Evaluating anisotropic properties in the porcine temporomandibular joint disc using nanoindentation

The objective of this study was to determine the viscoelastic properties present within the intermediate zone of the porcine temporomandibular joint (TMJ) disc using nanoindentation. A 50-μm conospherical indenter tip using a displacement-controlled ramp function with a 600 nm/s loading and unloading rate, a 3000-nm peak displacement with a holding period of 30 s was used to indent the samples. Experimental load-relaxation tests were performed on the TMJ disc to determine the response in three different directions; the mediolateral, anteroposterior, and articular surface directions. The experimental data were analyzed using a generalized Maxwell model to obtain values for short- and long-time relaxation modulus and of material time constants. The short time relaxation modulus E_I values were 180.92, 64.99, and 487.77 kPa for testing done on the articular surface, mediolateral, and anteroposterior directions, respectively. Corresponding values for the long-time relaxation modulus E∞ were 45.9, 14.97, and 133.5 kPa. The method confirmed anisotropy present within the central intermediate zone of the porcine TMJ disc due to the directional orientation of the collagen fibers.