INTRODUCTION: A medial malleolar osteotomy is often indicated for operative exposure of posteromedial osteochondral defects and fractures of the talus. To obtain a congruent joint surface after refixation, the oblique osteotomy should be directed perpendicularly to the articular surface of the tibia at the intersection between the tibial plafond and medial malleolus. The purpose of this study was to determine this perpendicular direction in relation to the longitudinal tibial axis for use during surgery. MATERIALS AND METHODS: Using anteroposterior mortise radiographs and coronal computed tomography (CT) scans of 46 ankles (45 patients) with an osteochondral lesion of the talus, two observers independently measured the intersection angle between the tibial plafond and medial malleolus. The bisector of this angle indicated the osteotomy perpendicular to the tibial articular surface. This osteotomy was measured relative to the longitudinal tibial axis on radiographs. Intraclass correlation coefficients (ICC) were calculated to assess reliability. RESULTS: The mean osteotomy was 57.2 +/- 3.2 degrees relative to the tibial plafond on radiographs and 56.5 +/- 2.8 on CT scans. This osteotomy corresponded to 30.4 +/- 3.7 degrees relative to the longitudinal tibial axis. The intraobserver (ICC, 0.90-0.93) and interobserver (ICC, 0.65-0.91) reliability of these measurements were good to excellent. CONCLUSION: A medial malleolar osteotomy directed at a mean 30 degrees relative to the tibial axis enters the joint perpendicularly to the tibial cartilage, and will likely result in a congruent joint surface after reduction.
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