What is a Baker's cyst, typically found in an adult patient with a history of knee pain or swelling, possibly due to osteoarthritis, rheumatoid arthritis, or a meniscal tear?

Baker's Cyst: Clinical Description

Definition and Anatomic Location

A Baker's cyst is a fluid-filled mass representing distention of a bursa in the popliteal fossa, most commonly the gastrocnemio-semimembranosus bursa, located in the posteromedial region of the knee between the medial head of the gastrocnemius muscle and the semimembranosus tendon. 1, 2, 3

• The cyst appears as a comma-shaped extension that can be visualized sonographically in the posterior transverse scan between the medial head of gastrocnemius and semimembranosus tendon 1, 4
• This bursa uniquely communicates with the knee joint via an opening in the joint capsule posterior to the medial femoral condyle, unlike other periarticular bursae 2
• The capsular opening creates a valve-like mechanism in the presence of knee effusion that contributes to cyst formation in adults 2, 5

Clinical Presentation

Patients present with swelling and tenderness behind the knee that may worsen with exercise and can be present at rest, causing a feeling of tightness or fullness in the popliteal fossa. 1, 6

Key Clinical Features:

• Visible and palpable swelling in the popliteal fossa, especially with knee extension 6
• Fluctuant mass that may increase in size with knee extension 6
• Pain that worsens with knee movement or exercise 6
• Symptoms are not intermittent like vascular claudication—they persist at rest 1, 6
• Position changes typically do not provide significant relief 1
• Possible limitation in range of motion of the knee joint 6

Important Clinical Pitfall:

Large knee effusions that are out of proportion to pain are typical of underlying intra-articular pathology associated with Baker's cysts. 1 The cyst itself may extend far into thigh and calf muscles 1, 6

Associated Conditions

In adults, Baker's cysts rarely manifest alone and are most often found in conjunction with intra-articular pathologies including osteoarthritis, meniscus tears, and rheumatoid arthritis. 2, 3, 5

• In rheumatoid arthritis patients with knee involvement, Baker's cysts occur in 15.3% of cases 7
• The occurrence is significantly related to local knee inflammation rather than systemic disease activity 7
• Patients with knee-only involvement and anemia are at higher risk for developing Baker's cysts 7

Pediatric Distinction:

• In children, popliteal cysts are only occasionally associated with intra-articular pathology and are more often incidental findings discovered during routine physical examination 2, 3
• These cysts generally do not have clinical relevance in the pediatric population 3

Complications

Rupture of a Baker's cyst can occur, causing sudden calf pain and swelling that clinically mimics deep vein thrombosis. 1, 6

• A ruptured cyst presents as pseudothrombophlebitis with dissemination of fluid into the calf 6, 2
• This is easily identified by ultrasound but represents a critical differential diagnosis requiring imaging confirmation 1, 6
• In rheumatoid arthritis patients with Baker's cysts, 26.8% develop rupture 7

Critical Differential Diagnosis Caveat:

Clinicians should not assume all popliteal masses are benign Baker's cysts and must obtain imaging to exclude popliteal artery aneurysm, especially in patients with a history of other arterial aneurysms. 6 Clinical prediction scores and D-dimer testing alone are insufficient to distinguish between ruptured Baker's cyst and DVT 6

Diagnostic Approach

Ultrasound is the preferred initial diagnostic tool to confirm the presence of a Baker's cyst, with comparable accuracy to MRI for this specific purpose. 4, 6

• Ultrasound can accurately diagnose a popliteal cyst, detect cyst rupture, and determine the vascularity of a mass 6
• Plain radiographs of the knee should be obtained first to evaluate for underlying joint pathology, including anteroposterior, lateral, sunrise/Merchant, and tunnel views 6
• MRI without IV contrast is recommended when additional evaluation is needed after ultrasound or when concomitant internal knee pathology is suspected 1, 6
• MRI accurately depicts the extent of effusion, presence of synovitis, and presence or rupture of a popliteal cyst 1

Imaging Detection Superiority:

• Ultrasound detects Baker's cysts 1.88-fold more frequently than clinical examination (range 1.17-2.5-fold) 1
• MRI demonstrates strong correlation with clinical synovitis (r=0.9, p=0.0001) 1

Red Flags for Malignancy

Although almost all knee cysts are benign, certain signs necessitate suspicion of malignancy: 3

• Symptoms disproportionate to cyst size 3
• Absence of joint damage (e.g., meniscal tears) that might explain the cyst's existence 3
• Unusual cyst topography 3
• Bone erosion 3
• Cyst size greater than 5 cm 3
• Tissue invasion of the joint capsule 3