Refractive Surgery is surgery designed to produce a change in the refractive nature of the eye with the presumed goal of correcting the 'refractive error' of the eye. In other words, to make the eye less nearsighted, or less farsighted, or have less astigmatism, or more focusing ability.

An understanding of optics and the way the eye works is essential to comprehending the purpose of refractive surgery...

The eye is one of our five sensory organs which include the ear, the nose, taste receptors, and sensory receptors in the skin, mucous membranes, and other tissues. The eye is the most well defined of these structures and, therefore, the easiest to understand...

The structure of the eye is like that of a house with a single, outwardly curved (convex) clear 'bubble window', the cornea, at the front and a lengthy 'fiberoptic' cable, the optic nerve, extending from the back. It is essentially an empty structure except for a doughnut shaped tissue, the colored iris, which allows varying amounts of light to pass to the back inside surface of the eye. The size of the opening in the iris, the pupil is controlled by muscles in the iris - when the amount of light is excessive, the muscles make the pupil smaller, and vice versa.

Light rays pass through the clear cornea, which because of its curved surface bends (refracts) the light rays, which are then squeezed closer together to pass through the pupil. Then, they pass through the normally clear lens (about the size and shape of an M&M) which has two (2) curved surfaces, the front and the back. Therefore, these light rays are bent (refracted) two more times on their trip to the back of the eye.

Most of the trip of the light rays to the back surface of the eye is through the vitreous, a clear jelly whichfills the space between the back of the lens and the retina, the inside lining of the back surface of the eye which contains specialized cells which convert light energy into electrical impusles. These cells are either called rods, which are for black and white images, or cones, which are for color images (like camera film.)

What is truly amazing about the eye is that part of these cells in the retina (photosensitive cells) is a six inch appendage of the cell, the axon, which joins with other axons to compose the optic nerve which travels to the brain stem, the very top of the spinal cord, located in the very center of the brain. There, each axon connects with (synapses with) a cell or cells, and the axon of the receiving cell(s) travels another six inches to the back of the brain, the occipital lobe, where it synapses with a brain cell(s) to produce what we call vision.

Therfore, the major functions of these parts of the visual system are:

• Cornea - Refracts light rays
• Pupil - Controls the amount of light entering the eye
• Lens - Refracts light rays
• Vitreous - Light traverses this space
• Retina - Converts light energy to electrical energy
• Optic Nerve - Transmits electrical energy from the retina to the brain stem
• Brain Stem - Intermediate 'relay station' for visual fibers
• Occipital Cortex - Final destination. Converts electrical energy to visual images

A 'Perfect Eye' would therefore have: (1) a clear and unobstructed path from the front of the eye to the back of the eye; (2) the proper balance between the length of the eye and the curvatures of the three refracting surfaces; and, (3) properly functioning cells in the retina and brain which allow the conversion of light energy to electrical energy, the transmission of this energy, and the interpretation of the energy into what we call vision. Unfortunately, most people do not have 'perfect eyes.'

Eyes that are too long or have too much refracting power (from the cornea and the lens) are nearsighted eyes, as images are focused in front of the retina. The image received by the retina is not a 'dot for dot' representation of what the image viewed by the eye. Instead, each of these 'dots' of light becomes enlarged to form a 'disc' of light with a consequent spread of the dot image to adjacent parts of the retina. This is what causes blurring of vision.

The opposite results when eyes or too short or have too little refracting poser. These eyes are farsighted, as images are focused (or would be) behind the retina. The same type of dot to disc representation occurs.

When light rays that are vertically oriented are not refracted the same amount as the light rays that are horizontally oriented, this condition is called astigmatism. An example would be when that eye looks at a building that is built as a square, it would appear as a rectangle with different vertical and horizontal dimensions being visualized. This example refers to strictly vertical (90 degrees) and strictly horizontal (0 degrees); astigmatism can occur at any angle between 0-180 degrees.

On a more dynamic level, the eye can possess no refractive error whatsoever but is unable to adjust to near images by increasing its refracitve powers. This condition is termed presbyopia and normally affects persons in their 40s and early 50s. The mechanisms within the eye that can increase the curvature of the lens (accommodation) become less efficient and external plus lenses are necessary to bring the near images into focus.

The eye is truly like a camera because it not only requires proper focusing (refraction) but it requires a clear media through which light rays must pass. Any loss of clarity of the structures through which these light rays must pass will interfere with their successful interpretation within the visual cortex of the brain. Examples of disorders which might cause this scattering or absorption of light rays are opacities or swelling of the cornea (scars, edema, abrasions, etc.), opacification of the lens (cataract), and cloudiness of the vitreous (hemorrhage or inflammation.)

Also, the receiving tissue, the retina must be functioning properly, as opposed to aged related macular degeneration, in which there is deterioration of the most important part of the retina, the macula, which is responsible for our finest and our reading vision.