KneeSchool/Imaging Academy/PLC Injuries
PLC Injuries
Page 5.10.3 · Lateral Knee MRI · imaging page
A pattern rather than a structure, so the report has to name which components are involved and the examination owns the instability.
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PLC Injuries
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What It Shows
A posterolateral corner injury involves several structures together: the collateral ligament, the popliteus, the popliteofibular ligament and the capsule. 2.10.4 covers the posterolateral corner.
Magnetic resonance imaging shows each of them, the smaller ones inconsistently. 5.5.9 covers the ligaments.
It matters because the injury is defined by which components are involved rather than by any single finding. 3.10.6 covers the posterolateral corner function.
How It Is Acquired
Use a protocol with thin coronal and axial images through the fibular head. 5.5.2 covers the sports knee protocol.
Assess each named structure separately and report each as intact, injured or not assessable. 5.20.1 covers structured reporting.
Assess the fibular head and styloid for avulsion. 2.3.2 covers the attachments.
Assess the cruciates, since a combined injury changes everything. 3.10.9 covers the combined PCL and PLC injury.
Assess the common peroneal nerve. 5.10.5 covers the common peroneal nerve.
What Normal Looks Like
Each lateral structure identifiable and continuous. 2.10.4 covers the posterolateral corner.
An intact fibular head and styloid. 2.3.2 covers the attachments.
No marrow change at the lateral attachments. 5.13.1 covers bone marrow oedema.
A normal common peroneal nerve. 2.10.6 covers the common peroneal nerve.
What Abnormal Looks Like
Injury of two or more lateral structures, which is the pattern. 2.10.4 covers the posterolateral corner.
A fibular styloid avulsion, which implies the deeper structures. 5.10.4 covers the popliteofibular ligament.
Oedema through the lateral soft tissues with the structures hard to identify individually. 5.5.9 covers the ligaments.
A cruciate injury alongside, usually the posterior one. 5.8.3 covers the complete PCL tears.
Nerve change, which alters the consent. 5.10.5 covers the common peroneal nerve.
Key Learning Points
- The injury is defined by which components are involved.
- Each structure is reported separately.
- A styloid avulsion implies the deeper structures.
- A combined cruciate injury is the usual pattern.
- The nerve belongs in the assessment.
MRCS Level
How It Is Acquired
In a viva the expected answer lists the components.
The expected points are the thin images through the fibular head, each structure named, the fibular avulsion, the cruciates, and the nerve.
Say that the grading is clinical and the imaging is structural. 4.13.4 covers the posterolateral corner assessment.
Say that the dial test and the recurvatum test report the instability. 4.13.5 covers the dial test.
Say that a missed corner injury is a recognised reason a cruciate reconstruction fails. 3.10.8 covers the PLC reconstruction biomechanics.
What Abnormal Looks Like
The reasoning expected is that these structures act together to resist varus and external rotation, so the functional deficit depends on the combination rather than on any one component. 3.10.6 covers the posterolateral corner function.
That is why the report lists components and why the examination grades the instability. 4.13.4 covers the posterolateral corner assessment.
Acute oedema obscures the individual structures, so an early scan can establish the pattern without resolving the parts. 5.5.9 covers the ligaments.
A fibular styloid avulsion is the single most useful indirect sign, because it implies attachments that cannot be seen directly. 5.10.4 covers the popliteofibular ligament.
How Good It Is
Magnetic resonance imaging is the best available structural assessment of this corner and does not grade the instability.
Its best contribution is the combination of components and the fibular avulsion. 2.10.4 covers the posterolateral corner.
It is limited because the small structures are inconsistently seen, because acute oedema obscures them, and because the deficit is rotational and clinical. 4.13.5 covers the dial test.
No figures appear for accuracy on this page, under decision 028, and no rotation values or grading thresholds appear either: those are clinical grades with published definitions and this environment cannot check them.
Key Learning Points
- The deficit depends on the combination of components.
- The report lists components and the examination grades.
- Acute oedema establishes the pattern and hides the parts.
- A styloid avulsion is the most useful indirect sign.
- The deficit is rotational and clinical.
FRCS Level
How Good It Is
At decision level the imaging decides whether a cruciate reconstruction needs the corner addressed at the same time. 3.10.8 covers the PLC reconstruction biomechanics.
A corner injury left untreated is a recognised cause of graft failure. 5.7.11 covers the graft failure.
A combined posterior cruciate and corner injury is a different operation again. 3.10.9 covers the combined PCL and PLC injury.
Alignment belongs in the chronic case, because varus loads the reconstruction. 5.17.4 covers the osteotomy.
The nerve changes the consent and the rehabilitation. 5.10.5 covers the common peroneal nerve.
Where It Misleads
The commonest error is reporting the lateral collateral ligament alone. 2.10.1 covers the LCL.
Absence of a structure from the report is read as that structure being intact. 5.20.8 covers common reporting errors.
A chronic injury with no oedema is read as normal. 5.5.9 covers the ligaments.
The fibular styloid avulsion is reported as an isolated fracture. 2.3.2 covers the attachments.
Rotational instability is inferred from the images. 4.13.5 covers the dial test.
Key Learning Points
- Reporting the collateral ligament alone is the error.
- Absence from the report is not evidence of integrity.
- A chronic injury with no oedema reads as normal.
- A styloid avulsion is reported as an isolated fracture.
- Rotational instability cannot be inferred.
Fellowship Level
Where It Misleads
At fellowship level the corner injury was missed at the index operation and the graft has failed. 5.7.11 covers the graft failure.
The original scan usually shows the injury that was not reported, which makes obtaining it more useful than repeating it. 5.20.1 covers structured reporting.
Chronic injury has no oedema and may have no identifiable structures, so the diagnosis becomes clinical and the imaging supports the plan. 4.13.4 covers the posterolateral corner assessment.
Varus alignment in a chronic lateral injury is part of the problem and part of the operation. 5.3.6 covers the long leg alignment films.
A rotational profile adds to the picture in a knee whose instability does not fit its structures. 5.16.3 covers the rotational profiles.
The useful report lists each component with its state, the fibular head, the cruciates, the nerve and the alignment. 5.20.1 covers structured reporting.
Key Learning Points
- The original scan usually shows what was not reported.
- A chronic injury may have no identifiable structures.
- Varus alignment is part of the problem and the operation.
- A rotational profile adds to a mismatched picture.
- The report lists components with their state.
Explore Further
Where this topic came from and where it goes next in the spiral.
- 2.10.4Posterolateral Corner
- 3.10.6Posterolateral Corner Function
- 4.13.4Posterolateral Corner Assessment
- 5.10.4Popliteofibular Ligament
- 5.10.5Common Peroneal Nerve
- 3.10.9Combined PCL and PLC Injury
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.
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