Ординатура / Офтальмология / Английские материалы / Wavefront Analysis Aberrometers and Corneal Topography_Boyd_2003

THE REFRACTIVE MEDIA OF

THE HUMAN EYE

THE HUMAN CORNEA

The maximum refraction in the eye occurs at the anterior surface of cornea1 due to its high curvature and due to the large difference between the refractive indices of media on its two sides, namely air and corneal substance (1 and 1.37). Refraction occurring at posterior surface of cornea is not very significant as its refractive index is quite similar to that of aqueous (1.37 and 1.33).1

Before we discuss the optical properties of this structure, it is important to discuss the shape of the corne, which also has a bearing on its optical properties.

Shape: The cornea comprises the central one sixth of the outer wall of the eye and measured externally is oval in shape, the average horizontal and vertical diameter being 12.6 and 11.7 mm respectively.2 It is a well-known fact that the cornea is not spherical in shape. Different areas of the cornea have differing shapes. Thus the central 4 mm or so is supposed to be spherical and this is one of the assumptions on which, keratometry is based. However even this area almost always shows a small degree of astigmatism and therefore it is strictly speaking toroidal in shape.1 It is also known that the cornea is flatter in the periphery and becomes progressively flatter as one goes away from the center1, rendering the surface aplanatic.2 This is one of the factors which correct the problem of spherical aberration in the eye, the other factor being the structure of the lens1. However, the peripheral part of the cornea is irregular and radially asymmetric and does not conform to any specific geometrical shape1. It is therefore usual to divide the cornea in two parts: the central zone of about 4 mm, also called the optical or

apical zone or corneal cap and a peripheral or basilar zone.3 The apical zone is classically defined as all those areas of the cornea which vary in power by no more than one diopter in normal eyes. Curiously, the shape of the cornea may apparently vary with time, e.g. the cornea is relatively flatter during early mornings.5

Curvature: The central 4 mm or the axial zone of the cornea has a radius of curvature of 7.8mm.1 This area is also called the optical or apical zone since the pupil allows only the rays passing through this area to reach the retina.1 The radius of curvature of the posterior surface of the cornea is 6.7 mm.1 Changes in the curvature of the cornea have profound effect on the refractive status of the eye, e.g. a 1 mm increase in the radius of curvature of the cornea causes 6 diopters hypermetropia and a 1 mm decrease in the curvature causes 6 diopters myopia.1 This type of curvature is also usually associated with significant astigmatism.

Power: The refractive index of the cornea is 1.376.2 From the radius of curvature and the refractive index, we can calculate the power of both the surfaces of cornea. The normal cornea has 49 diopters of convergence at the anterior surface and 6 diopters of divergence at the posterior surface, the total convergence of the cornea thus being 43 diopters.1 The total refractive power of the cornea is the sum of refractive powers across three interfaces: air/tear, tear/cornea and cornea/aqueous humor. The formula for calculating the power of across a given surface is Ps = (n2 – n1)/r where Ps is the power of the surface, n1 and n2 are refractive index of first and second media respectively and r is the radius of curvature of the surface in metres. Index of refraction of air is 1.00, tears 1.336, cornea 1.376 and aqueous 1.336. Thereore power at air/tear interface is (1.336 – 1.00)/ 0.078 = 43.1D. Similarly for tear/cornea interface it is +5.1D and cornea/aqueous -6.2D total = 42.0 D. In keratometry, a semiempirical

constant the keratometric index is used (constant combines the individual contributions of the cornea surfaces). Based on keratometric index of 337.5, dioptric power of cornea with average radius of 7.8 mm is 43.3D.5

Other Parameters: The thickness of the cornea is bout 0.52 mm in the axial area and about 0.66 mm in the peripheral part.2-5 It is therefore apparent that the curvature of the posterior surface of the cornea is higher than that of the anterior surface, which is indeed the case.

Astigmatism: The seat of astigmatism is usually in the cornea. A small degree of curvature astigmatism due to cornea, around 0.25 diopters, is almost invariable2. At birth the cornea is almost spherical; 68% kids at 4 years and 95% at 7 years have with the rule or direct astigmatism2 (where the vertical meridian is steeper than the horizontal meridian). In old age, this WTR astigmatism disappears or may even become inverse astigmatism. This change with age is considered to be due to the pressure of the lids or the tone of the orbicularis muscle, which changes with age. Acquired astigmatism may occur due to surgery including cataract, inflammation, ulceration, trauma and lid lesions2, which may all alter the shape of the cornea.

