Mechanical and physiological contributors to metatarsal structure and bone stress injury.

Metatarsal Bone Stress Injuries (BSI) are common in women and young adult athletes. Bone structure may be an important determinant of injury risk because bone is damaged with high stresses and strains. However, the factors that influence metatarsal bone structure and injury risk are not well understood. We investigated relationships between second metatarsal bone structure with past physical activity, mechanical loading, foot strength and structure, and metabolic factors. Seven injured and 34 healthy female runners (17-40 y; >10 miles/week) were analyzed. Questionnaires quantified past activity and factors related to Female Athlete Triad. Micro-CT measured metatarsal geometry and density. Across all runners using principal component analysis, greater metatarsal volumetric bone mineral density (BMD), cortical thickness, and trabecular microstructure were associated with shorter metatarsal lengths (r = -0.441, p = 0.008). We observed positive associations between arch doming strength and loading history (r = 0.394, p = 0.038). Greater moments of inertia and BMC were moderately associated with lower arch height index (r = -0.316, p = 0.064). An exploratory analysis investigated factors that distinguished injured (n = 7) versus performance-matched (n = 14) healthy runners. Injury aligned with longer metatarsals, greater Triad risk, earlier menarche, and high levels of adolescent bone loading physical activity. Our results suggest a potential pathway for high arches to increase risk of BSI through reduced metatarsal bone strength. Lack of structural differences between injured and healthy groups suggest that metatarsal BSI is driven by multiple interacting factors. These include running biomechanics that cause increased damage within the metatarsals, along with reduced capacity to repair bone microdamage.