Tensile Stress Relaxation Properties of Human TMJ Disc

Objectives: The objective of this study was to characterize the viscoelastic mechanical properties of the human TMJ disc under incremental tensile stress relaxation. In addition, this study investigated the effect of tissue anisotropy on the tensile mechanical properties, as well as differences between male and female subjects.
Methods: TMJ discs with no sign of pathology were extracted bilaterally from 6 male (age 67.5±8.4 years) and 6 female (age 70.3±8.7 years) cadavers. In the left discs, 2mm mediolateral strips were sampled, while the right discs were used to sample 2mm anteroposterior strips. Samples were placed in an environment control chamber (37°C) with PBS and loaded using a Bose 3200 axial tester. Each sample was loaded at a strain rate of 1% per second. Samples were loaded for 5, 10, and 15% strain for 15 minutes, 20% strain for 20 minutes, and 30% strain for 30 minutes. The stress relaxation responses for each strain level were recorded. The stress-strain plots were curve fitted to a viscoelastic model to determine the instantaneous and relaxed moduli.
Results: All modulus values are reported as average instantaneous/relaxed modulus. The average modulus for all subjects and orientations was 7.93/3.39 MPa. The anteroposterior modulus was 8.28/3.43 MPa compared to the mediolateral modulus of 7.57/3.34 MPa. The male average modulus was 7.03/2.77 MPa and the female average modulus was 8.90/4.05 MPa.
Conclusions: The human results for the tensile modulus were slightly lower compared to tensile modulus values for porcine tissues using a similar testing protocol. Although the anteroposterior modulus values were higher than those in the mediolaterial region, the difference was not significant (p>0.05). Similarly, no differences were found between sexes (p>0.05). A clear understanding of the mechanical properties of the TMJ disc may facilitate the development of new therapies aimed at repairing the tissue and restoring joint function.