Effet Of Auxiliary Resistance Forms On The Resistance And Marginal Fitness Of Complete Crowns For Short Molar Preparations:a Laboratory Study

ObjectiveTo evaluate the effect of different auxiliary resistance forms on the resistance and marginal fitness of complete crowns for short molar preparations. To evaluate the relationship between the marginal adaptation and resistance. To observe the internal fitness with the method of silion with different colours and to evaluate the value of this method.Materials and Methods70 Nissin resin teeth were prepared with 20-degree total occlusal convergence (TOC),2.5 mm of occlusocervical height and a shallow finish line on a milling machine. The milled preparations were then randomly assigned to seven groups of ten. The first group was as the control group.30 dies were modified by preparing 2 interproximal grooves with 0-degree,6-degree and 20-degree centered on the mesial and distal surfaces of the dies. The rest of teeth were prepared with occlusal pin holes in the center of the occlusal surface milled with the same burs to form 0-degree, 6-degree,20-degree pin holes. Cobalt-chromium copings were fabricated for all specimens. An orange colour light-bodied, silicone impression material was used to construct replicas of the space between the crowns and the preparations. The height of the silicone light-body at three points were measured under telecomicrocope. The marginal gap of specific points at axial surface were measured before and after cementation. The resistance of each specimen was evaluated by applying an external force at a 45-degree angle to the long axis of the die with a universal testing machine in a lingual to buccal direction. The maximum force applied before the crown dislodgement was measured. The data was analyzed with SAS 9.2 software.Results1. The snk test results indicated that the 0-degree groove group, 0-degree pin hole group and 6-degree pin hole group were effective to improve the resistance of the crowns(p<0.05). The 6-degree grooves group,20-degree grooves group and 20-degree pin hole group have no significant difference with the control group.2. The 0-degree grooves group,6-degree grooves group and 0-degree pin hole group, 6-degree pin hole group,20-degree grooves group adversely affected the seating of the crowns(p<0.05).20-degree grooves group had no significant difference with the control group.3. The figures indicated that better marginal adaptation resulted in greater resistance force.4. The data of internal fitness between the crown and preparation indicated that the occlusal discrepancy was the largest. The occlusal discrepancies of preparations with auxiliary resistance forms were much larger than the preparations without any resistance forms.5. The data measured with the method of silicone impression had significant difference with the data measured under microscope.Conclusion1. Auxiliary resistance forms with less degrees indicated greater resistance force.2. Auxiliary resistance forms resulted in worse marginal fitness. In clinical, if the resistance of a preparation is enough, the auxiliary resistance form should be avoided being used.3. Better marginal adaptation resulted in greater resistance force.4. The occlusal discrepancy was much larger than that of the other points. Esaping channel should be set to ensure appropriate marginal fitness.5. The method of silicone impression could reflect the internal fitness intuitively, but its marginal gap was different from that cemented with GIC cement.