Work in progress. This site is being built in the open. No page on it has yet been reviewed by a clinician, every page says so, and nothing here is medical advice or a substitute for seeing someone about your knee. What that means
KneeSchool

KneeSchool/Imaging Academy/PLC Structures

PLC Structures

Page 5.4.12 · Ultrasound of the Knee · imaging page

The posterolateral corner, the structure group ultrasound can see parts of and cannot assess.

Pipeline status. Editorial draft. Evidence verification did not run. Not consultant reviewed. Not for clinical use.

The same topic at 4 depths

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

PLC Structures

Choose a depth

For Medical Students

What It Shows

The posterolateral corner is a group of structures including the lateral collateral ligament, the popliteus and the popliteofibular ligament. 2.10.4 covers the posterolateral corner.

Ultrasound reaches the superficial members and not the deeper ones. 5.4.3 covers the lateral knee.

It matters because the corner functions as a unit, so an assessment of some of its parts does not assess it. 3.10.6 covers the posterolateral corner function.

How It Is Acquired

Assess the lateral collateral ligament, which is fully accessible. 5.4.10 covers the LCL.

Assess the biceps femoris and its insertion. 2.10.5 covers the biceps femoris.

Assess the popliteus sheath, accepting that the intra-articular tendon is not reachable. 5.4.8 covers the popliteus tendon.

Assess the common peroneal nerve, which is where ultrasound adds most here. 2.10.6 covers the common peroneal nerve.

Accept that the popliteofibular ligament and the deeper capsular structures are not reliably assessable, and report that. 2.10.3 covers the popliteofibular ligament.

What Normal Looks Like

An intact lateral collateral cord from femur to fibular head. 5.4.10 covers the LCL.

A biceps femoris tendon inserting cleanly. 2.10.5 covers the biceps femoris.

A popliteus sheath without fluid. 5.4.8 covers the popliteus tendon.

A normal fascicular common peroneal nerve at the fibular neck. 2.10.6 covers the common peroneal nerve.

What Abnormal Looks Like

Discontinuity or thickening of the lateral collateral ligament. 5.10.1 covers lateral collateral injuries.

A fibular head avulsion fragment. 5.3.14 covers avulsion injuries.

Fluid in the popliteus sheath. 5.4.8 covers the popliteus tendon.

An enlarged or displaced common peroneal nerve. 5.10.5 covers the common peroneal nerve on magnetic resonance imaging.

Key Learning Points

  • The corner is a group of structures functioning as a unit.
  • Ultrasound reaches the superficial members and not the deeper ones.
  • Assessing some parts does not assess the corner.
  • The common peroneal nerve is where ultrasound adds most.
  • An incomplete assessment should be reported as incomplete.

MRCS Level

How It Is Acquired

In a viva the expected answer lists what can and cannot be reached, which is the answer.

The expected points are the collateral, the biceps, the popliteus sheath, the nerve, and the inaccessible deeper structures.

Say that the corner's function cannot be assessed from this study. 3.10.6 covers the posterolateral corner function.

Say that the dial test is the functional assessment. 3.10.7 covers the dial test biomechanics.

Say that magnetic resonance imaging is the imaging study for the corner. 5.10.3 covers posterolateral corner injuries.

What Abnormal Looks Like

The reasoning expected is that the corner restrains external rotation and varus together, so its function depends on several structures acting in combination rather than on any one of them. 3.10.2 covers external rotation stability.

That is why a single intact structure proves nothing about the unit, and why a partial assessment is not a partial answer. 3.10.6 covers the posterolateral corner function.

The popliteofibular ligament is a key contributor and is among the least accessible, which is the worst combination. 2.10.3 covers the popliteofibular ligament.

The nerve is the one component where ultrasound outperforms the alternative, and it is not part of the restraint at all. 2.10.6 covers the common peroneal nerve.

How Good It Is

Ultrasound is the wrong study for the posterolateral corner and the right study for its nerve.

Its best contribution is the common peroneal nerve and a fibular head avulsion, both of which are specific findings. 5.3.14 covers avulsion injuries.

It is limited absolutely for the corner as a functional unit, because its key contributors are not reachable. 2.10.3 covers the popliteofibular ligament.

No figures appear for accuracy on this page, under decision 028, and the comparison is the claim: the corner is assessed less completely here than by any other modality, which is a comparison rather than a number.

Key Learning Points

  • The corner restrains external rotation and varus together.
  • Function depends on several structures acting in combination.
  • A single intact structure proves nothing about the unit.
  • The popliteofibular ligament is key and among the least accessible.
  • The nerve is the one component where ultrasound wins and is not a restraint.

FRCS Level

How Good It Is

At decision level a corner injury is a magnetic resonance and examination question, and the ultrasound answers a side question.

A suspected nerve lesion accompanying a corner injury is the clearest reason to scan. 2.10.6 covers the common peroneal nerve.

A fibular head avulsion is a direct finding with a surgical consequence. 5.3.14 covers avulsion injuries.

The corner's reconstruction plan depends on which structures are injured, which needs magnetic resonance imaging and the examination together. 5.10.3 covers posterolateral corner injuries.

A normal ultrasound here should never be reported as a normal corner. 5.20.1 covers structured reporting.

Where It Misleads

The commonest error is reporting the corner as intact from an ultrasound. 5.10.3 covers posterolateral corner injuries.

A single intact structure is taken to reassure about the unit. 3.10.6 covers the posterolateral corner function.

The nerve is not examined when a corner injury is suspected. 2.10.6 covers the common peroneal nerve.

Anisotropy in the obliquely running structures is read as injury. 5.2.10 covers anisotropy.

The inaccessible structures are not named as unassessed. 5.20.1 covers structured reporting.

Key Learning Points

  • The corner is reported intact from an ultrasound.
  • A single intact structure is taken to reassure about the unit.
  • The nerve goes unexamined when a corner injury is suspected.
  • Anisotropy in oblique structures is read as injury.
  • The inaccessible structures are not named as unassessed.

Fellowship Level

Where It Misleads

At fellowship level this page is the chapter's clearest statement that ultrasound should be requested for a structure and not for a region.

The corner's components differ in accessibility, and the two that matter most functionally are among the least accessible, so the study's coverage is inversely related to the question's importance. 2.10.3 covers the popliteofibular ligament.

A report naming the corner implies a completeness no ultrasound achieves here, which makes the wording a clinical safety issue rather than a stylistic one. 5.20.1 covers structured reporting.

The nerve finding is genuinely valuable and belongs to a different question from the one that prompted the scan, so it is usually found incidentally or not at all. 2.10.6 covers the common peroneal nerve.

Magnetic resonance imaging assesses the corner directly and performs inconsistently on the deeper structures, which means a negative corner study of either kind carries less weight than a positive one. 5.10.3 covers posterolateral corner injuries.

The practical conclusion is to request a nerve scan or a named ligament scan, and to take the corner question to magnetic resonance imaging and the dial test. 3.10.7 covers the dial test biomechanics.

Key Learning Points

  • Coverage is inversely related to the question's importance here.
  • A report naming the corner implies a completeness never achieved.
  • The nerve finding belongs to a different question and is found incidentally.
  • A negative corner study of either kind carries less weight than a positive one.
  • Request a named structure, and take the corner to magnetic resonance imaging.

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.