Simple Bone Cyst

  • Unicameral Bone Cyst
    • misleading term because some of these lesions may consist of several multilocular cavities

Definition

  • typically a unilocular cystic lesion filled with serous fluid found in a major long bone in skeletally immature patients

History

  • First recognized by Virchow

Epidemiology

  • Male>female 2:1
  • 80% of cases between 3 & 14, & uncommon after 20

Aetiology

  • Theories
    • Virchow felt that SBC were formed by central cystic softening of an enchondroma.
      • describes these cysts as developmental anomalies of the physis where there is a transient failure of ossification of physeal cartilage & cyst formation
    • Localized bleed
      • results in an intramedullary haematoma & localized venous stasis with development of a cyst

Localization

  • Proximal humerus 60-70%
  • Femur 15%
  • Pelvic cysts are seen in older adult patients.
  • Calcaneal cysts are also seen in adults rather than adolescents

Clinical

  • Pain
  • Pathological fracture
  • Swelling or deformity on occasion
  • Most are asymptomatic & go unrecognized

Investigations

Xray

  • Central lucency within the medullary cavity of the shaft of major long bone
  • Narrow zone of transition with well defined sclerotic margins
  • Cortex is not disrupted unless there is a pathological fracture & the lesion doesn’t extend into the soft tissues.
  • In a pathological fracture a segment of cortex may break off & drop into the distal part of the lesion, constituting the fallen fragment sign
  • Extension into the epiphysis can occur in skeletally mature patients
  • After resolution of a cyst there may be a residual disorganized pattern to the trabeculae

CT & MRI

  • will confirm the extent of the lesion & its cystic nature, with a very bright signal on T2 weighted images & low intensity on T1 weighted images

Pathology

Gross pathology

  • fluid is yellowish/serous, similar to serous effusions in other cavities
  • Usually consists of a single cyst but may be multilocular
  • Wall consists of paper thin, browny yellow fibrous tissue
  • pressure within the cyst is high

Microscopy

  • walls consist of fibrous tissue without a lining
  • fibrous tissue may contain giant cells, dilated vessels & scattered inflammatory cells

Classification

Neer's Classification of Simple Bone Cysts
Type Description
A: Active
  • usually juxta-epiphyseal
B: Inactive
  • cyst lies nearer the diaphysis (is more than 2cm removed from the physis)
  • Inactive cysts are less aggressive & more amenable to treatment

Treatment

  • Pathological fractures
    • should be allowed to heal prior to any regime of injections
    • Options
      • Closed
      • Flexible intramedullary nails
        • which may allow earlier movement & make casting unnecessary
  • Injections
    • Injection of methylprednisolone
      • successful in 90% of cases & can be repeated if required
      • described by Campanacci & Scaglietti
      • Typically 2 to 6 injections are required
    • Injection of iliac crest bone marrow
      • reported by Lokiec & Wientraub
      • 84% rate of healing with repeated punctures & iliac crest bone marrow
    • Injection of steroid & bone marrow injections
      • showed no benefit of bone marrow injections over steroid injections
    • Injection with demineralized bone matrix & bone marrow
  • Cannulated screw
    • allows continuous drainage of the cyst by leaving a cannulated screw protruding from the lesion
  • Bone Graft
    • Expanding lesions in weight bearing bones can be curetted & packed with bone graft
    • recurrence rate of 18-36%

Prognosis

  • Spontaneous healing may occur after pathological fracture, but is often incomplete & is also often complicated by repeated fracture
  • Growth arrest & coxa vara may result from the cyst or after curettage & grafting
  • Very rare malignant change noted, including to a chondrosarcoma
  • Simple cysts tend to spontaneously resolve in the late teens & rarely persist into adulthood
Return to top