LASIK Laser Eye Surgery Articles from
Mountain View Vision Specialist Dr. Shobha Tandon

Eye Anatomy and Refractive Errors
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NeoVision Eye Center

Recent advances in refractive surgery are designed to permanently reduce or eliminate the need for corrective eyewear to see objects clearly. Today, more than 3 million corneal refractive procedures are performed worldwide each year. Presently, more than 100 million patients wear contact lenses for correction of their refractive errors.

To better understand refractive surgeries, one should understand how a perfectly normal eye (20/20) focuses light and what are different types of refractive errors? Let us first understand the main components of eye.

Anatomy of the Eye

Cornea: It is the transparent front wall of the eye. Cornea is one of the most critical components of the eye and is the "window" of your eye, just like the anterior window of your camera. Cornea works to refract light rays from the object viewed so that they focus on the retina at the back of your eye. Refractive errors occur when the cornea fails to focus light rays precisely on the retina. Cornea is the tissue which is reshaped during laser surgery.

Iris: The iris is the colored portion of the eye. It is similar to the aperture of a camera. The iris regulates the amount of light entering the eye.

Pupil: The adjustable opening at the center of the iris that allows varying amounts of light to enter the eye. Dilation drops used by your eye doctor increase the size of the pupil so that the internal health of your eye can be evaluated.

Lens: The lens focuses the light entering through the pupil, directing it to the retina. The curvature of the lens adjusts to focus at different distances.

Optic Nerve: The optic nerve is similar to the cable that connects your video camera to the VCR. The image seen in the eye is transported to brain where the image is processed. The optic nerve head is the portion of the optic nerve that is seen by an experienced eye care professional to evaluate for eye diseases such as glaucoma, papilledema (increase pressure in the brain), and optic neuritis.

Retina: It is a nerve layer at the back of your eye that senses light and sends images to your brain. The retina is similar to the film in a camera. This is where the picture seen by the eye is converted to electrical signals and then transported to the brain via the optic nerve. The retina is made of cells called rods and cones. Rods are primarily used to aid with night and peripheral vision, while cones are designed to aid with color vision and crisp 20/20 vision.

Macula: The macula is a small area located at the posterior portion of the retina. It is comprised of cones and functions in crisp 20/20 vision. Diseases that affect this area can cause a decrease in vision. Disease processes such as macular degeneration, retinal detachments, diabetes, and hypertension can potentially cause blindness.

Sclera: The sclera is the white portion of the eye. It is comprised of tough connective tissue that gives the eye its shape.

Conjunctiva: It is a very thin layer covering the sclera. This layer carries blood vessels and very importantly it contributes to the innermost layer of a healthy tear film. Conditions affecting this layer such as surgery, infection etc. can affect your tear film.

Muscles: Ocular muscle alignment is critical in developing sharp three-dimensional vision. During a routine eye exam, an eye care professional evaluates the integrity of the muscles and the nerves which supply them. Muscle imbalance can cause lazy eye if it is not corrected during childhood.

Refractive Errors

Normal Eye

In a perfectly normal eye, the cornea and lens contribute to sharp focusing of incoming light rays from distant objects directly on the retina. The cornea acts as a natural lens and its curvature determines its power. Cornea is responsible for 80% and the lens is responsible for about 20% of the eye's refractive power. Nerve fibers in the retina transmit this image to the brain, where it is interpreted as vision. This perfect eye system sees 20/20. If the light rays do not come to a pinpoint focus on the retina, the patient has a refractive error. There are four types of refractive disorders. These are:

Myopia (also known as near-sightedness)

If a person can see objects nearby with no problem, but reading road signs or making out the writing on a bill board is more difficult, he may be near-sighted. Myopia occurs when light rays are focused in front of the retina, causing distant images to appear blurry. Far-objects appear blurry because the light rays are spread apart instead of focused when they strike the retina. Myopia is the result of an eyeball which is too long, or a cornea which is too curved.

Myopia is not a disease, nor does it mean that you have "bad eyes." It simply refers to a variation in the shape of your eyeball. Myopia usually starts in childhood and gets progressively worse through adolescence. It usually stops changing by the late teens, but it can sometimes continue to get worse into the mid-twenties. Corrective concave lenses ("negative" lenses that are thicker at the edge than in the center) or contact lenses are prescribed to focus light more precisely on the retina, where a clear image is formed.

Hyperopia or Hypermetropia (also known as far-sightedness)

Hyperopia, or far-sightedness, is the opposite of myopia. If a person can see objects at a distance clearly but has trouble focusing well on objects close up, he may be farsighted. It occurs when light rays are focused behind the retina, causing near images to appear blurry. Near-objects appear blurry because the light rays are spread apart instead of focused when they strike the retina. Very far-sighted people have difficulty seeing clearly the distant objects as well. Hyperopia is the result of an eyeball which is too short, or a cornea which is less curved. This creates blurred vision that is more noticeable for near objects than distant ones.

