Anterior Knee

For Medical Students

What It Shows

The anterior knee holds the quadriceps tendon, the patella, the patellar tendon and the bursae around them, all within reach of a high frequency probe. 5.2.7 covers the frequency selection.

Those structures are superficial and parallel fibred, which is the ideal combination for ultrasound. 5.2.8 covers reflection.

It matters because this is the one knee region where ultrasound resolves detail no magnetic resonance sequence in routine use matches. 5.5.10 covers the tendons.

How It Is Acquired

Scan with the knee slightly flexed, which tensions the extensor mechanism and removes the slack that mimics thickening. 5.4.5 covers the quadriceps tendon.

Use the highest frequency that reaches the target, which at this depth is the highest the probe offers. 5.2.7 covers the frequency selection.

Keep the beam perpendicular to the fibres and follow the curve, because an oblique beam darkens a normal tendon. 5.2.10 covers anisotropy.

Scan in two planes, long axis and short axis, so a focal change is confirmed. 5.4.6 covers the patellar tendon.

Add colour Doppler where an active process is the question. 5.2.11 covers Doppler imaging.

What Normal Looks Like

A bright fibrillar pattern running along the tendon, with fine parallel lines. 5.5.10 covers the tendons.

A smooth patellar cortex with a sharp bright surface. 5.2.8 covers reflection.

Thin anechoic bursae, which are present and barely visible when normal. 5.5.12 covers the bursa.

No colour Doppler signal within the tendon substance. 5.2.11 covers Doppler imaging.

What Abnormal Looks Like

Loss of the fibrillar pattern with a thickened hypoechoic tendon. 5.4.6 covers the patellar tendon.

A discrete anechoic cleft within the tendon, indicating a tear. 5.4.5 covers the quadriceps tendon.

A distended bursa with internal echoes. 5.5.12 covers the bursa.

Colour Doppler signal within the tendon, indicating an active process. 5.2.11 covers Doppler imaging.

MRCS Level

How It Is Acquired

In a viva this is the region to name when asked where ultrasound outperforms magnetic resonance imaging.

The expected points are the slight flexion, the highest frequency, the perpendicular beam, the two planes, and the Doppler.

Say that a dark area must be tested by angling onto it. 5.2.10 covers anisotropy.

Say that resolution here is better than any routine magnetic resonance sequence. 5.5.10 covers the tendons.

Say that the examination is live and the images are a selection from it. 5.2.9 covers refraction.

What Normal Looks Like

The reasoning expected is that resolution falls with depth and rises with frequency, so a structure within a few centimetres of the skin can be imaged at the highest frequency the probe offers. 5.2.7 covers the frequency selection.

A tendon's parallel collagen reflects strongly when struck squarely, which gives the fibrillar pattern and also creates the artefact that imitates a tear. 5.2.8 covers reflection.

Tensioning the tendon straightens those fibres, which is why position changes the appearance of normal tissue. 5.4.5 covers the quadriceps tendon.

Doppler adds activity to an anatomical picture, and in a tendon the vessels of interest are small enough that probe pressure abolishes them. 5.2.11 covers Doppler imaging.

How Good It Is

Ultrasound of the anterior knee is the strongest indication for knee ultrasound and the weakest for everything behind it.

Its best contribution is tendon detail and the dynamic and interventional work that follows from it. 5.4.16 covers the knee injection.

It is limited to what the probe reaches, so the cruciates, the central menisci and most cartilage are unavailable from here. 5.1.3 covers choosing the correct investigation.

No figures appear for accuracy on this page, under decision 028, and no probe frequencies or tendon thickness values appear either: those are equipment and protocol quantities and this environment cannot check them.

FRCS Level

How Good It Is

At decision level an anterior knee question is one of the few knee questions better answered by ultrasound than by magnetic resonance imaging.

A tendon question is answered faster, at higher resolution and with the opportunity to examine dynamically. 5.4.6 covers the patellar tendon.

An intervention is guided here better than by any other modality, because the needle and the target are seen together. 5.4.16 covers the knee injection.

A bursal collection can be characterised and aspirated in one sitting. 5.4.17 covers aspiration.

An intra-articular question is the wrong question for this probe position. 5.1.3 covers choosing the correct investigation.

Where It Misleads

The commonest error is reporting anisotropy as tendon pathology. 5.2.10 covers anisotropy.

An intra-articular structure is reported from a study that could not reach it. 5.1.6 covers imaging pitfalls.

Doppler absence is read as absence of vascularity when probe pressure caused it. 5.2.11 covers Doppler imaging.

A tendon scanned in slack is reported as thickened. 5.4.5 covers the quadriceps tendon.

One plane is accepted as confirmation. 5.4.6 covers the patellar tendon.

Fellowship Level

Where It Misleads

At fellowship level the anterior knee is where ultrasound's strengths and its structural weakness are both at their clearest.

The resolution advantage is real and the record is a handful of still images, so the advantage is largely unavailable to anyone who was not holding the probe. 5.20.1 covers structured reporting.

That makes a negative anterior knee ultrasound a statement about one examination, and the report almost never says whose. 5.2.6 covers sound waves.

Findings here are common in asymptomatic tendons, so the attribution of symptoms to an appearance needs the clinical picture rather than the image. 5.4.6 covers the patellar tendon.

Video capture and a structured template would address both and neither is standard, which locates the problem in practice rather than in the technology. 5.4.15 covers dynamic instability.

The practical conclusion, repeated from the physics pages, is to treat a knee ultrasound as a consultation with the person who performed it. 5.2.9 covers refraction.