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Orbit and accessory visual apparatus
Bony Orbit
The bony orbits are skeletal cavities located on either side of the root of the nose that serve as sockets for the eyes and associated tissues. The walls of each orbit protect the eye from injury, provide points of attachment for six extraocular muscles that 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.
By convention, each cavity is considered to approximate 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 and 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. 44.1 ); the remainder of the cavity is filled with extraocular muscles, vessels and nerves that are contained within and supported by orbital fat and connective tissue. In brief, the orbit transmits the optic, oculomotor, trochlear and abducens nerves, branches of the ophthalmic and maxillary divisions of the trigeminal nerve and the ophthalmic vessels. The ciliary parasympathetic ganglion is located towards the rear of the orbit, lateral to the optic nerve and medial to lateral rectus. The orbit also contains components of the lacrimal system, which is responsible for the production and drainage of tears.
There is a strong correlation between the pattern of orbital growth and eyeball growth. Orbital growth is most rapid during the first 12–24 months of life and most parameters reach 86–96% of adult values by the age of 8 years ( ). Orbital volumes are larger in boys than in girls throughout childhood ( ).
Roof
The roof of the orbit is formed principally by the thin orbital plate of the frontal bone ( Fig. 44.2 ). It is gently concave on its orbital aspect, which separates the contents of the orbit 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 fossa that houses the orbital part of the lacrimal gland. The roof slopes down 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 (see Fig. 44.2 ). This paper-thin, rectangular plate covers the middle and posterior ethmoidal air cells, providing a route by which infection can spread into the orbit. The ethmoid articulates with the medial edge of the orbital plate of the frontal bone at a suture that 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. 44.3 ). During development, the medial wall of the orbit doubles in length, with disproportionate enlargement of its anterior half. Growth is rapid during the first 6 years of life and gradual between 7 years and adulthood ( ).
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 (see Fig. 44.3 ). The floor is thin and forms most of the roof of 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 pure 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; infraorbital nerve involvement leads to ipsilateral sensory disturbance of the skin of the midface.
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. 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, and 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 (see Fig. 44.4 ). An inferior orbital fissure, 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 (see Fig. 44.2 ) 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 (see Fig. 44.4 ). The growth of the optic canal continues from the fetal period into adulthood with an increase in both the length and width 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 (see Fig. 44.2 ). 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 (see Fig. 44.4 ).
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 (see Fig. 44.2 ). 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 (see Fig. 44.4 ), 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 (see Fig. 44.2 ). The posterior foramen may be absent, and occasionally there is a middle ethmoidal foramen. The foramina open into canals that transmit their vessels and nerves into the ethmoidal sinuses, anterior cranial fossa and nasal cavity.
Orbitomeningeal (cranio-orbital) foramina
The orbit may also be connected with the cranial cavity via inconstant canals, including an A-type orbitomeningeal foramen with the anterior cranial fossa, or a metopic canal, Warwick’s foramen, accessory opening of the foramen rotundum or an M-type orbitomeningeal foramen with the middle cranial fossa ( ).
One or more orbitomeningeal (cranio-orbital) foramina are present in at least one orbital cavity in approximately 60% of individuals ( ). These foramina usually lie close to the superior orbital fissure, either in the frontal bone, or the greater wing of the sphenoid, or along the frontosphenoidal suture. They typically transmit the meningolacrimal artery (an orbital branch of the middle meningeal artery that supplies the lacrimal gland), but may also also transmit the anastomosis between the recurrent branch of the lacrimal artery and the middle meningeal artery. Although usually small in calibre, approximately 5% of orbits have an orbitomeningeal foramen large enough to transmit important vessels that could be a source of severe bleeding during deep orbital surgery.
