Muscle Wasting

For Medical Students

What It Tests

Quadriceps wasting indicates that the knee has been used less than the other one, for long enough for the muscle to respond.

It is therefore a duration finding rather than a diagnostic one. It says the problem is real and not recent, and it says nothing about which structure is responsible. 4.2.9 covers onset in the history.

It is also the finding most predictive of how the patient will do, and the only one in the inspection that rehabilitation reliably changes. 4.11.5 covers the functional assessment.

How It Is Performed

Lay the patient supine with both legs straight and exposed from the groin down.

Look at the two thighs from the front and from the side, comparing the contour and in particular the bulk of the vastus medialis above the medial side of the knee.

Measure the circumference of both thighs at the same distance above the joint line, using the patella's upper border as the landmark, and record the distance used.

Ask the patient to tighten the quadriceps and watch both sides, because wasting is clearer with the muscle contracting. 4.3.4 covers the sitting assessment.

Look at the calf as well, since a wasted calf points at disuse of the whole limb or at a nerve problem rather than at the knee. 4.13.4 covers the corner, where the nerve is at risk.

What a Positive Means

A visible difference in quadriceps bulk indicates a knee that has been used less for weeks or more.

The vastus medialis is usually the first part to look wasted, which is why the medial contour above the knee is where to look first.

Wasting with a normal examination otherwise suggests the problem is functional or painful rather than structural, and the history carries it. 4.2.1 covers pain.

Wasting that has not recovered after a technically sound operation is the finding most predictive of a persisting complaint. 4.7.10 covers post-operative range.

Calf wasting with quadriceps wasting suggests disuse of the whole limb, and calf wasting alone suggests a nerve or a more distal problem. 4.4.5 covers a proximal pattern.

MRCS Level

How It Is Performed

In a viva the measurement is what distinguishes a candidate who has used this finding from one who has read about it.

The expected points are the comparison of contours, the vastus medialis, the circumference measured at a stated distance above the joint line on both sides, and the muscle under contraction.

Say the distance you are measuring from and at, because a circumference without a stated landmark is not comparable at the next visit. 4.1.8 covers the record.

Say that you would assess the quadriceps strength as well as the bulk, since the two do not always correspond. 4.7.1 covers active motion.

Say that you would ask what rehabilitation the patient has actually completed, because a wasted quadriceps in an unrehabilitated knee is a treatment opportunity rather than a finding. 4.2.34 covers previous rehabilitation.

What a Positive Means

The reasoning expected is that the quadriceps is the knee's principal dynamic stabiliser, so its state is both a consequence of the problem and a determinant of how the knee copes. 4.11.5 covers that argument.

That dual role is why wasting predicts outcome better than laxity does in several of the conditions in this section, the posterior cruciate deficient knee most clearly. 4.11.6 covers it.

An effusion inhibits the quadriceps directly, so wasting in a chronically effused knee has a cause that must be addressed before the muscle can be rebuilt. 4.8.2 covers the mechanism.

Pain also inhibits it, which is why a painful knee wastes faster than a lax one and why wasting tracks pain rather than instability. 4.2.1 covers pain.

How Good It Is

Measured quadriceps circumference is the most reproducible finding in the inspection and one of the most reproducible in the whole examination.

It is sensitive and entirely non specific. Almost every chronic knee problem produces it, so it contributes to severity and duration rather than to diagnosis.

Its reproducibility depends on measuring at a stated landmark, and a circumference recorded without one is not comparable, which is the commonest reason the finding is wasted.

Isokinetic strength testing measures what this estimates and is a specialist rather than a clinic assessment. 4.4.10 covers functional screening.

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 quadriceps state is one of the few examination findings that changes whether an operation is offered now, later, or at all.

A wasted quadriceps in a patient who has not completed rehabilitation means the non operative treatment has not been tried, and that distinction separates a patient who needs an operation from one who needs a physiotherapist. 4.11.5 covers the assessment.

Reconstructing a knee with a significantly wasted quadriceps produces a worse result and a longer recovery, so the finding often changes the timing rather than the indication. 4.7.7 covers contractures.

In the posterior cruciate deficient knee the quadriceps is the compensating mechanism, so its state is close to being the decision. 4.11.6 covers that argument.

After surgery, persistent wasting is the finding most worth following, and following it is more useful than repeating the laxity measurement. 4.7.10 covers post-operative range.

Where It Misleads

The commonest error is noting wasting and not measuring it, which discards the only reproducible part.

The second is measuring without a stated landmark.

The third is accepting a patient's account of rehabilitation without establishing what was done, since wasting is evidence it was not completed.

The fourth is attributing wasting to the knee in a patient with hip pathology or a nerve problem, both of which produce it. 4.1.4 covers the structured approach.

The fifth is treating wasting as irreversible, which it usually is not, and which makes it the most useful finding in the inspection.

Fellowship Level

Where It Misleads

In referred practice unrecovered quadriceps wasting is the commonest explanation for a knee that is mechanically satisfactory and functionally disappointing.

A patient with a competent reconstruction, a normal examination and persistent wasting has a rehabilitation problem, and offering further surgery for it makes the knee worse. 4.14.3 covers the pattern that genuinely does need surgery.

Wasting that will not respond to rehabilitation in a knee with a normal examination raises pain inhibition, an effusion, or a nerve problem, and all three are treatable and none is a ligament issue. 4.8.2 covers the effusion.

In a knee with a previous common peroneal nerve palsy the wasting pattern extends below the knee, and attributing the whole picture to disuse misses the cause. 4.13.4 covers the corner.

Measured wasting at first presentation, recorded with a landmark, is one of the most useful entries in a record and one of the rarest, which makes serial comparison in a referred patient usually impossible. 4.1.8 covers the record.

In medicolegal work measured wasting is objective evidence of reduced use, and it is harder to dispute than a pain score or a laxity grade. 4.23.7 covers that setting.