The purpose of this study was to evaluate the availability of computer system for the measurement of tooth size in the model analysis through the comparison of two measurements: One was to use a computer; and the other was to use vernier calipers. 20 sets of casts were used, which showed a moderate degree of crowding and full eruption of all teeth. The mesio-distal width of 12 teeth from the left central incisor to the left first molar at each set of the casts were measured twice with vernier calipers and a computer respectively. This measurement was r´-peated two weeks later. First, for. the reproducibility analysis, the two computer measurements were compared then the vernier calipers measurements were compared. Second, all the teeth were sepapated into the region of mesiodistal contact points and its width was measured by a micrometer to obtain standard measurements. For the accuracy analysis, these standard measurements were compared with the measurements from the dental casts using two methods. The difference between them was defined as the measurement error. To investigate the cause of measurement error, an examination was made for the presence and degree of contact point deviation on each tooth in terms of upper and lower occlusograms, and the mesio-distal angulation of each tooth was measured with TARG. Following results were obtained through statistical analysis.
1. In the analysis for reproducibility; the measurements with vernier calipers showed significant differences in three out of twelve teeth while the computer measurements showed significant differences in one out of twelve teeth.
2. In the analysis for accuracy; compared with the standard measurements, the measurements with vernier calipers showed significant differences in three out of twelve teeth while the computer measurements showed significant differences in two out of twelve teeth.
3. Compared with the standard measurements, the measurements with vernier calipers were apt tp~ be larger at the upper first nnolar, and smaller at the lower first molar. The computer measurements, however, were apt to be larger at both upper and lower fast molars.
4. The measurements with vernier calipers showed the largest error at the lower first molar and the degree of error was variable according to the tooth while the difference of error was smau in the computer measurements.
5. In the analysis for the correlation of the degree of measurement errors with the contact point deviation index and the mesio-distal crown angulation of each tooth, the measurements with vernier calipers did not show significant correlation while the measurements with computer showed slight positive correlations.
The results of this study indicate; that a computer system may be useful for the measurement of tooth size in the model analysis.
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