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«O¤print from Advances and Technical Standards in Neurosurgery, Vol. 31 Edited by J.D. Pickard 6 Springer-Verlag/Wien 2006 – Printed in Austria – ...»

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O¤print from

Advances and Technical Standards in Neurosurgery, Vol. 31

Edited by J.D. Pickard

6 Springer-Verlag/Wien 2006 – Printed in Austria – Not for Sale

Anatomy of the Orbit and its Surgical Approach

G. Hayek, Ph. Mercier, and H. D. Fournier

Laboratory of Anatomy, Faculty of Medicine, University of Angers,

Angers, France

With 16 Figures

Contents

Abstract

...................................................................... 35 Introduction.................................................................. 36 Anatomy of the Orbit........................................................ 36 The Orbital Cavity......................................................... 36 The Orbital Fascia or Periorbita.......................................... 38 Orbital Contents........................................................... 39 Orbital Muscles............................................................ 40 The Arteries of the Orbit.................................................. 42 Veins of the Orbit.......................................................... 47 Nerves of the Orbit........................................................ 48 Lacrymal Gland........................................................... 55 Approach Routes to the Orbit............................................... 56 Incision.................................................................... 56 The Lateral Approach Route................................................ 57 Osteoplastic Techniques................................................... 58 Non-Osteoplastic Techniques...........

–  –  –

can explore all parts of the optic nerve even in the optic canal. The lateral compartment of the orbit could be exposed by the lateral approach above or below the lateral rectus muscle. It is the only route that could give access to the inferior part of the orbit. The supero lateral approach is the largest route and has advantages of the two preceding routes. It gives access to the superior part of the orbit but not the optic canal and gives also a good exposition to the lateral part of the orbit but less than the lateral route in the inferior part. These approaches could be used to remove all intra orbital lesions apart from those located in the infero medial part of the orbit.

Keywords: Anatomy; orbit; orbital anatomy; orbital approach; orbital tumor; surgical approach.

Introduction Even though it represents a confined space, the orbit is amenable to a variety of exploratory surgical techniques. The diversity of possible techniques is due to its location at the juncture of various di¤erent anatomical regions.

Containing the eyeball and located between the face and the cranium, the orbit is the meeting place of tissues and organs that are the focus of a range of di¤erent specialties, including ophthalmology, otorhinolaryngology and neurosurgery. The di‰culties associated with approaching the orbit are related to its relatively small volume, its irregular, four-sided pyramid shape and to its situation embeded in the craniofacial structures.

It is essential to be intimately familiar with the microanatomy of the orbit before undertaking surgery in this region. In this chapter we have undertaken detailed study of the microsurgical anatomy of the orbit in cadaver specimens. After a literature review, we have described three neurosurgical approaches to the orbit on the basis of this anatomical study with emphasis on microanatomical structures that could be encountered along these routes.

Anatomy of the Orbit The Orbital Cavity (Fig. 1) The orbits are two cavities located symmetrically on either side of the sagittal plane at the root of the nose. The shape of each is that of a four-sided pyramid with its axis set o¤ from the sagittal plane by an angle of 20.

The roof is thin and concave in a downward direction. It can be separated into two laminae by the frontal sinus: there is the lacrimal fossa in the anterolateral part, and the anteromedial part houses the fovea into which the trochlea of the superior oblique muscle is inserted.

The floor separates the orbital cavity from the maxillary sinus. It is Anatomy of the Orbit and its Surgical Approach 37 Fig. 1. Photograph of anterior aspect of the right orbital cavity showing optic canal (OC ), superior orbital fissure (SOF ), inferior orbital fissure (IOF ), infra orbital groove and foramen (IOGF ) and supra orbital notch (SON ) traversed by the infraorbital groove that runs from the back towards the front until it changes into a canal for the maxillary nerve.





The lateral wall is oblique outside and in front. The posterior twothirds is formed by the greater wing of sphenoid with the superior orbital fissure at the top and the inferior fissure at the bottom. This wall is very thick, especially at the front (the lateral pillar), and it separates the orbit from the cerebral temporal fossa behind and from the temporal fossa (which houses the temporal muscle) in front.

The thinnest of the walls of the orbit is the slightly sagittal medial wall that has, in its forward portion, the lacrimal groove that subsequently turns into the nasal canal.

38 G. Hayek et al.

The superolateral angle of the orbit corresponds in front (1/3) to the lacrimal fossa and behind (2/3) to the superior orbital fissure. This is a dehiscence between the two wings of the sphenoid bone in the shape of a comma, with an inferomedial bulge and a superolateral taper. The bulging end matches the lateral face of the body of the sphenoid bone between the origin of the roots of the wings, and the tapered part extends as far as the frontal bone between the two wings. This fissure represents a line of communication between the middle cerebral fossa and the orbit, providing a passage for the orbital nerves (but not the optic nerve) and corresponding to the anterior wall of the cavernous sinus. At the junction between the two parts, there is a small bony protruberance on the lower lip to which the common tendinous ring (Zinn’s tendon) is attached.

The superomedial angle is perforated by the anterior and posterior ethmoid canals. The posterior end is continuous with the medial wall of the optic canal.

The posterior two-thirds of the inferolateral angle correspond to the inferior orbital fissure, which provides a communication between the orbit and the pterygomaxillary (or pterygopalatine) fossa; this is covered by the periosteum.

On the superior orbital rim at the junction (1/3 medial and 2/3 lateral) is the supraorbital foramen for the supraorbital nerve and vessels.

