Instability

For Medical Students

What It Tests

Instability means the knee moves in a way the patient cannot control. That is a mechanical event and it is different from the knee feeling weak or unreliable.

Asking about it tests whether the joint is failing structurally, and it is the one question in the knee history that outperforms every other instrument available. 4.1.3 covers why.

Laxity and instability are not the same thing. Laxity is what an examination measures, instability is what the patient experiences, and many knees have one without the other.

How It Is Performed

Do not ask whether the knee is unstable. Ask what the knee does, and when.

Ask for a specific occasion: what they were doing, what the knee did, whether they fell, and whether it swelled afterwards.

Ask whether it happens on a straight line or on a turn, because the two point at different structures. 4.2.6 covers giving way in detail.

Ask how often, and whether it is getting more or less frequent, since a worsening frequency changes the urgency.

Ask what they now avoid, which measures the symptom's cost better than any frequency does.

What a Positive Means

An episode of the knee giving way on a turn or a pivot points to rotational instability and therefore to the cruciate or the lateral structures. 4.14.3 covers the pattern.

Giving way on a straight line, going down stairs or a slope, points to the extensor mechanism or to the posterior cruciate rather than to the anterior.

A single traumatic episode with immediate swelling points to an acute structural injury. 4.2.3 covers swelling.

Repeated episodes without swelling in a young patient point towards patellar instability or towards generalised laxity rather than towards a cruciate. 4.15.3 covers the patellar version.

MRCS Level

What a Positive Means

The distinction a viva is listening for is between true instability, pseudo-instability and apprehension, and all three are reported as the knee giving way.

True instability is a joint surface moving abnormally on another. It is structural and it is what reconstruction addresses.

Pseudo-instability is a knee that buckles because of pain inhibition or because the quadriceps gave out. It is common in degenerate and in painful knees and surgery for laxity does not help it.

Apprehension is the patient withdrawing from a movement they expect to fail. It is informative about confidence rather than about structure, and it responds to rehabilitation.

Separating the three is a history task, because examination measures laxity and all three can occur with the same laxity.

How Good It Is

A clear account of a mechanical giving way episode during a specific activity is one of the strongest pieces of information in the whole knee assessment.

It outperforms laxity measurement, because a measured laxity that the patient does not experience is not the problem and a modest laxity they do experience is.

It outperforms imaging, because a scan cannot see an event.

Where it performs badly is in distinguishing which structure, since the patient's account of direction is often unreliable and the examination has to settle it. 4.14.5 covers the patterns.

FRCS Level

How Good It Is

At decision level instability is the symptom that most often determines whether to reconstruct, and the history carries more weight in that decision than the examination does.

A knee with substantial measured laxity that has never given way does not need stabilising on the strength of the measurement.

A knee with modest laxity that gives way during the patient's work or sport does, and the laxity grade will understate the problem.

The feature that most strongly supports reconstruction is instability during an activity the patient cannot avoid. Instability only during high level sport is a different decision, because avoidance is available.

Frequency matters less than predictability. A knee that gives way unpredictably is more disabling than one that gives way more often in a known situation.

Where It Misleads

Instability misleads in two directions and both are costly.

Over-reported, where a patient with a painful degenerate knee describes buckling and is offered a reconstruction that addresses laxity they do not have.

Under-reported, where a patient has silently stopped doing everything that provokes it. A knee that no longer gives way because its owner no longer turns is not a stable knee, and the only way to detect this is to ask what they have given up.

The second is the commoner error in elective practice and it systematically under-estimates the symptom in less active and in older patients.

A third trap is rotational instability reported as pain. A patient who cannot describe an event but avoids pivoting may have rotational laxity nobody has tested. 4.14.1 covers the pivot shift.

Fellowship Level

Where It Misleads

Persistent instability after a reconstruction is the referral this page exists for, and the history has to separate four causes before anything is imaged.

A graft that has failed, which usually gives a clear return of the original symptom after a defined event.

A rotational component never addressed, which gives instability on pivoting with a knee that tests well for anterior translation. This is the commonest of the four and the easiest to miss, because the pre-operative record rarely contains a rotational assessment. 4.1.8 covers the record.

Pseudo-instability from quadriceps deficit, which is common after reconstruction and responds to rehabilitation rather than to revision.

And a patellofemoral cause misattributed to the tibiofemoral joint, where the knee gives way because the patella subluxes. 4.15.3 covers it.

The history separates these four better than imaging does, and the order of questioning is what makes that possible: the event first, the activity second, the direction third, and what the knee did afterwards last.