Stress-field translation in the healthy human temporomandibular joint

Movement over the surface of the temporomandibular joint (TMJ) disc produce s tractional forces. These forces potentially increase the magnitude of she ar stresses and contribute to wear and fatigue of the disc. Theoretically, tractional forces in all synovial joints are the result of frictional force s, due to rubbing of the cartilage surfaces, and plowing forces, due to tra nslation of the stress-field through the cartilage matrix as the joint surf ace congruency changes during motion. For plowing forces to occur in the TM J, there must be mediolateral translation of the stress-field as the condyl e moves dorsoventrally during jaw function. To test whether mediolateral st ress-field translation occurs in the intact TMJ, we measured stress-field p osition and translation velocities in ten normal individuals during rhythmi c jaw opening and closing. Magnetic resonance imaging and jaw tracking were combined to animate the three-dimensional position of the stress-field bet ween the articulating surfaces. This allowed for mediolateral translation v elocity measurements of the centroid of the stress-field The results showed that during jaw opening and closing at 0.5 Hz, the average peak mediolater al translation velocity was 35 +/- 17 mm/sec. When opening and closing incr eased to 1.0 Hz the average peak velocity was 40 +/- 19 mm/sec. Theoretical model estimates of the work done during such translation ranged from 6 to 709 mJ between the individual joints studied. The potential clinical import ance of this measure is that long-term exposure of the TMJ disc to high wor k may result in fatigue failure of the TMJ disc.