Study on the Superobese and Joint Replacement

Common sense suggests that extra body weight severely stresses bones and joints. But until the findings of a new Mayo Clinic study were presented at the annual meeting of the American Academy of Orthopedic Surgeons in Las Vegas, no one had reported how this higher, unprecedented level of obesity in the U.S. affects total hip and knee replacements.

Body Mass Index (BMI) is the measurement of choice for many physicians studying obesity. BMI uses a mathematical formula that takes into account a persons height and weight. In this Mayo study, patients diagnosed as superobese had a BMI greater than 50. Generally, a BMI of 30 indicates obesity. A person 6-feet tall with a BMI of 30 would weigh about 225 pounds. A person that same height with a BMI of 50 would weigh 369 pounds.

In the first study, 43 patients categorized as superobese underwent total hip arthroplasty from 1996 to 2006. This surgical procedure replaces a worn, fractured or damaged hip joint with a prosthetic implant. The patients had a BMI ranging from 50 to 77, and a mean age of 56. Mayo followed them an average of three years to determine outcomes.

Results showed:

More than half (56.5 percent) experienced either surgical or medical complications, including prolonged wound drainage.
Five of the 43 patients required a total of 15 re-operations to correct problems such as recurring dislocation of the implant, chronic infection, and new bone fractures around the device.

In the second study, which looked at complications associated with knee replacement, results showed:

Overall, 40.6 percent had surgical complications and 14.3 percent had medical complications — including two deaths after the surgery.
Surgical complications included 20 knees that had prolonged wound drainage —increasing the chance of deep infection of the implant.
Nineteen cases needed additional operations to trim slowly healing wound edges, treat deep infection, or repair failed implants.

As with hip replacement in the superobese patient, total knee arthroplasty also is associated with a markedly higher complication rate compared to patients of normal weights, says Mayo Clinic orthopedic surgeon David Lewallen, M.D., senior author on the study. The real concern here is that clinicians and patients need to be aware of that fact when they are making treatment decisions. In general, this group of patients is in desperate circumstances — and we really want to help them. To do that, we are trying to get the data out there that will guide management of the superobese patient. A big part of that has to be helping them lose weight prior to knee arthroplasty whenever possible.

In addition to Dr. Lewallen, the Mayo research team included Drs. David Polga, Aaron Altenburg and Robert Trousdale.

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