Examination Under Anaesthesia

For Medical Students

What It Tests

An examination under anaesthesia removes muscle guarding, so the ligament tests measure the ligaments rather than the patient's reaction to them. 4.10.10 covers it for the cruciate.

The examination is the same set of tests performed on a relaxed knee, with the findings recorded against the awake examination. 4.1.8 covers examination documentation.

It matters because guarding is the commonest reason a ligament examination is uninterpretable, and this removes it. 4.1.5 covers common examination pitfalls.

How It Is Performed

Record the awake examination first, since the comparison is part of the point. 4.1.8 covers examination documentation.

Examine both knees, because the contralateral side under the same anaesthetic is the only valid reference. 4.1.4 covers the structured approach.

Perform the anterior tests, since the Lachman is the test guarding most often defeats. 4.10.4 covers the Lachman test.

Perform the rotational tests, since the pivot shift needs a relaxed knee and is rarely interpretable awake. 4.10.6 covers the pivot shift test.

Perform the collateral stress tests in extension and in flexion. 4.12.1 covers valgus stress testing.

Perform the posterior and the posterolateral tests, which are the ones most often revised by anaesthesia. 4.11.2 covers the posterior drawer test.

Record every finding at the time, because none of it can be repeated later. 4.1.8 covers examination documentation.

What a Positive Means

A finding under anaesthesia is the knee's passive restraint without the patient's contribution.

A positive test that was negative awake usually means guarding masked it, which is the common and expected result. 4.14.6 covers it for rotational instability.

A larger excursion than the awake examination showed is not a new injury, and reading it as one is the error available here. 4.10.4 covers the Lachman test.

Laxity that is symmetrical with the other knee is that patient's normal rather than a finding. 4.23.3 covers the hypermobile patient.

A negative examination under anaesthesia in a patient with convincing symptoms moves the problem away from passive restraint. 4.2.6 covers giving way.

MRCS Level

How It Is Performed

In a viva this is the answer to what to do about an uninterpretable ligament examination, with the caveats attached.

The expected points are the awake record, both knees, the anterior tests, the rotational tests, the collaterals in two positions, the posterior and posterolateral tests, and the contemporaneous record.

Say that you would examine the other knee under the same anaesthetic, and give the reason. 4.1.4 covers the structured approach.

Say that you would record the findings at the time. 4.1.8 covers examination documentation.

Say that the dynamic and functional information is lost, not gained, under anaesthesia. 4.21.7 covers return to sport testing.

What a Positive Means

The reasoning expected is that a ligament test measures passive restraint plus muscle resistance, and anaesthesia removes the second term. 3.7.3 covers the secondary restraints.

The tests that depend most on relaxation gain most, which is why the rotational and posterolateral tests are the ones the examination changes. 4.13.4 covers the posterolateral corner assessment.

The knee's functional stability includes the muscle contribution, so the anaesthetised laxity is not the instability the patient experiences. 4.2.2 covers instability.

Physiological laxity is bilateral, so the contralateral knee under the same conditions is the reference that matters. 4.23.3 covers the hypermobile patient.

How Good It Is

The examination is the most accurate assessment of passive restraint available clinically.

Its best contribution is the rotational and posterolateral assessment, which is rarely interpretable awake. 4.13.4 covers the posterolateral corner assessment.

It says nothing about function, which is the quantity the patient reports.

Magnetic resonance imaging shows the structures and this shows their behaviour, so the two together are stronger than either. 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 this examination is performed when the awake findings would change the operation and cannot be obtained.

A suspected multiligament injury is the clearest indication, because the pattern decides the operation and guarding defeats the pattern. 4.23.6 covers the complex multiligament injury.

A suspected rotational component alongside a cruciate injury changes the procedure, and the pivot shift under anaesthesia is how it is established. 4.14.5 covers the combined instability patterns.

A discrepancy between the symptoms and the awake examination is an indication, and the result may be that the passive restraint is intact. 4.2.6 covers giving way.

The findings are recorded before the operation begins, since nothing afterwards reproduces them. 4.1.8 covers examination documentation.

Where It Misleads

The commonest error is not examining the other knee, which removes the only reference for that patient's physiological laxity. 4.23.3 covers the hypermobile patient.

A larger excursion than the awake examination is read as a new or worse injury. 4.10.4 covers the Lachman test.

Anaesthetised laxity is equated with instability, which omits the muscle contribution the patient lives with. 4.2.2 covers instability.

The findings are not recorded at the time and are reconstructed afterwards. 4.1.8 covers examination documentation.

The examination is performed and the awake examination is never documented, so the comparison is unavailable. 4.1.4 covers the structured approach.

Fellowship Level

Where It Misleads

At fellowship level the examination's accuracy about passive restraint is exactly what limits what it can decide.

It measures the knee the patient does not use, since the muscle contribution that anaesthesia removes is part of their stability every waking moment. 4.2.2 covers instability.

Grading by excursion under anaesthesia is a judgement by hand, so two surgeons record different grades on the same knee and the operative plan inherits the difference. 4.12.1 covers valgus stress testing.

Physiological laxity varies widely, so an absolute excursion means little and the contralateral comparison fails when the other knee is also injured or operated. 4.23.5 covers the revision patient.

Anaesthetic depth and muscle relaxation change the findings, which makes the examination's conditions a variable nobody records. 4.1.8 covers examination documentation.

The findings cannot be repeated or audited, so a disputed result has no appeal. 4.23.7 covers the medico legal setting.