The Ferrule Effect in Molars

The dental ferrule refers to a circumferential band of dentin of at least 1-2 mm of dentin coronal to the margin of the preparation for a full crown (Fig. 7.3). It has been suggested that the presence of a ferrule may reinforce endodontically treated teeth, the “ferrule effect,” especially in teeth restored with cast post and core [38]. It was also observed that an adequate ferrule lowers the impact of the other factors (post and core system, luting agents, or crown material) on the survival of endodon- tically restored teeth [39].

  • 7 Considerations for the Restoration of Endodontically Treated Molars
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Most research was done with single-rooted teeth and comparatively little data are available on molars. In the two major literature reviews that focus on the importance of the ferrule effect, one did not mention a single work devoted to molars [38] and the other reported the critical need for more clinical trials that investigate an effect on molars [39 ] . Only one clinical study focused specifically on endodontically treated, restored molars, and concluded that the amount of tooth structure available for ferrule (evaluated retrospectively from bitewing radiographs) may not be a significant prognostic parameter [40]. However, two other clinical studies included molars [41, 42] and both reported improved survival for teeth with ferrule (93 % at 3 years [41] and 98 % at 5 years [42]) compared to teeth without ferrule (74 % [41], 93 % [42]). Notably, in these two studies molars represented less than half of the teeth and a minority of the failures [42].

Therefore, while from a mechanical standpoint a ferrule is logical and recommended whenever possible, it is somewhat speculative to generalize and recommend ferrules for all molars based on these data alone. Moreover, it should be kept in mind that occlusal loads on molars are mostly axial, whereas the ferrule effect has mostly been demonstrated to be effective in protecting single-rooted teeth against lateral and oblique forces.

Interestingly, in an in vitro study comparing the importance of ferrule in end- odontically treated molars restored with full metal crowns, it was reported that the presence of a 2 mm ferrule was a determinant factor on the fracture resistance and fracture mode. However, the forces required to induce fracture were well above the

From left to right, an endocrown

Fig. 7.3 From left to right, an endocrown (no ferrule), a post and core (either fiber post and resin composite core or metal cast post and core) covered by a full crown (no ferrule), and a crown covering a post and core with apical extension of preparation margin, providing a 2 mm ferrule. The red arrow symbolizes the risks the apical extension may represent, here regarding the furcation, but in general regarding the respect of the biological width space physiological forces, or even parafunctional levels, either in the ferrule group (2035-2934 N) or in the no-ferrule group (1528-1879 N) [43]. As stated before, results based on one type of teeth should no longer be generalized to all endodonti- cally treated teeth without any distinction, as it is often the case in the literature.

Hence, given the lack of solid evidence in favor of the need of ferrule in molars, it is questionable whether it should be achieved at the expense of the remaining tooth structure (Fig. 7.3) and it is questionable to make the decision to extract a molar based solely on the lack of ferrule. Interestingly, when dentists are made aware of the additional tissue loss (3-45 %) associated with complete versus partial coverage restoration in molars, over 50 % of the clinicians altered their initial choice of restoration design from full to partial coverage [44]. Besides, the preparation of a ferrule at any cost in case of little remaining coronal tissue might be associated with additional problems. Notably, the subgingival margin placement can lead to a reduction of the biologic width space (distance between crown margin and alveolar crest) below the required 2-3 mm and to difficulties with the margin impression, excess luting cement removal, and cleaning of the area by the patient. The alternatives, crown lengthening or orthodontic extrusion, represent additional costs and delay for the patient; details of these procedures are beyond the scope of this text. Given the significant progress made in adhesive dentistry, preservation of coronal tissue should be contemplated instead of ferrule preparation in molars, and alternative approaches such as so-called endocrowns should be considered, as will be described further.

Admittedly, endodontic treatment in molars is typically rendered when a major portion of enamel and dentin has been already lost due to caries. Moreover, there are molars that may have a guarded prognosis (see also Chaps. 2 and 8) but still require a restoration (Fig. 7.4). Obviously, there is a range of clinical scenarios in the restoration of molar endodontics, and a variety of possible solutions will be discussed below.

 
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