Partial Knee Replacement (PKR), also known as unicompartmental knee arthroplasty (UKA), is a surgical option for people suffering from knee arthritis. Unlike a total knee replacement, which replaces all surfaces of the joint, PKR targets only the damaged area. This approach preserves healthy bone, cartilage, and ligaments, often leading to a more natural-feeling knee, less post-operative pain, and a faster recovery time. Eligibility requires a careful evaluation of the joint’s anatomy, the patient’s overall health, and their physical condition to maximize the chances of a successful outcome.
The Primary Requirement: Localized Arthritis
The prerequisite for partial knee replacement is that the arthritis damage must be confined to just one section of the knee joint. The knee is anatomically divided into three compartments: the medial (inner), the lateral (outer), and the patellofemoral (under the kneecap). For PKR to be considered, diagnostic imaging, such as X-rays and Magnetic Resonance Imaging (MRI), must confirm that significant degeneration is isolated to only one of these three surfaces.
The most common area for this isolated damage is the medial compartment, which bears the greatest load during walking and standing. If X-rays show healthy joint space and intact cartilage in the remaining two compartments, the patient may be a candidate. Preservation of the healthy compartments retains the native mechanics and stability of the knee. If the damage extends into a second compartment, or if there is diffuse arthritis across the entire joint, a total knee replacement becomes the more appropriate treatment option.
Patient Health and Physical Requirements
Beyond the location of the arthritis, a patient’s overall physical condition and joint stability are carefully assessed. The integrity of the major knee ligaments is a non-negotiable requirement for partial knee replacement. Specifically, the Anterior Cruciate Ligament (ACL) must be intact and fully functional to ensure the long-term stability of the joint after the procedure. Since PKR only resurfaces one side, the remaining ligaments are solely responsible for controlling the motion of the knee.
Weight management is another significant factor, as a high Body Mass Index (BMI) may negatively impact the success rate and longevity of the implant. Modern consensus suggests that gross obesity (BMI over 30 or 35) increases the rate of failure for unicompartmental implants. Surgeons prefer candidates who are not significantly overweight because excessive force on the implant can lead to earlier wear and loosening. Furthermore, a patient must possess a sufficient range of motion, requiring at least 90 degrees of flexion, for the smooth function of the replacement.
Conditions That Disqualify a Candidate
Several medical and structural conditions immediately disqualify a patient from receiving a partial knee replacement, making a total knee replacement necessary. Inflammatory types of arthritis, such as Rheumatoid Arthritis or Lupus, are contraindications because they cause diffuse, widespread joint damage across all three compartments of the knee. Since PKR is designed for localized osteoarthritis, systemic inflammatory disease rules out the partial procedure.
Severe, uncorrectable angular deformity, such as a significant knock-kneed or bowlegged presentation, also prevents a successful PKR. These severe deformities place abnormal loading forces on the resurfaced compartment, which can lead to rapid implant failure. Additionally, the presence of an active infection in the knee or a history of severe bone loss in the joint are strict contraindications for any joint replacement procedure. Patients with severe stiffness or a limited range of motion are typically channeled toward total knee replacement, as the partial procedure cannot effectively restore the necessary mobility.