http://orthopaedicprinciples.com/2010/12/unicameral-bone-cyst/

Unicameral Bone Cyst

Simple Bone Cyst (Unicameral Bone cyst)        

• Unicameral bone cysts are developmental anomalies of the physis where there is a transient failure of ossification of physeal cartilage and cyst formation
• Proximal humerus: MC site (50%) followed by proximal femur (25%) and calcaneum
• Other sites: proximal tibia and ilium
• Affects patients in the first and second decades of life (85%cases)
• M:F:: 3:1
• Anterior aspect of calcaneus and other flat bones like ilium may be involved in older adolescents and young adults.
• When it remains in contact with the physis it is termed “active” (Higher recurrence)
• When it separates and migrates towards the diaphysis it is termed “inactive” or latent.
• The cysts may occasionally cross the growth plate and destroy it over time, which leads to shortening
• Usually asymptomatic, unless there is a fracture.
• Spontaneously resolve in late adolescence and rarely persist into adulthood

Pathology:

• Fibrous membrane lined cyst containing matrix metalloproteinases, prostaglandins (PGE2), interleukin-1, interleukin-6, tumour necrosis factor-alpha and oxygen free radicals. Nitrate and nitrite are higher compared to serum.
• Fibrous membranes constitute fibrous tissues, occasional multinucleated cells and CD 68 foamy histiocytes
• Straw colored fluid which maybe blood tinged aspirated from the cyst and complete filling of  the radiolucent lesion with contrast material strongly favor the diagnosis of a unicameral bone cyst.
• Cyst wall is composed of fibrovascular tissue with giant cells and reactive bone.
• Cholesterol clefts and hemosiderin laden macrophages maybe seen in long standing cases and if there is a fracture.

X-ray:

• Solitary lytic lesion that is centrally located with pseudoloculated appearance and minimal marginal sclerosis
• Presence of bone fragment in the cyst gives the appearance of the “fallen fragment sign” showing that the cyst contains fluid. It is present only when there is a fracture and is pathognomonic of UBC
• The bone is widened but usually not to more than the width of the epiphyseal plate
• Cortical thinning
• No periosteal reaction

MRI:

• Homogenous fluid signal
• Peripheral rim enhancement only
• Blood products maybe identified if fracture has occurred- the hematocrit sign, the level of horizontal delineation between serum and cells

 Aspiration

• If cyst contains clear fluid- diagnosis confirmed
• If blood is aspirated- cytological examination
• If aspiration fails due to solid tissue- consider open biopsy

 Treatment

• Aspiration/injection with either bone marrow or corticosteroid (Methyl prednisolone). The injections are carried out with bone biopsy needles and are repeated three to five times at intervals of 2-3 months.-for upper limb UBCs
• Surgery in the form of curettage and bone grafting with augmentation using power burr and cryosurgery (mainly indicated for lesions that are large with risk of pathological fracture) especially for lower extremity long bone cysts
• Proximal femoral lesions require open curettage and internal fixation with bone grafting
• Flexible intramedullary nails maybe used for the treatment of unicameral bone cyst.
• Injections with bone marrow, calcium sulphate pellets, calcium phosphate bone cement and demineralized bone matrix is being investigated(1)
• Observation may be opted for humeral and calcaneal lesions discovered incidentally.
• In calcaneal cysts, rule out plantar fasciitis which maybe the source of pain
• Pathologic fracture:

            - 1/7th of lesions regress with healing of fracture.

            – For proximal humerus lesions allow fracture to heal and re evaluate  

                 radiographically prior to operative treatment.

             – In lower limb consider ORIF

Ref:

1. Rougraff BT et al: Treatment of active unicameral bone cysts with percutaneous injection of demineralized bone matrix and autogenous bone marrow. J Bone Joint Surg Am 2002;84-A(6):921