Foot and Ankle Offset in the Setting of Severe Rotational Foot and Ankle Deformities

Abstract

Category: Ankle; Hindfoot Introduction/Purpose: Foot and Ankle Offset (FAO) is a clinically relevant measurement technique used to objectively evaluate the foot and ankle that has been shown to be reliable and validated in common mild foot deformities. It represents a measurement of the offset between the body weight vector and the ground reaction force vector, thus making it a biomechanically relevant measurement. However, FAO has not been validated in the setting of severe ankle deformity. The goal of this paper was to evaluate the validity of FAO measurements in the setting of severe foot and ankle deformities by utilizing a novel rotational FAO measurement technique to account for the deformity. Methods: This study included 57 feet (36 patients) that had a history of severe cavovarus deformity. Each participant received a Weightbearing CT (WBCT) scan that was then used to measure FAO. This measurement was taken three times, once using the traditional measurement technique and two additional times using a modified technique with a 15-day washout period between each measurement. This modified technique allowed for alignment of the talus in a neutral position through rotational correction in the axial, coronal, and sagittal planes to identify the most proximal and central point of the talus. Patients were broken into three groups based on the alignment of their foot and ankle. Normal alignment was defined as a FAO of 2.3% ± 2.9%, varus alignment as -11.6% ± 6.9%, and valgus alignment as 11.4% ± 5.7%. The measurements from the different techniques were compared to identify validity between them and the intraobserver reliability was assessed. Results: The mean traditional FAO was 2.37 ± 4.65% (95% CI=1.16–3.59) and modified FAO was 2.51 ± 4.6 (95% CI=1.3–3.71). The mean modified FAO values between the different alignment groups were found to be significantly different (p <.0001). Significant differences were also found when comparing varus to valgus (p <.001), varus to physiologic (p = .002) and valgus to physiologic alignment (p=.002). Traditional FAO and modified FAO measurements were found to have a significant correlation between one another (r(54) = 0.92, p<.001). There was found to be a high positive correlation between the variables of the two techniques (r=0.92) with the intraobserver reliabilities (ICC=0.95) for FAO measurements being excellent. The agreement between traditional FAO and modified FAO measurements was considered excellent as well (ICC=0.99). Conclusion: The Traditional and Modified FAO methods produce significantly similar FAO values even in the setting of severe ankle deformities. Thus, Traditional FAO measures demonstrated the capacity to objectively portray disease progression in not only mild, but also severe forms of ankle deformities, despite the unique and severe physiological contortions of the foot and ankle in these patients. Therefore, the Traditional FAO measurement method could potentially be used to provide a more detailed depiction of the misalignment in the foot and ankle, and providers could more accurately treat these patients and potentially supply them with better outcomes.