Kinematic and contact stress analysis of posterior malleolus fractures of the ankle.

Objective: To determine if there are measurable dynamic contact stress aberrations and kinematic abnormalities (instability) that have not been observed in conventional static loading studies of posterior malleolar ankle fractures.

Design: Cadaveric fracture model.

Setting: Biomechanics laboratory.

Intervention: Seven fresh cadaveric specimens were fixed in an unconstrained testing apparatus and loaded to one body weight. The ankle was moved from 25 degrees of plantarflexion to 15 degrees of dorsiflexion. The model included the intact ankle and four fracture simulations (50% fracture without internal fixation, 2 mm gap and step malreductions, and anatomically fixed).

Main Outcome Measure: Motion at the ankle was monitored with an electromagnetic tracking device, and intra-articular contact stresses were measured using a real-time stress sensor.

Results: There were no kinematic abnormalities suggestive of tibiotalar subluxation in any of the fracture simulations. There was no increase in peak contact stress in any of the fracture models compared with the unfractured model. However, there was a shift in the location of the contact stresses to a more anterior and medial location following the fracture. When summed over the range of motion, these areas of cartilage bore significantly higher cumulative contact stresses relative to the nonfracture situation.

Conclusions: We found no talar subluxation and no increase in contact stresses near the articular incongruity, making it unlikely that these factors explain the increased incidence of arthrosis after trimalleolar fractures (OTA/AO classification 44 B3 fractures). Rather, we found that the joint remaining bears increased stress and that the center of stress shifts anteriorly, loading cartilage that normally sees little load.