Pathomechanic determinants of posttraumatic arthritis.

The etiology of posttraumatic arthritis is poorly understood. One possible mechanism involves a mechanical insult to the cartilage matrix that affects chondrocyte function. To better understand the etiology of posttraumatic arthritis, pathomechanic changes in articular contact mechanics resulting from injury during physiologic motion and loading need to be determined. Previous studies of injury-associated changes in articular contact mechanics, using static testing methods, have measured relatively modest increases in contact stresses. Static testing cannot measure transient loads associated with motion or loading rates. This testing method poorly simulates normal viscoelastic cartilage properties, and accounts for contact stress changes in a single or limited number of joint positions. In this study, time-variant contact stresses in two ankles with an anterolateral stepoff were measured during quasiphysiologic motion and loading. Contact stresses were integrated over the entire range of motion to estimate pathomechanic loads that accumulate over the entire motion cycle. Numerical techniques were applied to time-variant contact stress data to calculate contact stress directional gradients and contact stress rates of change. Contact stress directional gradients and rates of change were integrated over the complete motion cycle to estimate whole-cycle accumulation of these potential pathomechanic parameters.