What causes medial swelling on the knee?

Medial Knee Swelling: Causes and Differential Diagnosis

Medial knee swelling most commonly results from medial compartment osteoarthritis, medial collateral ligament (MCL) bursitis, meniscal pathology, or inflammatory conditions, with the medial compartment bearing 70-80% of joint load during gait making it the most frequently affected site. 1

Primary Structural Causes

Medial Compartment Osteoarthritis

• The medial compartment is by far the most frequently affected in knee OA because approximately 70-80% of joint load passes through this compartment during mid-stance phase of gait, as the center of mass is located medial to the knee joint center. 1
• Physical examination routinely reveals knee effusions in OA patients, and synovitis/effusion severity correlates significantly with frequent knee pain. 2
• Subchondral insufficiency fractures (previously termed spontaneous osteonecrosis) most commonly involve the medial femoral condyle in middle-aged to elderly females, though radiographs are often initially normal. 2

MCL Bursitis

• MCL bursitis presents as a distended and inflamed bursa between the superficial and deep portions of the MCL, causing painful swelling and tenderness over the medial side of the knee and proximal tibia. 3
• This is a rare but important cause of medial knee pain that must be considered in the differential diagnosis of increased medial knee pain, particularly in patients with knee OA. 3
• Pain is typically exacerbated by valgus stress testing. 3
• MRI shows a distended bursa with internal septations beneath the superficial portion of the MCL. 3

Meniscal Pathology

• Medial meniscal tears can cause localized medial joint line tenderness and swelling. 1
• Meniscal cysts may present with high signal surrounding the MCL fibers and can simulate an MCL tear on imaging. 4
• Meniscocapsular separation is another condition that may present with medial swelling. 4

MCL Injuries

• Grade I and II MCL injuries can occur as isolated injuries or in conjunction with other knee structures. 5
• Grade III or complete MCL tears may present with significant medial swelling. 5
• Pellegrini-Stieda syndrome (calcification at the proximal MCL following trauma) can cause medial knee pain and swelling. 6

Other Important Causes

Inflammatory and Infectious Conditions

• A variety of conditions including medial cellulitis, medial bursitis, and inflammatory arthropathies may present with high signal surrounding the MCL fibers and simulate structural pathology. 4
• Pigmented villonodular synovitis is a rare condition that can cause knee pain and swelling. 1, 2
• Aspiration of an acutely swollen knee can aid diagnosis (to rule out infection or crystal arthropathy) and help relieve pain. 7

Soft Tissue Pathology

• Medial plicae can cause anterior-medial knee pain and swelling. 1, 2
• Hoffa's disease, characterized by enhancing synovitis >2mm in Hoffa's fat, correlates with peripatellar pain. 1, 2
• Ganglion cysts and tumors can cause medial knee swelling. 1, 2

Diagnostic Approach

Physical Examination Findings

• Palpation-induced tenderness over the medial joint line typically matches the quality and location of pain experienced during activity. 8
• Inspect for erythema, warmth, and effusion—joint effusions are uncommon with isolated tendinopathy and suggest intra-articular pathology. 8
• Range of motion is often limited on the symptomatic side with significant pathology. 8
• Valgus stress testing helps identify MCL pathology. 3

Imaging Strategy

• Obtain anteroposterior and lateral knee radiographs first to exclude fractures, osteoarthritis, osteophytes, and loose bodies. 2, 9
• MRI without contrast is indicated when radiographs are negative or non-diagnostic and symptoms persist, as it can identify MCL bursitis, meniscal tears, bone marrow lesions, and synovitis. 1, 3
• Ultrasound is excellent for detecting knee joint effusions and can guide aspiration, though it has limited utility for comprehensive internal derangement evaluation. 1

Critical Pitfalls to Avoid

• Do not overlook referred pain from the hip or lumbar spine before attributing symptoms solely to knee pathology—evaluate these clinically if knee imaging is normal. 2, 9
• Not all meniscal tears are symptomatic, particularly in patients over 45 years. 2
• Multiple conditions can mimic MCL tears on imaging, including MCL bursitis, medial osteoarthritis, medial cellulitis, medial bursitis, medial meniscal cyst, meniscocapsular separation, and retinacular tear. 4
• Radiographs may be initially normal in subchondral insufficiency fractures, which later show articular surface fragmentation and subchondral collapse. 2

Treatment Considerations

• For MCL bursitis, ultrasound-guided corticosteroid injection into the bursa is an effective and safe treatment modality. 3
• Most grade I and II MCL injuries can be treated nonoperatively with rest, range of motion exercises, and NSAIDs. 5, 6
• For inflammatory conditions, aspiration can provide both diagnostic information and symptomatic relief. 7
• Bracing of the knee can be useful nonoperative treatment for persons with osteoarthritis predominantly involving the medial tibiofemoral compartment. 1