What are the possible diagnoses and management options for a patient presenting with pain and swelling behind the knees?

Pain Behind the Knee with Swelling: Diagnostic Approach

The most likely diagnosis is a Baker's cyst (popliteal cyst), which characteristically presents with swelling and tenderness behind the knee that may worsen with exercise and can be present at rest. 1

Primary Differential Diagnoses

Baker's Cyst (Most Common)

• Baker's cysts are fluid accumulations in the bursa of the gastrocnemius or semimembranosus muscles that frequently communicate with the knee joint space, presenting with swelling and tenderness in the popliteal fossa. 1
• Pain may worsen with knee movement or exercise, and patients often report a feeling of tightness or fullness behind the knee. 1
• The cyst has a characteristic comma-shaped appearance on ultrasound between the medial head of gastrocnemius and semimembranosus tendon. 1
• Visible and palpable swelling in the popliteal fossa is especially prominent with the knee extended, appearing as a fluctuant mass. 1

Ruptured Baker's Cyst (Critical to Exclude)

• A ruptured Baker's cyst can clinically mimic deep vein thrombosis (DVT) with sudden calf pain and swelling, making this a critical differential diagnosis. 1, 2
• Rupture causes dissemination of fluid into the calf and requires differentiation from DVT. 1
• Most patients respond well to conservative management. 2

Underlying Knee Pathology

• Baker's cysts are often secondary to intra-articular knee pathology, particularly osteoarthritis, meniscal tears, or synovitis. 1, 3
• Osteoarthritis is the most common cause of chronic knee pain and frequently coexists with Baker's cysts. 4

Diagnostic Algorithm

Step 1: Initial Imaging

• Obtain plain radiographs of the knee first (AP, lateral, Merchant/sunrise, and tunnel views) to evaluate for underlying joint pathology causing the cyst. 5, 1
• Radiographs are appropriate for evaluating osteoarthritis and other structural abnormalities. 5

Step 2: Confirm Baker's Cyst

• Ultrasound is the preferred initial diagnostic tool for confirming the presence of a Baker's cyst, with comparable accuracy to MRI for this specific purpose. 1
• Ultrasound can accurately diagnose a popliteal cyst, detect cyst rupture, and determine the vascularity of a mass. 1
• Ultrasound is also useful for confirming effusion and guiding aspiration. 6

Step 3: Advanced Imaging if Needed

• MRI without IV contrast is recommended when additional evaluation is needed after ultrasound or when concomitant internal knee pathology is suspected. 1
• MRI accurately depicts the extent of an effusion, presence of synovitis, and presence or rupture of a popliteal cyst. 1
• MRI can detect meniscal tears, cartilage lesions, and bone marrow edema not visible on radiographs. 5

Critical Pitfall to Avoid

Do not assume all popliteal masses are benign Baker's cysts—obtain imaging to exclude popliteal artery aneurysm, especially in patients with a history of other arterial aneurysms. 1

Management Approach

Conservative Management (First-Line)

• Initial management consists of exercise therapy, patient education, and analgesia with acetaminophen up to 4 g/day in adults. 5
• Most Baker's cysts, including ruptured cysts, respond well to conservative management. 2

Interventional Options

• Ultrasound-guided aspiration may provide temporary relief for symptomatic cysts. 1, 3
• Corticosteroid injection into the knee joint may be beneficial when there is associated synovitis. 1, 3
• Image-guided anesthetic injection can help identify the source of pain and aid in treatment planning. 1

Address Underlying Pathology

• If osteoarthritis is present, first-line treatment includes exercise therapy, weight loss if overweight, and self-management programs. 4
• If meniscal tears are identified, exercise therapy for 4-6 weeks is appropriate for most cases. 4