■ Part 1. Surgical Anatomy and General Considerations
The paired orbits represent a common meeting place of cranial and facial bones. At a key intersection, the orbits protect the eyes and their viscera (the so-called anterior visual systems) and, in the company of adjacent structures, establish upper facial proportion and dimension. As noted in Chapter 1, Cryer considered the zygomatic arch to be a “flying buttress,” supporting the greater wing of the sphenoid, the malar prominence, and temporal bone ( Fig. 8.1A,B ).
The orbits intersect the cranium and face on both a horizontal and vertical axis, as depicted in the art form ( Fig. 8.2 ).
By way of the nasal bones and the medial orbital frames (frontal processes of the maxillae), the orbits link the central upper face with the lateral upper face, represented by the zygoma, inferior and lateral orbital frame, frontozygomatic (FZ) suture, and the zygomatic arch. In similar fashion, the superior orbital frame and the roof of the orbit synchronize with the frontal bar and the nasofrontal suture. The orbits vertically align with the palate and maxillomandibular block below and the sphenoidal platform above by way of the anterior and posterior maxillary buttresses, respectively.
The General Architecture of the Orbit
The shape of the orbit can be likened to a pear or an Erlenmeyer flask, particularly in late adolescence and adulthood. The body of the flask or fruit is seated in the bilge of the floor and bilge of the roof ( Fig. 8.3A ).
The neck tapers toward the apex and the stem tilts upward, because the optic canal is at a higher level than the inferior orbital rim ( Fig. 8.3B ).
Contrary to common perception, the orbit is not triangulated, except at the apex. A model pine tree or triangulated Lego® (Billund, Denmark) construct, for example, is comparatively ill-fitting, except in the apex of the orbit ( Fig. 8.4A–C ).
Each of seven bones contributes to the orbit. The contributions are most evident in painted skulls, as the frontal, sphenoid, ethmoid, lacrimal, maxilla, palatine, and zygoma bones.1 – 4 An acronym to remember the seven bones is “ funky Susie Elberton likes munching p retzels at the zoo.” The palatine bone’s contribution is easily forgotten, so we have reminded students, residents, and fellows in training that they had “best remember the pretzels!” ( Fig. 8.5 ).
The temporal bone is only distantly involved, to the extent that it provides a zygomatic process to complete the zygomatic arch. The occipital bones offer no contribution to orbital structure.
Tessier and coauthors emphasize, as would we, that “orbital bones” (strictly speaking) “do not exist but, rather, are contributions made by bones of the cranial and facial skeleton.”1
The Orbital Roof
The frontal bone forms the anterior orbital roof, and the sphenoid bone forms the posterior orbital roof, as vividly depicted by painted skulls ( Fig. 8.6A–C ).
The posterior medial portion of the orbital roof adjacent to the outlet of the optic canal is thin and prone to fracture, as most evident in the illuminated skull and three-dimensional reformats.
Medial fractures may reach the floor and posterior table of the frontal sinus.5 Load forces concentrated in the more rigid bone of the posterior lateral portion of the orbital roof may reach the optic canal or extend to the anterior cranial vault or middle fossa.
The Medial Orbital Wall
The lamina papyracea of the ethmoid bone forms the medial orbital wall and anteriorly joins the lacrimal bone and maxillary process of the frontal bone, creating the posterior lacrimal crest. The middle and posterior ethmoid cells project into the orbit, subtly reducing the posterior orbital volume. Known as the ethmoid jut,6 , 7 the degree of curvature varies among individuals and is challenging to reconstruct; the convexity of temporoparietal split cranial bone best matches the jut when a bone graft is used. The medial wall gains only modest strength from the cross-struts (“septae”) within the middle and posterior ethmoid sinuses. The ethmoid jut is notably apparent when the orbital roof has been drilled away, providing a glimpse of the reduced orbital volume caused by its inward contour ( Fig. 8.7A,B ).
The anterior and posterior ethmoidal foramina, visible landmarks that define the boundary between the medial wall and roof (the frontoethmoid suture) of the orbit, lie in line with each other, some 25 and 35 mm, respectively, from the anterior lacrimal crest. They serve as guides to the location of the outlet of the optic canal, some 10 to 15 mm further inward. The ethmoid foramina are not found at the most superior portion of the orbit but are located at the level of the cribriform plate ( Fig. 8.8 ).
The Lateral Orbital Wall and the Orbitozygomatic Complex
The lateral wall of the orbit posteriorly is formed by the greater wing of the sphenoid and anteriorly by the orbital plate of the zygoma ( Fig. 8.9A,B ).
The two components of the lateral wall lie on a straight line directed toward the optic strut, the pillar-like column of bone immediately inferolateral to the optic foramen.4 , 6 , 7 The optic strut is part of the lesser wing of the sphenoid, lies at a 45-degree angle, and is strategically located to buttress the inferolateral aspect of the optic canal and superomedial aspect of the superior orbital fissure ( Fig. 8.10A,B ).
The greater wing of the sphenoid is curvilinear as the roof is engaged above and as the inferior orbital fissure is reached below. Reestablishment of the camber of the wing (sphenoidcant)4 , 6 , 7 is critical to ensure proper positioning of the fractured zygoma and to restore orbital volume ( Fig. 8.11 ).
The smooth continuity of the two components of the lateral wall is readily recognized when the orbital roof has been drilled away with cutting burrs ( Fig. 8.12 ).
Note the proximity of the middle fossa (and tip of the temporal lobe) deep to the greater wing of the sphenoid.
The body of the zygoma (orbitozygomatic complex) is underpinned by a lower process (the anterolateral midfacial buttress ) arising from the thickened periphery (alveolar process) of the palate (see Chapter 4). From the malar prominence arise four additional processes that are key to preoperative assessment and to operative finesse ( Fig. 8.13 ):
The frontal process of the zygoma leaves the malar prominence and joins the zygomatic process of the frontal bone at the FZ suture, as noted previously.
The inferior orbital process reaches medially, creating the lateral shape and contour of the inferior orbital rim. It ends at the infraorbital foramen.
The orbital plate of the zygoma that engages the greater wing of the sphenoid.
A temporal process leaves the zygomatic body to unite with the zygomatic process of the temporal bone. This combined span is referred to as the zygomatic arch. Their serrated intersection at midarch is called the zygomaticotemporal suture.
The zygomatic arch has a relatively flattened midsection, the length and linearity of which determines the projection of the body (malar prominence) of the zygoma. The arch is similar to the third (bottom) of three kitchen drawer pulls depicted ( Fig. 8.14 ).
The zygomatic arch is an important reference point in reestablishing correct anterior projection of the cheek during fracture repair. It is often manipulated from below with an instrument in one hand, and the linearity at the midaspect of the arch enforced by applying external, digital pressure over the arch with the other hand.8 This “trimanual” manipulation is similar to the “bimanual” reduction of a large nasal fragment by Manson.9 A coronal incision is required for more complex, unstable fractures of the arch; the linear architecture of the zygomatic arch is then readily reestablished with rigid fixation devices.