Principles of Knee Assessment

For Medical Students

What It Tests

Five principles govern every examination in this section, and learning them is worth more than learning any individual test.

Compare with the other side. The unaffected knee is the only normal you have for this patient, and published normal ranges are populations rather than people.

Examine the joint above and below. Hip pathology refers to the knee reliably, and in a child it is the first thing to exclude. 4.19 covers that version.

Work from least to most uncomfortable, because pain and guarding destroy the information every later test depends on.

Put the patient in a position where the muscle can relax. A test that requires a relaxed hamstring and is performed on a tense one measures the hamstring.

And test one thing at a time. A manoeuvre that loads three structures at once tells you that something moved.

How It Is Performed

Expose both knees fully and look at the patient standing, walking and lying, because each position shows something the others hide.

Standing shows alignment under load and any thrust during stance. 4.5 covers alignment.

Walking shows what the knee does when the patient is not thinking about it, which is often different from what it does when they are. 4.4 covers gait.

Lying lets the muscle relax and is where laxity testing becomes possible.

Stop and position again rather than pressing on through a tense examination, because the information does not improve with persistence.

MRCS Level

How It Is Performed

At viva level the examination is expected to be a sequence rather than a list, and the sequence is what is being marked.

Introduce, expose, inspect standing, assess gait, then move to the couch for palpation, range of motion, effusion and the special tests relevant to the history.

Say what you are looking for as you do it. An examiner cannot mark a silent examination and will assume you did not know.

Finish by stating what you would do next, which is usually to examine the hip, the other knee, or the neurovascular status rather than to request a scan.

How Good It Is

The principle that most improves a knee examination's reliability is side to side comparison, and it is the one most often skipped under time pressure.

Absolute laxity measurements vary widely between normal knees, so a figure means little and a difference means a great deal.

The second most valuable principle is relaxation. The difference between a relaxed and a tense hamstring on an anterior drawer is larger than the difference between an intact and a torn ligament.

That is why the Lachman, performed in slight flexion where the hamstrings are shortened and less able to resist, outperforms the anterior drawer so consistently.

FRCS Level

How Good It Is

The principles matter more at this level rather than less, because the examinations that change management are the ones where the findings are subtle.

Grading laxity is where discipline pays. A grade is a comparison against the other side with an endpoint described, and a grade recorded without an endpoint is almost uninterpretable later.

Flexion angle is the second discipline. Every laxity test in this section has an angle attached and the angle is part of the finding rather than part of the technique. 4.12.3 and 4.14.3 both turn on it.

The third is timing. An examination at presentation, at two weeks and at six weeks in the same knee produces three different pictures, and the one that should drive a reconstruction decision is rarely the first.

Where It Misleads

A principle applied mechanically misleads as readily as one ignored.

Side to side comparison fails where both knees are abnormal, which is common in generalised laxity and in bilateral degenerative disease, and the clinician who relies on it alone concludes that both knees are normal.

Examining the joint above fails to help when the hip is also degenerate, which in an older patient it frequently is. Two positive findings do not tell you which one is producing the symptom.

And testing one thing at a time is impossible for rotational instability, where the informative tests are deliberately combined loads. 4.14 is built around that exception.

Fellowship Level

Where It Misleads

The principle that breaks down most completely at this level is that an examination finding belongs to one structure.

In a multiligament knee the structures have failed together and each test is now loading an unknown combination, so the examination describes a pattern of laxity rather than a list of torn structures.

The useful discipline is to record what the knee does rather than what you think is torn: translation at which angle, rotation at which angle, and where the endpoint is. The structural attribution can be revised and a well recorded finding cannot be redone.

Generalised laxity is the other breakdown. A patient whose normal is loose makes every comparison in this section unreliable, including the side to side one, and the assessment becomes functional rather than structural.

In both situations the examination's job shifts from diagnosis to documentation, and the examination under anaesthetic becomes the one that decides. 4.14.6 covers it.