The orbit and accessory visual apparatus

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CHAPTER 39 The orbit and accessory visual apparatus

BONY ORBIT

The bony orbits are skeletal cavities located on either side of the root of the nose. They house the eyes, the paired peripheral organs of vision. The walls of each orbit protect the eye from injury, provide points of attachment for six extraocular muscles which allow the accurate positioning of the visual axis, and determine the spatial relationship between the two eyes, which is essential for both binocular vision and conjugate eye movements.

Each cavity approximates to a quadrilateral pyramid with its base at the orbital opening, narrowing to its apex along a posteromedially directed axis. Each orbit has a roof, floor, medial and lateral walls. The medial walls lie approximately 25 mm apart in adults and are nearly parallel. The angle between the medial and lateral walls is about 45°. The compromise between protection and ensuring a good field of view dictates that each eyeball is located anteriorly within the orbit. The eyeball thus occupies only one fifth of the volume of the orbit (Fig. 39.1): the remainder of the cavity is filled with vessels and nerves that are contained within and supported by orbital fat and connective tissue. In brief, the orbit also contains the extraocular muscles; the optic, oculomotor, trochlear and abducent nerves, and branches of the ophthalmic and maxillary divisions of the trigeminal nerve; the ciliary parasympathetic ganglion; the ophthalmic vessels; the nasolacrimal apparatus.

image

Fig. 39.1 T2-weighted axial MRI scan though the mid-orbit.

(By courtesy of Dr Timothy Beale FRCR, Royal National Throat Nose and Ear Hospital, London.)

Roof

The roof of the orbit is formed principally by the thin orbital plate of the frontal bone (Fig. 39.2). It is gently concave on its orbital aspect, which separates the orbital contents and the brain in the anterior cranial fossa. Anteromedially it contains the frontal sinus and displays a small trochlear fovea, sometimes surmounted by a small spine, where the cartilaginous trochlea (pulley) for superior oblique is attached. Anterolaterally there is a shallow lacrimal fossa which houses the orbital part of the lacrimal gland. The roof slopes significantly towards the apex, joining the lesser wing of the sphenoid, which completes the roof. The optic canal lies between the roots of the lesser wing, and is bounded medially by the body of the sphenoid.

Medial wall

The medial wall of the orbit is formed principally by the orbital plate (lamina papyracea) of the ethmoid bone (Fig. 39.2). This paper thin, rectangular plate covers the middle and posterior ethmoidal air cells, providing a route by which infection can spread into the orbit (see Ch. 32). The ethmoid articulates with the medial edge of the orbital plate of the frontal bone at a suture which is interrupted by anterior and posterior ethmoidal foramina. Posteriorly, it articulates with the body of the sphenoid, which forms the medial wall of the orbit to its apex. The lacrimal bone lies anterior to the ethmoid: it contains a fossa for the nasolacrimal sac that is limited in front by the anterior lacrimal crest on the frontal process of the maxilla and behind by the posterior lacrimal crest of the lacrimal bone (to which the lacrimal part of orbicularis oculi and lacrimal fascia are attached). A descending process of the lacrimal bone at the lower end of the posterior lacrimal crest contributes to the formation of the upper part of the nasolacrimal canal, which is completed by the maxilla (Fig. 39.3).

Floor

The floor of the orbit is mostly formed by the orbital plate of the maxilla which articulates with the zygomatic bone anterolaterally and the small triangular orbital process of the palatine bone posteromedially (Fig. 39.3). The floor is thin and largely roofs the maxillary sinus. Not quite horizontal, it ascends a little laterally. Anteriorly it curves into the lateral wall, and posteriorly it is separated from the lateral wall by the inferior orbital fissure, which connects the orbit posteriorly to the pterygopalatine fossa, and more anteriorly to the infratemporal fossa. The medial lip is notched by the infraorbital groove. The latter passes forwards and sinks into the floor to become the infraorbital canal, which opens on the face at the infraorbital foramen: the infraorbital groove, canal and foramen contain the infraorbital nerve and vessels. Proportionally more orbital fractures involve the floor, particularly in the region of the infraorbital groove. The classic ‘blowout fracture’ leaves the orbital rim intact and typically entraps soft tissue structures, leading to diplopia, impaired ocular motility and enophthalmos; infra-orbital nerve involvement leads to ipsilateral sensory disturbance of the skin of the mid face.

