Introduction
The eyelids protect the eyes. Disease which alters eyelid structure or function threatens sight and an understanding of eyelid anatomy and physiology is fundamental to good reconstructive surgery. The eyelids should not be studied in isolation but in the context of the surrounding structures – the forehead, temples and cheeks. Safe surgery in these regions of the face depends on an understanding of the sometimes complex anatomy.
Muscles arising from the bones of the facial skeleton insert either into the soft tissues of the face, the muscles of facial expression ( 1.5 ), or into the mandible, the muscles of mastication ( 1.6 ). The forehead and scalp muscles – the frontalis and occipitalis – function separately.
The spaces between the muscles are filled by fat pads which are discrete and individually named ( 1.7 ). Vessels and nerves weave around the muscles and at certain points they are at risk from the surgeon’s knife ( 1.15 , 1.17 ).
The actions of the muscles of the temple, forehead and face are supported and disseminated smoothly across the face by a multilayered sheet of fascia of varying thickness: the superficial musculo-aponeurotic system, or SMAS ( 1.7 ).
Supporting these facial structures are several short, strong, fibrous retaining ligaments ( 1.7 ) which arise from specific sites in the facial skeleton and insert into the overlying tissues and the skin. Progressive laxity in the retaining ligaments and loss of elasticity in the skin are responsible for many of the aging changes which prompt referral for cosmetic or functional advice.
1.1
The bony orbit ( Diags 1.1–1.3 )
The bony orbit is a roughly pyramidal space with its base anteriorly; in cross-section it is rectangular anteriorly and triangular posteriorly. Each orbit is about 4 cm deep and has a volume of about 30 mL. The apex is the optic foramen, enclosed between the two roots of the lesser wing of the sphenoid. The inferior root, a thin bar of bone, separates the optic canal from the superior orbital fissure laterally. The inferior orbital fissure extends inferiorly and laterally from just below the optic foramen. About midway along its length the infraorbital groove branches anteriorly.
The medial walls of the orbits are parallel to each other. The lateral orbital walls are at 45 degrees to the medial walls and 90 degrees to each other. The floor, narrow at the apex, broadens as it slopes down and laterally. It is separated from the lateral wall by the inferior orbital fissure and it is continuous with the medial wall. The junction of the medial wall and the roof is marked by the anterior and posterior ethmoidal foramina.
The lacrimal gland fossa is just posterior to the superolateral orbital rim. The lacrimal sac fossa is just posterior to the inferomedial orbital rim, bounded anteriorly by the anterior lacrimal crest, a continuation of the inferior orbital rim, and posteriorly by the posterior lacrimal crest, a continuation of the superior orbital rim.
Each orbital margin measures approximately 40 mm, although the horizontal margins are usually greater than the vertical. The lateral and inferior rims are posterior to the medial and superior rims ( Diags 1.3, 1.5 ) and this is more marked in children. The lateral rim is approximately 20 mm posterior to the medial and the plane between them has almost one-third of the eye in front of it. The superior orbital rim protrudes about 10 to 15 mm beyond the inferior rim. The adult corneal apex is 8 to 10 mm posterior to the superior rim and 2 to 3 mm anterior to the inferior rim and just reaches the plane between the two. Measured from the lateral orbital rim the corneal apex is about 13 mm in a child and up to 22 mm in an adult.
Just within the midpoint of the lateral rim, Whitnall’s (lateral orbital) tubercle may be palpated. The trochlea is palpable just within the superomedial rim. The supraorbital notch is at the junction of the medial third and the lateral two-thirds of the superior rim and the infraorbital foramen is about 5 mm below the midpoint of the inferior rim or just medial to this.
The orbits are lined by periosteum (periorbita) which can be lifted easily (see Figs 12.3c , 13.7c ) except at the orbital margins, at the sutures, fissures and foramina and at the margins of the lacrimal sac fossa. At the posterior lacrimal crest the periosteum splits to enclose the lacrimal sac and reunites at the anterior lacrimal crest.
The orbits offer protection and support for the eyes and they transmit nerves and vessels to the face.
1.2
Surface anatomy of the eyelids ( Figs 1.1–1.5 )
The average size of the palpebral aperture in an adult is 30 mm horizontally and 10 mm vertically between the centres of the lids. The point of maximum concavity is different in the two lids. In the upper lid it is just medial to the pupil. In the lower lid it is just lateral. With the eye in the primary position the upper lid covers 1 to 3 mm of the upper cornea and the lower lid is at or close to the lower limbus. Scleral show of up to 2 mm between the lower lid and the limbus can be considered a normal variation but excessive scleral show may indicate lower lid retraction, proptosis or anomalies of the midfacial skeleton.
