Myositis Ossificans / Heterotopic Ossification

Myositis Ossificans / Heterotopic Ossification, radiograph of heterotopic ossification around the hip
Radiograph of heterotopic ossification around the hip. Image by Tdvorak, Wikimedia Commons, CC BY-SA 3.0.

Definition

  • Formation of mature bone outside the skeleton
  • Different from ectopic calcification which consists of calcium salt deposits (mostly calcium phosphate) in tissues such as blood vessels, cornea & supraspinatus tendon. Relatively common in THJR (b/w 20 & 90 %)

Clinical Symptoms

Most asymptomaticPainful limitation ROM in up to 7 %

Aetiology

  • Burns
  • Head injury
  • Direct trauma to soft tissues
  • Total hip replacement
  • Acetabular fractures & surgery
  • Other articular injuries e.g. elbow

Pathology

  • 4 characteristic zones in heterotopic bone
    • innermost zone
      • highly mitotic cells, possibly surrounded by dead muscle fibres
      • hard to distinguish from sarcoma
    • surrounding layers of cells
      • producing osteoid
    • zone of osteoblasts
      • new bone with trabecular organisation
      • may include osteoclasts modelling bone adjacent to host bone
    • peripheral fibrous layer

Pathogenesis

  • process of bone formation is essentially the same as fracture healing
  • Injury, haematoma, haematoma organised, converted to osteoid, bone
  • ? reason -? role of BMP & other biologically active molecules

After Hip Replacement

  • post THR incidence reported 8-90%
  • only severe grades are symptomatic 1-33%
  • Risk factors
    • Previous HO on other side(up to 90 % risk in subsequent)
    • Revision procedures
    • Male sex
    • ‘hypertrophic Osteoarthritis’
    • trochanteric osteotomy
    • DISH

Classification

Brooker classification, AP pelvis radiograph

ClassDescription
IIsolated islands of bone in the soft tissues
IIBone spurs with a gap of at least 1 cm between opposing bone surfaces
IIINear complete bridging with a gap under 1 cm
IVApparent ankylosis

Original publication Brooker AF, Bowerman JW, Robinson RA, Riley LH Jr. Ectopic ossification following total hip replacement. Incidence and a method of classification. J Bone Joint Surg Am. 1973;55(8):1629-32.

Severe Type

  • Fibrodysplasia ossificans progressiva (myositis ossificans progressiva)
    • Rare condition
    • Heterotopic bone mass replaces muscle tissue & restricts chest movement leading to early death

Differential Diagnosis

  • bone tumour

Prophylaxis

  • NSAIDs Schmidt et al
    • No HO in 85 % of treated, 25 % of untreated
    • None of HO in treated group was high-grade (III – IV)
    • Dosage still under Ix
      • 25 mg tds indocid for 6 weeks
    • Maybe 2 weeks is adequate
    • Beware usual side effects
  • Radiotherapy
    • Single dose 700 – 800 cGy within 1 – 4 days of surgery
    • Or single preop dose of 800 cGy
    • No secondary malignancy in study of 90 patients to 8 years
    • Becoming more common prophylaxis because single shot & no compliance issues

Treatment

  • Once formed, delay surgery at least 6 – 12 months before attempted excision
  • Tc bone scan to assess activity
  • Excision very effective in improving ROM
  • Poor correlation between XR appearance & clinical ROM
  • Pain relief following excision is variable
  • NB: range of motion possible is, however, often significantly greater than one would expect from X-ray appearances alone. Many class 4 cases have an entirely functional range. The ankylosis is usually only apparent – remember the system is based on AP views only
  • Excision can improve range of motion, but has less effect on symptoms

Reviewed by Professor Phong Tran, Head of Orthopaedic Surgery, Western Health. Last updated 10 October 2026.