Clinical Significance of Measurements in Adult Acquired Flatfoot Deformity using Weight Bearing CT: A Matched Case-Control Study
Authors: Lintz Francois , Bernasconi Alessio , Lalevée Matthieu , Fernando Céline , Barg Alexej , Netto Cesar de Cesar 
Published: November 2020

Category: Hindfoot; Other Introduction/Purpose: Adult Acquired Flatfoot Deformity (AAFD) results in progressive foot collapse through peritalar subluxation. Numerous radiographic and Weight Bearing CT (WBCT) measurements have been described in the literature aiming to gauge the severity of the multiple components of the deformity. However, the real diagnostic power of each measurement is currently unknown. Moreover, novel measurements have recently been described such as 3D biometrics and multidimensional measurements. The objective of this case-control study was to individually assess the diagnostic accuracy of known 2D and 3D WBCT measurements and to compare it with a novel multidimensional measurement. We hypothesized that the latter would demonstrate superior diagnostic power than isolated 2D and 3D measurements. Methods: Retrospective case-control study, including 19 AAFD feet and 19 controls that were matched for age, gender and BMI (9 male, 10 female, mean age 54.4 years in both groups). All patients had standing WBCT imaging as baseline assessment of their foot pathology. 2D measurements assessed included: axial and sagittal talus-first metatarsal angles (TM1A), talonavicular coverage angle (TNCA), forefoot arch angle (FFAA) and middle facet incongruence angle (MF°) and uncoverage percentage (MF%). The 3D Foot and Ankle Offset (FAO) was calculated using semi-automatic software. A novel multiplanar biometric measurement (AAFD- MD) was calculated using a multidimensional mathematical algorithm that pooled multiplanar 2D measurements. Intra and interobserver reliabilities were assessed. Comparisons between variables were done using Student-t test or Wilcoxon rank-sum test. Receiver Operating Characteristic (ROC) curves were calculated to determine diagnostic accuracy, sensitivity and specificity of each measurement. Results: AUC for ROC curves were 1. for MF%, 0.96 for FAO, 0.94 for MF° and 0.92 for AAFD-MD. For MF%, a threshold value equal of greater than 28.1% was found to be diagnostic of AAFD with a sensitivity of 100% and specificity of 100%. FFAA were decreased in AAFD: 6.3° versus 15.2° in controls (p<0.001). Axial and sagittal TM1A were respectively 17.6° and 20.8° in AAFD, while in controls: 7.5° (p<0.001) and 6.3° (p< 0.001). The TNCA was increased in AAFD: 27.9° versus 15.6° in controls (p<0.001). In AAFD, MF° and MF% were respectively 13° and 49.4% compared with 5.3° and 10.6% in controls (p<0.001 for both). The FAO was 7.5% in AAFD and 1.1% in controls (p<0.001). Conclusion: The observed results did not confirm our hypothesis. The multidimensional measurement was not as accurate a diagnostic tool as Middle Facet uncoverage percentage which expresses the amount of subluxation of the MF. In that respect, this could mean that congruency of the middle facet could be the last frontier between asymptomatic Pes Planovalgus and symptomatic AAFD, leading to progressive foot collapse, secondarily affecting the FAO. These results also give insight into the meaning of the FAO, which appears here to be a more general assessment of the Foot and Ankle Complex alignment, rather than a marker for a specific pathology.