What is the differential diagnosis of suprapatellar fullness?

Differential Diagnosis of Suprapatellar Fullness

Suprapatellar fullness most commonly represents joint effusion or synovial proliferation, but the differential diagnosis includes Baker's cyst extension, suprapatellar bursitis, pathological synovial plica, loose bodies, and less commonly chondrocalcinosis or hemorrhagic bursitis. 1

Primary Diagnostic Considerations

Joint Effusion and Synovial Proliferation

• Small amounts of effusion can be detected in the suprapatellar pouch even when missed clinically, making ultrasound or MRI superior to physical examination for detection 1
• Inflammatory synovitis appears as synovial thickening with or without associated fluid, detectable by ultrasound with 2.18-fold greater sensitivity than clinical examination 1
• MRI synovitis detection is 1.71-fold more sensitive than clinical examination, with ultrasound showing suprapatellar bursitis detection at 1.7-fold clinical sensitivity 1

Baker's Cyst (Popliteal Cyst)

• Baker's cysts are fluid accumulations in the gastrocnemius or semimembranosus bursa that frequently communicate with the joint space and can extend into the suprapatellar region 1
• Ultrasound detects Baker's cysts with 1.88-fold greater sensitivity than clinical examination 1
• The comma-shaped extension between the medial gastrocnemius head and semimembranosus tendon on posterior transverse ultrasound scan confirms the diagnosis 1
• A ruptured Baker's cyst can clinically mimic deep vein thrombosis and is easily identified by ultrasound 1

Suprapatellar Synovial Plica

• Pathological suprapatellar plica results from congenital thickening of the synovial membrane that becomes symptomatic due to idiopathic, traumatic, or inflammatory conditions 2
• Clinical presentation includes anterior knee pain with possible clicking, swelling, giving way, and locking after prolonged knee flexion 2
• Complete suprapatellar plica can create a septum separating the subquadricipital recess from the knee joint, presenting as pseudotumoral suprapatellar mass 3
• Arthroscopy remains the gold standard for detecting all synovial plicae, though MRI can suggest the diagnosis 2, 3

Isolated Suprapatellar Pouch Pathology

• An enlarged suprapatellar pouch completely separated from the knee joint cavity by a suprapatellar membrane can cause persistent swelling extending 10 cm proximal to the patella 4
• Chondrocalcinosis in an isolated suprapatellar pouch presents as recurrent painless suprapatellar swelling 4
• The mechanism involves either primary effusion of an isolated pouch or secondary separation from incomplete involuted plica becoming adherent during recurrent inflammation 4

Loose Bodies

• Loose joint bodies can be detected sonographically in the suprapatellar pouch, though failure to detect them never rules out their presence 1
• (Osteo-)chondromatosis should be considered in the differential diagnosis 1

Hemorrhagic Bursitis

• Chronic hemorrhagic prepatellar bursitis can have complex MRI appearance due to blood products, potentially mimicking more concerning pathology 5
• The size and complex signal characteristics may create diagnostic confusion on imaging 5

Diagnostic Approach

Clinical Examination Limitations

• Very small effusions or synovitic proliferations are often missed clinically but can be demonstrated by ultrasound or MRI 1
• Physical examination alone has poor specificity for distinguishing between causes of suprapatellar fullness 1

Imaging Strategy

• Ultrasound is the initial imaging modality of choice for evaluating suprapatellar fullness, with longitudinal and transverse scans in neutral position while exerting pressure on the suprapatellar pouch by quadriceps tightening 1
• Suprapatellar transverse scan in maximal flexion optimally visualizes the femur and articular cartilage 1
• MRI provides superior soft tissue characterization when ultrasound findings are equivocal or when complex pathology is suspected 1
• Normal suprapatellar bursa on ultrasound appears as a well-defined hypoechoic band cranial to patella, with thickness ranging 1-4 mm and length 12-44 mm 6

Pathological Imaging Features

• Bursal thickness greater than 3-4 mm, asymmetry greater than 2 mm, irregular outline, and inhomogeneous contents should be considered pathologic 6
• Quadriceps contraction increases both thickness and length of bursal swelling by 1-2 mm on average 6

Key Clinical Pitfalls

• Do not assume all suprapatellar fullness represents simple joint effusion—consider pathological plica, especially when imaging shows intraarticular or periarticular lesions with persistent anterior knee pain 2
• Distinguish between prepatellar and suprapatellar pathology, as prepatellar bursitis occurs anterior to the patella while suprapatellar pathology is proximal to it 5
• Consider chondrocalcinosis in patients with recurrent painless suprapatellar swelling, particularly when the pouch appears isolated from the main joint cavity 4
• Recognize that complete suprapatellar plica can present as a pseudotumoral mass requiring differentiation from neoplastic processes 3