The skeletal framework around the anterior aperture of each orbit and the 4 walls of the orbital cavity can be conceived as a pyramid with a quadrangular base. This pyramid transforms into to a 3-sided posterior apex, because the orbital floor ends at the posterior basin of the inferior orbital fissure.
Bony openings (canals, grooves, fissures, foramina, notches) provide the pathways for the neurovascular structures linking the intraorbital structures inside the periorbital sac to the cranial cavity, ethmoid and skull base, infratemporal fossa, and face.
A system of 3 sagittal buttresses along the inferomedial orbital walls contributes to the stability (biomechanical resistance) of the internal orbit.
Besides the overall dimension of the orbital cavity, the contours and sloping of the inferomedial wall orbital surfaces, with the posteromedial bulge as predominantly, are the key determinants for globe height and globe projection.
The orbits are two inversely corresponding bony housings at the transition between the skull base and the midface. They contain the visual organ, consisting of the eyeballs with the refractive apparatus, retinal receptors and the auxilliary adnexa, in particular the lacrimal system and the extraocular muscles (EOM) and moreover the adipose body including innervation [optomotor, special (CN II) and general (CN V) somatosensory, parasympathetic and sympathetic] and vascular supply.
Orbital cavity—bony skeleton
Systematic skeletal description
Each orbit is assembled of 7 bones: zygoma, maxilla, palatine, ethmoid, lacrimal, sphenoid, and frontal. In highly simplified terms, the skeletal components outline a cone-shaped or pear-shaped cavity with a thick marginal rim framing the aperture at its base in contrast to the thin-walled internal orbit ( Fig. 1 A–D). Various bony openings (canals, grooves, fissures, foramina, and notches) are pathways for neural and vascular linkages to the cranial cavity, infratemporal fossa, paranasal sinuses, inner nose, and the face (see Fig. 1 A, B).
In geometric abstraction, the orbit can be described as a pyramid with a quadrangular base, which converts into a 3-sided tip or apex ( Fig. 2 A).
The open base, or aditus ad orbitam , projects frontolaterally and the apex posteromedially toward the optic foramen.
The junctions of the walls in the superomedial, superolateral, inferolateral, and inferomedial quadrants, or ”pyramidal corners”, are curved as a matter of fact ( Fig. 2 B, C).
The most posterior minor portion at the apex is formed by the lesser wing of the sphenoid (LWS). The orbital roof takes a triangle shape bent up into a concavity. The lacrimal fossa is a shallow depression anterolaterally for the lacrimal gland. The trochlear fovea conforms to the anteromedial adherence zone of the trochlear fiber condensations.
The inferior orbital wall (floor) incorporates the orbital plate of the maxilla as the major area, the orbital plate of the zygoma as anterolateral contribution, and the upper surface (plate) of the orbital process of the palatine bone at the posteromedial end position (see Fig. 1 B, C). Analogous to the orbital roof, it takes a triangular shape, however, with a limitation through the inferior orbital fissure (IOF). The orbital floor is shorter in anteroposterior extent than the 3 other orbitals walls and thus is missing in the orbital apex.
The medial wall is part of the centrofacial or naso-orbito-ethmoid unit and begins at the anterior lacrimal crest of the frontonasal process of the maxilla, followed by the lacrimal bone, the rectangular lamina papyracea of the ethmoid as the largest component, and the lateral sphenoid body posteriorly ( Fig. 4 A, B).
The frontoethmoidal suture line (FES) marks the level of the ethmoidal roof. The cribriform plate, however, may lie up to 10 mm caudal to the FES. A firm bony thickening results from support of the underlying basal (ground) lamina of the middle turbinate and reinforces the track of the maxilloethmoidal suture line. Therefore, it is referred to as inferomedial orbital strut (IOS).
The lateral orbital wall consists of the orbital plate (facies) of the zygoma anteriorly and the greater wing of the sphenoid (GWS) posteriorly (see Fig. 5 A,B and Fig. 6 ), forming a flat plane surface angulated approximately 45° toward the sagittal plane. The GWS separates the orbit from the middle cranial fossa and is part of the vertical pterygomaxillary buttress. Axial cross-sections unveil the posterior GWS as a central trigone, with a spongious bone space between the orbital, temporal, and cranial cortical surfaces ( Fig. 5 B). This potential space for surgical decompression is termed, the sphenoid door jamb (SDJ). More anteriorly, the lateral orbital wall turns into a monocortical layer with the zygomaticosphenoid suture (ZSS) line located in the thinnest portion. The ZSS is a reliable reference for the reduction of zygoma fractures from inside the orbit.
