Patellar Instability

For Medical Students

The Principle

Something stays in place because of its shape, because something is holding it, and because it is being pushed in the right direction.

If any one of those fails, it can come out. If more than one fails, it comes out easily.

That is why some people dislocate a joint from a bad tackle and others from turning over in bed.

At the Knee

The patella almost always dislocates outwards, and almost always near full extension. 3.12.1 covers why.

The groove is what holds it once the knee is bent. 3.12.4 covers the groove.

The medial patellofemoral ligament is what holds it before the groove takes over. 3.12.5 covers the ligament.

A patella that sits high enters the groove later, so it spends longer relying on the ligament alone. 3.12.12 covers height.

The direction of the quadriceps pull decides how hard the patella is being pushed outwards in the first place. 3.12.6 covers the Q angle.

What Changes It

The shape of the groove, which is set during growth.

Whether the ligament has been torn before. 6.59 covers the injury.

How high the patella sits.

Alignment and rotation of the leg. 3.12.11 covers them.

Clinical Relevance

A first dislocation in a normally shaped knee usually needs a real injury. 6.53 covers it.

A first dislocation in a shallow groove may need almost nothing, and is more likely to happen again. 6.54 covers recurrence.

This is why two patients with the same injury get different advice.

The assessment is mostly imaging, because three of the four factors are bone shape or position. 5.12 covers it.

MRCS Level

The Principle

Instability is best analysed as a set of independent contributors rather than as a single diagnosis, because the contributors need different answers.

Three of the four are skeletal and one is soft tissue, which is the reason a soft tissue operation does not suit every patient.

The contributors add. A knee with two mild abnormalities can be less stable than a knee with one severe one.

At the Knee

The four factors are trochlear dysplasia, patellar alta, increased tubercle to groove distance and medial soft tissue incompetence.

Rotational malalignment is a fifth in some classifications and acts through the tubercle to groove distance. 3.12.11 covers it.

Trochlear dysplasia is present in a large majority of recurrent dislocators, which is why it is listed first. 3.12.4 covers it.

Patellar alta acts by delaying engagement, lengthening the arc in which only the ligament is working. 3.12.12 covers it.

Medial soft tissue incompetence is usually acquired, from a previous dislocation, rather than congenital. 3.12.5 covers the ligament.

What Changes It

The number of factors present, which matters more than the severity of any one.

Flexion angle, since the vulnerable window sits near extension.

Generalised ligamentous laxity, which lowers the threshold across the board.

Skeletal maturity, because a growing knee changes and a mature one does not. 2.4.2 covers ossification.

Activity and demand, which decide how often the vulnerable arc is loaded under load.

How It Is Measured

Trochlear morphology on axial imaging and a true lateral radiograph. 5.12.1 covers it.

Patellar height by one of several ratios, which do not agree with each other. 5.12.2 covers them.

Tubercle to groove distance on computed tomography or magnetic resonance imaging, which varies with knee position at acquisition. 5.12.4 covers it.

Medial restraint by clinical glide and apprehension testing, and by imaging the ligament. 4.15.4 covers the tests.

Each index has a soft threshold, so a patient is usually described by a pattern of factors rather than by one abnormal number.

Clinical Relevance

Assess all four factors in every patient, because the pattern decides the operation. 6.54 covers the condition.

Expect a soft tissue reconstruction alone to work best where the skeletal factors are normal. 7.51 covers it.

Grade recurrence risk after a first dislocation by the factors present rather than by the severity of the event. 6.53 covers it.

State which indices are soft, because quoting a threshold as a cutoff overstates it.

FRCS Level

At the Knee

The mechanical account that holds up is a competition between a lateral displacing vector and the restraint available at that flexion angle.

The vector comes from tubercle position, femoral and tibial rotation and the dynamic pattern. The restraint comes from the groove and the medial complex. 3.12.7 covers the dynamic part.

Instability occurs where the vector exceeds the restraint, which is a narrow arc near extension in almost every knee. 3.12.1 covers the arc.

That framing explains why indices predict in groups and not individuals: each index measures one term in an inequality whose other terms are unmeasured.

It also explains why addressing two mild factors often works better than addressing one severe factor, which matches reported outcomes.

What Changes It

Which side of the inequality the dominant abnormality sits on, since vector problems and restraint problems need different operations.

Patellar height, which converts a restraint problem into a longer exposure. 3.12.12 covers it.

Rotational profile, which is under recognised and changes the vector without changing any axial index taken at one angle. 3.12.11 covers it.

Skeletal maturity, which decides whether a bony procedure is available at all. 7.94 covers paediatric practice.

Previous surgery, which may have moved the vector or the restraint, usually without documentation of which.

How It Is Measured

Each factor has its index and each index has its own acquisition dependence, so a single study at a single knee position underdetermines the problem. 5.12 covers the imaging.

Patellar height ratios disagree systematically, so the ratio must be named with the number. 5.12.2 covers them.

Tubercle to groove distance falls as the knee flexes during acquisition, which makes the commonly quoted threshold position dependent.

Robotic and cadaveric testing measure the inequality directly and cannot be applied to a patient. 3.16.2 covers them.

No clinical measurement expresses the vector and the restraint in the same units, which is why the assessment is a pattern rather than a calculation.

Clinical Relevance

Decide whether the dominant problem is vector or restraint, because that choice selects the operation. 7.53 and 7.51 cover the options.

Name the patellar height ratio alongside its value, since the ratios disagree.

Record the knee position at which the tubercle to groove distance was acquired.

Address two mild factors in preference to overcorrecting one, which matches both the mechanics and the reported outcomes.

Take the patient's age, physiology, comorbidities, knee demand, psychological state and their own ideas, concerns and expectations into the decision, because the indices describe a knee and the operation happens to a person.