Musculoskeletal ImagingFibrous dysplasia
a developmental abnormality of unknown cause in which osteoblasts fail to differentiate and mature normally. It may affect a single, a few or many bones. When the polyostotic variety (and rarely the monostotic type) is associated with endocrine dysfunction, typically manifested by precocious female sexual development and cutaneous pigmentation, the disease is known as the
Albrights syndrome.
The cutaneous findings consist of caf-au-lait spots, caused by increased amounts of melanin in the basal cells of the epidermis. The spots resemble those in neurofibromatosis but usually are fewer, contoured more irregularly and darker in colour.
Most patients have monostotic lesions, predominating in one of the ribs or the femur, tibia, gnathic bone, calvaria or humerus; polyostotic fibrous dysplasia more frequently involves the skull and facial bones, pelvis, spine and shoulder girdle. The ilium is involved only infrequently with solitary lesions. When multiple bones are affected, the lesions may be unilateral or bilateral and may affect several bones of a single limb or both limbs, with or without axial skeletal involvement. The polyostotic variety shows significantly more severe and extensive bone involvement, and patients with this form more typically have pain, a limp, a pathologic fracture and deformity.
In skull and facial bones, hypertelorism, displacement of the globe, exophthalmos, diplopia and visual impairment may be seen, and distortion of the sphenoid wing and temporal bone may lead to compromise of the internal auditory nerve. Radiographs show radiolucent or sclerotic lesions, with osseous expansion almost always occurring in an outward direction. The outer table of the cranial vault is convex, and a localized zone of relative radiolucency surrounded by a sclerotic rim may be seen (doughnut lesion).
In the spine, radiographic features include expansile radiolucent lesions with multiple internal septations in the vertebral body and sometimes the pedicles and vertebral arch. Rarely paraspinal soft tissue extension and vertebral collapse may lead to angular deformity and spinal cord compression.
Lesions in the long tubular bones are usually intramedullary and diaphyseal, radiolucent and hazy (ground glass appearance) (Fig.1). A zone of reactive sclerosis may also be present. Other occasional findings are endosteal erosion with focal cortical thinning and bone expansion, internally trabeculated configuration, and matrix that may be focally calcified or ossified.
A severe coxa vara deformity may be caused by pronounced curvature of the femoral neck and proximal portion of the femoral shaft (shepherds crook deformity). Fractures are encountered frequently after minor injuries, as are stress (insufficiency) fractures.
Other sites of involvement include the ribs, innominate bones, clavicle, scapula, sacrum, and carpal and tarsal bones. When the ribs are involved, unilateral, fusiform expansion of one or more bones is characteristic.
CT scanning and MR imaging are extremely valuable for an accurate assessment of the extent of bone involvement. The signal intensity characteristics of fibrous dysplasia vary. On T1-weighted spin-echo MR images, the lesions are of low signal intensity, whereas on T2-weighted images the lesions may be of low, intermediate or high signal intensity.
Rarely the skeletal lesions of fibrous dysplasia may undergo malignant transformation, most often to osteosarcoma or fibrosarcoma. Generally, such lesions have been irradiated.
Hypophosphataemic rickets and osteomalacia have also been noted in patients with either monostotic or polyostotic fibrous dysplasia, both with and without the McCune – Albright syndrome.
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