Knee Replacement: Understanding Medial Knee Pain, Causes, and Management

Why does the inside of my knee hurt after knee replacement?

Medial knee pain following a total knee replacement (TKR) is a common concern that can arise from a spectrum of factors, ranging from expected post-surgical healing processes and soft tissue irritation to more complex issues involving prosthetic components or rehabilitation challenges.

Anatomy Refresher: The Medial Knee

To understand why pain might occur on the inside of the knee after a replacement, it's crucial to briefly revisit the relevant anatomy. The medial, or inner, aspect of the knee is a complex region comprising:

• Medial Collateral Ligament (MCL): A primary stabilizer preventing the knee from gapping inwards (valgus stress).
• Pes Anserine Bursa and Tendons: A group of three tendons (sartorius, gracilis, semitendinosus) insert onto the medial tibia, often with a bursa beneath them, which can become inflamed.
• Medial Meniscus: Although typically removed or significantly trimmed during TKR, its former location and surrounding soft tissues remain relevant.
• Medial Femoral and Tibial Condyles: The bony structures that articulate, which are resurfaced or replaced in TKR.
• Medial Retinaculum: Connective tissue that supports the patella (kneecap) on its medial side.

These structures, along with the numerous nerves and blood vessels, can all contribute to pain sensation.

Understanding Total Knee Arthroplasty (TKA)

Total Knee Arthroplasty (TKA), also known as total knee replacement (TKR), is a surgical procedure that involves resurfacing the damaged cartilage and bone of the knee joint with prosthetic components. Typically, the ends of the femur (thigh bone) and tibia (shin bone), and often the back of the patella (kneecap), are replaced with metal and plastic implants. The goal is to alleviate pain, correct deformity, and restore function. While highly successful, it is a major surgery that involves significant tissue manipulation.

Common Causes of Medial Knee Pain Post-TKA

Medial knee pain after TKA can be attributed to several factors, some expected and others requiring specific attention:

Normal Post-Operative Inflammation and Healing

• Acute Post-Surgical Pain: In the immediate weeks to months following TKA, pain on the inside of the knee is often part of the normal healing process. Surgical incisions, tissue dissection, and bone preparation inevitably cause inflammation, swelling, and nerve irritation in the area.
• Tissue Remodeling: As the body heals, scar tissue forms. This process can be sensitive and may cause persistent discomfort as the tissues mature and adapt to the new joint mechanics.

Soft Tissue Irritation or Impingement

• Pes Anserine Bursitis/Tendonitis: The tendons of the pes anserine group, located on the medial side of the tibia just below the knee joint, can become irritated or inflamed. This is common due to increased activity, altered gait mechanics, or direct surgical trauma to the area.
• Medial Retinacular Irritation: If the patella's tracking is slightly off, or if there's excessive scar tissue in the medial patellar retinaculum, it can cause pain on the inner aspect of the kneecap, which can be perceived as medial knee pain.
• MCL Scar Tissue/Adhesions: While the MCL is typically preserved during TKA, scar tissue can form around it, leading to stiffness or pain, especially with certain movements.

Ligamentous Imbalance or Laxity

• Medial Collateral Ligament (MCL) Issues: The MCL is critical for knee stability. If it is either too tight or too loose after the TKR, it can cause medial pain. An MCL that is too loose might lead to a feeling of instability and increased stress on the medial side, while an overly tight MCL can cause pain due to excessive tension.
• Gap Balancing: During TKA, surgeons carefully "balance" the soft tissues to ensure proper tension in flexion and extension. Imbalances, though rare with experienced surgeons, can lead to uneven stress on the medial structures.

Prosthetic Component Related Issues

• Component Malposition: Even slight rotational or angulation errors in the placement of the femoral or tibial components can alter the biomechanics of the knee, leading to abnormal stress on the medial soft tissues or impingement of the components.
• Component Loosening: While less common in the early post-operative period, aseptic loosening (without infection) of the prosthetic components from the bone can cause pain, often described as a deep ache. This occurs when the bond between the implant and bone fails.
• Polyethylene Wear: Over many years, the plastic (polyethylene) insert between the metal components can wear down, leading to instability, inflammation, and pain. This is typically a late complication.

