| Objective:The objective of this study was to evaluate the fracture strength and fracture patterns of maxillary incisors which had been endodontically treated and then restored with different post-core and crown systems. The results are expected to providing useful experimental basis for clinical post-core design.Method:Twenty eight human maxillary incisors which were extracted for their serious periodontal disease were used in this study. They were selected for their integrated anatomical and similar configurations. Then they were divided into 4 groups randomly, each group included 7 samples. After root canal treatment finished for each tooth, different patterns of artificial teeth fracture were made and grouped as follows:Group Al:each tooth tissue were cut off at 8mm level above the cement-enamel junction (CEJ), supposed to equal the level about 2mm under cutting edges, then be restored with Ni-Cr alloy post-core; Group A2:each tooth tissue were cut off at 8mm level above the CEJ, supposed to equal the level about 2mm under cutting edges, then be restored with Au-Pd alloy post-core; Group B1:the tissue were cut off at 2mm above the CEJ, then restored with Ni-Cr alloy post-core; Group B2:the tissue were cut off at 2mm above the CEJ, then restored with Au-Pd alloy post-core. All the post-cores were cemented with ZPC, and then prepared for crowns, crown edges were all designed at 0.5mm above the labial CEJ and had lmm shoulder. All the Ni-Cr alloy crowns were fabricated and cemented with ZPC then. Each specimen was embedded in acrylic resin 2 mm below the CEJ and then be fixed with a special jig in CMT7104 universal load-testing machine. A compressive load was applied at the palatal junction of incisor third and middle third of crown with 135-degree angle to the long axis of the tooth, and kept on a speed of 1.0mm/min. Data of fracture strength were recorded just samples fracture occurred, fracture patterns were also recorded soon after.Results:1 Twenty eight samples failed in the test. A1 group fractures occurred at 614.79±39.03N, group A2:438.73±28.20N, group B1:430.13±32.99N, group B2:411.94±47.46N.2 Factorial analysis was used to determine if there were significant differences among the four groups. The results showed the difference was significant with material factor (p<0.05); and the difference was also significant with ferrule factor (p<0.05). There was an interactive relationship between these two factors. T-test analysis indicated that significant differences were exited between groupA1, A2 and groupA1, B1 (P<0.05). There was no significant difference between other two groups (P>0.05).3 The fracture types of specimens were classified as restorable type which the fracture occurred above cervical third of root, and destructive type which occurred below the cervical third of root. In group Al, two teeth showed restorable root fracture and five teeth showed destructive root fracture; Group A2, four teeth showed restorable root fracture and three teeth showed destructive root fracture; Group B1, one teeth showed restorable root fracture and six teeth showed destructive root fracture; Group B2, three teeth showed restorable root fracture and fore teeth showed destructive root fracture.4 Fisher's exact test showed that the incidence of destructive root fracture among each groups had no significantly difference (P>0.05) in statistically.Conclusions:1 Different post-core material had an effect on tooth fracture strength. The harder restored material can provide more protection to the tooth tissue when it meets great strength. But most of the fracture types of specimens are destructive type.2 The failed strength value in two different materials used in this study are higher than normal range of chewing force(30-300N), means both could meet the needs of patient daily practice.3 To make use of the post-core material which has a low elastic modulus, can reduce the incidence of destructive root fracture on a certain extent.4 The ferrule also had influence on fracture strength. Preserving health teeth tissue as much as possible could strengthen tooth against fracture occurring.5 There was an interactive relationship between different post-core materials and ferrule. |
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