Mid-shaft stress fractures are much more common in the second metatarsal than in the fifth. The aim of this dissertation was to study the effects of simulated muscle fatigue and plantar fasciotomy on bone strain and microdamage accumulation in second and fifth metatarsals.

A gait simulator loaded cadaver feet under conditions simulating normal walking, walking with muscle fatigue, and walking after plantar fasciotomy. Diaphyseal strains were measured on the dorsal, medial, and lateral aspects of second and fifth metatarsals. Explanted metatarsals were cyclically loaded in a configuration that reproduced the strains recorded during gait simulations. Experimental and contralateral control metatarsals were stained in basic fixchsin to label microcracks and assessed with light microscopy.

For normal walking conditions, peak second metatarsal strain (-1897 microstrain) was about twice the peak fifth metatarsal strain (-908 microstrain). Simulated muscle fatigue increased peak second metatarsal strain and decreased peak fifth metatarsal strain. Plantar fascia release caused greater changes:​ second metatarsal peak strain doubled (-3797 microstrain) and fifth metatarsal peak strain increased by 22%.

The majority (70%) of in vivo microcracks were located in the mid-cortical region, 27% in the endosteal region, and only 3% periosteally. There were no significant differences in crack density and crack surface density between males and females, bones, nor quadrants. Crack density was 0.35 mm^-2 in females, 0.28 mm^-2 in males, 0.24 mm^-2 in second metatarsals, and 0.35 mm^-2 in fifth metatarsals. Microcrack parameters were not significantly related to age.

Only fifth metatarsals were successfully loaded to 10,000 cycles and stained adequately for analysis. When fifth metatarsal microcrack parameters from the experimental bones were normalized by those of the control bones there were no significant gender, fatigue condition, or quadrant differences in crack density nor crack surface density. The mean normalized crack density and crack surface density were 2.32 ± 3.13 and 2.56 ± 4.03, respectively. Normalized crack parameters were not related to age, sex, or strain.

High strains are likely to play a role in stress fracture etiology. Microdamage accumulation was unable to explain the incidence of metatarsal stress fractures, suggesting that other factors, such as bone remodeling, are involved.