BACKGROUND: The residual root of anterior teeth are usually repaired by means of post and cores plus porcelain fused metal crown in clinic, and the retention force is mainly provided by the post because of the deletion of crown, so the retention force provided by the post is directly correlated with the long-term effect of repair.OBJECTIVE: To analyze the changes of retention of cast metal post and core restorations with different convergence angles, and investigate the influence of convergence angles on retention.DESIGN: A repeated observational measurement.SETTING: Department of Prosthodontics, West China School of Stomatology, Sichuan University.MATERIALS: The experiments were carried out in the building of high polymer material of Sichuan University in May 2005. Fifty-four simulated tooth roots were manufactured by the numerical control machine tools.They were divided into six groups according to the convergence angles of post being 0°, 3.93°, 5.71°, 7.48°, 11.31° and 14.71°, with nine specimens in each group.METHODS: ① Manufacture of the standard specimens: The tooth roots with the convergence angles of 0°,3.93°, 5.71°, 7.48°, 11.31° and 14.71° were prepared with the digital control precision machine. ② Manufacture of the cast post and cores: The post and core wax patterns of the specimens were made in the condition similar to the clinical practice. The post and cores were cast and tried in, necessary grinding was made to ensure the adoption of the posts and the specimens. Then the posts were luted to the specimens with ZPC after the posts and the simulated root canals having een cleaned by alcohol. The posts and the specimens were pressed until the ZPC solidified. ③ Retention Force test: After having been dipped in physiological saline for 24 hours, the bonded castings and specimens were subjected to a tensile testing at the drawing speed of 10 mm per minute.The retention force of post was the load recorded when the post was pulled out from the specimen. ④ Planar bonding strength test: We cut a PMMA stick into the same little cylinders and made angular milling on the flank of the cylinders. The angle between the milled bevel and the central axis of the cylinder was controlled to 0°, 1.9°, 3°, 6°, 9°, 12° and 15°. Afterwards we cut the brass into pieces. Eight wafers were cast with the same diameter of 8 mm and thickness of 1 mm. With the same procedure of bonding the posts to specimens, the brass pieces were bonded to the cylinder bevels and the wafers were adhered to the upside of some unmachinized cylinders.Immerged in physiological saline for 24 hours, their bonding force were tested at the speed of 10 mm per minute, and the load was recorded when the pieces and wafers were separated from the cylinders. ⑤ Calculation of the frictional and interlocking forces: As the post can be regarded as a frustum, the bonding surface area can be calculated with the formula calculating the flank area of frustum, and the area of the occlusal bonding surface is the sum of the annulus part of the root surface and the circular part of the post tip. Some regressions were made to the retention force, frictional strength, bonding strength per unit area, retention force per unit area and frictional force per unit area to convergence angles.MAIN OUTCOME MEASURES: ① Bonding strength of different convergence angles; ② Influence