Unicameral (Simple) Bone Cyst

The unicameral (simple) bone cyst is the most common benign lytic bone lesion in children: a solitary, fluid-filled, benign lesion in the metaphysis of a growing bone. It is a frequent source of pathological fracture, particularly through the proximal humerus. The fallen-fragment sign on a plain radiograph is a classic examination discriminator, because it allows a confident diagnosis without biopsy.

Who. Children, with a peak age of 5-15 years; the cyst is rare after skeletal maturity. Boys are affected two to three times as often as girls (2-3:1), and there is no strong laterality preference. The lesion accounts for approximately 3% of all bone biopsies.

Where. The cyst sits centrally in the metaphysis and does not cross the physis. By site:

• Proximal humerus - 50%
• Proximal femur - 25%
• Calcaneus - less than 5%
• Other sites (tibia, ilium, distal femur and elsewhere) - the remaining 20%

Cause. The exact cause remains uncertain. The leading theory is venous outflow obstruction, proposed by Cohen in 1970: blocked venous drainage from the metaphysis elevates intraosseous pressure, which leads to focal bone resorption and fluid accumulation. The cavity enlarges until the cortex is thinned, and fracture occurs when the mechanical load exceeds what the thinned cortex can bear.

Other theories include a defect in metaphyseal remodelling during growth, localised failure of bone formation, and altered osteoclast and osteoblast activity.

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Not a true cyst. The cavity contains serosanguineous fluid and is lined by a thin fibrovascular (connective-tissue) membrane with no epithelial lining, so the lesion is not a true cyst: the fluid is interstitial rather than secreted. Despite the name "unicameral", the cavity is often multiloculated by thin bony septa.

Histology. The diagnosis is made on imaging, but know the slide:

• Scattered multinucleated giant cells, haemosiderin-laden macrophages and reactive woven bone
• The hallmark: amorphous eosinophilic fibrinous material that mineralises into curvilinear "cementum-like" bodies (Liesegang-type structures), a useful clue distinguishing UBC from other lytic lesions

The fluid. Intracystic pressure is elevated, approaching local venous pressure, which is consistent with the venous-outflow-obstruction aetiology. The fluid is rich in bone-resorbing mediators - prostaglandins (PGE2), interleukins, oxygen free radicals, nitric oxide and matrix metalloproteinases - and these drive the local osteolysis that enlarges the cavity.

Why it matters for treatment. Decompression lowers the raised pressure, which is the basis of continuous-decompression techniques, and intracystic steroid reduces the inflammatory and resorptive mediators. Both interrupt the resorptive cycle rather than simply filling the cyst.

Activity (Neer). An active cyst abuts the physis and its cortex is thinned, so the risk of fracture is high. As the bone grows the cyst moves away from the physis toward the diaphysis, the cortex thickens and the fracture risk decreases: the latent phase, from which the cyst may resolve spontaneously at skeletal maturity.

Why activity matters. It guides treatment intensity and predicts recurrence. Active cysts require more aggressive treatment; latent cysts may be observed.

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Site. The anatomical location determines fracture risk, treatment urgency and the threshold for surgical intervention. The proximal femur is the single most important site to recognise early, because the consequences of a fracture there are far more severe than at the humerus.

History. The typical patient is a boy with acute pain after minor trauma, which is the pathological fracture: approximately 80% present this way. Between fractures the cyst is painless, and there may have been a previous fracture through the same area. There is usually no functional limitation unless the bone has fractured or a proximal femoral cyst is causing a limp.

The incidental cyst. Cysts are increasingly detected on radiographs performed for other reasons. The finding causes significant parental concern, and reassurance is important.

Examination. A normal child with no systemic symptoms: no weight loss, fever or malaise. After an acute fracture there may be swelling, bruising and tenderness at the fracture site, with limited and painful movement; otherwise the range is full. There is no palpable mass, no skin change and no warmth (rules out infection).

The neurovascular examination is normal, because UBC does not affect nerves or vessels, and the regional lymph nodes are normal.

Unicameral versus aneurysmal bone cyst. The two are separated by age, position in the bone, contents, MRI appearance and behaviour after treatment.

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Other lytic lesions in children. The rest of the differential, benign and malignant, and how each is told apart.

Radiographs. UBC is primarily a radiographic diagnosis, made on plain radiographs alone in the vast majority of cases, and biopsy is NOT required when the imaging is characteristic. Take AP and lateral views of the entire affected bone, including the full joint above and below.

The characteristic cyst:

• A well-defined, central, geographic lytic lesion in the metaphysis
• Cortex thinned but intact unless fractured
• Thin internal septa or ridges, which may make it look multiloculated
• No periosteal reaction, no soft tissue mass, and no crossing of the physis

Then look for anything that does not fit: periosteal reaction, a soft tissue mass, or cortical destruction outside a fracture. A well-defined central metaphyseal lesion with cortical thinning, no soft tissue mass and a normal healthy child is sufficient for the diagnosis, and a characteristic UBC is not routinely biopsied.