Lateral wall

The lateral wall of the orbit is formed by the orbital surface of the greater wing of the sphenoid posteriorly and the frontal process of the zygomatic bone anteriorly: the bones meet at the sphenozygomatic suture (Fig. 39.2). The zygomatic surface contains the openings of minute canals for the zygomaticofacial and zygomaticotemporal nerves, the former near the junction of the floor and lateral wall, the latter at a slightly higher level, sometimes near the suture. The orbital tubercle, to which the lateral palpebral ligament, the check ligament of lateral rectus and the aponeurosis of levator palpebrae are all attached, lies just inside the midpoint of the lateral orbital margin. The lateral wall is the thickest wall of the orbit, especially posteriorly where it separates the orbit from the middle cranial fossa. Anteriorly the lateral wall separates the orbit and the infratemporal fossa. The lateral wall and roof are continuous anteriorly but are separated posteriorly by the superior orbital fissure, which lies between the greater wing (below) and lesser wing (above) of the sphenoid, and communicates with the middle cranial fossa. The fissure tapers laterally but widens at its medial end, its long axis descending posteromedially. Where the fissure begins to widen, its inferolateral edge shows a projection, often a spine, for the lateral attachment of the common tendinous ring. An infraorbital sulcus which runs from the superolateral end of the superior orbital fissure towards the orbital floor is sometimes associated with an anastomosis between the middle meningeal and infraorbital arteries.

ORBITAL FISSURES AND FORAMINA

Optic canal

The lesser wing of the sphenoid is connected to the body of the sphenoid by a thin, flat anterior root and a thick, triangular posterior root. The optic canal lies between these roots (Fig. 39.2) and connects the orbit to the middle cranial fossa, transmitting the optic nerve and its meningeal sheaths, and the ophthalmic artery. The common tendinous ring, which gives origin to the four recti, is attached to the bone near the superior, medial and lower margins of the orbital opening of the canal.

Superior orbital fissure

The superior orbital fissure is the gap between the greater and lesser wings of the sphenoid, bounded medially by the body of the sphenoid, and closed at its anterior extremity by the frontal bone (Fig. 39.2). It connects the cranial cavity with the orbit and transmits the oculomotor, trochlear and abducens nerves, branches of the ophthalmic nerve and the ophthalmic veins.

Inferior orbital fissure

The inferior orbital fissure is bounded above by the greater wing of the sphenoid, below by the maxilla and the orbital process of the palatine bone, and laterally by the zygomatic bone (Fig. 39.2). The maxilla and sphenoid often meet at the anterior end of the fissure, excluding the zygomatic bone. The inferior orbital fissure connects the orbit with the pterygopalatine and infratemporal fossae and transmits the infraorbital and zygomatic branches of the maxillary nerve and accompanying vessels, orbital rami from the pterygopalatine ganglion and a connection between the inferior ophthalmic vein and pterygoid venous plexus. A small maxillary depression may mark the attachment of inferior oblique anteromedially, lateral to the lacrimal hamulus.

Ethmoidal foramina

The anterior and posterior ethmoidal foramina usually lie in the frontoethmoidal suture (Fig. 39.2). The posterior foramen may be absent, and occasionally there is a middle ethmoidal foramen. The foramina open into canals which transmit their vessels and nerves into the ethmoidal sinuses, anterior cranial fossa and nasal cavity.