The summit or apex of the orbit precisely coincides with the bulging portion of the superior orbital fissure. A little above and inside is the exocranial foramen of the optic canal. This canal, of 6–12 millimeters in length, forms a hollow at the origin of the small wing (between its two roots) on the body of the sphenoid bone. This canal is a site of communication between orbit and the anterior fossa of the cranium. It gives passage to the optic nerve with its meningeal sheath and for the ophthalmic artery traversing below this nerve from the inside to the outside.

The Orbital Fascia or Periorbita (Fig. 2) This corresponds to the orbital periosteum. Its bone attachment is very loose apart from at points around the optic canal and the superior orbital fissure where it is continuous with the dura mater. In front, it continues into the cranial periosteum on the orbital rim to which it is very strongly attached. Here it sends out extensions towards the peripheral tarsal rim to form the orbital septum, which delineates the orbit in front and separates the intraorbital fatty tissue from the orbicular muscle of the eye. Inside, it is attached to the posterior lacrimal crest and on top, it is traversed by the levator palpebrae superior muscle. The periorbita thus surrounds the contents of the orbit, forms a bridge over the top, and closes the inferior orbital fissure. It is perforated by the various vessels and nerves of the orbit.

Anatomy of the Orbit and its Surgical Approach 39 Fig. 2. Photograph of superior aspect of the right orbit after roof removal showing the transparent orbital fascia (periorbita) and underneath the frontal nerve

Orbital Contents

The orbit can be split into two parts, an anterior part containing the eyeball and a posterior compartment containing the muscles, the vessels and the nerves supplying the eyeball, all supported in a cellular, fatty matrix, the so-called adipose body of the orbit.

The eyeball does not touch any of the walls but is suspended at a distance of 6 mm outside and 11 mm inside. Its anterior pole is at a tangent to a straight line joining the upper and lower rims of the orbit, and it projects out beyond a line joining the medial and lateral edges, especially towards the outside. Finally, the anteroposterior axis of the eyeball (which is precisely sagittal) forms an angle of 20 with the axis of the orbit, oblique in front and outside.

From the optic nerve as far as the sclero-corneal junction, the eyeball is 40 G. Hayek et al.

covered by a two-layer fascia (Tenon’s capsule) with parietal and visceral sheets separating it from the orbital fatty tissue. There is a virtual space between the two sheets (the episcleral space) which forms a sort of lubricated joint system to facilitate the movements of the eye. The fascia is fused behind with the capsule of the optic nerve and in front with the sclera where it joins the cornea. In its anterior part, it is perforated by the muscles of the eye. The fascia turns back over these muscles to create their aponeurotic sheath.

Orbital Muscles (Fig. 3) The orbit contains seven muscles, the first being the levator palpebrae superior muscle and the other six controlling the eye movements: four rectus muscles (superior, inferior, lateral and medial) and two oblique muscles (superior and inferior).

The levator palpebrae superior is a fine, triangular muscle, which originates above and in front of the optic canal at which point it is fine and tendinous although it sharply broadens out and assumes a more muscular character. It runs along the upper wall of the orbit just above the superior rectus muscle (covering its medial edge). It terminates in an anterior tendon that spreads out in the form of a large fascia, which extends out to the eyelid. The edges of this fascia form extensions, including a lateral one which traverses the lacrimal gland between its palpebral and orbital parts and goes on to attach to the fronto-zygomatic suture.

Rectus muscles: these four muscles form a conical space that is closed in front by the eyeball. They arise in the common annular tendon (Zinn’s tendon); this tendon is located on the body of sphenoid near the infraoptic tubercle, and it surrounds the superior, medial and inferior edges of the optic canal, and then continues across the inferomedial part of the superior orbital fissure before inserting on a tubercle of the greater wing. It subsequently splits into four lamellae arranged at right angles to one another, from which the four rectus muscles arise respectively. The superolateral and inferomedial ligaments are solid but the other two are perforated: the one in the superomedial band lets the optic nerve and the ophthalmic artery through, and the other, which is larger, stretches between the inferomedial and superolateral bands passing through the inferolateral band.

This opening called the common tendinous ring (Zinn’s ring) or the oculomotor foramen corresponds to the bulging end of the superior orbital fissure and provides a passage for the nasociliary nerve, both branches of the oculomotor nerve, the abductor nerve and the sympathetic root of the ciliary ganglion. The superior ophthalmic vein can also pass through or above this opening, and the inferior ophthalmic vein may pass inside or below it.

Anatomy of the Orbit and its Surgical Approach 41 Fig. 3. Artist’s drawing of right orbital cavity with extra ocular muscles showing common annular tendon (CAT ), common tendinous ring (CTR), optic foramen (OF ), levator palpebrae superior muscle (LPS), the four recti muscles: superior (SR), medial (MR), inferior (IR), lateral (LR), superior oblique muscle (SO) and inferior oblique muscle (IO) 42 G. Hayek et al.

The rectus muscles then continue for four centimeters in a forward direction to terminate in tendons, which are attached to the anterior part of the sclera near the limbus.

The oblique muscles, of which there are two.

The superior oblique muscle arises as a short tendon attached inside and above the optic foramen. It runs along the superomedial angle of the orbit and then becomes tendinous again when it turns back at an acute angle over the trochlea. It then becomes once more muscular and turns backwards in a lateral direction, skirts the upper part of the eyeball passing under the superior rectus muscle to terminate on the superolateral side of the posterior hemisphere of the eye. The inferior oblique muscle, shorter than the superior, is located on the anterior edge of the floor of the orbit and arises outside the orbital opening of the lacrimal canal before passing outside, behind and upwards. It skirts the lower surface of the eyeball, passing under the inferior rectus muscle to terminate on the inferior, lateral side of the posterior hemisphere of the eye.



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