Do mobile-bearing designs offer a clinical advantage over fixed-bearing designs for unicompartmental knee arthroplasty (UKA)? That was the debate between Craig J. Della Valle, MD, and Richard D. Scott, MD, during the combined Knee Society/American Association of Hip and Knee Surgeons 2011 Specialty Day program.

No clinical advantage found
“Although the idea of a mobile-bearing design is very appealing, I do not believe that it offers a clinical advantage over a fixed-bearing design,” Dr. Della Valle stated. “The mobile-bearing design has two important potential benefits—a decreased rate of wear as well as the promise of decreased stresses at the interfaces between the cemented implant and bone that could translate into a lower rate of prosthetic loosening.

“But the real question in my mind,” he continued, “is whether there is something magical about a mobile-bearing UKA that makes the outcomes better or in some way different or makes survivorship longer? And is there a clinical benefit that leads to better patient outcomes? I don’t believe there is; I think there are some real negatives to a mobile-bearing implant as well as advantages to a fixed-bearing design.”

Although Dr. Della Valle acknowledged that mobile-bearing UKAs have been shown to have very low polyethylene wear rates, the promise of decreased rates of prosthetic loosening has not been realized. “Almost every series in the literature includes tibial component loosening as a failure mode as well as problems with femoral component loosening,” he said.

Technique is also an issue, according to Dr. Della Valle. “Almost every series in the literature that I’ve reviewed shows failures associated with dislocation of the mobile bearing. Although the rate of bearing dislocation is variable, the demanding nature of the technique makes it a particular problem early in the surgeon’s learning curve. Unfortunately, trying to decrease the risk of bearing dislocation by making the knee tighter via insertion of a thicker bearing can result in progressive arthritis of the lateral compartment.”

Switching his focus to clinical outcomes, Dr. Della Valle noted that several studies, including a meta-analysis, showed no differences between fixed- and mobile-bearing UKAs based on clinical outcome tools such as the Knee Society Score (KSS), the WOMAC score, and the SF 12 and SF 36 measures.

In expert hands, he concluded “both of these techniques work very well. There is nothing ‘magic,’ however, about a mobile bearing. Although wear is admittedly low, the surgical technique is more demanding, the risk of bearing dislocation is real, and all the other modes of failure are still possible. With a fixed-bearing design, particularly one that has a metal-backed tibial component, loosening is rare, reoperations for wear are present but infrequent, and survivorship is high.”

Mobile designs have an edge
“In theory, UKA is an attractive alternative to osteotomy or total knee arthroplasty in selected patients with osteoarthritis,” said Dr. Scott. “In the last decade, broader indications have enabled surgeons to perform UKAs in younger patients, which means a more conservative procedure and greater longevity of the polyethylene is needed.” He noted that the initial results of UKAs are as good as or better than TKAs, and second-decade survivorship is improving.

Dr. Scott identified poor patient selection, prosthetic design, and surgical technique as reasons for UKA failures, with wear, loosening, and degeneration of the opposite compartment as the modes of failure. “Wear is often design-related,” he explained. “The wear pattern of the prosthesis reproduces the preoperative wear pattern of the arthritic knee. Prosthetic designs, therefore, must accommodate this wear pattern.

“You cannot fight the predetermined wear pattern in a UKA with a conforming, fixed-bearing prosthesis,” he continued. “Fixed-bearing UKAs must be nonconforming—a round femoral component on a relatively flat tibial component. If you want a conforming design, it has to be mobile-bearing. That’s the simple fact.”

Dr. Scott supports the use of a mobile-bearing UKA because it has the potential to increase survivorship by improving contact stresses and increasing the longevity of the polyethylene articulation. Another plus, he said, is “the extra bonus of a metal-backed tibial component with a conservative, composite thickness as thin as 6 mm, the thinnest one on the market permitted by the U.S. Food and Drug Administration.”

Dr. Scott recommended starting any UKA with a conservative initial tibial resection to make potential revision easy. In preoperative planning, he suggested drawing a conservative resection line for a total knee replacement. “Where this line transects the periphery of the medial plateau marks a conservative initial tibial resection. In surgery, draw that line on the tibia for the initial tibial cut. If the procedure must be abandoned or revised in the future, a primary arthroplasty is the result,” he noted.

According to Dr. Scott, the literature supports the concept that mobile-bearing UKAs offer excellent longevity, even in the middle-aged patient. His personal experience confirms that the early results of both mobile- and fixed-bearing UKAs are similar for both range of motion and pain.

“The surgical technique for mobile bearings is probably less forgiving than for fixed-bearing UKAs, at least in my hands,” he said. But he again pointed to the option of a thin composite, metal-backed component and a potentially low long-term failure rate due to wear as attractive features for mobile-bearing designs.

“In summary, both fixed- and mobile-bearing UKAs are good alternatives to osteotomy and total knee replacement in selected patients,” he concluded. “With time we should be better able to define the indications for each and better assess their advantages and disadvantages.”

Disclosure information—Dr. Della Valle—Biomet; Smith & Nephew; CD Diagnostics; Pacira; Zimmer; Orthopedics Today; AAHKS; Arthritis Foundation. Dr. Scott—DePuy, A Johnson & Johnson Company; Conformis.

Bottom line

• Mobile-bearing designs for UKA have potentially lower wear and improved contact stresses compared to fixed-bearing designs.
• Both mobile- and fixed-bearing UKAs have similar initial clinical outcomes based on standard measures such as the KSS, WOMAC, SF-12, and SF-36.
• The surgical technique for mobile-bearing designs may be more demanding and less forgiving than for fixed-bearing UKAs.
• Literature reviews frequently find failure associated with dislocation of the mobile bearing.

Mary Ann Porucznik is managing editor of AAOS Now. She can be reached at porucznik@aaos.org