Viscoelastic characterization of the porcine temporomandibular joint disc under unconfined compression.

Pathophysiology of the temporomandibular joint (TMJ) disc is central to many orofacial disorders; however, mechanical characterization of this tissue is incomplete. In this study, we identified surface-regional mechanical variations in the porcine TMJ disc under unconfined compression. The intermediate zone, posterior, anterior, lateral, and medial regions of eight TMJ discs were sectioned into inferior and superior surface samples. Surface-regional sections were then subjected to incremental stress relaxation tests. Single strain step (SSS) and final deformation (FD) viscoelastic models were fit to experimental data. Both models represented the experimental data with a high degree of accuracy (R(2)=0.93). The instantaneous modulus and relaxation modulus for the TMJ disc sections were approximately 500 kPa and 80 kPa, respectively; the coefficient of viscosity was approximately 3.5 MPa-s. Strain dependent material properties were observed across the disc's surface-regions. Regional variations in stiffness were observed in both models. The relaxation modulus was largest in the inferior-medial parts of the disc. The instantaneous modulus was largest in the posterior and anterior regions of the disc. Surface-to-surface variations were observed in the relaxation modulus for only the FD model; the inferior surface was found to be more resistant to compression than the superior surface. The results of this study imply the stiffness of the TMJ disc may change as strain is applied. Furthermore, the lateral region exhibited a lower viscosity and stiffness compared to other disc regions. Both findings may have important implications on the TMJ disc's role in jaw motion and function.