Gross Anatomy of the Orbit PDF

Gross Anatomy of the Orbit MSc in Clinical Optometry: Principles of Therapeutics Unit 1: Part 1 GROSS ANATOMY OF THE ORBIT Description: Unit 1 of the Principles of Therapeutics module is concerned with the anatomy and physiology of the eye and orbit. This first part of the unit describes the gross...

Gross Anatomy of the Orbit MSc in Clinical Optometry: Principles of Therapeutics Unit 1: Part 1 GROSS ANATOMY OF THE ORBIT Description: Unit 1 of the Principles of Therapeutics module is concerned with the anatomy and physiology of the eye and orbit. This first part of the unit describes the gross anatomy of the orbit, specifically, its osteology and the organisation of orbital nerves, muscles and blood vessels. Hours: Eight Learning Outcomes Following successful completion of this module you should be able to: Identify the bones that make up the walls of the orbital cavity Recognise the relationship between the paranasal air sinuses and the orbital cavity Describe the general anatomical arrangement of tissues within the orbit Identify the origin, course and insertion of the extraocular muscles Describe the motor, sensory and autonomic innervation of the orbit Describe the arterial blood supply and venous drainage of the orbit Introduction Knowledge of the structure of the eye from what can be seen with the unaided eye (gross anatomy) is fundamental to understanding ocular function and how structure and function can be changed by disease. Over recent decades, there have been various methods for imaging living patients, e.g. computerised tomography (CT) and magnetic resonance imaging (MRI). As a result, knowledge of gross anatomy has become increasingly important in order to interpret these images and also enable the development of therapy that targets specific sites. From an optometrists’ perspective, knowledge of the normal ocular gross anatomy enables an understanding of the structure and function of the eye and surrounding orbit. Furthermore, it enables the practitioner to make more accurate diagnoses through appropriate interpretation of (i) a patient’s history and symptoms and (ii) the visible signs and symptoms of a particular disease. Bones of the orbit from several arteries and veins of the region, lymph Figure 1 represents most of the structures of the orbit drains to the submandibular nodes and it receives including its connective tissue. Of the four walls of the sensory and parasympathetic innervation from the orbit the medial wall is the thinnest and is often paper- infraorbital and alveolar nerves. Blood and nerve supply thin (Figure 2). The thickness of the roof, floor and to the other sinuses are described with those systems. lateral walls increases in that order and only the tough Figures 3 and 4 illustrate the position of the sinuses. outer wall is not apposed to a sinus. Medially, close to the apex of the orbit the large sphenoidal sinus lies The bones contributing to the orbit and the principle beneath the floor of the optic canal and is usually structural features are indicated in Figure 2. The separated from its pair by a thin lamina of bone. smoothed angles of the quadrilateral formed by the Immediately forward of the sphenoidal, the complex of margin, the sharp upper margin and the absence of a several ethmoidal sinuses lines the medial wall and, superciliary ridge identify the orbit as female. most anterior, the frontal sinus is located, extending laterally beneath the brow. The maxillary sinus, below the orbital floor, completes the group of paranasal sinuses and is the largest of them. It receives branches 1 Gross Anatomy of the Orbit 2 Gross Anatomy of the Orbit organised into compartments enclosing fat and provides support for traversing vessels and nerves. The fascial sheets are attached to the extraocular muscle sheaths, the dura mater of the optic nerve and the fascia bulbi, or Tenon’s capsule, enclosing the sclera of the eye. A thickened fascial sheath, the septum orbitale, is continuous inwards from the periorbita at the orbital margin, attaching to the perimeter of the tarsal plates of the eyelids confining orbital fat. Rupture of the septum, common in the elderly, causes herniation of orbital fat into the lids. Stronger fibroelastic bands, the so-called check ligaments, pass from the orbital aspect of the horizontal rectus muscle sheaths opposite the equator of the eye where they are thickest and attach to the orbital wall. In this position the muscle sheaths are attached to each other (the intermuscular fascia). Tenon’s capsule tightly