Soft Tissue
Fibroblastic / myofibroblastic tumors
Myositis ossificans

Author: Daniel Wimmer, D.O. (see Authors page)
Editor: Jerad M. Gardner, M.D.

Revised: 26 August 2016, last major update June 2013

Copyright: (c) 2002-2016, PathologyOutlines.com, Inc.

PubMed Search: Myositis ossificans [title] soft tissue

Definition / General
  • Focal intramuscular lesion composed of cellular fibrous tissue and metaplastic bone
  • See also Bone chapter
Terminology
  • Also called benign fibro-osseous lesion or heterotopic ossification (perhaps more accurate terms since may not necessarily involve muscle or inflammation)
  • In subcutis, called panniculitis ossificans; in fascia or tendons, called fasciitis ossificans
  • Various forms: circumscripta (localized, either traumatic or atraumatic) or progressiva (also called fibrodysplasia ossificans progressiva, an autosomal dominant inherited disorder with progressive crippling ossification involving muscle groups over time, eMedicine)
  • Related entities:
    • Fibro-osseous pseudotumor of digits: similar entity to myositis ossificans but more superficial and distal; affects fingers of older adults with occupations that require repetitive manual use (Ann Diagn Pathol 2008;12:21), less likely to contain fibrinous material (Int J Surg Pathol 2003;11:187)
    • Panniculitis ossificans: similar histologic features but involves subcutis rather than muscle
Epidemiology
  • Usually physically active young males with rapid growth of mass
  • 60 - 75% have history of trauma in prior 4 - 6 week
  • May also occur after elective surgery, severe burns, neurological injury
Sites
  • Upper extremity flexors, quadriceps, thigh adductors, gluteal muscles, soft tissues of hand
Radiology Description
  • Xray: periosteal reaction with eggshell calcification at periphery 3 - 6 weeks after injury; recommended to review Xrays before diagnosis
Case Reports
Treatment
  • Excision, although may regress without treatment; rarely recurs if incompletely excised
Clinical Images

Images hosted on PathOut servers:

CT scan (left two) and MRI (right two), courtesy of Mark R. Wick, M.D.



Images hosted on other servers:

Various images

Gross Description
  • Well circumscribed, soft center, gritty periphery, usually 3 - 5 cm (but may be as large as 15 cm)
Gross Images

Images hosted on other servers:

Bone fragment from abdominal wall

Micro Description
  • Cellular stroma with new bone, atypia and mitotic figures, rarely cartilage; zonation often present, although zones may be poorly demarcated
  • Early lesions (3 weeks): inner cellular zone resembling nodular fasciitis with short fascicles or haphazard fibroblasts that are uniform with faint eosinophilic cytoplasm, tapering processes, vesicular or finely granular nuclei and variable nucleoli, usually numerous mitotic figures but none atypical; stroma is vascular, myxoid or edematous with extravasated red blood cells, fibrin, scattered inflammatory cells and osteoclast-like giant cells; if highly cellular, may mimick sarcoma such as osteosarcoma; intermediate zone has osteoblasts depositing woven bone, and outer zone has mineralized trabeculae
  • Later: bone matures with formation of marrow and myofibroblasts are less prominent
Micro Images

Images hosted on PathOut servers:

Courtesy of Mark R. Wick, M.D.



AFIP Images:

Fibroblastic and myofibroblastic cells in center of lesion resemble nodular fasciitis, with nuclear uniformity

Osteoid is broader than in osteosarcoma

Osteoid undergoing mineralization



Mature bone at periphery, fibrous tissue resembling nodular fasciitis at center, osteoid in between

Osteoblasts have large
nuclei with prominent
nucleoli, but clues to benign
nature of lesion are
reactive fibrous areas

Osteoid deposition is
uniform, and nuclei are
enlarged but not pleomorphic



Images hosted on other servers:

2x

10x

40x

Various images of thigh lesion

Cytology Images

Images hosted on other servers:

Hip mass with FNA

Positive Stains
  • Fibroblasts and myofibroblasts express vimentin and variable actin and desmin, osteoclasts express vimentin
Electron Microscopy Description
  • Fibroblasts and myofibroblasts have dilated rough endoplasmic reticulum and aggregates of cytoplasmic filaments variably associated with dense bodies; osteoblasts have numerous mitochondria and abundant dilated rough ER
Molecular / Cytogenetics Description
Differential Diagnosis