University of Iowa researchers have determined that thigh size in obese people is a reason their hip implants are more likely to fail.
In a study, the team simulated hip dislocations as they occur in humans and determined that increased thigh girth creates hip instability in morbidly obese patients (those with a body mass index (BMI) greater than 40). The researchers propose that surgeons modify surgical procedures to minimize the chance of dislocation in obese patients and consider other designs for hip replacement implants.
"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," says Jacob Elkins, a UI graduate student and first author of the paper published in the journal Clinical Orthopaedics and Related Research. "Studies have shown up to a 6.9-fold higher dislocation rate for morbidly obese patients compared to normal weight patients.
Total hip replacement gives mobility back to people who experience debilitating hip joint pain. According to the National Institute of Arthritis and Musculoskeletal and Skin Disease (NIAMS), 231,000 total hip replacements are performed annually in the U.S. and more than 90 percent of these do not require follow-up repair or replacement. But when an implant fails, it is painful, and costly. Studies have shown that dislocation ranks as the most common reason for failed implants, according to Medicare hospital discharge data.
A hip implant is a ball-in-socket mechanism, designed to simulate a human hip joint. However, it lacks the connective tissue that stabilizes a normal hip joint, meaning the ball portion of the implant can sometimes "pop out."
Clinical studies point to an increased dislocation risk among obese patients with total hip replacements, but the reasons have remained unclear. Dislocation requires extreme range of motion, such as flexing at the waist. Given the reduced range of motion in the obese, why do they experience more dislocations?
Using a computational model he created to understand how a hip implant works in patients, Elkins and research collaborators analyzed 146 healthy adults and six cadaver pelvises. They examined the effects of thigh-on-thigh pressure on the hip implant during a wide range of movements from sitting to standing. With the ability to simulate movements in human bodies of varying sizes, the team could test different implants. They also looked at the various implants' performances in different body types. They used a hip-center-to-hip-center distance of 200 millimeters as a basis for their analyses of thigh girth for eight different BMIs, ranging from 20 to 55.