The total surface stress measured in vitro on acetabular cartilage when step-loaded by an instrumented hemiprosthesis are partitioned into fluid and cartilage network stresses using a finite element model of the cartilage layer and measurements of the layer consolidation. The finite element model is based on in situ measurements of cartilage geometry and constitutive properties. Unique instrumentation was employed to collect the geometry and constitutive properties and pressure and consolidation data. When loaded, cartilage consolidates and exudes its interstitial fluid through and from its solid network into the interarticular gap. The finite element solutions include the spatial distributions of fluid and network stresses, the normal flow velocities into the gap, and the contact network stresses at the cartilage surface, all versus time. Even after long-duration application of physiological-level force, fluid pressure supports 90 percent of the load with the cartilage network stresses remaining well below the drained modulus of cartilage. The results support the “weeping” mechanism of joint lubrication proposed by McCutchen.

Although reported frictional coefficients in synovial joints are very low, a computer model of the human hip joint in simulated walking predicted a temperature rise of several degrees Celsius. To confirm this prediction, physical experiments were conducted in vitro on intact human hip joints dynamically loaded and articulated as in walking. Thermisters were placed in subchondral bone in both the acetabulum and femoral head, just below the cartilage layers. The surrounding saline bath was maintained at 37°C. Measured temperatures as high as 2.5°C above the 37°C were recorded in the subchondral bone. Loading that simulated the stance/swing phases of gait but without articulation produced no significant increase in temperature; thus fluid flow perse is not significantly energy dissipative; the prime source is friction at the articulating surfaces.

A computer simulation of the human hip joint behaviour in the dynamic regime of walking was performed based on the geometry and the relevant dynamic constitutive properties of the cartilage layers in a particular joint. These were established in vitro, in situ by de nuovo techniques developed in this laboratory. Presented in this paper are the simulation results describing the fluid flow patterns within the interarticular space. Simulation generated flow patterns appear well correlated to split-line patterns on the femoral head. Hence our proposition that enhancement of the superficial layer anisotropy by the fluid flow and consequent reduction of the interarticular sealing is the primary mechanical aspect of the synovial joint degradation.

The time response of surface displacement and acoustic impedance of in situ layers of articular cartilage were measured by using pulse-echo ultrasound. Disturbances were introduced by altering the osmotic pressure. Strongly nonlinear behavior was observed near physiological equilibrium. A model of articular cartilage is proposed which relates our results to cartilage microstructure.