2 Applied Anatomy

Keywords: ophthalmic plastic surgery, anatomy, eyelids, eyebrows, orbits, lacrimal system, nose, face

2.1 Introduction

A sound working knowledge of eyelid, orbital, nasal, and facial anatomy is essential before a surgeon can embark on oculoplastic, oculofacial, orbital, and lacrimal surgery. The surgical procedures described throughout this book are much easier to understand and much more likely to be successful when a good working knowledge of this anatomy has been acquired. This chapter presents detailed and practical anatomical descriptions as they apply directly to these surgical procedures and to a variety of clinical problems.

2.2 The Eyelids

In adults the normal interpalpebral fissure varies in vertical height from 8 to 11 mm. The peak of the upper eyelid usually lies just nasal to the midline of the pupil. The horizontal length of the interpalpebral fissure measures approximately 30 to 32 mm. The angle between the upper and lower eyelids medially and laterally measures approximately 60 degrees. Laterally the eyelids should be in contact with the globe, but medially the eyelids are displaced away from the globe, creating a space—the lacus lacrimalis. Within the lacus lacrimalis lies the caruncle, and immediately lateral to the caruncle lies the plica semilunaris (▶ Fig. 2.1a). The plane between the caruncle and the plica semilunaris is commonly used to gain access to the medial orbital wall (e.g., to drain a subperiosteal abscess). This is referred to as a transcaruncular approach (▶ Fig. 2.1b,c).

The lateral canthal angle usually lies approximately 2 to 3 mm higher than the medial canthal angle (▶ Fig. 2.2). This angle and its location should be considered when surgery is undertaken on the lateral canthus (e.g., a lateral eyelid tightening procedure performed with a lower eyelid blepharoplasty).

The eyelid margin is covered by conjunctival epithelium along the posterior half and meets the anterior margin, which is covered with cutaneous epidermis, at the gray line. The meibomian gland orifices lie within the conjunctival epithelium, and the eyelashes emerge within the cutaneous epidermis.

The eyelid can be divided into five structural planes:

1. The eyelid skin and subcutaneous fascia.

2. The orbicularis oculi.

3. The orbital septum.

4. The eyelid retractor muscles.

5. The tarsal plates and conjunctiva.

The upper eyelid skin crease is created by attachments from the superficial aspect of the levator aponeurosis into the orbicularis muscle and the subcutaneous tissue (▶ Fig. 2.3).

The skin crease varies in height and tends to lie 5 to 6 mm above the eyelid margin centrally in men and 7 to 8 mm above the eyelid margin centrally in women. Medially the crease extends to within 3 to 4 mm of the eyelid margin; laterally it lies approximately 5 to 6 mm above the eyelid margin (▶ Fig. 2.4). This should be considered when marking the desired position of the upper lid skin crease for ptosis surgery or an upper lid blepharoplasty.

In patients of Asian descent, the upper lid skin crease is typically poorly developed or absent, because the orbital septum inserts onto the levator aponeurosis in a lower position (▶ Fig. 2.5). This anatomical arrangement allows the preaponeurotic fat to extend further into the eyelid.

2.2.1 The Eyelid Skin

The eyelid skin is the thinnest in the body and unique in having no subcutaneous fat (▶ Fig. 2.6).

This, along with the color match, makes the upper eyelid an ideal donor site for a small full-thickness skin graft to be used for lower eyelid reconstruction. The skin can be removed easily and quickly and does not need to be thinned, unlike skin harvested from other donor sites. The surgeon must be careful to avoid causing lagophthalmos when using the upper eyelid as a skin graft donor site.

2.2.2 The Orbicularis Oculi Muscle

The orbicularis oculi muscle is divided anatomically into three parts (▶ Fig. 2.7):

1. Orbital

2. Preseptal

3. Pretarsal

The orbital portion of the orbicularis muscle lies over the bony orbital margins. It arises from the frontal process of the maxillary bone in front of the anterior lacrimal crest, from the orbital process of the frontal bone, and from the medial canthal tendon. The fibers of the orbicularis pass around the orbital margins without interruption at the lateral canthus, and the bony insertions lie just below the points of origin.

The palpebral part of the orbicularis muscle extends from the orbital margins to the margins of the eyelids. The muscle fibers pass circumferentially around the eyelids and are fixed medially and laterally at the medial and lateral canthal tendons. The palpebral part of the orbicularis muscle is further subdivided into the preseptal and pretarsal orbicularis muscles.

The preseptal part of the orbicularis muscle lies over the orbital septum in the upper and lower eyelids. Its muscle fibers arise in a perpendicular fashion from the upper and lower borders of the medial canthal tendon. In the upper lid, the muscle arises by anterior and posterior heads. The anterior head is a broad extension from the superior surface of the common part of the medial canthal tendon. The posterior head arises from the superior arm and from the posterior arm of the medial canthal tendon. The inferior preseptal muscle arises as a single head from the whole length of the common medial canthal tendon.

The superior limb of the medial canthal tendon fuses to the fundus of the lacrimal sac by a layer of fibrovascular fascia (▶ Fig. 2.8). When the deep head of the preseptal muscle pulls on the lacrimal sac via this fascia, it contributes to the lacrimal pump mechanism. The preseptal muscles extend in an arc around the eyelids and insert along the lateral horizontal raphe.

The pretarsal orbicularis muscle lies over the tarsal plates. The fibers of the muscle in both the upper and lower eyelids arise from the medial canthal tendon by means of superficial and deep heads. The superficial heads extend from the eyelid and continue anterior to the upper and lower limbs of the medial canthal tendons. The muscles thicken and surround the limbs of the medial canthal tendon and the canaliculi superiorly, anteriorly, and inferiorly. Contraction of these muscle fibers also contributes to the lacrimal pump mechanism.

Close to the common canaliculus, the deep heads of the pretarsal orbicularis muscle fuse together to form a...