| With the development of preservation Stomatology, fixed partial denture is significantly more than ever as a prosthesis form. In the various fixed restorations, full crown takes the largest proportion because of shallow tooth surface cutting,pulp protection,good retention and other advantages. With the promotion of material science and technology, the development of full crown is also developing quite fast. In Clinic, full crowns which are made of various materials are widely used in restoring dental defects and deformities, protecting teeth and improving the teeth's appearance or as retainers of fixed bridges, and good results have been gained. Only when full crown restorations have sufficient retention with cement to maintain its position in the teeth, can chewing function and morphological appearance be effectively restored. Therefore, the retention of full crown has an important clinical significance of the prosthesis quality and effect.The main factors of retention of full crown are convergence angle, the height of the crown, the suitability between restoration and preparation, adhesive bonding and reasonable using, proper disposal of adhesive surface and so on. Convergence angle is entirely decided by dentists in actual clinical operations. It is also the most difficult indicator which directly influences the retention of full crown. Convergence angle is the angle between axial wall and the long axis of the abutment. The awareness of ideal convergence angle is not uniform yet at present. The convergence angle of full crown restoration recommended by Chinese scholars is 2—5°. Because of the restrictions operating in mouth, statistical results indicate that most of clinical operations are difficult in accordance with the ideal value in tooth preparation. Some scholars believe that different teeth and different designs have influence on convergence angle. It should be adjusted by different teeth and designs according to specific circumstances in mouth. It can't always be 2—5°. There isn't unified agreement on the largest convergence angle which can guarantee sufficient retention of full crown yet.Objective The aim of this study is to investigate the relationship between convergence angle of abutment and the retention of full crown, to investigate the influence of different convergence angles on the retention of full crown, to provide guidance and basis to clinical tooth preparation.Materials and methodsCollecting a maxillary first molar which is extracted because of periodontal disease and fresh, complete, with normal appearance and abrasion, witout caries. Enbeding the root of the tooth in a cylinder cradle under enamelo-cemental junction 0.5mm after removing calculus and periodontal ligaments. Doing routine tooth preparation. Stripping the occlusal surface 1mm uniformly. Maintaining the height of crown about 4mm. Prepare the cervix of the tooth to 0.5mm wide 90°chamfer. And making the cervical margine line above enamelo-cemental junction 0.5mm. Eliminating the undercut of axil wall above chamfer. Then making precise impression of preparation and cradle by silicone rubber impression material. Liquating base wax plate then fill the silicone rubber cavity block with it, then the wax patterns of preparation were gotten. Then 40 composite resin dies were made after flasking, heat treatment and bolting. Dividing them into 5 groups at random and different convergence angles (0°,2°,4°,6°,8°) were made for each group by dental parallel grinding instrument. Eight dies were made for each group of tooth preparation. Then metal full crown was fabricated for each resin die. Controlling all crowns in the same thickness basically and installing tension exerted device vertical to the occlusal surface. Fixing each crown with ZPC, and doing microtensile test on electronic universal testing machine which was controlled by microcomputer. Measuring the retention between each die and its corresponding crown before and after cementation, then doing statistical analysis for the data of each group. Statistically significant level was considered as "alpha equals 0.05".Results1. Before cementation, the retention of full crowns in 0°,2°,4°,6°,8°groups is (68.64±9.75)N,(64.75±8.91 )N,(60.23±9.58) N,(51.91±4.83)N,(39.10±6.27) N, respectively. After cementation, the retention of each group is (200.38±16.46)N,(196.87±13.65) N,(188.98±16.93) N,(180.79±14.13) N,(163.24±15.67) N, respectively.2. Before cementation, the retention of full crown decreases more obviously than after cementation with the increasing of convergence angle. The retention of 6°group is significantly weaker than that of 4°group (P=0.048). The retention of 8°group is significantly weaker than that of 6°group (P=0.003).3. After cementation, the retention of full crown also decreases with the increasing of convergence angle, but the extent of decreasing is significantly less than before cementation. Only the retention of 8°group is significantly weaker than that of 6°group (P=0.029).4. After cementation, the retention of 0°,2°,4°,6°and 8°group increases by (197.35±51.99)%,(207.84±37.34)%,(224.29±86.00)%,(249.62±26.67)%,(326.94±78.76)%, respectively. The retention of 8°group is significantly weaker than other groups. The extent of retention increasing also increases with the increasing of convergence angle. 8°group is the most obvious.5. Before cementation, the decreasing of retention between 2°group and 0°group, 4°group and 2°group, 6°group and 4°group, 8°group and 6°group is 5.67%, 6.98%, 13.81%, 24.68%, respectively. After cementation, the decreasing is 1.75%, 4.01%, 4.33%, 9.71%, respectively. With the same increasing of convergence angle, the larger convergence angle is, the larger decreasing extent of retention is. When the convergence angle is same, the decreasing extent of retention after cementation is significantly weaker than before cementation.Conclusion1. The retention of full crown decreases with the increasing of convergence angle both before and after cementation, there is an inverse linear relationship between them.2. With the same increasing of convergence angle, the larger convergence angle is, the more significant decreasing extent of retention is.3. After cementation, the decreasing extent of retention is significantly weaker than before cementation.4. The proportion of adhesion in the retention of full crown increases with the increasing of convergence angle.
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