A Functioning Eye (Emmetropia)The eye is the organ of sight. It is used in almost everything we do, from playing sport to reading. A normal and well functioning eye can focus objects and images, both near and far, perceive depth and adapt to changes in light. The perception of depth is due to having two separate eyes creating two separate images, while the ability to adapt to light change is attributed to the iris and the dilator muscles. However these two aspects of the eye are not involved in laser eye surgery and will not be discussed in reference.
The focusing of objects and images is very much part of laser eye surgery. In order for a clear visual image to be formed the image must come to a point on the retina. Light rays do not normally travel toward each other, usually the light rays either travel outwards or almost parallel, for this reason the light rays must be refracted. The cornea is the primary place of refraction, the bent light rays then travel through the aqueous humor and the pupil to the lens. Here the light is one again refracted even closer together, the light then goes through the vitreous humor and is projected onto the retina.
The focus of the lens should be aimed at the fovea centralis (a tiny pit in the middle of the macula). It is in this region that vision is most sharp. For this reason, instead of simply staring at one point the eye must constantly scan the area to focus the whole object or image. In order to accommodate the changing distances of the object or image the lens in the eye has to adjust, becoming thinner to focus distant objects and fatter to focus near by objects. This process, of changing the lens thickness is known as accommodation.
In order for this to occur the ciliary muscles contract and relax. The contraction fattens the lens and the relaxation stretches the lens. The eye functions on much the same principle as a camera. The iris, or coloured portion of the eye, acts as a shutter to regulate the amount of light admitted to the eye. The cornea (the clear window at the front of the eye) and the lens (located behind the pupil) serve to focus light rays from the object viewed onto the retina at the back of the eye. The retina then transmits the “picture” of the object viewed to the brain where the object is “seen”.
Clear vision is the result of light rays passing through the cornea, pupil and lens and focusing directly upon the retina. If the cornea is not round or it is too steep or too flat in relation to the length of the eye, light rays focus either in front of or behind the retina resulting in “refractive errors” such as nearsightedness (myopia), farsightedness (hyperopia) and astigmatism. Refractive surgery is a term for several procedures designed to treat these vision abnormalities. Eye DefectsMyopia – is the refractive condition where the farthest point of focus is located at a point near to the observer, and not at infinity.
When one is nearsighted, distance vision is blurred at all times while near vision is often excellent within a certain range. There are a number of explanations for this optical condition. The eyeball may be too long, causing the image to be focused short of the retina at the back of the eye. Or, the focusing lenses of the eye are too strong. The primary focusing lens is the cornea, the clear window at the very front of your eye. The internal lens, called the crystalline lens, is adjustable and alters your focus from distance to near. Sometimes, one of these two lenses may have a radius of curvature that is too steep.
In myopia, it is often the cornea that is too highly curved. It is this curvature which is altered in laser eye surgery. Shortening the eyeball has been tried, but it has not been without the potential of serious and permanent damage. Often in situations where the crystalline lens is forced into an excessive plus power curvature, myopia can result. This may occur from near vision stress, a spasm of the ciliary muscle, diet, medications, and even emotional fatigue. Hyperopia – The cornea and the lens work together to focus images from the visual world on the back of the eye (the retina).
If an image is out of focus, it is typically because the overall shape of the eye is incorrect or the cornea does not have the proper curvature. Farsightedness or hyperopia occurs when the eye is too small or the cornea is too flat. When this happens, visual images are focused behind the retina. A person with hyperopia is able to see objects at a distance, but has trouble with objects up close. Many people are not diagnosed with hyperopia without a complete eye exam. Astigmatism – is a vision condition that occurs when the front surface of your eye, the cornea, is slightly irregular in shape.
This irregular shape prevents light from focusing properly on the back of your eye, the retina. The light becomes unevenly refracted and falls short of the retina. As a result, your vision may be blurred at all distances. People with severe astigmatism will usually have blurred or distorted vision, while those with mild astigmatism may experience headaches, eyestrain, fatigue or blurred vision at certain distances. Most people have some degree of astigmatism. A comprehensive optometric examination will include testing to diagnose astigmatism and determine the degree.
Presbyopia – is a vision condition in which the crystalline lens of your eye loses its flexibility, which makes it difficult to focus on close up objects. Presbyopia may seem to occur suddenly, but the actual loss of flexibility takes place over a number of years. Presbyopia usually becomes noticeable in the early to mid-forties. Presbyopia is a natural part of the aging process of the eye. It is not a disease and it cannot be prevented. Some signs of presbyopia include the tendency to hold reading materials at arm’s length, blurred vision at normal reading distance and eye fatigue along with headaches when doing close work.
A comprehensive optometric examination will include testing for presbyopia. What is Laser Eye Surgery? Laser surgery reshapes the cornea, the clear window in front of the eye, which provides two-thirds of the focussing or refractive power of the eye (the lens located behind the iris and pupil provides one third). Shortsightedness is corrected by making the cornea flatter. Long-sightedness is corrected by making the corneal curve steeper while astigmatism is corrected by returning the cornea to a spherical shape. Excimer lasers were first developed by the computer industry for precisely etching microchips, are used.