Transmittance of Light: The cornea transmits radiation from approximately 310 nm in the ultraviolet to 2500 nm in the infrared region.6-8 The cornea is extremely sensitive to UV radiation at 270nm and corneal absorption of this radiation results in photokeratitis after exposure to welding arc.6-8 UV light reflected from the snow also causes corneal damage in contrast to the normal UV radiation from overhead sunlight that is shielded by the brows and upper lids.

The transparency of the cornea is crucial to the functioning of the eye and this is maintained by various factors among them being the crystalline lattice arrangement of the collagen fibrils within stro-

mal lamellae as proposed by Maurice8, the avascularity of the cornea3 and the Na/K pump of the corneal endothelium.

Nutrition: A major function of both the aqueous and the vitreous is to provide nutrition to the surrounding tissues. The aqueous for example provides substrates for and removes metabolites from the avascular cornea and lens. Some substances such as amino acids and glucose may also pass from the aqueous to the vitreous and may thus be associated with retinal metabolism as well.3

Applied Anatomy and Histology

of the Human Cornea

As we have emphasized before, the cornea is the most important refractive medium of the eye. Its transparency depends on the fact that it is relatively acellular and free of vessels and reveals a smooth, regular contour. It differs from the sclera in that it is covered by two epithelial layers, one anteriorly and one posteriorly. These cellular layers form semipermeable interfaces, which make possible an active metabolic interchange with the tear film externally as well as with the aqueous humor posteriorly. Proper hydration of the corneal stroma partially depends on the normally functioning tear-forming apparatus.

The tear-air interface is the most powerful refracting surface of the eye; curvature of tear film reflects that of the underlying cornea. Cornea becomes flatter paracentrally and peripherally, paracentrally there is a 2 mm increase in the radius of curvature and peripherally there is a 4 mm increase in the radius of curvature. Peripheral flattening of the cornea is more evident nasally than temporally. Glare produced by scattering bright light within the globe causes decreased vision. Thus corneal opacity produces light scattering within the globe which reduces intensity discrimination of images on the retina.9,10

rectly called a mesothelium because they are analogous to the mesothelial cells lining the various fluidcontaining body cavities, for example, the pericardium or peritoneum. Technically, the term endothelium should be reserved for the cellular linings of blood and lymph vessels. However, the term corneal endothelium remains firmly entrenched in clinical usage.

The endothelial layer functions as a regulator of corneal water content. Endothelial cells, which are rich with mitochondria, represent an active metabolic pump that performs this function in conjunction with the tear film. The cornea must remain relatively dehydrated to retain its transparency. If the water content of the ground substance increases significantly, the stroma will become edematous and opaque, and the two most important optical functions of the cornea, transparency and refractive power, may be severely altered.

In addition to endothelial competence that adjusts water content, the transparency of the cornea depends on the regular, parallel arrangement of the stromal collagen lamellae, the maintenance of smooth anterior and posterior border layers of the cornea, and the paucity of cellular elements and blood vessels within the stroma.

Nourishment to the cornea is provided by a limbal vascular arcade in conjunction with direct diffusion of nutrients from the tear film and the aqueous humor. Since the cornea is avascular, generally good transplantation results can be attained; it is relatively unresponsive to blood vessel-mediated immunologic rejection reactions. Only in pathologic states, for example, long-standing inflammation (keratitis), does one observe new vessel formation and vascularization within the cornea, usually caused by ingrowth from the limbal vessels.

THE AQUEOUS AND VITREOUS HUMOR1

The aqueous is the thin, watery fluid that fills the anterior chamber (space between the cornea and

the iris) and the posterior chamber. The anterior and posterior chamber aqueous volumes are 0.25 ml and 0.06 ml respectively. It is continually produced by the ciliary body, nourishes the cornea and the lens.

The vitreous is a thick, transparent substance that fills the center of the eye. The viscous properties of the vitreous allow the eye to return to its normal shape if compressed. In children, the vitreous has a consistency like a gel. With age it gradually thins and becomes more liquid. The vitreous is firmly attached to certain areas of the retina. As the vitreous thins, it separates from the retina, often causing floaters.

