Hydroxyapatite cranioplasty in fibrous dysplasia of the skull.
We describe the case of a woman who had complained of worsening headache during the previous year. Analysis of computed tomography imaging and biopsy led to a diagnosis of fibrous dysplasia of the skull. A symptomatic frontal bone lesion was removed, and the resulting defect was repaired with hydroxyapatite cement. Hydroxyapatite cement has several advantages over hydroxyapatite ceramic blocks and granules, and it is particularly well suited for calvarial reconstruction.
Fibrous dyplasia is a developmental anomaly of the bone that is characterized by the replacement of normal bone by fibro-osseous tissue. The decision on when, or whether, to perform surgery to correct the condition must be individualized according to each patient's particular clinical circumstances. When surgery is performed, repair of the resulting defect can sometimes be accomplished with hydroxyapatite cement. In this article, we describe one such case, and we briefly review the clinical, pathologic, and radiologic features of this disease.
A 33-year-old woman came to the otolaryngology clinic with a complaint of worsening right frontoparietal headaches during the previous year. Her medical history was positive for migraines during adolescence, but they had become rare during the previous several years. Her current headaches were distinctly different from her previous migraines.
Computed tomography (CT) of the head had been obtained prior to consultation. The CT revealed the presence of lesions of the right ethmoid, sphenoid, and frontal bones that were consistent with fibrous dysplasia. The head and neck examination revealed a large, firm, right middle turbinate and a bony lesion of approximately 5 cm in diameter on the right frontal bone that was easily palpable under the scalp.
The right middle turbinate was biopsied, and the diagnosis of fibrous dysplasia was confirmed. A facial CT was obtained to better define the lesions (figure 1). The CT demonstrated involvement of the right middle turbinate and the ethmoid and sphenoid sinuses, as well as the right frontal bone. Because the frontal bone lesion was symptomatic, it was removed via a bicoronal incision. At surgery, a 5 x 5 x 2-cm firm rubbery mass was found and removed completely. The inner table of the skull was found to be intact. The resulting defect was filled with hydroxyapatite cement. Pathologic analysis showed fibrous dysplasia without evidence of sarcoma (figure 2).
The patient's recovery was unremarkable. She was seen for followup 6 months after surgery, and she had remained free of headaches. A followup CT at that time clearly demonstrated the implanted hydroxyapatite cement (figure 3). The ethmoid and sphenoid lesions have remained asymptomatic, and they have been followed without treatment.
Fibrous dysplasia typically appears during the first two decades of life, during which time there is a progressive growth of lesions. The condition eventually stabilizes in early adulthood. There are three typical varieties of fibrous dysplasia: monostotic, polyostotic, and polyostotic with pigmented skin lesions and endocrine abnormalities (McCune-Albright syndrome).
The most commonly involved bones of the craniofacial skeleton are the maxilla and frontal bones.  Approximately 10% of the monostotic lesions involve the head and neck. More than 50% of patients who are affected with the polyostotic variety have craniofacial lesions.  Lesions of the head and neck typically do not respect sutures of the skull, and in these cases they are technically classified as polyostotic. It has been suggested that such lesions that are not associated with involvement of the extracranial skeleton represent a third type of polyostotic fibrous dysplasia. 
The histopathologic hallmark of fibrous dysplasia is a mixture of fibrous connective tissue and immature, woven bone with a paucity of osteoblasts. Fibrous dysplasia must be differentiated from other fibro-osseous entities--chiefly ossifying fibroma, which features mature lamellar bone rimmed by osteoblasts. The low risk of malignant transformation to osteosarcoma has been well described; transformation occurs in fewer than 1% of cases. [4,5]
The radiographic appearance of fibrous dysplasia is variable. Different lesions in the same patient can have different appearances on CT (figure 1). In our patient, the ethmoid and sphenoid lesions had the typical "ground glass" appearance, while the frontal bone mass had a predominately soft tissue density with irregular calcifications.
The treatment of craniofacial fibrous dysplasia must be tailored to the individual patient. Because these lesions often cross suture lines, complete excision might cause unnecessary morbidity. This risk must be balanced against the cosmetic and functional disturbances that arise as the lesions grow. Because the disease often arrests at puberty, surgery should be delayed until that time if possible. Surgery on actively growing foci is associated with increased blood loss and a relatively rapid recurrence. Moreover, incomplete excision is associated with a greater than 25% rate of recurrence. Radiation therapy is contraindicated because it increases the chance of malignant transformation. [2,5]
The case described here illustrates that no strict protocol can be applied to the disease. The frontal lesion was symptomatic and readily accessible, and therefore it was treated. On the other hand, aggressive treatment of the patient's sphenoethmoid disease, which was felt to be not amenable to endoscopic techniques, was avoided because the lesion was not causing symptoms.
Hydroxyapatite cement is a relatively new biomaterial available for cranial reconstruction. It has several advantages over previously developed hydroxyapatite compounds. Prior to the development of hydroxyapatite cement, hydroxyapatite was available only as dense ceramic blocks or granules. Neither of these formulations was particularly suited for cranial reconstruction because of difficulties with sculpting.
Hydroxyapatite cement is prepared as a thick paste that can be applied to the defect. It hardens in about 15 minutes. Experimental studies show that there is no loss of shape or volume over time and that the implant is slowly replaced from its periphery with native bone in a higher proportion than that seen with the other types of hydroxyapatite. And unlike methyl methacrylate, hydroxyapatite cement causes no exothermic reaction during the setting phase. Also, it does not induce the inflammatory response noted with methyl methacrylate.  Finally, the advantages of this cement over autogenous bone grafts are obvious.
It should be noted that the ultimate strength of implanted hydroxyapatite has not yet been established. Therefore, its use should be limited to non-stress-bearing bone.
From the Otolaryngology--Head and Neck Surgery Service, Womack Army Medical Center, Fort Bragg, N.C.
Reprint requests: MAJ Andrew Vories, MD, Otolaryngology Service, Darnall Army Community Hospital, Fort Hood, TX 76544. Phone: (254) 288-8497; fax: (254) 286-7105.
The opinions or assertions contained herein are the private views of the authors and are not to be construed as reflecting the views of the Department of the Army or the Department of Defense.
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