Influence of Foot Position on the Measurement of First Metatarsal Pronation Using the First Metatarsal Axial Radiographs

Abstract

Category: Other Introduction/Purpose: In hallux valgus (HV) surgery, the importance of correcting first metatarsal (1MT) pronation, as well as correcting adduction, has been increasingly recognized. A 1MT axial radiograph is a simple method to quantify 1MT pronation (see figure). However, this view does not provide the exact axial projection, and 1MT is angled on the image. Furthermore, the 1MT angle is dependent on the foot position and alignment. Therefore the measured pronation angle on the radiograph may not be the same as the “true” pronation along the 1MT long axis. The purposes of this study were to 1) quantify the difference between the measured 1MT pronation on the axial radiograph and the true pronation angle, and 2) determine the influence of foot position on the measurement. Methods: CT images of 10 feet from HV patients (HV group; age, 58 years; HV angle, 44°) and 10 feet from those without HV (non-HV group; age, 47years; HV angle, 12°) were obtained. Digitally reconstructed radiographs of the 1MT were generated from the CT images in a three-dimensional virtual space (see figure). 1MT was pronated along the long axis (true pronation angle). Then, images with different plantarflexion (25° to 35° in 5° increments) and adduction (-10° to 10°) angles were created. This procedure was repeated for different pronation angles (-10° to 30°), and 135 simulated 1MT axial radiographs were obtained from each bone. We measured the 1MT pronation angles in all images. Differences in the measured pronation angle and true pronation angle were calculated for each group. Correlations between the measurement difference and 1MT plantarflexion/adduction angles were assessed using the Spearman correlation coefficient. Results: The mean measurement differences were 0.7° and 1.0° for the HV group and non-HV group, respectively. The standard errors of the measurement were 0.5° and 0.6° for the HV group and non-HV group, respectively. There was no significant correlation between the measurement difference and plantarflexion angle (P = 0.92 and P = 0.92 for the HV group and non-HV group, respectively), nor between the measurement difference and adduction angle (P = 0.82 and P = 0.74). Conclusion: The measurement differences were low in both HV and non-HV feet, indicating that the 1MT pronation angle measured on the axial radiograph represented the true pronation angle along the long axis of 1MT. The measurement differences were consistent regardless of plantarflexion and adduction angles. Therefore, a variation of 1MT angle on the image, which can be caused by the misalignment of foot position while taking the radiograph and difference in foot alignment (such as flatfeet and cavus feet), did not affect the measurement. 1MT axial radiograph could be used as a valid and robust method to quantify 1MT pronation.