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The fallen-fragment sign. The sign is seen when a pathological fracture through the cyst produces a cortical fragment that settles to the dependent portion of the cavity. Only a fluid-filled cavity allows a fragment to sink; solid tumours (fibrous dysplasia, NOF, Ewing sarcoma) hold fragments in place.

The sign is pathognomonic, since no other bone lesion produces it, and when present it is considered virtually diagnostic, so biopsy is not required before treatment. Look for the fragment on a cross-table lateral.

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What to measure. Three measurements on the radiograph set the fracture risk and the treatment.

Cyst index. The cyst area divided by the square of the diaphyseal diameter. A cyst index above 4 indicates a high risk of pathological fracture.

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MRI with gadolinium is for atypical radiographic features, or when the differential includes malignancy. Sequences are T1, T2, STIR and T1 post-gadolinium with fat suppression. A UBC gives low T1 and high T2 fluid signal, with no soft tissue component, no peritumoural oedema, and a thin cyst wall with no solid nodular enhancement.

Fluid-fluid levels may appear after a recent fracture but are less prominent than in aneurysmal bone cyst. The key discriminator is the absence of solid enhancing tissue: if it is present, consider an alternative diagnosis such as telangiectatic osteosarcoma or ABC secondary to another tumour.

CT comes in when a fracture or surgery is planned. It assesses cortical thickness and integrity for fracture-risk stratification, showing fluid density within the cyst, cortical thinning and a precise measurement of the remaining cortical bone. Before surgery it determines the entry point for aspiration, the cortical window for curettage and the relationship to the physis. In the calcaneus CT is useful to map the extent of the cyst and assess fracture risk before weight-bearing.

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When red flags are present, stage fully and refer to a bone tumour centre before any biopsy:

• Full blood count, ESR and CRP
• Bone scan or whole-body MRI
• Chest CT and CT of the primary site

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

The principle. Treatment is guided by fracture risk, activity status and skeletal maturity. Pathological fractures through a UBC generally heal normally, and the site decides what the bone needs.

The proximal femur is fixed internally with flexible intramedullary nails or a plate, because the cyst weakens the bone and the risk of recurrent fracture is high.

ObservationInjectionSurgical

Not every UBC needs treatment. The goal is to prevent pathological fracture while allowing the cyst to resolve spontaneously where possible. A latent cyst away from the physis with adequate cortical thickness can be safely observed with serial radiographs.

The first visit. Take a full history and examination to exclude red flags, and AP and lateral radiographs of the entire bone to assess activity, cortical thickness and fracture risk. Observe a latent cyst with adequate cortex (greater than 5 mm) in the proximal humerus or another low-risk site. Consider early intervention for an active cyst with cortex less than 5 mm, especially in the proximal femur.

Follow-up. Repeat radiographs at 3-6 monthly intervals until skeletal maturity, watching cyst size, cortical thickness and distance from the physis. Proceed to treatment if the cyst enlarges, the cortex thins further or a pathological fracture occurs; a fracture is treated first, and the cyst addressed afterwards if it persists.

A cyst that persists at skeletal maturity with adequate cortex needs no further intervention.

Counselling. Explain to the family that the cyst is benign, does not undergo malignant transformation, and will likely stabilise or resolve, often at skeletal maturity. The child should avoid contact sports only while the cortex is critically thinned; once the cyst becomes latent and cortical thickness improves, normal activity may resume.

Aspiration and methylprednisolone acetate injection (modified Scaglietti technique) puts corticosteroid into the cyst cavity to promote healing and cortical thickening without open surgery. Its morbidity is low compared with open curettage, which is why it is first-line.

Technique. Under image-intensifier control:

1. Confirm the diagnosis on radiograph, with MRI if there are atypical features.
2. General anaesthetic in children, which provides muscle relaxation for optimal drainage. Position supine, with the image intensifier oriented for AP and lateral views.
3. Enter the cyst percutaneously with a large-bore needle or trocar, through normal cortex proximal or distal to the lesion, to avoid weakening an already thin wall.
4. Aspirate the contents completely and allow the cavity to collapse; the fluid is typically serosanguineous. Some surgeons lavage with saline to remove residual debris before injection.
5. Inject methylprednisolone acetate. The dose varies with cyst size, typically 80-200 mg, and the steroid remains in the cavity and is gradually absorbed.

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How the steroid works. It is thought to reduce inflammation, alter the permeability of the fibrous membrane, and promote bone healing through the cyst wall.

Aftercare. A short-arm or short-leg cast or splint for 4-6 weeks protects the bone if the cortex is very thin. Radiographs at 6 and 12 weeks assess healing: cyst filling and cortical thickening.

Repeat injections. If the cyst persists or recurs at 12 weeks, repeat the injection, at intervals of 6-12 weeks if needed; multiple injections (2-4) are common. Each subsequent injection is less effective than the first, so if 2-3 injections fail, proceed to open curettage and bone grafting.

Curettage and bone grafting, with or without internal fixation, mechanically removes the cyst lining, fills the cavity and stabilises the bone.

