Knee Ultrasound: Baker's Cyst, Tendons, Effusion, and Ligaments

Knee ultrasound covers four anatomic compartments and is excellent for soft tissue pathology — tendon tears, bursitis, effusion, cysts, and ligament injuries. The key is moving through the four quadrants systematically rather than chasing symptoms.

Why Ultrasound for the Knee?

• Dynamic assessment — test tendons under load, watch patellar tracking
• Sensitive for joint effusion (more sensitive than physical exam)
• Excellent for popliteal (Baker's) cysts
• Ligament evaluation (MCL/LCL visible; ACL/PCL requires MRI)
• Guides aspiration and injection in real time

What ultrasound cannot reliably show: Menisci (only the peripheral rim is accessible), ACL/PCL, intra-articular chondral defects — MRI required for these.

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Patient Position and Setup

Probe: 5–15 MHz linear transducer.

The four-compartment approach:

1. Anterior: quad tendon, patella, patellar tendon, pre-patellar bursa
2. Medial: MCL, medial collateral ligaments, medial meniscus peripheral rim
3. Lateral: LCL, iliotibial band, lateral meniscus peripheral rim
4. Posterior: Baker's cyst, popliteal vessels, semimembranosus and biceps femoris tendons

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Anterior Compartment

Patient Position

Knee flexed 30–45° over a pillow. More flexion puts the patellar tendon under tension, improving visualization of fibers.

Quadriceps Tendon

Probe: Longitudinal (long axis), just superior to the patella.

Normal: Hyperechoic fibrillar tendon with three layers (rectus femoris, vastus intermedius, and combined layer). Thickness at the proximal patella ≈ 5–8 mm.

Pathology:

• Tendinosis: Focal hypoechoic areas, tendon thickening, loss of fibrillar pattern — insertional (at patella) or proximal
• Tear: Discontinuity of fibers; complete rupture shows a gap with fluid
• Enthesopathy: Irregular cortex at the patella insertion, calcium deposits

Patella

Inspect the patellar cortex for irregularity (fracture, chondrocalcinosis). Measure any osteophytes.

Patellar Tendon (Ligament)

Position: Same — flexed knee.

Probe: Long axis from inferior pole of patella to tibial tuberosity.

Normal: Hyperechoic fibrillar band, 3–5 mm thick, courses from apex of patella to tibial tuberosity.

Pathology:

• Jumper's knee (patellar tendinopathy): Hypoechoic thickening at the proximal patellar tendon (inferior patella insertion). Most common finding in athletes.
• Tear: Focal hypoechoic defect or complete discontinuity
• Patellar bursitis (infrapatellar bursa): Fluid-filled sac at the deep or superficial infrapatellar position

Pre-Patellar Bursa

Position: Over the anterior patella.

Normal: Not visible (no fluid).

Bursitis (Housemaid's knee): Anechoic fluid collection anterior to the patella. Measure thickness, note if complex (infected bursitis shows internal echoes/septations).

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Medial Compartment

Patient Position

Leg in external rotation, hip abducted ("frog-leg" position or knee slightly externally rotated).

Medial Collateral Ligament (MCL)

Probe: Longitudinal over the medial joint line.

Normal: Echogenic fibrillar band — two layers (superficial, deep) with a potential space between them (MCL bursa).

Valgus stress test: Apply gentle valgus force while scanning the MCL. Look for abnormal joint gapping (> 1–2 mm compared to contralateral side).

Pathology:

• Grade I (sprain): Periligamentous edema, thickening, normal fiber continuity
• Grade II (partial tear): Hypoechoic area within the ligament; fibers partially disrupted
• Grade III (complete tear): Full discontinuity, marked periligamentous edema, joint gapping on stress

Medial Meniscus (Peripheral Rim)

The body and posterior horn of the medial meniscus are accessible at the medial joint line.

Normal: Triangular echogenic structure at the joint line. Posterior horn is larger than anterior.