Common tendinous ring
The common tendinous ring is a fibrous anulus that 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. 44.4 ). 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. 44.15 ). 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 that 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 that forms a supporting framework for the eyeball and also influences ocular rotations and compartmentalizes orbital fat ( Fig. 44.5 ). Certain regions have anatomical and clinical significance, including the orbital septum, fascial sheath of the eye, ‘check’ ligaments, suspensory ligament and periosteum. The notion that orbital connective tissues function as extraocular muscle pulleys and influence ocular motility has gained widespread acceptance ( , , ). FLOAT NOT FOUND
Orbital septum
The orbital septum is a weak membranous sheet, attached to the orbital rim where it becomes continuous with the periosteum (see Fig. 44.5 ). It extends into each eyelid and blends with the tarsal plates and, in the upper eyelid, with the superficial lamella of levator palpebrae superioris. The orbital septum is thickest laterally, where it lies in front of the lateral palpebral ligament. It passes behind the medial palpebral ligament and nasolacrimal sac, but in front of the pulley of superior oblique. The septum is pierced above by levator palpebrae superioris and below by a fibrous extension from the sheaths of inferior rectus and inferior oblique. The lacrimal, supratrochlear, infratrochlear and supraorbital nerves and vessels pass through the septum from the orbit en route to the face and scalp. Clinically, the septum is an important anatomical reference that differentiates pre- and post-septal (orbital) cellulitis.
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 (see Fig. 44.5 ; Fig. 44.6 ). 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. Injection of local anaesthetics via a cannula into the space between the fascia bulbi and the sclera (sub-Tenon’s anaesthesia) has become a popular technique for many ophthalmic surgical procedures ( ).
The fascia bulbi is perforated by the tendons of the extraocular muscles and is reflected on to each as a tubular sheath, 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 (see Fig. 44.6 ). Other extraocular muscles have less substantial check ligaments, and the capacity of any of them actually to 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 forwards 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 that extend from the fascia bulbi and the muscle sheaths to the periosteum of the orbit, and so provide compartments for orbital fat ( ). 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 that 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 connective tissue pulleys
There is mounting evidence that challenges the traditional view that the recti are attached only at their origin and scleral insertion. The concept that orbital connective tissue sheaths elastically coupled to the orbital walls function as pulleys was initially proposed as an explanation for the observed orbital stability of rectus muscle paths ( ). Each pulley consists of an encircling sleeve of collagen located within the fascia bulbi, near the equator of the globe. Elastic fibres and bundles of smooth muscle confer the required internal rigidity to the structure ( ). Although the original model described a passive pulley system, the current view is that fibres from the orbital surface of the muscle insert into the pulley sleeve to allow small longitudinal movements. This ‘active pulley hypothesis’ provides a better explanation for normal ocular kinematics ( , ).
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 (see Fig. 44.5 ; Fig. 44.7 ). 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 forms a socket within which the eye can rotate. Conditions resulting in an increased overall volume of orbital fat with associated swelling of the extraocular muscles, e.g. hyperthyroidism (Graves’ disease), may lead to forward protrusion of the eyeball (exophthalmos).
Extraocular Muscles
Extraocular muscles are specialized skeletal muscles with several unique morphological, cellular and molecular properties ( ). There are seven 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. Complete congenital absence of the extraocular muscles, thought to represent a severe form of congenital fibrosis syndrome, has been described ( ). Rarely, humans have deep orbital bands consistent with supernumerary extraocular muscles ( ).
Levator palpebrae superioris
Levator palpebrae superioris is a thin, triangular muscle that 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 (see Fig. 44.4 ). 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 (see Fig. 44.20 ).
The connective tissue sheaths of the adjoining surfaces of levator palpebrae superioris and superior rectus are fused (see Fig. 44.5 ). 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. 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.
Vascular supply
Levator palpebrae superioris receives its arterial supply both directly from the ophthalmic artery and indirectly from its supraorbital branch.
Innervation
Levator palpebrae superioris is innervated by a branch of the superior division of the oculomotor nerve that enters the inferior surface of the muscle. Sympathetic fibres to the smooth muscle component of levator palpebrae superioris (superior tarsal muscle) are derived from the plexus surrounding the internal carotid artery; these nerve fibres may join the oculomotor nerve in the cavernous sinus and pass forwards in its superior branch.
