Strains in the metatarsals during the stance phase of gait: Implications for stress fractures

Background: Stress fractures of the metatarsals are common overuse injuries in athletes and military cadets, yet their etiology remains unclear. In vi tro, high bone strains have been associated with the accumulation of microd amage and shortened fatigue life. It is therefore postulated that stress fr actures in vivo are caused by elevated strains, which lead to the accumulat ion of excessive damage. We used a cadaver model to test the hypothesis tha t strains in the metatarsals increase with simulated muscle fatigue and pla ntar fasciotomy. Methods: A dynamic gait simulator was used to load fifteen cadaveric feet d uring the entire stance phase of gait under conditions simulating normal wa lking, walking with fatigue of the auxiliary plantar flexors, and walking a fter a plantar fasciotomy, Strains were measured, with use of axial strain- gauges, in the dorsal, medial, and lateral aspects of the diaphysis of the second and fifth metatarsals as well as in the proximal metaphysis of the f ifth metatarsal, Results: When the feet were loaded under normal walking conditions, the mea n peak strain in the dorsal aspect of the second metatarsal (-1897 microstr ain) was more than twice that in the medial aspect of the fifth metatarsal (-908 microstrain), Simulated muscle fatigue significantly increased peak s train in the second metatarsal and decreased peak strain in the fifth metat arsal, Release of the plantar fascia caused significant alterations in stra in in both metatarsal bones; these alterations were greater than those caus ed by muscle fatigue. After the plantar fasciotomy, the mean peak strain in the dorsal aspect of the second metatarsal (-3797 microstrain) was twice t hat under normal walking conditions. Conclusions: The peak axial strain in the diaphysis of the second metatarsa l is significantly (p < 0.0001) higher than that in the diaphysis of the fi fth metatarsal during normal gait. The plantar fascia and the auxiliary pla ntar flexors are important for maintaining normal strains in the metatarsal s during gait. Clinical Relevance: Higher strains in the diaphysis of the second metatarsa l may explain why stress fractures are more common in this region than they are in the fifth metatarsal, Elevated strains in the metatarsals due to mu scle fatigue or loss of function of the plantar fascia may contribute to th e development of metatarsalgia and stress fractures.