Myositis ossificans

Soft Tissue
Fibroblastic / myofibroblastic tumors
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

Page views in 2019: 8,403

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Table of Contents

Cite this page: Wimmer D. Myositis ossificans. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/softtissuemyositis.html. Accessed May 30th, 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

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

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