Gender Differences in Tibia Size and Shape: Implications for Total Ankle Replacement Implant Fit and Coverage

Research Type: Level 4 – Case series Introduction/Purpose: Success after total ankle replacement (TAR) requires accurate implant sizing so the tibial component has anterior and posterior cortical support. Implant fit, however, can be challenging, especially in patients who are smaller and female. Because of medial-to-lateral (ML) constraints, surgeons may be forced to choose tibial components that are undersized in the anterior-to-posterior (AP) dimension. Prior studies reported gender differences in tibial morphology in healthy adults, but this has not been investigated in arthritic ankles. This study sought to quantify the distal tibia morphology in male and female TAR patients, and compare those measurements to their TAR implant sizes. We hypothesized that female patients would have narrower tibias than males, and these size differences would result in worse tibial implant coverage in the AP plane. Methods: Consecutive patients with preoperative weight-bearing computed tomography (WBCT) were included from a single-institution TAR registry. Using standard tools available from our institution’s picture archiving and communication system (PACS), we measured the mean tibial AP lengths and ML widths at a standardized distance from the ankle joint line. All measurements were performed after multiplanar reformatting according to standardized landmarks. Ratios of AP length:ML width were generated. Then, each patient’s AP and ML measurements were compared to the actual AP and ML dimensions of the TAR implant that the surgeon selected for that patient intraoperatively (Wright Inbone, Wright Infinity, or Exactech Vantage). This allowed us to quantify the actual implant coverage relative to each patient’s tibial morphology. Comparisons by gender were made using unequal variance t-tests with Satterthwaite approximation. Multiple linear regression analyses were performed incorporating gender, body size (height and weight), and AP and ML measurements. Results: 99 patients were included: 44 female (mean age 63.2 years, mean height 164.6cm, mean weight 79.4kg) and 55 male (mean age 63.1 years, mean height 178.5cm, mean weight 92.0kg). There was no difference in the ratio of AP to ML tibial size between men and women (P=0.68). However, there were significant differences in implant coverage between genders. Regardless of implant type used, tibial implants took up significantly greater amounts of the ML plane in women (mean 2.3mm difference), P< 0.01; meanwhile, the AP coverage of the tibia by implant used was similar between genders (P=0.81). In multiple regression analysis, these differences were primarily explained by body size, but gender still had a statistically significant effect on implant coverage (1.15mm [CI 0.28-2.02], P< 0.01). Conclusion: We found significant differences in TAR implant coverage between genders: tibial implants took up greater space in the ML plane in women compared to men in order to achieve similar AP cortical support. These differences were primarily due to females having smaller body size, but gender was also a significant factor. Therefore, tibial components may seem oversized in the ML plane relative to the AP plane in small female patients. The impact of this mismatch on outcomes remains unknown; further research may explore if more implant sizing options should be made available or if patient-specific implants may be beneficial.