The lateral canthus is higher than the medial canthus – a line drawn between the canthi is elevated about 0 to 7 degrees laterally, a mean of about 3.5 degrees. The distance between the medial canthi is approximately half the interpupillary distance ( Table 1.1 ).
| Birth | 8 years | 16 years | |
|---|---|---|---|
| Inner intercanthal distance | 20 (15–25) | 30 (24–34) | 32 (26–36) |
| Outer intercanthal distance | 67 (62–72) | 96 (86–106) | 105 (95–115) |
| Interpupillary distance | 39 (33–45) | 53 (46–60) | 59 (52–66) |
| Palpebral fissure length | 19 (17–21) | 28 (25–31) | 31 (28–33) |
| Angle IC to OC | 3.5 deg (0–7) | 3.5 deg (0–7) | |
| Globe protrusion | 13–22 children and adults | ||
Variations in children may reflect anomalies of facial development. The final dimensions of the palpebral apertures are achieved toward the late teens.
In the upper lid the delicate preseptal skin (inferior to the brow) and the pretarsal skin (superior to the lashes) meet at the level of the skin crease, a transverse crease 6 to 10 mm from the lash line in an adult, lower in a child. The skin crease is formed by the insertion of the levator aponeurosis into the orbicularis muscle at this level (see Diag. 1.16 ). It is occasionally twice this size. There is often redundant skin superior to the skin crease in the upper lid so that a fold of skin, the upper lid skin fold, is created which covers the skin crease ( Fig. 1.1 ). Superior to the skin crease the ‘fullness’ in the upper lid ( Fig. 1.2 ) is due to orbital fat. The lacrimal gland lies laterally. Immediately below the brow there may be some hollowing of the upper lid – the upper lid sulcus (see Fig. 1.1 ). This is often marked in the elderly, especially if there is a ptosis (see Fig. 9.1 Pre B ). If a skin crease is present in the lower lid, it is usually less obvious than the upper lid crease. It is formed approximately at the level of the lower border of the inferior tarsal plate, 4 to 5 mm from the lash line (see Diag. 1.15 ).
In the lower lid the junction of the lid and the root of the nose, the naso-jugal fold, may develop a shallow linear depression, the ‘tear trough’, which extends down and laterally from below the inner canthus ( Figs. 10.1g,l ). It deepens with age.
The brow position and profile are different in males and females. The brow lies just above the superior orbital rim in females and it tends to be slightly arched. In males the brow is flatter and deeper and it lies at a lower level, along the anterior aspect of the superior orbital rim. As the orbital rim descends laterally the downward curve of the brow is gentler. In contrast to the thin skin of the upper lid, brow skin is thick (see Fig. 10.7d ). It bears numerous hairs whose follicles are directed laterally at about 30 degrees, except at the medial end of the brow where they are directed upwards. Deep to the brow is a fat pad – the retro-orbicularis oculi fat or ROOF – which is variable in volume. It is more prominent in males but in both males and females the brow fat may spread inferiorly, especially laterally, causing a fullness in the upper lid which some find unaesthetic.
In profile view ( Fig. 1.3 ), the anterior surface of the adult cornea is approximately in line with the malar eminence or slightly posterior to it. If the cornea is anterior to the malar eminence the intrinsic support for the lower lid is weaker; this is known as a ‘negative vector’.
In upgaze ( Fig. 1.4 ) the action of the levator and Müller’s muscles lifts the upper eyelid. The action of the frontalis lifts the brow. The elevation of the brow contributes about 2 mm to the elevation of the upper lid. The lateral canthus rises slightly. The upper lid fold is accentuated.
In downgaze ( Fig. 1.5 ) the lower lid level is depressed by the pull of the lower lid retractors and the lower lid skin crease is accentuated. The lateral canthus moves down slightly. The upper lid fold is reduced, revealing the previously covered skin crease.
1.3
Eyelid skin
The skin of the eyelids is the thinnest in the body, less than 1 mm thick and almost transparent in places. It is attached quite loosely to the orbicularis muscle and more firmly to the region of the canthal tendons – especially the medial.
Apart from the lashes, the skin hairs are very fine. The sweat glands of Moll secrete between the lashes or into the ducts of the glands of Zeis. The sebaceous glands of Zeis secrete into the lash follicles.
Deep to the skin is a thin layer of loose connective tissue which contains no fat and which lies on the orbicularis muscle.
