPD should be selected. By continuing to use the existing RPD with a retrofitted crown, the economic burden for the patient and the number of visits to the hospital can be reduced. Various direct and indirect techniques for simplifying the process and improving accuracy have been reported for fabricating crowns to retrofit to existing clasps [3–8]. In one technique, a lost wax procedure is carried out for metal restorations, but there are still many problems. Since all laboratory processes are manually completed, the quality of the prosthesis is affected by the skill of the dental technicians [9–11]. It is difficult to give appropriate ideal contours, undercuts, rest seats, and guiding planes to the new retrofitted crowns using the conventional lost wax technique. Recently, nonmetal restorations have been attracting attention, due to esthetics demands and increased disease from suspected dental metal allergies. In many ways, dental ceramic for crown restorations is an excellent material – it has color similar to that of natural teeth, is fluorescent and permeable, and discoloration or coloration is unlikely to occur; it also has high biocompatibility [12,13]. The all-ceramic crown has been suggested to be a predictable and successful alternative to conventional metal and metal-fused ceramic crowns. In recent years, Computer-Aided Design/Computer-Aided Manufacturing (CAD/CAM) has been developed in industrial circles. It may enter the mainstream of restoration processing in the near future because the machining efficiency and accuracy are rapidly improving with growing research and the development of new products in the field of dentistry [14–17]. Particularly, a zirconia and lithium disilicate restoration demonstrates high intensity, high toughness, and resistance to compression [18–20]. Also, duplicating an existing crown and fabricating a surveyed crown can be performed with great accuracy using CAD/CAM. Although several fabrication methods have been introduced for restoring crowns retrofitted to the clasp, little is known regarding the fitting accuracy between the clasp and the crown, especially with CAD/CAM fabrication. In this in vitro study, remade crowns were fabricated to retrofit to the existing clasps using both conventional lost wax casting and CAD/CAM under two clinical conditions in cases of lost and present existing crowns. The four fabricating ways were quantitatively evaluated by measuring the fitness to the existing clasp and the retentive force. The aim of this research was to evaluate the fitness accuracy and retentive force of existing clasp and remade crowns in four fabricating technique.
2.
Materials and methods
2.1.
Fabrication of the specimens
2.1.1.
Experimental model
A commercial simulation model (Lower model with missing teeth; Nissin Dental Products Inc., Tokyo, Japan) with the right second premolar and the first molar missing in the lower jaw was used. The occlusal rests and guide planes were prepared on the adjacent teeth (the right first premolar and a second molar) as abutment teeth using a parallel milling machine (Frasgerat F1; Degussa, Essen, Germany). A working cast of the simulation model was fabricated using a silicone impression material (Duplicone, Shofu Inc., Kyoto, Japan) and extra-hard plaster (NEW FUJIROCK IMP; GC Corp., Tokyo, Japan).
2.1.2.
Fabrication of the clasp
An Akers clasp and a ring clasp were designed for the first premolar and the second molar, respectively, so that half of the clasp arm’s length was engaged on the undercut area of the abutment, and the tip was engaged at the 0.25 mm and 0.75 mm undercuts, respectively. After the blockout was done, the duplicate cast was made conventionally using silicone rubber impression material (Duplicone, Shofu Inc., Tokyo, Japan) and investment material (Heravest Speed, Heraeus Kulzer Japan, Tokyo, Japan). The framework pattern that connected both clasp tangs of the Akers clasp (tips: 2.0 mm width and l.0 mm height; shoulders: 2.4 mm width and l.3 mm height; lengths: 12.0 mm) and the ring clasp (tips: 2.0 mm width and l.0 mm height; shoulders: 2.4 mm width and l.3 mm height; lengths: 30.0 mm) was conventionally cast with a Co–Cr alloy (MC alloy; DeguDent, Hanau, Germany). After casting, the framework was conventionally finished and polished (Fig. 1). The fitness between the original crown and the clasp was measured using the method mentioned below.