invests the globe commencing at the limbus and where it is penetrated by the extraocular muscles it is continuous with their sheaths. Three prominent transverse fascial bands traverse the orbit opposite the eye, Whitnall’s ligament lying above the levator muscle, the intermuscular transverse ligament lying between the levator and the superior rectus and inferiorly, the suspensory ligament of Lockwood passing beneath the inferior rectus and also partly enclosing the inferior oblique, with an extension, the casulopalpebral fascia, continuing to the tarsal plate of the lower eyelid. Each of the transverse ligaments span the orbit and attach to bone near the orbital rim horizontally and, apart from Whitnall’s ligament, may be regarded as thickenings of Tenon’s capsule. Fascia and adipose tissue The greater part of the orbit is occupied by fat and At various sites smooth muscle is associated with the fascia as shown in Figure 1, (fat represented by grey fascia. The largest muscle, the orbital muscle of Müller, spots and fascia by blue lines). A complex of delicate lies in the periorbita bridging the inferior orbital fissure fascial sheets extending from the periosteum is and is probably redundant in man (Figure 5). Other 3 Gross Anatomy of the Orbit smooth muscle groups are present in the rectus muscle ligaments, perhaps only regularly at the medial rectus, and have no known function. The smooth muscle sheets of the upper and lower eyelids, the superior and inferior palpebral muscles (of Müller) contribute to eyelid retraction. Extraocular muscles Five of the six extraocular muscles and the levator palpebrae superioris have short tendonous origins in a common annulus (of Zinn). The complete annulus is shown in Figure 5, a few millimeters from its attachment to bone, bearing the origins of five muscles. The origin of the sixth, the inferior oblique muscle, is located anteriorly close to the orbital margin. The figure also shows clearly the nerves that enter the orbit within and without the annulus. It also shows the attachment of the annulus to the dura mater of the optic nerve on the medial side preventing movement of the nerve within its canal. In this position the superior and medial recti have their origins and the close attachment to the dura may be responsible for the pain associated with extreme rotations of the eye in retrobulbar neuritis. What the figure cannot show is that the annuIus twice bridges the superior orbital fissure (Figure 2) and attaches laterally to the spina recti lateralis, a small, often sharp bony elevation of the sphenoid bone varying in prominence. The rectus muscles have a roughly similar length of approximately 40mm (36-43mm) and tendon lengths at insertion vary more than twofold (3.6-8.4mm). Muscles has a slightly longer course from origin to insertion than lie close to the orbital wall, bowed outwards slightly the medial rectus, a greater length in apposition to the when relaxed and form an incomplete muscle cone with globe and a tendon more than twice the length (Figure the apex at the annulus and the base at the globe. The 6). As the muscle cone diameter increases anteriorly, medial orbital wall is in the sagittal plane and the lateral the approximately strap-like rectus muscles thicken and ° orbital wall is at 45 to it, and the lateral rectus therefore widen to a maximum of 8-11mm at their middle thirds. 4 Gross Anatomy of the Orbit The superior oblique muscle has a narrow 2.5mm wide origin becoming thicker and widening to a maximum of about 5mm (Figure 7). It advances to the trochlea, a U- shaped ring of hyaline cartilage located in the frontal bone at the upper medial angle of the orbit close to the margin. Anteriorly, the muscle thins becoming cylindrical and tendonous, reducing to about 1.5mm in diameter before entering the trochlea. Passing through the trochlea, the tendon is reflected making an angle of ° approximately 54 with its initial path, thins and widens and passes across the upper surface of the globe beneath the superior rectus muscle and attaches obliquely to the upper lateral posterior quadrant of the sclera (Figure 6). Its length to the trochlea is 32mm and from trochlea to the insertion it is 20mm. The trochlear nerve can be seen crossing above the levator obliquely and entering the lateral edge of the muscle (Figure 7). Their tendons insert in the anterior half of the globe 3- 8mm from the limbus. The muscles are depicted in several dissections and drawings (Figures 6, 7, 8 & 9). The inferior oblique arises as a short rounded tendon, 1- 2mm in