Mobile-bearing TKA no better than fixed-bearing - Academy News at the 2007 AAOS Annual Meeting

Mobile-bearing TKA no better than fixed-bearing

Although mobile-bearing total knee arthroplasty (TKA) was introduced to reduce polyethylene wear and periprosthetic osteolysis, there is no reliable evidence to prove the superiority of the mobile-bearing knee design over the fixed-bearing design, according to scientific paper 014.

In their study of 146 patients who had received fixed-bearing TKA in one knee and mobile-bearing TKA in the other knee, the researchers found no significant difference in clinical and radiographic results between the two knees at 13-year follow-up.

Polyethylene wear and periprosthetic osteolysis are emerging problems in fixed TKAs. Mobile-bearing TKAs were introduced to reduce contact stresses in the polyethylene and to decrease wear as well as to minimize bone-cement stress at the tibial surface.

“The long-term results of fixed-bearing and mobile-bearing TKA procedures have been reported in many large series independently, but we were aware of no studies that made a direct comparison of the long term results on fixed-bearing and mobile-bearing TKAs in the same patients,” said the presenters.

“A comparison of the results in the same patient eliminates variability introduced by differences in gender, age, weight, co-morbidities, bone quality and activity level,” they noted.

In a prospective randomized study, the authors compared the long-term results associated with fixed-bearing and mobile-bearing TKAs in 146 patients who had received an anatomic modular knee (AMK) fixed-bearing knee prosthesis in one knee and a low-contact stress (LCS) mobile-bearing knee prosthesis in the other knee. Eight patients were men, and 138 patients were women; the mean age was 69.8 years. Primary osteoarthritis was the diagnosis was for 276 knees (95 percent) and rheumatoid arthritis was diagnosed in eight knees (5 percent).

Each patient had a history recorded, a physical examination and both preoperative and postoperative knee scores using The Hospital for Special Surgery (HSS) knee scoring system. The knee rating system of the Knee Society (KS) was used to determine pre- and postoperative knee function scores. Wear of the polyethylene layer and osteolysis were also determined.

After a minimum follow-up of 11 years, the average HSS score in the AMK group was 89 points (50 points preoperatively), and the HSS score in the LCS group was 87 points postoperatively (49 points preoperatively). The mean KS function scores were 92 points in the AMK group and 90 points in the LCS group. Pain score, function score, range of motion, and the prevalence of polyethylene wear and periprosthetic osteolysis were comparable.

In the AMK group, two knees (1 percent) were revised for wear of tibial bearing, and two knees (1 percent) were revised for osteolysis, or loosening. In the LCS group, three knees (2 percent) were revised for wear of medial tibial bearing, and one knee (0.7 percent) was revised for medial tibial bearing dislocation.

“The concept that less wear debris is generated with a mobile-bearing total knee prosthesis than with a well-designed fixed-bearing total knee prosthesis was not proven from the results of this study,” the authors concluded. The research team includes Young-Hoo Kim, MD, Jun-Shik Kim, MD, and Dae-Ya Kim, MD—all of Seoul, Republic of Korea. The authors report no conflicts of interest.

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