Lower limb

Patella

Before you say anything

The largest sesamoid bone in the body, and the only one where the side depends on reading the articular surface rather than the shape of an end. Turn it over, find the vertical ridge, and see which facet is bigger. That larger facet is lateral, and it is also the reason the patella dislocates the way it does.

Which side? Three checks

Hold it like this first: Hold it with the apex downwards, the smooth articular surface facing backwards, and the larger of the two facets on that surface directed laterally.

Check 1 — Find the apex
One end is pointed, the opposite border is thick and flat
The pointed apex is inferior and the flat base superior
Check 2 — Turn it over
One surface is rough and convex, the other smooth and divided by a vertical ridge
The smooth articular surface faces backwards, towards the femur
Check 3 — Compare the two facets
The vertical ridge divides the articular surface into unequal facets
The larger facet is lateral, so it points to the side the bone belongs to

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Say these first

The largest sesamoid bone
A thick base above and a pointed apex below
A posterior articular surface divided by a vertical ridge into unequal facets

Do not confuse it with

A tarsal bone such as the calcaneus or talus
The patella has one rough non-articular surface and one articular surface divided into just two unequal facets by a single vertical ridge; tarsal bones have several articular facets facing in different directions.

Examiner trap

Reading the side straight off the larger facet while looking at the articular surface. That surface faces backwards, so looking at it reverses left and right.

Memory hook

Apex down, rough side out, larger facet lateral.

Why it matters

The patella dislocates laterally; this is resisted by the forward projection of the lateral femoral condyle, the larger lateral facet and the horizontal fibres of vastus medialis.
A bipartite patella is a normal variant, usually superolateral and bilateral, and should not be reported as a fracture.
A transverse fracture is pulled apart by the quadriceps, so the fragments separate; a comminuted fracture from a direct blow may not.

Senior’s 30-second takeaway

Patella — the whole bench in four lines. Read this on the way in.

If you see this: A flat triangular bone · A rough front and a smooth back · A vertical ridge dividing two unequal facets
If asked to identify it: This is a patella; the apex points downwards, the articular surface backwards and the larger facet laterally, giving the side.
If asked one more: It is the largest sesamoid bone and articulates only with the patellar surface of the femur.
If asked why it matters: It dislocates laterally, checked by the lateral femoral condyle and vastus medialis.

Then stop. Let the examiner ask the next question.

🩺 Before you say anything

Which side? Three checks

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Senior’s 30-second takeaway

Before you say anything