Different Positions of Weightbearing CT Images Can Influence the Hindfoot Alignment Evaluation Using 2-Dimensional Methodology

Abstract

Background:

Three-dimensional weightbearing CT (WBCT) has been widely used to assess foot and ankle alignment. However, most current measurement methods are based on 2-dimensional concepts—distance, long axis, angulation, etc—and are sensitive to changes in orientation of the WBCT images. This study demonstrated how changes in positioning of the image can significantly influence the evaluation of hindfoot alignment.

Methods:

WBCT scans of 10 feet without deformities were used. In the horizontal view, the long axis of the foot image was aligned to a neutral position and then rotated 5, 10, 15, and 20 degrees both internally and externally to simulate malposition. The Calcaneal Moment Arm (CMA) was measured by 2 investigators independently in the above positions. An intraclass correlation coefficient (ICC) model was used to assess the intra- and interobserver reliabilities. The correlation between the CMA and the rotation angle of the foot images was analyzed using linear regression.

Results:

The CMA measurements demonstrated that internal rotation of the image changed the neutral hindfoot into valgus, whereas external rotation led to varus. A 1-degree internal or external rotation of the image correlated with 0.48 (±0.03) mm increase or decrease in the modified WBCT-CMA value (P < .0001, R² = 0.6406).

Conclusion:

This study demonstrated that although 3D WBCT provides the ability to observe the foot from different perspectives, current alignment evaluation tools are limited to 2 dimensions. Therefore, positioning the WBCT images in a consistent orientation is important to generate correct data.