Durability of Diamond Burs for the Fabrication of Ceramic Crowns Using Dental CAD/CAM

Transcription

1 Dental Materials Journal 24 (1) : , 2005 Durability of Diamond Burs for the Fabrication of Ceramic Crowns Using Dental CAD/CAM Atsushi YARA1, Hideo OGURA1, Akiyoshi SHINYA2, Shoko TOMITA2, Takashi MIYAZAKI3, Yasuhisa SUGAI4 and Yuuki SAKAMOTO5 1Department of Dental Materials Science, School of Dentistry at Niigata, The Nippon Dental University, 1-8 Hamauracho, Niigata-shi, Niigata , Japan 2Department of Crown and Bridge, School of Dentistry at Tokyo, The Nippon Dental University, Fujimi, Chiyoda-ku, Tokyo , Japan 3Department of Oral Biomaterials and Technology, Showa University School of Dentistry, Hatanodai, Shinagawa-ku, Tokyo , Japan 4ADVANCE Co. Ltd., 5-7 Nihonbashi-kobunacho, Chuo-ku, Tokyo , Japan 5GC Corporation, 76-1 Hasunuma-cho, Itabashi-ku, Tokyo , Japan Corresponding author, yatushi@ngt.ndu.ac.jp Received September 22, 2004/Accepted February 23, 2005 The diamond burs of two dental CAD/CAM systems (GN and CD) were examined if they could be used to fabricate up to 21 ceramic full crowns without fracture. After one, 11, and 21 machining times, the surfaces of the diamond burs were observed and the number of captured on SEM pictures was counted. The average surface roughness of the crowns was also measured. All diamond burs could be used to fabricate 21 ceramic crowns without fracture. A significant decrease in the number of diamond was found on the surfaces of GN burs after 11 and 21 machining times, but not on those of CD burs. The average surface roughness of GN crowns significantly increased with increase in the number of machining times. A significant positive correlation was found between the average surface roughness and the number of diamond. Key words : CAD/CAM, Durability, Diamond bur INTRODUCTION In the past decade, CAD/CAM has been introduced to dentistry and is now commonly used to fabricate dental restorations. Although dental CAD/CAM systems offer many benefits and advantages, it has been pointed out by a report of the Japanese Society for Dental Materials and Devices1-2) that the fabrication cost which comprises the material cost of restoration and the cost of cutting instrument is relatively expensive. Diamond burs and tungsten carbide burs are used for machining of raw materials in the system. The former is for ceramics and the latter for titanium or titanium alloys. Both instruments need to be replaced with new ones after repeated machining. According to the manufacturers of two dental CAD/CAM systems (GN- I, GC Corp., Tokyo Japan; and Cadim, Advance, Tokyo, Japan), the diamond bur may be used to fabricate more than 10 ceramic full crowns which is the recommended, maximum number of repeated machining times. A very recent study by Hotta et al.3), however, indicated that the tungsten carbide bur can be used to fabricate up to 51 titanium crowns. This type of result may also be expected for diamond burs in the fabrication of ceramic crowns although the durability of diamond burs is lower than that of tungsten carbide burs. The purpose of this study was to evaluate the durability of diamond burs for the fabrication of ceramic crowns using dental CAD/CAM system and to investigate the surfaces of these burs as well as those of fabricated crowns after repeated machining. MATERIALS AND METHODS Ceramic crowns were fabricated using two dental CAD/CAM systems (GN- I, GC Corp., Tokyo, Japan; and Cadim, Advance, Tokyo, Japan) shown in Fig. 1. The codes of these systems are GN for GN- I and CD for Cadim. Although both systems used computer-assisted machining, their measuring devices were different: laser-measuring device for GN and contact-probe for CD. The ceramic blocks (GN- I Ceramic Block, GC Corp., Japan; and Cadim Block CE, Advance, Tokyo, Japan), which were exclusively prepared for the two CAD/CAM systems, were used for each system. The machining of a ceramic block was carried out using a pair of cylindrical diamond burs exclusively fitted for each system one for initial machining and the other for final machining. The length and diameter of these burs are listed in Table 1. For the GN- I system, four diamond burs are

