牙冠修形的錐角及與高度之相關性

　　　　　牙齒修形後的錐角是影響牙套固持性的重要因素之一，錐角越小則牙套固持性越 好。由實驗所得理想錐角應是4度到10度，但臨床上修形後的錐角在國外報告中大多超過這 個數值。本研究的目的在了解本院四個院區臨床上修形後牙齒的錐角以及牙冠高度與錐角之 間是否相關。在比較單齒模投影時的兩種定位方法並無明顯差異之後，我們利用壓克力製定 位座調整好222顆單齒模角度位置，經由投影，將近遠心面頰面外形描繪出來，再利用三角 形關係算出錐角角度，經t檢定或變異數分析（ANOVA）比較不同組之差異。結果顯示臨床上 牙齒修形後的錐角平均值由9.22度到24.76度，與國外報告相似；又只有近遠心面的牙冠高 度與錐角之間有負相關性，表示牙冠越長則修形後近遠心面錐角越小；同時發現後牙有固持 形態的比率較高。
　　　　　The wall taper of a crown preparation is one of the important factors in the retention of a prepared tooth. The ideal wall wall taper of 4 to 10 degrees is based on laboratory research established by Jorgenson, but it is rarely achieved in clinical preparation. The objectives of this study were to find the mean taper of clinical crown preparations at Chang Gung Memorial Hospital, and its relationship with the crown height. A total of 222 single casting crown dies were positioned and oriented on an acrylic stand. We projected the faciolingual and mesiodistal silhouettes of the dies on paper with an overhead projector, and traced the sharp images. Lines were drawn parallel to the traced axial walls in the gingival third and extended occlusally. A straight line crossed the axial lines and, according to the relation of the triangle, the wall taper was obtained mathematically. The results showed that mean taper of the clinical crown preparation varied from 9.22 to 24.76 degrees. There was a negative correlation between the crown height and the mesiodistal wall taper, that is, the higher the crown, the smaller the mesiodistal wall taper. Retentive forms, such as retentive grooves or occlusal wells, were found on molar dies more often than on the incisors and premolars. In conclusion, the ideal convergence angle of 4 to 10 degrees is seldom achieved in clinica practice, and there is a negative correlation between the wall taper and the crown height only on mesiodistal aspect.