Metatarsal Strains During Loading and Motion of the Foot and Ankle in a Cadaveric Model.

Metatarsal bone stress injuries (BSI) are painful, overuse injuries commonly seen in athletes and military personnel. High strain magnitudes combined with repetitive loading cycles causes bone fatigue and microdamage accumulation. To protect against BSI, it is necessary to determine how to limit bone strain magnitude without sacrificing physical activity. This study examined maximum and minimum principal dorsal strains of metatarsals 2-4 during tibial dorsiflexion/plantarflexion, external/internal rotation, and varus/valgus alignment with four underfoot conditions. Five lower-limb cadaveric specimens were instrumented with strain gauges on metatarsals 2-4 and a robotic actuator applied 25% bodyweight and moved the ankle/foot through its envelope of passive motion. Dorsiflexion and varus tibial motion produced the greatest increase in strain magnitudes during all underfoot conditions. Adding half-inch blocks beneath the medial and lateral forefoot caused principal strain magnitudes to increase threefold within the metatarsals adjacent to the block across all motions. Passive engagement of the windlass mechanism during dorsiflexion caused approximately a threefold increase in maximum principal strain of metatarsals 2-3 coinciding with a significant decrease in metatarsal 4. The results emphasize the role that the arch and its supporting structures play in modulating metatarsal loading even within a passive model.