Unicameral Bone Cyst

Unicameral (simple) bone cyst is a benign, fluid filled lesion of the metaphysis in a growing child, lined by a thin fibrous membrane and containing clear or straw coloured fluid. It is one of the commonest benign lesions of childhood bone, accounting for about 3% of primary bone tumours.

It presents most often between 5 and 15 years of age, with a male to female ratio of 2 to 3 to 1. About 50% are in the proximal humerus and 25 to 30% in the proximal femur, with the remainder in the calcaneus, ilium, tibia and fibula. In adults, the calcaneus and ilium are the commoner sites.

Cysts are called active when they lie within about 1 cm of the growth plate and grow, and latent when they have moved away from the physis as the child grows and tend to heal. Most (about 65%) present with a pathological fracture after minor trauma. Complications are recurrent fracture, growth disturbance (if the physis is damaged), deformity (varus of the proximal femur, shepherd's crook deformity), avascular necrosis of the femoral head (after a fracture or surgery), and recurrence after treatment (up to 20 to 50%).

Pathophysiology.

The cause is unproven. Venous obstruction of interstitial fluid drainage in the metaphysis raises pressure, so the cyst expands. The fluid contains prostaglandins, cytokines and enzymes that resorb bone. As the child grows, the cyst moves away from the physis, and some resolve after fracture, which releases the pressure.

Most cysts are painless until a fracture occurs: sudden pain, swelling and loss of function after minor trauma, and especially in the arm after throwing or a fall. Without fracture, there may be mild aching or an incidental finding on a radiograph. In the proximal femur, a limp and groin pain.

Examine the limb for swelling, tenderness, deformity, range of motion, limb length and neurovascular state.

• Radiographs: a central, well defined, symmetrical lytic lesion in the metaphysis with a thin, sharp sclerotic margin, mild expansion that does not exceed the width of the physis, and a thinned cortex. A "fallen fragment" sign (a bone fragment of cortex lying dependent in the cyst after a fracture) is pathognomonic.
• MRI shows a homogeneous fluid filled cyst (low T1, high T2) without solid component, and is used when the diagnosis is unclear.
• CT for the calcaneus, pelvis and proximal femur.
• Aspiration: clear or straw coloured fluid (blood stained fluid suggests aneurysmal bone cyst or another lesion).
• Biopsy: not needed when imaging is typical; curettage specimens are sent for histology.

Differential diagnosis.

• Aneurysmal bone cyst and fibrous dysplasia.
• Langerhans cell histiocytosis and non-ossifying fibroma.
• Chondroblastoma and enchondroma.
• Osteomyelitis (Brodie abscess) and intraosseous lipoma (calcaneus).

Management aims to heal the cyst and prevent fracture, while avoiding damage to the physis.

Observation and fracture care.

• Fractured upper limb cysts (proximal humerus): immobilise in a sling for 4 to 6 weeks; about 15 to 20% of cysts heal after fracture (up to 40% in some series), and the rest can be treated once the fracture has healed.
• Small, asymptomatic, latent lesions at low fracture risk may be observed with radiographs every 3 to 6 months.
• Lower limb cysts at risk of fracture (proximal femur, with cortical thinning of over 50%) need protection and often treatment: protected weight bearing, and prophylactic fixation.

Intralesional injection.

• Methylprednisolone acetate injected into the cyst after aspiration and decompression: 80 mg for a small cyst and up to 200 mg for a large cyst (maximum 200 mg in a session), repeated every 6 to 8 weeks for up to 3 to 4 injections; 50 to 70% heal or improve. Use sterile technique; avoid with infection at the site, and watch for local skin or fat atrophy, adrenal suppression with repeated doses, and hyperglycaemia in diabetes. Do not inject into the joint.
• Aspirated bone marrow injected into the cyst (autologous bone marrow concentrate) or demineralised bone matrix, with or without calcium based bone substitute, is an alternative with similar healing rates.

Decompression and surgery.

• Flexible intramedullary (elastic) nails with cyst decompression, or cannulated screws, for proximal humerus and femur lesions: relieves pressure, allows early use and prevents fracture.
• Curettage with bone grafting (autograft or allograft) for recurrent or large lesions, with internal fixation for the proximal femur (angled plate, locked plate or intramedullary device).
• Recurrence is 10 to 30% and higher in younger children and active cysts.

Operative antibiotic prophylaxis. Cefazolin 30 mg/kg to a maximum of 2 g intravenously (2 g in adults; 3 g when weight is 120 kg or over) within 60 minutes before incision as a single dose; with severe penicillin allergy, clindamycin 10 mg/kg (maximum 900 mg) intravenously or vancomycin 15 mg/kg intravenously starting 90 to 120 minutes before incision as a single dose.

Analgesia.

• Children: paracetamol 15 mg/kg orally every 4 to 6 hours (maximum 75 mg/kg daily and not exceeding 4 g daily) and ibuprofen 10 mg/kg orally every 6 to 8 hours with food (maximum 40 mg/kg daily and not exceeding 2400 mg daily), for up to 5 days; avoid ibuprofen with kidney impairment, dehydration or active bleeding.
• Adults: paracetamol 1 g orally every 6 hours as needed for up to 5 to 7 days (maximum 4 g daily; 3 g daily when weight is under 50 kg, with chronic alcohol use or hepatic impairment).

Monitoring. Radiographs at 6 weeks and 3 months after fracture or injection, then every 3 to 6 months. Healing is shown by sclerosis, cortical thickening and reduction in size; classify the result (Capanna and Neer grades). Growth, leg length and alignment until skeletal maturity.

Referral and escalation. Refer every unicameral bone cyst with fracture or a risk of fracture to paediatric orthopaedics. Urgent referral for a displaced fracture of the proximal femur, signs of infection, a lesion that grows, or blood stained aspirate.

Special groups.

• Young children with active cysts near the physis: higher recurrence; avoid damage to the growth plate.
• Proximal femur cysts: high fracture risk with varus deformity and avascular necrosis; protect and treat early.
• Adults: calcaneal and iliac lesions need exclusion of intraosseous lipoma and other tumours.
• Corticosteroid injection in diabetes: monitor glucose.