Fibrous dysplasia is a benign, slowly progressive, self-limiting bone disorder of unknown etiology. Normal bone tissue is replaced by fibrous tissue composed of homogeneous spindle cells and dysplastic woven bone trabeculae. This may result from arrested bone maturation during the woven bone stage, postnatal disruption of woven bone support, or defective differentiation of bone-forming mesenchyme. Clinically, this condition is not rare, accounting for approximately 25% of all bone neoplasms and 7% of all benign bone tumors. The monostotic form constitutes about 70% of cases, the polyostotic form without endocrine disorders about 30%, and the polyostotic form with endocrine dysfunction about 3%.

bubble_chart Etiology

Unknown. It may be related to trauma, infection, endocrine dysfunction, or certain factors causing local circulatory disturbances, but none have been confirmed. Currently, it is generally believed that this condition is not a true tumor.

bubble_chart Pathological Changes

The vascular supply varies significantly. The affected tissue generally appears white, grayish-white, or pale yellow, slightly softer than normal bone tissue, and exhibits a gritty or elastic sensation when cut. Large bone lesions often erode and expand outward from the bone marrow, leaving only two thin layers of cortical bone in tubular and flat bones. Removing the outer shell resembles peeling a membrane. Microscopically, the size, shape, and distribution of the trabeculae of woven bone are irregular, haphazardly embedded in loose or dense connective tissue rich in cells and blood vessels. This tissue resembles the result of connective tissue metaplasia. The trabeculae exhibit considerable morphological variation, often spherical, appearing curved, C-shaped, or arched in cross-section, with irregular edges and wide osteocyte lacunae. The trabeculae are densely packed, forming a bone network. They consist of coarse-fibered woven bone, appearing reticular rather than lamellar under polarized light. Occasionally, woven bone shows lamellar transformation, and sometimes arched trabeculae encircle a central blood vessel. Most trabeculae lack osteoblastic lining, which helps differentiate this condition from ossifying fibroma.

Clinical Classification Most authors agree with the classification proposed by Beleval and Schneider (1954), which divides the disease into three types: ① Monostotic type: Single or multiple lesions affecting one bone, with the maxilla being the most frequently involved (64%), followed by the mandible (36%) and craniofacial bones (10%). ② Polyostotic type without endocrine disorders: Multiple lesions affecting more than one bone. In moderately affected polyostotic cases, the incidence of craniofacial bone involvement is 5%, while in extensively affected polyostotic cases, it reaches 100%. Van Tilburg, after reviewing 144 reported cases, found that in both monostotic and polyostotic types, the frontal and sphenoid bones were most commonly affected in the craniofacial region, often simultaneously, followed by the ethmoid and temporal bones. The disease may occur unilaterally or bilaterally. ③ Polyostotic type with endocrine disorders: The ratio of this type to the monostotic type is 30:1. Lesions are scattered across multiple bones, often unilaterally distributed, and accompanied by larger skin pigmentation spots. It is more common in females, presenting with precocious secondary sexual characteristics.

Clinical Manifestations Approximately 60% of cases occur before the age of 20, with occasional occurrences in infants and individuals over 70. The male-to-female ratio is 1:2. Over 80% present with deformity and swelling of the affected bone. When the face is involved, it manifests as facial asymmetry, eyeball displacement or protrusion, nasal stenosis, loose teeth, alveolar ridge deformity, tearing, and palatal elevation. As the disease progresses, headache and occasional epistaxis may occur. Depending on the primary site and extent of involvement, corresponding clinical symptoms may appear. For example, involvement of the temporal bone often leads to enlargement and deformation of the temporal bone, narrowing of the external auditory canal, and conductive deafness. About 16% of cases with external auditory canal stenosis are complicated by sebaceous cysts. Those with sebaceous cysts often develop temporomandibular arthritis, deviation of the mouth, labyrinthitis, or intracranial complications. Involvement of the cochlea and internal auditory canal may result in sensorineural deafness. Invasion of the petrous bone can easily lead to symptoms of middle or posterior cranial fossa involvement. The disease may extensively invade the nasal sinuses, orbits, and the floor of the anterior cranial fossa, exhibiting a clinically malignant growth tendency, presenting as stuffy nose, hyposmia, facial asymmetry, eyeball protrusion or displacement, diplopia, visual impairment, and difficulty opening the mouth. Fibrous dysplasia in the sphenoid bone and sphenoid sinus area often causes severe frontal, parietal, or occipital pain. Due to the thin walls of the sphenoid sinus, lesions can easily extend to surrounding structures, affecting cranial nerves II, III, IV, V, and VI, leading to cranial nerve damage symptoms and signs. Larger lesions may cause brain atrophy or result in increased intracranial pressure.

