Partial Knee Replacement: Who is a Candidate and What are the Advantages and Disadvantages?

Moby Parsons, M.D.

The knee joint has three compartments each of which can develop arthritis individually or collectively. These include the medial (inside), lateral (outside) and patellofemoral (knee cap) compartments. A substantial number of knees develop arthritis that is limited to the medial compartment with preservation of the other areas. The picture to the right demonstrates these compartments. The white surface of the joint is the cartilage which provides a smooth gliding surface. When cartilage is damaged and wears, the underlying bone may be exposed and as this progresses, bone on bone contact can cause pain, stiffness and loss of function. This is osteoarthritis.

The x-rays shown here shows joint space narrowing between the femur and the tibia in the medial compartment. This indicates that the cartilage on the inside part of the joint has worn away allowing bone on bone contact. The outside part of the joint continues to have a well-preserved joint space indicating intact cartilage. When the ligaments of the knee, specifically the anterior cruciate ligament, are intact, this pattern of arthritis is called anteromedial arthritis. Patients with this pattern of joint wear who have well preserved range of motion, no history of an inflammatory arthritis and minimal deformity of the leg are candidates for a partial knee replacement.

Knee replacement involves resurfacing the worn joint surface. As the name implies, a partial knee replacement resurfaces only the medial compartment and leaves the preserved compartments intact. The x-ray to the right, below, shows a partial knee replacement. Not only does the introduce a restored joint space to the worn part of the knee, but it also realigns the leg which becomes slightly bow-legged from the asymmetrical wear pattern.

Advantages of the partial knee replacement are several. First, the implant can be inserted without having to disrupt the quadriceps tendon and without having to dislocate the knee. This permits earlier return to function and faster rehabilitation compared to a total knee replacement. It also leads to less postoperative pain which facilitates rapid recovery. Second, when the ACL and all other knee ligaments are preserved, the knee feels more normal especially during high level activities. Some patients with full knee replacements are aware the implant is not their normal knee and this tends to be much less common with a partial replacement as preservation of the ligaments guides the motion according to normal knee mechanics. Third, the implant can be put in through a smaller incision that requires less soft tissue disruption. This also facilitates a more rapid recovery.

Disadvantages of partial knee replacement include a slightly higher risk of earlier loosening and need for revision. This can be difficult to predict in any given patient but there are many patients who can expect to get 15-20 years of excellent function out of a partial implant. Careful attention to surgical technique is critical to ensure favorable long-term outcomes.

In conclusion, in properly selected patients with the correct indications, partial knee replacement is an excellent option that can provide a high level of comfort and function with sustainable outcomes. Results are critically dependent on adhering to proper selection criteria as placing a partial knee in a patient with more extensive cartilage wear.