Variables that Affect Tractional Force on Porcine TMJ Discs

Tractional forces on the surface of the TMJ disc impose mechanical work and strain energy on the collagenous matrix of the disc. This increases the likelihood of mechanical fatigue of the tissue. Objective: This study tested the hypotheses that tractional forces: (1) increased with velocity and aspect ratio of the translating stress-field, and compressive strain, and (2) varied depending on cartilage thickness. Methods: Porcine TMJ discs (n=187) received a 10 N normal static load via an acrylic indenter for 1, 5, 10, 30, or 60 s, followed by movement. Physical data describing instantaneous velocity, compressive strain [change in cartilage thickness/original thickness], and aspect ratio of the stress-field [diameter of stress-field/cartilage thickness] were recorded and analyzed using quadratic regression relations. Likelihood Ratio Tests determined whether or not there were significant differences in tractional forces between thicker and thinner discs. Results: Non-linear increases in tractional forces were found to be related to velocity, aspect ratio, and compressive strain when the stress-field moved onto relatively thicker (R²=0.83) and thinner cartilage (R²=0.86). Likelihood Ratio Tests found tractional forces were significantly higher (≤12% of peak, P < 0.05) when the stress-field moved onto thicker than thinner cartilage for conditions where velocity was >27 mm/s, and combined effects of aspect ratio and compressive strain3 was >0.09. Conclusion: The combined effects of geometry of the stress-field, compressive strain, velocity of translation, and cartilage thickness can significantly increase tractional forces on the surface of the TMJ disc. This work was supported in part by research funds from UNMC College of Dentistry. The authors wish to thank Farmland Foods of Crete Nebraska for their help in this project.