Results - Discussion - References.
The loads at which teeth failed are shown in Table 3.The load was similar in groups A, C and D. Group A had a mean of 209.20 N (SD ј 46.20), compared with 238 N for groups C and D (SD ј 57.51 and 43.43, respectively). For Group B, however, the load at failure was considerably higher at 271 N (SD ј 79.99). Statistical analysis revealed no significant differences between groups A, C and D, but the failure load for group B was statistically significantly higher when compared to group A (P ј 0.0239).There was no statistically significant difference between forces required to fracture groups B and C or groups B and D. The specimens with 3-mmretained coronal dentine outperformed the other groups.
The most prevalent mode of failure in all the groups was typically a fracture line, originating at the centre of the root surface, passing up in a lingual direction (down in a buccal direction) with partial decementation of the post (Fig. 2).Two cases in group B showed fracture of the die stone base only. In one case in group C, there was a complete avulsion of the tooth after fracture of the die stone base.
Table 3. Failure loads for each group in Newtons.Discussion.
Figure 2. An example of the mode of failure for specimen D which was similar to the other groups.
It was necessary to use three different tooth types in each experimental group of the study because problems were encountered in collecting similar teeth. There was no statistically significant difference between the mean buccolingual and mesiodistal dimensions in each group. Teeth were stored in 5% antifungal thymol agent because they had to be stored for an extended period as collection proceeded (Leary et al.1987). Zinc phosphate was considered an appropriate material for cementation of posts on the basis of previous studies (Chapman et al. 1985,Young et al.1985, Russell et al.1997).
The post length of 9 mm selected in present study was similar to that used by Patel & Gutteridge (1996), the 7.5 mm used by Volwiler et al. (1989) and the 8 mm used by Huysmans et al. (1992 a, b). The load was applied at 1308 from the horizontal with a steel rod, as previous studies have shown this to be the most clinically comparable angle of loading in anterior teeth (Eshelman & Sayegh 1983, Volwiler et al.1989, Patel & Gutteridge1996). A variety of cross-head speeds have been employed by other researchers but this does not seem to be a crucial factor as their results are largely in agreement. They range from 0.5 (Leary et al. 1987) to 76.2 mm min (Eshelman & Sayegh1983).
Preservation of sound coronal dentine tooth structure when restoring the root-filled tooth has been recommended, but no previous work has examined the fracture resistance of teeth restored with cast post and partial core designs supported by different heights of coronal tooth structure.
It was clear that specimens in group B, where the remaining buccal dentine core was 3 mm in height, fractured at a higher force than specimens in the other groups. This was probably due to the fact that incorporating sound dentine as part of the core increased cast core retention and reduced stress transmission to the root. Retained coronal dentine can also increase post length, improving retention and give resistance to rotation of the post and core restoration (Turner 1982). The findings of the current study are in agreement with Henry (1977) and Hunter & Flood (1989a). However, the results of the present study indicated that retaining dentine greater than 3 mm in height had no additional benefit in terms of strength, possibly as increasing the height of retained dentine without increasing the width at the base leaves the tooth susceptible to fracture, as seen with unsupported cusps in root-treated premolar and molar teeth.
It is common to find retained buccal dentine when preparing a post and partial core restoration for an anterior tooth, rather than retained palatal dentine, which is often sacrificed in the preparation of the access cavity. Thin portions of retained dentine make laboratory construction difficult, as the resulting thin section of die stone is prone to fracture. Further work should take into account different widths of retaining dentine and ensure that specimens are subjected to a range of forces found to occur in the mouth. It is also important to be aware of the development of new materials which are having a profound effect on clinical practice.
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