Characteristics

• Solitary or unicameral bone cysts occur in childhood, typically affecting the metaphysis of a long bone, most commonly the proximal humerus and femur.

• Simple cysts are not tumours and often heal spontaneously.

• Contain straw-coloured fluid and are surrounded by a flimsy fibrous tissue membrane which contain giant cells.

• In actively growing cysts there will be a zone of bone resorption ‘behind’ the cyst.

• M:F = 3:1.

Clinical features

• Often present with fracture through a cyst.

• Otherwise asymptomatic.

Radiological features

• Well-circumscribed (metaphyseal) radio-lucency, with a fine sclerotic border (narrow zone of transition), and long axis parallel to long axis of host bone.

• Often extends to the physis, associated cortical thinning and expansion of the metaphysis.

• A gravity-dependent fragment may be seen within the cyst after fracture (fallen fragment sign).

Management

• If asymptomatic (incidental diagnosis), observe with repeat X-rays. Reassure parents and warn about fractures – avoid contact sports.

• Actively growing cysts – aspiration and steroid injection. If the cyst still enlarges or fractures, then curettage and bone (or substitute) grafting.

• Fracture – treat the fracture non-operatively and the cyst is likely to fill in as the fracture heals.

• For recurrent fractures, curettage, grafting +/− stabilisation is the treatment of choice.