We use our eyes in virtually everything we do and depend on our vision perhaps more than any other sense we have. Sight is the most precious of the five senses, and many people fear blindness more than any other disability.
The eye allows us to see and interpret the shapes, colors, and dimensions of objects in the world by processing the light they reflect or give off. The eye is able to see in dim light or bright light, but it cannot see objects when light is absent. The eye changes light rays into electrical signals then sends them to the brain, which interprets these electrical signals as visual images.
The eye is set in a protective cone-shaped cavity in the skull called the orbit or socket and measures approximately one inch in diameter. The orbit is surrounded by layers of soft, fatty tissue which protect the eye and enable it to turn easily. Six muscles regulate the motion of the eye. Among the more important parts of the human eye are the iris, cornea, lens, retina, conjunctiva, macula, and the optic nerve.
Cornea
The cornea is sometimes referred to as the ‘window of the eye.’ It provides most of the focusing power when light enters your eye. The cornea is composed of 5 layers of tissue. The outer layer (the epithelium) is the eye's protective layer. This layer is made up of highly regenerative cells that have the ability to grow back within 3 days, and therefore, allow for fast healing of superficial injuries. Most of the inner layers provide strength to the eye. The laser vision correction procedure is performed on this part of the eye.
Lens
The lens is the clear structure located behind the pupil. Its primary function is to provide fine-tuning for focusing and reading. The lens performs this function by altering its shape to become thinner or thicker as necessary. Between the ages of 40 and 50, the lens becomes less flexible and presbyopia sets in. As people reach their 60’s or 70’s, the lens sometimes becomes cloudy and hard (cataract formation), preventing light from entering the eye.
Pupil
The pupil is the 'black circle' that you see in people's eyes. The primary function of the pupil is to control the amount of light entering the eye. When you are in a bright environment, the pupil becomes smaller to allow less light through. When it is dark, the pupil expands to allow more light to reach the back of the eye.
Iris
This is the colored part you see in people's eyes (i.e. blue/green/brown/hazel). The primary function of the iris is to control the size of the pupil. This is achieved through contraction or expansion of the muscles of the iris.
Vitreous Body
This is the clear 'gel like' substance located inside the eye's cavity. Its purpose is to provide a spherical shape to the eye. The vitreous may develop small clumps known as “floaters,” which are more common in nearsighted people than in the rest of the population.
Optic Nerve
The optic nerve carries images from the retina to the brain.
Retina
The retina consists of fine nerve tissue which lines the inside wall of the eyes and acts like the film in a camera. Its primary function is to transmit images to the brain. When your vision is perfect, the light rays coming into your eye focus precisely on this part of the eye.
Sclera
This is the 'white part' that we see in people's eyes. The sclera's purpose is to provide structure, strength and protection to the eye.
Clear vision is the result of light rays properly focusing on the retina of a normal eye. Your eye works like a camera -- light rays enter your eye through the cornea, or "window" of the eye. Ideally, the cornea bends the light rays to create a focal point that equals the length of the eye. You need vision correction when your eye cannot properly direct light rays on the retina. The cornea, at the front of your eye, provides most of the eye’s focusing power. The lens inside your eye provides the fine tuning of light, contributing to your ability to read. Light rays must focus precisely on the retina for you to see clearly. If you wear corrective lenses, you may have one of the following common refractive problems:
Presbyopia occurs as you reach your 40s or 50s. The lens inside your eye loses its elasticity, making it more difficult to read smaller print. Because laser vision correction does not deal with the lens of your eye, it can not correct this problem. Reading glasses help those with presbyopia.
Common refractive disorders of the eye such as myopia, hyperopia, astigmatism, and presbyopia are measured in units called diopters. Diopters represent the amount of correction you need to normalize your vision. The more nearsighted, farsighted, or astigmatic you are, the higher your prescription in diopters.
Your prescription is written in three numbers:
For example, -5.00 -1.50 x 180 represents a typical prescription. The first number (-5.00) identifies your degree of nearsightedness or farsightedness. The sign identifies whether you are nearsighted (- sign) or farsighted (+ sign).
The second number (-1.50) identifies your degree of astigmatism. The number can be written either with a + sign or a - sign.
The third number (180) is the axis, which indicates the direction of your astigmatism. An axis of 180 degrees, for example, means the astigmatism is horizontal.
Therefore, -5.00 -1.50 x 180 means that the patient is moderately nearsighted, with a moderate degree of astigmatism in a horizontal direction.
Most excimer laser studies have found the "typical" laser patient to be 30 to 35 years old, with a prescription between -3.00 and -4.00 diopters of myopia with less than 1.00 diopter of astigmatism.
Three variables can be altered to achieve clear vision. These are: changing the light as it enters the eye, altering the front curve of the eye, or changing the length of the eye.
For centuries, directing light rays with lenses has obtained clear vision. Glass or plastic lenses bend light the exact amount required to change the focal point to match the length of the eye, producing clear vision. Later, contact lenses would be used to achieve the same results.
Researchers have been trying for decades to come up with a more practical solution to create clear vision. Early research led to the development of several surgical procedures, including Radial Keratotomy (RK) and Automated Lamellar Keratectomy (ALK). These procedures offered some improvements but were limited in the degree of correction possible. With the advent of the excimer laser, more precise vision correction procedures such as Photorefractive Keratectomy (PRK) and Laser in Situ Keratomileusis (LASIK) were developed.
Many surgeons believe LASIK to be a superior technology for vision correction. Rather than bend light before it enters the eye, LASIK allows the surgeon to design a corneal shape that will bend incoming light to correspond with the length of the eye. This technological breakthrough has changed the way people think about vision correction. The LASIK procedure is performed as an outpatient procedure with only a topical anesthetic. The vast majority of patients are spectacle or contacts lens free the day following surgery!
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