Fibro-Osseous Lesions of the Orbit and Optic Canal

Osteoma

Osteomas are the most common benign tumor of the paranasal sinuses and show a predilection for men with a male-to-female ratio of 1.5–3:1. The frontal (70% to 80%) and ethmoid sinuses (20% to 25%) are most commonly involved, followed by the maxillary and sphenoid sinuses, respectively. Osteomas are most typically diagnosed between the third and fourth decade of life and are thought to have an incidence of up to 3% of the general population and are often discovered incidentally on imaging. The exact underlying etiology or pathophysiology is not well understood; however prevailing theories postulate osteomas are either developmental, form secondary to trauma, or form secondary to infection.

In general, osteomas are slow-growing solitary lesions with an average growth of 1.6 millimeter (mm) per year (range 0.44 to 6.0 mm per year). Between 4% and 10% of osteomas produce clinical symptoms, and the symptoms are typically related to the location of the tumor, size, and growth rate. When osteomas are symptomatic, headache localized to the area over the tumor is the most common presenting symptom. Other symptoms include facial pain, swelling or deformity, nasal discharge or obstruction, and sinusitis. Orbital symptoms, including epiphora, proptosis, diplopia, and visual loss, can also be observed.

The imaging modality of choice for osteomas is a thin-slice computed tomography (CT) scan, as it provides detail regarding the size, location, and concurrent sinonasal pathology. Osteomas appear as well-circumscribed, dense masses with either a homogeneous or heterogeneous appearance ( Fig. 28.1 ). Earwaker characterized multiple types of osteomas based on CT imaging, including the following:

• 1.

  Uniformly sclerotic

• 2.

  Target-like lesion

• 3.

  Partially corticated shell with heterogeneous matrix

• 4.

  Heterogeneous matrix without a well-defined shell

• 5.

  Laminated pattern

Uniformly sclerotic lesions are the most common. Depending on the histologic makeup of the osteoma, it may be hyperintense on T1-weighted magnetic resonance imaging (MRI), as in the case of sclerotic lesions, or it may demonstrate a signal void on all sequences, as in the case of heterogeneous lesions. MRI can be used as an adjunct to evaluate for mucocele formation or intracranial and intraorbital involvement.

Histologically osteomas are well-circumscribed lesions characterized by a variable amount of cancellous and compact, lamellar bone with haversian systems. They can be divided into ivory and mature types.

Surgical intervention is typically reserved for symptomatic patients, for cases when the osteoma is causing obstruction of the involved sinus, or if the lesion demonstrates rapid growth. In the instance of slow-growing, asymptomatic osteomas, conservative management with intermittent radiographic follow-up is recommended. When osteomas are complicated, symptomatic, or rapidly growing, complete excision is the treatment of choice. Recurrence rates of these lesions are low with complete excision. Small osteomas are often removed en bloc with curettes, whereas others require extensive drilling.

In giant osteomas, which are characterized as lesions more than 30 mm in largest dimension or 110 g, dura or periorbita are often encountered during resection. This has led to controversy over the optimal surgical approach. Overall, osteomas of the paranasal sinuses can be resected using endoscopic approaches, external approaches, or a combination of the two. Size and location typically define the approach. Endoscopic approaches have been used in all locations within the paranasal sinuses; however, traditionally open approaches have been used more frequently for frontal sinus lesions. Historical approaches for these masses included a Lynch frontoethmoidectomy or osteoplastic flap to facilitate access, visualization, and treatment of possible complications such as cerebrospinal fluid leak. Cosmetic and functional concerns, in addition to the advent of improved endoscopes, instruments (such as high-speed endoscopic drills, the ultrasonic aspirator), and intraoperative navigation systems, have challenged the need for open approaches. Chiu et al. identified three factors that limited endoscopic resection of osteomas from the frontal sinus: location of the base of attachment, relative size of the tumor to that of the frontal recess, and location in relation to a virtual sagittal plane through the lamina papyracea. Osteomas were classified into four grades based on these characteristics, which are summarized in Table 28.1 . Endoscopic resection was recommended for grades I and II disease, and open approaches were advised for grades III and IV. However, there have been documented reports of successful endoscopic resections of osteomas with far lateral extent or intraorbital involvement.

Purely endoscopic approaches have gained significant popularity in recent decades. Although familiarity of the endoscopic modified Lothrop procedure has redefined the parameters by which frontal osteomas can be resected endoscopically, a narrow anteroposterior diameter of the frontal sinus and tumors attached to the orbital roof or anterior table of the frontal sinus significantly increase the need for open or endoscopic-assisted procedures.