Benign developmental aberration characterized by a cartilage capped exostosis
Aetiology
Arises from the herniation & separation of a segment of epiphyseal growth plate cartilage through the periosteal bone cuff that normally surrounds the plate
Can arise after radiotherapy
Epidemiology
Usually presents in the first two decades of life, 48% in the second decade
M>F 1.5:1
8% of tumours but actual incidence much higher because many are asymptomatic
Pathology
Gross pathology
Base of lesion has a rim of cortical bone with central cancellous bone continuous with the underlying normal shaft
The cartilage cap varies considerably in thickness but is normally 2-3mm in thickness
Secondary chondrosarcomas are usually at least 2cm in thickness
A cartilage cap in excess of one cm thickness or 5cm diameter is suggestive of malignancy
Cystic change within the cartilage cap is cause for concern
The cartilage cap thins as the patient ages
Histology
Towards the base of the lesion the chondrocytes line up in columns simulating the appearance of the epiphyseal plate, & there is maturation into trabecular appearing bone
Spindle cell differentiation should suggest the diagnosis of parosteal osteosarcoma
Clinical
Presents as pain or a mass
There can be a clicking or inflammation of tendons running over the mass
May be an overlying bursa, which can be confused with a secondary chondrosarcoma
Usually occurs in long bones, particularly around the knee (distal femur, proximal tibia & the proximal humerus). Can occur in any bone that undergoes enchondral ossification
Uncommon in the bones of the hand
Usually occurs in the metaphysis
After adolescence & closure of the growth plate there is usually no further growth
Further growth at this stage could herald malignant change & the lesion should be excised
Malignant change probably occurs in 1%
Investigations
Xrays
Flattened (sessile) or stalk like (exostosis) protuberance in a juxta-epiphyseal location (i.e. in the metaphysis)
The protuberance normally points away from the joint
The cortex of the osteochondroma is contiguous with the normal cortex, & the medulla is contiguous with the medulla of the host bone
Extensive calcification with radiolucent irregularities of the cap implies possibly malignant change.
Treatment
Surgical excision flush with the host bone is indicated if the lesion is painful, unsightly, producing disability or may be undergoing malignant change.
Recurrence occurs in only around 2% & suggests that the original tumor was a chondroma
