MPFL Injury

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Student MRCS FRCS Fellow

For Medical Students

What It Shows

The medial patellofemoral ligament is the medial soft tissue restraint running from the medial femur to the medial border of the patella, and it fails when the patella dislocates laterally. 2.12.2 covers the MPFL.

It appears on axial fluid sensitive magnetic resonance images as discontinuity, thickening or fluid along its course. 5.5.9 covers the ligaments.

It matters because the tear is a record of the event, and the event is often what the history cannot establish. 3.12.5 covers the MPFL function.

How It Is Acquired

Use axial fluid sensitive sequences through the medial patella and the medial femoral attachment. 5.5.1 covers the standard protocol.

Follow the ligament across consecutive slices rather than judging it on one. 5.5.9 covers the ligaments.

Look at both attachments and the mid substance, because the site of failure differs between patients. 2.12.2 covers the MPFL.

Look for the accompanying findings rather than the ligament alone, since the bone bruising and the effusion often carry more weight. 5.11.7 covers bone marrow lesions.

Record how long ago the dislocation was, because the findings fade. 5.13.1 covers bone marrow oedema.

What Normal Looks Like

A thin continuous band running from the medial femur to the medial patellar border. 2.12.2 covers the MPFL.

No fluid along its course and no retraction of either end. 5.5.9 covers the ligaments.

An intact medial retinacular layer around it. 2.12.4 covers the retinacula.

No bone bruising at the medial patella or the lateral femoral condyle. 5.11.7 covers bone marrow lesions.

What Abnormal Looks Like

Discontinuity, a wavy contour or fluid signal along the ligament. 5.5.9 covers the ligaments.

Avulsion at the medial femoral attachment, which is the commonest adult pattern. 2.12.2 covers the MPFL.

Injury at the patellar attachment, sometimes with a small bony fragment. 5.3.12 covers patellar fractures.

Bone bruising of the medial patellar facet and the lateral femoral condyle, which is the impaction pair a lateral dislocation leaves. 5.11.7 covers bone marrow lesions.

An osteochondral fragment lying in the joint, which changes what happens next. 5.11.5 covers osteochondral lesions.

Key Learning Points

The ligament fails when the patella dislocates laterally.
The tear records the event rather than the tendency.
Both attachments and the mid substance must be looked at.
The impaction pair is the medial patella and the lateral femoral condyle.
A loose osteochondral fragment changes what happens next.

MRCS Level

How It Is Acquired

In a viva the expected answer separates the injury from the instability.

The expected points are the axial fluid sensitive sequence, the two attachment sites, the impaction pair, the effusion, and the search for a fragment.

Say that the scan after a first dislocation is requested mainly to find a fragment. 5.11.5 covers osteochondral lesions.

Say that the site of failure is reported because it is what a repair would address. 5.17.6 covers imaging after MPFL reconstruction.

Say that a healed ligament does not mean a stable patella. 3.12.8 covers patellar instability.

What Abnormal Looks Like

The reasoning expected is that the ligament fails at the weakest point for that patient, which is the femoral attachment in most adults and the patellar side more often in the child. 5.15.6 covers paediatric patellar instability.

A chronic injury looks different from an acute one, appearing thickened, lax or absent rather than oedematous. 5.5.9 covers the ligaments.

The bone bruising resolves over weeks, so a late scan has lost the clearest sign that a dislocation happened. 5.13.1 covers bone marrow oedema.

A further dislocation adds findings without separating them from the first, which is why the interval matters more than the count. 3.12.8 covers patellar instability.

How Good It Is

Magnetic resonance imaging is the best available test of the medial soft tissues after a dislocation and a poor test of the tendency to dislocate.

Its best contribution is the fragment and the injury pattern in the first weeks. 5.11.5 covers osteochondral lesions.

It is limited by the fading of the findings, by variation in where the failure is called, and by healing that restores the appearance without restoring the function. 3.12.5 covers the MPFL function.

No figures appear for accuracy on this page, under decision 028, and no ligament thickness or displacement values appear either: those are published quantities measured differently between studies and this environment cannot check them.

Key Learning Points

The ligament fails at the weakest point for that patient.
The adult fails at the femoral attachment more often than the child.
A chronic injury is thickened, lax or absent rather than oedematous.
The bone bruising resolves and takes the clearest sign with it.
Healing restores the appearance before it restores the function.

FRCS Level

How Good It Is

At decision level the scan contributes a fragment, an injury pattern and the bony measurements, and adds nothing to the recurrence argument that the measurements do not already carry.

A fragment is an operative finding and is the one result that changes the plan in the first week. 5.11.5 covers osteochondral lesions.

The bony risk factors rather than the ligament carry the recurrence argument. 5.12.1 covers trochlear dysplasia.

The site of failure is worth reporting because it is where a repair or a reconstruction attaches. 5.17.6 covers imaging after MPFL reconstruction.

The whole measurement set should be read off the same study rather than requested separately. 5.16.4 covers the TT-TG measurement.

Where It Misleads

The commonest error is treating the ligament injury as the explanation for recurrent instability when the groove is the explanation. 5.12.1 covers trochlear dysplasia.

A normal looking ligament late after a dislocation is read as evidence that no dislocation occurred. 5.13.1 covers bone marrow oedema.

Medial soft tissue oedema from any cause is read as this injury. 5.14.1 covers synovitis.

A retinacular injury is reported as a ligament injury although the layers differ. 2.12.4 covers the retinacula.

An incidental finding in a knee scanned for pain is read as an instability diagnosis. 3.12.9 covers patellofemoral pain.

Key Learning Points

The groove rather than the ligament carries the recurrence argument.
A fragment is the finding that changes the first week.
A late normal ligament does not exclude a dislocation.
Medial oedema from any cause is read as this injury.
The layers of the medial side are reported as one structure.

Fellowship Level

Where It Misleads

At fellowship level the scan arrives already reported, and the weakest part of the report is the part the referrer relies on.

The site of failure is stated with a confidence the images rarely support, and the operation is then planned around it. 5.17.6 covers imaging after MPFL reconstruction.

A reconstruction is considered for a patient whose ligament was never shown to be the failing structure. 3.12.5 covers the MPFL function.

In the skeletally immature knee the femoral attachment sits near the growth plate, which changes the significance of the same appearance. 5.15.2 covers the growth plates.

Serial scans across episodes are compared although each was acquired at a different interval from its dislocation, so the comparison measures timing rather than damage. 5.13.1 covers bone marrow oedema.

The useful report states the site of failure with its confidence, the fragment, the impaction pattern, and the interval since the episode. 5.20.1 covers structured reporting.

Key Learning Points

The site of failure is reported more confidently than the images support.
A reconstruction can be planned for a structure never shown to be failing.
The immature femoral attachment sits near the growth plate.
Serial scans across episodes compare timing rather than damage.
The report should state the interval since the episode.

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.

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