Refraction of Light Through the Eye
Most refraction in the eye occurs when the light rays travel from the objects within your visual field to form a clear image inside the eyes. When the visual system receives the light rays, possesses them into electrical impulses and sends them to the brain, you can see the objects.
Factors Affecting the Change in Refractive Power of the Eye
The eye's ability to refract or focus light sharply on the retina primarily is based on three eye anatomical features: the curvature of the cornea, the power of the lens inside the eye and the size (overall length) of the eye. If these eye anatomical factors have their necessary features, which lead to bend light correctly, you can have a normal vision without glasses. Otherwise, refractive errors occur, which include: myopia, hyperopia, astigmatism, and presbyopia. A diopter is an important unit of measurement in optometry that represents the amount of focusing power the lens of your eye requires to achieve proper vision quality. The more nearsighted or farsighted you are, the higher your prescription in diopters.
What is 10/10 eyesight?
Patients' eyes are measured with a panel containing different sizes of the letter E in different directions. Normal vision is 10/10. That means you will see a person at a distance of 6 meters in the row of 10/10. Seeing 5/10 means that you see the maximum of this row at a distance of 6 meters, while a natural person sees this row at a distance of 12 meters. In other words, a person with 5/10 eyesight has half of a normal vision.
There are three numbers in your eyeglasses prescription: for example, -5-1.5*180 . The first number indicates the level of hyperopia or myopia. The front sign ( minus or plus signs) represents a myopia (minus sign) or a hyperopia (plus sign). The second number (1.5) represents the level of your astigmatism. This number may be written by a plus or minus sign. The third number (180) shows the axis of astigmatism. For example, 180 represents the horizontal axis of astigmatism. Therefore, the number -5-1.5*180 means that the patient has moderate myopia and moderate astigmatism in the horizontal axis.
Types of Refractive Errors
In this disorder, due to the longer anterior-posterior diameter of the eyeball or an increase in the corneal curvature, the image of the objects is formed in front of the retina, and you are in trouble with viewing distant objects. This disorder, which is often hereditary, is usually diagnosed between the ages of 8 and 12. During puberty, with increasing growth, its severity increases, and remains almost constant at 20 to 40. Myopia, especially high myopia, increases the risk of retinal detachment.
In the disorder due to the short anterior-posterior diameter of the eyeball or the reduction of the corneal curvature, the image of the objects is formed behind the retina. For a person with hyperopia both of the near and distance objects are blurred, but the near objects look more blurred than distance objects.This disorder, which like myopia is often hereditary, is exacerbated by increasing age and reducing the lens adaptation ability.
At an early age, the lens of the eye is very flexible, and to look near and distance objects its diameter varies easily. After the age of 40, the lens becomes tighter and loses its ability to accommodation, it makes the person unable to read and do things that require near vision. This is called presbyopia, which is considered a natural state in the absence of other diseases. But it may be accompanied by other disorders such as myopia, hyperopia, or astigmatism.
Astigmatism is a very common eye disorder, and most people have some degree of it. Severe astigmatism is often inherited. The cornea is smooth and round and its curvature is the same in all its axes. Astigmatism usually occurs when the cornea surface has a non-symmetrical curvature. In this case, due to the non-symmetrical refraction of the light waves in different parts of the cornea, the images do not focus exactly on the retina, and the images of both distant and near objects are more blurred than normal. In this disorder, instead of a spherical shape, the cornea is similar to a section of the baseball, which causes the image of the objects to form on two separate points on the retina. Refractive errors correction methods • Glasses Using glasses is the easiest way to correct refractive errors. Besides protecting your eyes from the sunlight, the eyeglasses lens can be easily replaced by a new one with different score. For the correction of presbyopia, bifocal glasses can be used, in which the lower part is used for reading.
• Contact lenses
Several contact lenses are now available. Choosing the right type of lens depends on the type of refractive error and the way of life and activities of the individual. Contact lenses and glasses focusing light on the retina, correct refractive errors.
• Eye exercises, Vitamins and Medical treatment
There is not enough scientific evidence that these methods are effective in correcting refractive errors.
