Obese patients have a 2.4-fold greater risk for revision after minimally invasive UKA

However, this research does not infer that TKA will yield better results in obese patients than UKA.

Issue: September 2009

LAS VEGAS — Recently presented research identifies obesity as a risk factor for revision in patients undergoing minimally invasive unicompartmental arthroplasty using a fixed-bearing design.

“At 3.6 years follow-up, [we had a] revision rate of 6.2%,” William G. Hamilton, MD, said during his presentation at the American Academy of Orthopaedic Surgeons annual meeting. “At least in this study, the only variable that was statistically related to revision was body weight and BMI. Patients requiring revision, on average, were 16 pounds heavier and the relative risk of revision when your BMI is greater than 30 is 2.4.”

Same design

Hamilton and colleagues studied 507 consecutive medial unicompartmental arthroplasties performed by four surgeons using the Preservation unicompartmental device [DePuy Orthopedics].

During the study, 43 patients were lost to follow-up. The remaining cohort had a mean weight of 182 pounds at the time of surgery.

“We had a patient who was 325 pounds and a patient whose BMI was 58, as well as some deformities that were outside what I would consider to be classically indicated criteria for unicompartmental arthroplasty,” Hamilton said.

The investigators discovered a 6.2% revision rate at 3.6 years follow-up. An analysis of factors such as age, height, preoperative deformity and polyethylene thickness showed no correlation with revision. However, the investigators discovered that revised patients had significantly greater weight (197 vs. 181 pounds) and body mass index (BMI) (32±7 vs. 28.8±5.6) than those who did not undergo revision. The revision rate for patients with a BMI of less than 30 was 4.2%, and the rate for those with a BMI greater than 30 was 9.9%.

“Patients who failed by progression of their lateral disease, tended to be the obese patients,” Hamilton said. “Four out of the five patients were obese, and this did in fact approach statistical significance.” All of these patients also had a BMI greater than 30 and had an overcorrected mechanical axis, Hamilton said.

“However, I think that it is important to note that obesity alone does not explain all of the failures,” he said. “Twelve out of 38 of the revisions had a BMI of less than 30. This study does not tell us that total knee replacement would perform better in this subset of patients, which would be the conclusion that many would draw.”

Many factors

A confluence of factors could have contributed to the high, early failure rate. “At the same time we started using a new implant and relatively new surgical technique, we put the implant in suboptimal conditions and then we expanded our indications,” Hamilton said. “I think that all of these things have lead to our fairly high early failure rate.”

Craig H. Bennett, MD, a co-moderator of the session, asked Hamilton if the surgeons attempted to restore patient’s natural alignment or achieve neutral alignment.

Hamilton said that surgeons were using as thick a polyethylene insert as needed to fill the medial space.

“The reason the obese patients were overcorrected is that it is hard to protect the MCLs in those cases, and we know that the morbidly obese have a higher rate of MCL avulsion,” he said. “I think that we tended to compromise the fibers of the deep MCL more in the obese patients.”

For more information:

Craig H. Bennett, MD, can be reached at 2200 Kernan Drive, Suite 1154, Baltimore, MD 21207; 410-448-6273; e-mail: cbennett@umm.edu.

William G. Hamilton, MD, can be reached at Anderson Orthopaedic Clinic, 2501 Parkers Lane, Alexandria, VA 22306; 703-892-6500; e-mail: billhamilton@cox.net. He receives consultant fees and royalties from DePuy.

Reference:

Hamilton WG, Ammeen D, Engh GA. The effect of obesity on the revision rate of minimally invasive unicompartmental arthroplasty. Paper #402. Presented at the American Academy of Orthopaedic Surgeons 76th Annual Meeting. Feb. 25-27, 2009. Las Vegas.