Neurovascular Structures

For Medical Students

What It Shows

The popliteal artery and vein and the tibial and common peroneal nerves run behind and around the knee, and are within the field of view of every knee study. 2.10.6 covers the common peroneal nerve.

Vessels are identifiable by their course and their flow related signal, and nerves by their course and fascicular pattern. 5.2.18 covers gradient echo imaging.

It matters because these structures are present on every knee study, are rarely the question, and are occasionally the answer. 5.5.1 covers the standard protocol.

How It Is Acquired

Identify the popliteal vessels on the axial images, where their relationship to the capsule is clearest. 5.5.1 covers the standard protocol.

Follow the common peroneal nerve around the fibular neck, which is where it is vulnerable. 2.10.6 covers the common peroneal nerve.

Follow the tibial nerve through the fossa. 5.4.4 covers the posterior knee on ultrasound.

Look at these structures deliberately in a trauma or a post-operative study, where they are most likely to matter. 5.16.5 covers osteotomy planning.

Note that ultrasound resolves a peripheral nerve better, and this study shows its whole course. 2.10.6 covers the common peroneal nerve.

What Normal Looks Like

A popliteal artery and vein in their expected positions with normal calibre.

A common peroneal nerve as a small fascicular structure at the fibular neck. 2.10.6 covers the common peroneal nerve.

A tibial nerve following its course through the fossa without enlargement.

No fluid, mass or displacement around either structure. 5.5.12 covers the bursa.

MRCS Level

How It Is Acquired

In a viva these are the structures to mention when asked what else a knee study covers.

The expected points are the axial vessel identification, the common peroneal nerve at the fibular neck, the tibial nerve, the deliberate look in trauma, and the ultrasound comparison.

Say that a dislocation or a plateau fracture makes a vascular assessment urgent. 5.3.11 covers tibial plateau fractures.

Say that the common peroneal nerve is at risk in a corner injury and in an osteotomy. 5.10.5 covers the common peroneal nerve on magnetic resonance imaging.

Say that an aneurysm or a popliteal mass can present as a knee problem. 5.4.4 covers the posterior knee on ultrasound.

What Normal Looks Like

The reasoning expected is that the popliteal vessels lie close to the posterior capsule, so a posterior capsular injury or a posteriorly displaced fracture puts them at risk. 5.3.11 covers tibial plateau fractures.

The common peroneal nerve wraps the fibular neck superficially, which is why it is the structure injured in a corner injury, a proximal tibial osteotomy and a fibular neck fracture. 2.10.6 covers the common peroneal nerve.

A nerve's fascicular pattern makes it identifiable, and ultrasound resolves that pattern better while this study follows the nerve where a probe cannot. 5.4.3 covers the lateral knee on ultrasound.

These structures being incidental to most knee questions is exactly why they are missed in the minority where they are not. 5.1.6 covers imaging pitfalls.

How Good It Is

Magnetic resonance imaging shows these structures adequately and is rarely asked to.

Its best contribution is the whole course of a nerve and the relationship of the vessels to a fracture or a capsular injury. 5.3.11 covers tibial plateau fractures.

It is limited for fine nerve detail, which ultrasound gives, and for vascular assessment, which a dedicated angiographic study gives. 5.2.21 covers advanced magnetic resonance techniques.

No figures appear for accuracy on this page, under decision 028, and no vessel calibre or nerve diameter values appear either: those are published quantities and this environment cannot check them.

FRCS Level

How Good It Is

At decision level these structures matter in a small number of situations that each matter a great deal.

A knee dislocation or a high energy fracture makes the vascular status the first question, and imaging supports rather than replaces the clinical assessment. 5.3.11 covers tibial plateau fractures.

A proximal tibial osteotomy or a corner reconstruction puts the common peroneal nerve at risk, so its pre-operative state is worth knowing. 5.16.5 covers osteotomy planning.

A post-operative foot drop is a nerve question with a specific imaging answer. 5.10.5 covers the common peroneal nerve on magnetic resonance imaging.

A popliteal mass needs characterising before anything is put into it. 5.4.17 covers aspiration on ultrasound.

Where It Misleads

The commonest error is not looking at these structures at all. 5.5.1 covers the standard protocol.

A vascular question is answered from a knee study rather than a dedicated one. 5.2.21 covers advanced magnetic resonance techniques.

A nerve lesion is missed because the fibular neck was not reviewed. 2.10.6 covers the common peroneal nerve.

A popliteal mass is assumed to be a cyst. 5.4.4 covers the posterior knee on ultrasound.

Normal flow related signal is read as pathology. 5.2.18 covers gradient echo imaging.

Fellowship Level

Where It Misleads

At fellowship level these structures close the chapter on the point it opened with, that the protocol and the request decide what gets looked at.

They are in the field of view of every knee study and outside the question of almost all of them, so they are reviewed by habit rather than by protocol, and habits vary. 5.5.1 covers the standard protocol.

The situations where they matter are few and serious, which is the worst combination for a habit based review: low frequency and high consequence. 5.3.11 covers tibial plateau fractures.

A structured report with a mandatory field for them would close the gap, and no routine knee template has one. 5.20.1 covers structured reporting.

The nerve is better seen on ultrasound and the vessels better on a dedicated vascular study, so the complete answer always involves another study, which makes this one's role to raise the question. 5.1.3 covers choosing the correct investigation.

The chapter's conclusion is that a magnetic resonance study of the knee is a set of choices, and what it does not show is usually a choice that was made rather than a limit that was met. 5.5.1 covers the standard protocol.