Journal of Endodontics Research - http://endodonticsjournal.com
An in vitro investigation into the effects of retained coronal dentine on the strength of a tooth restored with a cemented post and partial core restoration
http://endodonticsjournal.com/articles/116/1/An-in-vitro-investigation-into-the-effects-of-retained-coronal-dentine-on-the-strength-of-a-tooth-restored-with-a-cemented-post-and-partial-core-restoration/Page1.html
By JofER editor
Published on 10/27/2008
 
A. Al-Wahadni & D. L. Gutteridge
Faculty of Dentistry, Jordan University of Science and Technology, Irbid, Jordan.
Division of Restorative Dentistry, Leeds Dental Institute, Leeds, UK .


Aim.
This study aimed to examine the fracture resistance of teeth restored with cast post and partial cores supported by different heights of coronal tooth structure.

Conclusions.
  1. In this in vitro study, retained coronal buccal dentine improved fracture resistance of teeth restored with partial post and cores when compared to teeth without retained buccal dentine.
  2. Teeth with retained buccal dentine of 3 mm had significantly higher resistance to fracture compared to control teeth with post and full cores, whereas longer lengths of retained buccal dentine did not produce statistically significant improvements.
  3. Crack formation and propagation were evident midway between buccal and lingual root margin and the junction of partial and dentine cores.

Introduction.
A. Al-Wahadni & D. L. Gutteridge
Faculty of Dentistry, Jordan University of Science and Technology, Irbid, Jordan.
Division of Restorative Dentistry, Leeds Dental Institute, Leeds, UK .


Introduction.
Posts are normally used to aid core retention. A direct relationship exists between the quantity of sound tooth structure and tooth strength and care is required when preparing teeth to accept posts (Hunter & Flood 1989a,b). The emphasis of many restorative techniques is now on the preservation of tooth structure, wherever possible. Following root canal treatment, where the provision of a post and core restoration may be indicated, a successful post restoration should minimize loss of tooth structure, reduce unnecessary stresses and maintain the seal of the root filling (Sokol1984).
The importance of retaining the maximum amount of sound dentine when restoring root-filled teeth has been emphasized (Trabert et al. 1978, Mattison 1982). Caputo & Standlee (1976) and Tjan & Whang (1985) made recommendations on the amount of sound dentine that should remain around a post. The importance of retaining sound dentine in the distribution of post stress to the remaining root has also been emphasized (Goerig & Mueninghoff1983, Halpern1985, Stokes1987).
Kafalias (1969) also recommended retaining self-supported coronal dentine as it produced an irregular joint between the tooth and casting which would increase retention and resistance to dislodgement. Perel &Muroff (1972) reported that dentine should not be sacrificed to be replaced by a cast core. Baraban (1967) recommended that remaining dentine should be well supported, and criticized the practice of removing coronal tooth structure to the level of the gingival margin prior to the provision of post and core restorations.
Although recommendations based on clinical experience have been made, little scientific evidence has been reported on the advantages of retaining sound coronal dentine. Ina photoelastic study, Henry (1977) found that stress transmission to the root became more favourable as coronal dentine was retained and stress concentration at the shoulder reduced. Hunter & Flood (1989a,b) agreed 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, improve retention and give resistance to rotation of the post and core (Mckerracher 1981). However, the effect on the strength of the restored tooth is more difficult to assess. In fact, Patel & Gutteridge (1996) concluded that retained coronal dentine would not strengthen a tooth restored with a cast post and partial core, reporting that retained buccal coronal dentine significantly reduced the fracture strength of samples in vitro. In conclusion, most authors agree that retaining sound coronal tooth structure has benefits in terms of retention and improvements in stress distribution, but the effect of preserving coronal dentine on the strength of the teeth restored with post and core restorations requires further investigation. The aims of this study were:
  1. To investigate the effect of retained sound coronal dentine on the strength of teeth restored with cemented post and partial core restorations under direct loading.
  2. To investigate the effect of different heights of residual coronal dentine on the strength of the teeth restored with a cemented post and partial core restoration.
  3. To observe the pattern of fracture and mode of failure of the teeth and restorations.

Materials and methods.
Sixty single-rooted teeth, including maxillary incisors, maxillary and mandibular canines and single-rooted maxillary and mandibular premolar teeth were collected from a number of sources and stored in a solution of water and 5% thymol at room temperature. They were examined under 2x magnification to ensure that they were free of caries, restorations or cracks, which might affect their resistance to experimental loading. Mesiodistal and buccolingual dimensions were recorded at the cervical margin with the aid of a digital caliper (Anglia, STMicroelectronics, Edinburgh, UK) to ensure that each experimental group would contain teeth of a similar size distribution. Forty teeth were selected from the initial 60 teeth and allocated to four groups (Fig.1).
  • Group A: The control group. Ten posts and cores were cemented in dowel channels with no retained buccal dentine.
  • Group B: Ten posts and cores were cemented in dowel channels with 3 mm of retained buccal dentine.
  • Group C: Ten posts and cores were cemented in dowel channels with 4 mm of retained buccal dentine.
  • Group D: Ten posts and cores were cemented in dowel channels with 5 mm of retained buccal dentine.
Each group contained 10 teeth with a similar distribution of mesiodistal and buccolingual dimensions. The measurements are given in Tables 1and 2.