The marginal orbital tubercle (Whitnall tubercle) on the frontal zygomatic process ( Fig. 6 ) binds the attachments of the lateral retinacular suspension complex. It is a rounded blunt eminence of 2 mm or 3 mm diameter arising 2 mm to 4 mm behind the orbital rim and approximately 1 mm below the zygomaticofrontal suture (ZFS).
Bony openings—fissures, canals, grooves, foramina, and connections encountered during periorbital dissection
The optic canal opens into the superomedial corner of the orbital apex, where the posterior medial wall extension meets with the roof. It has an elliptical cross-section (approximate diameter 4–9.5 mm) a length of over 5.5 mm up to 11.5 mm and passes upward (15°) and inward (45°) until it ends medial to the anterior clinoid process in the middle cranial fossa. The canal is formed by the sphenoid body inferomedially, by the anterior root of the LWS superiorly, and by the optic strut laterally ( Fig. 7 A–C). The optic strut is a bony abutment linking the sphenoid body and the anterior clinoid process that separates the optic canal from the superior orbital fissure (SOF).
Superior orbital fissure
The SOF frequently is a club-shaped or L-shaped gap interposed between the LWS and the GWS or the orbital roof and the lateral wall, respectively ( Fig. 7 A–D). It slopes from the lateral apex inferomedially, where it levels along the sphenoid body and at the top of the maxillary strut, which is a transverse bony confluence above the foramen rotundum. The ring of Zinn (common tendinous ring/annular tendon), which is the origin of the 4 extraocular rectus muscles, subdivides the SOF into 3 individual hubs. The lateral half of the tendon ring (lateral and inferior rectus muscles) encircles a large outlet at the central or midlevel SOF sector, the superolateral annular foramen. The upper and lower divisions of the oculomotor nerve (CN III), the abducens nerve (CN VI), the nasocliary nerve (branch of CN V2), sympathetic nerve fibers, and the superior ophthalmic vein pass through this foramen and thereupon into the intraconal space of the orbit. The upper and innermost half of the annular ring (superior and medial rectus muscles) girdles the superomedial foramen, which funnels the optic nerve at its entrance into the optic foramen. The narrow superior lateral SOF sector outside and above the annulus transmits the frontal and lacrimal nerves (branches of CN V2) as well as the trochlear nerve (CN IV) into the extraconal space.
The inferior SOF sector underneath the annulus gives passage to the inferior ophthalmic vein and the sympathetic root as well as the long root of the ciliary ganglion (CG). A posterior extension of Müller’s orbital smooth muscle recedes into the lower boundary of this sector and stretches over the posterior IOF basin.
The cranio-orbital foramen (COF) (orbitomeningeal foramen) (see Fig. 1 A) is the exit point of the recurrent meningeal artery, which is an inconstant vessel (occuring in approximately in 40%-50% only) with a rather varying branching pattern. The COF is located either separately in the GWS next to the superolateral end of the SOF or it is even merging with the latter. The COF may be joining the orbit to the anterior (A-type) or middle (M-type) cranial fossa.
Inferior orbital fissure
Prototypically, the IOF outlines display a silhouette resembling a cat-tongue chocolate inside the orbit, although overall it is a complex 3-dimensional (3-D) opening of varying shapes ( Fig. 5 A–C). The long axis runs a posteromedial to anterolateral route, starting at the maxillary strut, to a loop in-between the lateral and the medial surfaces of the zygoma. The IOF separates the orbital floor from the lateral orbital wall. The infraorbital neurovascular bundle and the zygomatic nerve ascend from the pterygopalatine fossa (PPF) through the posterior IOF basin to reach either the infraorbital groove or the lateral orbital wall. The narrowing in the center of the IOF (isthmus promontory) originates from a crescent-shaped overhang of the orbital floor next to the surface of the palatine bone.
