Artificial Knee Popping: Causes, When to Be Concerned, and Treatment

Why is my artificial knee popping?

Popping in an artificial knee can range from a benign, expected sound due to the nature of the prosthetic components to an indicator of a more significant issue such as soft tissue impingement, component wear, or malalignment, necessitating professional evaluation.

Understanding Total Knee Arthroplasty (TKA)

A Total Knee Arthroplasty, commonly known as knee replacement surgery, involves replacing damaged bone and cartilage in the knee joint with prosthetic components. These typically include a femoral component (metal cap on the thigh bone), a tibial component (metal plate on the shin bone), and a polyethylene (plastic) spacer that fits between them, acting as the new cartilage. In some cases, the kneecap (patella) may also be resurfaced with a polyethylene button. The goal of TKA is to alleviate pain, correct deformity, and restore function. However, unlike a natural joint with smooth cartilage and synovial fluid, an artificial joint relies on different materials and mechanics, which can sometimes lead to audible sounds.

Common Reasons for Popping in an Artificial Knee

The sounds emanating from an artificial knee can be varied, ranging from clicking and clunking to popping. Understanding the potential causes is crucial for determining if medical attention is required.

• Soft Tissue Impingement: This is a very common cause, especially in the early stages post-surgery. Tendons, ligaments, or scar tissue can occasionally catch or rub against the edges of the prosthetic components as the knee moves. As the knee flexes and extends, these tissues may momentarily become taut and then release, creating a popping sensation or sound. This often improves as the tissues adapt and scar tissue matures.
• Component Malalignment: Even minor degrees of rotational or angular malalignment of the femoral or tibial components, or issues with the patellar component, can lead to abnormal articulation. This means the prosthetic parts aren't gliding against each other perfectly, which can cause them to catch or clunk, resulting in a popping sound during movement.
• Wear and Loosening: Over time, the polyethylene spacer can wear down. This wear can lead to increased space between the metal components, allowing for more movement and potentially causing a popping sound. In rare cases, the components themselves can loosen from the bone, which may manifest as instability, pain, and audible clicks or pops, particularly during weight-bearing activities.
• Polyethylene Spacer Issues: The plastic insert is critical for smooth motion. If it becomes dislodged, fractured, or experiences uneven wear, it can lead to abnormal articulation between the metal components, resulting in popping, clicking, or grinding sounds.
• Patellofemoral Tracking Issues: The patella (kneecap) is designed to track smoothly within a groove on the femoral component. If the patella does not track correctly—perhaps due to component position, soft tissue imbalance, or an issue with the patellar button itself—it can rub or catch, causing a click or pop, especially during knee flexion and extension.
• Ligamentous Instability: The ligaments surrounding the knee provide stability. If these ligaments are too loose (laxity) or too tight, or if they have been damaged, it can lead to abnormal motion of the prosthetic components or the joint as a whole, potentially resulting in a popping sensation or sound.
• Bone Spurs (Osteophytes): While the goal of TKA is to remove damaged bone, occasionally residual bone spurs or new bone growth (heterotopic ossification) can form around the joint, leading to impingement and popping sounds during movement.
• Normal Post-Surgical Sounds: It's important to note that some degree of clicking or popping can be a normal occurrence after knee replacement. Unlike a natural knee with shock-absorbing cartilage, an artificial knee consists of harder materials (metal and plastic) that can inherently produce sounds during articulation. These sounds are often benign, painless, and simply a characteristic of the prosthetic joint.

When to Be Concerned and Seek Medical Attention

While some popping can be normal, certain accompanying symptoms warrant immediate medical evaluation by an orthopedic surgeon:

• Pain: Popping accompanied by new or increasing pain is a significant red flag.
• Swelling, Warmth, or Redness: These symptoms can indicate inflammation, infection, or other serious issues.
• Instability or Giving Way: If your knee feels like it's buckling or giving out, it could indicate component loosening or ligamentous instability.
• Loss of Range of Motion: A sudden decrease in your ability to bend or straighten your knee.
• Locking: If the knee gets stuck in a certain position.
• Sudden Onset: If the popping starts suddenly and is persistent, especially if it's different from any previous sounds.
• Audible Grinding: A grinding sensation often indicates significant wear or component issues.

Diagnostic Approaches for Knee Popping

When you present with concerns about knee popping, your orthopedic surgeon will typically follow a systematic diagnostic process:

• Physical Examination: The surgeon will assess your knee's range of motion, stability, and tenderness, and try to reproduce the popping sound.
• Imaging Studies:
  • X-rays: Standard X-rays can reveal component alignment, signs of loosening, wear patterns, or bone changes.
  • CT Scans: May be used for more detailed evaluation of component position and rotational alignment.
  • MRI: Less common for direct implant issues due to artifact, but can be useful for evaluating surrounding soft tissues if other causes are suspected.
• Blood Tests: If infection is suspected (e.g., with fever, redness, warmth), blood tests for inflammatory markers (ESR, CRP) may be ordered.
• Aspiration (Joint Fluid Analysis): In cases where infection or significant inflammation is suspected, fluid may be drawn from the knee joint for analysis.

Management and Treatment Options

Treatment for artificial knee popping depends entirely on the underlying cause:

• Observation and Reassurance: If the popping is painless, stable, and deemed benign, often no specific intervention is needed beyond continued monitoring.
• Physical Therapy: For soft tissue impingement or mild tracking issues, specific exercises to improve flexibility, strength, and patellar tracking can be beneficial.
• Activity Modification: Adjusting certain activities that aggravate the popping might be recommended.
• Anti-inflammatory Medications: Over-the-counter or prescription NSAIDs can help manage inflammation if present.
• Revision Surgery: For more significant issues like component loosening, severe wear, dislodged inserts, or persistent painful malalignment, revision surgery may be necessary to replace or reposition the problematic components.

Prevention and Long-Term Care

While not all causes of popping are preventable, adherence to post-operative guidelines and proactive long-term care can minimize risks:

• Adhere to Rehabilitation Protocols: Completing your prescribed physical therapy is crucial for regaining strength, flexibility, and proper joint mechanics.
• Maintain a Healthy Weight: Excess body weight places additional stress on the artificial joint, potentially accelerating wear.
• Avoid High-Impact Activities: Activities like running or jumping can put excessive stress on the implant and should generally be avoided unless specifically cleared by your surgeon.
• Regular Follow-ups: Attend scheduled follow-up appointments with your orthopedic surgeon to monitor the health of your artificial knee and detect any issues early.

Conclusion

Hearing sounds from your artificial knee can be unsettling, but it's important to remember that not all popping signifies a problem. While some sounds are a normal characteristic of the prosthetic joint, any new, persistent, or painful popping, especially when accompanied by other concerning symptoms like swelling, instability, or loss of motion, warrants prompt evaluation by your orthopedic surgeon. Early diagnosis and appropriate intervention are key to ensuring the long-term success and function of your knee replacement.