2 YARA at al. Fig. 1 Two CAD/CAM Table 1 Length and diameter of burs, both 135 systems. systems commonly used to fabricate one ceramic crown. But in the present study, the machining system of GNI was modified such that only two diamond burs were required to fabricate a ceramic crown one for coarse machining and the other for finishing (R1.5 ball-end mill and R0.5 ball-end mill respectively). A conical die (base diameter: 9.6 mm, taper: 1/ 10, Fig. 2) made of brass was used as the original model for the fabrication of ceramic crown (Fig. 3). The die had a V-shaped groove at 6 mm from the top, and the upper edge of the groove was used for crown margin. A CAD program was set up for each CAD/CAM system to fabricate a model-type crown (dotted line in Fig. 2), which had a shoulder-type margin (0.5 mm). An impression of the original model was made using a silicone rubber impression material (Exaflex Regular, GC Corp., Japan). A stone die was prepared from the impression using an exclusive stone (GN- I CAD stone, GC Corp., Tokyo, Japan) for GN and a dental stone (NEW FUJIROCK, GC Corp., Tokyo, Japan) for CD. Three dies were prepared for each system and their dimensions were measured using the respective measuring device in the CAD. For each stone die, a ceramic full crown (Fig. 3) was machined following the Fig. 2 Conical die for original crown (dotted line). model and outline of a manufacturer's instructions. Thus, three pairs of diamond burs were tested for each system. These diamond burs were examined to check if the bur could be used without fracturing to fabricate up to 21 ceramic crowns. The surface topography of each initial diamond bur (coarse machining bur) was observed after the machining of the 1st, 11th, and 21st crowns using a scanning microscope (JSM-25III, JOEL, Tokyo,

3 136 DURABILITY OF DIAMOND BURS FOR DENTAL CAD/CAM Fig. 3 Fig. 4 The measurement Fabricated of the ceramic Japan). With these SEM pictures input into a computer graphic program, the number of diamond was counted. A standard size frame, 1.2 mm high and 1.2 mm wide, was prepared in the computer system. It was mapped on the original size of the diamond burs (bur diameter: 2.0 mm for both GN and CD burs) in the computer display and was used to count the number of diamond. As shown in Fig. 4, its top margin and side margin were adjusted at 0.6 mm from the top of the bur and at 0.4 mm from the sidewall of the bur respectively. Then, the number of diamond within this frame crowns. number on the SEM pictures. area was counted. In addition to the evaluation of diamond burs, the surface of the fabricated crown was evaluated. The average surface roughness (Ra) of the occlusal surfaces after one, 11, and 21 machining times was measured using a profilometer (Surfcorder SE-40D, Kosaka Laboratory Ltd., Tokyo, Japan). The data for the counted as well as those of average surface roughness were statistically analyzed using two-way ANOVA and Tukey multiple comparison test. The correlation between the number of diamond and average surface

4 YARA roughness regression was also analysis. statistically analyzed using et All diamond burs for both systems could be successfully used without fracturing to fabricate 21 ceramic crowns. Fig. 5 shows the SEM pictures of the initial diamond burs after the 1st, 11th, and 21st machining. As shown in this figure, the size of the diamond on CD burs was relatively smaller than those on GN burs. Further, after the 11th and 21st machining, some diamond of the GN burs were detached from the bur. Table 2 shows the number of diamond Table SEM pictures 2 Number of the of the diamond diamond 137 for the initial burs at three different numbers of machining times. The results of two-way ANOVA showed that the two main factors (diamond bur and number of machining times) and their interaction significantly influenced the number of diamond (p<0.01 for two main factors, p<0.02 for interaction). Fig. 6 shows the number of diamond after different numbers of machining times for two different diamond burs. For CD diamond burs, no significant changes in the number of diamond were observed with increase in the number of machining times. On the other hand, for GN diamond burs, the number of diamond decreased as the number of machining times increased. A significant difference in the number of diamond was found between 1st and 11th single RESULTS Fig. 5 al. burs at three on the different burs machining times.

5 138 DURABILITY OF DIAMOND BURS FOR DENTAL CAD/CAM Fig. 6 Number of diamond on CD and GN burs. Fig. 7 Average surface roughness at different machining times. Table 3 The average surface roughness at different machining times machining as well as between 1st and 21st machining (p<0.05). Table 3 lists the data for average surface roughness. For CD crowns, their average surface roughness ranged from 0.8 to 1.6um; and for GN crowns, their average surface roughness ranged from 1.1 to 2.1um. The results of two-way ANOVA showed that the two main factors (diamond bur and number of machining times) and their interaction significantly influenced average surface roughness (p< 0.01 for diamond bur, p< 0.02 for number of machining times, and p< 0.03 for their interaction). Fig. 7 shows the average surface roughness at three different numbers of machining times for two different systems. At 1st machining, the average surface roughness was not significantly different between the two systems (p >0.05). After 11th and 21st machining, the average surface roughness of the GN crowns significantly increased (p< 0.05) - by a maximum difference of 0.7um compared with the 1st machining. As for CD crowns, there were no significant changes in surface roughness (p >0.05). The correlation between average surface roughness and the number of diamond was significant (p< 0.01), where the coefficient of correlation was A regression equation which was obtained from a single regression analysis was also significant (p< 0.01), and the average surface roughness increased with decrease in the number of diamond (Fig. 8). Fig. 8 Correlation between surface roughness and number of diamond on the bur. DISCUSSION Diamond burs have been frequently used for cavity preparation in dental practice, and their efficacy has been demonstrated by many studies. Recent studies4-8d have indicated that the curing shrinkage of dental composites and the oxidation of titanium castings are still a major concern in these forming procedures. With dental CAD/CAM systems, however, these flaws and disadvantages are eliminated - which is considered a chief benefit of the CAD/CAM system. Although a dental CAD/CAM system has provided the aforementioned advantages, the machining burs in the CAM have some limitation due to their fracture. Fracture of the bur blank has not been dealt with in these studies since this type of problem is rarely seen in daily dental practice. In dental CAD/CAM systems, however, the diamond burs are exposed to high machining load which could sometimes reach a maximum of kgf according to the manufacturer. This would result in the fracture of the bur blank. When we started this study, the diamond burs were assumed to fracture after about 20 machining times based on the information

