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KneeSchool/Clinical Examination Academy/Acute ACL Injury

Acute ACL Injury

Page 4.10.2 · ACL Examination · examination page

Examining a swollen, painful, guarding knee. What is answerable now and what has to wait.

Pipeline status. Editorial draft. Evidence verification did not run. Not consultant reviewed. Not for clinical use.

The same topic at 4 depths

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Acute ACL Injury

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For Medical Students

What It Tests

The acute examination tests three things that are answerable in a guarding knee, and it should not attempt more.

Whether there is a haemarthrosis, whether the extensor mechanism works, and whether there is a mechanical block to extension.

Laxity testing in an acutely injured knee under-reports, because pain and muscle spasm resist the translation the test is trying to produce. 4.1.5 covers the timing problem.

How It Is Performed

Inspect for swelling and for the position the knee is held in.

Confirm an effusion by the sweep test or the patellar tap, which are gentler than most manoeuvres. 4.8.5 covers the sweep test.

Ask the patient to straighten the knee against gravity, which screens the extensor mechanism. 4.7.8 covers extension lag.

Attempt gentle passive extension and characterise the endpoint, looking for a block. 4.9.8 covers it.

Then attempt a Lachman test gently, accepting that a negative result in this knee means very little. 4.10.4 covers the test.

Do not attempt a pivot shift in an acute knee. It will not be tolerated and the result will be uninterpretable. 4.10.6 covers it.

What a Positive Means

A haemarthrosis after a non-contact pivoting injury means a cruciate rupture, a patellar dislocation, an osteochondral fracture or a peripheral meniscal tear. 4.8.7 covers it.

A positive Lachman in this setting is strong, because a guarding knee under-reports and a positive has overcome that.

A negative Lachman is weak and should not be recorded as excluding the injury.

A block to extension is a displaced meniscal fragment and changes the urgency.

An inability to straighten against gravity is an extensor mechanism injury until proven otherwise.

Key Learning Points

  • Three things are answerable acutely: haemarthrosis, extensor mechanism, mechanical block.
  • Laxity testing under-reports in a guarding knee.
  • A positive Lachman acutely is strong; a negative one is weak.
  • Do not attempt a pivot shift in an acute knee.
  • A block to extension changes the urgency.

MRCS Level

How It Is Performed

In a viva the expected sequence for an acute knee is compressed and the candidate is marked on knowing why.

Say that you would not perform the full examination, and say which three questions you are answering.

Say that you would re-examine at two to six weeks, because the examination at that point is substantially more informative and is what should drive a reconstruction decision.

Say that you would aspirate only if the knee were tense and the diagnosis uncertain, and that aspiration is not routine.

Finish with the neurovascular assessment and with a statement about the energy of the injury, because a high energy mechanism changes the priorities. 4.23.6 covers multiligament assessment.

How Good It Is

Every laxity test in this chapter performs worse acutely, and the reason is uniform: they all depend on the patient permitting translation.

The Lachman test is the least affected and remains the best available test acutely, which is one of the reasons it outperforms the anterior drawer.

The anterior drawer is heavily affected, because the ninety degree position shortens the hamstrings and a guarding patient resists easily. 4.10.5 covers it.

The pivot shift is almost unobtainable acutely and is the test most transformed by anaesthesia. 4.10.10 covers examination under anaesthesia.

So the ranking of these tests is not fixed: it depends on when the examination is performed, and no figures are quoted for any of it, under decision 023.

Key Learning Points

  • Compress the acute examination deliberately and know which three questions you are answering.
  • Re-examine at two to six weeks, which is what should drive the reconstruction decision.
  • The Lachman is the least affected by guarding and the best test available acutely.
  • The anterior drawer is heavily affected by the hamstrings in its test position.
  • The ranking of these tests depends on when the examination is performed.

FRCS Level

How Good It Is

At decision level the acute examination's job is triage rather than diagnosis.

The findings that change the next few days are a mechanical block, an extensor mechanism injury, a high energy mechanism with multidirectional laxity, and a vascular or neurological deficit.

Everything else waits, and saying so is better practice than persisting with an examination the knee will not permit.

The reconstruction decision should rest on an examination at two to six weeks, when the effusion has settled and the patient is not guarding, and a decision taken on the first examination is taken on the least informative one available.

A repairable meniscal tear is the one thing that argues for earlier intervention, and the history establishes that rather than the examination. 4.2.9 covers onset.

Where It Misleads

The commonest error is recording a negative laxity examination in an acute knee and treating it as reassuring.

The second is attempting the full battery of tests, which hurts the patient and produces findings nobody should rely on.

The third is missing the combined injury, because the obvious cruciate diagnosis closes the assessment. 4.12.7 covers one combination.

And aspirating a tense haemarthrosis without considering that the blood itself is the diagnostic finding, which is a reason to record it before removing it.

Key Learning Points

  • The acute examination's job is triage rather than diagnosis.
  • Block, extensor mechanism, high energy laxity, and neurovascular deficit are what change the next few days.
  • The reconstruction decision should rest on an examination at two to six weeks.
  • Recording a negative acute laxity examination as reassuring is the commonest error.
  • The obvious cruciate diagnosis frequently closes the assessment too early.

Fellowship Level

Where It Misleads

The acute examination matters most at this level for what it failed to record, because the referred patient's pre-operative rotational state frequently was never assessed.

A guarding acute knee cannot be assessed rotationally, which is a legitimate reason and not a reason to leave the record blank: recording that rotation could not be assessed is different from recording that it was normal. 4.1.8 covers the record.

In a multiligament knee the acute examination is both the hardest and the most important, because the pattern of laxity recorded then is frequently the only pre-operative account available. 4.23.6 covers the assessment.

An examination under anaesthetic at the time of any urgent procedure is a free and under-used opportunity to record what an awake knee could not show. 4.10.10 covers it.

No figure here supports these claims, under decision 023.

Key Learning Points

  • Recording that rotation could not be assessed is different from recording that it was normal.
  • In a multiligament knee the acute laxity pattern is often the only pre-operative account.
  • An examination under anaesthetic during an urgent procedure is a free and under-used opportunity.
  • The referred patient's pre-operative rotational state was frequently never assessed.

Explore Further

Where this topic came from and where it goes next in the spiral.

References

No reference list is attached to this draft. Evidence verification has not run, because source retrieval was unavailable at generation time. Under the Operations Handbook a missing reference is acceptable and an invented one is a critical failure, so nothing has been cited and no citation marker appears in the text. This page carries no diagnostic accuracy figure, which is decision 023 rather than an omission. The comparative standing of these tests is standard teaching and a specific sensitivity written from memory is the same class of claim as an invented citation, so ACC-001 refuses the number and the page states the comparison. A reviewer with the literature can add the figures into How Good It Is without restructuring the page, and chapter 3.17 lists the studies the figures would come from. Any measurement here, including a flexion angle or a translation in millimetres, is written from standard teaching, has not been checked, and is registered under decision 006 in pipeline/config/figures with a verification state of UNVERIFIED_FROM_MEMORY.

Safety. KneeSchool is educational and does not give individual medical advice. Seek urgent assessment for a knee that cannot bear weight after an injury, a hot swollen knee with a fever, or new numbness, weakness or coldness in the leg or foot.