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, and so 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 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 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 recti
The four recti are approximately strap-shaped; each has a thickened middle part that thins gradually to a tendon ( Figs 44.8–44.9 ). 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 (see Fig. 44.4 ). 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. However, before their scleral attachment, the recti make functionally important connections with orbital connective tissue that influence muscle action (see p. 772 ).
Superior rectus
Superior rectus is slightly larger than the other recti. It arises from the upper part of the common tendinous ring, above and lateral to the optic canal. Some fibres also arise from the dural sheath of the optic nerve. The fibres pass forwards and laterally (at an angle of approximately 25° to the median plane of the eye in the primary position) to insert into the upper part of the sclera, approximately 8 mm from the limbus (see Fig. 44.8 ). The insertion is slightly oblique, the medial margin more anterior than the lateral margin.
Vascular supply
Superior rectus receives its arterial supply both directly from the ophthalmic artery and indirectly from its supraorbital branch.
Innervation
Superior rectus is innervated by the superior division of the oculomotor nerve that enters the inferior surface of the muscle.
Actions
Superior rectus moves the eye so that the cornea is directed upwards (elevation) and medially (adduction). To obtain upward movement alone, the muscle must function with inferior oblique. Superior rectus also causes intorsion of the eye (i.e. medial rotation). Because a check ligament extends from superior rectus to levator palpebrae superioris, elevation of the eye also results in elevation of the upper eyelid. For more detailed discussion of its actions, see page 775 .
Inferior rectus
Inferior rectus arises from the common tendinous ring, below the optic canal. It runs along the orbital floor in a similar direction to superior rectus (i.e. forwards and laterally) and inserts obliquely into the sclera below the cornea, approximately 6.5 mm from the limbus (see Fig. 44.9 ).
Vascular supply
Inferior rectus receives its arterial supply from the ophthalmic artery and from the infraorbital branch of the maxillary artery.
Innervation
Inferior rectus is innervated by a branch of the inferior division of the oculomotor nerve that enters the superior surface of the muscle.
Actions
The principal activity of inferior rectus is to move the eye so that it is directed downwards (depression). It also causes the eye to deviate medially and extorts the eye (i.e. produces lateral rotation). To obtain downward movement alone, inferior rectus must function with superior oblique. A fibrous extension from inferior rectus to the inferior tarsus of the eyelid causes the lower eyelid to be depressed when the muscle contracts. For more detailed discussion of its actions, see page 775 .
Medial rectus
Medial rectus is slightly shorter than the other recti but is the strongest of the group. It arises from the medial part of the common tendinous ring, and also from the dural sheath of the optic nerve, passing horizontally forwards along the medial wall of the orbit, below superior oblique (see Figs 44.8–44.9 ). It inserts into the medial surface of the sclera, approximately 5.5 mm from the limbus and slightly anterior to the other recti.
Vascular supply
Medial rectus receives its arterial supply from the ophthalmic artery.
Innervation
Medial rectus is innervated by a branch from the inferior division of the oculomotor nerve that enters the lateral surface of the muscle.
Actions
Medial rectus moves the eye so that it is directed medially (adducted). The two medial recti acting together are responsible for convergence of the eyes. For more detailed discussion of its actions, see page 775 .
Lateral rectus
Lateral rectus arises principally from the lateral part of the common tendinous ring and bridges the superior orbital fissure (see Fig. 44.4 ); some fibres also arise from a spine on the greater wing of the sphenoid. The muscle passes horizontally forwards along the lateral wall of the orbit to insert into the lateral surface of the sclera, approximately 7 mm from the limbus (see Fig. 44.8 ).
Vascular supply
Lateral rectus receives its arterial supply from the ophthalmic artery directly and/or from its lacrimal branch.
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