Mobile-bearing, fixed implants have similar outcomes
Although a mobile-bearing implant for total knee arthroplasty (TKA) permits deep flexion, the authors of scientific paper 12 found no solid data to show that mobile-bearing implants have better survivorship rates.
In fact, they recommend that surgeons avoid using mobile-bearing implants in patients older than 70 years of age, in patients with a body mass index greater than 30, or in patients with varus or valugus deformity of more than 20°.
Mobile-bearing implants have been advertised as providing a better functional outcome—including more range of motion (ROM) and better stability—as well as less polyethelyne wear, longer survivorship rates, and better kinematics. However, after comparing the results of 523 mobile-bearing TKAs to the results of 426 fixed-bearing TKAs (all performed between January 2001 and May 2004), these investigators found no evidence of superiority for a mobile-bearing implant in either ROM or functional performance.
All patients received the same femoral component, and all surgeries were performed by a single surgeon. The surgeon used a mobile-bearing implant from January 2001 until February 2002, when he switched to a fixed-bearing implant. Most patients (82 percent) had simultaneous bilateral TKAs.
Both preoperative and postoperative ROM were documented on lateral radiographs, and Knee Society Scores (KSS) were obtained two years postoperatively. The average KSS in the mobile-bearing group was 23 before surgery, and increased to 86 one year after surgery. In the fixed-bearing group, the average KSS was 21 before surgery and 85 a year postoperative.
Patients were defined as “frequent kneelers” if they knelt at least five times daily; 61 percent of patients who received the mobile-bearing implant were frequent kneelers, compared to 48 percent of patients who received the fixed-bearing implant. Full flexion was defined as the ability to flex the knee to at least 130°, enabling the patient to sit on the ground with the calf touching the thigh for at least one minute.
At one year postoperative, 68 percent of patients who received mobile-bearing implants obtained full flexion, compared to 55 percent of patients who received fixed-bearing implants. The authors note, however, that the majority of patient who obtained full flexion postoperatively had full flexion preoperatively. Researchers also compared their results to the Dundee data base and found that their patients had better maximum flexion both before and after surgery.
Complications were similar in both groups, except for dislocations. There were five dislocations in the mobile-bearing group and none in the fixed-bearing group. All disclocations happened within the first year; three occurred within a month of surgery.
Other complications included peroneal nerve palsy (two cases in the mobile-bearing group and one case in the fixed-bearing group), infections (two cases in each group), intra-operative tibial plateau fracture (mobile-bearing group), intra-operative supracondylar femur fracture (mobile-bearing group), supracondylar femur fractures (three in the mobile-bearing group and one in the fixed-bearing group), and patellar “clunk” (three cases in each group).
“Although the mobile-bearing implant carried a higher knee dislocation risk, patients who were active and expected to kneel postoperatively seemed more comfortable with the implant. The higher rate of dislocation can probably be justified because these patients are frequent kneelers,” noted the authors.
“Patients who had gross deformities preoperatively usually required extensive soft-tissue release, which increased the risk of knee dislocation. All the dislocations in this study occurred in patients who required extensive soft-tissue release during the surgery.”
The primary researchers are Samih Tarabichi, MD, and Marwan Hawari, MD, both of Dubai, United Arab Emirates. Drs. Tarabichi and Hawari received nonresearch-related funding from Zimmer.