Instability-associated changes in contact stress and contact stress rates near a step-off incongruity

Intra-articular fractures can result in articular surface incongruity and joint instability, both of which can lead to posttraumatic osteoarthritis. The purpose of this study was to quantify changes in contact stresses and contact stress rates in incongruous human cadaveric ankles that were either stable or unstable. It was hypothesized that joint instability, superimposed on articular incongruity, would cause significant increases in contact stresses and contact stress rates.

Intact human cadaveric ankles were subjected to quasi-physiologic stance-phase motion and loading, and instantaneous contact stresses were captured at 132 Hz. The anterior one-third of the distal part of the tibia was displaced proximally by 2.0 mm, and testing was repeated. Anterior/posterior forces were modulated during loading to cause incongruous ankles to either remain stable or become unstable during loading. Transient contact stresses and contact stress rates were measured for seven ankles under intact, stable-incongruous, and unstable-incongruous conditions. Peak and 95th percentile values of contact stress and contact stress rates for all three conditions were compared to determine the pathomechanical effects of incongruity and instability.

The addition of instability caused 95th percentile and peak contact stresses to increase approximately between 20% and 25% in the unstable-incongruous specimens compared with the stable-incongruous specimens. In contrast, the addition of instability increased the magnitude of peak positive and peak negative contact stress rates by 115% and 170% in the unstable-incongruous specimens compared with the stable-incongruous specimens. Similarly, the 95th percentile contact stress rates increased 112% in the unstable-incongruous specimens compared with the stable-incongruous specimens.

In human cadaveric ankles, instability superimposed on an existing articular surface incongruity causes disproportionate increases in contact stress rates compared with the increases in contact stresses.