December 2009

PFPS gender gap inspires researchers

By Linda Weber

Although the mechanisms behind patellofemoral pain syndrome (PFPS), like those of  anterior cruciate ligament (ACL) injuries, are still somewhat elusive, research is shedding  new light on the role gender plays in the onset of this chronic overuse condition.

A recent prospective study of 1,525 United States Naval Academy cadets found epidemiological evidence that confirmed what clinicians and researchers have known anecdotally but had little hard evidence to support: that PFPS occurs more often in women than in men. The study, published in September in the Scandinavian Journal of Medicine & Science in Sport, determined that  female cadets were 2.23 times more likely to develop PFPS than male cadets over a 2.5 year period.

“Until recently, our epidemiologic understanding of PFPS was limited. We were citing studies reporting the prevalence of PFPS done in the eighties, and there was a lack of incidence data,” said Michelle Boling, PhD, ATC, assistant professor in the department of clinical and applied movement sciences at the University of North Florida in Jacksonville and lead investigator on the study. “Since then, women’s participation in sports has changed, so we suspected that the rates of the disorder might be different.”

Researchers have been examining the effect of gender on anatomical and biomechanical variables that are thought to contribute to patellofemoral pain, analyzing healthy male and female subjects performing movement tasks that have been previously shown to provoke symptoms in PFPS patients.

“So far, we know that females have a larger Q-angle than males, which puts more lateral pull on the patella. We know females have weaker hip musculature and that their quadriceps and hamstrings are weaker than males’. And we know that females display more knee valgus than males during dynamic tasks like jumping or cutting,” Boling said.

In the October issue of the British Journal of Sports Medicine, investigators found that female high school athletes demonstrated significantly greater normalized vertical ground reaction forces and force loading rates than their male counterparts during the landing phase of repeated vertical single-leg hops.

An Australian study published in the April issue of the Journal of Electromyography & Kinesiology found that EMG activity at the vastus medialis oblique and vastus lateralis muscles during stair stepping was significantly associated with hip strength in both female and male subjects. However, the Australian investigators  found no difference between genders in terms of the timing or peak of EMG activation of the vasti or gluteus medius muscles.

A study in the September issue of the Journal of Orthopedic Research examined patellar tendon orientation and patellar tracking differences in male and female knees. Researchers from Massachusetts General Hospital in Boston used MRI and low dose x-ray imaging to analyze knee motion in 18 male and 13 female volunteers as they performed single-leg lunges.

The results showed no notable gender-related differences in patella movement within the groove of the femur, according to Kartik M. Varadarajan, a department of mechanical engineering researcher in MIT’s Bioengineering Lab at MGH and lead researcher of the study. But during the early part of the knee-bending activity, the tibia in female subjects underwent more external rotation than in male subjects. The tibiofemoral rotation in turn led to alterations in patellar tendon orientation, repositioning the tendon anteriorly, medially and tilted externally in female subjects

“Future research will determine whether this twisting and tilting of the patellar tendon in females leads to greater pulling force on the patella, and whether it could explain the higher incidence of kneecap dislocations and kneecap pain in females,” Varadarajan said.

Researchers hope that current and future research into discrepancies between male-female kinematics, kinetics, muscle strength, and postural alignment will lead to more accurate identification of risk gender-specific risk factors and much more effective treatment options.

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