Anatomy of the Orbits Carl Peter Cornelius MD, DDS , Florian Probst MD, DMD, PhD , Marc Christian Metzger MD, DMD and Peter J.J. Gooris MD, DMD, PhD, FEBOMFS Atlas of the Oral and Maxillofacial Surgery Clinics of North America, 2021-03-01, Volume 29, Issue 1, Pages 1-18, Copyright © 2020 Elsevier Inc. Key points 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. Introduction 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). Fig. 1 ( A ) Orbits—anterior view—rim circumference. The broad blending of the anterior and posterior lacrimal crest results in discontinuity and ambiguity at the medial orbital rim. The anterior view limits the visibility over the internal orbital surfaces. ( B ) Orbits—left antero-oblique view to get insight into the inferomedial and anterolateral orbital walls concurrently. ( C ) Posterior view of the maxillae (swung out) to appreciate the orbital process of palatine bones ( red ) as robust constituent of the posterior orbital floor. ( D ) Osseous model set up demonstrating the origin of the posterior ledge—the orbital process of the palatine bone ( red ) reaches to the rearmost portion of the orbital floor with its orbital plate (facies orbitalis) (corresponding to “posterior ledge”); it is contiguous to the perpendicular and horizontal plate. Ethmoid bone ( yellow ), sphenoid ( turquoise ) with GWS, LWS, and pterygoid process—outer plate and hamulus. ALC, Anterior Lacrimal Crest; CCPB, Conchal Crest of Palatine Bone; COF, Cranioorbital Foramen; FES, Frontoethmoidal Suture; FLS, Fossa for Lacrimal Sac; FOFN, Frontal Foramen / Notch; FPZB, Frontal Process of Zygoma Bone; GWS, Greater Wing of Sphenoid; HPPB, Horizontal Process of Palatine Bone; IMM, Infraorbital Margin of Maxilla; IOF, Inferior Orbital Fisssure; IOFIP, IOF Isthmus Promontory; IOG, Infraorbital Groove; IOMN, Infraorbital Margin; IPZ, Infraorbital Process of Zygoma; IT, Inferior Turbinate; LB, Lacrimal Bone; LOMN, Lateral Orbital Margin; LWS, Lesser Wing of Sphenoid; MES, Maxilloethmoidal Suture Line; MOMLP, Medial Orbital Margin - Lower Part; MOMUP, Medial Orbital Margin - Upper Part; OFC, Optic Foramen/Canal; OPE, Orbital Plate of Ethmoid; OPFB, Orbital Plate of Frontal Bone; OPPB, Orbital Process and Plate of Palatine Bone; OPZ, Orbital Plate of Zygoma, OPZLP, Orbital Plate of Zygoma Lower Part; OSM, Orbital Surface of Maxilla; OS, Optic Strut; PDPB, Perpendicular plate of palatine bone (nasal surface); PPPB, Pyramidal Process of Palatine Bone; PLC, Posterior Lacrimal Crest; SFS, Sphenofrontal Suture; SOF, Superior Orbital Fissure; SOFN, Supraorbital Foramen/Notch; SOMN, Supraorbital Margin; SPFG, Sphenopalatine foramen/groove; STS, Sphenotemporal Suture; ZFF, Zygomaticofacial Foramen; ZFS, Zygomaticofrontal Suture, ZMS, Zygomaticomaxillary Suture; ZSS, Zygomaticosphenoid Suture Geometric concept 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). Fig. 2 ( A ) Geometric scheme of the orbit—a quadrangular pyramid converts into a tetrahedron posteriorly. Accordingly, there is a triangular frontal cross-section in the apex. Dot matrix designates posteromedial bulge. ( B ) Exploded skull model of left orbit—walls of the orbital cavity and their bone components. Color code: floor—green, medial wall—yellow, roof—red, and lateral wall—blue/purple (matching Fig. 2C). ( C ) Color-marked cast specimens of the internal orbits (color coding of the walls [see Fig. 2B]). 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). Orbital walls The superior orbital wall (roof) to its largest extent consists of the orbital part of the frontal bone (see Fig. 2 B; Fig. 3 A, B). Fig. 3 ( A ) Orbital roof ( right side of anatomic specimen ) from below ( B ) Orbital roof—upside down/inferodorsal view revealing a look into the superomedial corner of the orbital apex with the optic foramen/canal (OFC) turned medially as upward just like the optic strut and the narrow superior lateral SOF sector. The optic strut and the narrow superior lateral SOF sector. ACP, Anterior Clinoid Process; AEF, Anterior Ethmoidal Foramen; EB, Ethmoid Bone; FPZB, Frontal Process of Zygoma Bone; LF, Lacrimal Fossa; MCF, Middle Cranial Fossa; OCR, Opticocarotid Recess; OS, Optic Strut; PEF, Posterior Ethmoidal Foramen; SB, Sphenoid Bone; SDJ, Sphenoid Door Jamp; TR, Trigone (GWS); TRF, Trochlear Fovea 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). Fig. 4 ( A ) Medial orbital wall—lateral to medial view offering a look into the optic foramen/canal (OFC) and the PPF along the medial IOF margin and retrotuber maxillary region. ( Inset ) Vertical extent of the postentry zone behind the infraorbital and supraorbital rim ( vertical green arrow ). Lazy S shape of ascending orbital floor ( green line ). ( B ) Medial orbital wall—lateral view, ethmoid, sphenoid, and frontal air cells opened. EBA, Ethmoid Bone/Air Cells; IFC, Infraoptic Canal; LPP, Lateral Pterygoid Plate; MAS, Maxillary Antrum/ Sinus; MES, Maxilloethmoidal Suture Line; OSM, Orbital Surface of Maxilla; PPF, Pterygopalatine Fossa; SPF, Sphenopalatine Foramen. 