Examination in the Era of MRI

For Medical Students

What It Tests

Magnetic resonance imaging shows the knee's tissues in detail that no examination approaches, and it did not make examination redundant. It changed what examination is for.

Before imaging was routine the examination had to establish what was torn. Now imaging does that better, and the examination establishes whether what is torn is the problem.

That is a harder question and a more important one, because treatment follows from it rather than from the image.

It also means the examination's errors cost more. A scan finding attributed to the wrong symptom leads to an operation on the wrong structure.

What a Positive Means

A positive scan finding and a positive examination finding mean different things.

A scan finding means the tissue is abnormal. That may be recent, old, or incidental.

An examination finding means the joint behaves abnormally today, under load, in this person.

When they agree the diagnosis is usually straightforward. When they disagree the examination is more often the one that should drive management, which is the opposite of most people's instinct.

MRCS Level

What a Positive Means

The reason this matters is the base rate of abnormal findings in people without symptoms.

Degenerate meniscal change, cartilage signal change and small effusions are common in asymptomatic middle aged knees, and their presence on a scan therefore carries limited diagnostic weight on its own.

The consequence is that imaging in that population cannot select who needs treatment, because it finds something in almost everybody. 4.9.11 covers the meniscal case.

In a young knee after a defined injury the base rate is much lower and a scan finding means more, which is why the same test is more useful in one population than another.

How Good It Is

Comparing examination and imaging as though they competed produces the wrong ranking, because they are good at different questions.

Imaging outperforms examination for meniscal and cartilage lesions, for bone, and for anything requiring localisation.

Examination outperforms imaging for functional instability, for range of motion, for extensor mechanism function, and for the question of which of several findings is symptomatic.

For cruciate rupture both perform well, and the examination has the advantage of being available immediately.

No figures appear here, under decision 023. The ranking is standard teaching and a quoted sensitivity written from memory would not be.

FRCS Level

How Good It Is

The decision-relevant question is what a scan adds to an examination that has already been done properly, and the answer varies more than practice reflects.

Where the examination is clear and the management does not depend on a detail only imaging supplies, a scan adds confirmation rather than information.

Where the examination is equivocal, or where the operative plan depends on which structure and where, imaging adds a great deal.

And where the examination and the scan disagree, the scan has added the most valuable thing it can, which is a reason to examine again rather than a conclusion.

Where It Misleads

Universal imaging introduced errors that did not previously exist, and they are now the dominant ones.

Attribution error. A finding is present, so it is treated as the cause. The frequency of incidental findings makes this the most common error in degenerate knees.

Contamination of the history. A patient who has read the report describes the diagnosis rather than the symptom, which is why the history should be taken first. 4.1.3 covers it.

Deskilling. A generation examining knees that have already been scanned examines them less carefully, and the skill degrades across a department rather than in an individual.

And premature closure. A scan arriving early stops the diagnostic process, including the second examination at two to six weeks that would have been the most informative assessment.

Fellowship Level

Where It Misleads

At this level imaging and examination are usually both abnormal and the question is which abnormality is operative.

In the revision knee imaging is least useful and examination most, because the previous operation guarantees abnormal signal and the useful information is what the knee now does. 4.10.12 covers the ACL version.

The reverse holds for planning. Once the decision to operate is made, imaging drives the plan and the examination contributes little to it, so the two instruments dominate different halves of the same episode.

The judgement that is hardest to teach is when to stop investigating. A patient with several abnormal findings and an unclear picture is usually better served by a period of observation and a repeat examination than by a further scan, and the pressure runs the other way.

What this page cannot supply is how often each of these errors occurs. The patterns are standard teaching, no frequency is cited, and nothing here has been checked against a source.