[Biomechanical mechanisms of overuse injuries of second plantar longitudinal arch in flatfoot]

Abstract

Objective: To research biomechanical mechanism of overuse injuries of second metatarsal and plantar aponeurosis of flat foot and normal foot, to contrast injury risk of flat foot with that of normal foot in gait cycle, and to offer quantitative academic base for clinical therapeutic and preventive measures.

Methods: Biomechanical MTS test, computational medicine means, finite element method and fatigue injury theory were adopted, fatigue behavior of flat foot and normal foot in gait cycle was simulated quantificationally by computer, their models successfully validated by test of stress concentration of foot bones and X-ray measurement of arch deformation.

Results: The finite element model of elastic arch of flat foot was established, peak values of fatigue stresses of second metatarsal and plantar aponeurosis in flat foot were individually 28.77 MPa and 12.53 MPa, these stresses of flat foot increased individually 8% approximately 21% and 21% approximately 51% than those of normal foot. Transfer of tension stresses of both flat foot and normal foot took place when plantar aponeurosis got infected or released partially, relative maximum of stress transfer occurred in plantar ligament, absolute maximum of stress transfer occurred in base of second metatarsal, the final stress of flat foot always overrun 0.26 approximately 5.32 MPa than normal foot.

Conclusion: Von Mises stresses of both flat foot and normal foot are concentrated on second metatarsal and plantar aponeurosis, but peak values of fatigue stresses of flat foot are higher than normal foot. Tension stress of plantar aponeurosis is maximum in all plantar soft tissues, plantar fasciitis or surgical partial release of plantar fascia leads to transfer of tension stresses, and can aggravate overuse injury of plantar muscles and plantar ligament, also can add risk of fatigue fracture of second metatarsal. Internal rotation of flat foot, arch collapse, and hammer toe of second ray caused by bunions all can increase incidence rate of overuse injury in flat foot.