2.1.3.
Fabrication of the crown
An abutment preparation of the right first premolar was performed on the working cast (Fig. 2). The crowns were fabricated to retrofit to the existing Akers clasp in a total of 4 different ways using the conventional casting method and the CAD/CAM system under two clinical conditions, namely, the loss and presence of the previous abutment tooth’s crown form (Fig. 3). All crowns were fabricated by one dentist with four years of clinical experience.
2.1.3.1. Loss of the previous abutment tooth crown form. After a thin plastic coping was fabricated on the abutment tooth with autopolymerized acrylic resin (pattern resin; GC Dental Industrial Corp., Tokyo, Japan), the same resin was applied on the coping to record the inner surface of the clasp using the brush-on technique. After the clasp was held for 3 min on the coping under a constant load of 9.8 N to prevent deformation, it was removed from the coping. The conventional crown was waxed up (Inlay Wax; GC) to the anatomical form while keeping the clasp’s inner surface on the coping (CO1) and contour to add a 0.2-mm undercut. The wax pattern of the crown (CO1) and the prepared die forms were scanned using a
Please cite this article in press as: Ozawa D, et al. Fabrication of crown restoration retrofitting to existing clasps using CAD/CAM: Fitness accuracy and retentive force. J Prosthodont Res (2015), http://dx.doi.org/10.1016/j.jpor.2015.01.002
JPOR-267; No. of Pages 8 journal of prosthodontic research xxx (2015) xxx–xxx
Fig. 1 – Experimental model and flame work.
zirconia, including yttrium discs (Aadva zirconia disc; GC) using the Aadva system (D810, Dental Designer, GM-1000; GC). Milled zirconia crowns were sintered in a furnace (SuperBurn; Motoyama Co., Osaka, Japan) based on the schedules shown in Table 1. Using the polishing kit (CeroPol Set, Hatho, Germany), the CAD/CAM crowns were conventionally finished and polished to similar surface roughnesses (Ra: approximately 0.85 0.07; NH-4H, Mitaka Koki, Tokyo). All crowns were placed on the prepared abutment tooth, and the fitting was confirmed on the working cast. Each of the 5 crowns was fabricated in four ways (CO1, CO2, CAD1, and CAD2), titanium and zirconia crowns were fabricated for CAD1 and CAD2, and four different undercuts were fabricated for CAD1, for a total of 45 crowns prepared for this experiment.
2.2.
Fig. 2 – Tooth preparation of the mandibular first premolar on the working cast.
digital scanner (D810; GC), and the design of the crown form was modified using computer-aided design (CAD). CAD crowns were designed to four forms with 0.2-, 0.3-, 0.4-, and 0.5-mm undercut values using design software (Dental Designer; GC) (CAD1).
2.1.3.2. Presence of the previous abutment tooth crown form. The silicone core of the previous abutment tooth crown form of the simulation model was made using pate-type silicone impression material (Exafine; GC), and it was overlaid on the prepared abutment crown on the working cast. The crown was waxed up similarly to the original crown form using the silicone core (CO2). For the CAD crown, the previous abutment tooth crown and the prepared die forms were scanned using the digital scanner. The scanned data were precisely overlapped with the bases of the remaining teeth without a divergence, both horizontally and vertically. The crown restoration data were morphed to the scanned previous crown form using design software (CAD2). Before the casting of the conventional crowns (CO1, CO2), cyanoacrylate adhesives (Startrite; Nissin Giken Co., Ltd., Tokyo, Japan) were applied to add one layer to the wax patterns as a margin for polishing. CO1 and CO2 crowns were casted with a 12% Ag–Pd–Au alloy (Castwell M.C. 12; GC) and were conventionally polished. For CAD/CAM crowns (CAD1 and CAD2), the completed design data were transferred to a milling machine (GM-1000; GC). CAD1 and CAD2 crowns were milled from titanium alloy (Aadva titanium disc; GC) and
3
Measuring accuracy
White high viscosity silicone impression material (Fit Checker; GC) was mixed (base: catalyst = 6:1) and amply applied on the internal surface of the clasp; the clasp was then placed on the remade crown. After the clasp was held for 3 min under a constant load of 9.8 N, it was removed from the crown. The black silicone material (Bite-Checker; GC) was then mixed (base: catalyst = 6:1) and poured inside the white silicone material on each clasp. The two combined silicone materials were removed from the clasp after 3 min. Each silicone block was sectioned using a razor blade buccolingually 1.0 mm from the end of the clasp tip, clasp shoulders, and rest regions. To measure the gap distance between the crown and the clasp as a fitness check, the thickness of the white silicone layer at the sections was observed using a profile projector (V-16E; Nikon, Tokyo, Japan) at a magnification of 50.
