Occlusion and Temporomandibular Disorders: Is There a Causal Relationship? A Critique of the Existing Scientific Literature

The hypothesis, “dental occlusion plays a significant causal role in temporomandibular disorders”, has been a central point of contention for many decades in the attempt to understand these disorders. Recently, certain authors have asserted that the scientific literature does not support this hypothesis. Yet many clinicians working in the TMD arena continue to believe that various dental occlusal factors do play a role, perhaps a significant role, in the development of some types of TMDs. This paper considers the quality of the scientific data on both sides of the issue, taking into account the epidemiologic requirements to demonstrate causation, as well as a wide variety of scientific principles that are considered central to the proof or refutation of a hypothesis. What emerges from this investigation is that, in spite of there being a considerable volume of literature, particularly in the past 20 years, the quality of a substantial portion of that literature does not meet the stringent requirements of Science to establish causation. Although a limited number of studies of a specific design appear to be supportive of the hypothesis, this evidence is, thus far, insufficient as “proof”. With regard to the original hypothesis, the only conclusion that can be drawn is that this hypothesis has not been refuted nor proven and, thus, remains a viable hypothesis that may and should be further challenged in the future by a higher level of scientific inquiry. Recent research suggests that the role of dental occlusion in temporomandibular disorders, quite apart from the issue of causation, may be more related to its potential as a perpetuating factor. Historically, there has been a belief among certain members of the dental profession that dental occlusal factors have a strong, perhaps dominant, causal relationship to temporomandibular disorders (TMD). The belief in this association was originally based primarily on direct clinical observation, with little scientific evidence to support it. Recently several investigators have asserted that the existing scientific literature does not support a causative association between dental occlusion and temporomandibular disorders. However, Storey 6 and others have noted that because of flaws in investigatory design, the causative association between dental occlusion and temporomandibular disorders has not been invalidated and remains an open question. The purpose of this paper is to consider the qualitative value of the science that addresses this question. Kirveskari and Alanen have stated that “Much, if not most, of the confusion about the role of occlusion is deeply rooted in a lack of appreciation of the problems in causal inference.” In addition to looking at the science that exists on both sides of the question of the role of occlusion in TMD, this paper will also consider concepts related to causation in a larger context, as defined by recognized scientific theorists and logicians in an attempt to clarify what is meant in Science by the term, “cause.” The Role of Hypotheses in Determining Causation Stating that dental occlusal factors have a causative role in the etiology of temporomandibular disorders is a hypothesis. According to Karl Popper, 9, 10 the highly regarded philosopher of science, evidence “for” a hypothesis is necessary but never sufficient to validate it and thus a hypothesis in inherently insufficient to become an accepted scientific fact. Rather, the hypothesis must be tested in an attempt to prove it wrong (to refute it). Only when it has been thoroughly tested by several means and has not been refuted, referred to as “survivability”, is it accepted as scientific probability. This concept is known as Popper’s principle of “refutation.” It is also referred to as “falsification.” One reason that evidence “for” a hypothesis is never