Measuring Crown Form According to Crown and Root Axes of the Maxillary Anterior Teeth, and Its Prosthodontic Significance (1981)

One hundred specimens each of extracted central incisors, lateral incisors, and canines of the upper anterior teeth were prepared under the identical conditions. Standardized photographs with the crown axis as the standard were taken and the crown and root axes were established by the schematic outlines of the teeth. Measurements of the crown form were taken and studied. The results were as follows : 1. The averages in axial inclination of the crown and root were : 178°for the central incisors, 177°for lateral incisors, and 174°for the canines, in labial aspect ; 179°, 183°, and 175°respectively in proximal aspect. Labially, the root axis was more distally inclined than the crown axis. The inclination became greater from the median to the distal side. Proximally, the upper central incisors were not as markedly inclined as their crown axis and were nearly straight. The lateral incisors were slightly inclined lingually than their crown axis. Contrarily, the canines were labially inclined. Angles of axial inclination of the crown and root provide guides for designing and making post cores. The significant difference found between the labial and proximal axial inclination must be duly considered in designing and making post cores in crown restoration of the lateral incisors. 2. In the labial contours of the cervical line area, the central incisors were rather greater, the canines next, and the lateral incisors slightest. This finding appears to present a guide for crown restoration of the cervical 1/6 portion of the labial surface. Especially the contours of the cervical portion of the lateral incisors must be taken into consideration. 3. The ratios of the length between the contact point and incisor edge to the length of the crown were : 0.22 for the central incisors, 0.26 for the lateral incisors, and 0.37 for the canines, in mesial aspect ; 0.32, 0.38, and 0.46 respectively in distal aspect. In every case, the mesial contact point was clearly closer to the incisor edge than the distal contact point. The contact point occupies the key point in mesial and distal contours. The above reatios are considered to be guides for recreating the embrasure of the correct adjacent relationships and for reproducing the proper appearance of the teeth. 4. The ratios of the curvature of cementoenamel junction on the mesial, proximal surface to the crown length were : 0.24 for the central incisors, 0.26 for the lateral incisors, and 0.21 for the canines. The ratios on the distal, proximal surface were 0.20, 0.22, and 0.14 respectively. The mesial cervical curvature of the crown was more marked than the distal one. This finding is considered to be helpful to prosthodontic restoration of the sublingual marginal portion. 5. In comparison of the greatest curvature of the cervical line area, with the crown axis as the standard, between the labial and lingual aspects, the curvature of the lingual aspect was slightly greater than that of the labial aspect for the central incisors, lateral incisors, and canines. A significant difference of 5% level was found.