Hyperopia is not a disease, nor does it mean that you have "bad eyes." It simply means that you have a variation in the shape of your eyeball. Hyperopia may be present in childhood but does not usually become apparent until people are in their late 20's or 30's when they can no longer see up close. When these patients get into their 40's or 50's, they begin to have difficulty seeing distant objects as well. Corrective convex lenses ("positive" lenses that are thicker in the center than at the edge) or contact lenses are prescribed to focus light more precisely on the retina, where a clear image is formed.

Astigmatism

Astigmatism is an overall inability of the eye to focus clearly at any distance. It occurs when cornea is shaped more like a football (with two different curvatures) than a well-rounded baseball. Images appear blurred or ghost-like because light rays are refracted unequally. In extreme cases, images both near and far, appear blurred. Many people who have myopia also have some degree of astigmatism. This creates eye strain. Many people live with minor astigmatisms that do not require any type of treatment. Others with more severe astigmatism can have it corrected by wearing glasses or contact lenses.

Presbyopia

Presbyopia is the inability to focus the eyes at near. In young individuals, the natural lens of the eye is soft and pliable. This innate flexibility permits the natural lens to change its shape, allowing it to focus on objects near the eye. As the years pass, the lens loses its flexibility and can no longer vary the focus of the eye. This condition usually becomes noticeable sometime between 40 and 50 years of age. People with myopia as well as hyperopia have presbyopia after age 40.

Laser Vision Correction (LVC)

Laser vision correction includes those refractive surgeries that use an Excimer laser to microscopically reshape the corneal curvature, (the outer window of the eye) so that the light rays focus more precisely on the retina. Following are the most popular LVC available today:

• Conventional Laser Assisted In-Situ Keratomileusis (Conventional LASIK)
• CustomLASIK / CustomCornea / WaveFront LASIK / CustomVue WaveScan LASIK
• IntraLase
• IntraLase with CustomLASIK
• Photorefractive Keratectomy (PRK)
• LASEK / Epi-LASIK

These procedures have either altogether eliminated or reduced the dependence on glasses or contact lenses for millions of people around the world. They are capable of correcting a wide range of refractive disorders such as near-sightedness (myopia), far-sightedness (hyperopia) and astigmatism. Note that these procedures can improve vision only if your vision can be improved with glasses or contact lenses.

Conventional LASIK

LASIK is the most commonly performed refractive procedure today. From 1990 to the present, over 10 million LASIK procedures have been performed worldwide establishing refractive surgery as the second most common ophthalmic surgical procedure, behind cataract (more than 10 million surgeries per year worldwide). In 2003, in USA alone, more than 1.2 million LASIK surgeries were performed.

There are no needles, no stitches, and no pain. Prior to the procedure, drops are placed in the eye to anesthetize it (like the ones used to check your eye pressure for a glaucoma test) and an instrument called speculum is placed in the eye to prevent you from blinking.

LASIK is a 3-step procedure:

Step 1

A specially trained ophthalmologist uses an automated instrument called a microkeratome (similar to a carpenter's plane) to create a thin, uniform flap in the cornea.

Step 2

The surgeon then folds the flap back (like the page of a book) to apply a computer-guided Excimer laser treatment to gently remove a thin layer of tissue from the exposed corneal surface to achieve a desired correction. This is a cold laser and it ablates or vaporizes tissue away to microscopically reshape the corneal surface.

Step 3

After the computerized laser therapy is applied, the surgeon replaces the flap, where it naturally re-adheres in its original position.

This treatment microscopically reshapes the cornea, resulting in greatly improved visual acuity without glasses or contacts. In just 24 hours following surgery, a vast majority of people see 20/40 or better. This continues to improve as the days pass, with the cornea healing with the assistance of medicated drops. Many people are able to return to work in one to two days. Discomfort is generally limited to some mild burning sensations or foreign body sensations in the eyes for a few hours.

Today, LASIK has become the procedure of choice because the healing occurs in the interior of the cornea, and no re-growth of surface cells is required. The corneal flap protects the treated area and there is usually little or no post-op discomfort. Visual rehabilitation is much quicker and regulation of the healing process with long term post-op drops is unnecessary since the center of the cornea tends to have much less of a healing reaction than the surface.

There are two new advancements to further improve the safety and precision of standard LASIK. These include CustomLASIK and IntraLase.

CustomLASIK / CustomCornea / WaveFront LASIK / CustomVue WaveScan LASIK

There are two types of optical distortions: low order and high order. Low order distortions include nearsightedness (myopia), farsightedness (hyperopia), and astigmatism. Higher-order aberrations are corneal irregularities, other than refractive errors, that can cause decreased contrast sensitivity or night vision, ghosting ("double" vision), starbursts, glare, shadows, and halos that some patients have had after the conventional LASIK surgery. Conventional LASIK surgery is limited to treating low-order aberrations. However, CustomLASIK (CustomVue) uses Wavefront technology to address both low-order and higher-order aberrations.