length, in a shallow depression in the floor of the orbit medially in the same vertical plane as the trochlea (Figure 6). The muscle passes posterolaterally ° and slightly upwards at an angle of 51 to the sagittal ° plane, or 75 to the axis of the orbit. Its width quickly increases and becomes a flat band passing beneath the inferior rectus muscle then curving upwards obliquely in close apposition to the sclera inserting into the inferior lateral quadrant of the globe posteriorly with a barely discernible tendon 10mm in width. It averages about 36mm in length. The position of the levator palpebrae superioris is shown in Figure 7. It is peculiar to mammals, passes from the annulus above the medial border of the superior rectus (Figure 5), widens slightly at first but then more expansively to completely cover the superior rectus and enter the upper eyelid as a thin aponeurotic band. The transition from muscle to tendon also marks a change in direction from horizontal to almost vertical. It becomes fan-shaped and extends the full width of the eyelid adjacent to the anterior aspect of the tarsal plate 5 Gross Anatomy of the Orbit to which it has fibrous attachments at intervals along its full length and others penetrate the orbicularis muscle possibly to the skin. At its medial and lateral extremes the aponeurosis has attachments to the orbital margin at the medial and lateral midpoints. A division of the oculomotor branch serving the superior rectus enters the belly of the levator. The transverse ligament of Whitnall is commonly regarded as a check ligament for the muscle. Human rectus muscles are traditionally regarded as inserting exclusively in the sclera with the single role of rotating the globe. Only the fibres on the global side of muscles were found to insert in this manner, the orbital fibres, nearly half the total, inserting into fibro-elastic muscle sleeves or pulleys with fibrous attachments to the wall of the orbit. The sleeves mark the position where muscles penetrate Tenon’s capsule to attach to the globe and they are continuous posteriorly with the muscle sheaths. The dynamic pulley hypothesis states that rectus muscles slide through the sleeves and that the orbital muscle fibres regulate sleeve position. Nerves i) Motor anterior ethmoidal that pass to skin; otherwise the The somatic motor nerves of the orbit are distributed to ethmoidals serve the mucosa of the upper paranasal the extraocular muscles, the trochlear and abducens sinuses and the nasal mucosa. Parasympathetic innervating the superior oblique and lateral rectus branches of the ubiquitous pterygopalatine rami respectively and the oculomotor innervates the other oculares accompany the ethmoidals. Only the long and four oculorotatory muscles and the levator palpebrae short ciliary nerves, the latter via the ophthalmic superioris. Access of nerves to their muscles is (sensory) root of the ciliary ganglion, innervate the eye indicated in several of the Figures only the trochlear to provide for ocular sensitivity. A sensory supply to the nerve advances external to the common annulus extraocular muscles is considered to be proprioceptive. (Figure 5), joining the superior oblique at its lateral margin (Figure 7). The others enter the muscle bellies There is unconfirmed evidence that filaments from the centrally except for the nerve to the inferior oblique (the maxillary nerve, the second branch of the trigeminal longest nerve) that enters the muscle’s posterior nerve, enter the orbit and may pass to the eye. A margin. branch of its infraorbital division innervates the skin of the lower eyelid. ii) Sensory The gross anatomy of the optic nerve is fixed at the iii) Autonomic hiatus of the optic canal and takes a slightly curved Sympathetic nerves of the orbit are all or mostly course to reach the eye. This provides a slack of 5-6mm postganglionic fibres of the superior cervical ganglion, permitting unhindered rotation of the eye. General the preganglionic fibres being housed in the ciliospinal sensitivity of the region is served by the ophthalmic centre of Budge of the spinal cord and their axons nerve - the smallest branch of the trigeminal (Figure 10). leaving the cord in T1-T3. They enter the base of the cranium from the ganglion as the internal carotid nerve The trigeminal ganglion lies in a pouch, Meckel’s cave, that divides to form the internal carotid and cavernous in the floor of the cranial cavity behind the orbit (Figures sinus plexuses. Orbital sympathetics pass from both of 10 and 11) and its uppermost and most medial