Drawbridge Seminars -- Bringing Research to Practice





"The clinician who looks only at occlusion is missing as much
as the clinician who never looks at occlusion"

Jeffery P. Okeson, DMD

A careful assessment of the available scientific literature regarding the causal relationship between dental occlusion and temporomandibular disorders clearly indicates, for multiple reasons, that this causal hypothesis has not been disproven, as has been suggested by certain “experts”. The primary reason is that a very large percentage of the literature related to this question is seriously flawed and does not rise to the stringent requirements of Science to refute the occlusal hypothesis regarding causation. However, thus far there is only limited scientific basis for establishing what, if any, causal relationship does exist.

Multiple occlusal factors have been studied with respect to their potential causal contribution to TMDs. Although many of these studies are flawed in any of several ways, it is worth looking at what factors have been considered. These include:

• Skeletal anterior open bite
• Overbite
• Overjet
• Cross bite
• Incisor inclination
• Missing teeth
• Posterior occlusal support
• Balancing-side interferences
• Working-side interferences
• Intercuspal interferences
• Symmetry of contacts in the retruded contact position,
• Slide between the retruded contact position and the intercuspal position

Very few of these studies considered more than inter-occlusal relationships upon which to draw conclusions. Only a very small number of studies have considered the relationship between occlusal factors and condylar position. Of particular interest in this respect are studies by Crawford in The Angle Orthodontist, Vol. 69 No. 2, 1999 and by Weffort in The Angle Orthodontist, Vol. 80 No. 5, 2010. [See Core Literature — Occlusion]

In the most recent edition of his text, Management of Temporomandibular Disorders and Occlusion, Okeson cites 57 studies that have looked at the causal relationship between occlusion and TMD. These studies include most, if not all, of the occlusal factors cited above. (However, the studies by Crawford, Weffort and other similar studies are not included in this group.) Although many of these studies demonstrate flaws of various types, it is interesting to note that, as presented in this text, the number of studies that showed a correlation between occlusion and TMD is 35 “Yes” findings (Okeson’s assessment) and 22 “No” findings; a ratio of 1.8545:1in favor of a causal correlation.

One of the most prevalent errors in such studies has to do with the number of subjects in the study. Because of the very high prevalence of potentially-significant occlusal factors in the general population, for studies to be able to demonstrate a significant statistical difference between TMD patients and controls,there need to be quite large numbers of subjects in any study . This is referred to as the “Critical Effect Size” of a study.

In the 57 studies cited by Okeson, the size of the studies (number of subjects) varied from 12 to 3428. Because a significant number of these studies have low numbers of subjects, it would seem worthwhile to examine what correlation might be found between occlusion and TMD if only larger studies were considered.

To do this, all studies with less than 200 subjects (number arbitrarily chosen) were excluded and the remaining studies were again compared as to the ratio of “Yes” responses vs. “No” responses. After excluding all studies with less than 200 subjects, there were 29 remaining. Of these, 21 were found to have a positive correlation (“Yes” responses) and 8 were found to have a negative correlation (“No” responses.) This results in a ratio of 2.625:1 in favor of a causal correlation between occlusal factors and TMD

Although this kind of analysis does not prove a clear correlation between occlusion and TMD, it is one of several strongly-suggestive observations that justify further consideration of a possible correlation, particularly of studies similar to that by Crawford and Weffort that consider occlusal factors related to condylar position.

For decades, a common observation has been made by dental clinicians with regard to the use of occlusal appliances in the management of temporomandibular disorders, suggesting that if occlusal appliances are consistently effective in reducing symptoms of TMD, because occlusal appliances alter the occlusal environment, this would seem to imply a correlation between a change in occlusion and a reduction in symptoms.

The problem with this is that the actual role of occlusal appliances in the management of temporomandibular disorders has continued to be debated. It has been suggested that occlusal appliances may have one or more of the following effects, thus complicating the issue considerably:

• Alteration of the occlusal condition
• Alteration of the condylar position
• Increase in the vertical dimension
• Cognitive awareness
• Placebo effect
• Increased peripheral input to the CNS
• Regression to the mean                             (from Okeson)

Further complicating this discussion have been several studies that seem to show limited benefit from the use of occlusal appliances in the management of TMD.

The causation of TMDs is clearly not the result of a single factor but a complex and dynamic process that is almost certainly different in each patient. As clinicians we strongly agree that the clinical evidence for a significant occlusal component to this complex picture, in most patients, cannot be dismissed.

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