Patellar Dislocation Event

For Medical Students

What It Tests

A patellar dislocation is the kneecap moving laterally out of the trochlear groove, usually during a twist on a flexed, loaded knee.

Asking about it tests whether the patellofemoral joint rather than the tibiofemoral joint is the problem, and patients usually describe it well because they can see and feel it.

The event matters more than most, because the first dislocation establishes the risk of recurrence and the opportunity to assess why it happened.

How It Is Performed

Ask whether the kneecap came out of place, and whether they or anybody else saw it to the side.

Ask how it went back: spontaneously on straightening the leg, by the patient pushing it, or by somebody else.

Ask whether it has happened before, and whether the other knee has ever done it.

Ask about immediate swelling, which is usual after a first dislocation.

Ask about family history and about general flexibility, since both bear on why it happened. 4.23.3 covers hypermobile patients.

What a Positive Means

A clear lateral dislocation that reduced on straightening the knee is a patellar dislocation, and the differential is short.

A first dislocation with a haemarthrosis raises an osteochondral fragment from the medial patellar facet or the lateral condyle, which is a reason to image acutely. 4.17.3 covers osteochondral lesions.

Recurrent dislocation points towards an underlying structural cause: trochlear dysplasia, patellar height, tubercle position or generalised laxity. 4.15.8 covers dysplasia.

A dislocation with no trauma at all, or in both knees, points more strongly to structural predisposition than to injury.

Many patients report the kneecap going out when the knee has actually given way at the tibiofemoral joint, so the account needs testing. 4.2.6 covers giving way.

MRCS Level

What a Positive Means

The reasoning expected is that a dislocation requires both a displacing force and a knee that permits displacement, and the history addresses both.

A high energy event in a normal knee is an injury. A trivial event in a knee with dysplasia is a structural problem that happened to be revealed.

The medial patellofemoral ligament is the primary restraint in early flexion and is torn in almost all first dislocations. 4.15.7 covers its assessment.

Trochlear dysplasia is the strongest structural predictor of recurrence, which is why the imaging after a first dislocation is about the shape rather than the soft tissue. 4.15.8 covers it.

A patient who has dislocated both patellae, or who has a family history, has a predisposition and the counselling differs.

How Good It Is

The patellar dislocation history is among the most reliable in the knee, because the event is visible and memorable and the reduction is distinctive.

It performs well for the diagnosis and poorly for the cause, which is why the imaging and examination that follow are about predisposition rather than confirmation.

The number of previous episodes is the most useful single feature for predicting further ones, and it is reliably recalled.

Where it performs badly is in distinguishing a subluxation from a dislocation, since the patient's account cannot separate them and the distinction matters for management.

FRCS Level

How Good It Is

At decision level the history after a first dislocation determines both the imaging and the counselling, and it is the assessment that most changes a young patient's next twenty years.

A first dislocation with a high energy mechanism, no structural predisposition and no fragment is usually managed non-operatively with a good prognosis.

A first dislocation with a trivial mechanism, dysplasia and a family history has a high recurrence risk, and saying so at the first consultation is better than discovering it at the fourth episode.

An osteochondral fragment changes the plan to an acute operation, and it is the one finding that makes a first dislocation surgical. 4.17.3 covers the lesion.

Recurrent dislocation needs the predisposing anatomy characterised before any stabilising procedure, because the operation has to address the reason. 4.15.8 and 4.15.4 cover the assessment.

Where It Misleads

The commonest error is treating a first dislocation as an isolated injury without assessing predisposition, which guarantees surprise at the second episode.

The second is accepting a report of the patella dislocating when the knee gave way, which sends the assessment to the wrong joint.

The third is attributing recurrent anterior pain after a dislocation to instability when it is chondral damage from the event, which is common and has a different prognosis. 4.15.10 covers patellofemoral change.

And in a skeletally immature patient a dislocation event raises a physeal or sleeve injury and the imaging differs. 4.19.4 covers paediatric patellar instability.

Fellowship Level

Where It Misleads

In referred practice the failed stabilisation is the presentation, and the history establishes which of several failures occurred.

Continued dislocation after a soft tissue stabilisation usually means the bony predisposition was not addressed, and the history of the original episodes will often reveal that they were atraumatic.

Pain without instability after stabilisation raises over-constraint, a chondral injury from the original events, or a medial patellofemoral reconstruction that is too tight, and these are separated by when the pain began relative to the operation. 4.20.6 covers the assessment.

A patient who dislocates medially after a lateral release or a tibial tubercle transfer has iatrogenic instability, which is rare, recognisable and almost entirely historical in its diagnosis.

In a hypermobile patient a stabilisation has a different prognosis and the history of other joints is what identifies them. 4.23.3 covers the assessment.

No figure here supports these claims, under decision 023.