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 of the proximal femur
• Large cyst occupying most of the bone diameter

Consent covers recurrence, growth disturbance (rare if the physis is respected), the possible need for repeat surgery, and donor-site morbidity if autograft is used.

Technique

1. Supine, with an appropriate tourniquet; an image intensifier is required.
2. Approach the cyst directly through a cortical window, kept small but adequate for thorough curettage.
3. Extend the curettage to every internal wall and use a high-speed burr to remove the fibrous membrane lining. The aim is to disrupt the membrane that perpetuates the cyst.
4. Fill the cavity with autograft (iliac crest cancellous chips), allograft, or a bone graft substitute (calcium phosphate or hydroxyapatite). Autograft has the best osteogenic potential.
5. Add flexible intramedullary nails (TENs) for proximal femoral lesions or large cysts at risk of fracture; they provide structural support while the graft incorporates.
6. Layered closure, over a drain if needed, with post-operative immobilisation in a cast or brace.

Afterwards. Cast or brace for 4-6 weeks, depending on how adequate the fixation is. Proximal femoral lesions need protected weight-bearing until the graft incorporates (8-12 weeks); humeral lesions mobilise early in a sling. Radiographs at 6 weeks, 12 weeks and 6 months assess graft incorporation and cyst resolution.

Healing and recurrence. The cyst is considered healed when the cavity is filled with mature bone and the cortex is restored. Recurrence typically occurs within the first year, and is treated by repeat curettage or by adjuvants at the margins (ethanol injection, phenol, or cryotherapy).

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Curettage and bone grafting alone

Indication

Recurrent cyst after injection, humeral lesions

Advantages

Definitive removal of cyst membrane, good healing rates

Disadvantages

Open surgery, donor site pain if autograft, recurrence

Curettage, grafting, and flexible intramedullary nails

Indication

Proximal femur lesions, large cysts at high fracture risk

Advantages

Mechanical support while healing, prophylaxis against fracture

Disadvantages

Hardware removal may be needed later; overgrowth risk in young children

Flexible intramedullary nails alone (no curettage)

Indication

Proximal femur cyst with pathological fracture

Advantages

Stabilises fracture, provides continuous reaming effect that may help cyst heal

Disadvantages

Does not remove cyst membrane; cyst may persist despite fracture union

Subtrochanteric valgus osteotomy with nail fixation

Indication

Proximal femur cyst at high risk with coxa vara tendency

Advantages

Corrects biomechanics, adds stability, nails provide continuous reaming

Disadvantages

More complex procedure; requires paediatric orthopaedic tumour expertise

Surgical Options Comparison

Procedure	Indication	Advantages	Disadvantages
Curettage and bone grafting alone	Recurrent cyst after injection, humeral lesions	Definitive removal of cyst membrane, good healing rates	Open surgery, donor site pain if autograft, recurrence
Curettage, grafting, and flexible intramedullary nails	Proximal femur lesions, large cysts at high fracture risk	Mechanical support while healing, prophylaxis against fracture	Hardware removal may be needed later; overgrowth risk in young children
Flexible intramedullary nails alone (no curettage)	Proximal femur cyst with pathological fracture	Stabilises fracture, provides continuous reaming effect that may help cyst heal	Does not remove cyst membrane; cyst may persist despite fracture union
Subtrochanteric valgus osteotomy with nail fixation	Proximal femur cyst at high risk with coxa vara tendency	Corrects biomechanics, adds stability, nails provide continuous reaming	More complex procedure; requires paediatric orthopaedic tumour expertise

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Continuous decompression with cannulated screws. A recognised minimally invasive technique that targets the raised intracystic pressure directly. One or more cannulated screws are placed percutaneously across the cyst and left in situ, providing ongoing drainage and decompression so the bone-resorbing fluid cannot re-accumulate.

It is a low-morbidity, day-case percutaneous procedure, with fewer repeat anaesthetics than the multiple sessions steroid injection often needs, and in the femur the screw also adds modest stability.

The multimodal variant combines percutaneous decompression or curettage with instillation of autologous bone marrow aspirate, demineralised bone matrix and/or an injectable calcium-phosphate or calcium-sulfate bone substitute: the substitute scaffolds healing while decompression prevents recurrence.

Decompression is an alternative or adjunct to repeated steroid injection for active cysts, with reported healing and recurrence rates at least comparable while needing fewer procedures. It does not replace open curettage, grafting and flexible nailing for the high-risk proximal femur.

Because the cyst does NOT cross the physis, limb-length discrepancy is rare.

Best prognosis. A latent cyst away from the physis, adequate cortical thickness, a proximal humeral location, and an older child closer to skeletal maturity.

Poorer prognosis. An active cyst abutting the physis, cortex less than 5 mm, a proximal femoral location, a very young child far from skeletal maturity, and a large cyst (greater than 80% of bone diameter).

The key principle. UBC does NOT undergo malignant transformation, and a cyst that persists remains benign. Treatment aims to prevent fracture and allow normal bone development.