Describe Simple & Aneurysmal Bone Cysts in detail.

Two Cystic Lesions

Two common benign cystic lesions of bone in the young must be distinguished: the simple (unicameral) bone cyst and the aneurysmal bone cyst (ABC).

Clinical Features & Management

A simple bone cyst is often symptomless until it presents with a pathological fracture (the ‘fallen fragment’ sign — a piece of cortex lying dependently in the fluid — is diagnostic). Many heal after fracture or respond to steroid/marrow injection or curettage and grafting. An ABC is a rapidly expansile, locally destructive blood-filled lesion causing pain and swelling; it is treated by curettage (with adjuvants) and grafting, sometimes with embolisation, and can recur. Secondary ABC change can overlie another tumour, so histology is important.

Behaviour & Pitfalls

The simple bone cyst is a self-limiting lesion that tends to migrate away from the physis and often heals with growth, so an incidental asymptomatic cyst may simply be observed; treatment (steroid or marrow injection, or curettage and grafting) is directed at painful or fracture-prone lesions, and recurrence is not unusual in young children. The aneurysmal bone cyst is more aggressive and expansile and can arise secondarily within another lesion (e.g. GCT, chondroblastoma, osteosarcoma), so representative tissue must be examined to avoid missing an underlying tumour.

Fallen leaf sign indicates pathological fracture of a simple cyst.

• Simple bone cyst: central, fluid-filled, proximal humerus/femur; ‘fallen fragment’ sign.
• ABC: eccentric, markedly expansile, blood-filled; fluid–fluid levels on MRI.
• Both present in the young, often with pathological fracture.
• Treat by curettage ± adjuvants/grafting (± injection for simple cyst); ABC can recur.