The intra-articular distance within the TMJ during free and loaded closingmovements

Previous studies on free opening and closing movements of the mandible have demonstrated that the opening movement traces of the condylar kinematic ce nter (i.e., the condylar point for which the protrusive and the opening mov ement traces coincide) lie closer to the articular eminence than the closin g traces. This indicates the presence of an intra-articular distance within the joint during free closing. Since the mandible behaves like a class III biomechanical lever, a counteracting mechanical load on the mandible durin g closing will press the condyle-disc complex against the articular eminenc e. Therefore, in this study the hypothesis was tested that the difference b etween opening and closing movement traces of the kinematic center is reduc ed when the closing movements are counteracted by a mechanical load. From 1 0 healthy participants, 20-second movement recordings were obtained by a si x-degrees-of-freedom opto-electronic jaw movement recording system (OKAS-3D ) for three types of movements: (1) free opening and closing movements, (2) free opening and loaded closing movements (i.e., the participants closed a gainst a small or high manually applied downward-directed force to the chin ), and (3) gum chewing. Off-line, the opening and closing movement traces o f the kinematic center were reconstructed, and the average difference betwe en the traces (the intra-articular distance) was calculated. The average in traarticular distance was significantly smaller during loaded closing than during free closing, whereas no significant differences were found in the i ntra-articular distances between the loaded situations of low and high manu al loading and contralateral chewing (ANOVA and post hoc Bonferroni multipl e comparisons of means test, p < 0.005). In conclusion, loading of the mand ible during closing movements reduces the intra-articular distance within t he temporomandibular joint.