Most refractive surgical procedures, by reshaping the cornea (transparent window in front of the eye) improve vision. Refractive corneal surgery corrects myopia by modifying the cornea's curvature to reduce its power and ensure the images form on the retina.The surgical procedure in reshaping the cornea curvature to correct hyperopia is the opposite of what is done to correct myopia.In astigmatism that the corneal curvature varies in one direction with its other directions, the surgery will correct corneal curvature in the astigmatism axis and curvature uniformity in all axes.
Common Refractive Laser Eye Surgeries
• Femto- LASIK
• Phakic IOL
LASIK is a combination of a microscopic surgery and excimer laser that is used to correct myopia, hyperopia, and astigmatism. At LASIK, first a very thin layer of the cornea (flap) is cut off by a special device called microkractome. Then the excimer laser is shifted to the bottom of the layer and again the flap is placed in its original position.The corneal flap sticks to the underlying corneal tissue within two to five minutes, and stitches are not needed.
It is one of the newest methods for the correction of myopia, hyperopia and astigmatism, and the latest in refractive error correction in which the cornea flap is cut off by using the femtosecond laser. From the point of view of surgical procedures, the femtosecond laser is very similar to LASIK. There is a fundamental difference between LASIK and femto-Lasik in how to create the corneal flap.In fact, femto-Lasik is the advanced LASIK method, in which the corneal flap is created by microkractome instead of laser, and is therefore also known as" all-laser LASIK"or bladeless surgery.Today femtosecond is one of the most popular refractive errors in Europe and the United States. Since femto-LASIK has all the benefits of LASIK, such as high speed visual acuity, rapid pain relief, and the possibility of aquick return to normal daily life, and in this technique a surgeon can do all the steps of the surgical procedure using laser, without the need for microkeratome, so femto-LASIK brings health and care more.
In this method to correct refractive errors the corneal epithelial layer (the outer corneal layer) is removed and reshaped by an excimer laser.
LASEK is a is a newly developed, modified PRK technique. In the LASEK method, the ophthalmologist uses an alcoholic solution to remove the epithelium of the cornea. The excimer laser is then transmitted to the cornea, and the epithelium layer is again placed in its initial position. This method now is less common and virtually is replaced by PRK.
Epi-LASIK, like PRK and LASEK, is a kind of the refractive corneal surgery. In this method, like LASEK, epithelium of the cornea is maintained at the end of the procedure, but in this method, instead of alcohol, a special blade called epikeratome is used to remove epithelium. This method has no particular advantage over PRK and is now almost obsolete.
Phakic Intra Ocular Lens (IOL)
Intraocular lens is used for people with high refractive errors or very thin corneas that their refractive errors can not be corrected with corneal laser surgery. These lenses are placed in different parts of the eye. The two types of lenses that are more commonly used nowadays are Artisan lenses and ICL lenses, which are first placed inside the anterior chamber and fixed with two clips on the iris fixer. The ICL lens is placed in the posterior chamber and on the natural lens of the patient.
What is the most appropriate method of refractive surgery?
Prior to determining the appropriate method, a complete eye examination should be done to detect such factors as refractive errors, corneal thickness and corneal topography and ensure the health of the eye. Then, with regard to age, occupation, physical condition, history of illness and other information, you can decide on the best choice. Your ophthalmologist can determine the most appropriate surgical procedure for you.
Who are the best candidates for refractive surgery?
• People who prefer not to wear eyeglasses or contact lenses
• People that their eye prescription(eye-glasses or contact lens numbers) have not changed for at least one year (changes of their eyeglasses prescription are less than (0.5-0.0 / 25 diopters))
• People who are older than 18 years old (Although people under 18 are not banned from undergoing refractive surgery,usually refractive laser surgery is not recommended for those under 18 because eyes tend to keep changing in to early adulthood, and the person in the following years may need another refractive surgery)
Some Important Points
• More than 95% of people who undergo refractive surgery can be successful in eye examinations for driver licenses without glasses.
• You may have to wear eyeglasses or contact lenses after surgery to see perfectly, this depends on your preoperative eyeglasses prescription, the topography and the thickness of the cornea.
• Surgery can not prevent the development of the presbyopia process. As a result, after 40 to 45 years of age you will need one pair of reading glasses for reading, writing or doing your close-up work.