MRI Analysis for Clarification of Mechanism of Ankle Osteoarthritis: When the Talar Tilt Exceeds 15 Degrees, the BME Pattern of the Ankle Are Changes

CATEGORY: Ankle Arthritis INTRODUCTION/PURPOSE: In recent years, total ankle arthroplasty (TAA) has been widely performed for severe ankle osteoarthritis (OA). However, TAA is not always successful in cases of advanced varus; in fact, some researchers have stated caution regarding its indication. Previously, to elucidate the pathological condition of ankle OA using MRI, we investigated that confirmed the existence of an association among the Takakura–Tanaka classification, foot and ankle alignment, and bone marrow edema (BME). In this study, we focused on the talar tilt angle and compared the cases of terminal ankle OA as per Takakura– Tanaka classification (stage 3b and 4) wherein this angle exceeded 15° with those wherein it did not exceed in terms of the mode of BME onset. METHODS: Of 616 cases of ankle OA diagnosed in our hospital between May 2009 and January 2018, we examined the MRI images of 52 feet of 50 patients diagnosed with severe ankle OA. The talar tilt angle with the ankle under load was measured using frontal X-ray, following which the presence/absence of BME was determined by dividing the talus, subtalar, and Chopart’s joints into 22 regions (areas 1–11 and 1’–11’). In statistical analysis, we first obtained the total number of BME incidences for each case. Then, after dividing this disease group into severe varus (SV; talar tilt angle = 15° or more) and mild varus (MV; talar tilt angle < 15°) groups, we compared t-test scores for the respective BME incidence rates. Furthermore, we used Fisher’s exact test to examine differences in terms of BME incidence rates between the two groups for each subdivided region. RESULTS: No significant differences were found between the two groups in terms of BME incidence rates for each case. However, the rates in each area were significantly lower in the SV group than in the MV group for area 2 (SV group, 14%; MV group, 57%) and area 4 (SV group, 7%; MV group, 39%), i.e. the SV groups corresponding to the outer surface of the trochlea talar. Conversely, in the subtalar joint, the rates were significantly higher in the SV group than in the MV group for area 10 (SV group, 36%; MV group, 11%) and area 10’ (SV group, 29%; MV group, 5%), i.e. the medial surface of the calcaneus. CONCLUSION: In cases of severe ankle OA wherein the talar tilt angle exceeds 15°, the load exerted on the outer side of the talus decreases in the talar joint, whereas a greater load is exerted on the medial subtalar joint located at the innermost side in the subtalar joint. Differences in terms of the mode of BME incidence in the subtalar joint, which is not replaced with normal TAA, may be a poor prognostic factor for postoperative TAA.