branch, them, those from the cavernous sinus plexus either the ophthalmic emerges from the cranial cavity through joining orbital-bound cranial nerves, possibly as the superior orbital fissure where it splits into three temporary conduits, or at first passing forward branches – the lacrimal, frontal and nasociliary nerves separately through the supraorbital fissure. Some of the (Figures 7 and 10). The proportion of ophthalmic nerve latter are distributed with the branches of the ophthalmic fibres distributed to the eye is quite small. The frontal artery within the orbit, others either join the ophthalmic and lacrimal branches are disposed peripherally, nerve or its nasociliary branch intracranially and external to the muscle cone adjacent to the periosteum continue to the eye in its long ciliary branch or join the and disperse in the facial skin. Only a small fraction of latter directly within the orbit (Figures 10 and 11). The lacrimal fibres pass to the gland. Even the nasociliary only orbital sympathetic nerves of internal carotid plexus nerve, the only branch entering the muscle cone, has an origin accompany the ophthalmic artery intracranially, infratrochlear branch and a terminal division of the entering the orbit with the artery through the optic canal. 6 Gross Anatomy of the Orbit 7 Gross Anatomy of the Orbit Some of these nerves pass to the eye. It is probable Blood Vessels that sympathetic nerves to eyelid smooth muscle and i) Arteries the lacrimal gland follow their vessels for access. Orbital Immediately after the internal carotid artery penetrates sympathetic nerves are mainly vasoconstrictors; some the dural roof of the cavernous sinus behind the orbit to enter the lacrimal gland but their role in the regulation of enter the cranial cavity, the ophthalmic artery (1mm in secretion is unclear and minor, at least with regard to diameter) branches from it anteriorly and enters the secretory volume. A final point worth noting is that the optic canal beneath the optic nerve. Its passage within internal carotid and cavernous sinus plexuses are not the orbit is somewhat variable (Hayreh,1962) but in purely of sympathetic nerve origin as it is now clear that most individuals it emerges from the canal on the lateral parasympathetic fibres contribute. Hence, where nerves side of the optic nerve and crosses medially above it are identified as sympathetic above, they may not be (Figures 8 and 13). The representation of its branches exclusively so, but no tangible information is available in Figure 13 serves the present purpose but the central on this point. retinal, lacrimal or ciliary artery may be the first branch, usually from a position below the optic nerve. The Parasympathetic nerves of the orbit issue from two central retinal artery continues forward beneath the ganglia, the ciliary and pterygopalatine (Figure 12). optic nerve, penetrating the nerve 5-17mm behind the Preganglionic fibres issue from the neurons of the globe to take up a central position before dividing to Edinger-Westphal division of the oculomotor nucleus in contribute to the familiar picture of the disc vessels. The the midbrain and synapse in the ciliary ganglion. They long posterior ciliary arteries penetrate the sclera leave the cranium in the oculomotor nerve, enter the obliquely, one on each side of the globe horizontally, to orbit through the superior orbital fissure, advance in the form the major iridic circle, whereas the more numerous inferior division of the nerve and enter the ganglion in short posterior ciliary arteries penetrate the sclera the short motor root, identifiable in Figure 12. The nearly orthogonally distributing exclusively in the postganglionic fibres, uniquely myelinated, pass to the choroid. Muscular arteries enter with the motor nerves eye in the short ciliary nerves and are distributed to the and in rectus muscles branches continue forward, ciliary muscle and the sphincter pupillae. They are penetrate to the orbital surface of the muscles and are responsible for accommodation and pupillary distributed to the bulbar conjunctiva. The palpebral constriction. conjunctiva receives most of its arteries from the palpebral arcades (Figure 13). The pterygopalatine ganglion is related to the facial nerve and the preganglionic fibres arise from a nucleus thought to lie lateral to the main facial nucleus and ventral to the superior salivatory nucleus and emerge in the sensory root (nervous intermedius) of the facial nerve. They continue in the greater petrosal nerve, an intraosseus branch of the facial nerve, and