I would suggest another reason for the high incidence of root fractures reported by Dr. Stephen Cohen and colleagues in their excellent article, "Vertical Root Fractures" (April JADA).
That misnamed phenomenon of occlusion we call cuspid guidance is more often seen as a sharing of the cuspid, first bicuspid, sometimes including the second bicuspid; occasionally, the mesial cusp of the first molar shares this "group function." In youth, the cuspid may play a solo role in guidance, but we wear as we age and the cuspid soon shares its function with its posterior neighbor(s).
Now picture the bicuspid restored with a crown following endodontic treatment and post placement. The laboratory has fabricated a bicuspid crown with ideal morphology, the steep cuspal inclines of youth, while the still natural canine is continuing to wear and even show some dentin through the thinning enamel on its guiding surface (the lingual for the upper, the labial for the lower).
With the hardness of porcelain to contend with, the forces of guidance movements are shifted away from the canines (which have the root morphology to deal with such forces) to the bicuspids (that have more slender, potentially weaker roots) that gradually are forced to absorb the majority of the guidance forces. Add a dash of bruxism and you have the recipe for vertical root bicuspids. Perhaps we should rethink the morphology of the cuspal inclines of restored bicuspids, especially those that have had endodontic post treatment.
