How the Eye Sees
To understand how we see clearly, we need to understand how the eye works. Light rays enter the eye through the clear cornea, pupil, and lens. These light rays are focused directly on to the retina, the light-sensitive tissue lining the back of the eye. The retina converts light rays into impulses sent through the optic nerve to the brain, where they’re recognized as images. Seventy percent of the eye’s focusing power comes from the cornea, and thirty percent from the lens.
Myopia, or near-sightedness: This is where the distance between the cornea and the retina is too long, so that light rays focus in front of the retina instead of on it. With myopia, close objects will look clear, but distant objects will appear blurred.
Hyperopia, or far-sightedness: This is where the distance between the cornea and the retina is too short. Here, light rays are focused behind the retina instead of on it. Distant objects will look clear, but close objects will appear blurred.
Astigmatism occurs when the cornea is curved unevenly, shaped more like a football than a basketball. Light passing through this uneven cornea is not properly focused on the retina. Distance and close vision may both appear blurry.
Presbyopia. This is a normal condition that typically starts around middle age, when the eye’s lens becomes less flexible and less able to focus on close objects. A typical sign of presbyopia is the need for reading glasses.
Over the past decades, a variety of surgical procedures (collectively known as refractive surgery,) has become an increasingly popular method of correcting refractive errors. In most cases, these procedures permanently alter the shape of the cornea, so that light rays are re-focused on the retina to improve vision.
With LASIK, your ophthalmologist creates a small flap in the front portion of your cornea, by gently separating the tissue. The corneal flap is then folded back. Once the flap is lifted to one side, laser energy is applied for a few seconds to a minute or so to re-shape the cornea. Then, the corneal flap is returned to its original position, where it adheres naturally. After the procedure, the re-shaped cornea focuses light more accurately on the retina.
The targeted beam of light is sent through your eye and focused on the retina. As the wave of light rays are reflected back from the retina through the eye’s vitreous, lens, pupil, and cornea, a sensor measures the irregularities in the waveform pattern of light as it emerges from your eye. Using this measurement, the waveform computer is able to create an accurate, three-dimensional map of your eye’s visual system, including specific imperfections in the cornea. This waveform data is used to program the excimer laser, allowing your ophthalmologist to customize the reshaping of your cornea.
With PRK, the epithelium, or outer surface layer of your cornea, is removed. An excimer laser controlled by a computer is programmed to re-shape your cornea. The laser removes tissue, either decreasing the curve of your cornea to correct near-sightedness, or increasing the curve of your cornea to correct farsightedness. After the procedure, a contact lens is placed over the cornea to promote healing. The re-shaped cornea focuses light more accurately on the retina.
A thin layer of cells, calls the epithelium, covers the front surface of your cornea. With LASEK, a special alcohol solution is used to loosen the epithelium, which allows it to be peeled back from the cornea. Once the epithelium is removed, an excimer laser controlled by a computer is programmed to re-shape your cornea. The laser removes tissue, either decreasing the curve of your cornea to correct nearsightedness, or increasing the curve of your cornea to correct farsightedness. After the laser treatment, the epithelium is placed back over the cornea and a contact lens in placed on the cornea to promote healing. The re-shaped cornea focuses light more accurately on the retina.
Intacs are made of two semi-circles of plastic. Your ophthalmologist creates a tiny opening in the cornea beneath the eyelid. The intacs are inserted through this opening, and rest between the layers of tissue in the cornea. The small opening in the cornea is closed with a suture which is removed two to four weeks after the procedure. Once in place, the intacs cause the peripheral cornea to become more steep, and the central cornea to flatten. This change in the central cornea’s shape corrects nearsightedness by shifting the point at which light is focused on the retina.
A tiny incision is made in the eye. The implantable contact lens is inserted through this incision, and in some cases unfolded, inside the eye. Once the lens is positioned in front of the eye’s natural clear lens, it provides the necessary correction to re-focus light rays onto the retina.
The optimal corneal shape is round. With astigmatism, the cornea is steeper in one direction than the other. This irregular shape prevents light from focusing correctly on the retina. With AK, the surgeon makes one or two incisions in the steeper part of the cornea, making these areas less steep, and the overall cornea shape more round. As a result, light rays are then focused directly on the retina.
Refractive surgery for monovision allows one eye to see clearly at a distance, while the other eye sees clearly up close. One eye is corrected surgically to allow clear distance vision, while the other eye is either not treated or is partially treated to allow clear vision close up.