An enhanced protocol to reduce error in electromagnetic tracking of first metatarsophalangeal joint motions

Abstract

Error associated with markers or sensors surface-mounted onto skin reduces the validity of kinematic models. Extensor tendon excursion at the first metatarsophalangeal joint (MTPJ) has been noted previously to be especially problematic. An enhanced skin mounting protocol is described and validated. A Fastrak system was used to obtain kinematic data concurrently from two pairs of sensors mounted on either side of the first MTPJ of nine freshly frozen cadaveric feet. One sensor pair was mounted directly into the medulla of each of the first metatarsal and proximal phalanx of the hallux, with the second pair attached to the skin over the same segments, using each of two mounting protocols. A standard mounting protocol was compared to a new enhanced protocol that secured the sensor on the hallux with a stabilizing cuff. The addition of a stabilizing cuff on the hallux sensor reduced root mean square error in first MTPJalpha rotations by 1.45 degrees during passive rotation and 1.61 degrees during active rotation compared with the standard protocol (P = 0.008 and 0.028, respectively). The cuff protocol improved CMC reliability coefficients for alpha rotations from 0.918 to 0.973 for passive MTPJ motion, and from 0.922 to 0.973 for active motion. Securing the hallux mounted sensor with a stabilizing cuff reduced error by more than one-third without reducing first MTPJ total range of motion.