Thigh Size Factor in Hip Implant Success

The patient’s thigh size is a major factor in determining if a hip implant is likely to succeed or fail, according to researchers at the University of Iowa. In their study, the research team simulated hip dislocations as they occur in humans and determined that increased thigh girth creates hip instability in morbidly obese patients. Morbidly obese patients were defined as those with a body mass index (BMI) greater than 40. The researchers propose that surgeons modify their surgical procedures to minimize the chance of dislocation in obese patients.

“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, ” he said.

According to the National Institute of Arthritis and Musculoskeletal and Skin Disease, 231, 000 total hip replacements are performed annually in the U.S. and more than 90% of these do not require follow-up repair or replacement. But when an implant fails, it is painful, and costly. Medicare hospital discharge data shows that dislocation ranks as the most common reason for failed implants.

Using a computational model he created to understand how a hip implant works in patients, Elkins and his collaborators analyzed 146 healthy adults and 6 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.

The team ran computations to examine the joint stability of several different hip implants and tested two femoral head sizes (28 and 36 millimeters), normal versus high-offset femoral neck, and multiple cup abduction angles.

The researchers report three main findings:

• thigh soft tissue impingement increased the risk of dislocation for BMIs of 40 or greater

“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.” he researchers advised surgeons treating obese hip implant patients to use the study findings to select better implant designs and modify their surgical procedures to minimize the chance of dislocation in obese patients.

“The number one thing surgeons can do is what is called a ‘high offset femoral stem, ‘” says senior author Thomas Brown, M.D., UI professor of orthopedic surgery, referring to the portion of the implant that attaches to the patient’s upper thigh bone. “Basically, the implant’s femoral stem is longer, so it effectively shifts the leg further away from the center rotation of the joint. The thighs then would need to move even further inward before they would abut one another and generate the forces necessary for dislocation.”