Lecture (6) Contents of Orbit (PDF)

Lecture (6): Contents of orbit ORBIT The orbits are a pair of a pyramidal-shaped bony cavities, on either side of the root of the nose and provides sockets for rotatory movements of the eyeballs. Each orbit is a four-sided pyramid with apex directed posteriorly at the optic canal(which transmi...

Lecture (6): Contents of orbit ORBIT The orbits are a pair of a pyramidal-shaped bony cavities, on either side of the root of the nose and provides sockets for rotatory movements of the eyeballs. Each orbit is a four-sided pyramid with apex directed posteriorly at the optic canal(which transmits the optic nerve with its meningeal sheath and the ophthalmic artery) and base forward, represented by the orbital margin (rim). The orbital entrance is rotated laterally. a)The bony orbit is pyramidal shape of the orbit. The base is situated anteriorly and the apex posteriorly. (b) The orbital entrance is rotated laterally.  The seven bones that form the orbit, four facial bones and three cranial bones are : frontal, zygomatic, sphenoid, palatine, ethmoid, lacrimal, and maxilla. The medial walls of the two orbits are parallel to the midsagittal plane and the long axis of each orbit (orbital axis) passes backwards and medially aspect of the eye. The margin is the strongest orbit. The following openings are associated with the margin: the supraorbital notch or foramen, the zygomaticofacial foramina and the infraorbital foramen. ▪ Roof of orbit: Formed by the orbital palate of frontal bone and the lesser wing of the sphenoid. *The frontal bone separates the orbit from the anterior cranial fossa. The roof has : ▪ The fossa for the lacrimal gland, located anterolaterally and the *trochlear spine for the trochlea (pulley) of the superior oblique muscle located anteromedially. ▪ Floor of orbit: The floor of the orbit is smaller than the roof and one of the thinnest wall of orbit. ▪ Formed by orbital surface of maxilla, and zygomatic bones and orbital process of palatine bone. The maxilla (Floor) separates the orbit from the underlying maxillary sinus. It is traversed by the infraorbital groove that leads to the infraorbital canal for the infraorbital neurovascular bundle, which passes anteriorly through the infraorbital canal and foramen onto face. The floor is separated from the lateral wall by the inferior orbital fissure, which connects the orbit with the pterygopalatine and infratemporal fossa. The infraorbital fissure transmits the zygomatic branch of the maxillary nerve, the inferior ophthalmic vein, and sympathetic nerves. Anatomy and relationships of the infratemporal and pterygopalatine fossae  The orbit’s medial wall Is vertical and separates the orbit from the nasal cavity and some of the paranasal sinuses. Formed by the frontal process of maxilla, lacrimal , ethmoid and sphenoid bones. The ethmoid bone separates the orbit from the ethmoid sinus. The anterior and posterior ethmoidal foramina are located between medial wall and the roof of the orbit and they transmit the anterior and posterior ethmoidal vessels and nerves through the ethmoidal canals into the anterior cranial fossa.. Anterior and posterior lacrimal crests which define the fossa for the lacrimal sac. This fossa communicates with the inferior meatus of the nose by means of the nasolacrimal canal. The lateral wall of the orbit Is the strongest (thickest) wall of the orbit and separates the orbit from both the temporal region and middle cranial fossa. *Formed by the zygomatic bone and greater wing of the sphenoid. The superior orbital fissure is found between the lateral wall and the roof of the orbit and transmits CN III, IV, the ophthalmic division of V and VI, and the superior ophthalmic vein which drains into the cavernous sinus. Foramina and Fissures Contents Supraorbital foramen Supraorbital nerve (V1) and vessles Infraorbital foramen Infraorbital nerve (V2) and vessles Oculomotor (II), trochlear (IV), abducens nerve (VI), ophthalmic Superior orbital fissure branches (V1), superior ophthalmic vein Maxillary nerve branches (V2), zygomatic nerve, sphenopalatine Inferior orbital fissure ganglion branches, infraorbital artery and vein& nerve, inferior ophthalmic vein (leading to pterygoid plexus) Optic canal Optic nerve (II), ophthalmic artery *Annulus of Zinn: the Anterior and posterior ethmoidal common tendinous ring, not Ethmoidal canals an anatomical bony foramen artery&nerve Superior division of cranial nerve III, nasociliary nerve (V1), sympathetic root ⁎ Annulus of Zinn of cervical ganglion, inferior division of cranial nerve III, cranial nerve VI, Major openings into orbit ORBITAL FASCIA OR PERIORBITA