Skin Changes

For Medical Students

What It Tests

The skin over the knee reports on inflammation, infection, circulation, previous surgery and systemic disease, and it reports before any of those are asked about.

Inspection of the skin is not a knee test. It is the part of the examination that decides whether a knee examination should proceed at all. 4.1.4 covers the structured approach.

Several of its findings are urgent rather than diagnostic, which makes this the page where looking first earns the most. 4.8.7 covers the acute haemarthrosis.

How It Is Performed

Expose both knees fully and compare them in the same light.

Look at the colour. Redness over a joint, particularly with swelling, raises infection and inflammatory arthritis. 4.8.9 covers synovitis.

Look at the surface for breaks: a wound, an abrasion, a puncture, or a healing scab, and note its relationship to the joint.

Look for bruising and note where it has tracked, since blood follows tissue planes and the pattern says where the injury was. 4.2.17 covers contact injury.

Look for skin dimpling or puckering, particularly medially, because that appearance in a grossly unstable knee suggests an irreducible dislocation. 4.11.7 covers that assessment.

Look at the surrounding limb for swelling, discolouration and venous distension, and feel the temperature of the foot. 4.6.2 covers temperature assessment.

What a Positive Means

Redness with swelling, warmth and an inability to move the knee is a septic joint until proven otherwise, and that conclusion is reached from the inspection rather than from a test.

A wound over or near the joint in an injured knee raises an open injury, which changes the urgency of everything that follows.

Bruising tracking posteriorly and distally after a twisting injury suggests capsular disruption rather than a simple sprain.

Skin dimpling in a grossly unstable knee is a recognised sign of an irreducible dislocation, and it changes the management before any imaging.

Psoriatic plaques, a rash, or nail changes point at an inflammatory arthropathy, which turns a mechanical presentation into a different diagnosis. 4.2.14 covers inflammatory pain.

MRCS Level

How It Is Performed

In a viva the skin is where a candidate demonstrates that they will not harm a patient, and saying what would stop you examining is as valuable as saying what you would find.

The expected points are the comparison in the same light, colour, breaks in the surface, bruising and its distribution, dimpling, and the state of the limb beyond the knee.

Say that you would not proceed with a ligament examination in a knee you suspected of being septic, and say why.

Say that you would look at the other joints and the skin elsewhere if an inflammatory cause were possible, since the knee is one joint of several in those diseases. 4.2.14 covers inflammatory pain.

Say that you would record the appearance in words rather than a label, because the appearance is what the next clinician needs. 4.1.8 covers the record.

What a Positive Means

The reasoning expected is that the knee is a superficial joint with thin overlying tissue, so inflammation and effusion are visible rather than inferred. 4.8.1 covers normal fluid physiology.

Blood tracks along fascial planes under gravity, which is why bruising appears distal and posterior to an injury rather than over it, and why its absence over a structure does not exclude injury to it.

The skin's blood supply around the knee is relevant to any planned surgery, so scars, grafts and previous radiotherapy change what incisions are available. 4.3.7 covers surgical scars.

A warm, red, swollen joint that cannot be moved is the presentation of a septic arthritis, and the inspection carries that diagnosis more than the examination does.

How Good It Is

Skin inspection is the most consequential low effort part of the examination, because its positive findings change management immediately and its negative findings cost nothing.

It is specific rather than sensitive for the urgent diagnoses. A normal looking knee can be septic, particularly early and in an immunosuppressed patient, so a normal inspection does not exclude it.

It is more reliable than any ligament test, because it requires no patient cooperation and no examiner judgement about force.

It is unreliable in poor light, through partial exposure, and in darker skin where erythema is harder to read, and in that last case temperature and swelling carry more weight. 4.6.2 covers temperature assessment.

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 the skin changes the plan more abruptly than any other examination finding, because the plans they lead to are urgent.

A septic knee stops the elective pathway and starts a different one, and the inspection is where that decision is made.

An open injury in a ligament injured knee changes the order of everything, and the ligament assessment waits. 4.11.7 covers that sequence.

In elective surgery the skin determines what is feasible. Previous incisions, their orientation and the state of the soft tissue envelope decide the approach, and a knee with multiple scars may not tolerate the operation that would otherwise be indicated. 4.3.7 covers scars.

An inflammatory skin finding changes the diagnosis, and operating on a mechanical presentation that is actually inflammatory produces a dissatisfied patient and no benefit. 4.2.14 covers it.

Where It Misleads

The commonest error is examining a swollen knee without looking at it, which is how a septic joint gets a ligament examination.

The second is reading the absence of bruising as the absence of injury.

The third is attributing redness to a mechanical cause, which is the error with the worst consequences on this page.

The fourth is missing dimpling in a grossly unstable knee, which is a short list of findings but a consequential one.

The fifth is recording scars without establishing what they were for, since a scar whose operation is unknown is a question rather than a finding. 4.2.30 covers previous surgery.

Fellowship Level

Where It Misleads

In referred practice the skin is where a knee that has been operated on too many times declares its limits.

A knee with several longitudinal scars, thin skin and a previous wound problem has a soft tissue envelope that constrains the revision far more than the ligament findings do, and that constraint is often the honest answer to the referral. 4.3.7 covers the scars.

A knee with a chronically warm, mildly swollen appearance after an arthroplasty or a reconstruction raises low grade infection, and that possibility has to be excluded before any revision rather than after it.

A patient with recurrent effusions, a rash and nail changes has an inflammatory arthropathy that the mechanical examination will keep failing to explain, and the skin is where that is noticed. 4.8.8 covers chronic effusions.

Lymphoedema and venous disease both change the appearance and the circumference of the limb, so a swollen limb is not the same finding as a swollen knee and the two are separated by looking at the whole leg.

In medicolegal work photographs of the skin at presentation carry more weight than any written examination finding, and their absence is often the gap in the record. 4.23.7 covers that setting.