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Fibrous Dysplasia
By Milena Ocon, B.S., Hrayr Shahinian MD, FACS
What is Fibrous Dysplasia?
Fibrous dysplasia is a congenital, non-hereditary, progressive, skeletal disorder by which normal bone is replaced by a variable amount of structurally weak fibrous and osseous tissue. It is characterized by the presence of woven bone. The normal formation of bone would involve a process by which woven bone appears first and later matures into lamellar bone. Under concentrated light, lamellar bone trabeculae (rows of cells bridging an intercellular space) show widely spaced parallel birefringent lines, rimmed with osteoblasts and osteoclasts. In fibrous dysplasia, where bone development ceases in the woven bone stage, the trabeculae can be recognized by random irregular, birefringence, surrounded by abundant fibrous tissue.
Monostotic disease is the most common type, occurring in 70 percent of cases and tends to occur most frequently on the long bones: femur, ribs and skull. Polyostotic disease, afflicting 30 percent of patients, occurs in two or more bones, involving the head and neck 50 percent of the time. The third type, occurring in 3 percent of cases is named McCune-Albright syndrome. This type is characterized by polyostotic fibrous dysplasia, skin pigmentation and precocious puberty in females.
Skull involvement occurs in 27 percent of monostotic and up to 50 percent of polyostotic patients. "Leontiasis ossea" is the specific name for fibrous dysplasia involving the facial and cranial bones. The common form is that in which one or more bone progressively increases, encroaching on the cavities of the orbit, mouth the nose and its sinuses. Abnormal protrusion of the eyeball (exophthalmos) may develop and eventually causes complete loss of sight due to compression of the optic nerve. There may also be interference of the nasal passage and with eating.
Fibrous dysplasia is a progressive disease, which typically occurs early in childhood and continues until skeletal growth ceases. It is first detected in young children where it manifests as a swelling of the jaw and a possible separation of teeth. It causes deformity and impingement and if occurring in the frontal and sphenoid bones it can eventually lead to deformation of facial features and skull shape.
Fibrous Dysplasia of the Skull Base
Fibrous Dysplasia of the frontal and sphenoid bones eventually leads to distortion of facial features and skull shape as a result of the proliferation of thick dense bone.
Craniofacial fibrous dysplasia is different in that it ignores suture lines; more than one bone is usually involved. It can also result in cranial nerve impingement. Disease of the temporal bone may present a patient with 80 percent hearing loss due to narrowing of the inner ear canal. It may also cause facial nerve (VII) paralysis or vertigo. Although any of our twelve cranial nerves and their cranial foramina can be involved with fibrous dysplasia, resulting in cranial nerve deficits, visual and hearing loss represent the more common and debilitating clinical presentations.
How does Fibrous Dysplasia present in radiographic appearance?
The radiologic appearance of fibrous dysplasia can appear as a lucent area with a sclerotic rim. In the skull base and facial bones, fibrous dysplasia manifests as marked sclerosis and bone thickening.
Treatment
It is estimated that patients with fibrous dysplasia are 400 times more likely than the general population to develop a malignant bone tumor. Therefore radiation is not recommended. Treatment would involve firstly management through observation, conservative means by surgical excision and shaving and a more curable treatment by complete bone resection. Surgical intervention is generally intended for cosmetic facial deformities and cranial nerve compressions. The abnormal bone must be completely removed. Surgery is usually delayed until adolescence, however if the progression of the disease comprises neurological function, a decompressive procedure should be considered early in childhood to preserve normal function.
Summary
Fibrous dysplasia is a non-hereditary, progressive and benign disease, in which treatment options are symptomatic. Some lesions are amenable to resection for a cure by a single procedure. More often, most lesions can be managed through staged procedures with overall very favorable outcomes and good long term prognosis.
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