Mobile-bearing unicompartmental knee arthroplasty may improve kinematics

Designer’s series shows a 98% survivorship at 10 years vs. independent series of 94% at 15 years.

Issue: December 2007

Since mobile-bearing unicompartmental knees gained FDA approval in 2004, some surgeons have looked to restore kinematics and potentially decrease polyethylene stresses and wear. At the Tenth Annual Insall Scott Kelly Institute Sports Medicine and Total Knee & Hip Symposium, Michael E. Berend, MD, discussed the patient selection criteria, obstacles and results of mobile-bearing unicompartmental arthroplasty. “I think that the mobile bearing uni is something that’s new to us. With it, I believe comes a whole body of information that I would suspect has gone under the radar for most of us,” Berend said during his presentation.

Michael E. Berend

Similar to a mobile-bearing total knee replacement (TKR), the mobile-bearing uni has increased conformity between the femoral component and the polyethylene. In cases with an intact PCL, Berend said the mobile-bearing uni knees may also improve knee kinematics. However, he noted that the replacements do not change how surgeons select patients, the anatomy of the knee or the importance of surgeon performance.

Criteria

He noted that the reported rate of potential unicompartmental knee arthroplasty (UKA) candidates range from 10% to 50%. “We looked at all of our total knee patients and used the Kozzin and Scott criteria to categorize them and found that only 4% to 6% of patients would be candidates just based on anatomic criteria,” Berend said.

He has now shifted from using anatomic criteria to using physiological criteria to determine care. “So prior to any uni, we get stress X-rays in the office and look for intra-articular correction of the deformity — where the lateral joint is maintained and the medial joint space can be restored,” Berend said. “I think that is one of the hallmarks of the ACL being intact. I also encourage you to look at the fragments you’re removing in your primary total knees. I’ll bet you’ll see when the ACL is intact, [there is] anteromedial arthrosis with intact posterior cartilage on both the femur and the tibia, and this is what you want to see to proceed with a uni.”

Obstacles

Challenges to performing a mobile-bearing uni knee include determining the amount of acceptable arthrosis in the patellofemoral joint, potentially jeopardizing the high success rate of TKR and operating on obese patients. “It takes me a little bit longer to perform this operation, and we get paid less, which is another challenge,” Berend said.

Berend noted there is short follow-up on the 16,000 unicompartmental knees performed in the United States. However, he said that mobile-bearing UKA has been a good addition to his practice. In a review of the 121 mobile-bearing UKAs performed at his center, Berend noted that there were no displacements or complications.

After surgeons divided the normal ACL and removed the normal lateral meniscus, this traditional total knee arthroplasty exposure shows anteromedial osteoarthritis. This case could undergo a unicompartmental knee arthroplasty with a smaller exposure.

Images: Berend ME

Outcomes

“There are obviously risks to any moving parts and that namely is dislocation of the bearing through impingement, either through retained bone or retained cement,” Berend said. Yet, he noted that long-term studies show less than a 1% rate of this occurrence. “The jump distance of the bearing is 3 mm in the back and 5 mm in the front, so it snaps in nicely and it is preserved as long as the bearing is maintained against the lateral rail.” He said this allows for anterior and posterior movement; however, if the bearing spins the jump distance may decrease to 1 mm. Therefore, eliminating bearing spin is important.

In an analysis of his fixed-bearing UKAs, Berend found that half had anterior tibial collapse in less than 1 year. These cases were associated with all-polyethylene components and excessive or under-tibial sloping, he said. “The conformity afforded by a mobile bearing, much like in total knees, will result in less loading and less wear compared to fixed-bearing designs,” he said. “And I would submit a hypothesis that the wear pattern in arthrosis, the area of the tibial collapse in and the predicted loading pattern are all in the same location, and this may be underlying some of the failure mechanisms that we have observed and are due to publish in the Journal of Arthroplasty.

Intraoperative image of a mobile-bearing unicompartmental knee implant
This intraoperative image shows a mobile-bearing unicompartmental knee implanted in a patient.

AP radiograph of a well-functioning mobile replacement
This AP radiograph demonstrates a well-functioning mobile unicompartmental replacement.

For more information:
Michael E. Berend, MD, can be reached at the Center for Hip & Knee Surgery, 1199 Hadley Road, Mooresville, IN 46158; 317-831-2273; e-mail: mikeberend@hotmail.com. He has indicated that he is a consultant and receives research grants and royalties from Biomet.
Reference:
Berend ME. Mobile-bearing unicompartmental arthroplasty. Presented at the Tenth Annual Insall Scott Kelly Institute Sports Medicine and Total Knee & Hip Symposium. Sept. 14-16, 2007. New York.