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Simple Bone Cyst (Unicameral)

                                                                                                                             Assistance provided by Nanni Allington MD.
- Discussion:
    - a membrane lined cavity containing a clear yellow fluid.
    - occurrs most often in children 4-10 years of age;
    - lesions remain asymptomatic unless complicated by fracture.
    - they enlarge during skeletal growth and become inactive, or latent, after skeletal maturity.
    - active cysts:
         - develop in patients under 10 years of age;
         - cyst arises adjacent to growth plate & may grow to fill most of metaphysis;
         - bone may be slightly expanded w/ thin cortical shell;
         - it will continue to enlarge during observation;
         - may cause pathologic fracture;
    - passive cysts:
         - patients are usually over 12 years of age;
         - cysts cease to expand;
         - become increasingly separated from growth plate (more than 1-2 cm);
         - have thicker bony wall than active lesions;
         - may show evidence of healing or ossification
         - less likely to result in frx;

- Location:
    - lesion appears to arise from the growth plate & in early stages, lesion is lies adjacent to growth plate;
          - typically the simple bone cyst will have a central location, whereas an ABC will have a slightly eccentric location;
    - predilection for the metaphysis of long bones;
    - proximal humerus (50% of cases)
    - proximal tibia;
    - proximal femur (40%)
    - foot:            
          - ref: Clinical Relevance of Calcaneal Bone Cysts: A Study of 50 Cysts in 47 Patients. 

- Radiographs: 
    - show a central, well marginated & symmetric radiolucent defect in metaphysis;
    - usually no bony separations or loculations;
    - may appear to have a slightly ballooned w/in the metaphysis;
    - metaphyseal bone does not remodel normally, & metaphysis is broader than normally seen but not
             broader than with width of epiphyseal plate;
    - thin rim of non reative bone borders the unicameral bone cyst;
    - when cyst becomes latent, epiphysis grows away from the lesion;
    - diff dx:
          - fibrous dysplasia;
                - monostotic fibrous dysplasia is usually eccentric rather than central and diaphyseal rather than metaphyseal;
                - periosteal reaction is greater in fibrous dysplasia than simple bone cyst;
          - aneurysmal bone cyst;
                - metaphysis is expanded, with marked cortical thinning that predisposes to fracture;
                - enlarge metaphysis to greater than width of the epiphyseal plate;

- Histologic Examination:
    - active cyst have a mesothelial membrane lining thin margin of bone;
    - inner wall of bone adjacent to membrane may be lined by osteoclasts;
    - between membrane & osteoclasts is a layer of areolar tissue containing fibroblastic and multinucleated giant cells;

    - goal of treatment is prevention of pathologic fracture;
          - conventional teaching is that the cyst will heal and resolve if a fracture occurs thru the cyst;
          - this has been called into question w/ some estimates that cyst healing occurs in less than 10% fractures;
    - simple cysts are treated with curettage and bone grafting;
    - recurrence is high for active cysts (50%) & low for latent cysts (10%);
    - alternative treatments:
          - steroids:
                  - 80-200 mg of methylprednisolone infused into cavity;
                  - Hashemi-Nejad A and Cole WG, 32 patients with unicameral cysts received multiple intralesional steroid injections;
                         - earliest time to healing was 3 months;
                         - at median review of 5 years, 13% of cysts had healed, 62% were paritally visible but sclerotic, 12.5% were
                                   visible but opaque, and 12.5% were clearly visible;
                         - healing response to intralesional steroids was unpredictable and was incomplete even after multiple injections;
                         - failure in wt bearing bones was high;
                         - authors questioned whether the results of the treatment were a result from multiple drill holes rather from the steroid;
                  - references:
                         - Incomplete healing of simple bone cysts after steroid injections
                         - Simple bone cysts. The effects of methylprednisolone on synovial cells in culture.
          - autologous bone marrow injection:
                  - reference:
                         - Simple bone cysts treated by percutaneous autologous marrow grafting. A preliminary report.
          - multiple drill holes:
                  - references:
                         - Simple bone cysts treated by multiple drill holes: 23 cysts followed 2-10 years
                         - Radiological evidence of healing  of a simple bone cyst after hole drilling

Solitary unicameral bone cyst: treatment with freeze-dried crushed cortical-bone allograft. A review of one hundred and forty-four cases.

Simple bone cyst. Treatment by trepanation and studies on bone resorptive factors in cyst fluid with a theory of its pathogenesis.

Unicameral bone cyst.

The fallen fragment sign in unicameral bone cyst.

Packing with high-porosity hydroxyapatite cubes alone for the treatment of simple bone cyst.

Unicameral bone cyst (simple bone cyst). 

Pathological fractures secondary to unicameral bone cysts.

Epiphyseal involvement of simple bone cysts.

Treatment of unicameral bone cyst: a comparative study of selected techniques.