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. Fig. 5 ( A ) Orbital floor ( right side of anatomic specimen )—anterocranial view (infraorbital margin tilted downward). The IOF communicates with the infratemporal and pterygopalatine fossa (PPF). Rather than a simple punched perforation in a shelf, the IOF is configured as a ravine with steep sides, basins, and affluents, such as foramen rotundum, pterygoid canal, and inferior orbital groove. ( B ) Orbital floor ( right ) from above. The posterior end of the IOF ends in a basin in front of the maxillary strut. Thus, the orbital floor does not contribute to the apex (ie, posterior orbit). ( C ) Assembly of maxilla, zygoma, and sphenoid bone (right)—inferolateral aspect showing retrotuber and infratemporal region. The ravine-like character of the IOF with a robust posterior bony border (sphenoidal trigone and medial IOF margin) is confirmed. ALIOF, Anterior Loop of Inferior Orbital Fissure; CIOF, Confluence/IOF Isthmus; FIT, Fossa Infratemporalis; FO, Foramen Ovale; FS, Foramen Spinosum; FR, Foramen Rotundum; IOFB, Inferior Orbital Fissure/Basin; IOFIP, Isthmus Promontory; LPP, Lateral Pterygoid Plate; MES, Maxilloethmoidal Suture Line; MS, Maxillary Strut; NLC, Nasolacrimal Canal/Groove; OPZLP, Orbital Plate of Zygoma Lower Part; ZB, Zygomatic Bone; ZTS, Zygomaticotemporal Suture 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). Fig. 6 Lateral orbital wall—medial view. ( Inset ) Oblique medial view. ZA, Zygomatic Arch. Bony openings—fissures, canals, grooves, foramina, and connections encountered during periorbital dissection Optic canal 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). Fig. 7 ( A ) Architecture of the orbital fissure system (on right side of specimen taking the form of an uppercase L, seen as mirrored image as shown here; on left hand side in form of an uppercase L, unmirrored (not shown)) extending into the apex and optic canal—anterior close up view of the major bony pathways in and out of the orbit. Besides the optic nerve (CN II), the optic canal transmits the ophthalmic artery and accompanying autonomous nerve fibers. ( B ) Orbital apex ( right side of specimen )—frontal bone slice—the triangular cross-section is defined by the lateral, medial, and superior orbital wall. The central and inferior SOF sectors lie below the OFC and are widened compared with the lateral superior sector. The lateral rectus spine (SRL) indicates the attachment of Zinn annular ring at GWS and the borderline between the upper and the central SOF sectors. ( C ) Intracranial view of apical tip (flip side of Fig. 7A). Anterior clinoid process (ACP) partially removed and lateral sphenoid sinus wall partially drilled away to show pneumatization of the optic struct, which is the equivalent to the optico-carotid recess. ( D ) Junction of orbital apex and lateral sellar region/middle cranial fossa ( right )—lateral view. The infraoptic groove (sulcus), tubercle or canal is the origin of the common annular tendon (Zinn ring). ( E ) Foramina in inferolateral orbital quadrant and surface of zygomatic body. ( F ) Inferolateral orbital quadrant—zygomaticofacial neurovascular bundles. FSS, Frontosphenoid Suture; FR, Foramen Rotundum; HF, Hypophyseal Fossa; OC, Optic canal, Intracranial End; PCP, Posterior Clinoid Process; SPF, Sphenopalatine Foramen; SRL, Spina Rectus Lateralis; SS, Sinus Sphenoidalis; VC, Vidian Canal / Pterygoid Canal; ZOF, Zygomaticoorbital Foramen; ZTF, Zygomaticotemporal Foramen. 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. Cranio-orbital foramen 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. Table 1 Bony openings related to nerval and vascular orbital pathways Abbreviations ; ICP, Internal Carotid Plexus; MB, Maxillary Bone; PB; Palatine Bone; ZB, Zygomatic Bone. 