Ferrule Effect – Studies with artificial crowns

Sixty extracted maxillary incisors were used to investigate the effect of six different tooth preparation designs on the resistance to failure (Sorensen & Engelman 1990). All the teeth had post and cores cemented, over which a crown was then cemented. Each tooth was loaded at1308 to its long a fixis until it failed which encompassed either displacement of the crown or post, or fracture of the root or post.
One design was a1308 sloped shoulder from the base of the core to the margin. Whereas this design has a ferrule between the crown and margins and also between the core and tooth, it did not increase resistance to failure or fracture.
Two groups had a 90 shoulder without any coronal dentinal extension, and one of these groups also had a 1-mm-wide608bevel finish line. The placement of a bevel at the crown margin in these groups did not increase fracture resistance. This was in accordance with thefindings of Tjan &Whang (1985).
Two groups had a significantly different mean failure load compared with the other four groups. These two designs had a 908 shoulder and a 1-mm-wide 608 bevel finish line. One of the groups had a1-mm coronal dentinal extension. The other hada2-mm-wide coronal dentinal extension and a 1-mm-wide 60 contra bevel at the tooth-core junction. As failure thres hold was not significantly different between these two groups, it may be inferred that the contra bevel is of no benefit. Furthermore, and most importantly, the coronal extension of dentine above the shoulder is the design feature which increases the resistance to failure, and thus imparts a ferrule effect.
Sorensen & Engelman (1990) advised that as much coronal tooth as possible should be preserved, and a butt-joint margin between the core and tooth be used, i.e. minimal taper. They went on to suggest that the ferrule effect be defined as ‘‘ . . . a 3608 metal collar of the crown surrounding the parallel walls of the dentine extending coronal to the shoulder of the preparation. The result is an elevation in resistance form of the crown from the extension of dentinal tooth structure.’’
An investigation of root fracture related to the post selection and crown design also considered the influence of the ferrule effect (Milot & Stein1992). Forty-eight resin maxillary central incisors were divided into three groups. One of the groups had a cast post and core, the other two groups used a direct post system with a cermet cement core. Half the teeth in each group had a 1-mm concave bevel apical to the margin. Although the dimensions of the tooth preparations were not disclosed, the illustrations indicated that finall teeth there was dentine extending coronal to the margin. Therefore, the ferrule effect would have been expected in both the control and test groups in this experiment.
Crowns were cemented on all the teeth, which were then loaded with a compressive force until they fractured. The force was applied at 1208 to the long axis of each tooth. The teeth with the bevel had an increased resistance to root fracture. Milot & Stein (1992) proposed that the bevel produced a ferrule effect and the gingival extension of the metal collar provided support at the point of leverage. The use of a cermet/glass-ionomer cement as a core material, as in this study, is not recommended owing to its lack of strength (McLean 1998). The authors noted some cracking and crazing of the cement cores prior to crown placement, but did not report any significant differences in fracture resistance between the teeth with cast and cermet cement cores.

Ferrule length.
Based on the effective ferrule demonstrated by Sorensen & Engelman (1990), the influence of ferrule length on resistance to preliminary failure was investigated (Libman & Nicholls 1995). The authors defined preliminary failure as the propagation of a crack in or around the luting cement of the crown. Twenty-five extracted maxillary central incisors were split into five groups; a control group and four test groups. The test groups had ferrule lengths of 0.5,1,1.5 and 2 mm. The teeth were prepared with1- mm-wide shoulders. The test teeth had cast post and cores cemented and the control group did not. All the teeth were restored with cast crowns. The teeth were subjected to cyclic loading until preliminary failure was detected, using a strain gauge. The control group and the teeth with 1.5- and 2-mm ferrules were found to be significantly better than the teeth with 0.5- and1- mm ferrules in resistance to preliminary failure. The authors concluded that1.5 mm should be the minimum ferrule length when restoring a root filled maxillary central incisor with a post- and core-retained crown.
Libman & Nicholls (1995) paid particular attention to the shortcomings of some methods of in vitro testing. The use of cyclic loading in their study was based upon the rationale that failure within the dental complex was associated with repeated fatigue loads rather than a single fracture-inducing load. The authors also acknowledged that their study did not duplicate the deformability of the periodontal ligament. Furthermore, the ferrule height in the study was constant around the circumference of the tooth, which may differ from the clinical situation where the finish line follows the morphology of the interproximal gingivae.
The conclusions of Libman & Nicholls (1995) were based on rigid parameters (Gegauff 2000). Their study used short (8-mmlong) and narrow (1.25-mmdiameter) posts and there was no simulation of periodontal support. Their finding of a1.5-mm minimum effective ferrule may not be the case clinically.
Cyclic loading was adopted in an investigation of the influence of post and ferrule length on resistance to failure (Isidor et al.1999). Using 9 0 extracted bovine teeth, they studied three post lengths (5, 7.5 and 10 mm) and two ferrule lengths (1.25 and 2.5 mm). Bovine teeth were selected in an attempt to reduce variability. The teeth were restored with prefabricated titanium posts and resin composite cores. The teeth were all crowned, then subjected to cyclic loading until the crown or post dislodged or the post or root fractured. The fracture resistance of the specimens increased with the ferrule length, but was not enhanced by increased post length. The authors note that this is of particular significance in re-evaluating studies which have investigated post length and design without a crown in their experiment methodology. Furthermore, it would suggest that an increase in post length, to achieve an increase in retention does not decrease resistance to failure.

Type of post and core.
Most of the studies investigating the ferrule effect have used cast post and cores. Milot & Stein (1992) used direct posts with cermet cores, but found no significant difference in fracture resistance compared with the teeth they restored with cast post and cores. The posts were all cemented with zinc phosphate cement. Al-Hazaimeh & Gutteridge (2001) investigated the effect of a ferrule on central incisors when a direct post and composite resin core was used. The prefabricated posts and composite resin cores were placed in 20 central incisor teeth. The posts and crowns were luted with a resin cement. Ten of the teeth had a 2-mm ferrule, the others had no ferrule. Analysis following compressive loading until failure demonstrated no significant difference between the two groups. The authors proposed that the strength afforded by the resin may have masked any benefit provided by the ferrule.
The authors observed a high mean failure load for both groups, which they attributed to the resin luting cement. Although there was no difference between the two groups’resistance to failure, the modes of failure differed. The group with a ferrule underwent oblique fracture, whereas the group without a ferrule largely underwent vertical root fracture.