THE JOINT-CONTACT AREA OF THE ANKLE - THE CONTRIBUTION OF THE POSTERIOR MALLEOLUS

Eight ankles from fresh cadavera were tested under simulated clinical conditions to determine the effect of increasing the size of the posterior malleolar fracture on the contact area of the ankle joint and on the distribution of joint pressure. The surface area of contact decreased with increased size of the posterior malleolar fragment. However, the documented changes were smaller than expected on the basis of the findings of Ramsey and Hamilton; they reported a 42 per cent reduction in contact area with only a one-millimeter lateral shift of the talus, which clinically would be associated with a similar one-millimeter shift of the distal tibial fragment. In addition, clinical experience has shown a high rate of post-traumatic degenerative arthritis associated with an inadequately reduced one-half-size posterior fragment. There were considerable changes in the load-distribution patterns, with increased confluence and concentration of loads as the size of the fragment was increased. In plantar flexion, many specimens had three separate areas of contact between the tibia and the talus. With increased size of the posterior fragment, the three areas of contact always joined to become one. Similarly, for all positions of the ankle, increased size of the posterior fragment caused decreases in the contact area. The maximum loss of contact area was 35 per cent for specimens with one-half-size fractures that were tested in the neutral position. CLINICAL RELEVANCE: The changes in load distribution and contact area that are associated with fracture of the posterior malleolus and with increased size of posterior fracture fragments may predispose ankles injured in this way to degenerative changes. Any inaccuracy in reduction or fixation of the posterior fragment in a non-anatomical position will increase the risk of post-traumatic arthritis.