Chapter 52

Mitchell P Weikert, MD MS

Department of Ophthalmology Laser Eye Surgeon Baylor College of Medicine Houston TX, USA

Chapter 170

Robert W Weisenthal, MD FACS

Clinical Professor of Ophthalmology Upstate Medical Center State University of New York at Syracuse DeWitt NY, USA

Chapter 73

Jayne S Weiss, MD

Professor of Ophthalmology and Pathology Kresge Eye Institute Detroit MI, USA

Chapter 70

Pongmas Wichiensin, MD

Attending Surgeon Department of Ophthalmology Rajavithi Hospital Bangkok, Thailand

Chapter 77

Kirk R Wilhelmus, MD PhD

Professor of Ophthalmology Department of Ophthalmology Cullen Eye Institute Baylor College of Medicine Houston TX, USA

Chapter 85

Steven E Wilson, MD

Professor of Ophthalmology Staff Cornea and Refractive Surgeon Director of Corneal Research The Cole Eye Institute The Cleveland Clinic Cleveland OH, USA

Chapter 164

Maria A Woodward, MD

Corneal Fellow Emory Eye Center Atlanta GA, USA

Chapter 168

Richard W Yee, MD

Clinical Professor Department of Ophthalmology and Visual Science University of Texas Science Center, Director of Corneal and External Diseases and Refractive Surgery Hermann Eye Center Memorial Hermann Hospital Houston TX, USA

Chapter 14

Sonya Yoo, MD

Associate Professor of Clinical Ophthalmology Bascom Palmer Eye Institute University of Miami Miller School of Medicine Miami FL, USA

Chapter 137

Volume 1

Fundamentals and Medical Aspects of Cornea and External Disease

Part I – Basic Science: Cornea, Sclera, Ocular Adnexa Anatomy, Physiology and Pathophysiologic Responses

Chapter 1 – Cornea and Sclera  :  Anatomy and Physiology

Teruo Nishida, Shizuya Saika

Introduction

The avascular cornea is not an isolated tissue. It forms, together with the sclera, the outer shell of the eyeball, occupying one-third of the ocular tunic. Although most of both the cornea and sclera consist of dense connective tissue, the physiological roles of these two components of the eye shell differ. The cornea serves as the transparent ‘window’ of the eye that allows the entry of light, whereas the sclera provides a ‘darkbox’ that allows the formation of an image on the retina. The cornea is exposed to the outer environment, whereas the opaque sclera is covered with the semitransparent conjunctiva and has no direct exposure to the outside. The differences in the functions of the cornea and sclera reflect those in their microscopic structures and biochemical components.

Interwoven fibrous collagen is responsible for the mechanical strength of both the cornea and sclera, protecting the inner components of the eye from physical injury and maintaining the ocular contour.^[1] The corneal epithelium forms an effective mechanical barrier as a result of interdigitation of cell membranes and the formation of junctional complexes such as tight junctions and desmosomes between adjacent cells. Together with the cellular and chemical components of the conjunctiva and tear film, the corneal surface protects against potential pathological agents and microorganisms.

The smooth surface of the cornea contributes to visual clarity. The regular arrangement of collagen fibers in the corneal stroma accounts for the transparency of this tissue.^[2] Maintenance of corneal shape and transparency is critical for light refraction, with the cornea accounting for more than two-thirds of the total refractive power of the eye. A functionally intact corneal endothelium is important for maintenance of stromal transparency as a result of regulation by the endothelium of corneal hydration.

Anatomy and Physiology

Structure of the cornea and sclera

The anterior corneal surface is covered by the tear film, whereas the posterior surface is bathed directly by the aqueous humor. The highly vascularized limbus, which is thought to contain a reservoir of pluripotent stem cells, constitutes the transition zone between the cornea and sclera. The anterior corneal surface is convex and aspheric (Fig. 1.1), and it is transversely oval as a result of scleralization superiorly and inferiorly.

The adult human cornea measures 11 to 12 mm horizontally and 9 to 11 mm vertically. It is approximately 0.5 mm thick at the center, with the thickness increasing gradually toward the periphery, where it is about 0.7 mm thick.^[3] The curvature of the corneal surface is not constant, being greatest at the center and smallest at the periphery. The radius of curvature is between 7.5 and 8.0 mm at the 3-mm central optical zone of the cornea, where the surface is almost spherical. The refractive power of the cornea is 40 to 44 diopters, constituting about two-thirds of the total refractive power of the eye.

The sclera, a tough and nontransparent tissue, shapes the eye shell, which is approximately 24 mm in diameter in the emmetropic eye. The anterior part of the sclera is covered with the bulbar conjunctiva and Tenon's capsule, which consists of loose connective tissue and is located beneath the conjunctiva (Fig. 1.2). The nontransparency of the sclera prevents light from reaching the retina other than through the cornea, and, together with the pigmentation of the choroid and retinal pigment epithelium, the sclera provides a dark box for image formation. The scleral spur is a projection of the anterior scleral stroma toward the angle of the anterior chamber and is the site of insertion for the anterior ciliary muscle. Contraction of this muscle thus opens the trabecular meshwork. At the posterior pole of the eyeball, where the optic nerve fibers enter the eye, the scleral stroma is separated into outer and inner layers. The outer layer fuses with the sheath of the optic nerve, dura, and arachnoid, whereas the inner layer contains the sievelike structure of the lamina cribrosa. The rigidity of the lamina cribrosa accounts for the susceptibility of retinal nerve fibers to damage during the development of chronic open-angle glaucoma. The sclera contains six insertion sites of the extraocular muscles as well as the inputs of arteries (anterior and posterior ciliary arteries) and outputs of veins (vortex veins) that circulate blood through the uveal tissues.

Optical properties of the cornea

The optical properties of the cornea are determined by its transparency, surface smoothness, contour, and refractive index of the tissue.^[4] If the diameter of (or the distance between) collagen fibers in the corneal stroma becomes heterogeneous (as occurs in fibrosis or edema), incident light rays are scattered randomly and the cornea loses its transparency. Given that the spherocylindrical surface of the cornea has both minor and major axes, changes in corneal contour caused either by pathological conditions such as scarring, thinning, or keratoconus or by refractive surgery render the surface regularly or irregularly astigmatic.

The total refractive index of the cornea is determined by the sum of refraction at the anterior and posterior interfaces as well as by the transmission properties of the tissue. The refractive indexes of air, tear fluid, corneal tissue, and aqueous humor are 1.000, 1.336, 1.376, and 1.336, respectively. The refractive power of a curved surface is determined by the refractive index and the radius of curvature. The refractive power at the central cornea is about +43 diopters, being the sum of that at the air–tear fluid (+44 diopters), tear fluid–cornea (+5 diopters), and cornea–aqueous humor (−6 diopters) interfaces. Most keratometry and topography measurements assume a standard refractive index of 1.3375.

Innervation

Innervation of the cornea is required for pain sensation as well as for tissue repair. In addition, autonomic innervation of the scleral spur and of blood vessels in the episclera, the surface of the sclera immediately beneath the subconjunctival connective tissue, plays an important role in the regulation of intraocular pressure.