By Samantha Rosenblum
Landing biomechanics have long been a focus of anterior cruciate ligament (ACL) injury research, but they may also be predictive of lower extremity stress fracture, according to a study presented in July at the American Orthopaedic Society for Sports Medicine (AOSSM) meeting in Chicago.
Researchers from the Keller Army Hospital in West Point, NY, the University of North Carolina at Chapel Hill, the University of Connecticut in Storrs, and the Uniformed Services University of the Health Sciences in Bethesda, MD, looked at data from the Joint Undertaking to Monitor and Prevent ACL Injury (JUMP-ACL) cohort, a prospective cohort study of military cadets who underwent detailed motion analysis during a jump-landing task at the beginning of each individual’s military career. The researchers looked at 1895 individuals from the graduating classes of 2009 to 2013 upon their entry into military service, then identified lower extremity stress fractures occurring in the four-year follow-up period.
During the follow-up period, 94 participants sustained an incident stress fracture. The most common sites of stress fractures were the foot and lower leg, primarily the metatarsal and tibial regions, which accounted for nearly 90% of all stress fractures in the observed population, according to Kenneth Cameron, PhD, MPH, ACT, the lead author of the study and the director of orthopedic research at Keller Army Hospital.
“This is a common distribution of stress fractures in a military population,” said Cameron, who presented the findings at the AOSSM meeting.
The researchers found that, compared to individuals with more than 5° of knee valgus at three points of the stance phase of the jump-landing task, the stress fracture incidence for those with neutral or varus knee alignment was 43% to 53% lower. In addition, compared to those with neutral or external rotational knee alignment at the same three time points, individuals with more than 5° of internal rotation had two to four times the incidence of stress fractures.
Individuals who developed stress fractures had less knee and hip flexion, primarily at the midpoint of the stance phase during the jump-landing task, compared to uninjured individuals. Those with stress fractures also had higher peak vertical ground reaction forces, which Cameron said may be due to the reduced sagittal plane motion at the knee and hip.
The increased incidence of stress fracture for those with increased knee internal rotation suggests the tibia is subjected to greater torsional loads, according to Richard Willy, PhD, PT, OCS, assistant professor of physical therapy at Ohio University in Athens. He noted that studies from the University of Delaware in Newark and the University of Tennessee in Knoxville, among others, have identified increased medial collapse in runners with a history of tibial stress fractures; these mechanics will shift the load at the knee to the lateral compartment and result in a bending moment on the tibia, Willy said.
Additionally, the JUMP-ACL researchers found that stress fractures occurred nearly three times more often in female participants than in their male counterparts. According to Cameron, this is consistent with published incidence values that are two to 10 times higher for women.
“We also observed that women who went on to sustain an incident stress fracture during the four-year follow-up period had significantly lower BMI [body mass index], significantly greater Q angle [alignment of the femur relative to the tibia], and were significantly weaker in external rotation strength at the hip when compared to women who remained injury-free,” Cameron reported.
Although some studies suggest that hormonal imbalances, menstrual irregularities, and diet may contribute to stress fracture risk, the specific and probably multifactorial mechanisms contributing to this gender difference are not fully understood, he said.
The study suggests that injury prevention programs could target the relevant biomechanical factors, specifically increasing knee flexion and extension strength, increasing knee and hip flexion, and limiting knee valgus and internal rotation. For women, increasing hip external rotation strength may also be important.
“Clinical trials are needed to determine the efficacy of movement enhancement and retraining programs in preventing the incidence of stress fracture in cadets and athletes who exhibit the high-risk movement patterns observed in the current study,” Cameron said.
Cameron KL, Peck KY, Owens BD, et al. Biomechanical risk factors for lower extremity stress fracture. Presented at American Orthopaedic Society for Sports Medicine annual meeting, Chicago, July 2013.
Pohl MB, Mullineaux DR, Milner CE, et al. Biomechanical predictors of retrospective tibial stress fractures in runners. J Biomech 2008;41(6):1160-1165.
Milner CE, Hamill J, Davis IS. Distinct hip and rearfoot kinematics in female runners with a history of tibial stress fracture. J Orthop Sports Phys Ther 2010;40(2):59-66.