Jordana Bieze Foster, Editor

Those investigating alleged improprieties associated with a 2007 study on hip protectors will tell you their efforts are all about ethics. But the way I see it, the more important lesson to be learned from this situation is one of biomechanics, specifically the clinical importance of the subtleties of asymmetry.

The study, conducted by the Institute for Aging Research in Boston and published by the Journal of the American Medical Association, called for nursing home residents to serve as their own controls, wearing a hip protector on one hip only. This type of study design is quite common in the lower extremity literature, but previous studies of hip protectors had all required bilateral device wear.

The unilateral strategy was intended to avoid bias on the part of caregivers, who might have favored pad-wearers over control participants. The researchers never expected that wearing just one hip protector would actually increase the elderly residents’ risk of hip fracture. But that seems to be what happened.

In the 334 participants with high levels of compliance, fractures occurred more frequently in protected hips than unprotected hips (5.3% vs 3.5%). In the subgroup of patients who were wearing the device at the time of injury, 13 fractures occurred on the protected side compared to just seven on the unprotected side.

The numbers were small and not statistically significant. But investigators now say the researchers should have recognized the potentially dangerous trend much earlier and halted the study for ethical reasons. The researchers, for their part, say there was no reason for them to think such a lightweight device, intended solely as a cushion with no biomechanical influence, could possibly have been contributing to fracture risk.

“These hip pads weigh less than the average man’s wallet—just several ounces,” coauthor Stanley Birge, MD, told the Boston Globe. “So we think it’s very unlikely that they’re going to cause some kind of imbalance.”

But lower extremity practitioners know that even very small asymmetries can have significant effects. A study from the Shelbourne Knee Center in Indianapolis, presented this month at the annual meeting of the American Orthopaedic Society for Sports Medicine, illustrates this point.

The Indianapolis study assessed range of motion symmetry in patients a mean of 10 years after anterior cruciate ligament reconstruction. Notably, the authors’ definition of “normal” symmetry was very strict: within 5° for knee flexion and just 2° for knee extension. But patients who didn’t meet that strict definition of symmetry were significantly more likely to demonstrate radiographic osteoarthritis at 10 years postsurgery.

A difference of 2° certainly isn’t an asymmetry that would be noticeable to a patient, or even to a practitioner who didn’t compare both limbs closely. One might think it wouldn’t be clinically significant. But one might be wrong.

And those patients were young and fit. So it’s not inconceivable that an equally subtle asymmetry could affect patterns of falls and fractures in a frail elderly population.

Obvious asymmetries can be a clinical challenge. But it’s the subtle asymmetries that can really get you in hot water.