Validating Talar Center of Migration Ratio: A Marker of Deformity in Ankle Arthritis

Research Type: Level 4 – Case series Introduction/Purpose: Coronal talar translation – the medial or lateral shift of the talus underneath the axis of the tibia – is increasingly recognized as an important marker of deformity in ankle arthritis. Adequate restoration of the talus position is considered essential to success in both joint-sparing and joint-sacrificing procedures like total ankle replacement (TAR). The current validated method for measuring this deformity is the talar center of migration (TCM), which requires a calibrated radiograph and does not account for variability in talar size. This study sought to develop a novel method to measure talar translation that could allow for accurate measurements across all talus sizes using non-calibrated radiographs. In addition, we aimed to describe correlations between talar migration, PROMIS scores, and intraoperative concomitant procedures during TAR. Methods: Patients with ankle arthritis were collected from a single-institution prospective TAR registry from 2023-2024. Inclusion criteria were preoperative calibrated weight-bearing radiographs. For each patient, 4 raters measured talar translation using 2 methods. The validated method, TCM, measures the distance from the center of the talus (along the bimalleolar axis) to the mechanical axis of the tibia. The novel method, TCM-Ratio (TCM-R) is a ratio of the distance between the center of a best-fit circle over the talus to the mechanical axis of the tibia divided by the radius of the talus. Intraclass correlation coefficients (ICC) were used to assess inter-rater agreement, and Pearson’s correlations were used to validate TCM-R to the validated standard, TCM. Multiple regression analysis, controlling for concomitant hindfoot fusion and coronal tibiotalar angle, assessed the relationship between TCM magnitude and preoperative PROMIS scores of pain interference and physical function. Finally, intraoperative concomitant procedures were reported. Results: Fifty-eight patients, 60.3% male, with mean age 64.8±9.4 years and mean BMI 30.0±4.7 kg/m2 were included. Agreement between the four raters was excellent for both TCM (ICC=0.914) and TCM-R (ICC=0.944). TCM-R was highly correlated with TCM (R=0.95, CI [0.91, 0.97], p< 0.001). The mean absolute value of TCM-R was 13.89±11.29, indicating that the talus was, on average, 14% medially or laterally translated underneath the tibia in ankle arthritis. The range of TCM-R was [-34.57% to +47.53%]. Positive values indicated medial translation, which was present in 43.1% (25/58) of patients. In regression analysis, TCM-R was not significantly associated with preoperative PROMIS scores. However, TCM-R was slightly higher in patients who underwent lateral ligament stabilization during TAR (18.33%±14.26%) compared to patients that did not (12.20%±9.60%). Conclusion: We described a new ratio-based method to measure coronal talar translation under the tibia, which obviates the need for calibrated radiographs and accounts for variations in talar size. This new measurement, TCM-R, had excellent inter-rater reliability and strongly correlated with the gold standard TCM. Although TCM-R was not associated with severity of patients’ preoperative pain or physical function as defined by PROMIS, it does reflect a deformity pattern that must be addressed intraoperatively by appropriate balancing of the ankle at the time of TAR. Future analysis will be critical to determine if inadequate TCM-R correction leads to worse postoperative outcomes.