Unicameral (Simple) Bone Cyst

Views: AP and lateral of the affected bone; include full joint above and below

Look for: Well-defined central lytic lesion in metaphysis, thin cortex, internal septa, proximity to physis, pathological fracture, fallen-fragment sign

Assess: Cortex thickness (less than 5 mm = high fracture risk), cyst size relative to bone width, distance from physis, any suspicious features (periosteal reaction, soft tissue mass, cortical destruction outside fracture)

Usually sufficient for diagnosis: UBC is primarily a radiographic diagnosis; biopsy is NOT required when imaging is characteristic

Indication: When radiographic features are atypical or differential diagnosis includes malignancy

Sequences: T1, T2, STIR, and T1 post-gadolinium with fat suppression

Findings in UBC: T1 low signal (fluid), T2 high signal (fluid), no soft tissue component, no peritumoural oedema, thin cyst wall with no solid nodular enhancement. Fluid-fluid levels may appear after recent fracture but are less prominent than in aneurysmal bone cyst.

Key discriminator: No solid enhancing tissue within the lesion. If solid enhancing tissue is present, consider alternative diagnoses (telangiectatic osteosarcoma, ABC secondary to other tumour).

Indication: Assess cortical thickness and integrity for fracture risk stratification; plan surgical approach

Findings: Fluid density within cyst, cortical thinning, precise measurement of remaining cortical bone

Pre-operative planning: Determine entry point for aspiration, cortical window for curettage, and relationship to physis

Indication: Systemic symptoms, atypical features, suspicion of malignancy

Includes: Full blood count, ESR, CRP, bone scan or whole-body MRI, chest CT, CT of primary site, referral to bone tumour centre before any biopsy

Critical: Do NOT biopsy a potentially malignant lesion outside a bone tumour centre. Improperly performed biopsy contaminates compartments and compromises limb-salvage surgery.

Clinical: Full history and examination to exclude red flags

Radiographic: AP and lateral of the entire bone, assess cyst activity (active vs latent), cortex thickness, fracture risk

Decision: If latent cyst with adequate cortex (greater than 5 mm) in proximal humerus or other low-risk site, observe. If active cyst with cortex less than 5 mm, especially proximal femur, consider early intervention.

Repeat radiographs: Monitor cyst size, cortical thickness, distance from physis

Counselling: Reassure family that UBC is benign and often resolves at skeletal maturity. Activity modification advised for high-risk cysts (avoid contact sports if cortex very thin).

Intervention threshold: Proceed to treatment if cyst enlarges, cortex thins further, or pathological fracture occurs.

Resolution: 20-30 percent of latent cysts resolve spontaneously

Persistent cyst: If cyst persists at skeletal maturity with adequate cortex, no further intervention needed

Pathological fracture: Treat fracture, then address cyst if persistent

Imaging: Confirm diagnosis on radiograph; MRI if atypical features

Anaesthesia: General anaesthetic in children (provides muscle relaxation for optimal drainage)

Equipment: Image intensifier, large-bore needle or trocar, syringes, methylprednisolone acetate 80-120 mg (variable dose)

Position: Supine, image intensifier oriented for AP and lateral views

Access: Percutaneous approach; enter the cyst through normal cortex proximal or distal to the lesion to avoid weakening an already thin wall

Drain fluid: Aspirate the cyst contents completely. Fluid is typically serosanguineous. Allow the cavity to collapse.

Cavity lavage: Some surgeons lavage with saline to remove residual debris before injection.

Inject: Methylprednisolone acetate (dose varies, typically 80-200 mg depending on cyst size)

Mechanism: The steroid is thought to reduce inflammation, alter fibrous membrane permeability, and promote bone healing through the cyst wall

Post-injection: The steroid remains in the cavity and is gradually absorbed

Immobilisation: Short-arm or short-leg cast or splint for 4-6 weeks for protection if cortex very thin

Follow-up: Radiograph at 6 weeks and 12 weeks to assess healing (cyst filling, cortical thickening)

Repeat injection: If cyst persists or recurs at 12 weeks, repeat the injection. Multiple injections (2-4) are common.

Indications: Recurrent cyst after failed injection (2-3 attempts), active cyst with very thin cortex at high fracture risk, pathological fracture requiring open reduction (especially proximal femur), large cyst occupying most of the bone diameter

Imaging: Confirm diagnosis; CT for cortical assessment and surgical planning

Consent: Discuss recurrence risk, growth disturbance (rare if physis respected), possible need for repeat surgery, donor site morbidity if autograft used

Position and tourniquet: Supine, appropriate tourniquet. Image intensifier required.

Approach: Direct approach over the cyst through a cortical window. Keep the window small but adequate for thorough curettage.

Curettage: Extend curettage to all internal walls. Use a high-speed burr to remove the fibrous membrane lining. The aim is to disrupt the membrane that perpetuates the cyst.

Bone graft: Fill the cavity with autograft (iliac crest cancellous chips), allograft, or bone graft substitute (calcium phosphate or hydroxyapatite). Autograft has the best osteogenic potential.

Internal fixation: Add flexible intramedullary nails (TENs) for proximal femur lesions or large cysts at risk of fracture. Fixation provides structural support while the graft incorporates.

Closure: Layered closure over a drain if needed. Post-operative immobilisation in a cast or brace.

Immobilisation: Cast or brace for 4-6 weeks depending on fixation adequacy

Weight-bearing: Protected weight-bearing for proximal femur lesions until graft incorporation (8-12 weeks); early mobilisation for humeral lesions with sling

Follow-up: Radiographs at 6 weeks, 12 weeks, 6 months to assess graft incorporation and cyst resolution

Resolution assessment: Cyst considered healed when the cavity is filled with mature bone and cortex is restored

Recurrence: Occurs in 10-30 percent after curettage and grafting, typically within the first year

If recurrence: Repeat curettage or consider adjuvants (ethanol injection, phenol, or cryotherapy at the margins)