Kneecap Surgery: Understanding Patellar Resurfacing and Patellofemoral Arthroplasty

Can they replace knee caps?

Yes, while a complete, isolated "replacement" of the entire kneecap (patella) is not a standard procedure, its undersurface can be resurfaced or replaced as part of a total knee replacement, or in a more focused partial knee replacement targeting the patellofemoral joint.

Understanding the Knee Cap (Patella)

The patella, commonly known as the kneecap, is a unique, triangular-shaped bone located at the front of the knee joint. Anatomically, it is classified as a sesamoid bone, meaning it is embedded within a tendon—specifically, the quadriceps tendon, which connects the powerful thigh muscles to the shin bone (tibia). The patellar ligament then extends from the bottom of the patella to the tibia.

The patella's primary functions are:

• Lever Arm Enhancement: It acts as a fulcrum, increasing the mechanical advantage of the quadriceps muscles, allowing them to exert more force and extend the knee more efficiently.
• Joint Protection: It shields the knee joint from direct impact and trauma.
• Smooth Movement: Its posterior surface is covered with a thick layer of articular cartilage, allowing it to glide smoothly within a groove on the thigh bone (femur), known as the trochlear groove, during knee flexion and extension.

The Concept of "Replacing" the Kneecap

When individuals ask about "replacing" the kneecap, they are typically referring to surgical interventions designed to alleviate pain and restore function in a severely damaged patellofemoral joint (the joint between the kneecap and the thigh bone). It's crucial to understand that a full, standalone replacement of the entire patella with an artificial bone is not a common or standard surgical procedure. Instead, interventions usually involve:

• Patellar Resurfacing: This is the most common scenario, performed as part of a Total Knee Arthroplasty (TKA).
• Patellofemoral Arthroplasty (PFA): A less common procedure that specifically replaces only the damaged surfaces of the patella and its corresponding femoral groove.

Patellar Resurfacing in Total Knee Arthroplasty (TKA)

In a Total Knee Arthroplasty (TKA), the damaged surfaces of the femur (thigh bone) and tibia (shin bone) are removed and replaced with metal and plastic components. During this procedure, the surgeon may also address the patella.

• What it is: Patellar resurfacing involves removing the damaged articular cartilage from the undersurface (posterior aspect) of the patella. This removed cartilage is then replaced with a smooth, dome-shaped component typically made of polyethylene (medical-grade plastic). This plastic "button" is cemented onto the remaining bone of the patella.
• Why it's done: This is performed when the articular cartilage on the back of the kneecap is significantly worn away due to conditions like osteoarthritis, causing pain and grinding sensations within the patellofemoral joint.
• Considerations: Not all total knee replacements include patellar resurfacing. The decision to resurface the patella is made by the orthopedic surgeon based on:
  • The extent of cartilage damage on the patella.
  • The patient's preoperative symptoms related to the patellofemoral joint.
  • The surgeon's preference and experience.

Patellofemoral Arthroplasty (PFA) / Partial Knee Replacement

Patellofemoral Arthroplasty (PFA), also known as patellofemoral joint replacement or partial knee replacement (specifically for the patellofemoral compartment), is a more targeted procedure.

• What it is: This surgery focuses solely on replacing the damaged articular surfaces of the patella and the trochlear groove of the femur. A metal component is implanted into the trochlear groove, and a polyethylene button is cemented onto the back of the patella.
• When it's considered: PFA is an option for individuals who have isolated arthritis or significant cartilage damage only in the patellofemoral compartment, with the other two compartments of the knee (medial and lateral tibiofemoral) remaining healthy. It's often indicated for severe, localized patellofemoral osteoarthritis, sometimes post-traumatic.
• Advantages:
  • Preserves healthy bone and cartilage in the rest of the knee.
  • Potentially less invasive than a total knee replacement.
  • May allow for a quicker recovery and more natural knee motion.
• Limitations: It is not suitable for widespread knee arthritis or for individuals with significant instability issues.

Conditions Leading to Kneecap Intervention

Surgical intervention involving the patella is typically considered when conservative treatments (e.g., physical therapy, medication, injections) have failed to provide adequate relief for severe pain and functional limitations caused by:

• Osteoarthritis: The most common cause, where the articular cartilage on the back of the patella and/or in the trochlear groove wears down over time.
• Post-traumatic Arthritis: Cartilage damage resulting from a previous injury, such as a patellar fracture or dislocation.
• Rheumatoid Arthritis and Other Inflammatory Arthropathies: Autoimmune conditions that can erode joint cartilage.
• Severe Patellofemoral Pain Syndrome with Cartilage Damage: Chronic pain that has progressed to significant cartilage erosion.
• Failed Previous Surgeries: In some cases, revisions may be necessary.

Surgical Considerations and Outcomes

The decision to proceed with any type of knee surgery involving the patella is complex and made collaboratively between the patient and their orthopedic surgeon. Factors influencing this decision include:

• Severity of Symptoms: The degree of pain, stiffness, and functional limitation.
• Extent of Cartilage Damage: Confirmed by imaging studies (X-rays, MRI).
• Patient Age and Activity Level: Younger, more active individuals may have different considerations.
• Overall Health: Presence of other medical conditions.
• Surgeon's Expertise: Experience with specific techniques and implants.

Outcomes: Both patellar resurfacing in TKA and PFA generally aim to provide significant pain relief, improve knee function, and enhance quality of life. However, like any major surgery, they carry potential risks, including:

• Infection
• Blood clots
• Component loosening or wear
• Patellar instability or dislocation
• Persistent pain or stiffness
• Nerve damage

The Role of Rehabilitation and Exercise

Regardless of the specific surgical approach, rehabilitation is absolutely critical for a successful outcome following any patellar intervention.

• Pre-habilitation (Pre-hab): Strengthening and improving range of motion before surgery can significantly aid post-operative recovery.
• Post-operative Physical Therapy: A structured program guided by a physical therapist is essential. It typically involves:
  • Early Range of Motion (ROM) Exercises: To prevent stiffness and restore mobility.
  • Strengthening Exercises: Focusing on the quadriceps, hamstrings, glutes, and core muscles to support the new joint and improve stability.
  • Gait Training: Re-learning to walk properly.
  • Balance and Proprioception Training: To improve stability and coordination.
  • Activity Progression: Gradually returning to daily activities and, eventually, more demanding physical endeavors.

Adherence to the rehabilitation protocol is paramount for regaining strength, flexibility, and optimal function of the knee.

Conclusion: A Nuanced Approach to "Replacing" the Kneecap

In summary, while a direct, full "replacement" of the entire kneecap as a standalone procedure is not the norm, the advanced field of orthopedic surgery offers effective solutions for a damaged patella. These solutions primarily involve resurfacing the undersurface of the patella as part of a total knee replacement or performing a partial patellofemoral replacement for isolated damage. The goal of these interventions is to alleviate debilitating pain, restore joint mechanics, and significantly improve the patient's ability to perform daily activities and engage in appropriate physical endeavors. Understanding these nuances is key to appreciating the sophistication of modern knee joint restoration.