CustomVue System, which includes the Wavefront Map and 3D ActiveTrak™ Excimer laser with Variable Spot Scanning (VSS™) is used to measure and treat the optical distortions. This system allows the surgeon to treat your optical distortion by performing Wavefront guided laser treatment according to your individual customized treatment plan. WaveScan transmits a safe ray of light into your eye. The light is then reflected back off the retina, out through the pupil, and into the device, where the reflected wave of light is received and arranged into a unique pattern that captures your lower- and higher-order aberrations.

The resulting refractive map (Wavefront map) used in CustomLASIK is as individual as a fingerprint. No two eyes are the same. Like your DNA or fingerprint, your vision is unique. This map provides information on more than just the front surface of the eye. It shows the variability of how light is focused in different areas of the eye. This information is then transferred to the Excimer laser, enabling the surgeon to customize the LASIK procedure to your unique visual requirements. There is no significant difference in how the actual LASIK procedure is done, whether Wavefront or conventional software is used. Like conventional LASIK, CustomLASIK won't cure all vision-related problems, so it's important to discuss its applications with your surgeon to determine if you are a good candidate.

Until now, laser vision correction (LVC) treatment was based on diagnostic technology similar to that used for the prescription eyeglasses or contact lenses. The new CustomLASIK takes LVC to an entirely new, personalized level by combining exclusive personalized diagnostic technology with the Excimer laser. The advanced diagnostic portion of the new Wavefront technology produces a precise, detailed analysis of your vision and provides a personalized LVC plan that addresses your individual needs. This WaveScan Map is coordinated with the VISX STAR S4 ActiveTrak™ Excimer Laser System to create one of the most advanced systems available. The 3-D eye tracker with auto-centration adds a new level of precision, comfort, and safety.

The benefits of CustomLASIK (CustomVue) include higher chance of seeing 20/20; the ability to treat people with larger pupils; reduced frequency of night vision problems following treatment; and clearer, crisper results compared to treatments performed without CustomLASIK.

IntraLase

In standard LASIK, an ophthalmologist uses an oscillating blade called a microkeratome to create the flap. Sometimes, microkeratome is a source of surgical complications during traditional LASIK. A new laser, called the IntraLase replaces the microkeratome. It may make LASIK even safer. IntraLase creates the corneal flap and then the surgeon uses the same Excimer laser to perform the rest of the LASIK procedure.

The "all-laser" LASIK approach, using the IntraLase and Excimer laser, not only avoids microkeratome-related complications such as corneal irregularities and scarring, but may also offer the potential for better vision than after standard LASIK. Precision and control in LASIK surgery have everything to do with accuracy of vision correction, quality of resulting vision and reproducibility among differing patients and surgeons. Replacing the mechanical microkeratome with a computer-guided laser is a significant advancement.

People previously "disqualified" based on the corneal thickness may now be eligible for LASIK with the IntraLase laser. The thickness of the cornea is an important parameter in determining the eligibility for LASIK. Most people have corneas about 550 microns thick. Most microkeratomes cut flaps that are about 130-160 microns thick. For people with thin corneas, LASIK surgeons tend to move cautiously, balancing the thickness of the cornea against the amount of vision correction needed. They may even decide that the procedure is inadvisable for a particular patient. The IntraLase laser can make thinner corneal flaps without compromising the safety of the procedure. This may allow surgeons to perform safe LASIK in people with thinner, steep or flat corneas.

IntraLase with CustomLASIK

CustomVue with IntraLase is the very best possible LASIK surgery today. It is a blade-free; all-laser individualized laser vision correction. It uses a computer-controlled laser rather than a hand held mechanical devise (microkeratome) to cut the corneal flap. This minimally invasive approach offers more safety and precision than ever imagined. With CustomVue, surgeons can now identify, measure, and correct
imperfectionsin an individual's eyes 25-times more precisely than with standard LASIK. CustomVue with IntraLase provides an unprecedented level of safety and precision.

PRK

PRK is a variation of LASIK, differing in that the surgeon does not create a corneal flap. Instead, the surface skin cells of the cornea (the epithelium) are gently removed, and then cornea is treated by the same Excimer laser. After this painless procedure, a contact lens is placed on the eye and left in place for three to four days, allowing the epithelium to grow back over the corneal surface. Although vision recovery is slightly slower compared with LASIK, some people are better candidates for PRK based upon corneal thickness and epithelial health.

LASEK / Epi-LASIK / E-LASIK

LASEK (laser epithelial keratomileusis) is a relatively new procedure that is a modified form of PRK. LASEK is mostly for people with corneas that are too thin or too flat for LASIK. It reduces the chances of complication that occur when the flap created during LASIK is not of the ideal thickness or diameter.

LASEK procedure requires the surgeon to loosen the outer layer of the cornea, called the epithelium. Unlike PRK, the epithelium is not removed in LASEK. The surgeon instead makes an epithelial flap by using a mechanical blade. Epithelial flap is then folded back so that the laser can reshape the exposed cornea. After laser application, the surgeon replaces the epithelial flap over the corneal bed and places a soft contact lens on top. Visual recovery after LASEK is generally faster than in PRK but slower than in LASIK. Currently, the use of the Excimer laser for LASEK is not FDA-approved.

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