Popliteal Fossa Examination

For Medical Students

What It Tests

The popliteal fossa contains the popliteal artery and vein, the tibial and common peroneal nerves, and the space in which a popliteal cyst forms. 2.10.6 covers the common peroneal nerve.

Examining it tests for a mass, for the pulse, and for the distal consequences of anything occupying the space.

It matters out of proportion to its size, because the two urgent diagnoses in this chapter live here: an arterial injury and a pulsatile mass. 4.11.7 covers the first.

How It Is Performed

With the patient supine and the knee flexed, feel for the popliteal pulse deep in the fossa, pressing towards the posterior tibia.

Palpate the fossa for a mass, noting its position, its consistency and whether it is pulsatile.

Stand the patient up with the knee extended and look at the back of the knee, which is when a popliteal cyst is most apparent.

Flex and extend the knee while palpating the mass, since a cyst becomes less tense in flexion and more tense in extension.

Examine the foot: the pulses, the temperature, the sensation and the motor function, because a mass or an arterial problem shows itself distally. 4.6.2 covers temperature.

What a Positive Means

A soft mass in the medial part of the fossa, most apparent standing with the knee extended, is a popliteal cyst, and it is secondary to something inside the joint. 4.8.8 covers chronic effusions.

A mass that does not change with knee position, or that is firm, needs imaging rather than reassurance, because a solid mass behaves differently.

An expansile pulsatile mass is an aneurysm, and that is a vascular referral rather than a knee diagnosis.

An absent or reduced popliteal or foot pulse in an injured knee is an arterial injury until excluded, which is the most urgent conclusion in Section 4. 4.11.7 covers the sequence.

Distal neurological symptoms with a mass in the fossa indicate compression, and the examination has to include the foot to find it. 4.6.8 covers the fibular head.

MRCS Level

How It Is Performed

In a viva the fossa is a place where a candidate can demonstrate safety, and naming the pulse and the distal examination is what does it.

The expected points are the pulse, the palpation for a mass, the standing inspection with the knee extended, the change with flexion, and the distal examination.

Say that a popliteal cyst is secondary and that the question is what is driving it, which is the clinically useful framing. 4.8.8 covers chronic effusions.

Say that you would image a mass that is firm or does not change with position, and say why. 4.1.6 covers that relationship.

Say that you would examine the calf, since a ruptured cyst mimics a deep vein thrombosis and the two have opposite treatments.

What a Positive Means

The reasoning expected is that a popliteal cyst arises from a communication between the joint and the gastrocnemius semimembranosus bursa, so it fills when the joint produces fluid. 4.8.1 covers normal fluid physiology.

That communication behaves like a valve, which is why the cyst empties poorly and why it becomes tense in extension when the capsule's volume is least. 4.8.2 covers the clinical significance.

Because the cyst is secondary, treating it without treating the intra-articular cause produces recurrence, which is the clinical consequence of the anatomy. 4.9.11 covers one common cause.

The artery lies deep in the fossa against the bone, which is why it is at risk when the knee dislocates and why a dislocated knee needs a documented vascular examination. 4.11.7 covers that assessment.

How Good It Is

Examination of the fossa is reliable for a substantial mass and for an absent pulse, and unreliable for a small cyst or a deep one.

The standing extended inspection is the most sensitive part and is the part most often omitted, which is a recurring pattern in this chapter.

It cannot distinguish a cyst from a solid mass with confidence, which imaging does, and the position change is a useful but imperfect discriminator. 4.1.6 covers that relationship.

Its most consequential use is the vascular examination, where a careful assessment takes seconds and a missed one costs a limb. 4.11.7 covers it.

No figures appear for any accuracy comparison on this page, under decision 023, and the comparison is the claim being made.

FRCS Level

How Good It Is

At decision level this examination contributes one urgent decision and one common mistake avoided.

The urgent decision is vascular. An abnormal pulse in a knee with a multiligament injury changes the whole pathway and has a time limit measured in hours. 4.11.7 covers it.

The mistake avoided is excising a popliteal cyst without addressing the intra-articular cause, which recurs, and the examination plus the intra-articular assessment is what prevents it. 4.9.11 covers a common cause.

A firm or non compressible mass changes the pathway to imaging and possibly to a tumour service, and the examination is where that is initiated.

In a knee being planned for posterior surgery the artery's position is the anatomical constraint, and the examination and the operation share the landmark. 2.10.7 covers surgical anatomy.

Where It Misleads

The commonest error is not examining the fossa at all.

The second is examining it supine only, where a cyst may be missed.

The third is treating a cyst as the diagnosis rather than as a consequence.

The fourth is not examining the foot, which is where a mass or an arterial problem declares itself.

The fifth is attributing calf pain and swelling to a ruptured cyst without considering a deep vein thrombosis, since the two present alike and are treated oppositely.

Fellowship Level

Where It Misleads

In referred practice the fossa produces two recurring referrals: the recurrent cyst and the mass that was assumed to be one.

A patient with a recurrent cyst after excision has an unaddressed intra-articular cause, usually a meniscal tear or compartment change, and the recurrence was predictable. 4.9.11 covers meniscal examination in osteoarthritis.

A patient with a firm mass called a cyst for months has had a diagnosis rather than an examination, and the position change test would have raised the question at the first visit.

A patient with a foot drop and a mass at the fibular neck has a compressive ganglion, often from the proximal tibiofibular joint, and it presents as a knee complaint. 4.6.8 covers the fibular head.

In a knee with previous posterior surgery the anatomy is altered and a mass may be a pseudoaneurysm, which is an urgent and specific diagnosis rather than a cyst. 4.2.30 covers previous surgery.

In medicolegal work an undocumented vascular examination in a knee that was dislocated is among the most serious omissions a record can contain, and this page is where it is prompted. 4.23.7 covers that setting.