Fibrous Dysplasia

Definition

  • Disease characterized by the replacement of normal bone & marrow by fibrous tissue & irregular minute bone spicules
  • Fibrous dysplasia, precocious puberty & café-au-lait patches form the McCune-Albright syndrome

Epidemiology

  • Patients tend to be in the second or third decade.
  • More common in females
  • Polyostotic fibrous dysplasia is much more common in females

Site

  • Most common site is the proximal femur
  • ribs & jaw are commonly involved. Involvement of the jaw may result in leontiasis & cherubism
  • lesion is typically intramedullary & diaphyseal

Aetiology

  • Failure of skeletal maturation, not hereditary

Clinical features

  • Many patients are asymptomatic
  • Patients may develop deformities,
    • particularly with polyostotic fibrous dysplasia
    • such as the classic shepherd’s crook deformity of the proximal femur
    • Fractures develop on the tension side of affected bones
  • Associations
    • Hyperthyroidism
    • hyperparathyroidism
    • hypophosphataemic rickets
    • osteomalacia

Investigations

Xray

  • Well demarcated zone of rarefaction, often surrounded by a rim of sclerotic bone
  • Expansion with thinning of the cortices is particularly likely in thin bones such as the ribs
  • There may be a hazy appearance, classically called a ground glass appearance

MRI

  • Low T1, variable T2. Cartilage is sometimes seen which will lead to ↑ T2

Bone scan

  • may be ↑ or ↓

Laboratory findings

  • Increased alkaline phosphatase

Pathology

  • Composed of dense fibrous tissue, surrounding bony trabeculae arranged in a meaningless fashion, known as Chinese alphabet.
  • Collections of foam cells forming little islands amongst the fibrous tissue are almost always seen
  • Giant cells may also be seen

Treatment

  • Usually expectant.
    • lesions tend to stop growing at skeletal maturity
    • however reactivation may occur during pregnancy or during estrogen therapy
  • Surgery
    • Large lesions in weight bearing bones need curettage & grafting, often with internal fixation
    • use of cortical allograft should be considered, because it provides structural support & is slower to resorb
  • Sarcomas may arise from fibrous dysplasia