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. Ethmoidal foramina 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. Infraorbital foramen 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. Nasolacrimal groove/canal 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. Fig. 8 ( A ) Sagittal internal orbital buttresses in parallel after removal of thin interjacent bony plates: IOS along the articulation of orbital floor and medial orbital wall with a strong anterior portion around the nasolacrimal duct, weaker midportion because of lamina papyracea as well as aeration by ethmoidal bulla and solid wide posterior palatine bone portion; midway floor strut surrounding the infraorbital groove and canal; lateral floor strut along anterior/lateral IOF edge. ( Inset ) Overview midface skeleton. ( B ) Sagittal internal orbital buttresses and soft tissue relations (left zygoma, anterior and middle skull base removed to expose LFS). CN II, Optic Nerve; CN V2, Trigeminal Nerve - Maxillary Nerve Division; CN V3, Trigeminal Nerve - Mandibular Nerve Division; CS, Cavernous Sinus; FIT, Fossa Infratemporalis; GG, Ganglion Gasseri / Trigeminal Ganglion; ICA, Internal Carotid Artery; IN, Infraorbital Nerve; IOC, Infraorbital Canal; IOS, Inferomedial Orbital Strut; LFS, Lateral Floor Strut; MFS, Midway Floor Strut; MS, Maxillary Strut; NLF, Nasolacrimal Fossa; NMP, Nasomaxillary Process; STS, Sphenotemporal Suture. Orbital dimensions, volume, surface contours, and 3-dimensional globe position—interrelationship Numerous factors determine the 3-D position of the ocular globe inside the orbit, its anterior projection, and radial relationships to the orbital rims; these are the overall dimension and volume as well as the geometry and surface contours of the internal orbit, such as slopes, angles, planes, concavities, and convexities—in particular, the so-called posteromedial bulge within the inferomedial wall transition. The overall dimensions ( Fig. 9 A) and volumes of the orbit show great variations owing to age, gender, and ethnicity—4-cm horizontal × 3.5-cm vertical are typical average values for the adult aditus, the anteroposterior depth is 4.5 cm to 5 cm approximately, and the overall volume of the orbital cavity measures up to 30 mL, including the ocular globe, a sphere of 22 mm to 27 mm in diameter, up to 7 mL in volume and 69 mm to 85 mm in equatorial circumference. Fig. 9 ( A ) Average dimensions and angular relationships of the orbits and interorbital space. ( Inset ) Routine 3-D reformatted CT scan showing that individual angulation and measurements can vary widely with the diploic space (SDJ) within GWS yielding a major contribution. ( B ) Frontal view of both orbits. Lateral orbital walls removed allowing a look into the diploic space (SDJ) within the GWS, and the infratemporal and temporal spaces. Inferomedial walls and ethmoidal cells partially removed showing maxillary sinuses and posterior ledges. Line sketch of so-called orbital angle ( red ), that is, the inclination of the orbital floor to horizontal plane. ( Inset ) Orbital angles (45°) along intact orbital floor surfaces. The sagittal projection of the globe (corneal apex plane and/or equator) is referenced relative to the sagittal projection of the orbital rims, traditionally to the retruded lateral orbital rim only (exophthalmometry) and more recently using 360° polar plots (CT morphometry) to measure distances to preassigned points all over the outer rim periphery (Eckstein and colleagues 2011). The globe projection in the coronal plane or the vertical/horizontal globe position is correlated to the delineation, shape, and curvatures of the orbital aditus (square, rectangle, trapezoid, polygon, or circular rim aperture) and the interorbital distance. The inclination of the medial part of the inferior rim is parallel to the angle of the orbital floor ( Fig. 9 B). The lateral parts of the rim and the floor both run in a horizontal plane. The lowest lateral point of the orbital floor is located next to the anterior IOF loop. With aging, both the lower rim and the floor shift posteriorly and inferiorly, so that the anteroposterior rim angle flattens, but the height and width of the aditus to the orbit do not change significantly. The lowest portion of the orbital floor coincides with a short-pathed, circumferentially running concavity just behind the orbital rims. Originating from the periorbital dissection sequence, this widening is named, the postentry zone (see Fig. 4 A). The orbital floor steadily ascends