A basic study on the accuracy of a zirconia coping fabricated by CAD/CAM system-Effect of abutment modificationH. HAMASAKA, M. SONE, Y. OKAWA, S. SOMEKAWA, S. UEDA, M. MASUDA, A. MATSUI, Y.
TOYOTA, F. NARUMI, T. MATSUKAWA, K. OKAMOTO, and S. OHKAWA
Division of Removable Prosthodontics, Department of Restorative and Biomaterials Sciences, Meikai
University School of Dentistry
Abstract
The purpose of this study was to evaluate the influence of 2 different abutment tooth forms (for metalcoping and CAD/CAM coping) on the fitting accuracy of zirconia coping. Zirconia copings were made of asemi-sintered zirconia block (Cercon® base, DeguDent) using the dental CAD/CAM system (Cercon® brain,DeguDent). Fitting accuracy was evaluated by a cement-replica technique with white and blue silicone
materials. After the cement-replica specimens were sectioned, the thickness of the cross-sectioned whitesilicone layer was measured at 5 points. For zirconia coping specimens, the mean discrepancy of fitting
gaps was over 120 ㎛ between the die and the zirconia coping at all points in each group. This study
suggested that the zirconia coping fabricated by this system requires more precise fitting accuracy.

Introduction
The dental CAM system and CAD/CAM system has been used generally to fabricate various prostheses inclinical dentistry. The progress of dental CAD/CAM technology has made it possible to use high-strength
ceramic materials such as zirconia, and zirconia ceramics have become popular materials in clinicaldentistry. Zirconia ceramics have excellent mechanical strength for prosthetic appliances and arebiocompatible with a significant reduction in plaque.1) These facts suggest that zirconia ceramics areclinically useful for keeper copings of magnetic attachments.
Nevertheless, the keeper copings of magnetic attachments were still cast by the lost wax method used indental casting. Our present study2) introduced the method of zirconia keeper copings fabricated by thedental CAM system. However, the adaption had poorer clinical acceptability than did other prostheses.
Thus, we prepared 2 different abutment tooth forms for zirconia keeper copings and investigated the fitting
accuracy.
The purpose of this study was to evaluate the influence on fitting accuracy of 2 different abutment toothforms of zirconia copings fabricated by the same CAD/CAM system.

Materials and Methods
Materials: The prepared epoxy resin lower canine tooth (338: NISSIN) was selected as the abutment tooth.
The abutment tooth forms for the metal coping (MC) and the abutment tooth form for the CAD/CAM
coping (CC) are shown in Figs. 1 and 2. As compared to the abutment tooth form for MC, the rotational
resistance groove was removed for the CC, giving additional reduction to gain more clearance androunding all orifice line angles.
The keeper copings were fabricated using the dental CAD/CAM system (Cercon® brain, DeguDent), and asemi-sintered zirconia block (Cercon® base, DeguDent) was employed for the keeper coping material(Table 1).

Fig.1 Abutment tooth form for the MC

Fig.2 Abutment tooth form for the CC

Table 1 Material composition
Methods (Cement-replica technique): Fitting accuracy was evaluated using a cement-replica technique.
Each keeper coping was placed in the master die with a white silicone material (FIT CHECKER
ADVANCED, GC). After curing the white silicone material, the keeper coping with the white silicone
material was removed from the master die. Then, the keeper coping with it was embedded in a blue silicone
material (EXAMIXFINE Regular Type, GC). The keeper coping was demounted from the cured silicone
material, and a blue silicone material filled in the space (Fig.3).

Fig.3 Procedure for fabricating specimens (cement-replica technique)
Determination of fitting accuracy: Each silicone-replica specimen was sectioned in the buccolingual
direction through the center of the coronal root. The measuring points of the white silicone layer are shownin Fig.4. The thickness of the white silicone layer was measured at each point (a: canal orifice of the lingualwall; b: center of the lingual wall; c: apical part of the post; d: center of the buccal wall; e: canal orifice ofthe buccal wall).

Fig.4 Locations of the measuring points

Results
The mean thicknesses of the white silicone layer, namely the mean fitting gaps, were 781±79 ㎛ at pointa, 167±69 ㎛ at point b, 193±31 ㎛ at point c, 168±58 ㎛ at point d, and 156±46 ㎛ at point e in the MCgroup (Fig.5). There were statistically significant differences between point a and other measuring points
by one-way ANOVA and the Tukey–Kramer method (p<0.05).
In the CC group, the mean fitting gaps were 300±147 ㎛ at point a, 171±19 ㎛ at point b, 250±67 ㎛
at point c, 127±43 ㎛ at point d, and 192±129 ㎛ at point e (Fig.6). There were no significant differencesin the mean thicknesses of the white silicone layers between the 5 measuring points by one-way ANOVA
and the Tukey–Kramer method (p<0.05).
Application of the Student t-test indicated that there was a statistically significant difference between theMC and the CC groups at point a (Fig.7).

Fig.5 Mean thicknesses of the white silicone layers at 5 measuring points (MC)Fig.6 Mean thicknesses of the white silicone layers at 5 measuring points (CC)Fig.7 Mean thicknesses of the white silicone layers at 5 measuring points in both groups

Discussion
Within the limitations of this study, it was concluded that modification of the abutment tooth form
improved fitting accuracy at point a in this system. This result indicates that removing rotational resistancegrooves and rounding all orifice line angles could have influenced the fitting accuracy of point a. However,the fitting accuracy of a zirconia keeper coping fabricated by this system, ether MC or CC, had
unacceptable marginal gaps of 100 ㎛.3) This result suggests that the adaptation of a zirconia keeper coping
fabricated by this system is not clinically recommended. Further investigation is needed to fabricate
CAD/CAM keeper copings.

References
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