Vitreous humor has the following composition: water (99%), a network of collagen fibrils, large molecules of hyaluronic acid, peripheral cells (hyalocytes), inorganic salts, sugar, ascorbic acid. The aqueous and vitreous humor have a similar index of refraction and have a passive role in that they maintain a transparent media for transmitting light that has been refracted at various interfaces.

Refractive index: of the vitreous is 1.3349, nearly the same as that of aqueous.2

Transmittance of light: Vitreous transmits 90% of light between 300 and 1400 nm and none above or below this range. The primary function of the vitreous is to allow unhindered transmission of light to the retina, which suffers no further refraction after leaving the posterior surface of the lens.1

Transparency: Vitreous transparency is achieved by an extremely low concentration of macromolecular solutes.The vitreous collagen is structurally different from collagen in other areas, which serves to organize it as fibers of small diameter minimizing the scattering of light. The presence of large hyaluronic acid molecules also serves to keep the collagen fibers widely dispersed, further minimizing the scattering of light. Normal vitreous also has an inhibitory affect on cell migration and proliferation, which also ensures vitreous clarity.1

Accommodation: A role for the vitreous in accommodation was suggested way back in 1851 when it was suggested that the vitreous pushes against the lens to alter its shape.12 It was later sug-

gested that contraction of the ciliary muscle causes a decrease in the diameter of the circular component of this muscle and moves the vitreous basezonulelens diaphragm forward as the ora serrata and choroid move forward.12-16 The anterior vitreous pushes on the posterior lens, causing it to protrude forward into a more steeply curved lens zonule diaphragm. Yet this was contradicted later when no differences were found in the accommodative range or movement of the anterior pole of the lens, between two eyes of a patient who had undergone unilateral vitrectomy. It is now considered that the vitreous has a passive role in accommodation as it has been seen to show a decrease in its axial length possibly due to posterior displacement of the lens and capsule into the vitreous. 12-16 Any change in the compressibility of the vitreous may thus affect accommodation.

THE HUMAN CRYSTALLINE LENS

Refractive Properties and Relevant

Anatomy-Histology:10,11,17

The crystalline lens forms one of the refractive media of the eye. It is a unique transparent, biconvex intraocular structure, which lies in the anterior segment of the eye suspended radially at its equator by the zonular fibers and the ciliary body, between the iris and the vitreous body (Figure 4). Enclosed in an elastic capsule, the lens has no innervation or blood supply after fetal development. Its nourishment must be obtained from the surrounding aqueous and vitreous; and the same media must also remove metabolic waste products. Therefore, disturbances in circulation of these fluids, or inflammatory processes in these chambers, play a large role in the pathogenesis of lens abnormalities. The aqueous

humor continuously flows from the ciliary body to the anterior chamber, bathing the anterior surface of the lens. Disturbances in permeability of the lens capsule and epithelium can occur, leading to the formation of cataracts. Posteriorly, the crystalline lens is supported by the vitreous (hyaloid) face and lies in a small depression called the "patellar fossa." In younger eyes, the vitreous comes in contact with the posterior capsule in a circular area of thickened vitreous, the ligamentum hyaloideocapsulare. The potential space between the capsule and the circle of condensed vitreous is called Berger’s space. The lateral border of the lens is the equator, formed from the joining of the anterior and posterior capsules, and is the site of insertion of the zonules.

The lens consists of three components: capsule, epithelium, and lens substance. The lens substance is a product of the continuous growth of the epithelium and consists of the cortex and nucleus. The transition between the cortex and nucleus is gradual. It does not reveal a concise line of demarcation when observed in histological sections. The lines of demarcation are often better visualized by slitlamp microscopy.

The adult crystalline lens measures approximately 9.6 ±0.4 mm in diameter with an approximate anterior-posterior diameter of 4.2 ±0.5 mm.11,18 The refractive index of crystalline lens 1.41. The diameter of ciliary sulcus 11.1 ±0.5-mm, according to studies performed at the Center for Research on Ocular Therapeutics and Biodevices, Storm Eye Institute, Charleston, SC, USA.18 The anterior and posterior poles form the optical and geometrical axis of the lens. Although the normal lens is transparent and clear in vivo, it is seldom completely colorless; even in childhood a slight yellowish tint is present that tends to intensify with age.

