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Slide 7 |
Mark R. Levine, MD
To fully understand the eyelid and orbital anatomy, a basic knowledge of the embryology of the eye and its adnexal structures is required. An important distinguishing feature in ocular development is the concept that the eye develops directly from an out-pouching of the embryonic brain (neural tube). The retina is an extension of the neural plate, first manifesting itself as the optic vesicle. Because the retina is formed directly from the brain, the optic nerve is not a typical nerve. The optic nerve is a fiber track that develops along a stalk of brain tissue. At 3½ weeks’ gestation, the primary optic sulcus first appears as a depression that occurs in the lateral portion of the neural plate. The optic sulcus invaginates and broadens to become the optic vesicle. The cranial neural crest cells are situated on either side of the invaginating neural folds. Cranial neural crest cells undergo extensive migration to envelop the optic primordium and form the mesenchyme of the globe and orbital tissues. By the fifth week of gestation, the dorsal portion of the vesicle flattens and invaginates to form the optic cup. The optic cup invaginates eccentrically towards its ventral margin, and a gap in continuity. The optic cup is created, forming the embryonic (choroidal) fissure. The embryonic fissure permits mesenchymal tissue to enter the globe, which forms its vascular supply.1
While invaginating, the layers of the optic cup rapidly differentiate, forming an outer pigmented layer (the future retinal pigment epithelium) and an inner sensory layer of the retina (the future sensory retina). The lens is formed by a proliferation of surface epithelium (lens placode) in a central portion of the optic cup. As the optic cup invaginates, the lens placode also invaginates to form the lens vesicle. The lens vesicle then separates from the surface epithelium of the cornea. The cranial neural crest cells migrate to form the mesenchyme of the globe and orbital tissues.
The orbit originates from mesenchymal folds that envelop the developing globe. The optic pit first appears at 3½ weeks’ gestation in the embryonic period. The optic pits develop on the lateral portions of the head and create a 180° orbital axis. Above the developing stomatodeum (primitive mouth), the mesenchyme’s overlying ectoderm proliferates and condenses to form the olfactory pits. Continued mesenchymal proliferation, which is particularly prominent around the olfactory pits, produces medial and lateral nasal folds. The two medial nasal folds coalesce to form the frontal nasal process, while the lateral nasal process is formed between the olfactory pit and optic vesicle.
The maxillary process originates as a triangular extension of the dorsal end of the mandibular arch. The maxillary process proliferates and extends medially until it meets with the lateral nasal fold. This occurs at approximately 5 to 6 weeks’ gestation. Ectoderm buried at the junction of the maxillary and lateral nasal processes form the anlage of the nasal lacrimal duct.
Following fusion with the lateral nasal process, the maxillary processes rapidly proliferate forward and upward and fuse in the midline. The development of the maxillary processes forces the orbital axis to a more anterior aspect. Therefore, at 6 weeks of gestation, the optic vesicles are in the same coronal plane and are directly 180º apart. At 7 weeks’ gestation, the angle formed by the orbital axis is 160º. By the eighth week, the angle is reduced to 120º. By 10 weeks’ gestation, the angle of the orbital axis is reduced to 72º. At birth, the orbital axis approaches 45º and remains stable thereafter.
The paired orbital cavities lie on each side of the midsagittal plane of the skull on close relation to the nasal sinus and cranial cavities. Each orbit consists of a portion of seven bones: frontal, sphenoid, zygoma, maxilla, ethmoid, lacrimal, and palatine. The bony orbit serves to isolate, support, and protect the eye and surrounding soft tissue while providing neurovascular communication with surrounding structures.2
The orbit is a nongeometric shape and most closely resembles a cross between a pair of four-sided pyramids that become three-sided near the apex as the floor is lost. Its adult capacity is 30 mL and its widest portion is approximately 1 cm behind the rim corresponding to the equator of the globe. The medial walls are almost parallel, whereas the lateral walls diverge at approximately 90º from each other or 45º from the mid sagittal plane. Thus, the divergent axis of each orbit becomes half of 45º, or 22.5º. The continuity of the bony orbit is interrupted by its large anterior marginal opening, the optic foramen, orbital fissure, and foramina.
The adult orbital margin approximates a rectangle with rounded corners and discontinuity at the lacrimal sac fossa medially. On average, the horizontal opening measures 40 mm and the vertical dimension is 35 mm. Superiorly, the orbital margin is formed by the frontal bone. The supraorbital ridge lies above the medial half of the orbit, representing an area of bony hypertrophy due to the strong attachment of the eyebrow. The medial aspect of the superior rim is indented by the supraorbital notch, which transmits the supraorbital artery and nerve. In some skulls, a complete bony foramen, rather than a notch, is found.
The medial orbital margin is formed by the maxillary process of the frontal bone and the frontal process of the maxilla. An indentation in the rim forms the lacrimal sac fossa with the anterior lacrimal crest (maxilla) continuous with the inferior rim and the posterior lacrimal crest (lacrimal bone) continuous with the superior rim. The lateral orbital margin comprises the zygomatic process of the frontal bone and the frontal process of the zygoma or malar bone. The zygoma is the strongest bone of the orbital margin and is the facial buttress. The lateral rim is recessed compared with the frontal plane of the medial rim, and thus allows a more temporal visual field.
On the orbital surface of the lateral rim is the lateral orbital tubercle (Whitnall tubercle), which marks the point of attachment of the lateral retinaculum of Hesser with contributions from the lateral canthal ligament, the orbital septum, the lateral horn of the levator aponeurosis, Lockwood suspensory ligament, and the lateral rectus check ligament.3
Inferiorly, the orbital margin is composed of the maxilla medially and the zygoma laterally. Approximately 1 cm below the inferior rim on the anterior face of the maxilla is the infraorbital foramen, the exit for the infraorbital neurovascular bundle.
