Is Condylar Position Predictor for Functional Signs of TMJ Hypermobility?

Temporomandibular joint (TMJ) hypermobility is usually only noted when it interferes with smooth mandibular movements. These interferences are thought to result from condylar dislocation in front of the eminence at wide mouth opening, or alternatively in front of the articular disc (posterior disc displacement, PDD). Objectives: The aim of this study was to test the hypothesis that condyles of symptomatically hypermobile persons are positioned more anteriorly or anterosuperiorly to the eminence during maximum mouth opening (MMO) than those of persons without TMJ hypermobility. Possible PDD was also evaluated. Methods: Nine symptomatically hypermobile and nine non-hypermobile persons were included, their diagnoses being based upon opto-electronic movement recordings. Condylar positions during MMO were analyzed on MRI with two slightly different methods. Both measured the angle between the top vertical axis of a coordinate system, superimposed over the summit lowest eminence point (E), and the line connecting E with the top of the condyle. In the first method, the x-axis of the system coincided with MRI's horizontal plane; in the second, with the line E–postglenoid tubercle. Results: No PDD's were found on any of the MRI's. After excluding an outlier and using both measurement methods, a small significant difference was found between the angles of the two groups of subjects (Mann-Whitney U test, P= 0.05). The condyles of all hypermobile persons traveled beyond the eminence. However, so did the condyles of nearly half of the non-hypermobiles. Conclusion: The large overlap between both groups suggests that condylar position alone is not a good predictor for symptomatic TMJ hypermobility. It is probably the combination of certain intraarticular relations with a particular line of muscle action, which gives rise to its functional signs. (Supported by the IOT.)