Bone Cyst

Types of bone cysts

Simple bone cysts

These are the most common bone cysts. They usually sit near the growth plate of a long bone in a growing child. As the bone grows, the cyst may move away from the growth plate. Many shrink or fill in after the teenage years. Their exact cause is unknown; a blockage of fluid drainage within bone is one theory.

Aneurysmal bone cysts

Despite the name, these are not aneurysms. They are expansile lesions filled with blood-filled spaces. They can be primary or arise next to another bone tumor. Most primary aneurysmal bone cysts involve rearrangements of the USP6 gene. They can grow quickly and erode bone, so they usually need treatment.

Other cyst-like bone lesions

• Intraosseous ganglion: a small cyst near a joint, related to ganglion cysts of soft tissue.
• Subchondral cysts: small cysts under the cartilage in osteoarthritis.
• Epidermoid cysts: in the fingertip bones or skull after injury.

Several other bone conditions can look like cysts on X-ray, including fibrous dysplasia, non-ossifying fibromas, giant cell tumors and, rarely, cancers.

Symptoms

• No symptoms in many cases.
• Dull bone pain or aching, sometimes worse with activity.
• Swelling or a lump over the bone, more typical of aneurysmal bone cysts.
• A limp or reduced use of an arm or leg in a child.
• A fracture after a minor fall or twist. This is how many simple cysts in the upper arm are discovered.
• Back pain or nerve symptoms with aneurysmal bone cysts in the spine.

Diagnosis

• X-ray: a simple cyst appears as a well-defined, central, clear area with thinned bone around it. A small piece of fractured bone lying at the bottom of the cyst, called the "fallen fragment" sign, is characteristic.
• MRI: shows fluid. In aneurysmal bone cysts, multiple compartments with fluid-fluid levels are typical.
• CT: helps assess fracture risk and spine lesions.
• Biopsy: recommended for aneurysmal bone cysts and any lesion that looks atypical, to exclude a rare cancer such as telangiectatic osteosarcoma, which can mimic it.

Treatment

Observation

Small simple cysts in non-weight-bearing bones, with plenty of surrounding bone, can be watched with repeat X-rays every few months. Children may be advised to avoid contact sports while the bone is weak.

Fracture care

A fracture through a simple cyst in the upper arm is treated with a sling or cast. The fracture usually heals in 6 weeks, and occasionally the cyst heals as well, though most persist.

Procedures

• Aspiration and injection: draining the cyst and injecting steroid, bone marrow, or bone graft substitute to stimulate healing. Several injections may be needed.
• Curettage and bone grafting: scraping out the cyst lining and filling the cavity with bone graft or substitute. Often combined with internal fixation (rods or screws) in the femur, where a fracture is more serious.
• For aneurysmal bone cysts: curettage with extra techniques to reduce recurrence (high-speed burr, cryotherapy or chemical agents); sclerotherapy injections; embolization for spinal or pelvic lesions; and in some resistant cases, the medicine denosumab.

Complications

• Fractures, the most common complication.
• Growth disturbance if the cyst or treatment involves the growth plate, causing leg-length difference or angulation.
• Recurrence after treatment, more common in younger children and lesions near the growth plate.
• Spinal cord or nerve compression from spinal aneurysmal bone cysts.

Outlook

Bone cysts are not cancer and do not spread. Most simple cysts heal eventually, often by skeletal maturity, and most aneurysmal bone cysts are cured with treatment, though recurrence can require repeat procedures. Children usually return to full activity once the bone has strengthened.

Frequently asked questions

Is a bone cyst cancer?

No. Simple and aneurysmal bone cysts are benign and do not spread. Because a few rare cancers can look similar on imaging, doctors may recommend a biopsy for atypical or aneurysmal cysts to be certain.

What causes bone cysts in children?

The exact cause is unknown. Simple cysts may form when fluid drainage in a growing bone is blocked. Most aneurysmal bone cysts have a specific gene rearrangement. They are not caused by injury, diet or anything a parent did.

Do bone cysts go away on their own?

Some simple bone cysts heal on their own, especially after a fracture or as the child stops growing. Aneurysmal bone cysts rarely go away without treatment. Your orthopedic surgeon will advise whether to watch or treat based on size, location and fracture risk.

Can my child play sports with a bone cyst?

It depends on the size and site of the cyst. Children with large cysts or cysts in weight-bearing bones may be advised to avoid contact and high-impact sports until treated and healed. Ask your orthopedic surgeon about specific activities.