A Partial Knee Replacement (PKR), also known as unicompartmental knee arthroplasty, differs from a Total Knee Replacement (TKR) because it addresses damage in only one section of the knee joint. This targeted approach involves resurfacing just the diseased part of the knee, preserving the healthy cartilage, bone, and ligaments in the other areas. The ability to perform this less-invasive surgery depends entirely on a patient meeting specific requirements, making candidate selection a precise part of the process. This article details the criteria surgeons use to determine eligibility.
Primary Medical Criteria for Partial Knee Replacement
The primary requirement for PKR is that joint damage must be confined to only one of the three knee compartments: the medial (inner), lateral (outer), or patellofemoral (kneecap) compartment. This condition is termed unicompartmental osteoarthritis, which means the patient has end-stage, “bone-on-bone” disease in a localized area while the rest of the knee remains relatively healthy. The procedure replaces only the surfaces of the affected compartment with a metal and plastic implant.
The knee’s ligamentous structures must be stable, especially the Anterior Cruciate Ligament (ACL). The ACL and Posterior Cruciate Ligament (PCL) provide stability that is necessary for the artificial joint components to track correctly and bear weight. If the ACL is deficient, the knee is considered too unstable for a PKR to be reliably successful.
The patient’s knee must also demonstrate a sufficient range of motion, requiring the ability to flex, or bend, the knee to at least 90 degrees. This preserved flexibility is evidence that the joint is not severely stiff, which is a contraindication for the procedure. The ability to fully extend the knee with no more than a slight (5-degree) fixed flexion deformity is also a factor for eligibility.
The knee should not have a severe angular deformity, or malalignment, which could place undue stress on the new implant and remaining healthy cartilage. While a small degree of varus (bow-legged) or valgus (knock-kneed) deformity may be acceptable, the surgeon must be able to passively correct this deformity during the physical examination. Finally, the bone density in the remaining compartments must be healthy enough to support the preserved joint surfaces.
Essential Diagnostic Steps in Candidate Evaluation
The evaluation begins with a review of the patient’s medical history, confirming the duration and location of pain and documenting previous non-surgical treatments. The surgeon will specifically note if the pain is localized to a single side of the knee, which is a strong indicator of unicompartmental disease. This history also provides context regarding the patient’s general health, activity level, and expectations for post-surgical recovery.
The physical examination assesses the knee’s mechanics, stability, and range of motion. The stability of the ligaments, particularly the ACL, is tested manually to confirm they are intact and functional. The surgeon also assesses the ability to correct any existing angular deformity by manipulating the leg, a test that helps predict the forces the implant will endure.
Imaging studies are required to confirm the extent and location of the damage identified during the physical examination. Weight-bearing X-rays are the standard imaging tool, as they show the joint space narrowing characteristic of advanced arthritis and allow the surgeon to assess the alignment of the leg under load. A Magnetic Resonance Imaging (MRI) scan may be ordered to provide a more comprehensive view of the cartilage and soft tissues, which can confirm the integrity of the ligaments and the health of the cartilage in the non-affected compartments.
When Partial Knee Replacement is Not Recommended
A patient is disqualified from PKR if the arthritis has spread beyond a single area, affecting two (bicompartmental) or all three (tricompartmental) areas of the knee. This widespread damage means a total knee replacement is the only viable option to address the entire diseased joint surface. The presence of inflammatory arthritis, such as Rheumatoid Arthritis, is a contraindication because these systemic diseases inherently affect the entire joint.
Patients with ligamentous instability, particularly a non-functional ACL, are not candidates for PKR because the partial implant design relies on the native ligaments for stability. Without this natural stabilization, the implant would be subject to excessive motion and premature failure. A severe fixed deformity that cannot be manually corrected by the surgeon suggests the soft tissues are too contracted or stretched to achieve the necessary alignment with a partial replacement.
Active joint infection, or an open wound or active skin disease near the surgical site, will postpone or disqualify a patient from the procedure until the infection is cleared. Attempting to implant artificial components in the presence of active bacteria increases the risk of a surgical site infection. Significant bone loss or poor bone quality, such as severe osteoporosis, can also prevent a PKR because the remaining bone may not be strong enough to securely anchor the implant components.