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Ferrule Effect

Restoring Endodontically Treated Teeth

When restoring a tooth one of the most important factors to the long-term prognosis and longevity of our treatment is the amount of remaining supragingival tooth structure. The Ferrule effect is described as a band that encircles the perimeter of the residual tooth and this is what has a crucial influence on fracture resistance, especially in decoronated teeth.

How? It reinforces the external surface of the tooth dissipating the forces that concentrate at the narrowest circumference of the tooth. Increasing the ferrule particularly on the palatal enhances resistance. This becomes increasingly important when restoring endodontically treated teeth.

 

What post material?

 

For many years, the cast gold post and core was regarded as the gold standard due to its superior success rate. Elastic Modulus that is close to dentine wil allow absorption and uniform distribution of stresses to the remaining root structure instead of concentrating them. Fibre posts have an elastic modulus closer to that of dentine (Fibre post = 20GPa, Dentine = 18 GPa, Prefabricated and cast metal posts = 200GPa, ceramic post = 150GPa and Titanium = 110GPa).

 

Having an elastic modulus similar to dentine allows the absorption and uniform distribution of stresses to the remaining root structure instead of concentrating them. Prefabricated posts with custom made composite cores simplifies the process as it can be done chairside with the same success if all adhesive protocols are followed.

 

Finite analysis with photoelastic and FEM studies show the forces on a root treated tooth are concentrated at the dentine crown interface and the post does not contribute till the bond between the composite core and dentine fails.

 

 

Fig 1. Zarow et al 2018. Finite analysis of cast posts (left) and prefabricated

fibre posts (right)

 

What does that mean? Posts with higher modulus of elasticity generates higher cervical stress than fibre posts (20Gpa) which has a similar modulus of elasticity to dentine (18Gpa). Metal posts have a modulus of elasticity close to 200 GPa.

 

Cast gold post and cores have less tensile stress compared with metal prefabricated post and composite core and in the presence of adequate ferrule does not effect the presence of the tensile forces.

If there is extensive tooth destruction and a lack of cavity walls then evidence suggest that a fibre post will fail frequently by post fracture and debonding. A properly executed ferrule must be 1.5-2.0mm high especially on the palatal and buccal walls. If the destruction renders an insufficient ferrule then and one must consider crown lengthening or orthodontic extrusion.

Zarow etl al 2018 developed a classification system that helps select the most appropriate treatment plan for restoring root filled teeth when choosing between a composite core alone, a composite core reinforced by fibre post, a cast gold post or implant treatment. Below is a summary of the decision making process in restoring endodontically treated teeth.

Class 0 - No Post – composite core build up

The use of posts is not mandatory for the restoration of non-vital tooth unless there is an obvious insufficient retention of the core.

 

The posterior teeth have pulp chambers that are large and can be used to increase the adhesive surface area and mechanical retention.

 

Unless the destruction has been extensive, the pulp chamber is enough for bonding a composite resin core. Aggressive post preparation is a risk for vertical root fracture. Magne 2017 found that posts are detrimental to the failure mode and should be absolutely avoided in incisors when ferrule is present.

Class 1 - Fibre post

 

Fibre posts are recommended in anterior teeth and premolars with less than 50% of tooth structure present and when there are two or fewer walls present.

 

Why? Premolars have pulp chambers that is difficult to retain a core unlike molars. Furthermore molars are the most frequently fractured teeth due to the presence of oblique forces.

 

Molars do not need posts unless it they are totally missing the coronal tooth structure and there is insufficient pulp chamber space present.

 

Evidence suggests that fibre posts should be luted for functional reasons to increase biomechanical properties as anterior teeth have to support more flexural stresses.

 

Class 2 - Pre-procedures are needed such as ortho extrusion or crown lengthening

The literature suggest that a post cannot compensate for total or partial ferrule loss. Anterior teeth and premolars should consider orthodontics extrusion as they are in the aesthetic zone and Buser et al 2007 suggested that in this zone, a good aesthetic outcome is challenging.

One must consider the added 3-6 month healing time, the discomfort and added costs.

 

Class 3 - gold cast post or silver-palladium cast post

 

The evidence suggests that when there is no ferrule effect, the tooth is likely to fail due to root fracture. However, as clinicians in private practice we do have patients that are willing to save the tooth at all costs. In these cases a gold cast post could be the alternative.

One must be aware that when a cast post fails, they do so catastrophically by root fracture. Technique wise, be aware that the provisionalisation is required and contamination of the root canal system could occur during that time.

Class 4 – Extraction

Generally speaking, if there is no ferrule, one should strongly consider extracting the tooth. Orthodontic extrusion and crown-lengthening can be considered but they are tempered by the fact that they incur additional healing time with unpredictable long-tern outcomes where there are really no long-term studies to support.

Thus, the decision making process in the restoration of root filled teeth is complex and should consider the following factors: amount and quality of tooth structure, tooth position in the arch and anatomy and function. According to the literature reviewed, in many root filled teeth there is no need for post placement. Fibre posts are recommended in anterior teeth and premolars with compromised tooth structure (<50%) and/or with high tooth length over bone crest (either for periodontal defect or surgical crown lengthening). A ferrule is highly beneficial for the prognosis of root filled teeth. In case of total or partial loss of ferrule, fibre posts can be placed, but only after orthodontic or surgical crown lengthening have been considered. If a ferrule cannot be provided and the patient still prefers to save the tooth, a gold cast could be a possible option. In cases where treatment and restoration of the compromised root filled tooth are too complex with poor prognosis and/or the tooth interferes in a large restorative reconstruction, extraction followed by implants or a FPD might be a valuable treatment option.

1. Juloski et al 2012 - Ferrule Effect: A Literature Review

2. Zarow et al 2018 - A New Classification System for the Restoration of Root Filled Teeth

3. Dietschi et al 2008 - Biomechanical Considerations for the Restoration of Endodontically Treated Teeth: A Systematic Review of the Literature, Part II.

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