Lens Capsule

The lens capsule is a basement membrane elaborated by the lens epithelium anteriorly and by superficial fibers posteriorly. By light microscopy the lens capsule appears as a structureless, elastic membrane, which completely surrounds the lens (Figure 5). It is a true Periodic Acid-Schiff (PAS) positive basement membrane, a secretory product of the lens epithelium.11,17,19 The capsule functions as a metabolic barrier and may play a role in lens shaping during accommodation. The lens capsule is of variable thickness in various zones. At its thickest regions the lens capsule represents the thickest basement membrane in the body. The relative thickness of the anterior capsule compared with the much thinner posterior capsule, may result from the fact that the former lies directly adjacent to and is actively

secreted by the epithelium, whereas the lens epithelium is not present on the posterior surface. Local differences in capsular thickness are important surgically, particularly because of the danger of tears or rupture of the thin posterior capsule during cataract surgery. Remnants of the tunica vasculosa lentis are common and appear as light-gray opacities (Mittendorf dots) at or near the posterior pole. These opacities are rarely responsible for significant visual loss.

Lens Epithelial Cells11,17,19

The lens epithelium is confined to the anterior surface and the equatorial lens bow. It consists of a single row of cuboidal-cylindrical cells, which can biologically be divided into two different zones with two different types of cells:

1.A-cells are located in the anterior–central zone, (corresponding to the central zone of the anterior lens capsule). They consist of relatively quiescent epithelial cells with minimal mitotic activity.

2.E-cells are located in the second zone, as a continuation of the anterior lens epithelial cells around the equator, forming the equatorial lens bow, with the germinal cells. These cells normally show mitotic capability, and new lens fibers are continuously produced at this site.

Lens Substance (Cortex and Nucleus)

The lens substance consists of the lens fibers themselves, which are derived from the equatorial lens epithelium. On cross-section these cells are

hexagonal, and are bound together by ground substance. After formation, the cellular nuclei of the lens fibers are present only temporarily. Subsequently they disappear, leaving the lens center devoid of cell nuclei except in certain pathologic situations, (e.g., the maternal rubella syndrome).

The original lens vesicle represents the primary embryonic nucleus; in later stages of gestation the fetal nucleus encircles the embryonic nucleus. The various layers surrounding the fetal nucleus are designated according to stages of growth. The most peripherally located fibers, which underlie the lens capsule, form the lens cortex. The designation of cortex is actually an arbitrary term signifying a peripheral location within the lens, rather than specific fibers.

SUMMARY AND CONCLUSIONS

The cornea, aqueous and vitreous humor, and the crystalline lens are the refractive media of the eye. The most significant refractive surfaces are the anterior and posterior cornea and the anterior and posterior crystalline lens. A sound understanding of the optical, refractive peculiarities as well as anatomico-physiological consideration of the refractive media of the eye is helpful for understanding the principles of the refractive surgery. The refractive errors are the most common of all vision problems in the United States afflicting about 120 million people. However with the advances in the wavefront analysis and laser technology, it is now possible to detect and surgically correct most of the optical aberrations and refractive errors of the cornea and other ocular media.

REFERENCES

1.Duke-Elder S. System of Ophthalmology, Vol V, Ophthalmic Optics. St. Louis, The C.V. Mosby Company, 1970. Chapter III, page 93 –145.

2.Pepose JS. The Cornea. In: Adler’s Physiology of the eye: Clinical application. Hart WM, Ed., 9th edition,

3.Martola A, Baum C. Central and peripheral corneal thickness. Arch Ophthlmol 1968; 79:28-30.

4.Boettner EA, Wolter JR. Transmission of the ocular media. Invest Ophthal 1962; 6:776.

5.Klyce SD, Beuerman RW. Structure and function of the cornea. In Cornea: Kaufman HF, Barron BA, McDonald MB, Eds., 2nd edition, ButterworthHeinemann. Boston 1998, Chapter 1.

6.Pitts DG. The human ultraviolet action spectrum. Am J Optom Arch Am Acad Optom 1974; 51:946.

7.Bergmanson JPG. Corneal damage in photokeratitis – why is it so painful? Optom Vis Sci 1990; 67:407.

8.Maurice DM. The strcucture and transparency of the cornea. J Physiol 1957; 136:263.

9.Trinkaus-Randall V, Edelhauser HF, Leibowitz HM, Freddo TF. Corneal disorders. Clinical diagnosis and management. Leibowitz HM, Waring GO III, Eds, 2nd edition, WB Saunders, Philadelphia, 1998, Chapter 1 PP 6-9.

