Fibrous dysplasia is a disease of bone maturation and remodeling in which the normal medullary bone and cortices are replaced by a disorganized fibrous woven bone. The resultant fibro-osseous bone is more elastic and structurally weaker than the original bone.
Fibrous dysplasia is conceptualized into four types: (1) monostotic type, (2) craniofacial type, (3) polyostotic (Jaffe-Lichtenstein) type, and (4) polyostotic (McCune-Albright) type. Each type usually presents as an asymptomatic, slowly expanding portion of one or more bones. The condition develops in children and teenagers primarily, with few, if any, cases beginning after the age of 20 years.
1. Monostotic fibrous dysplasia
which involves a single focus in one bone, accounts for about 75% of fibrous dysplasia cases. In the jaws, this may be seen most frequently in the body of the mandible or in the premolar-molar regions of the maxilla. It gives rise to a bony swelling caused by a poorly circumscribed area of fibro-osseous proliferation. This typically starts in childhood but usually undergoes arrest in adulthood. The jaws are the most frequent sites in the head and neck region. Males and females are almost equally frequently affected.
Clinically, a painless, smoothly rounded swelling, usually of the maxilla, is typical. The mass may become large enough to disturb function and cause malocclusion by displacing teeth.
Radiographically, there is typically an area of weak radiopacity, with a fine orange-peel texture and an eggshell-thin cortex of expanded bone. The degree of radiopacity depends on the amount of lesional bone which may give a predominantly radiolucent or more sclerotic appearance. The key feature is that the margins merge imperceptibly with the surrounding normal bone.
Polyostotic fibrous dysplasia
which involves two or more noncontiguous bones. This form is less common than monostotic fibrous dysplasia and may involve the skull, jaws, or facial bones together with ribs, long bones, or the pelvis. Two types of polyostotic fibrous dysplasia have been isolated.
2. Polyostotic (McCune-Albright) type
McCune-Albright syndrome encompasses polyostotic fibrous dysplasia with cutaneous melanotic pigmentations called cafe-au-lait macules and endocrine abnormalities. The most common of the endocrine abnormalities is precocious puberty. In fact, the youngest childbirth on record occurred when a 5-year-old Peruvian girl with McCune-Albright syndrome gave birth. Other endocrinopathies that may be part of this syndrome are hyperthyroidism, acromegaly, and hyperprolactinemia.
3. Polyostotic (Jaffe-Lichtenstein) type
The Jaffe-Lichtenstein type, less well known than McCune-Albright syndrome, describes polyostotic fibrous dysplasia with cutaneous melanotic pigmentations in the absence of endocrine abnormalities.
4. Craniofacial fibrous dysplasia
which involves two or more bones of the jaw-midface-skull complex in continuity without cafe-au-lait macules or precocious puberty. This type of fibrous dysplasia is seen relatively often in dental and oral and maxillofacial practices. It is frequently underestimated and thought to be a monostotic fibrous dysplasia of the maxilla, yet it often includes the zygoma, sphenoid, temporal bone, nasal concha, and clivus.
Nearly all cases of fibrous dysplasia will show a diffuse, hazy trabecular pattern that has been called the ground-glass appearance. However, some reports have described this pattern as radiolucent, whereas others have described it as mottled pagetoid.
Most radiographic and CT scan pictures of fibrous dysplasia show a homogenous, finely trabecular bone pattern replacing the medullary bone and diagnostically both cortices as well as sometimes the lamina dura. Its shape is fusiform and its margins are indistinct, showing a gradual blend into normal bone. It shows greater buccal than lingual expansion and does not displace the inferior alveolar canal.
The lesions consist of loose cellular fibrous tissue containing slender trabeculae of woven bone of variable shape which merge imperceptibly into surrounding normal bone. Osteoblasts are scattered throughout the substance of the trabeculae rather than surrounding them. Some lamellar bone or calcified spherules and occasional loose foci of giant cells can also be seen.
The single most important differential diagnosis for fibrous dysplasia is to distinguish it from an ossifying fibroma. Other entities that may resemble fibrous dysplasia include chronic sclerosing osteomyelitis, Paget disease, and sometimes osteosarcoma.
The disease is self-limiting, but grossly disfiguring lesions may need to be excised. This should be delayed if possible until the process has become inactive.
Radiotherapy is contraindicated in the treatment of fibrous dysplasias. Numerous cases of radiation sarcomas arising from radiotherapy have been documented.