Patellar Subluxation

For Medical Students

What It Shows

Patellar subluxation is lateral displacement of the patella relative to the trochlea without a complete dislocation. 2.12.1 covers the trochlea.

It is assessed on axial images by the patella's position in the groove at the imaged flexion angle. 5.5.1 covers the standard protocol.

It matters because subluxation happens near extension and the imaging is acquired with the knee flexed, so the displaced position is usually not the imaged one. 3.12.4 covers the trochlear function.

How It Is Acquired

Assess on axial images through the trochlea at a stated flexion angle. 5.5.1 covers the standard protocol.

Assess the patella's position relative to the groove's floor and facets. 2.12.1 covers the trochlea.

Look for the findings that record a past dislocation rather than the current position, which are more often present. 5.12.5 covers MPFL injury.

Assess the other measurements, since they explain why subluxation occurs. 5.12.1 covers trochlear dysplasia.

Accept that a normal position at the imaged angle excludes very little, and say so. 5.1.6 covers imaging pitfalls.

What Normal Looks Like

A patella seated centrally over the trochlear groove at the imaged angle. 2.12.1 covers the trochlea.

Symmetrical relationships between the patellar facets and the trochlear facets. 5.12.3 covers patellar tilt.

No findings of a previous dislocation. 5.12.5 covers MPFL injury.

Normal supporting measurements. 5.12.1 covers trochlear dysplasia.

What Abnormal Looks Like

A patella displaced laterally relative to the groove at the imaged angle. 2.12.1 covers the trochlea.

The findings of a previous dislocation, which are the commoner evidence: a medial patellofemoral ligament injury and the matching bone bruise pattern. 5.12.5 covers MPFL injury.

An impaction injury of the medial patella and the lateral femoral condyle. 5.11.5 covers osteochondral lesions.

Abnormal supporting measurements that explain it. 5.12.1 covers trochlear dysplasia.

MRCS Level

How It Is Acquired

In a viva the expected answer says why the imaging usually shows a normal position.

The expected points are the stated angle, the position in the groove, the past dislocation findings, the supporting measurements, and the limits of a normal study.

Say that the imaged position is the stable one. 3.12.4 covers the trochlear function.

Say that the bone bruise pattern records the mechanism. 5.11.7 covers bone marrow lesions.

Say that the supporting measurements explain the instability where the position cannot. 5.16.4 covers the tuberosity to groove measurement.

What Abnormal Looks Like

The reasoning expected is that the patella is unconstrained near extension and constrained once engaged, so a study acquired in flexion images the patella in the position it is held in rather than the position it escapes to. 3.12.4 covers the trochlear function.

That is why the durable evidence of instability is the injury pattern rather than the position: a medial patellofemoral ligament disruption with impaction of the medial patella and the lateral condyle records a lateral displacement that has already reduced. 5.12.5 covers MPFL injury.

The supporting measurements then explain why it happened, which is the part that bears on recurrence. 5.12.1 covers trochlear dysplasia.

A positive imaging finding of current subluxation is therefore unusual and informative, while a negative one is expected and uninformative. 5.1.6 covers imaging pitfalls.

How Good It Is

Magnetic resonance imaging documents a past dislocation well and a current subluxation rarely.

Its best contribution is the injury pattern, which is specific and durable. 5.12.5 covers MPFL injury.

It is limited because the position imaged is the stable one, so a normal study is the expected result in a genuinely unstable knee. 3.12.4 covers the trochlear function.

No figures appear for accuracy on this page, under decision 028, and no displacement or congruence values appear either: those are published quantities tied to specific flexion angles and this environment cannot check them.

FRCS Level

How Good It Is

At decision level this page is about what the imaging cannot be asked for.

A first dislocation with the characteristic injury pattern confirms the mechanism, which is useful where the history is unclear. 5.12.5 covers MPFL injury.

Recurrence risk is determined by the supporting measurements, so those are what the imaging contributes to the operative decision. 5.16.4 covers the tuberosity to groove measurement.

A current subluxation seen at the imaged angle is a strong finding precisely because it is unusual. 2.12.1 covers the trochlea.

A normal study in a patient with a clear history changes nothing, and the report should say so rather than reading as reassurance. 5.1.6 covers imaging pitfalls.

Where It Misleads

The commonest error is reading a normal patellar position as excluding instability. 5.1.6 covers imaging pitfalls.

The injury pattern is not sought in a suspected first dislocation. 5.12.5 covers MPFL injury.

The supporting measurements are not made. 5.12.1 covers trochlear dysplasia.

A chronic medial patellofemoral ligament appearance is read as an acute injury. 5.12.5 covers MPFL injury.

The flexion angle is not stated, so the position cannot be interpreted. 5.20.8 covers common reporting errors.

Fellowship Level

Where It Misleads

At fellowship level this is the page where the chapter's central problem is at its plainest.

The clinical question is what the patella does near extension during activity, and every available study images it flexed, still and unloaded, so the question is never actually asked. 5.18.6 covers dynamic magnetic resonance imaging.

What imaging supplies instead is a record of a past event and a set of morphological risk factors, which together support a recurrence estimate without observing the instability. 5.12.1 covers trochlear dysplasia.

Dynamic magnetic resonance imaging and ultrasound each observe part of the movement, neither is routine, and neither is loaded, so the three options fail differently and the examination keeps the question. 5.4.14 covers patellar tracking on ultrasound.

That makes the apprehension test and the observed J sign more informative about the behaviour than any image, which is an uncomfortable thing for an imaging chapter to conclude and is the honest position. 4.15.3 covers the patellar apprehension test.

The useful report documents the injury pattern, gives the measurements with their methods, and states that the position imaged is the stable one. 5.20.8 covers common reporting errors.