What are the key anatomic details to evaluate during knee echography (ultrasound)?

Anatomic Details on Knee Echography

The suprapatellar recess examined with the knee in 30° flexion using longitudinal and transverse scans provides the most sensitive anatomic window for detecting joint effusions and synovial proliferation, which are the most clinically significant findings on knee ultrasound. 1, 2

Standard Scanning Protocol and Anatomic Structures

Anterior Knee Structures

Suprapatellar recess scanning:

• Position the knee in 30° flexion with quadriceps contraction to maximize fluid detection—this identifies 54.2% more effusions than static scanning alone. 2, 3
• Perform both longitudinal and transverse scans in this position, applying parapatellar pressure to displace fluid into the suprapatellar pouch 1
• Visualize the femur, articular cartilage (appears as hypoechoic layer), and any joint cavity fluid 1
• The distal quadriceps tendon consists of 1-5 distinct laminas that should be individually assessed 4

Infrapatellar region:

• Scan for loose joint bodies, which may be present in the infrapatellar pouch 1
• Note that failure to detect a loose body cannot rule out its presence—this is a critical limitation 1

Medial Knee Structures

Medial retinaculum and supporting structures:

• The medial retinaculum has three anatomical layers: fascia, intermediate layer, and capsular layer 4
• At the medial patellofemoral ligament (MPFL) level, 1-3 layers may be visible 4
• The medial collateral ligament (MCL) shows a superficial band plus deeper meniscofemoral and meniscotibial bands 4
• The posterior oblique ligament (POL) is visualized along the posteromedial aspect 4

Semimembranosus tendon insertions:

• Identify the anterior arm, direct arm, and oblique popliteal arm—all can be differentiated with ultrasound 4

Lateral Knee Structures

Lateral supporting structures:

• Assess the iliotibial band and adjacent joint recesses 4
• The fibular collateral ligament is encircled by the anterior arms of the distal biceps tendon 4
• Along the posterolateral corner, visualize the fabellofibular, popliteofibular, and arcuate ligaments 4

Posterior (Popliteal) Structures

Popliteal fossa examination:

• Use posterior transverse scan to identify the pathognomonic "comma-shaped" extension of Baker's cysts between the medial head of gastrocnemius and semimembranosus tendon—this confirms the diagnosis. 1, 5
• Baker's cysts are fluid accumulations in the gastrocnemius or semimembranosus bursae that frequently communicate with the joint space 1, 5
• Assess for cyst rupture, which mimics deep vein thrombosis clinically and is easily identified by ultrasound 1, 5
• Define cyst shape, size, and extension into thigh or calf muscles 1, 5
• Detect loose joint bodies in the popliteal region 1, 5

Key Pathologic Findings to Document

Intra-articular pathology:

• Joint effusion/synovial proliferation (most common finding) 1
• Cartilage thinning or lesions 1
• Loose joint bodies in suprapatellar, infrapatellar, and popliteal regions 1

Bony abnormalities:

• Erosions, osteophytes, irregular bone surfaces 1
• Osteochondromatosis 1

Soft tissue pathology:

• Tendinitis, tenosynovitis, or tendon tears 1, 6
• Calcifications within bursae (appear as hyperechoic foci with posterior acoustic shadowing) 7
• Bursal inflammation (trochanteric, iliopectineal) 1

Technical Optimization

Transducer selection:

• Use high-frequency transducers of 10 MHz or higher to detect even minor synovitic lesions and small effusions that are missed clinically. 1, 7

Dynamic maneuvers:

• Active flexion/extension of the knee during scanning 1
• Quadriceps contraction to displace fluid (superior to parapatellar pressure alone) 3
• Dynamic assessment for vascular pathology when evaluating popliteal artery entrapment syndrome 5

Critical Pitfalls to Avoid

• Never assume absence of loose bodies based on negative ultrasound—ultrasound cannot rule out their presence if not visualized 1, 5
• Failing to scan at 30° flexion misses the optimal position for fluid detection 2
• Not using quadriceps contraction misses 54.2% of occult effusions 3
• Overlooking the comma-shaped extension between gastrocnemius and semimembranosus when evaluating for Baker's cyst 1, 5
• Missing small calcifications visible on ultrasound but not on plain radiographs 7
• Failing to perform dynamic maneuvers when assessing for vascular entrapment syndromes 5
• Confusing tendon calcification with bursal calcification, which require different treatment approaches 7