Fibrous dysplasia of bone

A bone disorder where normal bone is replaced with fibrous tissue

Fibrous dysplasia of bone is a skeletal disorder characterized by the replacement of normal bone with fibrous tissue, leading to structural weakness and deformity. This condition can affect one bone (monostotic) or multiple bones (polyostotic) and is caused by a post-zygotic mutation in the GNAS gene, which affects the production of osteoblasts.

Pathophysiology

Fibrous dysplasia results from a mutation in the GNAS gene, which encodes the alpha subunit of the stimulatory G protein (Gsα). This mutation leads to constitutive activation of the Gsα protein, resulting in increased production of cyclic AMP (cAMP). The elevated cAMP levels disrupt normal bone development and remodeling, causing the replacement of normal bone with fibrous tissue. This fibrous tissue is composed of fibroblasts and collagen, and it lacks the normal mineralization seen in healthy bone.

Clinical Presentation

The clinical manifestations of fibrous dysplasia vary depending on the number and location of affected bones. Common symptoms include:

• Bone pain
• Deformities, such as bowing of the limbs
• Fractures due to weakened bone structure
• Asymmetry of the face or skull when craniofacial bones are involved

In polyostotic fibrous dysplasia, patients may also experience endocrine abnormalities, such as precocious puberty, due to the involvement of the pituitary gland.

Diagnosis

Diagnosis of fibrous dysplasia is based on clinical evaluation, imaging studies, and sometimes biopsy. Imaging techniques such as X-ray, CT scan, and MRI are used to assess the extent of bone involvement and to differentiate fibrous dysplasia from other bone lesions.

Intermediate magnification of fibrous dysplasia

Histopathology

Histologically, fibrous dysplasia is characterized by the presence of fibrous stroma with irregularly shaped trabeculae of woven bone. The bone trabeculae are often described as "Chinese characters" due to their irregular shapes. The fibrous tissue is composed of spindle-shaped fibroblasts and collagen fibers.

Fibrous dysplasia showing fibrous stroma and bone trabeculae

Treatment

There is no cure for fibrous dysplasia, and treatment is primarily symptomatic. Management strategies include:

• Pain management with analgesics
• Surgical intervention to correct deformities or stabilize fractures
• Bisphosphonates to reduce bone pain and prevent fractures

Prognosis

The prognosis for individuals with fibrous dysplasia varies. Monostotic fibrous dysplasia often has a better prognosis than polyostotic forms. The condition is generally not life-threatening, but it can lead to significant morbidity due to pain, deformity, and functional impairment.

Very low magnification of fibrous dysplasia

Related pages

• Bone disease
• Osteoblast
• Endocrine disorder