Figure 1.

Mechanical and frictional testing. (A) Picture of the condylar head of a left porcine temporomandibular joint (TMJ) with the articular disc lifted, exposing the silky inferior surface of the disc. The rough surface of the condylar cartilage is covered with a layer of visible crossed collagen bundles. Collagen fibers in the disc are predominantly aligned in the anterior-posterior direction. Five testing regions (anterior, posterior, central, lateral, and medial) are marked on the condyle head. The corresponding contact regions on the TMJ disc were also tested. (B) Schematic of the custom-built micro-tribometer. Sliding of stainless-steel spherical tip on the articular surface is achieved by reciprocation of the piezoelectric bottom stage. Normal loading on the tissue is controlled by the top stage and the capacitance sensor on the cantilever beam. Friction force is recorded by another orthogonal sensor at the end of the beam. (C) A table summarizes the testing parameters. The first row includes the testing modes for all 5 regions, and the second row lists the extra tests performed on the central region to examine the effects of sliding speed and normal force on the friction coefficient. (D) Schematic of the custom-built micro-indenter employed for the indentation test. The step size of the piezoelectric motor is 60 nm (Physik Instrumente, Karlsruhe, Germany).