In contrast to the SOF, densely packed with nerves and vessels ( Table 1 ), the IOF encloses a continuation of the buccal fad pad into the orbital cavity (see Fig. 17 A). The fatty IOF backfill tissue intermingles with smooth muscle fibers (musculus orbitalis Müller), vessels (infraorbital vein/plexus pterygoideus, minor arterial branches), and parasympathetic nerve endings.
Foramen rotundum–maxillary strut–pterygoid (Vidian) canal
The foramen rotundum penetrates the base of the pterygoid process (see Figs. 2 B, 5 A, and 7B ) and is the path of communication for the maxillary nerve (CN V2) between the middle cranial fossa and the PPF. The maxillary strut relates to the bony bridge across the foramen rotundum. The opening of the pterygoid (Vidian) canal lies inferomedially to the foramen rotundum. It traverses the base of the medial pterygoid plate and exits into the PPF.
The ethmoidal foramina (EF) are laid out in an anterior-posterior row along the FES line (see Figs. 4 A, B and 13 A, B). Most of the time, there are 2 foramina —an anterior EF (AEF) and posterior EF (PEF). They may be supernumerary up to a maximum of 6. The topography of the holes (discussed later) varies not only in the sagittal direction but also vertically, with an intrasutural (frequency 85%) or extrasutural position, below or above the FES.
Awareness of the proximity of the posterior-most foramen to the optic canal is regarded as critically important in the prevention of optic neuropathy and amaurosis. Injuries of the ethmoid arteries may result in massive hemorrhage or retroorbital hematomas. Predictably, deroofing the ethmoidal canals above the FES level carries increased risk for bleeding and accidental entry into the anterior cranial fossa.
The infraorbital canal opens with an identically named foramen at the anterior facial wall (see Fig. 1 A, B, D). The canal begins as a conduit suspended on a bony beam underneath the anterior orbital floor and turns into the infraorbital groove (sulcus) as it runs backwards to the floor surface and its end at the IOF rear sink (posterior basin). Typically, the foramen is located 7 mm to 10 mm below the infraorbital margin on a plumb line passing through the midpupil.
Supraorbital and frontal foramina/notches
The supraorbital margin can embody supraorbital foramina and frontal foramina and/or passages formed as incisurae (notches) (see Figs. 1 A, B and 3A ). Their vertical heights differ—notches are contiguous with the margin; foramina are located superior to the rim. Although supraorbital foramina/notches (SOFNs) are a constant finding, frontal foramina/notches (FOFNs) are facultative, with varying frequency. The sizes and shapes of the apertures are rather diverse. Supraorbital emerging points are located on an imaginary borderline between the medial third and the lateral two-thirds of the bony rim, while frontal exits lie in the superolateral quadrant within a short range (0.5 cm) of SOFNs.
Zygomatico-orbital, zygomaticofacial, and zygomaticotemporal foramina
The openings and subsequent intrabony courses of the zygomatico-orbital foramen (ZOF), zygomaticofacial foramen (ZFF), and zygomaticotemporal foramen (ZTF) follow various patterns. As the name indicates, ZOF refers to single or multiple openings in the inner surface of the anterior inferolateral orbital quadrant ( Fig. 7 E, F). Each ZOF represents the entrance to a separate or an interconnected canal, which exits at the facial (ZFF) and/or temporal (ZTF) zygomatic surface. Hence, there can be an array of independent canals (ie, ZOF–ZFF and ZOF–ZTF) exclusively as well as a principal interconnected canal system with a Y-type division/subdivision standing either alone or with additional independent connections.
The frontonasal maxillary process, the lacrimal bone, and the lacrimal process of the inferior turbinate configure the nasolacrimal groove/canal (NLC) with the anterior and posterior lacrimal crests encompassing the fossa for the lacrimal sac at the medial orbital wall (see Fig. 5 A, B and 4 A, B). The amphora-like inlet of the membranous sac converges into the lacrimal duct (diameter 4–5 mm), which continues over a distance of at least 12 mm to the lower outlet, that is, the lacrimal plica underneath the anterior lower nasal concha.
Internal orbital buttresses
A set of 3 buttresses running in parallel stabilizes the orbital floor in the sagittal direction ( Fig. 8 A, B), the IOS along the maxillo-ethmoidal suture line, the intermediary bony underpinning of the infraorbital groove/canal, and the reinforcement along the medial IOF margin laterally. The involvement and fragmentation of the buttresses are indicators of the severity of the trauma.