Modified collar and buccal dentine thickness

Modified collar and buccal dentine thickness.
The effectiveness of a cervical collar with a thicker buccal dentine wall was investigated (Joseph & Ramachandran 1990). Forty extracted maxillary central incisors were divided into four groups; 1and 2 mm of buccal dentine, with and without 608 bevels. The subsequent testing w as similar to that used by Tjan & Whang (1985). The authors concluded that the use of a 2-mm collar increased the resistance of the tooth to root fracture. The authors noted there was no significant difference between the mean failure load of the two groups with cervical collars, regardless of the amount of remaining buccal dentine.

Collar with resin teeth.
The effect of a cervical collar was investigated using photoelastic stress analysis on resin teeth resembling canines (Loney et al.1990). Resinwas necessary to conduct the photoelastic analysis but it must be emphasized that resin has a stiffness between a sixth to a quarter that of dentine. Eight teeth were duplicated from a mould of a master tooth. The master tooth was prepared with 2- mm incisal reduction,1.5-mm buccal reduction and an angled shoulder, and a 0.5-mm lingual chamfer. The master tooth was then reduced another 4 mm to form a flat plane for the core. Four of the resin teeth replicated this preparation; the other four had a 1.5-mm bevel around the flat plane. Cast post and cores were cemented in all the teeth. Crowns were not placed. Each tooth was then placed under a 400-gload at 1528 to its long axis, sectioned and then examined at five points. The shear stresses were less varied in the collared group, however, the teeth without collars exhibited significantly lower shear stresses at the three of the five points examined. From their findings, Loney et al. (1990) concluded that the used of a core collar did not produce a ferrule effect.

Bonded posts and broken down teeth.
The influence of a metal collar on the structurally compromised teeth, with and without resin reinforcement has been examined (Saupe et al. 1996). Forty extracted maxillary central incisors were divided into two categories: resin reinforced and non-reinforced. These were further divided into roots with and without collars. The collar design was similar to that used by Barkhordar et al. (1989), having a 2-mm collar with 38 of taper on the root wall. All the teeth were prepared to simulate structurally compromised roots, with only 0.5-0.75 mm of dentine at the cementoenamel junction. The resin-reinforced roots were prepared using a visible light-cured resin composite bonded to the internal root surface. Cast post and cores were cemented in all teeth using a resin cement. All the other in vitro studies (without crowns) reviewed used zinc phosphate as a luting cement. Prior to loading the teeth, the roots were coated with rubber to simulate the periodontal ligament, then embedded in resin. The teeth were loaded until failure, which was detected by a sudden release of the load on the test tooth. Resin reinforcement significantly increased the resistance to failure, but the use of the collar in the resin reinforced group was found to be of no benefit.

Torsion.
The use of a cervical collar on a post was found to be of particular benefit in increasing the resistance of the post and core to torsional forces (Hemmings et al. 1991).Where a very tapered bevel of 458 was used, an increase in resistance to torsional forces 13 times that of the control group was seen. The study did not use crowns over the cores. The authors made the important point that a metal cervical collar may be aesthetically unacceptable where the metal is visible at the gingival margin.

Summary of studies without use of artificial crowns.
These studies give mixed results as to the effectiveness of a cervical collar in producing a ferrule effect (Fig.1). Even so, the results of these studies should be interpreted with caution, as none of them used artificial crowns on the cores. This was noted by Loney et al. (1990), who felt that such tests were still valid, as they helped to determine an optimal post and core form. They believed this would be of importance in crowns that had lost cement or become loose, resulting in the post and core being placed under increased forces.
Figure 1. The provision of a cervical collar as part of the core without an artificial crown. Although a beveled collar/ferrule has been depicted, a butt finish or chamfer could also be used.

Despite the confusion over the usefulness of a cervical collar, cores that are made with a ferrule create technical problems. The design requires increased casting expansion at the ferruled part of the core, and a decrease in casting expansion in the post. In a comparison of casting techniques, it was demonstrated that this could not be achieved to a clinically acceptable level, despite the authors trying five different casting techniques (Campagni et al. 1993). Their results raise concern about the findings in the studies which didemploya core with a ferrule, as the technical problems encountered with casting may have influenced the outcomes of testing.