Solitary
Bone Cyst
The Solitary Bone Cyst (SBC) also known
as the simple bone cyst & unicameral bone cyst (UBC) is found mostly in the
5-15 year old range with a male predominance.
Most lesions are found in the proximal humerus (50%) and upper femur
(25%). SBCs are always found on the
metaphyseal side of the physis. In
fact, these lesions have an intimate relationship with the growth plate and are
considered “active” if they are in
continuity with the physis and “inactive”
if the have separated from it. This
relates to the pathophysiology of the lesion in that active lesions are
actively expanding due to hydraulic dynamic expansion from fluid being excreted
by the physis. Symptoms are usually
absent until enough bone destruction has occurred to threaten or cause a
pathologic fracture.
Radiographically, these lytic lesions are located centrally within the metaphyseal portion of bones and can
have a pseudoloculated appearance and thin cortical rim (Figure #5). One classic finding is the “fallen leaves” sign where small
particles of fractured cortex are found within the most inferior aspect of the
cavity. The cysts are grossly filled
with serous fluid and the inner lining is a fibrous membrane with giant cells
and foam cells.
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Figure
#5
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Pathologic
fractures are a frequent problem in these lesions and steps to avoid this
complication are warranted if anticipated.
Most fractures will heal without major complication because the
periosteum of the bone remains intact.
However, most cysts will persist following a fracture.
Initial management of most lesions
include an initial aspiration to
ensure the diagnosis and concurrent injection with bone marrow or steroid. If aspiration does not produce serous fluid,
then re-evaluation of the diagnosis and open biopsy is required to rule out
more aggressive lesions such as sarcoma.
A large bore needle such as a
bone marrow biopsy needle should be employed and these injections should be
repeated three to five times over 3 months.
Results with this treatment tend to be more favorable in younger
patients with active lesions as the macrophage
activity is elevated and helps with remodeling. Failure of response to the injections advances treatment to the
next level, which includes aggressive curettage and bone grafting. Load bearing bones such as the femur require
more aggressive therapy initially, especially if there is an impending
fracture. These require aggressive
curettage and bone grafting from the start.
These bones will often fail dramatically and displace with pathologic
fractures leaving a difficult reduction and limited fixation capability due to
poor bone stock. Active lesions will
recur 50% of the time with this treatment, but inactive lesions fare much
better.
