Biomechanical Risk Factors Identified for Fractures

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WEST POINT, NY — Stress fracture injuries disproportionately affect military servicemembers, yet little is known about the biomechanical risk factors associated with them.

That’s why a team led by researchers from Keller Army Hospital in West Point, NY, prospectively examined the association between neuromuscular and biomechanical factors, using entry to military service as a baseline and then following up over four years.

For the study, researchers analyzed data from the JUMP-ACL cohort, an existing prospective cohort study of military cadets. JUMP-ACL conducted detailed motion analysis during a jump landing task at the initiation of each subject’s military career. Analysis was limited to 1,895 subjects in the class years 2009-2013 (i.e., those who completed baseline testing in 2005-2008).

During that period, 52 cadets reported a history of stress fracture at baseline and were excluded from further analysis, leaving 1,843 subjects.

Incident lower-extremity-stress fracture cases were identified through the Defense Medical Surveillance System and the Cadet Injury and Illness Tracking System during the follow-up period, with review and confirmation by an adjudication committee of two sports medicine fellowship trained orthopedic surgeons. The primary outcome measured was the incidence rate of lower-extremity stress fracture during the follow-up period.

Overall, 94 (5.1%, 95%CI: 4.14, 6.21) subjects sustained an incident stress fracture during the follow-up period. The incidence rate for stress fracture injuries among females was nearly three times greater when compared to males (IRR=2.86, 95%CI: 1.88, 4.34, p<0.001).

Compared with those with greater than 5° of knee valgus, subjects with neutral or varus knee alignment experienced incidence rates for stress fracture that were 43%-53% lower at IC (IRR=0.57, 95%CI: 0.29, 1.11, p=0.10), T50 (IRR=0.47, 95%CI=0.23, 1.00, p=0.05), and T85 (IRR=0.53, 95%CI: 0.29, 0.98, p=0.04). Subjects with greater than 5° of internal knee rotation exhibited rates for stress fracture that were 2-4 times higher at T15 (IRR=2.31, 95%CI: 1.01, 5.27, p=0.05), T50 (IRR=3.98, 95%CI: 0.99, 16.00, p=0.05), and T85 (IRR=2.31, 95%CI: 0.86, 6.23, p=0.10), when compared with those with neutral or external knee rotation alignment.

“Several potentially modifiable biomechanical factors at the time of entry into military service appear to be associated with the subsequent rate of stress fracture,” the authors write. “It is possible that injury prevention programs targeted to address these biomechanical movement patterns may reduce the risk of stress fracture injury in athletes and military service members.”

1Cameron KL, Peck KY, Owens BD, Svoboda SJ, et. al. Biomechanical Risk Factors for Lower Extremity Stress Fracture. Presented at the 2013 American Orthopaedic Society for Sports Medicine Annual Meeting in Chicago.

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