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Myositis Ossificans / Heterotopic Ossification

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 Symtoms

  • Most asymptomatic
  • Painful 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 System - Radiographic appearance of AP pelvis
      • Class I
        • isolated islands of bone
      • Class II
        • Bone spurs with a gap of at least 1 cm between opposing bone surfaces
      • Class III
        • Near complete bone bridging (gap < 1 cm)
      • Class IV
        • Apparent ankylosis

    Severe Type

    • Fibrodysplasia ossificans progressiva (myositis ossificans progressiva)
      • Rare condition
      • Heterotopic bone mass replace muscle tissue & restrict 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 2o 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
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