HPB Bolhuis, PhD and P Pallav, PhD

Our view on root posts has changed. Posts do not reinforce roots (Krejci et al., 2003),

And, as preparing the canal to fit a post is very risky (Ross et al., 1991; Kuttler et al., 2004), these should only be used when the extra grip is really necessary for the build up (see: Ferule).

Post shape and selection

Cast metal build-ups always require much tissue loss, because all undercuts must be removed. This solution seems appropriate only with long and solid roots (upper centrals and canines), where little tissue needs to be removed. 
A clinical example could be an occlusion with very deep frontal contacts where loads occur at a rather unfavorable angle at the level of the entry of the root canal.

When prefabricated posts are used, it is important to understand that a post should be selected that requires absolutely minimal preparation of the root canal. Adapting the root canal to the post is always risky. There is a clear development where long parallel precisely fitting posts are abandoned and more anatomical, i.e. shorter, more conical, and more freely fitting posts are preferred (Qualtrough and Mannocci, 2003).

Although Titanium posts are stronger in a simple tensile test, these don’t add more strength to the final structure than many carbon or fiberglass posts do. This is largely because the post should have a substantial cross-section at the level of the entry of the canal (Asmussen et al., 1999) combined with the flexibility of dentin. Dentin can handle much more elastic deformation than many engineering materials. 

Because of this, the elastic limit, i.e. the elastic deformability of the post is of greater importance than the strength. Especially the popular Titanium posts have lower elastic limits than carbon or fiberglass posts and break more easily under fatigue loads.
At 45 and 90 degrees load angles, the elastic limits of the black carbon fiber, and the tooth colored fiberglass posts are similar and corrosion occurs with neither, so the last are more popular for the esthetics.

Build-ups made of materials, which are much more rigid (higher E-modulus) than dentin (e.g. stainless steel, gold alloys, titanium, zirconia), tend to work as stress raisers i.e. these lead to stress concentrations. Particularly with upper (second) premolars these unfavorable stress distributions may cause fracture (Martinez-Insua et al., 1998; Lustig et al., 2000).

Beware with short and rigid posts. Problems of stress concentrations in the dentin caused by the rigidity of the post are greater with shorter posts (Holmes at el., 1996).

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With build up composites and fiberglass posts, which both have E-modules similar to that of dentin, it seems possible to avoid most stress concentrations (Ferrari et al., 2000; Bolhuis et al., 2004).

Adhesive composite cements compensate the reduced mechanical grip of shorter posts (Nissan et al., 2001; Hagge et al., 2002). Therefore conical and not too long posts should be selected, with the advantage of a shorter post that more of the endodontical seal is left intact (Metzger et al., 2000).

For 62 % of the teeth that are lost, the reason is fracture of the root, as was found in a 30-years clinical study (Axelsson et al., 2004) It should be noted that not only rigid posts, but also endodontical procedures are reasons for later root fractures (Wu et al., 2004).

The presence of posts did not significantly influence the in vitro fracture strength of endodontically sealed and decapitated premolars, which were restored with a solid composite crown and build up (Fokkinga et al., 2005).

The decision to use a post depends on individual phenomena like remaining tooth structure, occlusal forces, and ordinary variability and has to be made in each individual case (Fernandes and Dessai, 2001). The required strength and stiffness of the build up depends on many factors. Front teeth are often loaded with considerable transverse forces and molars are exposed to much greater, but more axial loads. Solitary crowns have different requirements than supports of fixed bridges or anchor teeth of removable appliances.