Orbital Vascular And Lymphatic Systems

The anatomy of the orbital vascular bed is complex, with tremendous individual variation. The main arterial supply to the orbit is from the ophthalmic artery, a branch of the internal carotid artery. The external carotid artery normally contributes only to a small extent. However, there are a number of orbital branches of the ophthalmic artery that anastomose with adjacent branches from the external carotid artery, creating important anastomotic communications between the internal and external carotid arterial systems. The venous drainage of the orbit occurs mainly via two ophthalmic veins (superior and inferior) that extend to the cavernous sinus, but there are also connections with the pterygoid plexus of veins, as well as some more anteriorly through the angular vein and the infraorbital vein to the facial vein. A working knowledge of the orbital vasculature and lymphatic systems is important during orbital, extraocular, or ocular surgery. Knowing the anatomy of the blood supply helps one avoid injury to the arteries and veins during operative procedures within the orbit or the eyelid. Inadvertent injury to the vasculature not only distorts the anatomy and disrupts a landmark but also prolongs the surgery and might compromise blood flow to an important orbital or ocular structure. Upon entering the cranium, the internal carotid artery passes through the petrous portion of the temporal bone in the carotid canal and enters the cavernous sinus and middle cranial fossa through the superior part of the forame lacerum . It proceeds forward in the cavernous sinus with the abducens nerve along its side. There it is surrounded by sympathetic nerve fibers (the carotid plexus ) derived from the superior cervical ganglion. It then makes an upward S-shaped turn to form the carotid siphon , passing just medial to the oculomotor, trochlear, and ophthalmic nerves (V1). After turning superiorly in the anterior cavernous sinus, the carotid artery perforates the dura at the medial aspect of the anterior clinoid process and turns posteriorly, inferior to the optic nerve.

Orbital Nerves

The orbit contains a vast array of motor, sensory, sympathetic, and parasympathetic nerve fibers. Some of these fibers can be seen during eyelid or orbital surgery and are often landmarks of one’s location within the orbit. It is important to know the various nerve pathways, appreciate that there might be some individual variation, and preserve these pathways during orbital surgery. The discussion of nerves begins with their superficial brainstem origin, proceeds to their intracranial course, and ends with their intraorbital course and eventual termination. The following nerves enter the orbit: 1. Optic nerve (cranial nerve II). 2. Oculomotor nerve (cranial nerve III). This motor nerve gives fibers to the levator, inferior oblique, and three of the four rectus muscles. It carries parasympathetic fibers destined for the ciliary ganglion. These fibers will eventually synapse in the ciliary ganglion and then travel to the iris sphincter muscles (sphincter pupillae). Sympathetic fibers have also been recently identified in this nerve. 3. Trochlear nerve (cranial nerve IV). This motor nerve distributes fibers to the superior oblique muscle. Sympathetic fibers have recently been identified within this nerve. 4. Trigeminal nerve (cranial nerve V). a. Ophthalmic division (V 1 ) . This sensory division gives fibers to the eyeball (iris, ciliary body, cornea), lacrimal gland, conjunctiva, and eyelids, as well as to the forehead. It also carries sympathetic nerves. b. Maxillary division (V 2 ) . As it enters the orbit, the maxillary division is known as the infraorbital nerve and lies beneath the periorbita. It gives off the zygomatic nerve, which is an important branch carrying parasympathetic and sympathetic fibers to the lacrimal gland. Within the infraorbital canal, alveolar nerves arise and provide sensation to the incisor and canine teeth. The infraorbital nerve provides sensation to the lower eyelid, nose, cheek area, and upper lip. 5. Abducens nerve (cranial nerve VI). This motor nerve goes to the lateral rectus muscle. Sympathetic fibers have recently been identified within this nerve.