It is the periosteum of the bony orbit and is loosely attached to the bones. It forms a funnel-shaped fascial sheath that encloses the orbital contents. It also sends out extensions toward the peripheral tarsal rim to form the orbital septum, which delineates the orbit in front and separates the intraorbital fat from the orbicular muscle of the eye. At the optic canal and superior orbital fissure, posteriorly, thickens, creating a tendinous ring known as the annulus of Zinn, for the tendinous attachments of extraocular muscles , Medially it is attached to the posterior lacrimal crest and forms the lacrimal sac. The periorbita, posteriorly expanding around the optic canal and superior orbital fissure, continuous with the optic nerve sheath, and then finally ends up united with the dura mater.  Structures passing through the superior orbital fissure into the orbit and above the common tendinous ring are the: lacrimal nerve & frontal nerve of CN V1, CN IV (trochlear nerve), and the superior ophthalmic vein. Structures passing through the superior orbital fissure and the common tendinous ring are the: superior and inferior divisions of CN III (oculomotor), nasociliary branch of CN V1, sympathetic roots of the ciliary ganglion, and CN VI (abducens). Anatomy of the left orbital apex, highlighting the extraocular muscle origins and the contents of the superior orbital fissure. Sensory nerve supply to the orbit  ORBITAL FASCIA At the optic canal and superior orbital fissure, posteriorly, it thus becomes continuous with the interior of the skull (endosteal layer of cranial dura). Thus, the subarachnoid space filled with CSF is extended out in the orbit to the eyeball’s optic disc. Changes in the CSF pressure will be transmitted to the optic disc and can produce a swelling of the disc called papilledema. Throughout, the fascia is perforated by the various vessels and nerves of the orbit and closes the inferior orbital fissure. CONTENTS: 1. Eyeball (most important content). 2. Muscles. 3. Fascia bulbi. 4. Nerves: (a) Optic (b) Oculomotor (c) Trochlear (d) Abducent (e) Ophthalmic (f) Ciliary ganglion. 5. Ophthalmic artery. 6. Ophthalmic veins. 7. Lacrimal gland. 8. Orbital fat. EXTRAOCULAR MUSCLES VOLUNTARY MUSCLES: seven voluntary muscles in the orbit. Six muscles move the eyeball and one muscle moves the upper eyelid. The muscles moving the eyeball are four recti: a)Superior rectus, (b) Inferior rectus, (c) Medial rectus, and (d) Lateral rectus and two oblique muscles: (a) Superior oblique, (b) Inferior oblique. The one which moves the upper eyelid is called: levator palpebrae superioris. INVOLUNTARY MUSCLES: There are three involuntary/smooth muscles; superior tarsal or Muller's muscle, inferior tarsal and orbitalis.  The Seven extraocular muscles are associated with the bones of the orbit. All but the inferior oblique muscle originate at the orbital apex. The inferior oblique, originates at the floor of the orbit in a depression on the orbital floor near the orbital rim. All of the rectus muscles originate at the annulus of Zinn. The superior oblique originates just medial to the optic foramen. This is the muscle that travels through the trochlea, forming a pulley that allows for intorsion of the globe. All of the extraocular muscles insert on the globe except the levator palpebrae superioris, which broadens anteriorly to become the levator aponeurosis, and is responsible primarily for eyelid elevation. Recti muscles Origin: All the recti arise from the corresponding margins of the common tendinous ring (encloses the optic canal and middle part of the superior orbital fissure) and attached to the apex of the orbit. Insertion: All the recti are inserted into sclera little posterior to the limbus (corneoscleral junction) infront of the equator of the eyeball. Oblique muscles: Origin: Superior oblique, (from the sphenoid bone), inferior oblique (from the floor of the orbit). Insertion: in the sclera, behind the equator ACTIONS OF MUSCLES ON THE EYEBALL Superior rectus: elevation, adduction, Medial rectus: adduction. Inferior rectus: depression, adduction, Lateral rectus: abduction. Superior oblique: depression, abduction, Inferior oblique: elevation, abduction. All the extraocular muscles are supplied by oculomotor nerve (CNIII), except the lateral rectus, supplied by abducent nerve (CN VI) and superior oblique, supplied by trochlear nerve (CN IV).  