1.4
Eyelid structure
The eyelids are conveniently divided into two anatomical lamellae (see Diags 1.15 , 1.16 ). The anterior lid lamella includes the skin and the orbicularis muscle. The posterior lamella is formed by the tarsal plate and the conjunctiva. A grey line, visible transversely along the middle of each lid margin, marks the junction of the anterior and posterior lamellae (see Fig. 3.16 ). These lamellae are very important in eyelid surgery. Between the lamellae there is a layer of connective tissue.
The margins of the eyelids are 2 mm wide. The posterior lid margin is sharp and applied to the globe. The anterior lid margin is rounded and holds the eyelashes. The mucocutaneous junction is at the Meibomian gland openings, just posterior to the grey line at the margin of the lid.
1.4
Eyelid structure
The eyelids are conveniently divided into two anatomical lamellae (see Diags 1.15 , 1.16 ). The anterior lid lamella includes the skin and the orbicularis muscle. The posterior lamella is formed by the tarsal plate and the conjunctiva. A grey line, visible transversely along the middle of each lid margin, marks the junction of the anterior and posterior lamellae (see Fig. 3.16 ). These lamellae are very important in eyelid surgery. Between the lamellae there is a layer of connective tissue.
The margins of the eyelids are 2 mm wide. The posterior lid margin is sharp and applied to the globe. The anterior lid margin is rounded and holds the eyelashes. The mucocutaneous junction is at the Meibomian gland openings, just posterior to the grey line at the margin of the lid.
1.5
Muscles of facial expression, the mimetic muscles ( Diags 1.4 , 1.5 )
These muscles are derived from the second branchial arch and they are innervated by the seventh cranial nerve.
1.5.1
Muscles and tendons of the eyelids
(a)
The muscles – orbicularis oculi ( Diags 1.6 , 1.7 )
The orbicularis oculi muscle closes the eyelids. The muscle is a flat sheet of fibres which encircles the palpebral aperture spreading out beyond the orbital rim. It is divided into two concentric zones – orbital (overlying the orbital rims) and palpebral (overlying the lids). The palpebral part is further divided into a preseptal part (anterior to the orbital septum) and a pretarsal part (anterior to the tarsal plate).
The orbital part arises from the medial orbital rim and its fibres sweep laterally in concentric bands to join at the lateral orbital rim. The palpebral part arises from the lateral canthal tendon and inserts medially. At the lid margins the pretarsal muscle extends posteriorly as far as the Meibomian gland openings and the muscle of Riolan (see Diags 1.15 , 1.16 ).
The medial attachments of the palpebral part of the orbicularis oculi muscle are complex ( Diag. 1.7 ).
The pretarsal muscles, firmly attached to the tarsal plates, insert medially by a superficial head and a deep head. The superficial head from each lid blends with a fibrous component to form the anterior part of the medial canthal tendon. The deep head from each lid is also known as the pars lacrimalis, or Horner’s muscle. Its fibres begin at the medial ends of the tarsal plates and insert into the posterior lacrimal crest a few millimetres behind the lacrimal sac. Contraction of the deep head pulls the lid medially and posteriorly.
The preseptal muscles, less firmly attached to the orbital septum, also insert medially by a superficial head and by a deep head. The superficial head from each lid inserts into the superficial part of the medial canthal tendon. The deep heads insert into the fascia overlying the lacrimal sac and on the medial orbital wall above and below Horner’s muscle. Contraction of the deep heads pulls the lacrimal fascia laterally.
There is some discussion about the detailed anatomy of the medial canthus. In practice the individual muscle insertions described previously are not usually identified at operation.
At the lateral canthus the pretarsal muscles join and insert by a common tendon into Whitnall’s tubercle. The preseptal muscles join laterally to form a lateral raphe which is connected to the underlying tendon.
(b)
The canthal tendons (also known as palpebral ligaments)
(i)
The lateral canthal tendon ( Diag. 1.8 )
Deep to the muscle insertions described above a Y -shaped fibrous thickening in the orbital septum joins the lateral ends of the tarsal plates to Whitnall’s tubercle. These muscular and fibrous structures together form the lateral canthal tendon.
(ii)
The medial canthal tendon ( Diag. 1.7 )
The medial canthal tendon also has a fibrous and a muscular component. The muscular component was described in detail previously.
The fibrous component is attached laterally to the medial ends of the tarsal plates as two limbs of a Y . It has a superficial and a deep component. The superficial component inserts medially on the frontal process of the maxilla just anterior to the anterior lacrimal crest, level with the upper part of the lacrimal sac. It has a definite inferior border but the superior border blends with the periosteum. The deep component leaves the deep surface just lateral to the anterior lacrimal crest and inserts into the posterior lacrimal crest behind the lacrimal sac. This deep component of the tendon is the main medial anchor of the lids.
