Fibrous dysplasia is a rare, nonheritable, noncancerous disease of bone where normal bone is replaced with fibrous bone tissue leading to abnormal growth or swelling of bone leading to altered mechanical quality of bones is and increased risk of fracture especially in long bones.Incidence of fractures is around 50% of cases. Patients also have an increased risk of malignant tumors such as osteosarcoma, fibrosarcoma, chondrosarcoma, and malignant fibrohistiocytoma. The risk of malignancy is higher in patients with the polyostotic form, or McCune Albright syndrome. The term fibrous dysplasia was suggested by Lichtenstein and Jaffe in 1942.
Fibrous dysplasia is of two types
- Monostotic fibrous dysplasia affects single bone or adjacent bones, like both the upper and lower jaw.
- Polyostotic fibrous dysplasia affects multiple bone.
70-80% cases are monostotic.
The polyostotic form is occasionally associated with precocious puberty, fibrous dysplasia, and cafe-au-lait skin lesions (McCune-Albright syndrome) or with myxomas of skeletal muscle (Mazabraud syndrome).
Monostotic fibrous dysplasia becomes inactive after puberty. It may reactivate during pregnancy. Polyostotic disease typically remains active throughout life.
Fibrous dysplasia of bone may also be an associated abnormality in Neurofibromatosis type II.
This disorder is usually diagnosed in childhood or early adulthood. Usual presentation is in children and adolescents, with a median onset age of 8 years. Fibrous dysplasia makes about 5% of all benign bone tumors. As many atients are asymptomatic, so the true incidence of this disorder is unknown.
Males are affected more often than females, except in McCune-Albright syndrome, in which females are affected more often than males.
Fibrous dysplasia is caused by the sporadic mutation of the GNAS1 gene. As a consequence of the mutation, there is a substitution of cysteine or histidine in amino acids of the osteoblastic cells, by arginine. Osteoblastic cells that express this mutation have a higher DNA synthesis than normal bone cells and the growth of these cells is faster, leading to an inappropriate differentiation of mesenchymal cells.
As a result, the medullary cavity of affected bones is filled with fibrous tissue, causing the expansion of the areas of bone involved.
Clinical presentation depends on the bone involved. Mostly the patients with fibrous dysplasia have no symptoms and are often diagnosed during during investigations for an unrelated problem. When symptomatic, pain is most common symptom. It is caused by expansion of bone. Irregular bone growth can lead to deformities. Sometimes, the patient may present with pathological fractures due to weakening of the bone. Skull or facial bones involvement can cause visible deformities.
In 3% of cases, people suffering from fibrous dysplasia also have endocrine diseases[hyperthyroidism, Cushing disease, hyperparathyroidism, and hypophosphatemic rickets] and skin pigmentation[McCune Albright syndrome] These endocrine diseases include early puberty.
The femur is the most common bone to be involve followed by tibia, maxilla, and skull.
Serum alkaline phosphatase levels are often increased in during active phases of this disease. Serum alkaline phosphatase levels can also be used to asses patients treated with bisphosphonates.
Patients might show vitamin D deficiency. Rickets should be ruled out with serum calcium, phosphate, and vitamin D levels are useful to exclude rickets.
In patients with precocious puberty, gonadotropins and gonadosteroids levels are assessed.
In McCune-Albright syndrome thyroid levels and other endocrine hyperfunction like gigantism, or hypercortisolism.
Lesions are typically medullary and occur in diaphysis with extension towards metaphysis. The cortices appear expanded and the matrix of the lesion has a ground-glass appearance. In upper end of femur, shepherd’s crook deformity is name given to the varus deformity that occurs with this lesion.
Technetium-99m methylene diphosphonate bone scan shows increased uptake due to osteoblastic activity. The bone scan is useful to determine if the disease is monostotic or polyostotic.
CT is helpful in differentiating fibrous dysplasia from a malignancy. MRI tells about medullary extent better.
Microscopy shows a fibrous/collagenous matrix with randomly oriented bone or fiber trabeculae that are formed by osseous metaplasia of spindled stromal cells. This trabecular pattern has an appearance of Chinese letter appearance.
Asymptomatic patients of fibrous dysplasia do not require treatment. There is no medical management for fibrous dysplasia. Symptomatic treatment in form of NSIADs may be given to control pain.. Bisphosphonates have been found to be effctive in decreasing chronic pain in patients with fibrous dysplasia need further investigations.
Surgical treatment of fibrous dysplasia is indicated in the prevention or treatment of fractures or major deformity. The most common surgical indications are fracture of a weight-bearing bone or a progressive disease. Patients with involvement of upper limb rarely require surgical management.
Intamedullary fixation is recommended in cases of fibrous dysplasia for stabilization and deformity prevention. Use of plates, curettage, or bone grafting to fill the cavity is no more recommended as the recurrence rates over the period are almost 100%
In severe deformities, correction of the deformity should be done.
After surgery, patients are followed on yearly basis for deformity evaluation and monitor disease progression
Fracture and deformity in weight-bearing bones are most common complications. Malignant transformation may occur in less than 0.5% of cases especially in if polyostotic disease or following radiation therapy.
More than half of patients atients with McCune-Albright syndrome have a high incidence of scoliosis
Molecular diagnosis using the techniques of polymerase chain reaction analysis with peptide nucleic acid can help in diagnosis of fibrous dysplasia or McCune-Albright syndrome could be helped by identification of this mutation in the peripheral blood.
Xray of Fibrous Dysplasia of Femur and Tibia
Following xrays are of 16 years old girl who came to out patient department with deformity of thigh. Her xrays revealed bowing of femur, coxa vara [decrease in neck shaft angle and multiple radiolucent shadows in tibia and femur suggestive of fibrous dysplasia.
Here is the xray of fibrous dysplasia of upper femur
Xray of fibrous dysplasia of tibia of same side.
Deformity was main complaint of the patient. Patient was advised to follow up regularly and to take analgesics as and when needed.
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