Knee Replacement Pain: Causes of Side Pain, Symptoms, and Management

Why does my knee replacement hurt on the side?

Lateral or medial knee pain after a total knee arthroplasty (TKA) can stem from various sources, including soft tissue irritation, implant-related issues, nerve impingement, or less commonly, infection, and requires professional evaluation for accurate diagnosis.

Understanding Knee Anatomy Post-Replacement

A total knee replacement involves resurfacing the ends of the femur (thigh bone) and tibia (shin bone) with metal components and placing a plastic (polyethylene) spacer between them. The patella (kneecap) may also be resurfaced. While the primary goal is to alleviate pain and restore function, the complex network of surrounding soft tissues—ligaments, tendons, muscles, bursae, and nerves—remains. Pain on the "side" of the knee typically refers to either the medial (inner) or lateral (outer) aspect, areas rich in structures that can become irritated, inflamed, or compromised post-surgery.

Common Causes of Lateral or Medial Knee Pain Post-TKA

Identifying the precise cause of pain on the side of a knee replacement requires a thorough clinical assessment, but several common culprits are frequently observed:

• Soft Tissue Irritation or Inflammation: These are among the most frequent causes, often due to altered biomechanics, overuse, or scar tissue formation.

  • Iliotibial Band (ITB) Syndrome: Common on the lateral side. The ITB is a thick band of fascia running from the hip to just below the knee. After TKA, changes in gait or muscle activation patterns can cause it to rub excessively over the lateral femoral condyle (the outer part of the thigh bone), leading to inflammation and pain.
  • Pes Anserine Bursitis or Tendinitis: Common on the medial side. The pes anserine bursa and tendons (sartorius, gracilis, semitendinosus) are located on the inner aspect of the tibia. Inflammation here can be due to overuse, altered gait, or muscle imbalances.
  • Collateral Ligament Irritation: The Medial Collateral Ligament (MCL) on the inner side and the Lateral Collateral Ligament (LCL) on the outer side help stabilize the knee. While often preserved during TKA, they can become stretched, irritated, or impinged by the prosthetic components, leading to pain.
  • Patellar Tendinopathy or Fat Pad Impingement: Although primarily anterior, pain from the patellar tendon (which connects the kneecap to the shin bone) or the infrapatellar fat pad can sometimes radiate to the sides of the knee, especially during activity.

• Implant-Related Issues: Though less common in the immediate post-operative period, these can develop over time.

  • Component Malalignment or Malpositioning: If the prosthetic components are not perfectly aligned or sized, it can lead to uneven loading on the soft tissues, excessive stress on ligaments, or impingement, causing pain on one side.
  • Component Loosening: Over time, the bond between the implant and the bone can weaken, leading to micromotion and pain. This is often progressive and may be accompanied by a feeling of instability.
  • Polyethylene Wear or Debris: The plastic spacer can wear down, releasing tiny particles that irritate the surrounding joint tissues, causing inflammation and pain.
  • Impingement: Soft tissues (like scar tissue or a portion of the joint capsule) can get "pinched" between the prosthetic components during movement.

• Nerve-Related Pain: Nerves around the knee can be affected during surgery or become entrapped in scar tissue.

  • Neuroma Formation: A neuroma is a benign growth of nerve tissue that can form at the site of a cut or damaged nerve, leading to localized, sharp, or burning pain.
  • Nerve Entrapment/Irritation: Specific nerves, such as the saphenous nerve (medial aspect) or branches of the common peroneal nerve (lateral aspect), can become compressed or irritated by scar tissue or swelling.

• Infection: While less common, periprosthetic joint infection (PJI) is a serious complication that can cause pain, swelling, warmth, and redness, often accompanied by systemic symptoms like fever. Pain can be generalized but may manifest strongly on one side.

• Referred Pain: Pain originating from other areas can sometimes be felt in the knee.

  • Hip or Lumbar Spine Issues: Problems in the hip joint or lower back (e.g., sciatica, degenerative disc disease) can refer pain down to the knee, including its sides.

• Scar Tissue Formation (Arthrofibrosis): Excessive scar tissue can limit range of motion and cause pain, sometimes localized to the sides if it restricts movement of structures like the ITB or patella.

• Overuse or Inappropriate Activity: Attempting too much activity too soon, or engaging in activities with improper biomechanics, can overload the healing tissues and lead to inflammatory pain on the sides of the knee.

When to Seek Medical Attention

While mild, transient pain after a TKA can be part of the recovery process, certain symptoms warrant immediate medical evaluation:

• Sudden onset of severe pain, especially if it's worse than your typical post-op pain.
• Significant swelling, redness, or warmth around the knee.
• Fever or chills, which could indicate an infection.
• Inability to bear weight on the affected leg.
• New instability or feeling of "giving way" in the knee.
• Persistent pain that does not improve with rest, ice, or over-the-counter pain relievers.
• Pain that worsens significantly with activity or at night.

Management and Prevention Strategies

Once the underlying cause of side knee pain is diagnosed by a medical professional, management strategies can be tailored. General approaches often include:

• Adherence to Physical Therapy: This is paramount. A structured rehabilitation program will focus on restoring range of motion, strengthening the muscles around the knee and hip, and improving gait mechanics.
• Gradual Activity Progression: Avoid pushing too hard too soon. Listen to your body and gradually increase activity levels as tolerated.
• RICE Protocol: For acute inflammation, rest, ice, compression, and elevation can help manage pain and swelling.
• Pain Management: Over-the-counter anti-inflammatory medications (NSAIDs) or prescribed pain relievers may be used under medical guidance.
• Targeted Strengthening and Flexibility:
  • For ITB syndrome, focus on hip abductor strengthening (gluteus medius) and ITB stretching.
  • For pes anserine issues, address hamstring and quadriceps imbalances and strengthen hip external rotators.
• Biomechanics and Footwear: Work with your physical therapist to correct any gait deviations. Ensure you wear supportive, appropriate footwear.
• Weight Management: Maintaining a healthy body weight reduces stress on the knee joint.

Conclusion

Pain on the side of a knee replacement can be a frustrating experience, but it is a common complaint with a range of potential causes. From soft tissue irritation to more complex implant-related or neurological issues, an accurate diagnosis is crucial for effective management. If you are experiencing persistent or concerning side knee pain after TKA, consult with your orthopedic surgeon or a qualified healthcare professional. They can perform a comprehensive evaluation, including imaging if necessary, to determine the precise cause and guide you toward the most appropriate treatment plan to restore comfort and function.