FIG. 4.
Hydrodynamic lubrication is not the only operating regime of the joint during walking. Knee joint reaction force and angular velocity (A) and minimum film thickness (B) vary during a typical walking cycle. The film thickness between the cartilage surfaces was estimated using biomechanical contact data (adapted from Komistek et al.104 and Winter105) and the formula for loaded rigid contacts,106 h0/R = 4.9ηυ/W, where h0 is the minimum film thickness, R is the radius of the equivalent cylinder (approximated as 2.56 × 10−2 m for tibiofemoral contact), η is the viscosity (1 N·s/m2; Wright and Dowson34), υ is the sliding speed,105 and W is the normal load per unit length in the travel direction (i.e., joint reaction force104 times the body weight105 divided by the square root of the contact area107). Hydrodynamic theory fails to predict a film thickness larger than the surface roughness of cartilage (typical arithmetic average roughness Ra = 200 nm) for the duration of the walking cycle. Lubrication of the joint is largely due to elastohydrodynamic, boundary, and/or mixed lubrication mechanisms.