Validating 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 the proper rotation of the talar component to restore native, non-osteoarthritic anatomy. The foot center axis is a unique anatomic concept that considers the position of the native foot relative to the ankle. The present pilot study sought to evaluate the reproducibility of axial alignment measurements, including the foot center on weight-bearing CT in patients without ankle osteoarthritis. 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=8 patients were included for a total of n=10 limbs. Average age was 50.1 +/- 14 and there were 38% 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 by n=4 observers. Intraclass correlation (ICC) was produced using a two-way random effects model with absolute agreement and other statistical testing was performed by IBM SPSS Statistics for Windows, Version 27.0. Armonk, NY. RESULTS: The average gutter bisector angle was 6.2 deg +/- 2.0 and the average angle between the medial base of the 1st metatarsal head, lateral base of the 5th metatarsal head and the posterior calcaneus was 33.0 deg +/- 2.6. Amongst the observers, the ICC for these landmarks were 0.95 and 0.96 respectively. The average cobb angle measurement between the gutter bisector angle and the second metatarsal was 8.3 deg +/- 7.3, with a corresponding ICC of 0.96. The average cobb angle measurement between the gutter bisector angle and the anatomic foot center was 5.6 deg +/- 4.3 and the average cobb angle measurement between the medial gutter line and the anatomic foot center was 6.23 deg +/- 6. Amongst the observers, the ICC for these measurements were 0.81 and 0.81 respectively. CONCLUSION: In a small pilot study of patients without osteoarthritis who underwent weight-bearing CT scans there was good (ICC>0.75-0.90) to excellent (ICC>0.90) agreement amongst observers for measurements of foot center axis and ankle rotation. These findings validate the reproducibility and agreement of these manually constructed measurements for assessing the relationship between the foot center axis and ankle rotation. Future studies will expand upon the use of these measurements to define non-arthritic anatomy. The anatomic foot center may be a useful guide for establishing intra-operative foot orientation when performing total ankle arthroplasty.