A rare patella anatomical abnormality causing locking in an adolescent girl.

Unlabelled: Locking is an inability to fully extend the knee and is most commonly associated with meniscal pathology and loose bodies. The locked knee is an infrequent presentation in paediatric orthopaedics, and the presence of a patella protuberance as a cause has never been reported in children or adults. An adolescent female presented with a 4-year history of intermittent locking of the right knee associated with a painful audible 'clunk' on full extension.