Posterior Knee

The same topic at 4 depths

Each tier deepens the one below it and never repeats it. Choose the level you want.

Choose a depth

Student MRCS FRCS Fellow

For Medical Students

What It Shows

The posterior knee holds the popliteal fossa, its vessels, the gastrocnemius and semimembranosus tendons and the bursa between them. 2.9.5 covers the semimembranosus anatomy.

A popliteal cyst arises from that bursa and communicates with the joint through a characteristic neck. 5.5.12 covers the bursa.

It matters because ultrasound characterises a popliteal cyst better and faster than any other modality, and reaches almost nothing else back here. 5.2.7 covers the frequency selection.

How It Is Acquired

Scan with the patient prone and the knee extended, which opens the fossa. 5.2.6 covers sound waves.

Identify the popliteal artery and vein first, because they are the landmark and the structure that must not be mistaken. 5.2.11 covers Doppler imaging.

Find the neck between the semimembranosus and medial gastrocnemius tendons, which is what identifies a popliteal cyst. 5.5.12 covers the bursa.

Use a lower frequency than anteriorly, since the fossa is deeper. 5.2.7 covers the frequency selection.

Apply Doppler to any cystic structure, to exclude an aneurysm. 5.2.11 covers Doppler imaging.

What Normal Looks Like

A patent popliteal artery and vein with normal Doppler flow. 5.2.11 covers Doppler imaging.

Gastrocnemius and semimembranosus tendons with normal fibrillar pattern. 5.5.10 covers the tendons.

A bursa that is not distended and is barely visible. 5.5.12 covers the bursa.

No cystic structure in the fossa.

What Abnormal Looks Like

An anechoic collection with a neck between the semimembranosus and medial gastrocnemius, which is a popliteal cyst. 5.5.12 covers the bursa.

Internal echoes or loose bodies within the cyst. 5.14.6 covers synovial chondromatosis.

A pulsatile structure with Doppler flow, which is an aneurysm rather than a cyst. 5.2.11 covers Doppler imaging.

A solid mass, which takes the lesion out of the cystic category entirely. 5.13.7 covers bone tumours.

Key Learning Points

The fossa holds the vessels, the tendons and the bursa between them.
A popliteal cyst has a characteristic neck between two tendons.
Ultrasound characterises it better and faster than any other modality.
The vessels are the landmark and the structure not to mistake.
Doppler on any cystic structure excludes an aneurysm.

MRCS Level

How It Is Acquired

In a viva this is the region to name when asked how to characterise a popliteal swelling.

The expected points are the prone position, the vessels, the neck, the lower frequency, and the Doppler.

Say that the neck is what makes it a popliteal cyst rather than any cyst. 5.5.12 covers the bursa.

Say that Doppler excludes an aneurysm, which is the lesion not to aspirate. 5.2.11 covers Doppler imaging.

Say that a cyst implies an intra-articular cause that ultrasound cannot find. 5.6.3 covers horizontal meniscal tears.

What Abnormal Looks Like

The reasoning expected is that the bursa communicates with the joint through a valve like neck, so a cyst is joint fluid that has been pushed out and cannot easily return. 5.5.12 covers the bursa.

That makes the cyst a sign of an intra-articular process rather than a primary lesion, and the process is usually a meniscal tear or synovitis the ultrasound cannot see. 5.14.1 covers synovitis.

Treating the cyst without the cause is therefore a recurrence, which is why the imaging question continues past the cyst. 5.1.3 covers choosing the correct investigation.

A solid or vascular lesion in the fossa is a different problem and the Doppler is what separates them at the time of scanning. 5.2.11 covers Doppler imaging.

How Good It Is

Ultrasound characterises a popliteal cyst definitively and finds its cause not at all.

Its best contribution is confirming a cystic structure with a neck and excluding a vascular lesion, in one examination with no contrast. 5.2.11 covers Doppler imaging.

It is limited because the intra-articular cause is out of reach, so a positive study still needs a magnetic resonance study. 5.6.3 covers horizontal meniscal tears.

No figures appear for accuracy on this page, under decision 028, and no cyst size thresholds appear either: those are published quantities and this environment cannot check them.