6 YARA et al. 139 from the manufacturers. However, the burs were visually sound after the 21st machining, and thus further machining is now being continued. It has been shown by Hartley et al.9), Borges et al.10), and Grajower et al.11) that dental diamond burs lost their diamond after repeated cavity preparation or grinding. This phenomenon was also found on GN burs after the fabrication of 11th and 21st ceramic crowns, as shown in Figs. 5 and 6. For CD burs, on the other hand, there was no significant decrease in the number of diamond. This might be related to a difference in mechanical property between CD and GN ceramic blocks. According to manufacturers' information, Vickers hardness was for CD ceramic block and about 500 for GN ceramic block. It was very probable that the higher hardness of GN ceramic block also resulted in the detachment of diamond from GN burs. The average surface roughness of the fabricated crowns significantly varied with different CAD/CAM systems and with the number of machining times. A significant increase in average surface roughness was found for GN crowns whereas no significant changes were found for CD crowns. However, the highest value yielded by GN crowns was only 2.1,um a value considered negligible for polishing. It should be noted that the average surface roughness increased with decrease in the number of diamond (Fig. 8). This result suggested that increase in the surface roughness of GN crowns was due to the detachment of diamond from the bur. The detached diamond would induce more adhesive removal of the ceramic material, hence affecting the accuracy of the fabricated crown. From this point of view, a further investigation is needed to clarify the effect of detached diamond on the accuracy of fabricated crown. CONCLUSION In the present study, the diamond burs of two dental CAD/CAM systems (GN and CD systems) were examined if they could be continuously used to fabricate up to 21 ceramic full crowns without fracture. The purpose of which was to evaluate their durability. As such, the surfaces of the diamond burs as well as those of the fabricated crowns were examined after one, 11, and 21st machining times. The results showed that all diamond burs could be used to fabricate 21 ceramic crowns without fracture. A significant decrease in the number of diamond was found on the surfaces of GN burs after the 11th and 21st machining, whereas no significant changes were found on CD burs.likewise, no significant changes in average surface roughness were found for CD crowns. As for GN crowns, their average surface roughness significantly increased with increase in the number of machining times. But the highest surface roughness value was 2.1,um - a value considered negligible for polishing. A significant correlation was found between the average surface roughness and the number of diamond, where the former increased with decrease in the latter. REFERENCES 1) The Committee on Evaluation and Status Reports of Dental Materials and Devices. A Review and the Future in Dental Cast Au-Ag-Pd alloys. Part 1: Literature survey and alternatives to dental Au-Ag-Pd alloys. J J Dent Mater 2003; 22: ) The Committee on Evaluation and Status Reports of Dental Materials and Devices. A Review and the Future in Dental Cast Au-Ag-Pd alloys. Part 2: The basic properties database of Au-Ag-Pd-Cu alloys. J J Dent Mater 2003; 22: ) Hotta Y, Miyazaki T, Lee WS, Kobayashi Y. Accuracy of the ceramic crown fabricated by the newly developed CAD/CAM system. J Showa Univ Dent Soc 1996; 16: ) Erosy M, Civelek A, L'hotelier E, Say EC, Soyman M. Physical properties of different composites. Dent Mater J 2004; 23(3): ) Kikuchi M, Takahashi M, Okabe T, Okuno O. Grindability of dental cast Ti-Ag and Ti-Cu alloys. Dent Mater J 2003; 22 (2) : ) Meng Y, Nakai A, Goto S, Ogura H. Study of resinbonded calcia investment. Part 3: Hardness of titanium castings. Dent Mater J 2004; 23 (1) : ) Sato H, Komatsu M, Miller B, Shimizu H, Fujii H, Okabe T. Mold filling and microhardness of 1% Fe titanium alloys. Dent Mater J 2004; 23 (2) : ) Kikuchi H, Onouchi M, Miyanaga K, Wakashima M, Okuno O. The thickness effects of titanium castings on the surface reaction layer. Dent Mater J 2004; 23 (3) : ) Hartley JL, Hudson DC, Sweeney WT, Diekson G. Methods for evaluation of rotating diamond-abrasive dental instruments. JADA 1957; 54: ) Borges CFM, Magne P, Pfender E, Heberlein J. Dental diamond burs made with a new technology. J Prosthet Dent 1999; 82: ) Grajower R, Zeitchick A, Rajstein J. The grinding efficiency of diamond burs. J Prosthet Dent 1979; 42: