A novel method for analyzing foot motion during circumduction using an electromagnetic tracking system

Circumduction of the hindfoot does not occur primarily in one of the traditional anatomic planes and can be difficult to describe precisely. The purpose of this study was to measure foot bone motion quickly and objectively to subsequently characterize differences among feet of varying shapes. As such, we have developed a quantitative characterization of foot bone motion during circumduction using electromagnetic tracking sensors. Five of these sensors were attached to the foot on specific bony landmarks, and one was attached to a footplate. The lower leg was held by padded clamps in a custom non-ferrous jig, and the foot was moved through a full range of circumduction. To describe the motion of the bones of the foot during circumduction, the sensor positions were fitted to 2D ellipses and 3D curves. A repeatability study on multiple feet (n = 7) demonstrated that multiple raters (n = 3) introduced more error than a single rater; therefore, a single rater was used for all subsequent data collection. Results from five neutrally aligned subjects demonstrated that bone motion was quantifiable by fitted ellipse parameters. Additional modeling with a paraboloid surface described the motion with improved accuracy. A further reduction in error was obtained using a 3D eighth-order Fourier series expansion fit. This method holds promise as a means for characterizing differences in foot bone motion among foot types during a clinical exam.