- Unicameral Bone Cyst
- misleading term because some of these lesions may consist of several multilocular cavities
Definition
- typically a unilocular cystic lesion filled with serous fluid found in a major long bone in skeletally immature patients
History
- First recognized by Virchow
Epidemiology
- Male>female 2:1
- 80% of cases between 3 & 14, & uncommon after 20
Aetiology
- Theories
- Virchow felt that SBC were formed by central cystic softening of an enchondroma.
- describes these cysts as developmental anomalies of the physis where there is a transient failure of ossification of physeal cartilage & cyst formation
- Localized bleed
- results in an intramedullary haematoma & localized venous stasis with development of a cyst
- Virchow felt that SBC were formed by central cystic softening of an enchondroma.
Localization
- Proximal humerus 60-70%
- Femur 15%
- Pelvic cysts are seen in older adult patients.
- Calcaneal cysts are also seen in adults rather than adolescents
Clinical
- Pain
- Pathological fracture
- Swelling or deformity on occasion
- Most are asymptomatic & go unrecognized
Investigations
Xray
- Central lucency within the medullary cavity of the shaft of major long bone
- Narrow zone of transition with well defined sclerotic margins
- Cortex is not disrupted unless there is a pathological fracture & the lesion doesn’t extend into the soft tissues.
- In a pathological fracture a segment of cortex may break off & drop into the distal part of the lesion, constituting the fallen fragment sign
- Extension into the epiphysis can occur in skeletally mature patients
- After resolution of a cyst there may be a residual disorganized pattern to the trabeculae
CT & MRI
- will confirm the extent of the lesion & its cystic nature, with a very bright signal on T2 weighted images & low intensity on T1 weighted images
Pathology
Gross pathology
- fluid is yellowish/serous, similar to serous effusions in other cavities
- Usually consists of a single cyst but may be multilocular
- Wall consists of paper thin, browny yellow fibrous tissue
- pressure within the cyst is high
Microscopy
- walls consist of fibrous tissue without a lining
- fibrous tissue may contain giant cells, dilated vessels & scattered inflammatory cells
Classification
| Type | Description |
|---|---|
| A: Active | usually juxta-epiphyseal |
| B: Inactive | cyst lies nearer the diaphysis (is more than 2cm removed from the physis) Inactive cysts are less aggressive & more amenable to treatment |
Treatment
- Pathological fractures
- should be allowed to heal prior to any regime of injections
- Options
- Closed
- Flexible intramedullary nails
- which may allow earlier movement & make casting unnecessary
- Injections
- Injection of methylprednisolone
- successful in 90% of cases & can be repeated if required
- described by Campanacci & Scaglietti
- Typically 2 to 6 injections are required
- Injection of iliac crest bone marrow
- reported by Lokiec & Wientraub
- 84% rate of healing with repeated punctures & iliac crest bone marrow
- Injection of steroid & bone marrow injections
- showed no benefit of bone marrow injections over steroid injections
- Injection with demineralized bone matrix & bone marrow
- Injection of methylprednisolone
- Cannulated screw
- allows continuous drainage of the cyst by leaving a cannulated screw protruding from the lesion
- Bone Graft
- Expanding lesions in weight bearing bones can be curetted & packed with bone graft
- recurrence rate of 18-36%
Prognosis
- Spontaneous healing may occur after pathological fracture, but is often incomplete & is also often complicated by repeated fracture
- Growth arrest & coxa vara may result from the cyst or after curettage & grafting
- Very rare malignant change noted, including to a chondrosarcoma
- Simple cysts tend to spontaneously resolve in the late teens & rarely persist into adulthood