Orbit and orbital adnexae

Eyelids

From superficial to deep, the eyelid consists of skin, subcutaneous tissues (prominent in the Asian preorbicularis fat pad), striated muscle fibres of orbicularis oculi, areolar connective tissue of the orbital septum, and the tarsal plates; above the tarsal plates, the superior tarsal smooth muscle (Müller's muscle) lies on the upper forniceal conjunctiva ( Fig. 7.1 ).

The upper eyelid also receives insertion from levator palpebrae superioris, which acts as a lid retractor, the muscle being inserted through an aponeurosis that descends into the upper lid posterior to the orbital septum. Some aponeurotic fibres cross the orbital septum and merge with fibres of the pretarsal head of orbicularis oculi and with the anterior surface of the tarsal plate. The lateral horn of levator indents the medial surface of the lacrimal gland, thereby defining a smaller palpebral lobe (not actually in the eyelid, despite being termed ‘palpebral’) that carries the lacrimal ductules into the upper conjunctival fornix, and a larger orbital lobe situated in the lacrimal fossa superolaterally. Levator palpebrae superioris runs parallel to superior rectus, and they share a common fascial sheath that forms part of the larger intermuscular orbital septa of Tenon's capsule. The common sheath between these two muscles inserts anteriorly around the upper conjunctival fornix, thereby preventing prolapse of the upper fornix on up-gaze; this complication may arise if the sheath is damaged during ptosis repair.

The anatomy of the lower eyelid is similar in structure to the upper lid, but with the absence of a levator muscle. The lower lid retractors – part of Tenon's capsule – consist of a sheet of fibrous tissue that extends from the sheath of inferior rectus, splits to enclose inferior oblique where it blends with the inferior suspensory (Lockwood's) ligament of the globe, and runs forward to the lower border of the tarsal plate, accompanied by smooth muscle of the inferior tarsal muscle (see Fig. 7.1 ).

The horizontal extremities of the tarsal plates are attached to the orbital margins by the canthal ligaments. The medial canthal ligament comprises the preseptal and pretarsal heads of orbicularis oculi, and each of these has a superficial and deep component. The superficial heads fuse medially to form that part of the medial canthal ligament attached to a point about 2–5 mm anterior to the anterior lacrimal crest ( Fig. 7.2 ), and the deep heads attach to the posterior lacrimal crest. Complete loss of tendinous fixation leads to an unopposed action of orbicularis and canthal dystopia, with rounding of the affected canthus, hiding of the caruncle, a narrow horizontal palpebral aperture (<28 mm in adults) or an increased intercanthal distance.

Orbital roof

The roof is composed of the orbital plate of the frontal bone and the lesser wing of the sphenoid. The superior margin has a supraorbital notch or foramen, transmitting similarly named vessels and nerves, and, in 50% of the population, a frontal notch lying more medially. The trochlea is a complex connective tissue sling anchoring the tendinous part of superior oblique to the orbital wall at the trochlear fovea, a small depression lying close to the superomedial margin.

Lateral orbital wall and orbital floor

The lateral wall is composed of the greater wing of the sphenoid, the orbital surface of the zygomatic bone, and the zygomatic process of the frontal bone.

The superior orbital fissure lies between the greater and lesser wings of the sphenoid. It transmits the oculomotor, trochlear and abducens nerves, the ophthalmic division of the trigeminal nerve, and the superior ophthalmic vein. The fissure is at least 28 mm from the fronto­zygomatic suture at the rim and, due to this depth and the curvature of the lateral wall, is rarely at risk during orbital surgery.

The floor is composed of the large orbital plate of the maxilla, the zygomatic orbital plate anterolaterally and the orbital process of the palatine bone.

