HPB Bolhuis, PhD and P Pallav, PhD

It has long been thought that removing the pulp had a strongly negative influence on the biomechanical properties of dentin (dehydration, brittleness, loss of tensile and flexural strength) and was major cause for cusp and root fractures of endodontically treated teeth.

A considerably greater loss in strength however, is caused by the loss of interlocking bond between the cups, as Sedgley and Messer (1992) found much more influence from the loss of marginal cristas, and loss of tissue over the pulp.

Ferrule

An interlocking bond, where the cervical millimeters of the crown embrace dentin rather than build up material, is an important contribution to the strength of the final structure.

If enough coronal tissue remains, the pulp chamber and the irregular shape of the excavated dentin surface, together with adhesive techniques often provide adequate bond for a composite core build up, without using a post (Assif et al., 1993; Bolhuis et al., 2001).

In dentistry the term ferrule indicates the vertical height of the cervical dentin ferrule surface available for bonding to the crown restoration, i.e. the height between the cervical edge of the crown and the occlusal edge of the remaining dentin.

A crown with no ferrule is prone to failure, as it depends entirely on the bond of the root canal post. The ferrule should be at least 1.5 mm (Stankiewicz and Wilson, 2002). Other studies indicate that with a ferrule in excess of 2 mm, a root post may be omitted and that for fracture resistance the ferrule height is more important than the length of the root post (Assif et al., 1993; Isidor et al., 1999; Bolhuis et al., 2001).

Therefore a balance will have to be found between creating sufficient ferrule and sacrificing healthy dental tissue. This is important especially in the absence of a rigid cast metal build up, as loads on the crown are transferred to the cervical dental tissue (Pierrisnard et al., 2002).

This may require crowns with thin edges and to prevent over-shaping, Pfm crowns (Porcelain fused to metal) with metal edges. Full metal crowns generally require less sacrifice of dental tissue than Pfm crowns with porcelain edges or full ceramic crowns.

The importance of the preservation of healthy tooth tissue was re-confirmed with a 5-years study of the durability of Pfm crowns with various types of (post) build-ups. Not the type of build up, but the amount of remaining tooth tissue determined the durability (Creugers et al., 2005).

Overlay (partial) restorations vs. full crowns

Especially the anatomy of pre- and molars requires endodontical procedures, which may cause considerable loss of strength. The buccal and palatinal walls may be thinned to the extent that a restoration covering the cusps is needed to handle occlusal loads (Reeh et al., 1989; Panitvisai en Messer, 1995).

This may be achieved with metal or ceramic partial crowns just covering the cusps with, indeed, potential esthetical (metal) or mechanical (porcelain) problems.

Composite restorations, covering the cusps sacrifice the smallest amount of dental tissue (Fokkinga et al., 2003). Unfortunately, some dentists still avoid this solution, because they think of it as not strong enough and time consuming (or underpaid!). Clinically, the 3-years survival of endodontically treated premolars with substantial loss of tissue, restored with composite material and a carbon fiber root post is about equal to the same structure, but with a full crown (Mannocci et al., 2002).

More often, a full crown is made, with arguments such as discolorations from previous amalgam restorations or dated root seals.

A large amount of dental tissue has to be removed for full-ceramic layered crowns reinforced with zirconia bases, or Pfm crowns. Especially with premolars, too little tooth structure may remain and root posts in one or even more canals are needed for adequate grip. The figures illustrate the gravity and irreversibility of these solutions.