The Spatial Orientation of the Foot: A Critical Step in Hindfoot Alignment Evaluation on Weight Bearing CT

Abstract

Category: Hindfoot; Ankle Introduction/Purpose: Weight Bearing CT (WBCT) has been proven to be effective in hindfoot alignment assessment. However, we have noted that in both WBCT examination and evaluation, positioning of the foot has not been sufficiently prioritized. We hypothesized that changing the foot orientation will significantly affect the measurement of the hindfoot, which will in turn affect further clinical decision making. Methods: WBCT scans of ten feet without any foot and ankle deformity were loaded in CubeVue software. The foot was aligned to a neutral (0°) position based on the 2nd ray and the heel, and was then rotated 5°, 10°, 15° and 20° both externally and internally to simulate malposition. The 'semi' three dimensional hindfoot Moment Arm (HMA) was evaluated with the foot in these different positions. For measured parameters, varus alignment was recorded as a negative value, and valgus alignment was recorded as a positive value. Each measurement was repeated twice to minimize bias. A linear regression model was used to analyze the relationship of the HMA value and the rotation angles. An intraclass correlation coefficient (ICC) model was used to assess the intra- and interobserver reliability. Statistical analysis was completed using SAS 9.4. Results: When rotating the foot from external 20° to internal 20°, the HMA demonstrated a linear change in hindfoot alignment from varus to valgus. A 1° increase in the rotation angle caused 0.52 (±0.03) mm increase in the HMA value (Figure 1). The HMA measurements had excellent inter- and intra-observer reliability (ICC >= 0.9) (Table 1). Both CT images and measured values confirmed that internal rotation changed a neutral into a valgus hindfoot, and external rotation changed a neutral hindfoot into varus (Figure 2). Conclusion: When using current two or 'semi' three-dimensional methods to evaluate hindfoot alignment on WBCT scans, positioning the foot in the correct orientation is critical. Malrotation and malposition of the foot will cause significant impact on measurements and generate false data as well as misdiagnoses. This study was performed on normal feet but is clinically relevant when applied to deformities, particularly those associated with abduction or adduction of the midfoot and forefoot while current knowledge about deformed foot positioning is limited. This is an important topic for future investigation.