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 subtalar 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 placed 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 secured to the distal end of the fibula; the device allowed for individual or combined displacements consisting of shortening, lateral shift, and external rotation of the fibula. The ankle was maintained in neutral flexion. The ligamentous injury associated with a pronation-lateral rotation fracture of the ankle was simulated by dividing the deep fibers of the deltoid ligament, the anterior-inferior tibiofibular ligament, and the interosseous membrane to a point that was an average of fifty-three millimeters proximal to the ankle joint. Baseline contact area and contact pressure in the joint mere determined, followed by measurements after two, four, and six millimeters of shortening of the fibula; after two, four, and six millimeters of lateral shift of the fibula; and after 5, 10, and 15 degrees of external rotation of the fibula. The three types of displacement were tested individually as well as in combination. The simulated deformities were found to cause a shift of the contact pressure to the mid-lateral and posterolateral quadrants of the talar dome, with pressures as high as 4.1 megapascals. A corresponding decrease in the contact pressures was noted in the medial quadrants of the talar dome. The highest pressures were recorded for maximum shortening of the fibula, the combination of maximum shortening and lateral shift, the combination of maximum shortening and external rotation, 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: Previous biomechanical studies have demonstrated mixed results regarding the effect of lateral displacement of the talus on contact pressures in the ankle joint. We believe that we are the first to evaluate the individual and combined effects of shortening, lateral displacement, and malrotation of the fibula while load was applied through the tibial plateau - that is, while the tibia and fibula were loaded in a more physiological manner than accomplished previously The findings of the present study confirm that substantial displacement of the fibula (two millimeters or more of shortening or lateral shift or 5 degrees or more of external rotation) increases the contact pressures in the ankle joint. Therefore, displacement of the fibula in these injuries should not be accepted.