Myositis Ossificans

Courtesy: Prof Nabile Ebraheim, University of Toledo, Ohio, USA

Introduction

• Myositis ossificans is currently more accurately termed heterotopic ossification.

• It is a reactive, non-neoplastic lesion occurring in soft tissues.

• The condition is characterized by fibrous tissue proliferation, followed by cartilaginous and osseous metaplasia.

• The entity was first described in 1883 by Riedel.

• The terminology is derived from Greek:

  • Hetero meaning other

  • Topos meaning location

  • Ossificans meaning bone formation

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Types of Heterotopic Ossification

• Genetic forms (rare)

• Non-genetic forms (most common)

Classification Based on Etiology

• Neurogenic heterotopic ossification

• Traumatic heterotopic ossification

• Fibrodysplasia ossificans progressiva (also known as Munchmeyer disease)

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Classification Systems

Brooker Classification

1. Presence of isolated islands of bone within soft tissues

2. Bone spurs arising from the pelvis or proximal femur with a gap greater than one centimeter between opposing surfaces

3. Bone spurs with opposing surfaces separated by less than one centimeter

4. Complete bony ankylosis of the joint

Della Valle Classification

• Includes the first three grades of the Brooker classification

• Does not include complete ankylosis

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Etiology and Pathogenesis

• Heterotopic ossification represents aberrant tissue repair following surgery or traumatic insult.

• The process may be triggered by immobilization or external stimuli.

• Injury to soft tissues around a joint or long bone leads to:

  • Influx of inflammatory cells

  • Release of signaling molecules

  • Activation of mesenchymal stem cells

• These cells undergo osteogenic or osteochondrogenic differentiation.

• Bone formation occurs through membranous or endochondral ossification pathways.

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Anatomical Locations

Heterotopic ossification may develop in:

• Skeletal muscles

• Fascia

• Tendons

• Ligaments

• Subcutaneous tissue

• Skin

• Vessel walls

• Any site containing connective tissue

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Epidemiology

• Most commonly affects young adults.

• Male to female ratio is approximately 3:2.

• Frequently associated with a prior insult such as:

  • Trauma

  • Surgery

  • Repeated massage

Common Associated Conditions

• Hip arthroplasty (approximately 40 percent)

• Bone fracture or joint dislocation (approximately 30 percent)

• High-energy extremity trauma

• Traumatic brain injury

• Spinal cord injury (up to 50 percent)

• Neurological disorders

• Severe burns

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When to Suspect Heterotopic Ossification

• History of trauma, surgery, or forceful massage

• Progressive stiffness around a joint

• Stiffness at common sites of injury such as:

  • Elbow

  • Hip

  • Pelvis

  • Knee

• Association with autoimmune conditions such as dermatomyositis or systemic sclerosis, where skin involvement is common

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Clinical Presentation

Early Inflammatory Phase

• Localized pain

• Tenderness

• Swelling

• Rapid increase in size of the lesion

Maturation Phase

• Progressive maturation of bone tissue

• Lesion becomes firm and well localized

• Increasing restriction of joint movement

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Investigations

Plain Radiographs

• No visible ossification in early stages

• Early lesions may show ill-defined soft tissue opacities without clear zonal maturation

• Mature intramuscular heterotopic ossification shows:

  • Well-demarcated radiodense mass

  • Peripheral ossification with central lucency

  • Typical “eggshell” pattern of calcification

• Lesions usually involve soft tissue alone

• When attached to the bone surface, the condition is termed parosteal heterotopic ossification:

  • Initially seen as a narrow stalk

  • Later develops a broad bony attachment with cortical continuity

• Advanced stages may show joint ankylosis

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Computed Tomography

• Best modality to demonstrate zonal maturation.

• Early lesions appear as low-density soft tissue masses.

• Follow-up imaging demonstrates peripheral ossification.

• Useful for assessing proximity to neurovascular and other vital structures.

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Magnetic Resonance Imaging

• Appearance varies with stage of maturation.

• Early lesions show:

  • Ill-defined mass

  • Heterogeneous signal intensity

  • Surrounding soft tissue edema

  • Possible fluid-fluid levels due to hemorrhage

• Contrast-enhanced imaging shows central enhancement due to vascularity.

• Mature lesions demonstrate peripheral rim enhancement.

• Key distinguishing feature:

  • Heterotopic ossification shows central vascularity with peripheral ossification

  • Malignant sarcomas show central ossification with peripheral vascularity

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Positron Emission Tomography

• Usually performed in combination with computed tomography.

• Uses radiolabeled fluoride or radiolabeled glucose.

• Radiolabeled fluoride binds to hydroxyapatite and detects early bone formation.

• Useful for identifying metabolically active lesions.

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Histopathology

• Early lesions are hypercellular with spindle-shaped cells and minimal bone matrix.

• May resemble nodular fasciitis or granulation tissue.