The infraorbital foramen, lying about halfway along the inferior rim, is more or less vertically aligned with the supraorbital notch and is continuous with the infraorbital canal. The anterior (and occasionally middle superior) alveolar nerves join the infraorbital nerve within the canal; if they are damaged – e.g. during blowout fractures of the orbital floor or during orbital decompression – there may be numbness of the upper lip and ipsilateral three front teeth. The entrance to the nasolacrimal canal lies anteromedial on the floor, just behind the lower part of the anterior lacrimal crest; close to the canal, a pit marks the origin of inferior oblique, which is the only extraocular muscle with an origin in the anterior part of the orbit.

Medial orbital wall

The medial wall is composed of the frontal process of the maxilla, the lacrimal bone, the lamina papyracea of the ethmoid and the body of the sphenoid.

Anteromedially, the lacrimal sac fossa is demarcated by the anterior and posterior lacrimal crests. The anterior and posterior ethmoidal vessels and nerves are located at similarly named foramina situated at the level of the frontoethmoidal suture. Although their position is variable, a ‘rule of 24–12–6’ has been suggested as a guide when operating along the medial wall, based on the approximate distance (in millimetres) from the anterior lacrimal crest to the anterior ethmoidal foramen, from the anterior to the posterior ethmoidal foramen, and from the posterior ethmoidal foramen to the optic canal, respectively. This means that the optic canal is found approximately 42 mm pos­terior to the anterior lacrimal crest.

Nasolacrimal system

Nasolacrimal drainage begins with the superior and inferior lacrimal puncta, which are portals to the vertical ampullae and then horizontal segments of the respective canaliculi. The individual canaliculi fuse to form the common canaliculus 0–5 mm from the entrance to the lacrimal sac at the ‘common internal opening’; the common canaliculus passes anteroinferiorly through the thick lateral fascia of the lacrimal sac to enter the lumen of the sac about 4 mm below its apical fundus. The lacrimal sac lies in the lacrimal fossa, formed by the lacrimal bone and the frontal process of the maxilla. The lacrimal fascia, mingling with the various heads of the medial canthal tendon, is attached behind to the posterior lacrimal crest of the lacrimal bone and in front to the anterior lacrimal crest of the maxilla. It lies between the sac and the medial palpebral ligament anteriorly and the lacrimal part of orbicularis oculi posteriorly. The angular vein crosses the anterior surface of the medial palpebral ligament about 8 mm medial to the inner canthal commissure.

Blood supply to the globe and optic nerve

Two distinct arterial systems are involved in supplying the eye. The central artery of the retina and its branches are distributed to the inner two-thirds of the retina. The outer one-third of the retina, which includes the photoreceptors, is avascular and nourished by tissue fluid derived from the choroidal vessels. The fovea centralis is completely avascular. The optic nerve head is supplied by both the central retinal artery and a circumpapillary anastomosis from the short posterior ciliary arteries.

The anterior segment of the eye, which includes the iris, ciliary body and insertions of the recti muscles, has a generous blood supply due to extensive anastomoses between branches of the anterior ciliary arteries and the anterior uveal network derived from the two long posterior ciliary arteries. For this reason, extensive anterior segment ischaemia is rare, unless the surgeon carries out simultaneous surgery on three or more rectus muscles.

The central retinal artery, a branch of the ophthalmic artery, enters the orbital segment of the optic nerve about halfway along its length and typically from the undersurface. This arrangement confers a dual blood supply on the anterior portion of the nerve, both from the central retinal artery and from perforating vessels arising in the dural sheath. In contrast, the posterior portion of the optic nerve is at greater risk of ischaemic neuropathy following surgery because it has a single dural arterial supply.

Venous drainage from the orbit tends to be a vascular network that is largely directed posteriorly towards the superior and inferior orbital fissures; the four vortex veins from the globe drain into this network. Enlargement of the superior ophthalmic vein and orbital swelling can occur with raised pressure in the ipsilateral cavernous sinus (as with a dural shunt), or where the superior fissure is embarrassed by masses in sphenoidal wings, as with a sphenoidal wing meningioma.