Bone Cyst

Overview

A bone cyst is a fluid-filled, hollow space that develops inside a bone. Most bone cysts are benign (non-cancerous) and occur most often in children and young adults, frequently in the long bones of the arm or leg. Because they grow slowly and often cause no symptoms, many are discovered by chance on an X-ray taken for another reason.

The two main types are simple (unicameral) bone cysts, which contain clear fluid, and aneurysmal bone cysts, which contain blood and can expand more aggressively. Although bone cysts themselves are not cancer, they can weaken the surrounding bone enough to cause a fracture, which is sometimes the first sign that a cyst is present.

Symptoms

Many bone cysts cause no symptoms and are found incidentally. When symptoms do occur, they may include:

• Aching or pain in the affected bone, sometimes worse with activity
• Swelling or a noticeable lump over the bone
• A limp, if a leg bone is involved
• Reduced movement near a nearby joint
• A fracture after minor injury (a "pathologic" fracture through the weakened bone)

A broken bone caused by a cyst can produce sudden pain, swelling, and difficulty using the limb. Aneurysmal bone cysts tend to grow faster and are more likely to cause pain and swelling than simple cysts.

Causes

The exact cause of most bone cysts is not fully understood. Current understanding includes:

• Simple bone cysts: Thought to result from a disturbance in normal bone growth or fluid drainage within the bone, often appearing during the growing years.
• Aneurysmal bone cysts: May arise on their own or develop in response to another bone lesion; they involve blood-filled spaces and can be linked to certain genetic changes in bone cells.

Bone cysts are generally not caused by injury, diet, or lifestyle, although a minor injury may draw attention to a cyst by causing a fracture through the weakened bone.

Risk Factors

Bone cysts are most associated with:

• Being a child or adolescent, when bones are actively growing
• Male sex, as simple bone cysts are somewhat more common in boys
• Active participation in sports, which can lead to a fracture that reveals a cyst

There are no well-established lifestyle risk factors. Most bone cysts occur in otherwise healthy young people without a known predisposing condition.

Diagnosis

Bone cysts are usually identified with imaging. Diagnostic steps include:

• X-ray: Often the first test, showing a well-defined, hollow area within the bone.
• MRI or CT scan: To define the cyst's size, type, and effect on the bone, and to help distinguish a simple cyst from an aneurysmal cyst or other lesion.
• Biopsy: Sometimes performed to confirm the diagnosis and rule out other bone lesions, particularly when the imaging appearance is unusual.

Careful evaluation is important because a few other bone conditions, including tumors, can look similar on imaging, and confirming a benign cyst guides appropriate, often conservative, management.

Treatment

Treatment depends on the cyst's type, size, location, symptoms, and the risk of fracture. Options include:

• Observation: Small, symptomless simple cysts are often monitored with periodic X-rays, as some heal on their own, especially in children.
• Injection: Steroid or bone-marrow injections into a simple cyst to encourage healing.
• Curettage and bone grafting: Scraping out the cyst and filling the cavity with bone graft or bone substitute, used for larger or symptomatic cysts.
• Surgery for aneurysmal cysts: Removal with grafting, and sometimes additional measures, because these are more likely to recur.
• Fracture care: Treating any broken bone, after which the cyst itself may also need treatment.

Follow-up is important because some cysts, particularly aneurysmal bone cysts, can come back after treatment.

Prevention

Bone cysts cannot generally be prevented because their cause is not well understood and they are not linked to lifestyle. The focus is on preventing complications rather than the cyst itself:

• Following the treatment and monitoring plan to reduce fracture risk
• Avoiding high-impact activities on a weakened bone until cleared by a doctor
• Attending follow-up imaging so any growth or recurrence is caught early

Once a cyst has healed or been treated successfully, normal activity can usually resume as advised by the orthopedic team.

When to See a Doctor

See a healthcare provider if you or your child has persistent bone pain, swelling, or a lump over a bone, or a limp without a clear cause. These should be evaluated even if mild.

Seek prompt medical care for a sudden, severe pain or inability to use a limb after a minor injury, which may indicate a fracture through a weakened bone. While most bone cysts are benign, any new or growing bone lesion should be assessed by a doctor to confirm the diagnosis and rule out other conditions.

Related Topics

Frequently Asked Questions

References

1. American Academy of Orthopaedic Surgeons.
2. Mayo Clinic.
3. MedlinePlus, U.S. National Library of Medicine.