On the ferrule effect and the biomechanical stability of teeth restored with cores, posts, and crowns

Abstract

An abutment for a fixed partial denture may not contain enough tooth structure, such that the abutment does not provide an adequate 'ferrule effect'. A crown or bridge dental prosthesis that is cemented onto such an abutment/s may undergo biomechanical failure. Here, the tooth, core, and post complex, on which the crown is cemented, may fracture off from the abutment, causing the crown to separate from the abutment, while the cement that bonds the crown to the tooth, core, and post complex remains intact, such that the tooth, core, and post complex remains inside the crown when the crown separates from the abutment. This article reviews the dentistry literature on the ferrule effect, and presents alternative definitions for terms such as ferrule, the ferrule effect, and the ferrule tooth structure. The article also explains how the use of a surgical operating microscope, or high magnification binocular surgical loupes of ×6-8, or greater magnification improve the ability of a dentist to assess how much ferrule tooth structure an abutment contains, compared to the use of unaided vision.

Keywords: Crown; dentistry; ferrule; microscopes; prosthodontics.

Conflict of interest statement

Conflict of Interest: None declared

Figures

Figure 1

Various cross sections at different levels of a tooth/core/post abutment complex shows how much cross-sectional area consists of gutta percha (red), post material (green), core material (blue), and natural tooth structure (yellow)

Figure 2

The cantilever on this bridge increased torque forces on the bridge and contributed to the bridge separation from the abutments. The premolar abutment shows catastrophic fracture of ferrule tooth structure approximately at the level of the abutment margin. In the mouth, the canine abutment debonded due to extensive caries

Figure 3

Longitudinal cross section of an unprepared, endodontically treated tooth (before core placement) shows the imaginary axes (in blue) of an ‘ideal’ cylinder preparation of this tooth and also shows the ferrule tooth structure, highlighted in green. Tooth structure that is not part of ferrule tooth structure is indicated as: (A) height of contour tooth structure; (B) the soffit; (C) occlusally arching tooth structure; (D) CEJ overhang material

Figure 4

A crown has fractured off at the level of the crown margin. The fracture interface combines fractures of a cement interface binding dissimilar materials, and chemical bonds within tooth, post and core materials spanning the interface between the sub-crown-margin and supra-crown-margin tooth, core, and post complex

Figure 5

This maxillary central incisor crown initially failed due to fracture of the ferrule tooth structure around the post, even though the cement luting the crown to the supra-crown-margin tooth structure did not fail. The crown, post, and core was then re-cemented as is, without ferrule tooth structure; a few months later the cement apical to the crown margin failed, while also fracturing the root