Size really does matter, at least when it comes to hip implants. That’s the chief finding of a new study by University of Iowa researchers that determined that thigh size in obese people is a key reason their hip implants are more likely to fail.
For the study, published in the journal Clinical Orthopaedics and Related Research, the UI team simulated hip dislocations as they occur in humans and determined that increased thigh girth creates hip instability in obese patients. The findings suggest surgeons should modify surgical procedures to minimize the chance of dislocation in obese patients and consider other designs for hip replacements.
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"We have shown that morbidly obese patients' thighs are so large that they are actually pushing each other outward and forcing the implant out of its socket," said lead researcher Jacob Elkins. "Studies have shown up to a 6.9-fold higher dislocation rate for morbidly obese patients compared to normal weight patients."
Total hip replacement is a common procedure in the U.S., designed to increase mobility and reduce pain in people with arthritis and other problems. A hip implant is a ball-in-socket device, like a human hip joint. But because it lacks the connective tissue that stabilizes a normal hip, the implant’s ball can sometimes "pop out."
According to the National Institute of Arthritis and Musculoskeletal and Skin Disease, 231,000 total hip replacements are performed annually in the U.S. Most don’t require follow-up repair or replacement, but failures are painful, and costly. Medicare records have shown that dislocation ranks as the most common reason for failed implants and that obese patients have an increased dislocation risk.
For the new study, Elkins and colleagues analyzed 146 healthy adults and six cadaver pelvises. The findings showed: "The larger your legs are, the more force that goes through the hip joint," Elkins said. "It's a simple concept. When your thighs are real big, they push on the hips."
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Surgeons treating obese hip implant patients can use the study findings to select better implant designs and modify their surgical procedures to minimize the chance of dislocation in obese patients, he added.
The National Institutes of Health, the Veterans Administration, and the National Center for Resource Resources funded the research.