[A study on tooth preparation by means of the newly developed parallelometer and its educational effect, with primary regard to convergence angles of abutment tooth].

Proper tooth preparation is one of the fundamental requirements in crown & bridge prosthodontics. However, it is difficult for students in particular to prepare adequate convergence angles of abutment tooth in the limited environment of patient's mouth. Therefore, the parallelometer was developed to improve the preparations as a supplementary device which informs the person by a signal of alarming simultaneously when a bur is not properly installed to convergence angles of abutment tooth. Sixty subjects were selected from students of Kanagawa Dental College and they were divided into two groups (A and B) to prepare abutment tooth of lower right 1st molar for full cast crown on typodont mounted into the manikin. A preliminary training was performed for once without the parallelometer for group A and with it for group B prior to a comparative experiment. The following results were obtained with primary regard to convergence angles of abutment tooth. 1. Related to each of four convergence angles, group B is superior to group A in all mean values with a statistical significance (P less than 0.01). 2. Related to a total of four convergence angles by the maximum number of subjects, group B is also superior to group A in a mean value by about ten degrees. 3. A newly developed parallelometer proved to be an efficient supplementary device for training of students to master a technique of preparation for adequate abutment tooth.