Myositis Ossificans







Sites






  • Lower extremity: quadriceps and gluteus muscles


  • Upper extremity: brachialis muscle


Clinical Symptoms and Signs






  • First stage – pain or tenderness, hours or days after the injury:



    • Rarely localized erythema in the skin over the lesion


  • Second stage – circumscribed mass, a week or 2 later.


  • Third stage – the mass acquires a stony consistency at palpation, at third week.


  • In some patients, white blood count, sedimentation rate, and alkaline phosphatase are slightly elevated.


  • The lesion commonly develops in as short time at a week or 2. This rate of development is a diagnostic clue because sarcomas infrequently grow so fast.



Image Diagnosis



Radiographic Features



Early Stage






  • Slight increase in soft tissue density


  • Ill-defined mass


Approximately Fifth to Sixth Week






  • Floccular radiopacities – “dotted veil” pattern.


  • Cortex not involved.


  • Periosteal reaction may be seen in deeper lesions.


  • At deeper locations and with only one X-ray projection may have the false appearance of attachment to the bone.


Approximately Third to Fourth Month






  • Well-circumscribed lesion


  • More dense mineralization at the periphery


  • Less mineralized at the center


  • Zonation appearance


Mature Lesions






  • Increasingly calcified lesion


  • Smaller


  • Attached to the cortex


CT Features






  • Cortex not involved


MRI Features






  • In early stages, a nonhomogeneous mass appears.


Bone Scan






  • Hot spot


Image Differential Diagnosis



Osteochondroma






  • In sessile osteochondromas, sometimes the differential is more difficult.


  • Lower density.


  • Shows irregular calcification of the cartilage-capped surface.


  • The underlying cortex is classically discontinued, and the metaphyseal spongiosa is continuous with the tumor spongiosa.


Parosteal Osteosarcoma






  • Early MO shows:



    • X-ray: cortex not involved


    • CT: cortex not involved


  • Mature MO shows:



    • Increasingly calcified lesion


    • Smaller lesion


    • Attachment to the cortex


  • Parosteal osteosarcoma shows:



    • Densely ossified juxtacortical round or oval mass attached to the underlying cortex


    • Absent well-formed periosteal reaction


    • A frequent characteristic finding which is a radiolucent line between the cortex and the tumor, except at its site of attachment


    • In CT and MRI, lytic areas, most commonly in the surface corresponding to neoplastic cartilage


    • Occasionally, cortical thickening under the tumor


    • In advanced cases, cortical destruction and medullary invasion


Pathology



Gross Features






  • In early stages, the lesion is soft, grayish white or reddish, and hemorrhagic, with gritty calcifications. In more mature lesions, the cut surface shows two different zones: one peripheral zone, hard and with cancellous bone, and a central zone, soft grayish reddish with hemorrhagic areas and cystic zones.


  • Less than 5 cm in diameter.


  • Stony consistency, mature lesion.


Histological Features



Central Portion or Early Stage






  • Active and immature vascular fibroblastic proliferation. Plump spindle cells. Numerous mitotic figures.


  • Cellular pleomorphic areas.


  • Although very active, the lesion lacks cytological atypia.


  • Inflammatory cells, macrophages.


  • Acute inflammatory cells are not seen.


  • Multinucleated giant cells, osteoclastic type.


  • Dilated vessels.


  • Hemorrhage and fibrinous material.


  • Necrosis of the proliferated tissue is absent.


Intermediate Portion






  • Proliferating spindle cells with collagen deposition.


  • Osteoid trabeculae and ectatic vessels


  • Immature osseous trabeculae and more mature lamellar trabeculae in a loose fibrous tissue.


  • Masses of cartilage with different stages of differentiation and endochondral ossification are occasionally present.


Peripheral Portion






  • A more organized shell of woven and parallel lamellar bone.


  • In the outer layers, active osteoblasts are seen around immature small bone trabeculae.


  • The lesion is separated from the surrounding muscle by a zone of myxoid fibrous tissue.


  • The peripheral muscle shows atrophic changes and inflammatory infiltrates.


  • Mature lesion shows fatty and hematopoietic bone marrow between the lamellar trabeculae.

The clue and hallmark for diagnosis is the presence of this zoning phenomenon or zoning effect.

Subungual exostosis, Nora’s lesion, florid reactive periostitis, and fibro-osseous pseudotumor of digits are probably closely related lesions of the heterotopic ossification type.


Pathologic Differential Diagnosis


Jan 2, 2017 | Posted by in ORTHOPEDIC | Comments Off on Myositis Ossificans
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