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?

Engineering a better hip implant

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.

The research team ran computations to examine the joint stability of several different hip implants. They 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: 1) thigh soft tissue impingement increased the risk of dislocation for BMIs of 40 or greater; 2) implants with a larger femoral head diameter did not substantially improve joint stability; 3) using an implant with a high-offset femoral stem decreased the dislocation risk.

“The larger your legs are, the more force that goes through the hip joint,” Elkins says. “It’s a simple concept. When your thighs are real big, they push on the hips.”

Recommendations for surgeons

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, the researchers say.

“The number one thing surgeons can do is what is called a ‘high offset femoral stem,’” says senior author Thomas Brown, UI professor of orthopaedic surgery, referring to the portion of the implant that attaches to the patient’s upper thigh bone, or femur. “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.”

The study, titled “Morbid obesity may increase dislocation in total hip patients: A biomechanical analysis,” was published online on Aug. 21. Elkins is in the UI’s College of Engineering Biomechanical Engineering Program and the Carver College of Medicine’s Medical Scientist Training Program.

Other authors are Matej Daniel, assistant professor at Czech Technical University in Prague, Czech Republic, and former Fulbright Research Scholar at UI Hospitals and Clinics; Douglas Pedersen, UI research associate professor of orthopaedic surgery; Bhupinder Singh, UI doctoral candidate in physical therapy; John Yack, UI associate professor of physical therapy; and John Callaghan, UI professor of orthopaedic surgery.

The National Institutes of Health, the Veterans Administration, and the National Center for Resource Resources funded the research.