Significance of operative definitions in studies of the risk from occlusion

The dismissal of the causal role of occlusal interference may be the result of the erroneous conception of occlusal interference and functional disorders, including temporomandibular disorders. The majority of researchers measure or assess variation in occlusion in a way that does not correspond to the concept of interference that is applied by many clinicians. Interferences are practically universally acquired risk factors, the result of a soft diet and minimal functional tooth wear. The lack of interference-free subpopulations needs to be taken into account in the design of studies of occlusion. Studies that fail to show the association between occlusion and temporomandibular disorder suffer from methodological bias concerning not only occlusal variation but probably also the nature of temporomandibular disorder. The basic assumption has been that temporomandibular disorder is a self-limiting pain disorder. The Research Diagnostic Criteria for temporomandibular disorders exclude several signs and symptoms associated with pathofunction. In clinical opinion, they often foreshadow painful conditions. Painless patients at risk of becoming pain patients are classified as ‘healthy’ according to these criteria. Randomised clinical trials testing the causal role of occlusion on the assumption that occlusal interferences are universal risk factors have so far clearly failed to exclude occlusion from the causal complex of functional disorders. Elimination of the risk from occlusion remains a valid form of therapy when the cost–benefit assessment is favourable.