Figure 1. The completed specimens.

The completed specimens

Table 1. Mesiodistal dimensions (mm) of sample teeth.

Mesiodistal dimensions of sample teeth

Table 2. Buccolingual dimensions (mm) of sample teeth.

Buccolingual dimensions of sample teeth

Teeth in group A were sectioned15 mm from the apex, 2 mm coronal to the amelo-cemental junction, at a level corresponding to the clinical gingival margin (Turner 1982, Patel & Gutteridge 1996). In groups B, C and D, the teeth were sectioned18, 19 and 20 mm, respectively, from the apex. Post channels were prepared using the Parapost system (Whaledent Int., NY, USA) using the black1.5-mmdrill under constant irrigation with saline, to create a post space of 9 mm in group A, with the root face at right angles to the channel and a potential post length of12,13 and14 mm in groups B, C and D, respectively. As differing tooth types were used in each group, it was felt that retaining 3, 4 and 5 mm of coronal dentine in the experimental groups would be most likely to demonstrate significant effects on the strength of the tooth, and on this occasion, smaller 1- and 2-mm preparations were not included.
A shoulder was prepared around the most coronal retaining dentine in groups B, C and D. To facilitate this, a jig was waxed up on a smooth, black, plastic Parapost to produce a flat central button, covering an area of root face dentine, 1.5 mm wide, buccally and lingually. Mesially and distally, the width of the dentine core covered was reduced to1mmto accommodate the oval anatomical outline of the root face. These dimensions were selected to produce a clinically realistic shoulder width of approximately1mm. Once cast, the jig was positioned in the prepared posthole and the shoulder prepared around it, ensuring that the amount of retaining coronal dentine was standardized. The shoulder was prepared with a depth limited diamond bur and finished with a tapered, plain cut tungsten carbide bur, sectioned to leave 3, 4 or 5 mm of cutting length. An antirotational notch was cut on the lingual aspect of teeth in all the groups.
Each specimen was embedded indie stone (Begostone, BEGO Bremer Goldschl gereiaeilh, Bremen, Germany), 2 mm from the shoulder in groups B, C and D and 2 mm from the sectioned tooth surface in group A, contained within a specially made stainless steel tube. The same tube was used to hold specimen under loading. A surveyor (Degussa, AG, Geschaftsbereich Dental, Frankfurt I, Germany)was used to align the post channel with the outside of the steel tube by ensuring that the analysing rod positioned within the post hole was parallel to the outside of the tube.
Post and cores were then waxed up for each tooth using black, laboratory burn-out posts and a standard mould for waxing of the core section. This was a silicone index of an initial wax-up of a tooth in group A. After casting with nickel-chrome alloy (Heraeus, Heraenium NA, Heraeus Kulzer GmbH, Hanau, Germany) and finishing, the post and cores were cemented using zinc phosphate cement (Elite cement 100, GC corporation, Tokyo, Japan), spun down the channels with a spiral paste filler (Dentsply Maillefer Instruments, SA, Ballaigues, Switzerland) prior to seating the castings firmly with digital pressure for 3 min. Specimens were stored in 100% humidity at room temperature and tested 1week later (Tjan & Whang 1985). As the study was designed to test the strength of a tooth restored with a partial post and core where the height of retained coronal dentine was the only variable, specimen preparation was simplified by omitting the construction of a crown over the post and core (Volwiler et al.1989, Patel & Gutteridge1996). In the test groups B, C and D, cementation of a crown would have introduced a ferrule effect as compared to the control group A and this would have made interpretation of the results more difficult.
The stainless steel tube containing the specimen was mounted in a specially designed retaining arm of an Instron universal testing machine (Instron1195, Instron Limited, Buckinghamshire, UK). The load was applied at1308 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 & Gutteridge 1996). The point of load application was standardized by the inclusion of a small mark in the silicone mould used to construct the core. The cross-head speed was 10 mm min_1, and the specimens were loaded to failure with a chart speed of 100 m min. Results a represented in the form of force at failure (N) from the force defection curve. Subsequently, a representative specimen from each group was selected and photographed to record the pattern of coronal fracture.
Comparison of results from the four test groups was carried out using one-way analysis of variation (anova) and Fisher’s PLSD (Protected Least Significant Difference) test to demonstrate differences between pairs of groups.

Results - Discussion - References.
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.

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.

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.

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