The Lacrimal System

The lacrimal system is made up of both the lacrimal secretory system and the nasolacrimal drainage system. The secretory system consists of the glands that make up the tear film (the lacrimal gland, the accessory glands of Krause and Wolfring, and the meibomian glands). The nasolacrimal drainage system consists of the puncta, canaliculi, lacrimal sac, and nasolacrimal duct. These two systems provide for the production and maintenance of the precorneal tear film as well as the drainage of tears from the eye. The normal functioning of these two systems is essential for proper optical refraction, preservation of corneal integrity, and ocular comfort. The physiology of tear production and distribution requires normal eyelid anatomy and mobility. Blinking spreads the tears vertically over the ocular surface. It also adds two important components to the tear film: lipid from the meibomian glands and mucin from the conjunctival goblet cells. The horizontal flow of tears to the medial canthus is along the tear meniscus at the eyelid margin. This requires normal contour and eyelid apposition to the globe and an adequately functioning orbicularis pump mechanism. Both of these functions can be compromised by horizontal and vertical eyelid laxity or by eyelid margin deformities. The lacrimal gland begins in embryologic development as epithelial buds arising from the conjunctiva of the superior temporal fornix. Canalization of the epithelial buds to form ducts begins in utero, but full development of the gland does not occur until three to four years postnatal. The lacrimal gland provides the principal aqueous secretory component of the tear film. It is located just behind the superolateral orbital rim within a depression in the lateral aspect of the orbital roof (the lacrimal gland fossa). The gland’s convex/concave shape reflects its location between the roof of the orbit and the globe. The gland is divided into a larger orbital lobe and a smaller palpebral lobe by the lateral horn of the levator aponeurosis. The orbital lobe lies behind the orbital septum and immediately above the lateral horn of levator.

The Extraocular Muscles

Whereas skeletal muscles generally perform specific limited roles, extraocular muscles (EOMs) have to be responsive over a wider dynamic range. As a result, EOMs have fundamentally distinct structural, functional, biochemical, and immunological properties as compared to other skeletal muscles. At birth, the extraocular muscles are at approximately 50 % to 60 % of their final dimension. Their relative growth within the enlarging orbit and their angular relations with the globe remain nearly constant from infancy to adulthood. The adult rectus muscles are approximately the same length (40 mm) but differ in thickness and in the length of their tendons. There are six extrinsic, or extraocular, muscles of the eye: four recti and two obliques. Only the horizontal and vertical recti insert on the eyeball in front of its equator. Both obliques have their insertions behind the equator of the globe. All six muscles consist of striated muscle fibers with abundant elastic fibers. The EOMs have muscle fibers and innervations that differ from those of skeletal muscle. There are three distinct types of muscle fibers (fine, granular, and coarse) that contribute to the action of the EOMs. The fine fibers are thought to be responsible for slow twitch movements, the granular fibers for fast twitch movements, and the coarse fibers for slow tonic movements. The EOMs are more richly innervated than other voluntary muscles of the body and have three types of nerve terminals: single endplate (driving eye movements), multiple endplates (tonic tension), and palisade endings (can be sensory receptors). In addition, there are both singly and multiply innervated nerve fibers present. EOMs are able to vary their contractile force by small increments. The maximum firing frequency of ocular motor units is about four times greater than those of limb muscle motor units. To allow them to operate at a higher frequency, EOMs also have faster contractile properties, with their time to peak tension and their one-half relaxation time being at least half of those in limb muscles.

Orbital Connective Tissue

A diffuse connective tissue framework exists within the orbital space that supports various orbital structures, maximizing their function and maintaining anatomic relationships between them. Some connective tissue septa are aligned with directions of force and serve to resist displacement of the extraocular muscles during contraction. Other fascial septa suspend and support delicate orbital vascular and neural elements. All orbital structures, including the periorbita, globe, optic nerve, and extraocular muscles, are involved in the organization and suspension of these extensive connective tissue septal systems. This intricate framework has several important components that are oft en difficult to see clinically and during orbital surgery but which must always be kept in mind as playing a role in the clinical presentation of a particular orbital problem. For example, a blowout fracture of the orbital floor with restricted motility in upgaze might be due to entrapment of the inferior rectus muscle within the fracture, but more commonly it is due to entrapment of the connective tissue framework + / − fat in the inferior orbit. Enophthalmos, a deep superior sulcus, ptosis, and lower eyelid malposition are commonly seen following enucleation surgery. Loss of volume (of the globe) certainly plays a role, but other contributing factors, including a disruption of the connective tissue framework (which helps support the globe), might be playing a role. Trauma to Tenon’s capsule, Whitnall’s ligament, Lockwood’s ligament, or check ligaments associated with the recti muscles, as well as the intermuscular fascial connections, might also contribute to the postenucleation socket abnormalities mentioned above, as their disruption allows the shifting of orbital tissues to occur. Evisceration surgery is associated with less disruption to the connective tissue framework, and as a result features of postenucleation socket syndrome other than loss of volume are not as common. Historically, four major connective tissue components in the orbit have been described: 1. the bulbar fascia, or Tenon’s capsule, which surrounds the globe; 2. the fascial sheaths of the extraocular muscles; 3. the intermuscular septum (the common muscle sheath connecting the four rectus muscles); and 4. the medial and lateral check ligaments.

