Bone strain and microcracks at stress fracture sites in human metatarsals

Microcracks in bone have been implicated in the development of stress fract ures. The goal of this study was to evaluate bone strain and microcracks at locations where stress fractures are common (second metatarsal diaphysis) and rare (fifth metatarsal diaphysis) in an attempt to increase our underst anding of the pathogenesis of stress fractures. A dynamic gait simulator wa s used to simulate normal walking with cadaver feet. The feet were loaded o ver the entire stance phase of gait and diaphyseal strains were recorded in second and fifth metatarsals. Microcrack density (Cr.Dn) and surface densi ty (Cr.S.Dn) were determined in metatarsal cross sections from the contrala teral feet. Bone strain was significantly higher in second metatarsals (-18 97 +/- 613 microstrain) than in fifth metatarsals (-908 +/- 503 microstrain ). However, second metatarsal Cr.Dn (0.23 +/- 0.15 #/mm(2)) was not signifi cantly different from fifth metatarsal Cr.Dn (0.35 +/- 0.19 #/mm(2)). There was also no significant difference between Cr.S.Dn in second (17.64 +/- 10 .99 mum/mm(2)) and fifth (26.70 +/- 15.53 mum/mm(2)) metatarsals. There wer e no significant relationships between the microcrack parameters and peak s train in either metatarsal, Cracks that occurred in trabecular struts (92 /- 33 mum) were significantly longer than those found ending at cement line s (71 +/- 15 mum) and within osteons (57 +/- 16 mum). There were no signifi cant relationships between the microcrack parameters and age in either meta tarsal. Peak strain was more than twofold greater in second metatarsals tha n in fifth metatarsals for simulations of normal walking; however, microcra ck parameters were unable to explain the greater incidence of second metata rsal stress fractures. (C) 2000 by Elsevier Science Inc. All rights reserve d.