Muscle Primary Action Secondary Action Tertiary Action Lateral rectus Abduction - - Medial rectus Adduction - - Superior rectus Elevation Intorsion Adduction Inferior rectus Depression Extorsion Adduction Superior oblique Intorsion Depression Abduction Inferior oblique Extorsion Elevation Abduction Levator palpebrae Elevation of upper - - superioris lid Action of extraocular muscles Clinical correlation: Strabismus/squint: Unilateral paralysis of an individual muscle due to involvement of the nerves produces strabismus or squint (deviation of eye to the opposite side) and may result in diplopia (double vision). The real image falls on the macula of the unaffected eye and false image falls on the peripheral part of retina of the paralyzed eye. The examples are: Muscle paralyzed Nerve involved Effects Lateral rectus Abducent Medial squint Medial rectus Oculomotor Lateral squint Levator palpebrae superioris muscle Origin: from undersurface of sphenoid bone at the apex of the orbit, Insertion: skin of upper eyelid, the superior tarsal plate (smooth muscle part), and the superior fornix of the conjunctiva. Nerve supply: 1. The skeletal muscle part is supplied by the upper division of oculomotor nerve. 2. The smooth muscle part is supplied by the postganglionic sympathetic fibers from the superior cervical ganglion. Actions: Elevation of the upper eyelid to open the eye. Intrinsic muscles: made of smooth muscles inside the eye ball. 1. Sphincter pupillae of iris (Edinger–Westphal nucleus of CNIII) : constriction of pupil. 2. Dilator pupillae of iris (sympathetic nervous system): dilation of pupil. 3. Ciliary muscle of ciliary body (Edinger– Westphal nucleus of CNIII): focus the lens for near vision (accommodation). OPHTHALMIC ARTERY It arises from internal carotid artery as it emerges from the roof of the cavernous sinus close to the optic canal. The artery enters the orbit through optic canal inferolateral to the optic nerve, both lying in a common dural sheath. It then runs forwards along the medial wall of the orbit tortuously. Near the medial angle of the eye it terminates by dividing into two branches; supratrochlear and dorsal nasal. Branches: 1. Central artery of the retina, 2. Lacrimal artery, 3. Posterior ciliary arteries, 4. Supraorbital artery, 5. Posterior ethmoidal artery, 5. Anterior ethmoidal artery, 6. Dorsal (external) nasal artery, 7. Supratrochlear artery, 8. Medial palpebral branches. ophthalmic nerve is the first division of the trigeminal nerve (CN V) and responsible for conveying sensory information from: The globe, upper eyelid, lacrimal gland, conjunctiva, nasal mucosa, frontal sinus, external nose, and scalp. The ophthalmic nerve passes through the superior orbital fissure. Near this point, it divides into more specialized nerves: Frontal, lacrimal branches passes superior to the tendinous ring and nasociliary branch pass through the tendinous ring.  The ophthalmic nerve divides into three major branches which pass through the superior orbital fissure: frontal nerve supraorbital nerve supratrochlear nerve lacrimal nerve nasociliary nerve posterior ethmoidal nerve anterior ethmoidal nerve external nasal nerve long ciliary nerves infratrochlear nerve communicating branch to ciliary ganglion  The oculomotor (CN III), trochlear (CN IV), and abducent (CN VI) nerves also enter the orbit through the superior orbital fissure. The oculomotor and abducent nerves pass through the tendinous ring, while the trochlear nerve enters the orbit and passes just superior to the tendinous ring to enter the superior oblique muscle. EYEBALL Location: The eyeball occupies the anterior one- third of the orbital cavity and is embedded in the fat. It is enclosed in the thin fibrous sheath (Tenon’s fascia), which separates the eyeball from the fat. The optic nerve emerges from it, a little medial to its posterior pole. TUNICS OF THE EYEBALL: The eyeball consists of three concentric coats: 1. An outer fibrous coat consisting of sclera and cornea. 2. A middle vascular coat consisting of choroid, ciliary body, and iris. 3. An inner nervous coat consisting of the retina. OUTER FIBROUS COAT OF THE EYEBALL Sclera: The sclera is the posterior five- sixth of the outer coat. It consists of dense fibrous tissue. It is opaque and seen as the white of the eye. The sclera is continuous anteriorly with the cornea at corneoscleral junction. Structure Piercing the Sclera: 1. Optic nerve, 2. Posterior and anterior ciliary vessels and 3. choroidal veins. Cornea: The cornea is the anterior one-sixth of the outer coat. It is transparent and more convex than sclera. Features: 1. It is avascular. 2. It not only permits the light to enter the eye but also reflects the entering light. 