Results.
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.



Figure 2. An example of the mode of failure for specimen D which was similar to the other groups.

Discussion.
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.

References.

Baraban DJ (1967) Restoration of pulpless teeth. Dental   Clinics of North America 11, 633-53.
Caputo AA, Standlee JP (1976) Pins and posts why, when and how? Dental   Clinics of North America 20, 299-331.
Chapman KW, Worley JL, Fraunhofer JAV (1985) Retention of prefabricated posts   by cements and resins. Journal of Prosthetic Dentistry 54, 649-51.
Eshelman Jr EG, Sayegh FS (1983) Dowel materials and root fracture. Journal   of Prosthetic Dentistry 50, 342-4.
Goerig AC, Mueninghoff LA (1983) Management of the endodontically treated   tooth.Part1.Concept for restorative design. Journal of Prosthetic Dentistry   49, 340-5.
Halpern BG (1985) Restoration of endodontically treated teeth. A conservative   approach. Dental Clinics of North Americ a 29, 293-303.
Henry PJ (1977) Photoelastic analysis of post core restorations. Australian   Dental Journal 22, 157-9.
Hunter AJ, Flood AM (1989a) The restoration of endodontically treated teeth.   Part 2. Posts. Australian Dental Journal 34, 5-12.
HunterAJ, Flood AM (1989b) The restoration of endodontically treated teeth.   Part 3. Cores. Australian Dental Journal 34, 115-21.
Huysmans MCDNJM, Peters MCRB, Plasschaert AJM, Van der Varst BGT (1992a) Failure   characteristics of endodontically treated premolars restored with a post and   direct restorative materials. International Endodontic Journal 25,121-9.
Huysmans MCDNJM, Van Der Varst BGT, Schafer R, Peters MCRB, Plasschaert AJM,   Soltese U (1992b) Fatigue behavior of direct post and core restored premolars.   Journal of Dental Research 71, 145-50.
KafaliasMC (1969) Abutment preparation in crown and bridgework. Australian   Dental Journal 14, 1-7.
Leary JM, Aqullino SA, Svare CW (1987) An evaluation of post length with   in the elastic limits of dentine. Journal of Prosthetic Dentistry 57, 277-81.
Mattison GD (1982) Photoelatic stress analysis of cast gold endodontic posts.   Journal of Prosthetic Dentistry 48, 407-11.
Mckerracher PW (1981) Rational restoration of endodontically treated teeth.   Part I. Principles, techniques and materials. Australian Dental Journal   26, 205-8.
Patel A, Gutteridge DL (1996) An in vitro investigation of cast post and   partial core design. Journal of Dentistry 24, 281-7.
Perel ML, Muroff FI (1972) Clinical criteria for post and cores. Journal   of Prosthetic Dentistry 28, 405-11.
Russell MD, Masood M, Cunningham L (1997) The behaviour of post-retained   core materials supported by coronal tooth structure in vitro. International   Endodontic Journal 30, 408-12.
Sokol DJ (1984) Effective use of current core post concepts. Journal   of Prosthetic Dentistry 52, 231-4.
Stokes AN (1987) Post crowns: a review. International Endodontic Journal   20, 1-7.
Tjan AH, Whang SB (1985) Resistance to root fracture of dowel channels with   various thickness of buccal dentine walls. Journal of Prosthetic Dentistry   53, 496-500.
Trabert KC, Caputo AA, Abou-Rass M (1978) Tooth fracture: a comparison of   endodontic and restorative treatments. Journal of Endodontics 4, 341-6.
Turner CH (1982) The retention of dental posts. Journal of Dentistry10,   154- 5.
Volwiler RA, Nicholls JI, Harrington GW (1989) A comparison of three core   buildup materials used in conjunction with two post systems in endodontically   treated anterior teeth. Journal of Endodontics 15, 355-61.
Young HM, Shen C, Maryniuk GA (1985) Retention of cast posts relative to   cement selection. Quintessence International 5, 357-60.