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The triangular superior orbital wall or roof is formed primarily by the orbital plate of the frontal bone with a 1.5-cm contribution posteriorly by the lesser wing of the sphenoid. Fossae in the roof house the lacrimal gland anterolaterally and the trochlear anterior medially. The optic foramen enters the roof in the orbital apex at an angle of approximately 45º from the midline. Near the frontal sphenoidal suture, the meningeal foramen may be found. The meningeal foramen contains an arterial anastomoses between the lacrimal artery and branches of the external carotid circulation.
The medial wall is small, thin, and formed by four separate bones. From anterior to posterior are the frontal processes of the maxilla, the lacrimal bone, the ethmoid bone, and the body of the sphenoid. The lamina papyracea of the ethmoid comprises the bulk of the medial wall, and it sutures with a frontal bone above and maxilla below mark the upper and lower limits of the medial wall. Pneumatization of the ethmoid allows the honeycomb pattern of the air cells to be seen through the lamina papyracea. The fossa of the lacrimal sac lies anteriorly and in the medial wall between the lacrimal crests. Along the frontal ethmoidal suture, 20 mm and 35 mm posterior to the anterior lacrimal crest, are the anterior and posterior ethmoidal foramen, respectively. Each conveys an arterial branch and nerve bearing the same name as the foramen.
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The lateral wall is triangular in shape and is made up of the zygoma anteriorly and the greater sphenoid wing posteriorly (Slide 7). Its boundaries along the apex of the triangle posteriorly are the superior and inferior orbital fissures. Behind the thick lateral rim, the lateral wall thins at the vertically oriented zygomatic sphenoid suture, a common landmark for bone removed in lateral orbitotomy. The lateral orbital tubercle (Whitnall tubercle) is located just inside the lateral rim.
The inferior orbital wall or floor is the shortest of the walls and is formed by three bones: maxilla, zygoma, and palatine (Slide 8). This is the only wall without a contribution from the sphenoid bone. The orbital plane of the maxilla forms most of the floor with small additions by the palatine posteriorly and the zygoma anterolaterally. The palatine bone is often hard to identify because the suture uniting it with the maxilla is commonly obliterated. The medial border is defined by the maxilloethmoidal suture and extends from it anteriorly and posteriorly. The floor is lost at approximately two thirds of the orbital depth as the orbit changes from four to three sides. The infraorbital groove, canal, and foramen course anteriorly from the lateral portion of the apex towards the central infraorbital rim. The area of conversion from the groove to the canal is variable; both are covered by periorbita. Thus, the infraorbital neurovascular bundle is separated from the orbital contents. The floor is strong lateral to the infraorbital nerve, but it thins and weakens medially due to expansion of the maxillary sinus. On the anteromedial aspect of the floor, inside the rim and lateral to the nasal lacrimal canal, is a shallow fossa for the origin of the inferior oblique muscle. The inferior oblique muscle is the only extraocular muscle without a posterior orbital origin.
The orbital apex is an extremely busy area due to the convergence of the orbital walls and the entrance and exit of many structures from and to the middle cranial fossa, pterygopalatine fossa, and sinuses. The orbital fissures and canals allow these connections. Central to the osteology of the apex is the sphenoid bone. Its greater wing, lesser wing, and body contribute to the framework for the apex as the posterior extension of the lateral, superior, and medial orbital walls.
The superior orbital fissure is a transverse opening between the roof and lateral wall of the orbit, corresponding to the gap between the greater and lesser sphenoid wings. It is widest medially beneath the optic foramen. There is considerable individual variation in the shape of the superior orbital fissure. A more narrowed portion exists superiorly and temporally through which the lacrimal, frontal, and trochlear nerves pass. An anastomosis between the middle meningeal artery and the lacrimal artery may enter here if not through the meningeal foramen. Other structures transmitted through the superior fissure enter within the annulus of Zinn, the origin of the rectus muscles, and include the superior and inferior divisions of the ocular motor nerve, the abducens nerve, nasociliary nerve, sympathetic nerve fibers, and the superior ophthalmic vein. Medial to the superior orbital fissure lie the optic foramen and canal between the two roots of the lesser wing of the sphenoid. Through it passes the optic nerve and ophthalmic artery. The axis of the foramen is downward (about 38º lateral to the sagittal plane). The optic strut is the inferior root to the lesser sphenoid wing, which articulates with the body and separates the optic foramen from the superior orbital fissure. The inferior orbital fissure is a 20 mm long, bony opening between the orbital floor and lateral wall in the posterior third of the orbit. The fissure is bordered laterally by the greater wing of the sphenoid and medially by the palatine and maxillary bone. The axis of the fissure parallels that of the optic foramen. The anterior content of the infraorbital fissure is about 15 mm to 20 mm behind the infraorbital rim, a more anterior projection than that for the superior orbital fissure. The infraorbital fissure is closed by periorbita and limits the subperiostal dissection along the orbital floor during surgery. The inferior orbital fissure is continuous posteriorly with a foramen rotundum through which the maxillary nerve exits at the base of the cranium, and it also has communications with a pterygopalatine and infratemporal fossa. Transmitted through the fissures are the maxillary nerve and its branches (zygomatic, alveolar, infraorbital), infraorbital artery, inferior orbital vein, and automatic nerve branches from the pterygopalatine ganglion.3
