Patellar Instability

For Medical Students

What It Tests

Patellar instability in a young patient presents as the patella giving way laterally, often with a story of the knee collapsing and something moving at the front. 4.2.22 covers the dislocation event.

The examination is deciding whether the patella is unstable, how far it translates, and which anatomical features are allowing it. 4.15.1 covers patellar tracking.

It differs from the adult version because the underlying shape of the trochlea and the alignment are more often the cause than any single injury. 4.15.8 covers trochlear dysplasia.

How It Is Performed

Take the history: the age at first episode, how many episodes, what position the knee was in, and whether the patella had to be put back. 4.2.20 covers a direct blow.

Ask about family history and about generalised laxity, since both are common in this group. 4.19.6 covers developmental variants.

Assess generalised laxity with the standard joint assessment, because it changes the interpretation of the knee findings. 4.23.3 covers the hypermobile patient.

Look at the standing alignment and the rotational profile, since valgus and internal femoral rotation both push the patella laterally. 4.19.7 covers torsional assessment.

Watch the patella as the knee extends, looking for the lateral deviation near full extension. 4.15.1 covers tracking.

Assess patellar translation in extension and at thirty degrees, comparing with the other side. 4.15.4 covers patellar glide.

Assess apprehension on lateral translation, which is the finding that distinguishes instability from laxity. 4.15.3 covers the apprehension test.

What a Positive Means

Increased lateral translation with apprehension, in a knee with valgus alignment or an abnormal rotational profile, is the characteristic picture.

Translation without apprehension in a generally lax child is laxity rather than instability, and the distinction decides whether anything needs doing. 4.23.3 covers the hypermobile patient.

Lateral deviation of the patella near extension indicates maltracking, which predisposes to the dislocation rather than resulting from it. 4.15.1 covers tracking.

A high riding patella or a shallow groove inferred from the examination raises dysplasia, which changes the treatment. 4.15.8 covers trochlear dysplasia.

An episode requiring reduction is a dislocation rather than a subluxation, and the history is the only source of that distinction. 4.2.22 covers the dislocation event.

MRCS Level

How It Is Performed

In a viva this is the adolescent patellar instability station and the expected answer covers the anatomy before the test.

The expected points are the age and episode history, the family history, the generalised laxity assessment, the standing alignment, the rotational profile, the tracking, the translation at two angles, and the apprehension test.

Say that you would assess generalised laxity formally, since it is part of the decision. 4.23.3 covers the hypermobile patient.

Say that you would assess the rotational profile, because femoral internal rotation and tibial external rotation both contribute. 4.19.7 covers torsional assessment.

Say that you would examine the other knee, since the predisposing features are usually bilateral. 4.1.4 covers the structured approach.

What a Positive Means

The reasoning expected is that patellar stability comes from the trochlear shape, the alignment, the soft tissue restraints and the quadriceps vector, and instability follows a deficiency in any of those. 3.12.8 covers patellar instability.

Near full extension the patella sits above the groove and is restrained by soft tissue alone, which is why the instability and the apprehension appear there. 3.12.4 covers trochlear function.

A valgus vector and internal femoral rotation increase the lateral force on the patella, which is why the rotational profile is examined. 4.19.7 covers torsional assessment.

Generalised laxity reduces the soft tissue contribution, so a given trochlear shape becomes unstable in a lax patient and stable in a stiff one. 4.23.3 covers the hypermobile patient.

How Good It Is

The examination establishes instability reasonably well, by combining translation with apprehension.

Its best contribution is the apprehension test, because it is the finding that separates instability from the common physiological laxity. 4.15.3 covers the apprehension test.

It cannot assess the trochlear shape, the patellar height or the tibial tubercle position, all of which the treatment depends on. 4.15.8 covers trochlear dysplasia.

Radiographs and cross sectional imaging measure those, so the examination establishes the instability and the imaging establishes the anatomy. 4.1.6 covers that relationship.

No figures appear for any accuracy comparison on this page, under decision 023, and the comparison is the claim being made.

FRCS Level

How Good It Is

At decision level the examination sorts instability by the number of episodes, the presence of predisposing anatomy and the degree of laxity.

A first dislocation with normal anatomy and no laxity is a different decision from a recurrent one with dysplasia, and the examination contributes to both halves. 4.15.8 covers trochlear dysplasia.

Recurrent instability with a valgus and rotational abnormality moves the discussion towards correcting the alignment rather than only the restraint. 4.19.7 covers torsional assessment.

Generalised laxity changes the expected result of any soft tissue procedure, so it is recorded as part of the decision. 4.23.3 covers the hypermobile patient.

Skeletal immaturity restricts the procedures available, so maturity is assessed alongside the instability. 4.19.1 covers the growth plate assessment.

Where It Misleads

The commonest error is calling physiological laxity instability, since translation alone is common in this age group. 4.23.3 covers the hypermobile patient.

Apprehension is reported by an anxious adolescent on any firm examination, which inflates the test. 4.15.3 covers the apprehension test.

The rotational profile is omitted, so a torsional contribution is attributed to the soft tissues. 4.19.7 covers torsional assessment.

A history of the knee giving way is taken as patellar when it can be cruciate, and the two are distinguished by what the patient felt move. 4.2.6 covers giving way.

Bilateral findings are read as normal because they are symmetrical. 4.1.4 covers the structured approach.

Fellowship Level

Where It Misleads

At fellowship level the examination has to apportion the instability between shape, alignment, rotation and soft tissue, and it can only see the last two directly.

Trochlear dysplasia is the strongest predisposing feature and the examination only infers it, so the inference is checked rather than acted on. 4.15.8 covers trochlear dysplasia.

Patellar height is not clinically measurable with any reliability and it changes the treatment, so the examination substitutes the tracking for it. 4.15.1 covers patellar tracking.

Rotational contributions from the femur and the tibia can oppose each other, so a normal foot progression angle can hide both. 4.19.7 covers torsional assessment.

Generalised laxity is a spectrum, and the formal assessment reduces it to a score whose threshold is a convention rather than a biological boundary. 4.23.3 covers the hypermobile patient.

A previously operated knee examines against its own baseline, and the examination's normal references do not apply to it. 4.20.6 covers the reconstructed restraint.