COMMON TENDINOUS RING

The common tendinous ring is a fibrous ring which surrounds the optic canal and part of the superior orbital fissure at the apex of the orbit, and gives origin to the four recti (Fig. 39.4). The optic nerve and ophthalmic artery enter the orbit via the optic canal, and so lie within the common tendinous ring. The superior and inferior divisions of the oculomotor nerve, the nasociliary branch of the ophthalmic nerve, and the abducens nerve, also enter the orbit within the common tendinous ring, but they do so via the superior orbital fissure (see Fig. 39.15). The trochlear nerve and the frontal and lacrimal branches of the ophthalmic nerve all enter the orbit through the superior orbital fissure but lie outside the common tendinous ring. Structures which enter the orbit through the inferior orbital fissure lie outside the common tendinous ring. The close anatomical relationship of the optic nerve and other cranial nerves at the orbital apex means that lesions in this region may lead to a combination of visual loss from optic neuropathy and ophthalmoplegia from multiple cranial nerve involvement.

ORBITAL CONNECTIVE TISSUE AND FAT

The orbit contains a complex arrangement of connective tissue which forms a supporting framework for the eyeball and also acts to limit ocular rotations and compartmentalize orbital fat (Fig. 39.5). Certain regions have anatomical and clinical significance, including the orbital septum, fascial sheath of the eye, ‘check’ ligaments, suspensory ligament and periosteum.

FASCIAL SHEATH OF THE EYEBALL

A thin fascial sheath, the fascia bulbi (Tenon’s capsule), envelops the eyeball from the optic nerve to the corneoscleral junction, separating it from the orbital fat, and forming a socket for the eyeball (Figs 39.5, 39.6). The ocular aspect of the sheath is loosely attached to the sclera by delicate bands of episcleral connective tissue. Posteriorly, it is traversed by ciliary vessels and nerves. It fuses with the sclera and with the sheath of the optic nerve where the latter enters the eyeball: attachment to the sclera is strongest in this position and again anteriorly, just behind the corneoscleral junction at the limbus. The fascia bulbi is perforated by the tendons of the extraocular muscles and is reflected on to each as a tubular sheath called the muscular fascia. The sheath of superior oblique reaches the fibrous pulley (trochlea) associated with the muscle. The sheaths of the four recti are very thick anteriorly but are reduced posteriorly to a delicate perimysium. Just before they blend with the fascia bulbi, the thick sheaths of adjacent recti become confluent and form a fascial ring.

Expansions from the muscular fascia are important for the attachments they make. Those from the medial and lateral recti are triangular and strong, and are attached to the lacrimal and zygomatic bones respectively: since they may limit the actions of the two recti, they are termed the medial and lateral check ligaments (Fig. 39.6). Other extraocular muscles have less substantial check ligaments, and the capacity of any of them to actually limit movement has been questioned.

The sheath of inferior rectus is thickened on its underside and blends with the sheath of inferior oblique. These two, in turn, are continuous with the fascial ring noted earlier and therefore with the sheaths of the medial and lateral recti. Since the latter are attached to the orbital walls by check ligaments, a continuous fascial band, the suspensory ligament of the eye, is slung like a hammock below the eye, providing sufficient support such that, even when the maxilla (forming the floor of the orbit) is removed, the eye will retain its position.

The thickened fused sheath of inferior rectus and inferior oblique also has an anterior expansion into the lower eyelid, where, augmented by some fibres of orbicularis oculi, it attaches to the inferior tarsus as the inferior tarsal muscle: contraction of inferior rectus in downward gaze therefore also draws the lid downward. The sheath of levator palpebrae superioris is also thickened anteriorly, and just behind the aponeurosis it fuses inferiorly with the sheath of superior rectus. It extends forward between the two muscles and attaches to the upper fornix of the conjunctiva.

