Tip-SEE-Tue: The Fuss Around Ferrule Effect

We often come across clinical scenarios, where an abutment tooth for a fixed partial denture may not contain enough coronal tooth material to support the core build-up material. In such a scenario, a post and core restoration is indicated.

Of utmost value to the survival of such a compromised tooth is the ‘ferrule effect’, which is that part of tooth structure that encircles the portion of the tooth above the preparation margin.

The abutment is most resistant to fracture if it provides a height of 1.5-2.0 mm of dentin above the projected ferrule margin, this is for the ferrule or crown to grab onto.

Without this band encircling the tooth, force vectors contacting the tooth cause flexion at a variety of different tooth-post-core levels. This puts immense stress on potential fracture planes within the tooth root and/or coronal structure. Now, if a crown or bridge dental prosthesis is cemented onto a tooth without an adequate ferrule, it makes the assembly prone to biomechanical failure. Here, the abutment tooth can undergo catastrophic fracture, separating it from the rest of the submerged tooth.

A positive ferrule prevents the shattering of the root and aids in providing resistance to the dislodgment of the prostheses. The final crown that sits on top adds to the bracing effect, thereby protecting the remaining tooth structure against breakage.

When a tooth is ferruled, the forces that would otherwise be generated in multiple flexure stress planes (in the non-ferruled tooth) in fact get distributed over a wide area. Hence, a ferrule can potentially improve the biomechanical stability of a tooth, by ‘shifting’ the interfaces that resist stresses, from weak tooth-post-core interfaces to a strong tooth-post-core interface that is located at the buttressed ferrule margin.

In this video, I explain this rather complex topic in a simple manner using illustrations and clinical pictures.

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