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 forpathological fracture in that age group. The bone cyst is consideredto be a hamartomatous developmental process with an unknownetiology. It may be a degenerative process seen in a pre-existingfibrous lesion of bone, such as monostotic fibrous dysplasia, thatoccurs between the ages of 5 and 15 years, more common in boysthan girls, with half the cases in the proximal humerus and half inthe proximal femur. These patients are usually asymptomatic untilthe time of fracture. The cysts are usually located in the metaphysealportion of long bones, immediately adjacent to the growth plateduring the early years when the lesions are considered active. Asthe patient approaches maturity, the cyst will start to migrate awayfrom the growth plate and go into what is referred to as the inactivestage. Radiographically, the lesions are well marginated with a thinsclerotic edge at the periphery. They are centrally located in the
metaphysis with thinning and slight dilatation of the surroundingcortex and no matrix calcification. The cysts are filled with a clear serous fluid with increasedpressure during the active phase that some experts feel may causethe cyst to enlarge by a hydraulic dynamic mechanism. The cyst islined by a fibrous membrane studded with macrophages and,occasionally, foam cells. In some cases, tissue similar to fibrousdysplasia can be found at either end of the lesion. The periostealcovering over the cyst is normal and for this reason, the pathologicalfractures go on to heal without difficulty. Because of the potentialfor repeated pathological fractures, surgeons are tempted to carryout some type of bone grafting procedure during the early, activephase of the disease but become frustrated with a recurrence rate of30-50 per cent. For this reason, the more common and current wayto handle bone cysts in the early age group, before bone maturity,is by simple aspiration with a double needle technique with theinstillation of methylprednisolone into the cyst cavity that inhibits
the macrophage activity and reduces the chance of local recurrence.It slows down the active lytic process of the disease. This injection isusually required on multiple occasions, every 3 to 6 months untilthe patient reaches maturity, at which time the disease tends tobecome inactive. 3 to 8 injections over this period of time may berequired to avoid the necessity of a bone grafting procedure at a laterdate. The success rate is approximately 85%. In patients over the age15 years, steroid injections in the inactive phase of the disease are notbeneficial because the macrophage activity has disappeared. At thispoint, the only acceptable treatment would be a classic bone graftingprocedure to strengthen the bone and reduce the chance of patho-logical fracture in the future. Physicians must be aware of thepossibility that an apparent bone cyst on radiographic examinationmay masquerade as a malignant tumor such as an osteosarcoma. Ifone is unable to obtain fluid at time of aspiration, a biopsy shouldbe performed to rule out this possibility.
CLASSICCase #190 sclerosis10 year femaleunicameral cystproximal humerus
Aneurysmal Bone Cysts The aneurysmal bone cyst is another clinical entity that presentsas a hemorrhagic pseudotumor in bones in the pediatric age group,most typically in the femur, tibia, pelvis and spine. It has many ofthe characteristic clinical features of the giant cell tumor, except ina younger age group, and is frequently associated with other wellknown neoplastic conditions such as giant cell tumor, chondro-blastoma, osteoblastoma, fibrous dysplasia, or in some cases ofosteosarcoma, especially the hemorrhagic type. It is a very lytic,destructive lesion of bone occurring in patients between the agesof 10 and 20 years. Its characteristic feature is an aneurysmalappearance seen on radiograph that, in the early stages, is extremelydestructive,osteolytic and permeative, taking on the radiographicfeatures of a malignant tumor such as a hemorrhagic osteosarcoma.In two thirds of the cases in the spine, the aneurysmal bone cyst isseen most commonly in the posterior elements, but in one third theywill be seen in the vertebral body. In the case of the giant cell tumor
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 amountof hemorrhage in a cystic lesion that has a very friable, mossy liningat the periphery and one sees reactive bone as it forms a shell aroundthe hemorrhagic cyst. Microscopically, the mossy tissue will beloaded with large, reactive-type giant cells and the backgroundstromal tissue will be made up of benign-appearing spindle cells witha large amount of interstitial hemorrhage and reactive bone formation,and even a few mitotic figures will be noted similar to the situationseen in hemorrhagic osteosarcoma. However, in hemorrhagic osteo-sarcoma, the number of mitotic figures would be very large. One mustbe very careful when sampling a biopsy of this type of lesion tomake sure that one obtains multiple specimens, especially from themore fleshy portion of the tumor, looking for the possibility of anadjacent 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
group. However, because of early progressive destruction and painassociated with these osteolytic lesions, surgeons will usually curettethe 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 orlarge spinal lesions that are difficult to resect surgically, intra-arterialembolization is a good technique to stimulate rapid involution of thetumor. Radiation therapy is a very effective in controlling massivelesions of the pelvis and spine, however, one runs the risk of asecondary sarcoma arising 5 to 15 years later.
CLASSICCase #1989 year femaleABC proximal fibula
growth plateResection specimen cut in path lab
Epidermoid Cyst The least common cyst in bone is the epidermoid cyst. It isusually seen in the distal phalanx or in the skull. In the case of thedistal phalanx, the lesion usually occurs as the result of a crushingtrauma to the distal phalanx that drives nail bed epithelium downinto the subadjacent bone where it implants and produces an ectopic,squamous epithelial cystic lesion formed by a keratinized outershell. It is filled with clear fluid and creates a surface erosion of theadjacent distal tuft of the phalanx that shows a very characteristicradiographic appearance. The lesions will transluminate with aflashlight. Other lesions that have a similar radiographic appearanceinclude enchondroma, glomus tumors of the distal phalanx andperhaps a neurofibroma. Treatment consists of a simple curettementand packing of the defect with autologous bone graft.
CLASSIC Case #205 35 year female with epidermoid cyst distal phalanx