Other extensions of the fascia bulbi pass medially and laterally and attach to the orbital walls, forming the transverse ligament of the eye. This structure is of uncertain significance, but presumably plays a part in drawing the fornix upwards in gaze elevation and may act as a fulcrum for levator movements. Other numerous finer fasciae form radial septa which extend from the fascia bulbi and the muscle sheaths to the periosteum of the orbit, and so provide compartments for orbital fat. Many of the fasciae contain smooth muscle cells. The ocular and orbital fasciae are arranged to assist in the location of the eye within the orbit without obstructing the activities of the extraocular muscles, except possibly in the extremes of rotation. They also prevent the gross displacement of orbital fat, which could interfere with the accurate positioning of the two eyes that is essential for binocular vision.

The periosteum of the orbit is only loosely attached to bone. Behind, it is united with the dura mater surrounding the optic nerve and in front it is continuous with the periosteum of the orbital margin, where it gives off a stratum which contributes to the orbital septum. It also attaches to the trochlea, and, as the lacrimal fascia, forms the roof and lateral wall of the fossa for the nasolacrimal sac.

ORBITAL FAT

The spaces between the main structures of the orbit are occupied by fat, particularly in the region between the optic nerve and the surrounding cone of muscles (Figs 39.5, 39.7). Fat also lies between the muscles and periosteum and is limited anteriorly by the orbital septum. Collectively, the fat helps to stabilize the position of the eyeball and also acts as a socket within which the eye can rotate. Conditions resulting in an increased overall volume of orbital fat, e.g. hyperthyroidism (Graves’ disease), may lead to forward protrusion of the eyeball (exophthalmos).

EXTRAOCULAR MUSCLES

There are seven skeletal extraocular (extrinsic) muscles associated with the eye. Levator palpebrae superioris is an elevator of the upper eyelid, and the other six, i.e. four recti (superior, inferior, medial and lateral), and two obliques (superior and inferior), are capable of moving the eye in almost any direction.

LEVATOR PALPEBRAE SUPERIORIS

Levator palpebrae superioris is a thin, triangular muscle which arises from the inferior aspect of the lesser wing of the sphenoid, above and in front of the optic canal, and separated from it by the attachment of superior rectus (Fig. 39.5). It has a short narrow tendon at its posterior attachment, and broadens gradually, then more sharply as it passes anteriorly above the eyeball. The muscle ends in front in a wide aponeurosis. Some of its tendinous fibres pass straight into the upper eyelid to attach to the anterior surface of the tarsus, while the rest radiate and pierce orbicularis oculi to pass to the skin of the upper eyelid. A thin lamina of smooth muscle, the superior tarsal muscle, passes from the underside of levator palpebrae superioris to the upper margin of the superior tarsus.

The connective tissue sheaths of the adjoining surfaces of levator palpebrae superioris and superior rectus are fused (Fig. 39.5). Where the two muscles separate to reach their anterior attachments, the fascia between them forms a thick mass to which the superior conjunctival fornix is attached: this is usually described as an additional attachment of levator palpebrae superioris. Traced laterally, the aponeurosis of the levator passes between the orbital and palpebral parts of the lacrimal gland to attach to the orbital tubercle of the zygomatic bone. Traced medially, it loses its tendinous nature as it passes closely over the reflected tendon of superior oblique, and continues on to the medial palpebral ligament as loose strands of connective tissue.

Actions

Levator palpebrae superioris elevates the upper eyelid. During this process the lateral and medial parts of its aponeurosis are stretched and thus limit its action: the elevation is also checked by the orbital septum. Elevation of the eyelid is opposed by the palpebral part of orbicularis oculi. Levator palpebrae superioris is linked to superior rectus by a check ligament, thus the upper eyelid elevates when the gaze of the eye is directed upwards.