• Findings include:

  • Mitotic figures

  • Multinucleated giant cells

  • Inflammatory infiltrate

  • Extravasated red blood cells

• With maturation:

  • Zonal architecture becomes evident

  • Peripheral woven bone with osteoblastic rimming develops

• Gradual transition from woven bone to mature lamellar bone distinguishes heterotopic ossification from osteosarcoma.

• Mature lesions show:

  • Well-circumscribed mass

  • Fibrous pseudocapsule

  • Thick-walled blood vessels

  • Lamellar bone with fatty marrow

  • Presence of Haversian systems and Volkmann canals

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Differential Diagnosis

• Parosteal osteosarcoma

• Soft tissue sarcomas, including malignant fibrous histiocytoma

• Synovial sarcoma

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Management

Prophylaxis

• Radiation therapy

• Non-steroidal anti-inflammatory drugs

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Radiation Therapy

• Used prophylactically after hip arthroplasty in high-risk patients such as those with:

  • Ankylosing spondylitis

  • Diffuse idiopathic skeletal hyperostosis

  • Hypertrophic osteoarthritis

  • Prior history of heterotopic ossification

• Typical dose ranges between 400 and 700 centigray.

• Potential adverse effects include joint stiffness.

• Long-term oncogenic risk remains controversial and requires cautious use.

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Non-Steroidal Anti-Inflammatory Drugs

• Indomethacin administered at 25 milligrams three times daily for six weeks has been shown to reduce heterotopic ossification formation.

• Gastrointestinal irritation and gastritis are recognized complications.

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Surgical Management

• Surgery should only be performed after complete maturation of the lesion.

• Early excision increases the risk of recurrence.

• Challenges include:

  • Poorly defined tissue planes

  • Encasement of neurovascular structures

  • Incomplete excision leading to recurrence

• Full maturation may take six months or longer, during which joint ankylosis may occur.

• Acceptable functional outcomes have been reported even in ankylosed joints.

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Neurogenic Heterotopic Ossification

• First described in 1918 by Dejerine and Ceillier in patients with spinal cord injury during the First World War.

• Common sequela of spinal cord injury.

• Also associated with:

  • Traumatic brain injury

  • Stroke

  • Hypoxic encephalopathy

  • Encephalitis

  • Tetanus

  • Poliomyelitis

  • Syringomyelia

  • Burns

• Develops between three and twelve weeks after injury.

• Bony injury is not required for development.

• More common in complete spinal cord injuries.

• Occurs below the level of neurological injury.

• Hip is the most frequently affected joint.

• Often extensive and may involve multiple sites.

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Fibrodysplasia Ossificans Progressiva

• Rare, slowly progressive genetic disorder.

• Inherited in an autosomal dominant pattern, usually due to spontaneous mutation.

• Caused by mutation in the ACVR1 gene, most commonly the R206H mutation.

• Leads to excessive bone morphogenetic protein signaling and endochondral ossification.

• Presents in early childhood.

Clinical Features

• Progressive ossification of muscles, tendons, and ligaments

• Severe joint stiffness and loss of mobility

• Disease spreads from:

  • Neck and shoulders

  • Trunk

  • Limbs

• Temporomandibular joint involvement causes difficulty with eating and speech.

• Most patients become wheelchair-bound by early adulthood.

• Diaphragm, tongue, and extraocular muscles are spared.

Skeletal Abnormalities

• Congenital malformation of the great toe

• Hallux valgus deformity

• Clinodactyly

• Short and broad femoral necks

• Costovertebral joint ankylosis

• Intercostal muscle ossification

• Sensorineural and conductive hearing loss

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Atypical Fibrodysplasia Ossificans Progressiva (FOP Plus)

• Involvement of head and neck with sparse scalp hair

• Cognitive impairment and seizures

• Cranial and cerebral malformations

• Ocular abnormalities including cataracts and glaucoma

• Additional skeletal disorders

• Endocrine and hematological abnormalities

• High mortality due to restrictive lung disease, pneumonia, malnutrition, or right-sided heart failure

• Median life expectancy is approximately forty years

• Pregnancy carries high maternal and fetal risk

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Treatment of Fibrodysplasia Ossificans Progressiva

• No definitive curative treatment exists.

• Surgical excision worsens disease progression.

• Short courses of corticosteroids during flare-ups may provide limited benefit.

• Etidronate and non-steroidal anti-inflammatory drugs may help symptom control.

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Preventive Strategies

• Lifestyle modification to minimize trauma

• Avoidance of contact and impact sports

• Household safety measures to prevent falls

• Respiratory physiotherapy to prevent pulmonary complications

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Progressive Osseous Heteroplasia

• Rare genetic disorder

• Caused by inactivating mutations in the GNAS1 gene

• Inherited in an autosomal dominant pattern

• Presents with dermal ossification in childhood

• Progresses to deep connective tissue and skeletal muscle involvement

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Albright Hereditary Osteodystrophy

• Genetic syndrome with multiple systemic manifestations

• Short stature and obesity

• Rounded facial features

• Subcutaneous ossifications

• Shortening and widening of metacarpals and metatarsals, especially the fourth and fifth rays

• Temporomandibular joint ankylosis