The unique and complex anatomy of the orbit means that small errors can produce suboptimal functional and cosmetic results. Thus, thorough knowledge of the anatomy of the orbit and its contents is critical for successful orbital reconstruction. The operative plan must be constructed on the basis of an accurate physical examination of the soft tissues, facial skeleton, and visual sensory system, combined with high-resolution computed tomographic scans in the axial and coronal planes with three-dimensional reconstruction. The addition of medical modeling adds the benefit of hands-on models on which to base the reconstructive plan. The overall success of the reconstructive procedure depends on adequate repositioning or reconstruction of the bony orbit and correction of any soft tissue defects. In the oncologic setting, extensive tumor resection may necessitate adequate soft tissue coverage of large defects.

21.2 Anatomic Considerations

Familiarity with the correct anatomic relationships of the orbit is essential in maintaining proper form and function.

In adults, the lateral orbital walls are approximately 90◦ from each other and angle 45◦ from anterior to posterior. The medial orbital walls are nearly straight anteroposterior, angling only slightly medially anteriorly. The divergent axis of each orbit is 23◦ (Fig. 21.1).

The orbit is widest 1 cm behind the orbital rim, which corresponds to the equator of the globe. The depth of the orbit is approximately 45 mm, but there is substantial variation between individuals and slight interindividual variation between the left and right orbits.

Fig. 21.1 Horizontal section through orbits. From Lemke BN, Lucarelli MJ. Anatomy of the ocular adnexa, orbit, and related facial structures. In: Nesi FA, Lisman RD, Levine MR, eds. Smith’s Ophthalmic Plastic and Reconstructive Surgery. St. Louis, MO: Mosby-Year Book; 1998; p.4. Reprinted with permission

21.2.1 Orbital Margin

The orbital margin is a discontinuous spiral. It is roughly rectangular with a horizontal dimension of 40 mm and a vertical dimension of 32 mm. The zygomatic bone forms most of the lateral margin and the lateral half of the inferior rim. This is the facial buttress and provides significant protection to the globe. The zygomatic bone is also an important aesthetic component, providing elevation and projection to the midface. Any reconstruction should focus on returning as much symmetry as possible to the face.

The frontal bone encompasses the superior orbital margin and extends laterally and medially to form portions of the lateral and medial borders. In most skulls, the supraorbital neurovascular bundle rising to the forehead forms the medial superior notch. In some patients, the frontal bone actually forms a foramen.

The maxillary bone, rising to meet the maxillary process of the frontal bone, forms the medial orbital margin inferiorly. The lacrimal sac complicates the medial rim by indenting the maxillary bone and forming anterior (maxillary bone) and posterior (lacrimal bone) crests.

The infraorbital neurovascular bundle exits the maxilla via a foramen approximately 1 cm below the inferior orbital rim.

21.2.2 Nasal and Paranasal Sinuses

The orbital roof, floor, and medial wall are intimately related to the nasal cavity. These bones are pneumatized by paranasal sinuses arising from and maintaining communication with the nasal cavity.

The maxillary sinus is the largest paranasal sinus (15 mL) and is the first to develop. The roof of the maxillary sinus is the orbital floor. Medially, the orbital floor is thin and prone to fracture. It thickens laterally near the infraorbital canal. The roof of the sinus (orbital floor) declines from the medial wall to the lateral wall at an angle of 30◦. The pterygopalatine fossa lies posteriorly with the internal maxillary artery intimately related to the posterior sinus wall. The nasal cavity lies medially except where the nasolacrimal canal and then inferior turbinate intervene.

The ethmoidal sinuses are the most exuberantly growing sinuses and may pneumatize the frontal, sphenoid, palatine, and lacrimal bones. Normally, 3–15 air cells expand from each lateral border of the cribriform plate. The air cell masses convolute medially to form the middle, superior, and supreme (if present) turbinates. The anterior and middle air cells drain into the middle meatus. The posterior air cells drain into the superior meatus.

The frontal sinus evaginates from the frontal recess superior to the nasal hiatus semilunaris. Pits are present at birth, but the infundibulum is not well developed or radiographically evident until approximately 6 years of age. The frontal sinus then expands until early adulthood, with greater expansion occurring in males. The frontonasal duct drains into the anterior middle meatus.