(c)
The lacrimal pump
During blinking the deep heads of the pretarsal muscles (Horner’s muscle) pull the medial ends of the eyelids medially, shortening the canaliculi, while the lacrimal fascia and sac wall are pulled laterally by contraction of the deep heads of the preseptal muscle. The puncta close and the tears in the ampullae of the canaliculi are forced medially and are sucked into the sac. As the deep insertions of the orbicularis muscle relax at the end of the blink the lacrimal fascia and sac wall move medially again, the medial ends of the lids move laterally, the puncta reopen and the ampullae refill with tears. Drainage of tears from the lacrimal sac into the nasolacrimal duct is not influenced directly by the lacrimal pump mechanism and is mainly due to gravity.
1.5.2
Muscles of the forehead and scalp ( Diags 1.4 , 1.5 )
The occipitalis muscle posteriorly and the frontalis muscle anteriorly are joined by an aponeurosis, the galea aponeurotica or epicranial aponeurosis. Laterally, it blends with the temporoparietal (superficial temporal) fascia which together form part of the superficial musculo-aponeurotic system (SMAS, see 1.7.1 and Diag. 1.9 ). The frontalis muscle fibres insert into the orbicularis muscle and the skin of the brows. The occipitalis arises from the occipital bone.
The corrugator supercilii muscle ( Diag. 1.4 ) arises from the medial end of the superciliary ridge, lateral to the origin of the procerus muscle, and passes upwards and laterally through both frontalis and orbicularis muscles to insert into the skin of the middle of the brow. It draws the brow in and down. The superficial and deep branches of the supraorbital nerve pass either side of the muscle, approximately at its midpoint, as they ascend into the forehead.
The procerus muscle arises on the nasal bones and inserts into the skin of the lower forehead and bridge of the nose. It wrinkles the nose.
These muscles are all innervated by the frontal branch of the facial nerve.
1.5.3
Muscles of the mouth ( Diags 1.5 , 1.11 )
Several small muscles deep within the cheek arise from the facial skeleton below and lateral to the eye and converge on the angle of the mouth. They and their anatomical relationships are important in any surgery in the mid face. The zygomaticus major and minor muscles arise from the zygomatic bone. The levator labii superioris and the levator anguli oris respectively arise from above and below the infraorbital foramen. A number of other smaller muscles in the mid face are less relevant surgically. They include the levator labii superioris alequae nasi which arises from the frontal process of the maxilla just anterior to the orbicularis oculi.
1.6
Muscles of mastication ( Diag. 1.5 )
These muscles are derived from the first branchial arch and they are innervated by the motor fibres in the mandibular division of the fifth cranial nerve.
1.6.1
Temporalis muscle
This fan-shaped muscle arises from a wide origin on the side of the skull – the inferior temporal line. It also has attachments to the strong overlying temporal fascia which inserts into the superior temporal line. The temporalis muscle fibres descend and converge to insert on the coronoid process and anterior part of the ramus of the mandible.
1.6.2
Masseter muscle
The masseter muscle, which can be easily palpated in the cheek when the teeth are clenched, arises from the lower border of the zygomatic arch and inserts on the angle and ramus of the mandible. The anterior border of the parotid gland wraps around the posterior border of the masseter. The parotid duct passes forward across the middle of the muscle and winds around its anterior border to pierce the buccinator muscle and enter the mouth at the level of the second upper molar tooth.
1.7
Facial fat and fascia
Subcutaneous fat throughout the body is separated into a superficial, continuous layer of fat, just deep to the dermis and of variable thickness, and a deeper, discontinuous layer which is formed of collections of fat between the muscles.
The superficial fat layer is thickened in the cheek where it is known as the malar fat pad. It also has a deep component between the facial muscles.
Of the deep fat pads in the face, a number are important during surgery in the periocular region.
These superficial and deep fat layers are separated by a layer of thin superficial fascia.
Fascia is also found at a deeper level where it is of variable thickness. It invests the muscles of facial expression (the mimetic muscles), binds the deeper structures together, forms intermuscular septa between muscles or groups of muscles and binds muscles or tendons to deeper structures.
This system of superficial and deep layers of fascia is the superficial musculo-aponeurotic system, or SMAS. The facial nerve pierces the deep layer of the SMAS in the mid cheek to innervate the enclosed muscles of facial expression.
1.7.1
The superficial musculo-aponeurotic system (SMAS) ( Diags 1.9 , 1.10 )
The multilayered sheet of fascial tissue which forms the SMAS extends from the galea aponeurotica (epicranial aponeurosis) in the scalp to the platysma muscle in the neck. It splits, en route, to enclose the muscles of facial expression, binding them together so that their action is disseminated and their effect is smoothly coordinated.
