Half Knee Replacement: Procedure, Candidates, Benefits, and Recovery

What is a Half Knee Replacement?

A half knee replacement, technically known as unicompartmental knee arthroplasty (UKA), is a surgical procedure that replaces only the damaged part of the knee joint, rather than the entire joint, preserving healthy bone, cartilage, and ligaments.

Understanding Knee Anatomy and Osteoarthritis

The knee is a complex hinge joint formed by the articulation of three bones: the femur (thigh bone), tibia (shin bone), and patella (kneecap). It is divided into three main compartments: the medial (inner), lateral (outer), and patellofemoral (front, under the kneecap) compartments. Each compartment is lined with articular cartilage, a smooth, slippery tissue that allows for effortless movement and acts as a shock absorber.

Osteoarthritis (OA) is a degenerative joint disease characterized by the breakdown of this articular cartilage. As the cartilage wears away, bones begin to rub directly against each other, leading to pain, stiffness, swelling, and reduced mobility. While OA can affect any joint, it commonly impacts the knee, often progressing in one specific compartment before affecting the entire joint.

What is a Half Knee Replacement (Unicompartmental Knee Arthroplasty)?

A half knee replacement, or unicompartmental knee arthroplasty (UKA), is a less invasive surgical option compared to a total knee replacement (TKR). Instead of replacing all three compartments of the knee, a UKA targets only the single, most severely damaged compartment—most commonly the medial compartment.

During the procedure, the orthopedic surgeon removes the worn-out cartilage and a small amount of underlying bone from the affected compartment. This damaged tissue is then replaced with a metal and plastic (polyethylene) implant, which resurfaces the joint and restores smooth, pain-free movement. The healthy cartilage, bone, and ligaments in the other two compartments are left intact, preserving the natural mechanics of the knee to a greater extent than TKR.

Who is a Candidate for a Half Knee Replacement?

Not everyone with knee osteoarthritis is suitable for a half knee replacement. Ideal candidates typically meet specific criteria, including:

• Localized Osteoarthritis: The arthritis must be confined to only one compartment of the knee, with the other two compartments and their cartilage being healthy.
• Intact Ligaments: The anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL), crucial for knee stability, must be intact and functional.
• Minimal Deformity: The knee should not have significant angular deformity (e.g., severe bowleg or knock-knee).
• Age and Activity Level: While there's no strict age limit, UKA is often considered for younger, more active individuals who wish to maintain a higher level of function, or older individuals with isolated arthritis.
• Conservative Treatment Failure: Non-surgical treatments like physical therapy, medication, and injections must have failed to provide adequate pain relief.
• Good Range of Motion: The patient should have a relatively good range of motion in the knee prior to surgery.

Benefits of a Half Knee Replacement

Compared to a total knee replacement, UKA offers several distinct advantages for appropriate candidates:

• Less Invasive: The surgical incision is typically smaller, and less bone and tissue are removed, leading to less blood loss.
• Quicker Recovery: Patients often experience less post-operative pain and a faster rehabilitation process, allowing for an earlier return to daily activities.
• Preserved Knee Kinematics: By retaining healthy bone, cartilage, and ligaments in the unaffected compartments, the knee's natural motion and feel are often better preserved.
• Improved Range of Motion: Many patients report a more natural-feeling knee and achieve a greater range of motion compared to TKR.
• Reduced Risk of Complications: While still a major surgery, some studies suggest a lower risk of certain complications like blood clots or infection compared to TKR.
• Future Options: If the UKA eventually fails or arthritis progresses in other compartments, a total knee replacement can still be performed.

Potential Risks and Complications

While generally safe, a half knee replacement, like any surgery, carries potential risks and complications. These can include:

• Infection: Though rare, infection can occur at the surgical site.
• Blood Clots: Deep vein thrombosis (DVT) in the leg or pulmonary embolism (PE) in the lung are possible.
• Implant Loosening or Wear: Over time, the implant components can loosen or the plastic insert can wear out, potentially requiring revision surgery.
• Progression of Arthritis: Arthritis may develop in the previously healthy compartments of the knee, necessitating further surgical intervention.
• Nerve or Blood Vessel Damage: Rare but possible during surgery.
• Persistent Pain or Stiffness: Some patients may still experience pain or limited range of motion.

The Recovery Process and Rehabilitation

Recovery from a half knee replacement is generally faster than from a total knee replacement.

• Immediate Post-Op: Patients typically begin walking with assistance within hours of surgery. Pain management is crucial.
• Hospital Stay: Often, patients are discharged within 1-2 days.
• Physical Therapy: A structured physical therapy program is vital for restoring strength, flexibility, and range of motion. This typically begins shortly after surgery and continues for several weeks to months. Exercises focus on knee flexion and extension, quadriceps strengthening, and gait training.
• Return to Activities: Most individuals can resume light daily activities within a few weeks. Return to more demanding activities, such as sports, is usually cleared by the surgeon after several months, depending on individual progress and the sport's demands.

Half Knee Replacement vs. Total Knee Replacement

The choice between UKA and TKR depends heavily on the individual's specific condition and lifestyle.

Long-Term Outlook and Activity

The long-term success of a half knee replacement is generally very good, with studies showing high patient satisfaction rates. While not as durable as some total knee replacements, many UKA implants last 10-15 years or more, especially in well-selected patients who adhere to post-operative guidelines.

Patients with a UKA can often return to a more active lifestyle than those with a TKR, including activities like hiking, cycling, swimming, and even some low-impact sports like golf or doubles tennis. High-impact activities such as running, jumping, or contact sports are generally discouraged to preserve the implant and the remaining natural knee structures. Regular follow-up with your orthopedic surgeon is essential to monitor the implant's health and the progression of any arthritis in the unaffected compartments.

Conclusion

A half knee replacement offers a valuable, less invasive surgical option for individuals suffering from osteoarthritis confined to a single compartment of the knee. By preserving healthy bone and ligaments, it aims to restore natural knee mechanics, provide quicker recovery, and enable a return to an active lifestyle. However, careful patient selection and adherence to rehabilitation protocols are crucial for maximizing the benefits and ensuring the long-term success of this specialized procedure. Consulting with an orthopedic surgeon is essential to determine if you are a suitable candidate for this targeted approach to knee pain relief.