[Effect of auxiliary resistance forms on the resistance and marginal fitness of complete crowns for short molar preparations].

OBJECTIVE: To evaluate the effect of different auxiliary resistance forms on the resistance and marginal fitness of complete crowns for short molar preparations. METHODS: A total of 70 Nissin resin teeth were prepared with 20 degrees total occlusal convergence, 2.5 mm of occlusocervical height, and a shallow finish line on a milling machine. The milled preparations were then randomly assigned to 7 groups of 10. The first group was used as the control group. A total of 30 dies were modified by preparing interproximal grooves with angles of 0 degrees , 6 degrees , and 20 degrees centered on the mesial and distal surfaces of the dies. The rest of the teeth were prepared with occlusal holes in the center of the occlusal surface milled with the same burs to form 0 degrees , 6 degrees , and 20 degrees holes. Cobalt-chromium copings were fabricated for all specimens. The marginal gap of specific points on the axial surface was measured before and after cementation. The resistance of each specimen was evaluated by applying an external force at an angle of 45 degrees to the long axis of the die by using a universal testing machine in a lingual to buccal direction. The maximum force applied before crown dislodgement was measured. Data were analyzed using the SAS 9.2 software. RESULTS: The results showed that the 0 degrees groove, 0 degrees hole, and 6 degrees hole were effective in improving the resistance of the complete crowns (P<0.05). The 0 degrees groove, 6 degrees groove, 0 degrees hole, 6 degrees hole, and 20 degrees hole had significant difference with the control group in terms of marginal discrepancies (P<0.05). CONCLUSION: Auxiliary resistance forms with less degree indicate greater resistance force but worse marginal fitness. In clinical practice, if the resistance of a preparation is enough, the auxiliary resistance forms should be avoided from being used.