Errors in measuring sagittal arch kinematics of the human foot with digital fluoroscopy

Although fluoroscopy has been used to evaluate motion of the foot during gait, the accuracy and precision of fluoroscopic measures of osseous structures of the foot has not been reported in the literature. This study reports on a series of experiments that quantify the magnitude and sources of error involved in digital fluoroscopic measurements of the medial longitudinal arch. The findings indicate that with a global distortion correction procedure, errors arising from image distortion can be reduced threefold to 0.2° for angular measurements and to 0.1 mm for linear measures. The limits of agreement for repeated angular measures of the calcaneus and first metatarsal were ±0.5° and ±0.6°, indicating that measurement error was primarily associated with the manual process of digitisation. While the magnitude of the residual error constitutes about ±2.5% of the expected 20° of movement of the calcaneus and first metatarsal, out-of-plane rotation may potentially contribute the greatest source of error in fluoroscopic measures of the foot. However, even at the extremes of angular displacement (15°) reported for the calcaneum during running gait, the root mean square (RMS) error was only about 1°. Thus, errors associated with fluoroscopic imaging of the foot appear to be negligible when compared to those arising from skin movement artefact, which typically range between 1.5 and 4 mm (equating to errors of 2° to 17° for angular measures). Fluoroscopy, therefore, may be a useful technique for analysing the sagittal movement of the medial longitudinal arch during the contact phase of walking.