Osteochondroma of acromioclavicular joint.

Osteochondroma is the most common type of benign bone tumour. It is a benign chondrogenic lesion derived from aberrant cartilage from the perichondral ring, and it commonly presents in the proximal humerus, proximal femur and knee. Osteochondroma is usually solitary but can be multiple with patients with hereditary multiple exostoses. Malignant changes happen in approximately 1% of cases. Osteochondroma usually causes local pain or swelling. We discuss a unique case of an osteochondroma that highlights the fact that osteochondroma can occur in the most unlikely places, and they should be properly visualised via radiography to evaluate any extensions and compromised surrounding structures before surgical intervention. © BMJ Publishing Group Limited 2019. No commercial re-use. See rights and permissions. Published by BMJ.