4. Unicameral Bone Cysts
The unicameral or solitary bone cyst is a very common pseudo-
tumor seen in bone in children and is the most common cause for
pathological fracture in that age group. The bone cyst is considered
to be a hamartomatous developmental process with an unknown
etiology. It may be a degenerative process seen in a pre-existing
fibrous lesion of bone, such as monostotic fibrous dysplasia, that
occurs between the ages of 5 and 15 years, more common in boys
than girls, with half the cases in the proximal humerus and half in
the proximal femur. These patients are usually asymptomatic until
the time of fracture. The cysts are usually located in the metaphyseal
portion of long bones, immediately adjacent to the growth plate
during the early years when the lesions are considered active. As
the patient approaches maturity, the cyst will start to migrate away
from the growth plate and go into what is referred to as the inactive
stage. Radiographically, the lesions are well marginated with a thin
sclerotic edge at the periphery. They are centrally located in the
5. metaphysis with thinning and slight dilatation of the surrounding
cortex and no matrix calcification.
The cysts are filled with a clear serous fluid with increased
pressure during the active phase that some experts feel may cause
the cyst to enlarge by a hydraulic dynamic mechanism. The cyst is
lined by a fibrous membrane studded with macrophages and,
occasionally, foam cells. In some cases, tissue similar to fibrous
dysplasia can be found at either end of the lesion. The periosteal
covering over the cyst is normal and for this reason, the pathological
fractures go on to heal without difficulty. Because of the potential
for repeated pathological fractures, surgeons are tempted to carry
out some type of bone grafting procedure during the early, active
phase of the disease but become frustrated with a recurrence rate of
30-50 per cent. For this reason, the more common and current way
to handle bone cysts in the early age group, before bone maturity,
is by simple aspiration with a double needle technique with the
instillation of methylprednisolone into the cyst cavity that inhibits
6. the macrophage activity and reduces the chance of local recurrence.
It slows down the active lytic process of the disease. This injection is
usually required on multiple occasions, every 3 to 6 months until
the patient reaches maturity, at which time the disease tends to
become inactive. 3 to 8 injections over this period of time may be
required to avoid the necessity of a bone grafting procedure at a later
date. The success rate is approximately 85%. In patients over the age
15 years, steroid injections in the inactive phase of the disease are not
beneficial because the macrophage activity has disappeared. At this
point, the only acceptable treatment would be a classic bone grafting
procedure to strengthen the bone and reduce the chance of patho-
logical fracture in the future. Physicians must be aware of the
possibility that an apparent bone cyst on radiographic examination
may masquerade as a malignant tumor such as an osteosarcoma. If
one is unable to obtain fluid at time of aspiration, a biopsy should
be performed to rule out this possibility.
7. CLASSIC
Case #190
sclerosis
10 year female
unicameral cyst
proximal humerus
121. Aneurysmal Bone Cysts
The aneurysmal bone cyst is another clinical entity that presents
as a hemorrhagic pseudotumor in bones in the pediatric age group,
most typically in the femur, tibia, pelvis and spine. It has many of
the characteristic clinical features of the giant cell tumor, except in
a younger age group, and is frequently associated with other well
known neoplastic conditions such as giant cell tumor, chondro-
blastoma, osteoblastoma, fibrous dysplasia, or in some cases of
osteosarcoma, especially the hemorrhagic type. It is a very lytic,
destructive lesion of bone occurring in patients between the ages
of 10 and 20 years. Its characteristic feature is an aneurysmal
appearance seen on radiograph that, in the early stages, is extremely
destructive,osteolytic and permeative, taking on the radiographic
features of a malignant tumor such as a hemorrhagic osteosarcoma.
In two thirds of the cases in the spine, the aneurysmal bone cyst is
seen most commonly in the posterior elements, but in one third they
will be seen in the vertebral body. In the case of the giant cell tumor
122. in the spine, the lesions are almost always found in the vertebral body.
At the time of surgical biopsy, the surgeon will note a large amount
of hemorrhage in a cystic lesion that has a very friable, mossy lining
at the periphery and one sees reactive bone as it forms a shell around
the hemorrhagic cyst. Microscopically, the mossy tissue will be
loaded with large, reactive-type giant cells and the background
stromal tissue will be made up of benign-appearing spindle cells with
a large amount of interstitial hemorrhage and reactive bone formation,
and even a few mitotic figures will be noted similar to the situation
seen in hemorrhagic osteosarcoma. However, in hemorrhagic osteo-
sarcoma, the number of mitotic figures would be very large. One must
be very careful when sampling a biopsy of this type of lesion to
make sure that one obtains multiple specimens, especially from the
more fleshy portion of the tumor, looking for the possibility of an
adjacent osteosarcoma.
As far as treatment is concerned, this is a lesion that tends to in-
volute spontaneously as the child matures into the young adult age
123. group. However, because of early progressive destruction and pain
associated with these osteolytic lesions, surgeons will usually curette
the tumor and then repair the defect with bone graft or, in some cases,
bone cement and Steinman pins. In the case of large pelvic lesions or
large spinal lesions that are difficult to resect surgically, intra-arterial
embolization is a good technique to stimulate rapid involution of the
tumor. Radiation therapy is a very effective in controlling massive
lesions of the pelvis and spine, however, one runs the risk of a
secondary sarcoma arising 5 to 15 years later.
335. Epidermoid Cyst
The least common cyst in bone is the epidermoid cyst. It is
usually seen in the distal phalanx or in the skull. In the case of the
distal phalanx, the lesion usually occurs as the result of a crushing
trauma to the distal phalanx that drives nail bed epithelium down
into the subadjacent bone where it implants and produces an ectopic,
squamous epithelial cystic lesion formed by a keratinized outer
shell. It is filled with clear fluid and creates a surface erosion of the
adjacent distal tuft of the phalanx that shows a very characteristic
radiographic appearance. The lesions will transluminate with a
flashlight. Other lesions that have a similar radiographic appearance
include enchondroma, glomus tumors of the distal phalanx and
perhaps a neurofibroma. Treatment consists of a simple curettement
and packing of the defect with autologous bone graft.
336. CLASSIC Case #205
35 year female with epidermoid cyst distal phalanx
