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Myositis Ossificans (M.O) / Hetrotropic Ossification

  1. Definition:
    It is hetrotropic calcification and ossfication in muscle tissue. The name is misnomer as there is no myositis (inflammation of muscle) and rarely ossflcation in the muscle (because the mineral phase differs from that in bone and no true bone matrix is formed).
  2. Etiology
    1. Injury (trauma) is an important factor and m.o. associated with trauma is called as post traumatic ossification it is seen in Elbow > hip
    2. Trauma around elbow eg. fracture supracondylar humerus, dislocation or surgery.
    3. Surgical trauma specially total hip replacement,
    4. Massage to the elbow and vigorous passive stretching to restore movements is aggravating factor.
    5. Repeated microtrauma and overuse injuries. This type usually involve soleus muscle in ballerians.
    6. It occurs in muscles which are vulnerable to tear under heavy loads, such as quadriceps, adductors, brachialis, biceps, and deltoid,
  3. Pathogenesis

    1. Bone formation in muscle represents metaplasia of fibroblast at the site of injury
    2. Partial rupture & avulsion of muscle, ligament & joint capsule from bone usually cause periosteal elevation with the formation of subperiosteal hemato,na and the wide exposure of cells with osteogenic potential. It inevitably ,it subperiosteal ossification, It has four microscopic zones (Ackerman Zonation) with centre most zone of undifferentiated, highly active (mitotic) cells and the outermost zone of well oriented bone encapsulated by fibrous tissue.
    3. It must be distinguished from extaskeletal osteosarcoma. The latter usually occur in elderly, lacks zonation, and the most peripheral regions of osteosarcoma are most cellular & primitive, which is reverse of myositis ossificans.
  4. Clinical features
    1. Mostly asymptomatic but may present with tenderness, palpable swelling, pain on range of motion, stiffness and increased warmth
  5. Imaging  
    1. Radiographs initially are normal, but by 10 days to 4 weeks fine calcification (dotted veil/cotton wool appearance) is seen.
  1. There is peripheral ossification and central lucency of the mass.
  2. The mass is usually seperated from underlying bone by at least a thin line & lesion are usually located in the diaphysis. if the lesion is in continuity with the bone it is not myositis ossificans and the possibility of tumor or infection arises.
  3. CT is better than X- ray
  1. Treatment of Myositis Ossificans
    1. In acute phase the treatment consist of limiting motion.
    2. Passive stretching and not active exercise is responsible for stimulating new growth formation. Passive exercises thus are contraindicated.
    3. Low dose irradiation, Bisphosphonates & indomethacin prevents hetrotopic ossification.
    4. Surgical excision in toto is not done until a year or 2 after the acute phase of disease, at a time when radiograph reveal that the bone is fully mature and bone scan show either a return to normal uptake or decreasing activity in the lesion.

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