Soft tissue
Fibroblastic / myofibroblastic
Myositis ossificans
Authors: Daniel Wimmer, M.D., Jerad M. Gardner, M.D.
Topic Completed: 1 June 2013
Minor changes: 14 December 2019
Copyright: 2002-2018, PathologyOutlines.com, Inc.
PubMed Search: Myositis ossificans [title] soft tissue
Table of Contents
Definition / general | Terminology | Epidemiology | Sites | Radiology description | Case reports | Treatment | Clinical images | Gross description | Gross images | Microscopic (histologic) description | Microscopic (histologic) images | Cytology images | Positive stains | Electron microscopy description | Molecular / cytogenetics description | Differential diagnosisCite this page: Wimmer D. Myositis ossificans. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/softtissuemyositis.html. Accessed August 6th, 2020.
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
- 9 1/2 year old boy with pain in his left proximal medial thigh (University of Pittsburg Case 72)
- 10 year old girl with myositis ossificans progressiva (Internet Journal of Orthopedic Surgery 2009;12(1))
- 11 year old boy with myositis ossificans circumscripta (Eur J Pediatr 2009;168:523)
- 33 year old man with pain and swelling of right upper thigh post trauma (Case of the Week #449)
- 46 year old man with post-traumatic myositis ossificans of the sternocleidomastoid (Cases J 2008;1:413)
- Adult autopsy case with marked myositis ossificans (Leg Med (Tokyo) 2008;10:274)
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
Image hosted on PathOut server, courtesy of Dr. Sajna VM Kutty:
COW #449
Images hosted on other servers:
Bone fragment from abdominal wall
Microscopic (histologic) 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
Microscopic (histologic) images
Images hosted on PathOut server:
COW #449, courtesy of Dr. Sajna VM Kutty
Courtesy of Dr. Mark R. Wick
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:
Various images of thigh lesion
Cytology images
Images hosted on other servers:
Hip mass with FNA
Positive stains
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
- Usually polyclonal (Virchows Arch 2005;446:438), but some cases have clonal USP6 rearrangements, and may be better classified as soft tissue aneurysmal bone cysts (Skeletal Radiol 2008;37:321, Cancer Res 2004;64:1920)
- Progressiva variant due to mutations in ACVR1 and NOG genes (Genet Couns 2009;20:53)
Differential diagnosis
- Extraosseous osteosarcoma: rare, age 40+ years, different radiologic findings, malignant cytology, atypical mitotic figures, no or reverse zonation, may have necrosis
- Juxtacortical osteosarcoma: bone tumor, no zonation
- Osseous muscle metastasis: see AJR Am J Roentgenol 2001;176:1165