Forehead, Eyebrows, Eyelids, And Canthi

Numerous oculoplastic surgical procedures are performed on the forehead, eyebrows, eyelids, and canthi. An understanding of the anatomy of these structures, as well as of the nearby temporal artery and facial nerve, is essential for the surgeon working in this region. In this chapter, the surface anatomy is described first, followed by a more detailed description of the tissue beneath. The skin of the forehead and cranium, or the “scalp,” is traditionally considered as five layers (Fig. 1-1): skin, subcutaneous tissue, galea aponeurosis, loose areolar tissue, and periosteum. These layers are present consistently throughout the head, with some slight modifications in certain areas (e.g., the brow area). These layers can be easily remembered with the mnemonic “SCALP” (S = skin, C = subcutaneous tissue, A = galea aponeurotica, L = loose areolar tissue, P = pericranium). The skin of the forehead and temporal region is usually relatively thick and rich in sebaceous glands. However, in some elderly individuals the skin of the temporal forehead can be quite thin and consequently requires a greater degree of care during surgical procedures (e.g., endobrow lift ) and resurfacing (e.g., laser or chemical). Although the eyebrows are technically part of the scalp rather than of the eyelids, they have important functional and surgical relationships to the lids. The eyebrows lie at the junction between the upper eyelids and the forehead. They lie over each superior orbital rim, are separated by the glabella, and are formed by thick, strong, skin-bearing hairs. The underlying muscle fibers, with their cutaneous insertions, move the brows freely over a suborbicularis fat pad adjacent to the underlying periosteum. Each brow has a head, a body, and a tail. The head lies between the supraorbital ridge and the orbital margin, overlying the frontal sinus. The medial brow hairs are almost vertical; the body of the brow follows the supraorbital margin and has hair in a more horizontal direction.

Orbital Bones

The paired orbital cavities are formed by the facial bones and serve as sockets for the eyes. The orbital bones and the structures contained within the orbit (connective tissue, fat, nerves, vessels) act to support, protect, and maximize the function of the eye. In form, the orbit is roughly a quadrilateral pyramid with rounded angles and resembles a pear. Its volume in the average individual is 30 cc, of which the eyeball contributes about 7.5 cc (range: 6.9–9.0 cc). There are four surfaces: the roof, floor, lateral wall, and medial wall. The base of the pyramid is the opening onto the face (orbital entrance) and is circumscribed by the orbital margin (or orbital rim). The orbit narrows inward to its termination, the apex. The widest portion of the orbital cavity lies 5 to 10 mm behind the orbital rim. The orbit is made up of seven bones: frontal, sphenoid, zygomatic, malar, palatine, lacrimal, and ethmoid. Superiorly, the orbit is bordered by the anterior cranial fossa and the frontal sinus. Nasally, the ethmoid sinus is separated from the medial orbital wall by the thin lamina papyracea of the ethmoid bone. Inferiorly, the maxillary sinus lies beneath the orbital floor. The lateral orbit is bordered anteriorly by the temporalis fossa, and posteriorly it borders the middle cranial fossa. The lateral and medial walls of each orbit form an angle of approximately 45 ° with each other. The two medial walls diverge somewhat posteriorly but are almost parallel to each other (being about 3 mm farther apart posteriorly than at the orbital margin). The lateral orbital walls of the two orbits form a 90 ° angle with each other. The four walls of each orbit converge posteriorly toward the apex, where the optic canal and superior orbital fissure pass into the middle cranial fossa. The overall dimensions of the orbit, especially its depth, are quite variable. An orbital surgeon cannot rely on precise measurements as a guide to the exact location of the optic canal or superior orbital fissure.