INTRODUCTION: With the increased use of computed tomography scans in cases with trimalleolar ankle fractures, bone fragments between the posterior malleolus and the rest of the articular surface tibial plafond surface - described as intercalary fragments (ICFs) - can be recognized. The aim of this study was to determine the ICF size threshold for a significant change in the pressure distribution at the ankle joint, having a considerable impact on the remaining cartilage of the joint.

DESIGN AND METHODS: Eight human cadaveric lower legs were used, and a posterior malleolus Bartoniček/Rammelt II fracture was created with sequential 2 mm, 4 mm, 6 mm, and 8 mm ICFs. The posterior malleolus was fixed with a 3.5 mm one-third tube buttress plate and each specimen was mounted in a custom-made frame for axial loading under 700 N in neutral position, 30° plantar flexion, and 30° dorsiflexion of the foot. Using electronic foil sensors, pressure measurements were performed to define the contact area, centre of force, and peak pressure at the ankle joint in loaded condition.

RESULTS: Compared to the situation without ICF, contact area decreased significantly after 4 mm, 6 mm and 8 mm ICFs. Anteroposterior center of force shift was significant after 6 mm and 8 mm ICFs. Mediolateral center of force shift was significant after 2 mm, 4 mm, 6 mm and 8 mm ICFs.

CONCLUSION: Posterolateral defects of the tibial plafond alter ankle joint pressure characteristics. Regardless of the direction of ankle joint flexion, increasing the ICF defect size results in decreased contact area, increased mediolateral center of force migration and higher peak joint forces. Malreduction or removal of ICFs larger than 2 mm should be avoided to preserve physiological ankle pressure characteristics.