from the bottom of the postentry zone until it reaches the convex top site of the orbital process of the palatine bone (posterior ledge). Because this process curves down abruptly backward into the posterior IOF basin, the paramedian portion of the orbital floor assumes an undulating shape in the sagittal direction, which is termed, the lazy S configuration . The bony surface relief in the posterior transition of the orbital floor into the medial orbital wall is upraised by the focal convexity, as aforementioned, the posterior medial bulge ( Fig. 10 A–C). This spot in the surface profile is referred to as Beat Hammer’s key area , because this author [Hammer, 1995] has stressed its tremendous importance for the appropriate surgical reconstruction of the internal orbit ( Fig. 11 A, B). The difficulty of procuring accurate and reproducible anatomic boundaries of the posterior medial bulge long has been recognized in statistical shape analysis. In consequence, still no mapping technique exists that displays increasing contour lines to estimate the steepness gradient of the all-important sloping hillside and select an appropriate implant immediately. Fig. 10 ( A ) Orbit (right)—staggered partitions after midline sagittal plane section. Lazy S becomes obvious along the medial orbital floor. The lateral orbital wall reveals as a flat plane. ( Inset ) Convexity of the posterior inferomedial wall with key area outlined provisorily ( red rectangle ). ( B ) Exposure of lazy S and key area—by superior retraction of the orbital soft tissue contents and swinging of the eyelids anteriorly (zygoma and GWS/SDJ removed). ( C ) Globe height and globe projection—graphic illustration based on 3-D reformatted CT scan of left orbit. The bulge configuration of the posterior inferomedial orbital walls (key area) in conjunction with the angulated plane of the lateral orbital wall are the primary determinants for the vertical and sagittal globe position. HM, Horner's Muscle; PMB, Posterior Medial Bulge. Fig. 11 ( A ) Globe height and globe projection—the fanlike divergence of the orbital walls is comparable to a pair of hands holding the globe in its forward position. Demonstrated by Paul Tessier and John Clark (Jack) Mustardé—on the occasion of a University of California Los Angeles meeting about orbital and eyelid surgery in the late 1970s: “Jack Mustardé said that posttraumatic enophthalmos was not correctable, which was the dogma at that time. Tessier said, No Jack, you are wrong, and he got behind him and pushed him forward by bringing his arms together. He said my arms are the walls of the orbit.” ( B ) Globe height and globe projection—in line with history the orbital wall—globe relationship replayed by Paul Manson and Beat Hammer—on the occasion of 3rd European Advanced Symposium in Orbital Reconstruction, Chalkididi/Greece, April 2008. Traumatic orbital wall defects involving the key area most of the time pass by the orbital process of the palatine bone, so that this intact bony ledge can serve as support in reconstruction. The internal surface of lateral orbital wall equates to a straight and even plane between the lateral rim and the SOF entrance (see Figs. 9 A and 10A ). Anterior orbit—midorbit—posterior orbit (apex) Often, the length (anteroposterior extension) of the orbit is divided into thirds. This division is intuitive and not based on anatomical references or numerical distances between measuring points, and the shorter orbital floor length is not taken into consideration. A more pragmatic approach is a partitioning into an anterior orbit, a midorbit, and a posterior orbit using the IOF ( Fig. 12 ). The extent of the IOF between the anterior loop and the maxillary strut designates the boundaries of the midorbit, whereas the anterior orbit lies in front and the posterior orbit in the rear of the IOF. Fig. 12 Start and ending of IOF correspond to the spatial boundaries of the concentric circles of the anterior, mid, and posterior orbit. Deep orbit versus rule of halves: 24 to 12 to 6 The anterior IOF loop has been proposed as an anatomic landmark to define the point of entrance into the deep orbit in posttraumatic surgical dissection and repair, with the 2 arguments that the frontal orbital cross-section begins to taper backward continuously from there on, making a subperiosteal (periorbital) dissection progressively difficult and with the high frequency at which it is involved by relevant traumatic defects. The