Hindfoot alignment assessment by the foot-ankle offset: a diagnostic study.

Abstract

Foot-ankle offset (FAO) is a three-dimensional (3D) biometric measurement of hindfoot alignment (HA) measured on images from weight-bearing computed tomography (WBCT). Our aim was to investigate its distribution in a large cohort of patients, hypothesizing that threshold FAO values in valgus or varus could be identified as markers for increased risk of associated pathologies. Prospective, monocentric, level II study including 125 subjects (250 feet) undergoing bilateral WBCT [58.4% female; mean age, 54years (18-84)]. Patients were clinically assessed and pathologies were classified according to anatomic location (valgus- or varus associated). HA was measured using FAO on 3D datasets and tibio-calcaneal angles (TCA) on two-dimensional Saltzman-El-Khoury views. Threshold FAO values and area under the receiver operating characteristics curve (AUC) were calculated for predicting increased risk of medial or lateral pathologies. Mean FAO was 1.65% ± 4.72 and mean TCA was 4.15° ± 7.67. Clinically, 167 feet were normal, 33 varus and 50 valgus with FAO values of 1.71% ± 3.16, -4.96% ± 5.30 and 5.79% ± 3.77, respectively. Mean FAO was 0.99% ± 3.26 for non-pathological feet, -2.53% ± 5.05 for lateral and 6.81% ± 2.70 for medial pathologies. Threshold FAO values of -1.64% (51.4% sensitivity, 85.1% specificity, AUC = 0.72) and 2.71% (95% sensitivity, 82.8% specificity, AUC = 0.93) best predicted the risk of lateral and medial pathology, respectively. Patients with FAO between -1.64% and 2.71% had the least risk of degenerative foot and ankle pathology. This interval could be considered a target for patients undergoing realignment procedures. A "safe zone" for Foot Ankle Offset was described between -1.64% and 2.71%, for which the risk of foot and ankle pathologies is lower. II-Diagnostic study.