The relative skin movement of the foot: a 2-D roentgen photogrammetry study

OBJECTIVE: This paper describes a validation of the relative motions of skin-mounted markers on the foot. DESIGN: The movement of a skin-mounted marker on the foot was evaluated by the use of 2-D roentgen photogrammetry. BACKGROUND: Validations of skin-mounted markers are needed in order to assess whether clinical use of motion analysis systems is accurate. METHODS: Six healthy volunteers participated in the study. Measurements were made with 2-D roentgen photogrammetry from the medial aspect of the foot. Spherical lead markers, with a diameter of 2 mm, were glued on the skin over the following landmarks: the medial malleolus, the navicular bone, the medial part of calcaneus, the base and the head of the first metatarsal bone, and the base of the fifth metatarsal bone. The foot was placed on a platform that was adjustable in three directions: horizontal, 20 degrees upward (towards dorsal flexion), and 30 degrees downward (towards plantar flexion). RESULTS: In relation to the underlying bones, the markers were found to move up to 4.3 mm. The markers with the largest movements were the ones over the medial malleolus, the navicular bone and the calcaneus. For the distally placed markers i.e. at the base and the head of the first metatarsal bone, and the base of the fifth metatarsal bone, the displacements were a maximum 1.8 mm. CONCLUSIONS: Markers mounted on the foot moved between 1.8 and 4.3 mm corresponding to the underlying bones. The most proximal attached markers demonstrated the largest movements.