concept of the deep orbit (Evans and Webb, 2007) suggests using the following 4 hard and soft tissue structures for orientation and pathfinding during dissection of the inferior and lateral wall: Infraorbital nerve (canal—groove/sulcus) IOF GWS (central trigone part/sphenotemporal buttress) Upper plate of the orbital process of the palatine bone These structures concenter at the confluence of the orbit, which corresponds to the area in direct proximity of IOF isthmus (see Figs. 5 A, B, 10 A, and 15 ). Often, orbital depth gauging data derived from anthropometric studies cannot provide appropriate guidelines for safe distance dissection because of variation, particularly in trauma due to severe multifragmentation, displacement, soft tissue disruption, and fat herniation. So, the preferences is to dissection in a subperiosteal (subperiorbital) plane to identify the lead structures. Apart from the EF, the medial orbit does not feature any orientation aid to prevent interference with the optic foramen and nerve. Despite the well-known uncertainties in terms of number and zonal location of the EF, their potential distances in relation to the lacrimal crest, among each other, and to the optic foramen ( Fig. 13 A) are referred to by a well-known mnemonic—the rule of halves: 24 mm to 12 mm to 6 mm (Rontal and colleagues, 1979) brings one to remind the distances ( Fig. 13 B). Fig. 13 ( A ) Variation of EF—periorbital sac emptied to show the sleevelike extensions to AEF, IEF, PEF, and optic foramen/canal (OFC). ( B ) Rule of halves—24 mm to 12 mm to 6 mm—average distance of 24 mm from anterior lacrimal crest to AEF, 12 mm between AEF and PEF, and another 6-mm span from PEF to OFC. AEF, Anterior Ethmoidal Foramen; IEF, Intermediate Ethmoidal Foramen; OFC, Optic Foramen / Canal; PEF, Posterior Ethmoidal Foramen. Orbital soft tissue contents Orbital septum The aditus to the orbit is closed off by the septum, a fibroelastic multilayered diaphragm separating the eyelids (preseptal) from the orbital contents (postseptal/retroseptal). The septum lies deep to the orbicularis oculi muscle (suborbicular). It is one of the components building up the middle lamellae of the upper and lower eyelids. In the upper eyelid, it fuses with the anterior layer of the levator aponeurosis and in the lower eyelid to the capsulopalprebral fascia below the tarsal plate. The thickness of the orbital septum is not homogenous. The lateral portion is thicker than the medial portion. Periorbita—periosteal lining—periorbitum The periosteal lining of the internal orbit is denoted as periorbita, or by some investigators as periorbitum, taking the final syllable of periosteum into linguistic consideration. The periosteal/periorbital sac is a thin but resistant fibrous envelope that provides protection to the orbital soft tissue contents ( Fig. 14 ). It sends sleeves and prolongations into the canals, foramina, and fissures (see Figs. 13 A and 14 ) to blend with the dura or the adjacent periosteum. The fusion zone over the orbital rims is thickened and sets up the arcus marginalis, which is the origin of the orbital septum. The infraorbital neurovascular bundle is completely wrapped in kind of a mesoperiosteum to keep it in direct contact with the orbital floor lining along the infraorbital groove. Tiny branches arising from the infraorbital artery penetrate the sheath to anastomose with muscular branches of the ophthalmic artery near the inferior rectus and oblique muscles ( Fig. 15 A, B). Fig. 14 Periorbital sac—intracranial view after removal of lateral orbital wall and roof ( left ). Medial cranial fossa with exposed ganglion Gasseri and trigeminal nerve divisions. CN III, Oculomotor Nerve; CN IV, Trochlear Nerve; CN V1, Trigeminal Nerve - Ophthalmic Nerve Division; HF, Hypophyseal Fossa; ICA, Internal Carotid Artery; PL, Periorbital Lining. Fig. 15 ( A ) Both orbital floors viewed from above with periorbital coverage intact. Perforators run along the course of the infraorbital neurovascular bundle on its visceral side (this means in effect within the intraperiorbital space). Left orbit: IOM originating immediately lateral to NLC, the long branch of the inferior division of CN III to IOM runs in parallel to the IOG, IOC and the lateral border of the IRM ( striped lines ). ( B ) Orbital contents and infraorbital neurovascular bundle perforators—medial approach. IOM and attached CN III motor branch hook retracted. The arterial strand divides