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Snapshot diagnosis - what's this swelling at the back of the knee?

A budding young tennis player has noticed a bulge on the back of his knee. It’s not causing him any pain but needs a diagnosis. Dr Mike Wyndham describes how he made it

A budding young tennis player has noticed a bulge on the back of his knee. It's not causing him any pain but needs a diagnosis. Dr Mike Wyndham describes how he made it

The patient

This nine-year-old boy is a keen tennis player and has hopes of being the next Andy Murray. In fact, the first time the swelling was noticed by his mum was when they were playing in a family mixed doubles match.

He had noticed the bulge at the back of his knee a month earlier but hadn't made much of it, as it wasn't painful. His mother was a shrewd lady and pointed out to me that her own mother had suffered a similar swelling at the back of her knee, which her doctor had called a Baker's cyst. I explained that this was a condition usually associated with osteoarthritis, which wasn't likely to be the case here.

First instinct

It's hard to be reassuring when you're not absolutely certain what the problem is. Anatomically, the swelling was smooth and appeared to be coming from the back of the knee. Reassuringly, it was not causing any pain and hadn't grown in the last month. Anyway, on the basis of first principles, I suggested that it was almost certainly a benign lesion but it would need investigating.

Differential diagnosis

• Traumatic tear of the gastrocnemius muscle

• Osteogenic sarcoma

• Haemarthrosis

• Popliteal cyst (fluid in the semimembranosus bursa)

There certainly had been no history of injury before the swelling had appeared and so a traumatic muscle tear could be excluded. A haemarthrosis was always a possibility. But with the lack of trauma and no bleeding diasthesis, this was highly unlikely. Additionally, the swelling was confined to the back of the joint rather than the whole joint.

Osteogenic sarcomas do develop from the lower femur or upper tibia, but one of the most common symptoms from the tumour is bone pain – which was absent in this patient's case. The swelling was no warmer than the surrounding tissues and so this diagnosis seemed unlikely.

This left popliteal cyst as the most likely cause.

Getting on the right track

Ultrasound confirmed the diagnosis as a fluid-containing swelling consistent with a bursitis.

Semimembranosus bursitis may occur in children as well as adults. Although in adults the swelling usually persists, in children it may disappear spontaneously. This is exactly what happened in this case.

Dr Mike Wyndham is a GP in Edgware, north London

What's causing this swelling at the back of the knee? What's causing this swelling at the back of the knee? Snapshot diagnosis - what's this swelling at the back of knee (front view)

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