of different convergence angles on the area and bonding strength of the post; ③ Retention force and frictional force of posts with different convergence angles; ④ The results of the one-way analysis of variance (ANOVA) of bonding strength, retention force and frictional force per unit area.RESULTS: ① The bonding strength of different convergence angles: It was 0.309 0 N/mm2 for the occlusal plane, 0.128 3 N/mm2 for the axial plane with a convergence angle of 0°, 0.108 7 N/mm2 for the 1.9°axial plane, 0.107 2 N/mm2 for the 3°axial plane, 0.084 9 N/mm2 for the 6°axial plane, 0.056 7 N/mm2 for the 9°axial plane, 0.046 3 N/mm2 for the 12°axial plane, 0.027 4 N/mm2 for the 15°axial plane. ② Bonding strength and bonding area of posts with different convergence angles: The post with a 0°convergence angle had a total post area of 108.047 mm2 and bonding strength of 19.041 N; The most tapered post with 14.71°had a total post area of 90.245 mm2 and bonding strength of 5.131 N. ③ The retention force of cast post and the bonding strength of ZPC declined with the increasing of convergence angles of the post. For the parallel post (convergence angle=0°), the retention force was 321.60 N, the frictional force was 302.559 N, retention force per unit area was 2.976 N/mm2,frictional strength was 3.885 N/mm2; for the most tapered post (convergence angle=14.71°), the retention force was 9.93 N, frictional force was 54.799 N, retention force per unit area was 0.664 N/mm2, frictional force was 0.681 N/mm2. ④ A one-way ANOVA showed that there were significant differences in the bonding strength, retention force and frictional force per unit area among the groups (P ＜ 0.05).CONCLUSION: The retention force, frictional force and the bonding strength of ZPC decrease with the increasing of convergence angles of posts, so the convergence angles of posts should be reduced as much as possible to obtain greater retention.%背景:临床上通常采用桩核加金瓷冠的方式修复前牙残根,因牙冠的缺失,修复体的固位力主要由根桩提供,因此根桩所提供的固位力大小直接影响到远期修复效果.目的:分析不同聚合度铸造金属桩的固位力的变化,探讨桩聚合度对固位力的影响.设计:重复观察测量.单位:四川大学华西口腔医学院修复科.材料:实验于2005-05在四川大学高分子材料楼完成.制备标准尺寸的有机玻璃,模拟上中切牙牙根试件54个,分为桩道聚合度0°,3.93°,5.71°,7.48°,11.31°,14.71°组,9个/组.方法:①试件的制备:用数控精密机床分别进行聚合度为0°,3.93°,5.71°,7.48°,11.31°,14.71°的桩道预备.②黏接桩核的铸造:按临床操作制作桩核,确保试件与桩密合.在铸造金属核上磨一小洞,作为拉伸时拉钩穿过的位置.试件桩道、根面、铸造桩核用乙醇处理后吹干,磷酸锌水门汀黏固,黏固时加压直至黏接剂凝固.③桩固位力实验:将试件用生理盐水浸泡24 h后进行拉伸加载测试,加载速度10 mm/min,铸造桩核被拉出时的载荷即为桩核的固位力.④平面黏接力测试:制备有机玻璃圆柱,圆柱侧面上制备与中心长轴成0°,1.9°,3°,6°,9°,12°,15°斜面,制备矩形铜片.再铸造8个直径为8 mm、厚1mm的小圆片.按临床操作情况将圆柱斜面、铜片喷沙面、铸造圆片清洁吹干后磷酸锌水门汀黏固,浸泡于生理盐水中24 h后进行拉伸加载测试,加载速度10 mm/min,铜片或铸造圆片从圆柱上脱落时的载荷即为不同聚合度的黏接力.⑤摩擦力和约束力的计算:桩可近似看作圆台,轴面黏接面积按圆台侧面积公式计算,桩与牙根的牙合面黏接面积为牙根面面积与桩道底面积之和.固位力、摩阻力、单位面积黏接力、单位面积固位力、单位面积摩阻力对聚合度之间进行回归分析.主要观察指标:①不同聚合度的黏接力测定.②不同聚合度对桩的面积和黏接力的影响.③不同聚合度桩核的固位力和摩阻力测定.④单位面积黏接力、固位力、摩阻力的单因素方差分析结果. 结果:①不同聚合度的黏接力测定:牙合面黏接强度为0.3090N/mm2.不同聚合度时的轴面黏接强度为0°时0.128 3 N/mm2,1.9°时0.108 7 N/mm2,3°时0.107 2 N/mm2,6°时0.084 9 N/mm2,9°时0.056 7 N/mm2,12°时0.046 3 N/mm2,15°时0.027 4 N/mm2.②不同聚合度对桩的面积和黏接力的影响:0°时总面积是108.047 mm2,总黏接力是19.041 N;14.71°时总面积是90.245 mm2,总黏接力是5.131 N.③不同聚合度桩核的固位力和摩阻力测定:0°时固位力为321.60 N,摩阻力为302.559 N,单位面积固位力为2.976 N/m一,单位面积摩阻力为3.885 N/mm2;14.71°时固位力为59.93 N,摩阻力为54.799 N,单位面积固位力为0.664 N/mm2,单位面积摩阻力为0.681 N/mm2.④单位面积黏接力、固位力、摩阻力的单因素方差分析结果:对单位面积黏接力、固位力和摩阻力的单因素方差分析结果概率P＜0.05,各组间差异有显著性意义.结论:桩固位力、摩阻力、磷酸锌黏接力随聚合度的增大而减小,因此若要获得较大的固位力,应尽可能减少桩的聚合度.
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