into numerous smaller branches (perforators) which connect to the periorbita and IRM along the top side of the infraorbital groove. These vessels must be meticulously bipolar cauterized during deep subperiorbital dissection of the orbital floor. ( Inset ) Distribution of infraorbital neurovascular bundle and vessel entrance into the periorbital sac. CN III Inf D, Oculomotor Nerve Inferior Division; CN VI, Cranial Nerve VI - Abducens Nerve; IOM, Inferior Oblique Muscle; IRM, Inferior Rectus Muscle. A distinction has to be drawn between the extraperiosteal space or bony side and the intraperiosteal space or visceral side of the periorbita. As a whole, the periorbita is only loosely adherent to the bony surfaces. It is fixed more firmly to the trochlea, Whitnall tubercle, and the deep limb of the medial canthus/lacrimal sac and the infraorbital groove. An intimately linked framework of diversified connective tissue components extends from the inner or visceral side of the periorbita to support the ocular globe and the entirety of muscular and neural and vascular elements (ie, Koornneef system of radial and interlocking septa [Koornneef 1976, 1977, Demer 2002, Miller 2019], extraocular muscle pulleys, and many more other concepts). To avoid confusion, the term, periorbita , generally is used to refer to the area around the orbit and the globes (forehead, hairy eyebrows, brow eyelids continuum, eye spacing, palpebral fissures, lacrimal puncta, eyelids, midcheek, and temples) in the widest sense of a periocular or periorbital region instead of the periosteal covering inside the orbit. Dissection of the periorbital lining A systematic dissection, separating the periorbital lining from the underlying bony surface relief into the deep orbit, comes across landmark structures of particular note, some of them associated with potential risk for injury ( Fig. 16 ). Unlike classic anatomy, which typically describes the soft contents of the orbit in craniocaudal layers after removal of the floor of the anterior cranial fossa, namely, the orbital roof, a periorbital dissection proceeds in anteroposterior direction without osteotomies of the bony rims or walls starting at the aditus over the midorbit toward the entrance into the apex along the lower and/or upper bony circumference. The extent of bone that can be exposed depends on the placement and size of the skin incision and subsequent surgical access. As a consequence, any look into the orbital cavity is restricted to a limited segment, and a 360° visualization of the whole internal orbit is achievable only by piecing together the perspectives from all sides to a panorama in one’s mental imagination. Fig. 16 Axial plane after transverse head section at intermediate height level of the orbits—view from above. Left lower circumference displays bone components making up the orbital floor and bordering the IOF. Right lower circumference with markings of relevant landmarks/structures at risk along the periorbital dissection route on the floor surface. Successive transection of the periorbital invaginations and through the contents of the IOF starting at its anterior loop widens the view within the inferolateral orbital quadrant. There is no harm to the posterior portion of the infraorbital nerve, because this is settled low in the IOF basin/PPF and does not reach up to the level of the orbital floor ( Fig. 17 A–C). Fig. 17 ( A ) Content of the IOF—periorbital invagination and fat filling the IOF for the whole length—retracted upwards. Orbital fat and buccal fat pad communicate. The infraorbital neurovascular bundle pulled out and upward of the IOG. ( Inset ) Topographic overview. ( B ) Augmented exposure of inferolateral orbital quadrant after transsection of the IOF contents ( dotted outline ). ( C ) Posterior portion of infraorbital nerve residing in the posterior IOF basin/PPF below the level of the IOF and orbital floor. IN, Infraorbital Nerve; LRM, Lateral Rectus Muscle; ZFF, Zygomaticofacial Foramen; ZFN, Zygomaticofacial Nerve. The landmarks and structures at risk along the 4 orbital walls appear in a prototypical sequence, with the periorbital dissection going posteriorly ( Fig. 18 ). Fig. 18 Periorbital dissection sequence—silhouettes of right periorbital sac (inferior–medial–superior–posterior) to indicate approximate position of landmarks and related structures at risk. Further thought should be given to the fact that all attempts to push the limits of the