Meniscal cyst: Anechoic or complex cystic structure at the joint line, often adjacent to a meniscal tear. If present, assess the meniscus carefully.

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Lateral Compartment

Patient Position

Leg in internal rotation.

Lateral Collateral Ligament (LCL)

Normal: Cord-like hyperechoic structure running from lateral femoral condyle to fibular head. Less accessible than MCL due to overlying iliotibial band.

Probe: Longitudinal, lateral joint line — adjust probe placement to separate IT band (superficial) from LCL (deep).

Iliotibial Band (IT Band)

Thick fibrous band lateral thigh/knee — can become painful (IT band syndrome in runners). Scan in long and short axis.

IT band syndrome: Periligamentous fluid between the IT band and lateral femoral condyle; tendon thickening; tenderness on probe pressure.

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Posterior Compartment

Patient Position

Patient prone (lying face down) with knee extended or slightly flexed. This is the key position for posterior knee evaluation.

Baker's Cyst (Popliteal Cyst)

The most common posterior knee mass. A Baker's cyst is a distended semimembranosus-gastrocnemius bursa located in the medial popliteal fossa between the medial head of gastrocnemius and the semimembranosus.

Sonographic appearance:

• Anechoic or hypoechoic fluid-filled structure in the medial popliteal fossa
• Has a characteristic neck connecting it to the joint (the "stalk" extending between the medial gastrocnemius and semimembranosus)
• If the cyst ruptures: fluid tracks down the calf between the gastrocnemius and soleus ("dissecting" Baker's cyst) — mimics DVT clinically

Measure: Transverse × AP × craniocaudal dimensions. Note contents (simple vs complex — debris, rice bodies in RA, loose bodies).

Tip: Neck identification confirms it's a popliteal cyst and not a ganglion or other mass.

Complex cyst: Internal echoes, septations, or solid components → consider ganglion, synovial cyst with synovitis, or other pathology.

Popliteal Vessels

While in the posterior position, identify the popliteal artery (deep, pulsatile) and popliteal vein (superficial, compressible). Assess for DVT (compress the popliteal vein) and popliteal artery entrapment if clinically suspected.

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Joint Effusion

Small joint effusions are best visualized in the suprapatellar recess.

Probe: Longitudinal, suprapatellar, above the patella on the anterior thigh.

Normal: Minimal anechoic fluid in the recess.

Effusion: Fluid extends and lifts the prefemoral fat pad. Measure the AP dimension of the suprapatellar fluid collection.

Simple vs complex:

• Simple (anechoic) → mechanical or early inflammatory
• Complex (echoes, septations) → hemarthrosis, septic arthritis, crystalline disease (gout — look for hyperechoic deposits on cartilage)

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Key Images to Capture

1. Quadriceps tendon — long axis and short axis
2. Patellar tendon — long axis (proximal, mid, distal)
3. Pre-patellar bursa region
4. Suprapatellar recess — effusion evaluation
5. MCL — long axis ± stress
6. Medial joint line — meniscal rim
7. LCL — long axis
8. Posterior fossa — Baker's cyst (two planes, measure)
9. Popliteal vessels — DVT exclusion
10. Any focal pathology — two planes with measurements

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Reporting Language

Baker's cyst: "3.8 × 2.1 × 4.4 cm anechoic popliteal cyst in the medial popliteal fossa consistent with a Baker's cyst. A communicating neck is identified between the medial head of gastrocnemius and semimembranosus. No internal debris or septations. No evidence of rupture."

Patellar tendinopathy: "Proximal patellar tendon hypoechoic thickening measuring 8 mm at the inferior pole of the patella (normal ≤ 5 mm) with loss of normal fibrillar echogenicity. No complete tear. Findings consistent with patellar tendinopathy."

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SonoBuddy's MSK guides include shoulder, knee, and ankle ultrasound protocols.