A partial knee replacement or Unicompartment knee arthroplasty is a surgical procedure that resurfaces a single compartment of a damaged knee with metal and plastic components called prostheses. Often times known as a knee resurfacing, this procedure is performed less often than a total knee replacement due to the requirements that only one of the three knee compartments is damaged to the point of needing resurfacing and that the normal function of the ligaments (ACL, PCL, MCL, LCL) is present.
The knee is divided into a medial (inside) compartment, a lateral (outside compartment), and a patello-femoral (kneecap) compartment. The medial compartment is the most often resurfaced due to the technical difficulties associated with resurfacing the lateral compartment in balancing the rotational motion as the knee travels from flexion to extension (screw home motion). Patello femoral resurfacing is also possible but is less utilized.
The ends of our femur, tibia, and undersurface of the patella are lined with specialized tissue called articular cartilage. It is this articular cartilage which allows the joints to move in a near frictionless fashion providing a smooth travel of the surfaces without causing any reactions such as a formation of heat or the sensation of pain with weight bearing. This articular cartilage is made of up tissue called hyaline cartilage. Unfortunately we as adults are unable to regenerate or grow hyaline cartilage if our own cartilage gets injured. Under some circumstances we are able to move good hyaline cartilage from an area of non-weight bearing to an area of weight bearing such as with an OATS transfer. However, when the amount of articular cartilage is that significantly damaged our best option of treatment is to resurface the involved area.
While a total knee replacement is a method of resurfacing all three of the joint compartments to make up for the damaged hyaline (articular) cartilage the partial knee only resurfaces one. The premise is very straightforward. We remove a small amount of bone with the articular cartilage covering the end of the femur and the tibia on the involved side (medial more commonly than lateral) or in the case of the patella femoral joint, we remove a small amount of bone from the femoral trochlea and the undersurface of the patella. We save the medial and lateral collateral ligament and the anterior and posterior cruciate ligament. When we resurface one compartment of the knee with the unicompartmental knee components, we make up the difference between the cartilage and bone that was lost due to the injury or arthritis with the components that are replaced. By saving the ligaments and replacing the bone loss we can provide a stable smooth joint resurfacing to allow return to normal activities with only a few restrictions.
Dr. Kagan has performed the partial knee replacement for almost 30 years with the earlier fixed bearing designs but now utilizing the mobile bearing knees (Oxford by Biomet) as well as fixed bearing knee (Zook by Zimmer and Vanguard by Biomet) . Dr. Kagan is also involved with the upcoming release of the iBalance unicompartment knee by Arthrex. As improvements continue with the instruments to perform the procedure and with the materials that the prosthesis are made of I hope we can look forward to a more durable and longer lasting lifetime of this procedure.
New minimally invasive surgical techniques including quadriceps sparing approaches along with better pain relief techniques has greatly improved the speed of recovery and limited the pain involved. This procedure is usually done in the hospital with a 1-2 day stay but could be done in an outpatient setting with proper patient selection.