Comparison of Foot Center Axis and Ankle Rotation using Weight-Bearing CT

CATEGORY: Ankle; Ankle Arthritis INTRODUCTION/PURPOSE: Even as its popularity increases, there remain many unanswered questions regarding total ankle arthroplasty. One debate considers how best to restore proper rotation of the talar component relative to native, non- osteoarthritic anatomy. At present, two common schools of thought are to parallel the talar component rotation with the medial gutter line or to choose a line that bisects the gutters. These axis' are then considered with respect to a chosen foot axis, often defined by the second metatarsal or second toe clinically. The present study sought to define the foot center axis relative to the ankle on weight-bearing CT in patients without ankle osteoarthritis to inform talar component positioning in total ankle arthroplasty. METHODS: Following IRB approval, charts were reviewed to identify patients without ankle osteoarthritis, trauma or other deformity who received a weightbearing CT scan. A pilot group of n=34 patients were included for a total of n=50 limbs. Average age was 50.3 +/- 7 and there were = 54% female patients. All measurements were made on MPR (multiplanar reformation) axial reconstructions. Cobb angle measurements between the anatomic foot center, the second, the fourth metatarsal and the gutter bisector or medial gutter line were recorded. Statistical analysis was performed by IBM SPSS Statistics for Windows, Version 27.0. Armonk, NY. RESULTS: The mean gutter bisector angle was 5.7 deg +/- 2.0. The mean cobb angle between the anatomic foot center and the medial gutter line was 6.1 deg +/- 4.4 and between the anatomic foot center and gutter bisector was 6.3 deg +/- 4.8, there was no statistical difference between measurements (p=0.62, Table 1). The mean cobb angle between the second metatarsal and the gutter bisector was 9.9 deg +/- 6.8, whereas the mean cobb angle between the fourth metatarsal and the gutter bisector was 7.5 deg +/- 5.2, which was not statistically significant (p=0.053). There was no statistical difference between mean cobb angle measurements between the anatomic foot center and the gutter bisector and the fourth metatarsal and the gutter bisector (p=0.243), but there was statistical difference between mean cobb angle measurements of the second metatarsal and the gutter bisector and the anatomic foot center and the gutter bisector (p=0.003). CONCLUSION: The axial alignment of the foot center axis relative to the ankle in patients without deformity or osteoarthritis is most collinear with the medial gutter line. In general, the axial alignment of the ankle joint relative to the foot in patients without osteoarthritis tends towards <10 degrees of difference on weight-bearing CT, regardless of method chosen. Due to both the consistency of measure and reproducibility of measure on pre-operative weight-bearing CT, the anatomic foot center may be a useful guide for establishing intra-operative foot orientation when performing total ankle arthroplasty.