Mou Shu-Hui, Chai Tsongi, Wang Juo-Song, Shiau Yuh-Yuan
Department of Prosthodontics, School of Dentistry, National Taiwan University, Taipei, Taiwan.
J Prosthet Dent. 2002 Mar;87(3):248-55. doi: 10.1067/mpr.2002.122011.
Because of an imagining principle called active triangulation in the Cerec system, a shadow is cast distal to the illuminated objects. This distal shadow may be enlarged when the occlusal-cervical height of the prepared tooth is increased. Depth data of the shadow are unreliable, so the internal fit of Cerec crowns has been questioned.
This study evaluated the influence of different convergence angles and tooth preparation heights on the internal adaptation of Cerec crowns.
Tooth preparations were made on typodont teeth with different combinations of convergence angles and occlusal-cervical heights: Group I = 20 degrees angle, 6 mm height; Group II = 20 degrees angle, 4 mm height; Group III = 12 degrees angle, 6 mm height; and Group IV = 12 degrees angle, 4 mm height. Ten Cerec crowns were fabricated for each type of tooth preparation. Measurements of the internal fit were performed with the cement space replica technique and an image analysis system. Three-way analysis of variance was used to analyze the differences in cement space with different tooth preparations and the number of times that milling tools were used to prepare the Cerec crowns (P<.05). Multiple comparisons were made to evaluate differences between groups (P<.0083).
Cerec crowns with a 12 degrees convergence angle demonstrated the best internal fit (cement space in Groups III and IV = 121 +/- 41 microm and 115 +/- 42 microm, respectively). The difference between the 2 convergence types was within the range of the scanning error (25 microm) produced by the Cerec camera. The number of times that milling tools were used had no significant effect on internal fit (P=.78). Tooth preparation height equal to or shorter than 6 mm occlusal-cervically with both 12 degrees and 20 degrees convergence angles also had no significant effect on internal fit (P>.0083). Cement space at distal walls (185 +/- 28 microm) was the thickest among all axial walls (P=.0001) and was twice as thick as that at the facial (90 +/- 14 microm) and palatal walls (92 +/- 15 microm).
Within the limitations of this study, there was little difference in the internal fit of Cerec crowns prepared with convergence angles of 12 degrees and 20 degrees. Distal shadows influenced the thickness of the cement spaces, particularly at the distal walls. However, tooth preparations with an occlusal-cervical height not greater than 6 mm did not exaggerate the effect of the distal shadows.
由于Cerec系统中一种名为主动三角测量的成像原理，在被照亮物体的远中端会产生阴影。当预备牙的咬合-颈向高度增加时，这个远中阴影可能会扩大。阴影的深度数据不可靠，因此Cerec全冠的内部适合性受到质疑。
本研究评估不同聚合角和牙体预备高度对Cerec全冠内部适合性的影响。
在模型牙上进行牙体预备，采用不同聚合角和咬合-颈向高度的组合：第一组 = 20度角，6毫米高度；第二组 = 20度角，4毫米高度；第三组 = 12度角，6毫米高度；第四组 = 12度角，4毫米高度。每种牙体预备类型制作10个Cerec全冠。采用粘结剂空间复制技术和图像分析系统进行内部适合性测量。使用三因素方差分析来分析不同牙体预备和制备Cerec全冠时使用铣削工具的次数对粘结剂空间的差异（P<0.05）。进行多重比较以评估组间差异（P<0.0083）。
聚合角为12度的Cerec全冠显示出最佳的内部适合性（第三组和第四组的粘结剂空间分别为121±41微米和115±42微米）。两种聚合类型之间的差异在Cerec相机产生的扫描误差（25微米）范围内。铣削工具的使用次数对内部适合性没有显著影响（P = 0.78）。咬合-颈向高度等于或小于6毫米、聚合角为12度和20度的牙体预备对内部适合性也没有显著影响（P>0.0083）。所有轴向壁中，远中壁的粘结剂空间（185±28微米）最厚（P = 0.0001），是颊侧（90±14微米）和腭侧壁（92±15微米）的两倍。
在本研究的局限性内，聚合角为12度和20度制备的Cerec全冠在内部适合性方面差异不大。远中阴影影响粘结剂空间的厚度，尤其是在远中壁。然而，咬合-颈向高度不大于6毫米的牙体预备不会夸大远中阴影的影响。
