ABSTRACT
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.
ABSTRACT
Fifty-four simulated tooth roots made of polymethacrylate (PMMA) were divided into six groups according to the root canal convergence angles of 0 degrees, 3.93 degrees, 5.71 degrees, 7.48 degrees, 11.31 degrees, 14.71 degrees. Cast post and cores were manufactured and cemented with Zinc phosphate cement (ZP). The retention capacities of the posts were tested. The bonding strength of ZP at different convergence angles were tested with some casted wafers, and linear regression was made. So the effect of interlocking and the proportion could be calculated. With these results, we made some curvilinear regressions. The results showed that the retention of cast post declined and the bonding strength of ZP fell with the increase of convergence angles. There were significant differences in the bonding strength and retention among groups, P<0.05. The experiment indicates that the dentists should reduce the convergence angles of posts as much as possible to obtain greater retention.