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Mobile-bearing knee implants more likely to be revised than fixed-bearing knees
The recent findings contradict previous studies which considered mobile-bearing results favorable.
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Despite previous studies reporting favorable mid-term clinical results with mobile-bearing knees, a recent study has found that mobile-bearing implants are 3.1 times more likely to be revised.
The findings were presented by Robert S. Namba, MD, at the American Academy of Orthopaedic Surgeons annual meeting.
The study, according to Namba, was developed to compare the short-term survivorship of mobile-bearing and fixed-bearing total knee arthroplasty (TKA).
“Theoretical advantages of mobile-bearing knees include improved wear, improved clinical function and improved fixation, though none of these concepts have been proven clinically,” Namba said.
Study parameters and cases
Namba and colleagues identified 2,599 (38% LCS, 62% RP-PFC Sigma, both DePuy) mobile-bearing TKA and 26,321 fixed-bearing TKA procedures performed between April 2001 and March 2007.
“We evaluated patient demographics, surgeon case volume, hospital case volume, the type of knee implant bearing, fixation of the implants, complications and the outcome of revision surgery,” Namba said.
According to the study, cumulative revision rate (CRR) was estimated by Kaplan-Meier, relative risk of revision (RR) and multivariate Cox regression methods. The Cox regression model also included age, gender, patient co-morbidities, implant design, surgical case volume, fixation and geographical region.
High failure rate
Namba reported the mean implant survival time to be 5.51 years. Overall CRR at 5.5 years was 1.4%, with 49 mobile-bearing TKA cases and 129 fixed-bearing TKA cases being revised. Instability (27%), aseptic loosening (22%), pain (14%), arthrofibrosis (14%) and extensor mechanism complications (10%) were all named as causes for mobile-bearing TKA revision.
“A significantly higher rate of revision was seen with the mobile-bearing knee group,” Namba said. “We could not determine a specific mode by which mobile-bearing knees failed; however, the reasons for revision knee surgery are often not precise.”
The results, the study concluded, point to the mobile-bearing design representing an independent and significant risk factor for revision surgery with an RR of 3.1.
The failures can be attributed to a number of factors, Namba said.
“The self-aligning feature of mobile bearings was thought to be a forgiving feature,” he said. “However, a difference in surgical feel with mobile bearings, less experience or the added complexity of another moving part might lead to a lack of surgical reproducibility.”
Namba’s study received the most audience attention out of any in its group, and a number of audience members questioned various aspects of the study.
“My concern is that the percentage of cases performed was only 10% of your total series,” said David A. Fisher, MD. “The surgeons clearly have a much greater exposure to fixed-bearing knees in your group.
“There are clearly two different surgical techniques for a mobile bearing vs. fixed bearing,” he added. “Do you think some of that may have been the early learning curve for many surgeons who adopted mobile-bearing knees in that group?”
Namba responded, “That is true, but we also identified a trend with increased failure of the LCS — which is an older design —rather than the RP-PFC, which speaks against the learning curve issue. The differences between both designs were not statistically significant.”
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- David A. Fisher, MD, is the Director of the Total Joint Center at the Indiana Orthopaedic Hospital. He can be reached at Indiana Orthopaedic Hospital, 8450 Northwest Boulevard, Indianapolis, IN 46278; 317-802-2828; e-mail: askdrfisher@aol.com. He is a consulting physician with DePuy Orthopaedics, Inc.
- Robert S. Namba, MD, is an orthopedic surgeon with Kaiser Permanente. He can be reached at 6670 Alton Parkway, Irvine, CA 92618; 949-932-5190; e-mail: robert.s.namba@kp.org. He has no direct financial interest in any of the companies or products mentioned in this article.
- Namba RS. Mobile-bearing total knee arthroplasty: a risk factor for early aseptic failure. Paper #271. Presented at the 2009 Annual Meeting of the American Academy of Orthopaedic Surgeons. Feb. 25-28, 2009. Las Vegas.