2.3.
Measurement of retentive forces
The tensile force (N) of the Akers clasp from each remade crown was measured using the tensile test apparatus (EZ-S; Shimadzu, Kyoto, Japan) at a crosshead speed of 50 mm/min. The maximum tensile force obtained when the clasp was separated from the crown was evaluated as the retentive force.
2.4.
Statistical analysis
The data of the fitness accuracy and the retentive force were analyzed using the SPSS statistical package (version 12.0; SPSS, Inc., Tokyo, Japan) by one-way ANOVA and Tukey’s multiple comparison test at a significance level of a = 0.05.
3.
Results
3.1.
Fitness accuracy
The fitness accuracy between Akers clasp and previous abutment tooth crown and retrofitted crowns in each fabrication method (CO1, CO2, CAD1 Ti, CAD2 Ti) is shown in Fig. 4. CAD1 showed the thinnest silicone film layer in all regions (clasp tip, 55 8 mm; clasp shoulder, 70.8 7 mm; and rest, 39.5 7 mm) as compared to the other fabrication
Please cite this article in press as: Ozawa D, et al. Fabrication of crown restoration retrofitting to existing clasps using CAD/CAM: Fitness accuracy and retentive force. J Prosthodont Res (2015), http://dx.doi.org/10.1016/j.jpor.2015.01.002
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Fig. 3 – Flowchart for the fabrication of crown restorations to the existing clasp in four different ways (CO1, CAD1, CO2, and CAD2).
Table 1 – Sintering schedules for zirconia crowns in the furnace.
Temperature (8C) Time (h)
Warming
Warming
Insulation
Cooling
1000 2
1450 4.5
1450 2
1000 1
Fig. 4 – Differences in the fitness accuracy of each crown with four fabrication methods. (a) The tip region, (b) the shoulder region, and (c) the rest region (*p < 0.05).
methods including original crown (claspt tip, 70 4.5 mm; clasp shoulder, 85 11 mm; and rests, 49 5 mm). Conversely, CO1 showed the thickest film layer in all regions (clasp tip, 117.8 15.8 mm; clasp shoulder, 146 16.9 mm; and rest, 208.7 34.6 mm). There were significant differences between the CAD/CAM (CAD1 and CAD2) and conventional fabrications (CO1 and CO2) (P > 0.05). However, no significant difference was found between CAD1 and CAD2 or between CO1 and CO2 (P > 0.05). Of the regions, the clasp tip tended to show the best fitness accuracy. As for the rest, CO1 and CO2 showed the worst accuracy with CAD1, while CAD2 demonstrated the best accuracy. Fig. 5 shows the differences of the fitness accuracy between Akers clasp and the CAD/CAM crowns with titanium and zirconia. When comparing each region, similar thickness was shown in both materials titanium and zirconia measured tips,
55 9 mm and 52 8 mm; shoulders, 75 7 mm and 74 7 mm; and rests, 43 6 mm and 42 7 mm, respectively. In both CAD1 and CAD2, there was no significant difference between the two materials (P > 0.05).
3.2.
