Viscoelastic Shear Properties of Porcine TMJ Discs

Objectives:  The temporomandibular joint (TMJ) disc, a fibrocartilaginous tissue, is a major component of jaw function by serving stress distribution and lubrication in the joint. This study determined viscoelastic shear properties of porcine TMJ discs.  The effects of frequency and shear strain on dynamic shear properties were investigated.  We also investigated the regional variation in both equilibrium and dynamic shear properties.  

Methods:  Twelve fresh porcine TMJ discs from young adult pigs (10~12 months) were used.  Cylindrical samples (5mm diameter) with uniform thickness (~1.2mm) were prepared from the intermediate, posterior, anterior, lateral, and medial regions of the TMJ disc.  Torsional shear tests were performed under 10% compressive strain.  Dynamic shear was applied in two methods: (1) a frequency sweep test over the frequency range of 0.01-10 rad/s with constant shear strain amplitude of 0.025 rad, and (2) a strain sweep test over the range of 0.005-0.05 rad at a constant frequency of 10 rad/s.  Transient stress-relaxation test were also performed to determine the equilibrium shear properties.

Results:  The dynamic shear complex modulus increases as the frequency increased, with values ranging from 100 to 450 kPa.  The phase angle, ranging from 12 to 25 degree, displays a unique pattern of regional variation as the frequency increased.  The dynamic shear modulus decrease as the shear strain increased.  The equilibrium shear modulus has values ranging from 35 to 65 kPa.  The anterior and intermediate regions have high values for both equilibrium and dynamic shear modulus, however the lateral and medial regions displays low shear modulus.

Conclusion:  Our results showed that TMJ disc displays intrinsic viscoelastic shear behavior which is region dependent.  These results suggest that the inhomogeneous shear characteristics of TMJ disc play a crucial role in determining local strain of the TMJ disc under mechanical loading.  (Supported by NIH P20RR-016461 and P20RR-017696).