Medial and Lateral Malleoli Impose Anatomical Constraints on Total Ankle Arthroplasty Design

CATEGORY: Ankle INTRODUCTION/PURPOSE: Contemporary total ankle arthroplasty (TAA) techniques reconstruct the tibiotalar articulation while preserving the medial and lateral malleoli. The malleoli place anatomic constraints on the design of the talar component of the TAA system. If these constraints are not respected, then proper placement of the talar component may be compromised intra- operatively, or unintended contact between the prosthesis and bone may occur post-operatively. The goal of this study was therefore to quantify the medial and lateral malleolar boundaries. METHODS: The orientation of the talus from a frontal view was quantified based on CT scans of left leg, non-arthritic specimens (n=89; 52M/37F) (Figure 1a). The talar dome was identified as the portion of the talus superior to the talar neck. The frontal profile of the dome defined by a plane positioned through the medial and lateral high points was extracted for each specimen. Statistical shape analysis was performed to identify the modes of variation of the frontal profile. Medial and lateral lines were fit to each profile, and resulting angles relative to the superior-inferior axis were measured. A paired student t-test (P<.05) was used to assess differences between the medial and lateral malleolus. RESULTS: Figure 1b shows the average frontal profile of the talar dome, as well as ±1 and ±2 standard deviations. The average medial and lateral taper angles of the talus were 15.5° ± 5.9° and 7.6° ± 2.9°, respectively, with the lateral taper angle being significantly smaller than the medial taper angle (P<.001). The medial taper increased with talus size (as measured by the medial- lateral width of the dome) whereas the lateral taper decreased with talus size (Figure 1c), though both regressions were weak (R2 < 0.1). No statistically significant difference in taper angles was found between genders. CONCLUSION: Restoration of the bicondylar articulation geometry of the tibiotalar joint is an important design goal for TAA. One aspect of this geometry is the anatomic constraints imposed by the medial and lateral malleoli, to minimize unexpected impingement of the TAA prostheses with surrounding anatomic structures. Here, we have quantified those constraints through analysis of the medial and lateral taper of the talus, showing an increased taper angle on the medial side of the talus as compared to the lateral side.