Medial Knee Swelling Causes and Diagnostic Approach

Primary Structural Causes

The American College of Radiology notes that medial compartment osteoarthritis is 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
The American College of Radiology recommends that physical examination routinely reveals knee effusions in OA patients, and synovitis/effusion severity correlates significantly with frequent knee pain, with a strength of evidence level of moderate 2
Subchondral insufficiency fractures most commonly involve the medial femoral condyle in middle-aged to elderly females, though radiographs are often initially normal, according to the American College of Radiology 2

Medial meniscal tears can cause localized medial joint line tenderness and swelling, as reported by the Journal of the American College of Radiology 3
Medial plicae can cause anterior-medial knee pain and swelling, according to the Journal of the American College of Radiology and the American College of Radiology 3, 2
Hoffa's disease, characterized by enhancing synovitis >2mm in Hoffa's fat, correlates with peripatellar pain, as noted by the Journal of the American College of Radiology and the American College of Radiology 3, 2

Palpation-induced tenderness over the medial joint line typically matches the quality and location of pain experienced during activity, with a strength of evidence level of high, according to the American College of Radiology 4
Inspect for erythema, warmth, and effusion—joint effusions are uncommon with isolated tendinopathy and suggest intra-articular pathology, as recommended by the American College of Radiology 4
Obtain anteroposterior and lateral knee radiographs first to exclude fractures, osteoarthritis, osteophytes, and loose bodies, according to the American College of Radiology 2, 5
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, with a strength of evidence level of moderate, as reported by the Journal of the American College of Radiology 3

Bracing of the knee can be useful nonoperative treatment for persons with osteoarthritis predominantly involving the medial tibiofemoral compartment, as noted by the American Academy of Physical Medicine and Rehabilitation 1