Simple Bone Cyst (Juvenile)

Simple bone cyst (also called unicameral or juvenile cyst) is a benign lesion characterized by a single cavity filled with serous fluid within the bone.

It is one of the most common benign bone lesions in children, with peak incidence between 5 and 15 years and male predominance (2:1 ratio).

Most common locations:

• Proximal humerus (near the shoulder): approximately 50% of cases
• Proximal femur (near the hip): approximately 25% of cases
• Less commonly: tibia, calcaneus, radius

Pathogenesis: The exact cause is not yet completely understood. The most accepted theory is a local blockage of venous drainage with accumulation of interstitial fluid that progressively erodes the bone. The cyst grows slowly, thinning the bone cortex.

Classification:

• Active cyst: located near or in contact with the growth plate (physis). Tends to grow and has higher recurrence risk after treatment.
• Latent (inactive) cyst: separated from the physis by normal bone. Tendency toward spontaneous resolution with skeletal maturity.

The "fallen fragment sign": Pathognomonic radiographic sign visible when the cyst fractures: a fragment of cortical bone "falls" to the bottom of the cystic cavity, confirming the liquid nature of the contents.

Main risk: Pathological fracture, which can occur even from minor trauma or during normal sports activities. Paradoxically, the fracture sometimes stimulates spontaneous cyst healing.