Yoon Stephan S, Cheong Chan, Preisser John, Jun Sangho, Chang Brian M, Wright Robert F
Resident, Department of Orthodontics, University of California-San Francisco, San Francisco, Calif.
Resident and Post-doctoral Fellow, Department of Prosthodontics, Harvard School of Dental Medicine, Boston, Mass.
J Prosthet Dent. 2014 Aug;112(2):285-92. doi: 10.1016/j.prosdent.2014.01.021. Epub 2014 Apr 14.
Total occlusal convergence of crown preparation is an important didactic and clinical concept in dental education.
The purpose of this study was to compare the discrepancy between the total occlusal convergence of dental students' typodont crown preparations and the ideal range (4 to 10 degrees) in 3 different regions of the mouth and in 4 different planes of the teeth.
The dental students of the Class of 2012 at Harvard School of Dental Medicine were asked to prepare typodont teeth for crowns on 3 different teeth, the maxillary left central incisor (ceramic), mandibular left first molar (complete metal), and mandibular left first premolar (metal ceramic), during their third year preclinical summative examination and the Comprehensive Clinical Examination in their fourth year. Eighteen students prepared 3 teeth in their third and fourth years, whereas 19 students participated only in their fourth year, for a total of 55 sets of 3 teeth. By using custom fit die bases to reproduce the position, a novel procedure of measuring each tooth was accomplished in 4 different planes: the faciolingual, mesiodistal, mesiofacial-distolingual, and mesiolingual-distofacial. The total occlusal convergence of each image was measured with a computer screen protractor. The gingival 2 mm of the axial wall was used to determine the taper of each wall. Linear mixed model analysis was used to estimate and compare the total occlusal convergences of different teeth and planes (α=.05). Bonferroni corrections were used to adjust for post hoc multiple comparisons.
The mean total occlusal convergence varied by tooth and plane (2-way interaction; P<.001). For the first molar, dental students excessively tapered in all 4 planes; the model-predicted 99% CIs for the total occlusal convergence were as follows: faciolingual (12.7, 19.4), mesiodistal (14.0, 19.3), mesiofacial-distolingual (13,4, 19.4), and mesiolingual-distofacial (13.7, 19.1). For the central incisor, 99% CIs for the total occlusal convergence were (15.9, 24.4) for the faciolingual measurement, providing strong evidence of excessive tapering, and (4.1, 8.0) for the mesiodistal measurement, which was within the ideal total occlusal convergence range. The mesiofacial-distolingual and mesiolingual-distofacial planes in the central incisor and all 4 planes in the first premolar had mean total occlusal convergences that exceeded 10 degrees; however, excessive tapering could not be statistically established, because their CIs included values within the ideal range.
The present study found significant evidence of excessive tapering in a study comparing the total occlusal convergence values of crown preparations with those of the ideal preparation for 3 different teeth in 4 different planes. The total occlusal convergence for the molar preparations had the highest mean values.
牙冠预备的全牙合聚合度是牙科教育中一个重要的教学和临床概念。
本研究的目的是比较牙科学生在全牙合聚合度方面，在口腔3个不同区域以及牙齿的4个不同平面上，其在模型牙上预备牙冠的情况与理想范围（4至10度）之间的差异。
哈佛大学牙医学院2012级的牙科学生被要求在其三年级临床前总结性考试以及四年级的综合临床考试期间，为3颗不同的牙齿预备模型牙冠，即上颌左中切牙（陶瓷）、下颌左第一磨牙（全金属）和下颌左第一前磨牙（金属陶瓷）。18名学生在三年级和四年级各预备3颗牙齿，而19名学生仅在四年级参与，总共55组3颗牙齿。通过使用定制贴合的代型底座来重现位置，在4个不同平面上完成了一种测量每颗牙齿的新方法：颊舌向、近远中向、近颊 - 远舌向和近舌 - 远颊向。使用计算机屏幕量角器测量每个图像的全牙合聚合度。轴向壁的龈缘2mm用于确定每个壁的锥度。采用线性混合模型分析来估计和比较不同牙齿及平面的全牙合聚合度（α = 0.05）。使用Bonferroni校正来调整事后多重比较。
全牙合聚合度的平均值因牙齿和平面而异（双向交互作用；P < 0.001）。对于第一磨牙，牙科学生在所有4个平面上都过度锥化；模型预测的全牙合聚合度的99%置信区间如下：颊舌向（12.7，19.4），近远中向（14.0，19.3），近颊 - 远舌向（13.4，19.4），以及近舌 - 远颊向（13.7，19.1）。对于中切牙，颊舌向测量的全牙合聚合度的99%置信区间为（15.9，24.4），提供了过度锥化的有力证据，而近远中向测量的置信区间为（4.1，8.0），处于理想的全牙合聚合度范围内。中切牙的近颊 - 远舌向和近舌 - 远颊向平面以及第一前磨牙的所有4个平面的平均全牙合聚合度均超过10度；然而，由于其置信区间包含理想范围内的值，因此无法在统计学上确定过度锥化。
本研究在一项比较牙冠预备的全牙合聚合度值与4个不同平面上3颗不同牙齿的理想预备值的研究中，发现了过度锥化的显著证据。磨牙预备的全牙合聚合度平均值最高。
