Subtalar Joint Behavior After Total Ankle Arthroplasty

CATEGORY: Ankle Arthritis; Hindfoot INTRODUCTION/PURPOSE: Proper prosthetic alignment for Total Ankle Arthroplasty (TAA) surgery has proven to improve implant survival and prevent complications. Unfortunately, most of the previously published data present information from non weight- bearing computed tomography (WBCT) scans or lack a correlation of the effects of TAA on hindfoot. To the author's knowledge, subtalar joint behavior after TAA is not completely understood. Therefore, the aim of this study is to assess subtalar joint alignment correction after TAA, and its correlation with the tibiotalar alignment correction using WBCT scan. METHODS: After obtaining IRB approval, our prospectively collected surgical database was queried for patients undergoing TAA for end stage ankle osteoarthritis. Patients were included if they had preoperative WBCT scan and underwent TAA using an Infinity Prosthesis with standard instrumentation. Patients were excluded if supramalleolar osteotomy, calcaneal osteotomy or revision surgery had been performed. Twenty patients met the inclusion/exclusion criteria. All patients were brought back to the clinic for WBCT scan and to sign informed consent for the specific needs of this study. Eight coronal radiographic parameters were assessed pre and postoperatively on WBCT imaging. Three fellowship-trained musculoskeletal radiologists realized all measurements in two time-frames separated for one month each. Cohen's inter and intraclass coefficients were calculated to estimate the amount of agreement that occurred by chance. RESULTS: Median age was 62 years (range 50-69), 37.5% female patients, median IMC 28 (range 25-33), 10 right feet. Good to excellent intraobserver (ICC 0.89, 0.75, 0.90) and interobserver (ICC 0.82) reliability were reported. Tibiotalar axis changed in 80% of patients, subtalar axis in 100% and hindfoot axis in 80%. Preoperative WBCT scan measurement showed median Medial Distal Tibial Angle (MDTA) of 93.6° (range 83.5°-105°) and median Talo-Calcaneal Angle (TCA) of 6.1° (range -2.4°-20.9°). Postoperative WBCT scan measurement showed median MDTA of 91.0° (range 82.6°-97.5°) and median TCA of 9.0° (range 0.8°- 15.7°). There was no correlation between the lower tibial platform axis (TAS) and subtalar or hindfoot axis. The change in the upper talus platform axis (TTS) showed a direct correlation with the change in the hindfoot axis (p <0.05). The change in joint congruence (TTAC) showed a direct correlation with change in subtalar axis (p <0.01). CONCLUSION: This is the first study that assesses subtalar joint alignment correction after TAA and its correlation with the tibiotalar alignment correction using WBCT scan. Tibiotalar alignment is often corrected after TAA. Subtalar joint axis and hindfoot axis change after TAA in response to tibiotalar axis, and this change occurs in the same direction as the tibiotalar correction. Future studies should focus on studying the cause of this lack of compensation.