Kneecap Replacement: Procedure, Recovery, and Risks Explained

How do you replace a knee cap?

Replacing a knee cap, medically known as patellofemoral arthroplasty or as part of a total knee replacement, is a surgical procedure performed to alleviate severe pain and dysfunction caused by damage to the articular cartilage or bone of the kneecap and its groove.

Understanding the Patella and Its Role

The patella, commonly known as the kneecap, is a sesamoid bone embedded within the quadriceps tendon. Its primary biomechanical function is to act as a fulcrum, increasing the mechanical advantage of the quadriceps muscle by extending the lever arm. This significantly enhances the force with which the quadriceps can extend the lower leg. The patella glides within a groove on the end of the femur, known as the trochlear groove, facilitating smooth knee extension and flexion. Healthy articular cartilage covers the undersurface of the patella and the trochlear groove, allowing for frictionless movement.

When is Kneecap Replacement Necessary?

Kneecap replacement is typically considered when conservative treatments, such as physical therapy, pain medication, injections, and activity modification, have failed to provide adequate relief for severe, debilitating pain originating from the patellofemoral joint. The most common conditions necessitating such a procedure include:

• Severe Patellofemoral Osteoarthritis: Degeneration of the articular cartilage on the back of the patella and/or in the trochlear groove. This can result from age-related wear and tear, previous injury, or inflammatory conditions.
• Post-Traumatic Arthritis: Arthritis developing after a significant injury to the patella or knee joint, such as a fracture or dislocation.
• Rheumatoid Arthritis or Other Inflammatory Arthropathies: Chronic inflammation that erodes joint cartilage.
• Failed Previous Surgeries: In some cases, if prior surgical interventions for patellofemoral pain have not been successful.

The Surgical Procedure: Patellofemoral Arthroplasty (Kneecap Replacement)

Patellofemoral arthroplasty (PFA), often referred to as a "kneecap replacement," is a form of partial knee replacement. Unlike a total knee replacement, PFA specifically targets only the damaged surfaces of the patellofemoral joint, preserving the healthy parts of the knee, including the tibiofemoral (thigh bone to shin bone) compartments.

• What is Replaced: The surgeon removes the damaged cartilage and a small amount of underlying bone from the back (undersurface) of the patella. This is then replaced with a smooth, dome-shaped plastic component. Simultaneously, a metal component is typically implanted onto the trochlear groove of the femur to provide a new, smooth surface for the new patellar component to glide against.
• Materials: The implants are typically made of highly durable materials like cobalt-chromium alloys for the femoral component and ultra-high molecular weight polyethylene for the patellar component.
• Goal: The aim is to restore smooth, pain-free gliding of the patella within its groove, thereby improving function and reducing pain.

The Surgical Procedure: Total Knee Arthroplasty (TKA) and the Patella

In many cases of total knee arthroplasty (TKA), where all three compartments of the knee (medial, lateral, and patellofemoral) are significantly affected, the patella's articular surface is also addressed.

• Patellar Resurfacing: During a TKA, the surgeon will often resurface the back of the patella, removing the damaged cartilage and bone and replacing it with a polyethylene button. This component then articulates with the new femoral component of the total knee replacement.
• Decision to Resurface: The decision to resurface the patella during a TKA depends on the extent of patellar damage, the surgeon's preference, and patient-specific factors. In some cases, if the patellar cartilage is healthy, it may not be replaced.

The Surgical Process: What to Expect

The surgical process for kneecap replacement involves several stages:

• Pre-Surgery Evaluation: Comprehensive medical evaluation, imaging (X-rays, MRI), and discussions with the surgical team to ensure the patient is a suitable candidate and understands the procedure.
• Anesthesia: The procedure is performed under general anesthesia, spinal anesthesia, or a combination, often with a nerve block for post-operative pain control.
• Incision: An incision is made over the front of the knee to access the joint.
• Resection and Implantation: The damaged cartilage and bone are precisely removed from the patella and/or trochlear groove. The prosthetic components are then securely cemented or press-fit into place.
• Closure: The incision is closed with sutures or staples, and a sterile dressing is applied.
• Immediate Post-Op: Patients are monitored in a recovery area. Pain management strategies are initiated, and early mobilization, often with the help of a continuous passive motion (CPM) machine or physical therapist, begins almost immediately.

Rehabilitation and Recovery: A Kinesiology Perspective

Rehabilitation is a critical component of a successful kneecap replacement, demanding consistent effort and adherence to a structured physical therapy program. The goal is to restore range of motion, strength, proprioception, and functional mobility.

• Phase 1: Early Post-Operative (Weeks 0-4):
  • Focus: Pain and swelling management, protecting the surgical site, restoring gentle knee range of motion (flexion and extension), and initiating quadriceps activation.
  • Exercises: Gentle knee bends, ankle pumps, isometric quadriceps sets, straight leg raises, and controlled knee flexion within pain limits.
  • Goal: Achieve near full extension and at least 90 degrees of flexion, reduce swelling, and regain basic muscle control.
• Phase 2: Progressive Strengthening (Weeks 4-12):
  • Focus: Gradual increase in weight-bearing, strengthening the quadriceps, hamstrings, glutes, and calf muscles, and improving balance.
  • Exercises: Wall slides, mini-squats, leg presses, hamstring curls, calf raises, step-ups, stationary cycling, and balance exercises (e.g., single-leg stance).
  • Goal: Improve walking mechanics, increase strength to perform daily activities without assistance, and enhance endurance.
• Phase 3: Advanced Function and Return to Activity (Weeks 12+):
  • Focus: Restoring higher-level functional movements, agility, power, and preparing for return to specific activities or sports (if appropriate).
  • Exercises: Lunges, plyometrics (if cleared by surgeon/PT), sport-specific drills, and progression to more dynamic activities.
  • Emphasis: Restoring optimal patellar tracking through balanced quadriceps and hip muscle strength.
• Role of Physical Therapy: A qualified physical therapist will guide the patient through each phase, adjusting the program based on individual progress and ensuring proper biomechanics. Consistency with home exercises is paramount.

Potential Risks and Considerations

While generally safe and effective, kneecap replacement carries potential risks, including:

• Infection
• Blood clots (DVT)
• Stiffness or limited range of motion
• Persistent pain
• Loosening or wear of the implants over time
• Nerve or blood vessel damage
• Fracture

Realistic expectations are crucial. While the procedure aims to significantly reduce pain and improve function, it may not restore the knee to its pre-injury state, especially for high-impact activities.

Life After Kneecap Replacement

Most individuals experience significant pain relief and improved quality of life following kneecap replacement. Long-term success depends on adherence to rehabilitation, maintaining a healthy weight, and avoiding high-impact activities that could accelerate implant wear. Regular follow-up appointments with the orthopedic surgeon are essential to monitor the implant and overall knee health. Continued participation in low-impact exercises like walking, swimming, and cycling is encouraged to maintain strength and joint mobility.