Lower-extremity stress fractures often occurred in association with potentially modifiable muscular and biomechanical factors, according to a study of 1,800 military cadets.
A detailed motion analysis of jump landing showed that landing with greater than 5 degrees of knee valgus or greater than 5 degrees of internal knee rotation significantly increased the likelihood of stress fracture.
Women were almost three times as likely as men to have lower-extremity stress fractures, Kenneth L. Cameron, PhD, of Keller Army Hospital in West Point, N.Y., reported at the American Orthopaedic Society for Sports Medicine meeting in Chicago.
"Several potentially modifiable muscular and biomechanical factors may be playing a part in the increased rates of stress fractures in athletes and military personnel," Cameron said in a statement. "It's possible that injury prevention programs targeted to address these movement patterns can help reduce stress fracture risk."
Reduced knee and hip flexion angles and increased vertical and medial ground reaction forces also were associated with an increased risk of stress fractures involving the lower extremities.
Athletes and military personnel account for a disproportionate share of lower-extremity stress fractures, and the frequency appears to be increasing. Whether stress fracture is associated with potentially modifiable factors had not been studied extensively.
Cameron reported findings from a retrospective analysis of 1,843 U.S. Military Academy cadets who completed baseline testing during 2005 to 2008, placing them in class years 2009 to 2013. Stress fracture during follow-up was documented by records of the Defense Medical Surveillance System and the Cadet Injury and Illness Tracking System.
Lower-extremity stress fractures identified in the records were reviewed and confirmed by orthopedic surgeons with sports medicine fellowship training.
At the beginning of cadet training, physical assessment included a detailed motion analysis study at various stages of landing after a jump. Investigators studied the association between stress fracture and sagittal, frontal, and transverse plane hip and knee kinematics at initial contact and at 15% (T15), 50% (T50), 85% (T85), and 100% (T100) of stance phase.
Cameron reported that 94 cadets had lower-extremity stress fractures during the study period. Women had a significantly higher incidence than did men (IRR 2.86, 95% CI 1.88-4.34, P<0.001).
Analysis of data for the different stance phases showed that cadets with neutral or varus knee alignment had a significantly lower risk of stress fracture compared with cadets who had >5 degrees of knee valgus. The difference ranged from 43% lower at initial contact (P=0.10) and T85 (P=0.04) to 53% lower at T50 (P=0.05).
Internal knee rotation >5 degrees more than doubled the likelihood of lower-extremity stress fracture as compared with cadets who had neutral or external knee rotation alignment. Incident rate ratios ranged from 2.31 at T15 and T85 (P=0.05, P=0.10, respectively) to IRR 3.98 at T50 (P=0.05).
Lower-extremity movement patterns and strength have previously been associated with stress fractures and overuse injuries," said Cameron. "However, our study is one of the first to identify dynamic knee rotation and frontal plane angles as important prospective risk factors for lower-extremity stress fractures.
"We hope that by better understanding the movement patterns associated with lower-extremity stress fracture injury we can help create programs for prevention."
Disclosure information was not available.
- Reviewed by Robert Jasmer, MD Associate Clinical Professor of Medicine, University of California, San Francisco