are joined by the sympathetic deep petrosal nerve in the floor of the cranium to form the Vidian nerve; the combined nerve enters the pterygoid canal and joins the pterygopalatine ganglion in the pterygopalatine fossa. In the dissection of Figure 12 all bone is removed including the pterygoid canal so that the Vidian nerve is exposed. The ganglion is attached to the maxillary nerve medially within the fossa where it has several branches (Figure 12). Only the dorsal branches, the rami orbitales, are of immediate interest as, among other targets, they conduct postganglionic fibres to the eye, the lacrimal gland and the orbital blood vessels. Some sympathetic fibres from the deep petrosal nerve may be present but the rami are not regarded as significant conduits for the orbital sympathetic supply. Branches of the rami are identified by their recipient structures. Fine rami oculares enter the eye posteriorly close to the ciliary nerves and distribute mainly to the choroid and have a vasodilatory function, but there is evidence that some also contribute to the innervation of the dilatator pupillae. The rami lacrimales represent a direct secretomotor parasympathetic pathway to the gland. The rami vasculares and oculares are vasodilatory. 8 Gross Anatomy of the Orbit The ophthalmic artery has a number of connections with iii) Lymphatics branches of the external carotid artery i.e. it is not an Over the years a number of speculative reports have end-artery. They are common but variable and include appeared purporting to demonstrate lymphatics in the anastomoses between frontal or supraorbital and orbit, including the eye, but vessels showing typical external maxillary (angular), anterior lymphatic structure are present only in the conjunctiva ethmoidal/sphenopalatine and lacrimal/middle and eyelids. Lymphatic drainage of the paranasal air meningeal arteries. The latter two arteries are joined by sinuses occurs via submandibular and deep cervical the recurrent meningeal artery - sometimes called the lymph nodes. accessory ophthalmic artery (Figures 8 and 13). Although variable in size it is the most common of the Conclusion junctions and may be constant. It can be as large as the In this first part of Unit 1 of the Principles of ophthalmic or even replace it with exclusive Therapeutics module, the gross anatomy of the orbit responsibility for orbital arterial supply. The clinical has been discussed in detail. An appreciation of the significance of the anastomoses is that should an anatomical arrangement of orbital tissues is essential to internal carotid artery become occluded through disease understand orbital disease, including its pathogenesis or surgical intervention, experience shows that vision and clinical presentation. Diseases of the orbit will be and ocular mobility are likely to be preserved. No doubt covered at various points within the programme. re-routed blood from the opposite internal carotid is partly responsible but it is argued that the ipsilateral Acknowledgements and further reading external carotid is mainly responsible through the Amin N., Syed I., Osborne S. Assessment and anastomoses. management of orbital cellulitis. (2016) Br J Hosp Med (Lond).77(4):216-20. ii) Veins There are two principle veins of the orbit, the large Brady, S.M., McMann, M.A., Mazzoli, R.A., Bushley, superior and the inferior orbital. Four or more vortex D.M., Ainbinder, D.J., and Carroll, R.B. (2001) The veins and the central retinal vein drain blood from the diagnosis and management of orbital blowout fractures: eye, the vortex veins draining to the nearest of the update 2001. Am.J.Emerg.Med., 19, 147-54 ophthalmics (Figure 14). The latter join apically where Demer, J.L., Oh, S.Y. and Poukens, V. (2000) Evidence they have a lateral location (Figures 5 and 7) and for active control of rectus muscle pulleys. Invest. continue intracranially to join the cavernous sinus. Ophthalmol. Vis. Sci., 41, 1280-1290. Otherwise orbital venous branching is variable and the pattern revealed in one dissection offers limited clues to Gillilan, L. A. (1961) The collateral circulation of the the pattern in the next. There are no valves present and human orbit. Arch. Ophthalmol., 65, 684-694 because orbital veins are connected anteriorly to the Hayreh, S.S. and Dass, R. (1962) The ophthalmic artery facial, inferiorly to the deep facial veins and posteriorly II. Intra-orbital course. Br. J. Ophthamol., 46, 165-185 to the cavernous sinus, the direction of bloodflow is Koornneef, L. (1977) Spatial aspects of orbital musculo- dictated