Total Knee Arthroplasty (TKA), commonly known as knee replacement surgery, is a successful procedure for treating end-stage arthritis. Although most patients achieve substantial pain relief and improved mobility, some report persistent discomfort after the initial recovery. Pain specifically located on the medial, or inside, aspect of the knee is a common concern. Understanding the causes of this medial-sided pain, which range from routine healing to complex mechanical issues, is necessary for finding a solution.
Acute and Expected Post-Surgical Discomfort
Some medial knee pain immediately after surgery is a predictable part of the healing process. The surgical incision and manipulation of surrounding tissues trigger an inflammatory response. This inflammation causes postoperative swelling (edema), leading to generalized aching and tenderness around the joint, including the inside area.
The peak of this discomfort typically occurs within the first few weeks following the operation. Pain management protocols help control these acute symptoms as the body begins the repair process. This swelling and related pain should progressively decrease over the initial few months as the surgical wound heals. If this pain persists beyond the expected three-to-six-month recovery window or suddenly worsens, it suggests a secondary underlying cause beyond normal surgical recovery.
Specific Soft Tissue Irritations
Pain localized below the joint line on the inner side of the leg often points to specific non-prosthetic soft tissue structures.
Pes Anserine Bursitis (PAB)
A common source of localized discomfort is Pes Anserine Bursitis (PAB), which is the inflammation of a small fluid-filled sac located on the upper shinbone. This bursa cushions the tendons of three hamstring muscles. Changes in gait following TKA or the surgical approach can irritate this bursa, causing a dull ache or burning sensation, especially with activity or at night. Diagnosis is confirmed by pinpoint tenderness upon palpation, sometimes aided by ultrasound imaging. Treatment involves conservative measures, such as rest, anti-inflammatory medication, or a targeted corticosteroid injection.
Another anatomical source of medial pain involves the Medial Collateral Ligament (MCL), which runs along the inside of the knee and provides stability. The MCL is routinely manipulated during TKA to ensure proper tension and balance of the new joint components. This necessary surgical step can result in post-operative strain or tenderness along the inner joint line.
The artificial joint itself can also irritate the MCL. A slight overhang of the tibial component (the metal tray fixed to the shinbone) can physically rub or impinge upon the medial soft tissues and the ligament. This mechanical irritation leads to chronic inflammation and persistent soreness on the inside of the knee.
Mechanical and Implant-Related Problems
When soft tissue causes are ruled out, attention turns to the prosthetic components and their mechanical function.
Component Malalignment
A significant cause of localized pain is component malalignment, where the artificial joint pieces are not perfectly positioned. Small rotational or angular errors in placing the femoral or tibial components alter the forces transmitted across the joint. For example, an internally malrotated tibial component places undue strain on the medial soft tissues and ligaments, leading to chronic pain. If the tibial component is sized or placed incorrectly, a medial overhang can occur, physically impinging on surrounding tissue. These mechanical issues may require specialized imaging, such as a computed tomography (CT) scan, for accurate assessment.
Another type of mechanical failure is aseptic loosening, which refers to the implant losing its secure fixation to the bone without infection. This happens when the bond between the implant and the patient’s bone fails over time. The resulting micromotion of the implant against the bone causes a deep, aching pain, often exacerbated by weight-bearing activities.
While aseptic loosening is more common years after surgery, early loosening can occur due to poor fixation technique. The polyethylene liner (the plastic spacer between the metal components) can also contribute to pain if it is incorrectly sized or if scar tissue forms around it, causing friction or internal soft tissue irritation.
Serious Systemic and Neurological Causes
Less common, but more serious, causes of post-TKA medial pain involve systemic or neurological issues requiring immediate medical evaluation.
Periprosthetic Joint Infection (PJI)
Periprosthetic Joint Infection (PJI) is a complication where bacteria colonize the joint and the implant. Acute infections often present with systemic signs such as fever, chills, and a warm, severely swollen, and painful knee. Chronic infections can be more subtle, presenting only as persistent, unexplained pain that does not improve. Ruling out infection is a priority, typically involving blood tests for inflammatory markers, such as erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP).
Another systemic concern is Deep Vein Thrombosis (DVT), or blood clots, which are a risk following any major lower-extremity surgery. While DVT-related pain is often centered in the calf, severe swelling of the entire leg and general leg pain are warning signs that must be investigated quickly.
A distinct source of medial pain can be neurological irritation, specifically involving the saphenous nerve. The infrapatellar branch of this nerve runs along the inner side of the knee and is susceptible to injury or entrapment during the surgical approach. Damage or scarring around this nerve can lead to a painful neuroma, causing chronic, sharp, or burning pain.
Diagnosis and Next Steps for Persistent Pain
Determining the exact cause of persistent medial knee pain is methodical and begins with a thorough physical examination. Certain symptoms warrant immediate attention, including high fever, drainage from the incision, sudden severe swelling, or pain that prevents weight-bearing. These “red flags” indicate a need for urgent care.
The diagnostic workup starts with plain X-rays to assess the position and alignment of the prosthetic components and look for signs of loosening. If infection is suspected, blood tests for inflammatory markers (ESR and CRP) are ordered, followed by joint aspiration for laboratory analysis.
For suspected soft tissue issues like bursitis or nerve irritation, ultrasound imaging or a diagnostic injection of a local anesthetic helps confirm the source of the pain. Mechanical problems, such as severe malalignment or aseptic loosening, often require revision surgery to correct component positioning or secure the implant.