Stance-phase aggregate contact stress and contact stress gradient changes resulting from articular surface stepoffs...

Determine how stepoff incongruities of the distal tibia affect aggregate (whole-cycle) contact stresses and contact stress gradients for a complete motion cycle in human cadaveric ankles.

Ten human cadaveric ankles were subjected to quasiphysiologic forces during stance-phase range of motion. Each specimen was loaded intact, with anatomic reduction of the anterolateral quarter of the distal tibia, and with increasing stepoffs of the anterolateral fragment up to 4.0mm. Transient contact stresses were measured using a custom-built, real-time stress transducer that sampled stresses at 132Hz at 1472 separate foci (sensels). Aggregate stresses were calculated by summing the sequential transient stress values multiplied by the transient sampling duration for the complete motion cycle at each sensel. Transient contact stress gradients were calculated at each sensel using a central-differencing formula applied to adjacent transient stress measurements. Aggregate contact stress gradients were calculated by vector summation of sequential transient stress gradients multiplied by the sampling duration.

Compared to the intact configuration, anatomic reduction of the fragment caused minimal changes in aggregate contact stresses and stress gradients (30% increase compared to intact values). In contrast, stepoffs caused substantial increases (200% increase compared to intact values) in peak and mean whole-cycle stresses and gradients.

Aggregate contact stresses and stress gradients quantify loading history for the complete motion cycle. Incongruity-associated changes in aggregate stresses and gradients are a surrogate for "accumulated" damage over a motion cycle in stepoff specimens. These loading abnormalities may be important determinants of posttraumatic arthritis.