Examination Imaging studies are of special significance in the diagnosis of this disease. Based on X-ray findings, the disease is classified into three types: ① **Pagetoid Type**: Often presenting as polyostotic lesions, characterized by cranial thickening with unilateral bubble-like expansion of the outer table and parietal bone. The inner table bulges into the diploë and cranial cavity. The thickened skull commonly shows coexisting localized and diffuse radiolucent and radiodense areas, resembling the mixed osteolytic and sclerotic features of Paget's disease. The skull enlarges and hardens, extending from the frontal bone to the occipital bone. Facial involvement may lead to orbital and nasal stenosis and obliteration of sinus cavities. This type accounts for about 56%. ② **Sclerotic Type**: This type often manifests as maxillary hypertrophy, causing dental misalignment and compression-induced narrowing of the nasal cavity and sinuses. The maxilla is more frequently affected than the mandible, usually presenting as a monostotic lesion. The lesions appear sclerotic or ground-glass-like. In contrast, mandibular lesions are more common in polyostotic cases, presenting as smooth-walled, radiolucent isolated defects. This type accounts for about 23%. ③ **Cystic Type**: The skull exhibits single or multiple circular or rose-flower-shaped defects, starting from a thin sclerotic margin, with diameters reaching several centimeters. Solitary lesions may resemble eosinophilic granuloma, while multiple defects can be mistaken for Hand-Schüller-Christian disease. Occasionally, multiple radiographic patterns may coexist in the same individual. This type accounts for about 21%. CT or MRI can precisely determine the location and extent of the lesions and reveal their relationship with surrounding soft tissues. Regular follow-up examinations are crucial for dynamically monitoring disease progression, guiding surgical approach selection, reducing complications, and estimating prognosis.

Except for the early stages of the monostotic type, which are difficult to detect, this disease can generally be diagnosed based on a combination of medical history, location, signs, and imaging studies, often without the need for histological evidence.

bubble_chart Treatment Measures

This disease, especially the monostotic type, is primarily treated with surgical resection, as radiotherapy may induce malignant transformation. Given the slow clinical progression of the disease, for patients with smaller lesions or no symptoms, surgery may be temporarily deferred, but close follow-up observation is essential. Rapidly progressing lesions, accompanied by significant deformities and functional impairments, should be considered indications for surgery. While radical resection is the optimal treatment, it carries the risk of functional impairment and cosmetic defects. Conservative partial resection is prone to recurrence, with rates of 21% for the monostotic type and up to 36% for the polyostotic type. The choice of surgical method and approach should be flexible, based on the primary site, extent of invasion, and degree of functional damage. The principle is to remove the diseased tissue as thoroughly as possible while preserving physiological function and cosmetic outcomes to the greatest extent.

During surgery, general anesthesia with oral intubation is preferred for children and patients with extensive lesions. For localized lesions, resection under local anesthesia is also feasible.

There are various surgical incisions, which can be selected based on the lesion's condition: ① Caldwell-Luc approach: Suitable for lesions extensively involving the maxilla, nasal cavity, infraorbital wall, ethmoid sinus, and sphenoid sinus. ② Weber-Fergusson approach: Suitable for patients with extensive invasion of the maxilla, infraorbital wall, zygomatic bone, hard palate, and sphenoethmoid sinus. ③ Craniofacial combined approach: Includes bifrontal or unilateral frontal flap + Weber-Fergusson incision, suitable for extensive lesions originating in the anterior cranial fossa floor or involving the nasal sinuses and orbital walls mutually. ④ Fish approach: Suitable for lesions originating in the temporal bone, with involvement of the external auditory canal, middle ear, inner ear, petrous bone, and middle cranial fossa floor.

Surgery is best performed using a flat chisel, round chisel, or large curette for stepwise resection. Intraoperative bleeding may be significant, so bone wax for hemostasis and necessary blood transfusion are advisable, especially for children.

The prognosis for surgical resection of this disease is favorable. Therefore, during surgery, excessive resection of lesions near critical nerves and vascular areas at the skull base or intracranial regions should be avoided to prevent complications.

bubble_chart Differentiation

During the diagnostic process, attention should be paid to differentiating the following diseases:

1. **Ossifying Fibroma** It has been clarified in recent years that this disease is completely distinct from fibrous dysplasia of bone. Clinically, it grows slowly and presents as an isolated lesion, more commonly affecting the mandible than the maxilla, and occasionally seen in the frontal and ethmoid bones. It occurs more frequently in females than males, with a predilection for ages 15–26. Radiographically, it appears as a well-defined, expansile, radiolucent lesion with a speckled or opaque central area. Microscopically, it is predominantly composed of fibrous tissue, with irregular trabeculae of bone haphazardly distributed within the fibrous stroma, forming the center of woven bone. However, osteoblasts are present at the periphery of lamellar bone and along the occlusal margin.

2. **Eosinophilic Granuloma** This is a benign, solitary, non-neoplastic osteolytic lesion originating from the reticuloendothelial system. It commonly involves the frontal, parietal, and mandibular bones, predominantly occurring before the age of 30 and more frequently in males. Histologically, it consists of dense aggregates of foamy histiocytes, accompanied by varying numbers of eosinophils and multinucleated giant cells. The nuclei of histiocytes contain small vacuoles, eosinophils exhibit fine vacuolation, and the giant cells are of the Langerhans or foreign-body type. These cells form focal aggregates.

3. **Gardner Syndrome** This syndrome involves multiple bone tumors affecting the maxilla, mandible, skull, and occasionally long bones, accompanied by intestinal tumors, dermoid cysts, fibromas, and focal undulating cortical thickening of long bones.

4. **Gigantiform Cementoma** Typically involving the entire mandible, it can cause cortical expansion. Radiographic examination reveals dense, lumpy accumulations. It is often hereditary, with no infectious source identified histologically.

5. **Exophytic Bone Tumors** Malignant tumors and cysts of the paranasal sinuses, among others, should also be carefully differentiated to avoid misdiagnosis.

6. **Polyostotic Fibrous Dysplasia** Additionally, it should be differentiated from hyperthyroidism, Paget's disease, neurofibromatosis, and jaw hypertrophy, among other conditions.