A young male football player presented with an insidious onset of posterior knee pain. Other than a mild antalgic gait his knee (and hip) examination was normal. An knee x-ray series appeared normal. Given that he was quite impaired, and because there was an uncertain diagnosis, an MRI was ordered.
As outlined an initial x-ray series was reported to be normal (although with the benefit of hindsight the desmoid can probably be seen). At the posteromedial aspect of the medial femoral condyle, at the site of attachment of the medial head of gastrocnemius, there is an elliptical focus of subcortical high signal seen on MRI measuring approximately 11 mm (transverse) x 12 mm (craniocaudal length). There is a small amount of surrounding bone marrow oedema. Appearances are consistent with an active cortical desmoid. There is also a small amount of oedema within the medial head of the gastrocnemius.
This player’s symptoms settled quickly with a short period of rest. No specific treatment was required. He was able to return to football after six weeks of reduced activity.
Cortical desmoids are one of the most frequent incidental findings on x-rays and a common finding in MRI. They are seen in up to 5% of young women and in up to 10% of young men. They are more common on the left and are bilateral in up to 35% of cases. A symptomatic desmoid is best demonstrated on T2 MRI images as intermediate to high signal surrounded by sclerosis (a rim of low signal intensity). This problem can generally be treated with a short period of rest and is not associated with ongoing issues.
In this age group it is also important to consider whether the hip is the source of the patient’s symptoms as referred pain is relatively common. The differential diagnosis also needs to include malignancy (osteosarcoma) as the MRI findings can be similar.
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