Simple Bone Cyst (Unicameral)

What is a simple bone cyst?

A simple bone cyst (also called a unicameral bone cyst or UBC) is a fluid-filled cavity within a bone. It is benign — not cancerous — and is most commonly found in children and adolescents. The most common sites are the proximal humerus (upper arm bone near the shoulder) and the proximal femur (upper thigh bone near the hip), though they can occur in other bones.

What causes it?

The exact cause is unknown. Simple bone cysts are not hereditary and are not caused by injury. They are thought to arise from a localised disturbance of bone development near the growth plate. They fill with a clear or blood-stained serous fluid. The cyst wall is thin and the cavity is typically a single chamber (hence unicameral).

How is it found?

• Often discovered incidentally on X-ray taken for another reason (such as after a fall)

• Pathological fracture — a fracture through the weakened bone is a common first presentation (approximately 50-75% of patients)

• Mild aching pain or discomfort in the affected area

Is it dangerous?

Does a simple bone cyst need treatment?

Not all simple bone cysts require immediate treatment. The decision depends on:

• Location — proximal femur cysts carry higher fracture risk and usually require treatment

• Size — large cysts occupying most of the bone diameter are at higher fracture risk

• Symptoms — pain or a previous fracture

• Activity level of the child

Small, asymptomatic cysts in low-risk locations may be observed with serial X-rays.

Treatment options

1. Observation

For small, asymptomatic cysts not at high fracture risk, regular X-ray review every 6-12 months to monitor for healing or enlargement is appropriate. Some cysts heal spontaneously, particularly after a fracture through the cyst (the fracture stimulates healing).

2. Steroid injection (methylprednisolone)

Injection of corticosteroid (methylprednisolone) into the cyst has been widely used for decades. The mechanism is not fully understood but it reduces the fluid-producing activity of the cyst lining and encourages bone healing.

• Performed under general anaesthetic or sedation

• One or more injections may be required — typically 2 to 3 at 2-3 month intervals

• Success rate: approximately 50-60% with a single injection; higher with repeated injections

• Can be repeated if the cyst does not respond

• Minimally invasive — no incision required

3. Demineralised bone matrix or bone marrow injection

Injection of demineralised bone matrix (DBM) or bone marrow aspirate has been used as an alternative or adjunct to steroid injection. These materials contain growth factors that stimulate bone repair. Results are comparable to steroid injection in many studies.

4. Curettage and bone grafting

For larger cysts, cysts that fail injection treatment, or cysts in high-risk locations, surgical curettage and bone grafting is performed. The cyst is opened through a cortical window, the lining is removed (curetted), and the cavity is packed with bone graft — autograft (own bone), allograft (donor bone), or synthetic bone substitute.

5. Intramedullary decompression with elastic nail or cannulated screw

A technique that has gained popularity involves placing a flexible intramedullary nail or cannulated screw through the cyst — this decompresses the cyst by providing a drainage channel and the implant helps support the bone against fracture. Particularly useful in the proximal femur and humerus where fracture risk is high.

Treatment after pathological fracture

If your child has already sustained a fracture through the cyst:

• The fracture is treated first — immobilisation in cast or internal fixation depending on fracture severity and location

• Some cysts partially heal following the fracture

• Cyst treatment (injection or surgery) is usually performed once the fracture has healed

Risks of surgical treatment

• Wound infection

• Bleeding or haematoma

• Fracture through the bone during or after surgery — minimised by careful technique

• Graft resorption or non-union

• Recurrence of the cyst — commoner with injection treatment than surgical curettage

• Anaesthetic risks

Recovery

Note: Reassurance — and the key risk 

Simple bone cysts are benign and do not turn into cancer. The main risk is a pathological fracture — a fracture through the weakened bone caused by even minor injury. Fractures through simple bone cysts in the proximal femur are particularly serious as they can be difficult to treat. The goal of treatment is to reduce fracture risk and encourage the cyst to heal.

Typical recovery

Observation

No restrictions unless cyst is large —   activity guidance at review.

Injection

Day case. Activity restriction 4-6 weeks.   X-ray review at 3 months.

Curettage and graft

Day case or overnight. Partial   weight-bearing 4-6 weeks. X-ray at 6-8 weeks.

Intramedullary nail

Overnight stay. Weight-bearing as directed.   X-ray monitoring.

Return to sport

Typically 6-12 weeks once healing confirmed   on X-ray.

Note:When to seek urgent help 

Sudden severe pain in the bone after minor injury — may indicate a fracture through the cyst Inability to bear weight on the leg or use the arm normally Temperature above 38.5°C after surgery

This information is for general guidance only and is not a substitute for a consultation with Mr Wadia. If you are worried about your child, please seek medical advice.