Exostosis

An exostosis, also known as a osteochondroma, is a benign chondrogenic lesions derived from aberrant cartilage from the perichondral ring. Exostoses can cause chronic pain ranging from mild to moderate, but rarely severe, depending on the shape, size, and location of the lesion, though most are asymptomatic and are found serendipitously on plain x-ray taken for other reasons. It is most commonly found in pre-teens through early 20s adjacent to the physes of the distal femur and proximal tibia but can be found adjacent to other physes, most notably the distal phalanx of the finger, where it presents as a subungual mass. larger growths can occur on places like the ankles, knees, shoulders, elbows and hips. Very rarely are they on the skull.Though rare, malignant transformation can occur into a chondrosarcoma.

They normally form growing stalks angling away from the physis toward the mid shaft of a long bone.

When used in the phrases "cartilaginous exostosis" or "osteocartilaginous exostosis", the term is considered synonymous with osteochondroma. Some sources consider the two terms to mean the same thing even without qualifiers, but this interpretation is not universal.