Tractional Forces, Work and Energy Densities in the Human TMJ.

The role of mechanics in degenerative joint disease of the temporomandibular joint (TMJ) is largely unknown. Objectives were to: 1) develop an empirical model to relate variables of cartilage mechanics and tractional forces; and 2) use the empirical model to estimate tractional forces for calculations of work done (mJ) and energy densities (mJ/mm³) in living human TMJs. Sixty-four porcine discs were statically, then dynamically loaded. Aspect ratios and velocities of stress-fields, compressive strains, and tractional forces were recorded and fit to a quadratic equation to derive the empirical model. Aspect ratios and velocities of stress-fields and cartilage thicknesses then were measured via dynamic stereometry in 15 humans with healthy TMJs and 11 with TMJ disc displacement. These data were used in the empirical model to estimate tractional forces for each TMJ, and then mechanical work done and energy densities were calculated. Mechanical work (mJ) was on average 20 times greater in TMJs with disc displacement than in healthy TMJs (P<0.02). TMJs with disc displacement showed 350% more mechanical work (mJ) and 180% higher energy densities in women compared to men (P<0.02). A power analysis (α=0.05, β=0.90) indicated that 40 women and 40 men would be required to detect a 50% difference in TMJ energy densities between genders. Mechanical work was significantly higher (P≤0.05) in TMJs with disc displacement compared to healthy TMJs, and mechanical work done and energy densities were significantly higher (P≤0.05) in TMJs with disc displacement in women compare to men.