ferrule effect

[drsushant]

Introduction. Successful restoration of root-filled teeth requires an effective coronal seal, protection of the remaining tooth, restored function and acceptable aesthetics. A postretained crown may be indicated to full these requirements. However, one mode of failure of the post-restored tooth is root fracture. Therefore, the crown and post preparation design features that reduce the chance of root fracture would be advantageous. A ferrule is a metal ring or cap intended for strengthening. The word probably originates from combining the Latin for iron (ferrum) and bracelets (viriola) (Brown 1993). A dental ferrule is anencircling band of cast metal around the coronal surface of the tooth. It has been proposed that the use of a ferrule as part of the core or articial crown may be of benefit in reinforcing root￠filled teeth. A protective, or ‘ferrule effect’could occur owing to the ferrule resisting stresses such as functional lever forces, the wedging effect of tapered posts and the lateral forces exerted during the post insertion (Sorensen & Engelman1990). A literature search was conducted using the Medline database to ￠find papers that have examined the ferrule effect or made reference to it. Papers were found by searching for the word ‘ferrule’. Those pertaining to dentistry were then obtained and read to see whether they contributed in examining the ferrule effect. Some of the references used in these papers provided further articles of interest. Laboratory-based investigation of the ferrule effect. Most research investigating the ferrule effect has been conducted in the laboratory. The complexity of the oral environment prevents clear extrapolation owing to the simplicity of the experiments. Studies without use of artificial crowns. The concept of an extracoronal ‘brace’ has been proposed (Rosen 1961) and defined as a‘‘. . . subgingival collar or apron of gold which extends as far as possible beyond the gingival seat of the core and completely surrounds the perimeter of the cervical part of the tooth. It is an extension of the restored crown which, by its hugging action, prevents shattering of the root.’’ Rosen & Partida-Rivera (1996) tested this concept using7 6 extracted maxillary lateral incisors that had the crown sectioned to a level 1mm coronal to the cementoenamel junction. Half of the teeth were further prepared with a bevelled shoulder 2 mm high and 0.25 mm wide at the base, having an angle of convergence of 68. A gold casting, which represented the collar portion of a crown, was then cemented onto these teeth. Screw posts were then inserted and tightened with incremental torque until the root or post fracture occurred. The collar significantly reduced the incidence of root fracture. However, the rotational application of force in a continuous manner would be rarely present in the mouth, and implies independent movement of the post and collar. Buccal dentine thickness. The influence of dentine thickness (buccal to the post space) on the resistance to root fracture has been investigated (Tjan & Whang 1985). This study used 40 extracted maxillary central incisors divided into four groups. The control group had 1mm of remaining buccal dentine. One of the test groups also had 1mm of remaining buccal dentine and a 608 bevel. The other two test groups had 2 and 3 mm of remaining buccal dentine, and no bevel. Cast post and cores were cemented into the test teeth, but no crowns were placed. The teeth then underwent compressive loading until they failed. The incorporation of a bevel produced a core that provided a metal collar. The authors concluded from their study that the incorporation of the metal collar did not increase resistance to root fracture. No significant differences were noted between the varying dentine wall thickness, although both the groups with only 1mmof dentine all failed owing to fracture rather than cement failure. This is of particular interest as different modes of failure may be easier to manage, i.e. a loose post versus a fractured root. Modified collar. The effect of a cervical metal collar was re-examined (Barkhordar et al.1989). This study was based on that of Tjan & Whang( 1985) but used a modified collar design. Twenty extracted maxillary central incisors were divided into two groups; those with and those without a collar. Both the groups had 1mm of buccal dentine, but the test group had a 2-mm collar preparation with approximately 38 of wall taper, and a total convergence of 68. Cast post and cores were then cemented but no crowns were used. The teeth then underwent compressive loading until root fracture. Barkhordar et al. (1989) found that a metal collar significantly increased resistance to root fracture. They also observed different fracture patterns in the collared teeth compared to those without collars. The collared group predominantly underwent patterns of horizontal fracture whereas the teeth without collars mainly exhibited patterns of vertical fracture (splitting).