3. It is highly sensitive and supplied by the ophthalmic division of trigeminal nerve. MIDDLE VASCULAR COAT OF THE EYEBALL (uveal tract) The middle coat is often called vascular coat because it contains most of the blood vessels of the eyeball. This coat also contains a large number of melanin-containing cells. It consists of three parts; from behind forwards these are: choroid, ciliary body, and iris Choroid: The choroid is the posterior part of the vascular coat of the eyeball. It is brown, thin, and highly vascular membrane lining the inner surface of the sclera. Anteriorly, it is connected to the iris by the ciliary body and posteriorly, it is pierced by the optic nerve. Ciliary Body: The ciliary body is the thickening in the vascular tunic. It is continuous with the choroid behind and the iris in front. It is situated posterior to the corneoscleral junction. The ciliary bodies suspend the lens via suspensory ligaments. The ciliary muscle: is a small (smooth) muscle mass consisting 1. Outer radial fibers. 2. Inner circular fibers. Its main function is to focus the lens for near vision and cause more refraction needed for accommodation. Iris: The iris is a contractile diaphragm between the cornea and the lens. An opening in its center is called the pupil. It contains a layer of smooth muscle, which consists of two parts: (a) Constrictor pupillae – (b) Dilator pupillae INNER NERVOUS COAT OF THE EYEBALL (RETINA): The retina is the innermost coat of the eyeball and consists of two layers, 1. An outer pigment layer (insensitive to light). 2. An inner sensory layer: is sensitive to light and is made up of photoreceptors cells called rods and cones; as well as numerous relay neurons; bipolar neurons and ganglion cells. The nerve fibers arising from its ganglion cells collect in the inferomedial region, where they pierce the outer two coats of eyeball and emerge as the optic nerve. Histologically, the retina is composed of 10 layers. Optic disc (1.5 mm in diameter): Is a white spot about 3 mm medial to the macula. The depressed area in the center is called physiological cup. The nerve fibers from retina meet and pass through this region (optic disc) of the eyeball to form the optic nerve. The blood vessels of retina also pass through this spot. Since there are no photoreceptor cells in the optic disc, it does not respond to the light, therefore, it is also called blind spot. N.B: The congestion of the optic disc from increased intracranial pressure is called papilledema in which the optic cup is obscured, and the disc margin is blurred. COMPARTMENTS OF THE EYEBALL: The interior of the eyeball is divided by the lens into: a small anterior compartment and a large posterior compartment. Anterior compartment: It is divided into two chambers: A smaller anterior chamber, lies between the iris and cornea and a larger posterior chamber lies between iris and lens. The two compartments communicate with each other through pupil. The two chambers are filled with an aqueous humour, which helps in maintaining the intraocular pressure. Posterior compartment: It is behind the lens and is filled with colorless, transparent jelly-like substance called vitreous humour/ vitreous body. LENS: It is a transparent, biconvex body, 1 cm in diameter and 4 mm thick, placed between the anterior and posterior compartments of the eyeball. Cataract: The opacity of the lens is termed cataract. With increasing age and in certain disease states, the lens becomes opaque. LACRIMAL APPARATUS: concerned with secretion and drainage of lacrimal (tear) fluid. The lacrimal apparatus consists of the following structures: 1. Lacrimal gland. 2. Ducts of lacrimal gland. 3. Conjunctival sac. 4. Lacrimal puncta. 5. Lacrimal canaliculi. 6. Lacrimal sac. 7. Nasolacrimal duct. Lacrimal gland: The lacrimal gland consists of upper large orbital part (in the lacrimal fossa of orbit) and lower small palpebral part (in the lateral part of the upper eyelid). They are continuous around the lateral margin of the levator palpebrae superioris. Functions of lacrimal fluid 1. Flushes the conjunctiva and keeps the cornea moist. 2. Provides nourishment to cornea. 3. Prevents infection (for it contains bactericidal lysozyme). Nerve supply of lacrimal gland Is supplied by parasympathetic (secretomotor), as follows: (a) The preganglionic parasympathetic fibers arise from lacrimatory nucleus in the pons, reaching the pterygopalatine ganglion, where they relay. (b) The postganglionic fibers arise from the cells of the pterygopalatine ganglion and pass successively through maxillary nerve, zygomatic nerve, zygomaticotemporal nerve and lacrimal nerve to reach the lacrimal gland. EYELIDS Each eyelid consists of five layers, from without inwards : Skin. 2. Superficial fascia. 3. Orbicularis oculi (palpebral fibers). 4. Tarsal plate and palpebral fascia. 5. Conjunctiva.