CONTACT CHARACTERISTICS OF THE SUBTALAR JOINT AFTER A SIMULATED CALCANEUS FRACTURE

A simple calcaneus fracture consisting of two parts was modeled in nin e fresh cadaver hindfoot specimens to assess changes in subtalar joint contact characteristics with increasing plantar depression of the pos terolateral fracture component. To perform the experiment, rods were p laced in the tibial and fibular shafts of each specimen, which was mou nted in a frame in neutral stance. A pneumatic cylinder was used to de liver a vertical compressive load through the rods into the foot while permitting free motion of the foot in the horizontal plane. Seared pa ckets of pressure-sensitive film were inserted into the anterior-middl e and posterior facets of the talocalcaneal articulation, and a 700-N load was applied. After testing of the intact foot, a primary fracture line was created using a microoscillating saw. The osteotomized poste rolateral component was anatomically reduced and fixed, the film inser ted, and the load reapplied. The test was repeated after the posterola teral fragment was displaced 2, 5, and 10 mm in a plantar direction. T he resulting pressure prints were scanned along with pressure/color de nsity calibration strips using a flat-bed scanner, and an image analys is system was used to determine contact areas within specified pressur e intervals. The contact area (>0.5 MPa) of the posterior facet was si gnificantly decreased with 2, 5, and 10 mm displacements of the poster olateral calcaneus fracture component. The ratio of high-pressure area (>5.0 MPa) to contact area in the posterior facet was significantly i ncreased only with displacements of 5 and 10 mm. There were no signifi cant changes in any contact parameters in the anterior-middle facet. B ecause of the changes in contact characteristics measured, these resul ts suggest that a calcaneus fracture with 2 mm of fracture component d isplacement in the posterior facet should be reduced.