Knee Ultrasound

Reference: Musculoskeletal Ultrasound: Anatomy and Technique, Springer 2008.

Setup

• High-frequency linear probe.
• Rest the edge of your hand or little finger on the patient to steady the probe.
• Common teaching is scanning in a fixed order (Anterior, medial, lateral, posterior). But for this note, I would go straight to the area of symptoms, as this approach is for MSK clinician rather than for radiologist or sonographer.
• Anterior: patient supine with a rolled towel under the knee for slight flexion. This tightens the extensor mechanism and reduces anisotropy. Medial and lateral: roll the patient partway to each side. Posterior: patient prone.
• Always confirm findings in the second plane.

Scan sequence

1. Anterior (Supine)

Place probe in long axis, above the patella: quadriceps tendon (bright, fibrillar, layers often fuse into one) and the suprapatellar recess (normally collapsed or a thin dark line).

Slide down: patellar tendon and Hoffa’s fat pad.

Trochlear groove. With the knee maximally flexed, place the probe in short axis just distal to the pole of the patella. Trochlear cartilage is a thin dark layer over the femur. Don’t misinterpret the dark layer as fluid.

Possible findings: effusion (within the suprapatellar recess), quadriceps tendon injury, thinning of trochlear groove cartilage, thickened patellar tendon (Jumper’s knee affects proximal more than distal).

2. Medial (Supine, Hip slightly externally rotated)

Start by palpating the medial joint line, then place the probe over the joint line.

Assess medial collateral ligament (MCL) superficial layer and the peripheral of the medial meniscus.

Meniscus: bright triangle between femur and tibia. Normally, it should be within the border of knee joint or minimally extruded.

MCL: bright, compact fibrillar band a few mm thick lying over it. It runs from the adductor tubercle, several cm above the joint, to the tibia up to about 5 cm below the joint (around the level of tibial tuberosity).

Follow the MCL down to the tibia. The pes anserinus tendons (SGT: sartorius, gracilis, semitendinosus) lie as small ovals on top of it. Look here for pes anserine bursitis.

3. Lateral (Supine, Hip slightly internally rotated)

Oblique long axis just lateral to the patellar tendon: iliotibial band running to Gerdy’s tubercle on the tibia. Follow it up over the distal femur, the site of ITB friction syndrome.

Move back onto the femur to find the groove in the lateral condyle. The popliteus tendon sits in it, and the lateral collateral ligament (LCL) attaches just above it.

Keep the top of the probe on the femur and swing the lower end backward until the fibula appears. This lengthens the LCL, a thin uniform fibrillar band.

4. Posterior (Prone)

Baker’s cyst: start in short axis at the distal third of posterior thigh. Look for the septum dividing lateral hamstring (Biceps Femoris) and medial hamstring (Semitendinosis & Semimembranosis). Move to the medial then slide down towards the knee joint, until you can see ‘Egg in a Bowl’ (SemiT is egg, SemiM is bowl). Slide further down until you see SemiM, medial head of gastrocnemius and medial posterior femoral condyle. Baker’s cyst should appear between SemiM and medial head of gastrocnemius.

Pitfalls

• Anisotropy of the medial gastrocnemius tendon can imitate a small Baker’s cyst.
• Pressing too hard hides bursal fluid.

Checklist

• Quadriceps tendon, suprapatellar recess, patellar tendon
• Effusion assessed
• MCL full length, medial meniscus
• Pes anserinus tendons and bursa
• ITB to Gerdy’s tubercle, and over the distal femur
• LCL, popliteus groove
• Baker’s cyst region