A bone cyst is a non-cancerous, fluid-filled or soft tissue-filled sac that develops within a bone. These lesions are typically benign and do not spread to other parts of the body. Often, bone cysts are discovered by chance during imaging tests for unrelated medical conditions. They represent an area within the bone that has not fully developed into solid bone tissue.
Types of Bone Cysts
Unicameral bone cysts (UBCs) and aneurysmal bone cysts (ABCs) are two of the most common types of bone cysts. Unicameral bone cysts, also known as simple bone cysts, are fluid-filled cavities that generally contain a single chamber. These cysts frequently appear in the long bones of children and adolescents, often near a growth plate, and may resolve on their own over time.
Aneurysmal bone cysts are characterized by multiple blood-filled compartments within the bone. These cysts tend to be more aggressive in their growth and can cause localized pain and swelling. ABCs are more commonly observed in adolescents and young adults, and unlike UBCs, they can occur in nearly any bone in the body, including the spine.
How Bone Cysts are Discovered
Bone cysts frequently do not cause noticeable symptoms and are often found incidentally during imaging for other health concerns. However, some individuals may experience pain or swelling if the cyst has weakened the bone enough to cause a pathological fracture—a break in an already compromised bone.
When a bone cyst is suspected, doctors use various imaging techniques for diagnosis. X-rays are typically the initial step, providing clear images of the cyst’s location and characteristics. A computed tomography (CT) scan offers a comprehensive view of bone integrity, while magnetic resonance imaging (MRI) is valuable for assessing soft tissue involvement and cyst contents, helping differentiate types.
Why Bone Cysts Form
The precise reasons why bone cysts develop are often not fully understood. For unicameral bone cysts, the exact cause is largely unknown, often described as idiopathic. One prevailing theory suggests that UBCs might arise from developmental anomalies within the bone or from localized disturbances in blood flow, such as a blockage in a vein, leading to fluid accumulation.
Aneurysmal bone cysts also have an unclear origin, but theories include localized vascular malformations or reactive processes within the bone. In some instances, an aneurysmal bone cyst may form secondary to another pre-existing, typically benign, bone lesion. Despite ongoing research, a definitive single cause for all bone cysts has not yet been identified.
Managing Bone Cysts
Managing a bone cyst depends on its type, size, location, and whether it causes symptoms or weakens the bone. Small, asymptomatic cysts may be observed with periodic X-rays. Many unicameral bone cysts in children can resolve spontaneously.
If treatment is necessary, non-surgical options include injecting corticosteroids directly into unicameral bone cysts to promote healing. For significant or symptomatic cysts, surgical intervention is often pursued. This typically involves curettage, where the cyst lining is scraped out, followed by bone grafting to fill the cavity and encourage new bone growth. Recurrence is possible after treatment, especially for aneurysmal bone cysts or in younger patients.