by posture. fibrous tissue in man. Amsterdam, Swets & Zeitlinger. Lawrenson J. G and Douglas R.H. (2015). The eye. In Gray’s anatomy: the anatomical basis of clinical practice st 41 Edition (Ed. Sandring, S.), Churchill Livingstone Ruskell, G.L. (2003) Access of autonomic nerve through the optic canal and their distribution in the orbit. Anat Rec., 275 (1), 973-8 Smith, B. and Regan, W.F. (1957) Blow-out fracture of the orbit. Am.J.Ophthalmol., 44, 733-739 Tovilla- Canales, J.L., Nava, A. and Tovilla y Pomar, J.L. (2001) Orbital and periorbital infections. Curr. Opin. Ophthalmol., 12, 335-41 9 Gross Anatomy of the Orbit Clinical Note “Blow-out” fracture of the orbit Orbital floor fractures can occur as isolated injuries or in combination with fractures of the orbital rim or other parts of the facial skeleton. The term “blow-out fracture” was introduced by Smith and Regan in 1957 with specific reference to an orbital floor fracture without involvement of the orbital rim, but with entrapment of one or more soft tissue structures, leading to impaired vertical motility, diplopia and enophthalmos. Two possible causative mechanisms have been proposed for this injury. The “hydraulic theory” suggests that blunt trauma to the globe causes a rapid increase in intra-orbital hydraulic pressure that is directly transmitted to the orbital walls. The alternative “buckling theory” proposes that the fracture most likely results from indirect forces transmitted to the orbital walls from a blow to the orbital rim. Proportionally more “blow-out” fractures involve the orbital floor, particularly in the region of the infra-orbital groove (Figure 2). Since the orbital floor overlies the maxillary sinus it lacks reinforcement. By contrast, the ethmoid bone, although also paper thin, is reinforced by the walls of the ethmoid air cells Orbital floor factures frequently involve the infra-orbital nerve (Figures 9 and 12), a branch of the maxillary division of the trigeminal nerve which runs in the infra-orbital groove, and then within a canal in the maxilla. Infraorbital nerve dysfunction leads to an ipsilateral sensory disturbance of the mid face, and may be the only sign in patients with a pure orbital floor fracture. Detection of orbital fractures is facilitated by thin-cut coronal computerised tomography (CT), which allows excellent visualisation of orbital bones, and is diagnostically superior to plain radiographs. Once a diagnosis has been made there are two management options: surgical and non- surgical. Patients without significant enophthalmos or tissue entrapment can be managed conservatively with antibiotics and a short course of steroids to reduce oedema. If restorative surgery is indicated, a transconjunctival approach is most appropriate. Clinical Note Orbital and periorbital infections Pre-septal cellulitis and orbital cellulitis are the most significant infections of the ocular adnexa and orbital tissues. They can be differentiated on the basis of whether the soft tissue infection lies in front or behind the septum orbitale. The orbital septum is a thickened sheet of connective tissue that extends from the orbital margins to the perimeter of the tarsal plates, thus forming a barrier between the eyelid and the orbit (Figure 1). Orbital cellulitis is less common, but more aggressive than pre-septal cellulitis. It usually affects children and young adults, and typically presents with painful lid swelling of sudden onset, with associated proptosis and ophthalmoplegia. In 60- 80% of cases, orbital cellulitis arises as a result of the spread of infection from the paranasal air sinuses. The anatomical dissections illustrated in this module (Figures 3, 4 and 9) demonstrate the close proximity of the sinuses to the orbital cavity. Infected material can gain access to the orbit through communicating foramina or across the thin orbital bones. Another important route for the spread of infection is through the venous drainage system. Veins draining the mid-face and the sinuses anastomose with orbital veins (Figure 14). The clinical significance of this route is that infection can potentially spread to the cavernous sinuses and cranial cavity, leading to meningitis and cavernous sinus thrombosis. High resolution CT scanning, including axial and coronal views, is an essential diagnostic tool in orbital cellulitis. This may be supplemented by magnetic resonance imaging (MRI) if cavernous involvement is suspected. The standard management of orbital cellulitis is immediate hospitalisation and intensive intravenous antibiotics. 10