Definition
Fibrous dysplasia of bone,a disturbance of medullary bone maintenance in which bone undergoing physiologic lysis is replaced by abnormal proliferation of fibrous tissue, resulting in asymmetric distortion and expansion of bone; may be confined to a single bone (monostotic fibrous dysplasia) or involve multiple bones (polyostotic fibrous dysplasia).
Dysplasia: Abnormal tissue development. See also heteroplasia.
Radiographic Appearance
Fibrous dysplasia is an intramedullary diaphyseal /metaphyseal lesion that blends with a thinned, slightly bulged cortex. The cortex of the affected bone may be thinned by endosteal erosion which creates a scalloped pattern. In long bones, the lesions are usually metaphyseal in location, extending into the rnid-diaphysis. The lesion has a homogenesis density @g it a so-called “ground glass” appearance. An angular deformity in the bone is often present at the level of the lesion. Periosteal reaction is absent except when a pathologic fracture is present. Bone scans show increased uptake-can assess the extent and multiplicity of involvement. CT scan is helpful in differentiating eosinophilic granuloma, osteomyelitis, and unicameral bone cyst. These show lower Hounsfield units (O to 40), compared to fibrous dysplasia (70 to 130). Few studies other than plain X-rays are required in a typical case of polyostotic disease.
Scintigraphy
The majority of lesions in fibrous dysplasia are tracer avid on Tc-99m MDP bone scans. Machida et al(3) analyzed 59 lesions in 26 patients with fibrous dysplasia. Four (14%) of 29 cystic lesion and two (7%) of 30 “ground glass” lesions had radiotracer uptake equivalent to normal bone. The remainder showed supra-normal tracer uptake. Bone scanning can be helpful in conjunction with radiography to detect polyostotic involvement. Intense uptake of gallium is seen in the dysplastic bone. No preferential uptake of thallium or indium-labeled leukocytes is seen.
Pathology
Fibrous dysplasia is the most common bone abnormality that occurs during the years of growth and development (as opposed to osteoporosis, the most common bone abnormality during the years of our bodies’ decline). Dysplasia means abnormal development. Fibrous dysplasia is a disease of the bone in which the outer layers of the bone become thin, and the inner bone marrow is replaced by a gritty fibrous tissue containing sharp, needle-like fragments of bone.
Fibrous dysplasia is usually first apparent in late childhood. It can occur in only one bone (monostotic fibrous dysplasia) or in several or many bones (polyostotic fibrous dysplasia). Monostotic fibrous dysplasia is up to six times as common as the polyostotic form (Orthopedics, August 1996). In polyostotic fibrous dysplasia, up to 75% of the skeleton may be involved. Often fibrous dysplasia is discovered when a bone fractures from relatively minor trauma. Unfortunately, fractures through dysplastic bone do not heal well until the fibrous tissue is surgically removed.
For many with fibrous dysplasia, the bony spicules in the marrow cause bone pain, disability, and slowly progressing deformity. Fibrous dysplasia acts like a benign bone tumor that usually continues to grow until the adolescent growth spurt is completed. Once full growth is achieved, the progression of the disease often stops, but people are left with one or more weakened areas of bone. These are sometimes removed surgically, depending on their extent.
When fibrous dysplasia occurs in the jaw, the swelling of the angle of the jaw (sometimes accompanied by upturned eyes from facial fibrous dysplasia), gives a cherubic look to the face. In fact, fibrous dysplasia of the jaw has its own name — cherubism.
Treatment:
Observation — with prevention of deformity and fracture. Pathologic fracture managed by standard methods according to the site of fracture. Suboptimal alignment will not remodel with time and should not be tolerated.
Surgical treatment. Indications-Fractures that cause significant or progressive deformity that jeopardizes the integrity of the bone. Cosmetically disfiguring or symptomatic fractures causing pain. Recurrent fractures usually require internal fixation with intramedullary rods and grafts. Progressive coxa vara deformity–early surgery by curettage and bone grafting or medial displacement osteotomy is recommended.
Shepherd’s Crook deformity and coxa vara–treated by valgus-medial displacement osteotomy combined with excision of lesional tissue, bone grafting and internal fixation. Severe angular deformities–treated with closing wedge osteotomies, bone grafting, and internal fixation.
Leg length discrepancy–treated with epiphysiodesis of contralateral limb at the appropriate age or valgus-medial displacement osteotomy of the involved limb if there is a small difference.
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