Retentive forces
Fig. 6 compares the retentive forces of the Akers clasps from retrofitted crowns in 4 fabrication methods (CO1, CO2, CAD1 Ti, CAD2 Ti). The CAD/CAM fabrications with a 0.2-mm undercut showed approximately 1.5–2 times higher retentive forces than those of conventional fabrications. In the conventional crowns, CO2 tended to show a higher retentive force than did CO1 without significant difference (P > 0.05). CAD1 and CAD2 demonstrated similar retentive forces, and there was no significant difference between them (P < 0.05).
Please cite this article in press as: Ozawa D, et al. Fabrication of crown restoration retrofitting to existing clasps using CAD/CAM: Fitness accuracy and retentive force. J Prosthodont Res (2015), http://dx.doi.org/10.1016/j.jpor.2015.01.002
JPOR-267; No. of Pages 8 journal of prosthodontic research xxx (2015) xxx–xxx
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Fig. 5 – Differences in the fitness between two crown restoration materials.
Fig. 6 – Differences in the retentive forces of clasp from crowns in 4 fabrication methods (undercut 0.2 mm) (*p < 0.05).
Fig. 7 shows the retentive force of the Akers clasp from each crown material and the influences of the undercut values on the retentive forces. Although the zirconia crown using CAD1 tended to indicate the greatest retentive force among all of the crowns, there were no significant differences between them (P > 0.05). As the undercut value was increased, the retentive forces became significantly greater (P < 0.05). In each undercut value, similar retentive forces were obtained for titanium and zirconia (P > 0.05).
4.
Discussion
4.1.
Experimental conditions
To focus on a comparative investigation of the fitness and the retentive forces under the same condition, an in vitro study was performed because it has the advantage of providing a standardized condition with respect to the preparation design, technique, and experimental performance, which results in a more repeatable assessment. For
the first premolar and the second molar, an Akers clasp and a ring clasp were selected, respectively, since the Akers and ring clasps are basic designs of the cast clasp and have three elements—support, bracing, and retention [21]. Generally, the simulation models or working casts were made of plaster, metal, and epoxy resin, or extracted teeth were used for the in vitro study. In the present study, the working cast was made of extra-hard plaster, since it is little influenced by irregular reflections during laser scanning. Regarding the crown alloys in this study, the CAD/CAM crowns were milled from titanium alloy, but an Ag–Pd–Au alloy was chosen for the conventional method since the accuracy of cast titanium could not be enough due to the low castability of the thin area [22–24]. Many measurement methods have been reported for evaluating the accuracy of the clasp fitness [25–30]. In the present study, fitness accuracy was evaluated by measuring the film thickness using high viscosity white silicone impression material in accordance with the methods of Osada [27] and Shimpo [28]. Limitations of this study were that the pathway of insertion/removal, the fitness, and the
Please cite this article in press as: Ozawa D, et al. Fabrication of crown restoration retrofitting to existing clasps using CAD/CAM: Fitness accuracy and retentive force. J Prosthodont Res (2015), http://dx.doi.org/10.1016/j.jpor.2015.01.002
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Fig. 7 – Differences of the retentive forces of clasp from two crown materials and four undercut values (*p < 0.05).
designs of the denture were not considered. Thus, the fitness accuracy and the retentive force could not be confirmed definitively as in the in vivo study.
4.2.