Patellofemoral Issues (Referred Pain)

• Patellar Tracking Problems: The patella (kneecap) must track smoothly in the trochlear groove of the femoral component. If it tracks laterally (to the outside), it can pull on the medial retinaculum or cause general anterior knee pain that refers to the medial side.
• Patellar Component Loosening/Wear: If the patella was also resurfaced, issues with its component can cause pain.

Rehabilitation Challenges and Biomechanical Factors

• Muscle Weakness and Imbalance: Weakness in the quadriceps, hamstrings, or hip abductors can alter gait mechanics, placing undue stress on the medial knee structures.
• Altered Gait Mechanics: Patients often adopt compensatory walking patterns post-surgery. These abnormal mechanics can lead to overuse injuries or strain on the medial soft tissues.
• Overuse or Premature Return to Activity: Engaging in too much activity too soon, or with improper form, can exacerbate inflammation and pain in the healing tissues.

Less Common but Serious Causes

• Infection: A serious complication, infection can manifest as persistent or worsening pain, redness, warmth, swelling, fever, or drainage from the incision site.
• Deep Vein Thrombosis (DVT): While typically affecting the calf or thigh, a blood clot can cause pain, swelling, and warmth, and should be ruled out if symptoms are present.
• Complex Regional Pain Syndrome (CRPS): A rare but severe chronic pain condition that can develop after trauma or surgery, characterized by disproportionate pain, swelling, skin changes, and sensitivity.

When to Seek Medical Attention

While some medial knee pain is expected after TKA, certain symptoms warrant immediate medical evaluation:

• Sudden, severe, or worsening pain: Especially if it was previously improving.
• Increased swelling, redness, or warmth: Around the knee or down the leg.
• Fever or chills: Indicating possible infection.
• Drainage from the incision site: Any pus or foul-smelling fluid.
• Calf pain, tenderness, or swelling: Potentially indicating a DVT.
• New instability or a feeling of the knee "giving way."
• Inability to bear weight or perform activities previously possible.

Management Strategies and Rehabilitation

Effective management of medial knee pain post-TKA typically involves a multi-faceted approach:

• Adherence to Physical Therapy: This is paramount. A structured rehabilitation program focuses on regaining range of motion, strengthening surrounding musculature (quadriceps, hamstrings, hip abductors), and improving gait mechanics.
• Pain Management: Over-the-counter pain relievers (NSAIDs if appropriate), prescribed medications, and RICE (Rest, Ice, Compression, Elevation) can help manage inflammation and discomfort.
• Activity Modification: Temporarily reducing activities that exacerbate pain and gradually reintroducing them as tolerance improves.
• Targeted Exercises: Specific exercises to address muscle imbalances, improve patellar tracking, and stretch tight soft tissues.
• Orthotics/Footwear: In some cases, custom orthotics or appropriate footwear can help optimize lower limb alignment and reduce stress on the knee.
• Injections: For localized soft tissue inflammation (e.g., pes anserine bursitis), corticosteroid injections may provide temporary relief.
• Surgical Revision: In rare cases where conservative measures fail and a specific structural issue (e.g., component malposition or loosening) is identified, revision surgery may be considered.

Conclusion

Pain on the inside of the knee after a total knee replacement is a common, though often complex, issue. While many cases relate to the normal healing process or minor soft tissue irritation, it is crucial for patients and their healthcare providers to carefully evaluate persistent or worsening symptoms. A thorough clinical examination, potentially supported by imaging studies, is essential to identify the underlying cause and guide an appropriate, evidence-based management strategy, ultimately aiming to optimize the long-term success and patient satisfaction with the knee replacement.