Fibrous dysplasia is a developmental disorder of maxilla in which immature woven bone is formed directly from abnormal fibrous tissue. It is characterised by an expansile lesion of fibro osseous tissue. Fibrous dysplasia is of two types:
1. Mono ostotic fibrous dysplasia
2. Poly ostotic fibrous dysplasia
Mono ostotic fibrous dysplasia affects a single bone commonly affecting a rib or facial bones. It is also more common than poly ostotic fibrous dysplasia. This condition may be associated with systemic conditions like precocious puberty, and skin pigmentation. This condition if associated goes under the name McCune - Albright Syndrome. This lesion is active while the bone is growing. The lesion becomes inactive when the growth stops. The activity of the disease may increase later during pregnancy.
Poly ostotic variety affects many bones nearly 75 % of skeletal tissue may be affected. This lesion may remain active throughout life.
Pathology: Disorganised immature bone surrounded by immature fibrous tissue is seen in the dysplastic areas. This tissue doesnot have the capacity to mature into a lamellar bone. Hence the normal mechanical integrity of the bone is lost affecting the weight bearing bones in polyostotic fibrous dysplasia. Pain may be disabling. Fractures are common. Fractured bones heal with more immature bone tissue thereby accentuating the condition.
Rarely fibrous dysplasia may become more aggressive and dedifferentiate causing (desmoplastic fibroma). Sometimes the tissue may undergo sarcomatous transformation.
Risk factors for malignant transformation include:
1. Polyostotic form
2. Post radiation sequelae
3. Facial bone involvement
4. Albright's syndrome
Usually fibrous dysplasia affects affects people in their childhood / teens. Males are commonly affected than females. Albright syndrome is an exception wherein female preponderance is more.
Involved bones consists of immature and relatively undifferentiated fibrous connective tissue that fails to produce normal amounts of collagen. The collagen produced are not oriented appropriately to withstand the pressure stress.
Fibrous dysplasia of maxilla:
Fibrous dysplasia of the facial skeleton commonly involves the maxilla. It commonly involves one maxilla. The patient manifests with unilateral swelling of cheek (firm in consistency). Patient may have unilateral proptosis because dysplasia involves the boundaries of the orbit reducing the space available for the orbital contents. Some of these patients may suffer loss of vision due to entrapment of optic nerve in the dysplastic process.
1. Serum alkaline phosphatase levels are often elevated depending on the extent of bony disease.
2. Serum calcium phosphate may also be elevated
3. Pituitary gonadotropins and gonadosteroids are assessed to assist in the workup of precocious puberty.
Xray shows an expansile bony lesion. Bone walls are intact. Maxillary sinus is obliterated.
CT scan shows a lesion that is confined to the interior of the bone with no soft tissue component. It is helpful in distinguishing fibrous dysplasia from a malignancy. Features of malignancy include osteolysis, destruction of sclerotic margins, and cortical destruction with soft tissue extension. The bony lesion shows a homogenous matrix with obliteration of maxillary sinus cavity.
Image of a patient with fibrous dysplasia of left maxilla
Coronal CT Paranasal sinuses image showing fibrous dysplasia of left maxilla
Technetium Tc 99m methylene diphosphonate (MDP) bone scan can be done to distinguish mono ostotic from poly ostotic lesions. Increased uptake is more common in poly ostotic lesions.
Medical: Underlying endocrine deficiency if any must first be treated. Vitamin D and bisphosphonates (after physeal closure) may be helpful in ameliorating pain and possibly in reconstituting lesions with normal bone.
Surgical: Curettage of the affected bone with filling up of the defect using a bone graft is the advised surgical procedure. In the case of maxilla the affected bone bleeds extensively when an attempt is made to curette it. Adequate amount of blood must be reserved before surgery. The surgical procedure is known as Parring of the maxilla.