The kneecap, or patella, is a specialized bone positioned at the front of the knee joint. It is the largest sesamoid bone in the human body, embedded within the tendon of the powerful quadriceps muscle group. The patella acts as an anatomical pulley, significantly increasing the leverage and efficiency of the quadriceps muscle during leg extension. When damage is limited to this specific area, the kneecap can be replaced alone through a specific procedure that targets only the affected joint surfaces.
Patella Replacement: Defining Patellofemoral Arthroplasty
The isolated replacement of the kneecap and its corresponding groove is known as a Patellofemoral Arthroplasty (PFA). This partial knee replacement is designed exclusively for damage confined to the patellofemoral compartment, the joint formed by the kneecap and the thigh bone (femur). Unlike a Total Knee Replacement (TKR), which resurfaces all three compartments of the knee, PFA preserves the healthy cartilage and ligaments in the rest of the joint. This makes PFA a less invasive and more bone-conserving option for selected patients.
During the surgery, the damaged cartilage and a small amount of underlying bone are removed from the back surface of the patella and the trochlear groove, the channel in the femur where the kneecap tracks. The trochlear groove is resurfaced with a thin metal component that acts as a new track. The underside of the kneecap receives a dome-shaped replacement, often called a patellar button, typically made of high-density polyethylene.
Both the metallic and polyethylene components are secured in place with bone cement, creating a new, smooth gliding surface for the kneecap. This targeted approach offers advantages over Total Knee Replacement, including a smaller incision, reduced blood loss, and the preservation of the natural knee ligaments. Modern PFA implant designs aim to recreate the motion of the native joint, which can lead to a more natural feeling knee function post-surgery.
Conditions Indicating Isolated Patellar Surgery
The most common indication for PFA is advanced, isolated patellofemoral arthritis, where the protective articular cartilage is severely worn down only in this compartment. This form of arthritis can result from severe chondromalacia patellae, which is the softening and breakdown of the cartilage on the kneecap’s underside. The surgery is generally reserved for patients whose symptoms, such as pain when climbing stairs or sitting for long periods, have not responded to non-surgical treatments.
Another reason for this specific replacement is damage resulting from severe trauma, such as a kneecap fracture that disrupts the joint surface, or persistent patellar instability. When the kneecap repeatedly dislocates or tracks incorrectly, the continuous friction and impact can lead to accelerated cartilage erosion.
A primary selection factor is the health of the rest of the knee joint, as the procedure is only appropriate if the medial and lateral compartments remain free of significant arthritis. If X-rays and MRI scans reveal generalized arthritis affecting all three compartments, a Total Knee Replacement is typically the recommended procedure. PFA is a treatment for localized damage, often yielding the best results in younger, more active individuals with post-traumatic or post-instability arthritis.
Treatment Options Before and Instead of Full Replacement
Before considering joint replacement, doctors typically recommend a trial of non-surgical treatments to manage pain and improve function. Physical therapy is a primary intervention, focusing on strengthening the quadriceps, hip extensors, and hip abductors to improve the kneecap’s alignment and tracking. Patients are also advised to modify activities to avoid excessive stress, such as limiting deep squatting or prolonged periods of knee flexion.
Anti-inflammatory medications, specifically Nonsteroidal Anti-Inflammatory Drugs (NSAIDs), can help manage pain and swelling associated with joint inflammation. Injections are another non-operative option, with corticosteroid injections providing temporary anti-inflammatory relief directly to the joint. Viscosupplementation involves injecting a gel-like substance that mimics the natural lubricating fluid of the joint, aiming to reduce friction and cushion the surfaces.
When non-surgical management is insufficient, less invasive surgical procedures may be considered as alternatives to replacement, especially for smaller cartilage defects or alignment issues. Cartilage restoration techniques, such as microfracture, involve creating small holes in the underlying bone to stimulate the growth of new fibrocartilage. For patients with kneecap malalignment, an osteotomy may be performed, which involves surgically cutting and repositioning the bone to improve the kneecap’s path.
The Recovery Process
Recovery following a Patellofemoral Arthroplasty is generally faster than that of a Total Knee Replacement due to the less extensive nature of the surgery. Patients are typically allowed to place full weight on the operated leg immediately after the procedure, often with the assistance of a cane or crutches. Pain management is controlled to allow the patient to participate fully in the subsequent rehabilitation program.
Physical therapy is a mandatory component of the recovery, beginning shortly after surgery to focus on restoring the knee’s range of motion and strengthening the quadriceps muscles. Regaining quadriceps strength is important for the long-term success of the implant, as these muscles govern the movement of the new kneecap component. Most patients are able to return to driving within two to six weeks, depending on the limb operated on and the type of vehicle.
The return to work and daily activities is progressive, with many individuals resuming office-based work within one to two weeks. More strenuous manual labor may require a longer period of six to eight weeks. While a full recovery can take several months, the goal is to allow a return to low-impact activities like cycling and swimming relatively quickly. The artificial joint is expected to function well for many years, offering a durable solution for isolated kneecap damage.