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Myositis Ossificans (Sterner’s Tumor)

- See:
      - Heterotopic Ossification
      - Ossification of Soft Tissue:

- Discussion:
    - extra-skeletal ossification that occurs in muscles & other soft tissues
    - can be traumatic (75%), non traumatic, or progressive;
    - severe blunt trauma causing an intra-muscular hematoma may result in delayed ossification;
    - involved muscles: quadriceps, brachialis, and deltoid are involved most often;
    - typically lesions are distant from joints, have decr pain w/ time;
    - typically occurs in adolescents as a painless, enlarging mass in upper arm, thigh, or buttocks;
    - associated with an intact cortex;
    - demonstrate a zonal pattern;
    - histology:
           - microscopic, hypercellular spindle cells, peripheral woven bone, surrounding diseased & often trapped muscle, and normal osteoblasts making
                      bone are typical;
           - zonation from central immature fibrous tissue to peripheral mature bone is characteristic, and helps distinguish myositis ossificans from osteosarcoma,
                      where the central bone is likely to be mature;
    - diff dx:
           - osteosarcoma:
           - proliferative myositis

- Radiographs:
    - soft tissue ossification not attached to bone is common;
    - x-rays show round mass w/ distinct peripheral margin of mature ossification & a radiolucent center of immature osteoid & primative mesenchymal tissue;
          - this peripheral maturation, reverse of that seen in a malignant tumor, is characteristic of myositis ossificans

- CT Scan:
    - calcification of the herterotopic bone proceds from the outer margin and progresses centrally;

- Bone Scan:
    - active myositis ossificans appears as intense para-osseous accumulation of tracer activity in acutely damaged muscle on delayed images;

- Prognosis:
    - over time, the volume of heterotopic bone will diminish;

- Managment:
    - indomethacin

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