Implications of Size and Gender for Implant Fit and Coverage in Total Ankle Replacement.

Background: Success after total ankle replacement (TAR) requires accurate implant sizing so the tibial component achieves adequate cortical support. Implant fit is challenging in smaller and female patients because medial-lateral (ML) constraints may force selection of tibial components undersized in the anterior-posterior (AP) dimension, although this has not been investigated in arthritic ankles. This study sought to quantify distal tibia morphology in TAR patients and compare those measurements to implant dimensions. Methods: Ninety-nine consecutive TAR patients with preoperative weightbearing computed tomographic (CT) scan were retrospectively reviewed. Multiplanar reformatting aligned axial slices orthogonal to the tibial anatomical axis. Tibial AP length and ML width were measured at 8 and 10 mm resection depths. Manufacturer specifications for the implanted tibial tray were used to obtain AP and ML dimensions. Dimensional mismatch was defined as anatomic measurement minus implant dimension (negative = overhang, positive = undercoverage). Sex-based comparisons used unequal-variance Results: There were significant differences in implant coverage between genders. Tibial implants took up significantly greater amounts of the ML plane in women (mean 2.3 mm difference), Conclusion: In this cohort, we found that tibial trays consistently exceeded native AP dimensions while leaving limited ML clearance, particularly in smaller and female tibiae. These constraints may approach thresholds associated with medial malleolar fracture and gutter impingement. Implant systems decoupling AP and ML sizing or offering narrower ML options may better accommodate distal tibial morphology. Clinical implications remain to be established. Level of Evidence: Level III, retrospective cohort study.