The position of the eyelids depends on reciprocal tone in orbicularis oculi and levator palpebrae superioris, and on the degree of ocular protrusion. In the opened position the upper eyelid covers the upper part of the cornea, while the lower lid lies just below its lower margin. The eyes are closed by movements of both lids, produced by the contraction of the palpebral part of orbicularis oculi and relaxation of levator palpebrae superioris. In looking upwards, the levator contracts and the upper lid follows the ocular movement. At the same time, the eyebrows are also usually raised by the frontal parts of occipitofrontalis to diminish their overhang. The lower lid lags behind ocular movement, so that more sclera is exposed below the cornea and the lid is bulged a little by the lower part of the elevated eye. When the eye is depressed both lids move: the upper retains its normal relation to the eyeball and still covers about a quarter of the cornea, whereas the lower lid is depressed because the extension of the thickened fascia of inferior rectus and inferior oblique pull on its tarsus as the former contracts.

The palpebral apertures are widened in states of fear or excitement by contraction of the smooth muscle of the superior and inferior tarsal muscles as a result of increased sympathetic activity. Lesions of the sympathetic supply result in drooping of the upper eyelid (ptosis), as seen in Horner’s syndrome.

THE FOUR RECTI

The four recti are approximately strap-shaped; each has a thickened middle part which thins gradually to a tendon (Figs 39.8, 39.9). They are attached posteriorly to a common tendinous ring that encircles the superior, medial and inferior margins of the optic canal, continues laterally across the inferior and medial parts of the superior orbital fissure, and is attached to a tubercle or spine on the margin of the greater wing of the sphenoid (Fig. 39.4). The tendinous ring is closely adherent to the dural sheath of the optic nerve medially and to the surrounding periosteum. Inferior rectus, part of medial rectus and the lower fibres of lateral rectus, are all attached to the lower part of the ring, whereas superior rectus, part of medial rectus and the upper fibres of lateral rectus are all attached to the upper part. A second small tendinous slip of lateral rectus is attached to the orbital surface of the greater wing of the sphenoid, lateral to the common tendinous ring.

Each rectus muscle passes forwards, in the position implied by its name, to be attached anteriorly by a tendinous expansion into the sclera, posterior to the margin of the cornea.

THE OBLIQUES

MOVEMENTS OF THE EYES

Movements of the eyes involve rotations around a centre of rotation within the globe. For practical purposes this can be considered to lie 13.5 mm behind the corneal apex. Normal eye movements are binocular. Movements of the eyes in the same direction are termed versions, whilst those in opposite directions are termed vergences. Eye movements are often accompanied by corresponding movements of the eyelids, particularly in upgaze where the activity of levator palpebrae superioris is closely coupled to that of superior rectus. The following section describes the ocular motor system in terms of the actions of individual extraocular muscles, the diversity of eye movements and their neural control.

Actions of the extraocular muscles

Levator palpebrae superioris elevates the upper lid, and its antagonist is the palpebral part of orbicularis oculi. The degree of elevation, which, apart from blinking, is maintained for long periods during waking hours, is a compromise between ensuring an adequate exposure of the cornea and controlling the amount of incident light. In bright sunshine, the latter can be reduced by lowering the upper lid, so limiting glare. Electrically, the levator discharges steadily for a given fixation, but with increasing rates with upward lid position, and relaxes during closure of the palpebral fissure. The role of the superior tarsal muscle is less clear. Its tonus is related to sympathetic activity, and since ptosis is a consequence of impairment of its sympathetic nerve supply, it may function as an accessory elevator of the upper eyelid.

The six extraocular muscles all rotate the eyeball in directions dependent on the geometrical relation between their bony and global attachments (Fig. 39.10), which are altered by the ocular movements themselves. For convenience, each muscle will be considered in isolation, but it must be appreciated that any movement of the eyeball alters the tension and/or length in all six muscles. It is useful to consider the four recti and two obliques as separate groups, (remembering always that they act in concert), because they form more obvious groupings as antagonists or synergists. The extrinsic ocular muscles collectively position the eyeball in the orbital cavity and prevent its anteroposterior movements, other than a slight retraction during blinks, because the recti exert a posterior traction while the obliques pull the eyeball to some degree anteriorly. They may be assisted by various ‘check ligaments’ (see above). A simplified description of the actions of the extraocular muscles is summarized in Figure 39.11.

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