INTRODUCTION
The eye manifests its refractive power via several curved surfaces; each is separated by a media with different indices of refraction. The most significant refractive surfaces are the anterior and posterior surfaces of the cornea and crystalline lens, while the aqueous and the vitreous humors play the role of passages.
In the emmetropic eye (without any refractive error), the range of corneal refracting power is between 39 and 48 diopters (D), while the refracting power of the crystalline lens is between 15 and 24 D. In the emmetropic eye, the axial length (from the anterior corneal surface to the retina) varies from 22 to 26 mm. The following are clear ocular media (through which light passes) and their respective indices of refraction: tear film 1.33, cornea 1.37, aqueous humor 1.33, crystalline lens 1.41, and vitreous humor 1.33. The important features which determine their refractive power are the radius of curvature, the refractive index and the distance between various interfaces.
CORNEA
Maximum refraction in the eye occurs at the anterior surface of the cornea due to its high curvature and due to the large difference between the refractive indices of media on its two sides (air 1 and corneal substance 1.37). Refraction occurring at the posterior surface of the cornea is not very significant as its refractive index is quite similar to that of the aqueous humor (1.37 and 1.33 respectively).
Before we discuss the optical properties of the cornea, it is important to discuss its shape, which also has a bearing on its optical properties.
Shape
The cornea comprises the central sixth of the outer wall of the eye. It is oval in shape, the average vertical and horizontal diameter being 11.7 mm and 12.6 mm respectively.
It is a well-known fact that the cornea is not spherical in shape. Thus, the central 4 mm or so is supposed to be spherical, and according to this assumption the keratometer is based. However, even this area shows a small degree of astigmatism and therefore it is strictly speaking toroidal in shape. It is also known that the cornea is flatter in the periphery and becomes progressively flatter as one goes away from the center. This is one of the factors that compensate for the spherical aberration in the eye, the other factor being the structure of the lens. However, the peripheral part of the cornea is irregular and radially asymmetric and does not conform to any specific geometrical shape. It is therefore usual to divide the cornea into two parts: the central zone of about 4 mm, also called the optical or the apical zone, and the peripheral or basilar zone. 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. These changes usually remain unnoticed by individuals with normal corneas. Some conditions like corneal dystrophies, ocular hypotony, radial keratotomies or contact lens use can make them apparent.
Small changes in corneal shape do occur throughout life:
- In infancy, the cornea is fairly spherical
- In childhood and adolescence, probably due to eyelid pressure on a young tissue, cornea becomes slightly astigmatic with-the-rule (WTR)
- In the middle age, cornea tends to recover its sphericity
- Late in life, against-the-rule (ATR) astigmatism tends to develop.
Geography
The cornea is divided into four geographical zones:
- The central zone or the axial zone (central 4 millimeters): It overlies the pupil and is responsible for the high definition vision. The central part is almost spherical and is called the apex.
- The paracentral zone: Where the cornea begins to flatten.
- The peripheral zone.
- The limbal zone.
Curvature
The central 4 mm, or the axial zone of the cornea, has a radius of curvature of 7.8 mm. The radius of curvature of the posterior surface of the cornea is 6.7 mm. Changes in curvature of the cornea have profound effect on the refractive status of the eye, e.g. a 1 mm change in the radius of curvature of the cornea causes 6 D change in refractive power. There are some physiological conditions in which corneal curvature is slightly affected (Table 1.1).
Power
The refractive index of the cornea is 1.376. By using the radius of curvature and the refractive index, we can calculate the power of both surfaces of the cornea. The refractive power of normal cornea is 49 D at the anterior surface and −6 D at the posterior surface; since the latter plays the role of a concave lens; the total refractive power of the cornea is 43 D in average.
Other Parameters
Thickness of the cornea is bout 0.52 mm in the axial area and is about 0.66 mm in the peripheral zone. Therefore, the curvature of the posterior surface of the cornea is higher than that of the anterior surface.
Astigmatism
The seat of astigmatism is usually in the cornea. A small degree of curvature astigmatism due to anterior corneal surface, around 0.25 D, is due to the fact that the vertical meridian is steeper than the horizontal meridian, and is known as WTR. At birth, the cornea is almost spherical; 68 percent of kids at 4 years and 95 percent at 7 years have WTR astigmatism. In old age, this WTR astigmatism disappears or may even become ATR astigmatism. This change with age is considered to be a result of the pressure of the lids or the tone of the orbicularis muscle, which changes with age. Acquired astigmatism may occur after surgery including cataract, inflammation, ulceration, trauma and lid lesions, 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. The cornea is extremely sensitive to UV radiation at 270 nm and corneal absorption of this radiation results in photo keratitis after exposure to welding arcs. UV light reflected from the snow also causes corneal damage in contrast to the normal UV radiation from overhead sunlight which 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 the crystalline lattice arrangement of the collagen fibrils, the avascularity of the cornea, the demyelinated corneal nerves and the Na/K pump of the corneal endothelium.
Nutrition
Corneal nutrition depends mainly on the aqueous humor, which provides substrates and removes metabolites.