Key Learning Points

The bursa communicates with the joint through a valve like neck.
A cyst is joint fluid pushed out that cannot easily return.
The cyst is a sign of an intra-articular process, not a primary lesion.
Treating the cyst without the cause invites recurrence.
Doppler separates a cyst from a vascular lesion during the examination.

FRCS Level

How Good It Is

At decision level a posterior ultrasound settles what the swelling is and leaves the reason open.

A cystic structure with a neck and no Doppler flow is a popliteal cyst, and that diagnosis is secure enough to act on. 5.5.12 covers the bursa.

A pulsatile lesion with flow is a vascular emergency rather than a knee problem, and recognising it during the scan is the point. 5.2.11 covers Doppler imaging.

A solid lesion needs characterising, and ultrasound's role is to establish that it is solid. 5.13.7 covers bone tumours.

The intra-articular cause needs a magnetic resonance study, because recurrence after aspiration follows from treating the cyst alone. 5.4.17 covers aspiration.

Where It Misleads

The commonest error is aspirating a cystic structure without Doppler. 5.2.11 covers Doppler imaging.

A cyst is treated as the diagnosis and the intra-articular cause is not sought. 5.6.3 covers horizontal meniscal tears.

A deeper structure is reported from a study optimised for a shallow one. 5.2.7 covers the frequency selection.

The neck is not identified, so a different cystic lesion is called a popliteal cyst. 5.5.12 covers the bursa.

A normal posterior ultrasound is taken to exclude posterior knee pathology. 5.1.6 covers imaging pitfalls.

Key Learning Points

A cystic structure is aspirated without Doppler.
The cyst is treated as the diagnosis.
A deep structure is reported from a shallow study.
Without the neck, another cystic lesion is called a popliteal cyst.
A normal study is taken to exclude posterior pathology.

Fellowship Level

Where It Misleads

At fellowship level the posterior knee is the clearest case in this chapter of ultrasound answering one question completely and the surrounding question not at all.

The cyst is characterised definitively and its cause is intra-articular and invisible, so the study is complete and insufficient at the same time. 5.6.3 covers horizontal meniscal tears.

That combination invites treating the visible thing, and aspiration without addressing the joint has a recurrence rate that makes the imaging pathway part of the outcome. 5.4.17 covers aspiration.

The vascular differential is the one knee ultrasound finding that is urgent, and it depends on the operator applying Doppler to a structure they have already decided is a cyst. 5.2.11 covers Doppler imaging.

Deeper posterior structures, including the posterior cruciate and the posterior capsule, are beyond useful reach, which bounds the region's role permanently. 5.8.1 covers the normal posterior cruciate.

The honest framing is that this is a focused characterisation study, and calling it a posterior knee assessment overstates what it covers. 5.1.3 covers choosing the correct investigation.

Key Learning Points

The cyst is characterised definitively and its cause is invisible.
Treating the visible thing invites recurrence.
The vascular differential is the one urgent knee ultrasound finding.
It depends on applying Doppler to a presumed cyst.
This is a focused characterisation study rather than a regional assessment.

Explore Further

Where this topic came from and where it goes next in the spiral.

References

No reference list is attached to this draft. Evidence verification has not run, because source retrieval was unavailable at generation time. Under the Operations Handbook a missing reference is acceptable and an invented one is a critical failure, so nothing has been cited and no citation marker appears in the text. This page carries no diagnostic accuracy figure, which is decision 028 rather than an omission: the comparative standing of the modalities is standard teaching and a specific sensitivity written from memory is the same class of claim as an invented citation, so ACC-001 refuses the number and the page states the comparison. A reviewer with the literature can add the figures into How Good It Is without restructuring the page. Any measurement here, including a sequence parameter, an angle or a millimetre, is written from standard teaching, has not been checked, and is registered under decision 006 in pipeline/config/figures with a verification state of UNVERIFIED_FROM_MEMORY.

Safety. KneeSchool is educational and does not give individual medical advice. Seek urgent assessment for a knee that cannot bear weight after an injury, a hot swollen knee with a fever, or new numbness, weakness or coldness in the leg or foot.