Clinical history: 14 year old boy presents with a firm lump over back of right leg. There was no known history of trauma to the site prior to the onset of the lesion.
What is your diagnosis?
Discussion and answer
Grossly,the lesion consisted of a 4cm mass with a whitish, solid cut surface.
Focal haemorrhagic areas were also noted.
On Microscopic examination, the lesion shows a characteristic zonal pattern. The central area consists of richly vascular fibroblastic tissue resembling granulation tissue, with scattered osteoclastic giant cells, surrounded by an intermediate zone active fibroblasts, osteoblasta and varying amounts of osteoid. The peripheral zone consists of calsifying osteoid which evolves into mature lamellar bone.
The diagnosis is myositis ossificans.
It is very important to distinguish myositis ossificans from extra skeletal osteosarcoma.
This is best accomplished by observing the characteristic zonation present in myositis ossificans.