Correlation Between Three-Dimensional Biometric Weightbearing CT Assessment of Hindfoot Alignment and Conventional Measurements of Adult Acquired Flatfoot

Abstract

Category: Hindfoot; Ankle; Other Introduction/Purpose: Semi-automatic three-dimensional (3D) biometric weightbearing CT (WBCT) tools have been shown to accurately demonstrate the relationship between the center of the ankle joint and the tripod of the foot. The measurement of the Foot and Ankle Offset (FAO) represents an optimized biomechanical assessment of foot alignment. The objective of this study was to evaluate the correlation between FAO and traditional adult acquired flatfoot deformity (AAFD) markers, measured in different planes. We hypothesized that the FAO would significantly correlate with other radiographic markers of pronounced AAFD. Methods: In this retrospective comparative study, we included 113 patients with stage II AAFD, 43 men and 70 women, mean age of 53.5 (range, 20 to 86) years. Three-dimensional coordinates (X, Y and Z planes) of the foot tripod (most plantar voxel of the first and fifth metatarsal heads, and calcaneal tuberosity) and the center of the ankle joint (most proximal and central voxel of the talar dome) were harvested by two blinded and independent fellowship-trained orthopedic foot and ankle surgeons. The FAO was automatically calculated using the 3D coordinates by dedicated software. Multiple WBCT parameters related to the severity of the deformity in the coronal, sagittal, and transverse plane were manually measured. Results: We found overall good to excellent intra (range, 0.84-0.99) and interobserver reliability (range, 0.71-0.96) for manual AAFD measurements. FAO semi-automatic measurements demonstrated excellent intra (0.99) and interobserver reliabilities (0.98). Hindfoot moment arm (p<0.00001), subtalar horizontal angle (p<0.00001), talonavicular uncoverage angle (p=0.00004) and forefoot arch angle (p=0.0001) were the only variables found to significantly influence and correlate with FAO measurements, with an R-squared value of 0.79. A value of hindfoot moment arm of 19.8mm was found to be a strong threshold predictor of increased values of FAO, with mean values of FAO of 6.5 when the HMA was lower than 19.8mm and 14.6 when the HMA was equal or higher than 19.8mm. Conclusion: We found that 3D WBCT semi-automatic measurements of Foot and Ankle Offset (FAO) significantly correlated with traditional markers of pronounced AAFD. HMA, subtalar horizontal angle, talonavicular uncoverage angle and forefoot arch angle were found to explain 79% of the variations in FAO measurements. FAO Measurements were also found to be more reliable than the manual measurements. The FAO offers a more complete biomechanical and multiplanar assessment of the AAFD, that that accounts for relative positioning of the foot tripod and the center of the ankle joint, representing in a single measurement the three-dimensional components of the deformity.