Prospective study of biomechanical risk factors for second and third metatarsal stress fractures in military recruits.

Objectives: This prospective study investigated anatomical and biomechanical risk factors for second and third metatarsal stress fractures in military recruits during training.

Design: Prospective cohort study.

Methods: Anatomical and biomechanical measures were taken for 1065 Royal Marines recruits at the start of training when injury-free. Data included passive range of ankle dorsi-flexion, dynamic peak ankle dorsi-flexion and plantar pressures during barefoot running. Separate univariate regression models were developed to identify differences between recruits who developed second (n=7) or third (n=14) metatarsal stress fracture and a cohort of recruits completing training with no injury (n=150) (p<0.05). A multinomial logistic regression model was developed to predict the risk of injury for the two sites compared with the no-injury group. Multinomial logistic regression results were back transformed from log scale and presented in Relative Risk Ratios (RRR) with 95% confidence intervals (CI).

Results: Lower dynamic arch index (high arch) (RRR: 0.75, CI: 0.63-0.89, p<0.01) and lower foot abduction (RRR: 0.87, CI: 0.80-0.96, p<0.01) were identified as increasing risk for second metatarsal stress fracture, while younger age (RRR: 0.78, CI: 0.61-0.99, p<0.05) and later peak pressure at the second metatarsal head area (RRR: 1.19, CI: 1.04-1.35, p<0.01) were identified as risk factors for third metatarsal stress fracture.

Conclusions: For second metatarsal stress fracture, aspects of foot type have been identified as influencing injury risk. For third metatarsal stress fracture, a delayed forefoot loading increases injury risk. Identification of these different injury mechanisms can inform development of interventions for treatment and prevention.