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. Plain film radiographs and computed tomography showed a posteriorly projecting bony protuberance over the inferior aspect of the patella. The protuberance was removed via an arthrotomy, and the patient made significant improvement in pain and function with no further locking episodes experienced.
Level of evidence: IV.