Measurement of fitness
It is conceivable that the clasp functions appropriately to retain RPDs by offering suitable fitness, retention, and supporting-tissue health. The fitness of the CAD/CAM crowns indicated a similar tendency to the previous crown. The CAD/ CAM crowns showed better fitness results in all regions as compared to conventional crowns. The fitness of CO1 and CO2 at rest was worse than that of the original crown because casting shrinkage would cause the clasp to float. Conversely, CAD1 and CAD2 demonstrated better fitness at rest. This occurs because the clasp settles correctly on the crown as there is no shrinkage factor during CAD/CAM procedures. The margin and internal fitness between the ceramic and metal crown restorations and the abutment teeth have been measured [29–33], and most of the CAD/CAM crowns showed better fitness as compared to conventional crowns. In addition, the conventional crowns demonstrated greater standard deviation values than those of the CAD/CAM crowns. Conventional crowns might have greater measurement errors caused by technicians’ inexperience, the type of casting alloys used, corrections of casting shrinkage, and treatment of the oxide layer on the surface of the castings [9,26,34,35]. Conversely, CAD/CAM crowns are fabricated by milling; thus, few machining errors might occur at every laboratory step. This should be considered a huge advantage of using CAD/CAM. There was no significant difference in the fitness accuracy between titanium and zirconia crowns. As for the contraction tolerance during the calcination of zirconia, the shrinkage correction value and the calcination temperature against the zirconia framework are effective. Bu¨chi et al. reviewed the accuracy produced by the zirconia fixed prostheses and mentioned few worse influences on clinical applications [32]. According to the results of this study and those reports, it was supposed that isotropic shrinkage and machining errors caused by contraction during the calcination of zirconia did not affect the accuracy of fitting the crowns to the existing clasps.
4.3.
Retentive forces
Although the same undercut value (0.2 mm) was used in all crown restorations in this study, CAD/CAM crowns showed retentive forces approximately twice as high as for conventional crowns. The main reason was that the CAD/CAM crowns showed superior fitness to conventional crowns. The appropriate retentive force of the clasp should be determined by many factors, namely, the partially edentulous pattern, tooth conditions, loss of periodontal tissue retention required for one RPD, number of retainers, etc. Many clinicians have reported that approximately 20 N per one RPD would be adequate to hold it in place when sticky foods are chewed and still allow the patient to easily remove the RPD. For the clasps, retention would be approximately 5–10 N, so that harmful forces are not applied to the abutment teeth [36,37]. In this study, as the undercut values were increased, the retentive force was also proportionally increased. From viewpoint of suitable retention, the CAD/CAM crowns should ideally be given 0.2–0.3-mm undercut values to maintain appropriate retention. The decrease of clasp retention was caused by daily use of the RPD. Tokue [37] also reported that Co–Cr clasps lost their retention force after 10,000 insertion-removal cycles. It may be derived from some other factors, such as microstructural phases or mechanical properties as well as wear and fatigue of the clasp materials. To recover retention for the clasp in the case of lost retentive force, fabrication of a new crown to retrofit the clasp using CAD/CAM might be a possible procedure instead of clasp adjustment and denture reproduction. In the future, dental treatments might only be possible with computer assistance. With innovations in optical impression technology, the technical work of taking impressions could be omitted in the future. Further studies are of interest for fabricating new crowns retrofit to the existing clasps using only digital data from the process of impression taking to that of milling the crowns.
5.
Conclusions
This study has evaluated various fabrication methods for crown restorations retrofit to the existing clasp using conventional and CAD/CAM procedures by measuring the
Please cite this article in press as: Ozawa D, et al. Fabrication of crown restoration retrofitting to existing clasps using CAD/CAM: Fitness accuracy and retentive force. J Prosthodont Res (2015), http://dx.doi.org/10.1016/j.jpor.2015.01.002
JPOR-267; No. of Pages 8 journal of prosthodontic research xxx (2015) xxx–xxx
fitness accuracy and the retentive forces. Within the limitations of this study, the following conclusions can be drawn: 1. For fitness and retentive forces, the CAD/CAM method showed more favorable results than did the conventional method. 2. As the undercut value was increased using CAD software, the retentive forces increased significantly. In each undercut value, similar retentive forces were obtained for titanium and zirconia crowns. 3. When a crown restoration must be remade to retrofit the existing clasp, CAD/CAM fabrication can be recommended so that both appropriate fitness and retentive force are obtained.
Conflict of interest This study was funded by GC Corp (Tokyo, Japan). The sponsor of this study had no role in the study design, conduct of the study, data collection, data interpretation or preparation of the report.
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