10.Pavan-Langston D. Manual of ocular diagnosis and Therapy. Little Brown and company, 1991, PP 67-172

11.Apple DJ, Rabb MF. Ocular pathology: Clinical applications and self assessment. St. Loius: Mosby Inc., 1998

12.Fincham EF, The mechanism of accommodation. Br J Ophtthalmol 1937; 8:70.

13.Araki M, Tokoro T, Matsuo C. Movement of the ciliary body associated to accommodation. Acta ophthalmol Jpn 1964; 68: 1852.

14.Sebag J. The Vitreous. In: Adler’s Physiology of the eye: Clinical application. Hart WM, Ed., 9th edition, St. Louis, The CV Mosby 1992, Chapter 9

15.Fisher RF. Is the vitreous necessary for accommodation in man? Br J Ophthalmol 1983; 67:206.

16.Beauchamp R, Mitchell B. Ultrasound measures of vitreous chamber depth during ocular accommodation. Am J Optom Physiol 1985; 62:523.

17.Pandey SK, Thakur J, Werner L, Wilson ME, Werner LP, Izak AM, Apple DJ. The Human Crystalline Lens, Ciliary Body and Zonules: Their Relevance to Presbyopia. In: Agarwal A, ed., Presbyopia: A Surgical Text. Slack Inc., Thorofare, NJ, USA 2002, Chapter 2, PP 15-25

18.Assia EI, Castaneda VE, Legler UFC, et al. Studies on cataract surgery and intraocular lenses at the Center for Intraocular Lens Research. Ophthal Clin North Am 1991; 4:251-266.

19.Pandey SK, Werner L, Apple DJ. Posterior capsule opacification: Etiopathogenesis, clinical manifestations, and management. In: Garg A, Pandey SK, eds., Textbook of Ocular Therapeutics. Jaypee Brothers, New Delhi, India 2002, PP: 408-425

CONTRIBUTORS

Vidushi Sharma, M.D., F.R.C.S. (Edin).

Senior Resident, Pediatric Ophthalmology and Orbit, Oculoplastic, Reconstructive Surgery section, Dr. Rajendra Prasad Center for Ophthalmic Sciences, All India Institute of Medical Sciences, New Delhi, India. Email: vidushi@vsnl.in

Dr. Rajendra Prasad Center for Ophthalmic Sciences, All India Institute of Medical Sciences, New Delhi, India;

Suresh K. Pandey, M.D.

Instructor, David J. Apple, MD, Laboratories for Ophthalmic Devices Research, John A. Moran Eye Center, Department of Ophthalmology and Visual Sciences, Fifth Floor, University of Utah, 50 North Medical Drive Salt Lake City, Utah-84132, USA. Phone, (801) 231 8375, E-mail: suresh.pandey@hsc.utah.edu

David J. Apple, MD Laboratories for Ophthalmic Devices Research, John A. Moran Eye Center, University of Utah, Salt Lake City, UT, USA.

Liliana Werner, M.D., PhD

Assistant Professor, David J. Apple, MD, Laboratories for Ophthalmic Devices Research, John A. Moran Eye Center, Department of Ophthalmology and Visual Sciences, Fifth Floor, University of Utah, 50 North Medical Drive Salt Lake City, Utah-84132, USA.

David J. Apple, MD Laboratories for Ophthalmic Devices Research, John A. Moran Eye Center, University of Utah, Salt Lake City, UT, USA.

David J. Apple, M.D.

Professor of Ophthalmology and Pathology, David J. Apple, MD, Laboratories for Ophthalmic Devices Research, John A. Moran Eye Center, Department of Ophthalmology and Visual Sciences, Fifth Floor, University of Utah, 50 North Medical Drive Salt Lake City, Utah-84132, USA.

David J. Apple, MD Laboratories for Ophthalmic Devices Research, John A. Moran Eye Center, University of Utah, Salt Lake City, UT, USA.

The authors have no financial or proprietary interest in any product mentioned in this chapter.

Supported in part by an unrestricted grant from Research to Prevent Blindness, Inc., New York, New York. NY, USA.

Corresponding author: Suresh K. Pandey, MD, David J. Apple, MD Laboratories for Ophthalmic Devices Research, John A. Moran Eye Center, Fifth Floor, University of Utah, 50 North Medical Drive Salt Lake City, Utah-84132, USA.

Phone, (801) 231 8375,

E-mail: suresh.pandey@hsc.utah.edu