THE EFFECT OF FIBULAR MALREDUCTION ON CONTACT PRESSURES IN AN ANKLE FRACTURE MALUNION MODEL

Nine fresh-frozen cadaveric specimens were disarticulated through the knee, and the soft tissues, except for the interosseous ligaments and interosseous membrane, mere removed to the level of the ankle. The sub talar joint was secured with screws in neutral position (approximately 5 degrees of valgus). Contact pressures in the tibiotalar joint were measured with use of low-grade pressure-sensitive film, which was plac ed through an anterior capsulotomy. For each measurement, 700 newtons of load was applied to the specimen for one minute. The film imprints were scanned, and the contact pressures were quantitated in nine equal quadrants over the talar dome. A fracture-displacement device was sec ured to the distal end of the fibula; the device allowed for individua l or combined displacements consisting of shortening, lateral shift, a nd external rotation of the fibula. The ankle was maintained in neutra l flexion. The ligamentous injury associated with a pronation-lateral rotation fracture of the ankle was simulated by dividing the deep fibe rs of the deltoid ligament, the anterior-inferior tibiofibular ligamen t, and the interosseous membrane to a point that was an average of fif ty-three millimeters proximal to the ankle joint. Baseline contact are a and contact pressure in the joint mere determined, followed by measu rements after two, four, and six millimeters of shortening of the fibu la; after two, four, and six millimeters of lateral shift of the fibul a; and after 5, 10, and 15 degrees of external rotation of the fibula. The three types of displacement were tested individually as well as i n combination. The simulated deformities were found to cause a shift o f the contact pressure to the mid-lateral and posterolateral quadrants of the talar dome, with pressures as high as 4.1 megapascals. A corre sponding decrease in the contact pressures was noted in the medial qua drants of the talar dome. The highest pressures were recorded for maxi mum shortening of the fibula, the combination of maximum shortening an d lateral shift, the combination of maximum shortening and external ro tation, and the combination of maximum shortening, lateral shift, and external rotation. In general, increases in each displacement variable corresponded to increasing contact pressures. CLINICAL RELEVANCE: Pre vious biomechanical studies have demonstrated mixed results regarding the effect of lateral displacement of the talus on contact pressures i n the ankle joint. We believe that we are the first to evaluate the in dividual and combined effects of shortening, lateral displacement, and malrotation of the fibula while load was applied through the tibial p lateau - that is, while the tibia and fibula were loaded in a more phy siological manner than accomplished previously The findings of the pre sent study confirm that substantial displacement of the fibula (two mi llimeters or more of shortening or lateral shift or 5 degrees or more of external rotation) increases the contact pressures in the ankle joi nt. Therefore, displacement of the fibula in these injuries should not be accepted.