The aneurysmal bone cyst (ABC) is a cystic lesion that usually occurs in individuals in their second decade of life and can affect any bone in the body. Even though it is benign, the ABC can be dangerous and can lead to substantial weakening of bone structure and impinge on the surrounding tissue.
World Health Organization has defined the ABC as a benign tumor-like lesion. It is defined as "an expanding osteolytic lesion consisting of blood-filled spaces of different size seperated by connective tissue septa with trabeculae or osteoid tissue and osteoclast giant cells." Even though ABC is a benign lesion, it can grow rapidly and can be a destructive lession. The expanding nature of the cyst can cause symptoms like pain, swelling, deformity, fractures of the growth plates, neurological symptoms (depending on location) and a pathological fracture.
- Incidence: ABC is generally considered rare, only 1-6% of all primary bone tumors. The actual incidence is difficult as many cases regress spontaneously and many cases are clinically silent.
- Age: ABC has been the second most common tumor or a tumor-like lesion in children. While ABC can occur in people of all age groups, it is usually affects the younger age groups, with 70-86% of patients younger than 20 years.
- Sex: A slightly greater incidence is seen in women as compared to males.
Most researchers believe that ABC is the result of a vascular malformation in the bone, but the ultimate cause of the anomaly is still controversial. Three theories are proposed and they are as follows:
ABC can be a result of a secondary reaction to another bony lesion. This theory was proposed because of the high incidence of other tumors in 23-32% of the cases of ABC. Giant cell tumors are commonly the accompanying tumors. However, many other benign and malignant tumors may be present as an accompanying condition. These ABC accompanied with of another tumors are called secondary ABC. Treatment of secondary ABC is based on what is appropriate for the underlying tumor.
ABC can occur without evidence of other lesions and they are classified as primary ABC.
ABC can occur in an area of previous injury.