periorbital dissection far back through the concentrically aligned walls of the midorbit toward the tight apex using instrumentation and spatulas build up soft tissue pressure. The compressing effects may be as detrimental to the optic nerve or the SOF contents as direct mechanical impingement at the optic foramen ( Fig. 19 ). Fig. 19 Frontal section at the transition of midorbit into posterior orbit—view of both apical cross-sections and their contents (fat removed). The tip of the spatula elevating the periorbita might exert indirect compressional effects on CN II, the SOF fillings and extraocular muscles. Ipsilateral pupillary mydriasis and anisocoria encountered during deep periorbital dissection and fracture repair are well-known but still distressing phenomena. Manipulation and interference with the long inferior oblique muscle (IOM) branch of the inferior oculomotor division (see Fig. 15 A) and/or the CG represent possible causes of intraoperative pupillary dilatation. Ciliary ganglion The CG is the pre–postganglionic relay center of the CN III parasympathetic pathway. It lies embedded in fat between the lateral aspect of CN II and the lateral rectus muscle, most often in the midhalf between Zinn ring and the back of the eyeball ( Fig. 20 ). It can have a round, ovoid, or an starlike flat-end shape. The mean size approximates to 2.5 mm in horizontal diameter, 1.5 mm in vertical height, and 0.5 mm to 1 mm in thickness. Fig. 20 CG—lateral view in posterior left orbit. Starlike appearance with multiple nerval inlets and outlets, including the parasympathetic radix (PSR). ant, anterior; post, posterior; sup, superior; inf, inferior; CG, Ciliary ganglion; CN III Inf D, Oculomotor Nerve Inferior Division; CN III Sup D, Oculomotor Nerve Superior Division; PSR, Parasympathetic Radix; SCN, Short Ciliary Nerve. CG has inputs by sensory, sympathetic, and parasympathetic fiber projections, which join it from the posterior aspect and is why they are named roots. The sensory root runs through a single branch diverting of the nasociliary nerve proximal to Zinn ring. The sympathetic root corresponds to a thin filament containing fibers from the intracavernous internal carotid plexus (ICP) reaching the CG via the SOF. The parasympathetic root commonly is single and emerges from the long IOM branch of the inferior oculomotor division (ie, ramus n. oculomotorii ad ganglion ciliare). The CG is traversed by the sympathetic and sensory fibers. The preganglionic parasympathetic fibers with their origin in the Edinger-Westphal nucleus synapse within the ganglion. The postganglionic neurons leave the CG and combine with the main bundles of up to 8 to 10 short ciliary nerves and innervate the sphincter pupillae of the iris and the ciliary muscle to control pupil constriction and regulate ocular accommodation. The short ciliary nerve bundles are of a tiny (0.2-mm) diameter. In their initial course, they stay in a lateral position to CN II. Then they undergo further divisions, which revolve around the whole CN II circumference to pierce the sclera at the posterior globe. The sensory fibers contained in the short ciliary nerve bundles convey sensation from the eyeball and the cornea; the sympathetic fibers are in charge of vasoconstriction. The long ciliary nerves, side branches of the nasociliary nerve, mingle with the short ciliary nerve bundles and carry sympathetic fibers. These fibers supply the pupillary dilator muscle. Clinics care points Dissection of the periorbital lining away from the bony surfaces of the internal orbit must proceed with forethought and precaution, while watching out for neurovascular structures and suspension (eg, trochlea and retinacula) or insertion (eg, inferior oblique muscle) sites. Topographic orientation in reference to anatomic landmarks is the preferable safeguard as opposed to average depth-gauge measurements. The approach into the tight spatial confines of the deep orbit using spatulas generates compressive forces to the intraorbital portion of the optic nerve beyond the risk of direct mechanical nerve injuries next to the optic foramen and superior orbital fissure. Acknowledgments A big kudos and many thanks go to the photographers Gerhard Poetzel and Rudolf Herzig, OMFS/FPS Departement Ludwig-Maximilians-University, Munich, for their enormous help! Disclosure The authors have nothing to disclose. Dissections of anatomic specimens were conducted in compliance with state and local laws.