A unicameral cavity filled with clear or sanguineous fluid and lined by a membrane of variable thickness, which consists of loose vascular connective tissue showing scattered osteoclast giant cells and sometimes areas of recent or old hemorrhage or cholesterol clefts.

It is also called solitary bone cyst and unicameral bone cyst.

The exact cause of a simple bone cyst is elusive.

Mirra hypothesized intraosseous synovial cyst in which a small amount of synovial tissue becomes entrapped in an intraosseous position during early infant development or secondary to trauma at birth.

Jaffe and Lichtenstein postulated that cyst results from a localized failure of ossification in the metaphyseal area during the period of rapid growth.

Cohen proposed cause of cyst was blockage of circulation and drainage of interstitial fluid in rapidly growing bone.

Cyst fluid itself may be both a factor causing cyst formation and an obstacle to healing. Bone resorption factors such as prostaglandin, interleukin 1 and lysosomal enzyme are found in the cyst fluid

A simple bone cyst almost always occurs during the first two decades of life and most often between 4 and 10 years of age.

Boys are more affected by ratio 2:1.

The majority of cysts are located in the metaphyseal region of the proximal humerus and femur.

Clinical presentations are as follows –

Asymptomatic cyst, which is diagnosed as an incidental finding on the radiograph.

More often presents with mild pain, reflective of microscopic pathological fracture.

More abrupt presentation with severe pain, due to pathologic fracture.

Often plain radiograph of the affected part is enough to make the diagnosis. The radiographic finding is characterized by a well outlined radiolucent metaphyseal lesion. The fallen fragment sign is pathognomonic of a simple bone cyst.

However, on rare occasions, computed tomography and magnetic resonance imaging are needed to evaluate the lesion.

Overtreatment should be avoided in the skeletally mature individuals, the sufficiently thick cortex is present, and when the lesion is located in the upper extremity. In these cases, a watchful observation is enough.

More aggressive treatment is indicated in young children and when a cyst is located in the weight-bearing bone of lower extremity.

Various treatment modalities include corticosteroid injection, autologous bone marrow injection, multiple drill hole and drainage of the cavity, and curettage of the membranous wall followed by bone grafting or bone substitute with intralesional bisphosphonates.

The progressive healing and resolution are seen in 60 to 70% of lesion.

Partial healing is seen in 20 to 30% of cases.

No healing or recurrence is seen in 10 to 20% of cases.