The same cold ultraviolet beam with the assistance of computer technology accurately removes corneal tissue, altering its shape. To correct most vision problems the amount of corneal tissue removed is less than a third of the thickness of a human hair. History of the Excimer LaserThis recent development in the application of lasers to ophthalmology was the reshaping of the cornea in a procedure known as photorefractive keratectomy. The excimer laser itself, a device that often uses a mixture of argon and fluorine gases(other gases such as Xenon and Chlorine can also be used), was developed in the mid-1970s.
A number of researchers were involved in early work with the excimer laser and its application to ophthalmology. In the late 1970s and early 1980s, John Taboada found that corneal epithelium is extremely sensitive to the excimer’s 193-nanometer wavelength light. Also in the early 1980s, R. Srinivasan, an IBM researcher, was using an excimer laser to etch microscopic circuits in computer chips and discovered that it could also be used to cut and remove biological tissue, with extreme precision and most importantly without substantial thermal damage.
Stephen Trokel, of Columbia University, worked with Srinivasan and in 1983 published his work on applying this precision to create linear “excisions” in the cornea. How the Excimer Laser WorksIn the development of laser technology, excimer lasers operating in the ultra-violet may be considered as the third generation of industrial lasers. The short wavelength, or, in other words, the high photon energy of excimer lasers, leads to a wide range of new applications. Excimer stands for excited dimer, a diatomic molecule usually of an inert (noble) gas atom and a halide atom, which are bound in excited states only.
These diatomic molecules have very short lifetimes and separate releasing the excittion energy through UV photons. The Excimer Laser used is an Ultraviolet Light Laser. This light has a high degree of energy associated to it. This energy does not produce appreciable heat but is capable of breaking the bonds between the complex molecules that make up the corneal tissue. A powerful computer guides the Laser beam during the procedure to obtain the degree of accuracy required to remodel the cornea. An electrical discharge is used to bring energy into the laser.
The current required for laser operation is very high, so the discharge becomes inherently unstable after a certain time. The laser can therefore only operate in pulses. With a sophisticated electrical circuit the discharge can be kept stable for 400ns (nano-seconds). The corresponding optical pulse of 250 is very long compared to other lasers. The discharge is build up in three phases: 1) X-ray pre-ionisation provides a homogeneous distribution of electrons in the discharge area. 2) A high voltage pre-pulse with a very fast rise time multiplies these electrons in an avalanche process, providing a homogeneous ignition of the discharge.
Finally the main pulse gives the high current at moderate voltage to create the laser output. The energy per pulse is 1 Joule. With a repetition rate of 1 kHz this makes an average power of 1 kW. The efficiency of the laser is about 2 %, so 50 kW of electrical input power is required. To reach 1 kHz repetition rate a large flow loop, as in the drawing below, is required. 3) Between every shot the gas needs to be refreshed to start with homogeneous conditions. The used gas is turbulent and contains all kinds of charged particles. A centrifugal blower circulates the gas.
The heat exchanger removes the heat from the blower and the discharge. The settling chamber reduces turbulence and the contractor speeds up the gas flow to refresh the gas between the electrodes within 1 ms (micro-second). The diffuser converts the gas speed back into pressure and then the gas enters the blower again. Types of Laser Eye SurgeryPhotorefractive Keratectomy (PRK) – treats lower degrees of myopia and astigmatism problems. The procedure is done under what is known as topical anesthesia. Anesthetic eye drops are administered to numb the eye before the surgery begins.
The protective layer of the cornea, known as epithelium, is removed. Then, using the excimer laser, a precise amount of tissue is removed to correct the specific refractive error. The laser removes less than 1/1000 of a millimeter of tissue, less than the thickness of a human hair. For nearsighted people, tissue is removed from the central cornea, thus have the effect of flattening the cornea. For farsighted individuals, tissue is removed from the peripheral cornea a steepening effect on the cornea. For astigmatism, the curvature of the cornea is evened out by sculpting tissue along a specific area or axis.
A contact lens, known as a bandage contact is then put in place and will be removed three to four days later, after the epithelium has regenerated. Vision is generally blurry for several days after the surgery but improves after the contact is removed and gradually improves further over the next several weeks. The entire procedure usually takes about 20 to 25 minutes per eye, with a short recovery period after the surgery. Laser Assisted in Situ Keratomileusis (LASIK) – is a procedure used to correct nearsightedness, farsightedness, and astigmatism.
The operation is done under what is known as topical anesthesia. Anesthetic eye drops are administered to numb the eye before the surgery begins. With LASIK, the cornea is reshaped under a flap, thus changing the way the light entering your eye is focused. LASIK is able to correct to correct almost all degrees of refractive errors from low to high. LASIK offers the advantage of quicker visual recovery, because it is done under a flap. A specially designed instrument called a microkeratome is used to create a thin flap of corneal tissue. The flap is then deflected back to expose the underlying corneal tissue.
Then, using the excimer laser, a precise amount of tissue is removed to correct specific refractive error. The laser removes less than 1/1000 of a millimeter of tissue, less than the thickness of a human hair. For nearsighted individuals, tissue is removed from the central cornea, thus have the effect of flattening the cornea. For farsighted individuals, tissue is removed from the peripheral cornea, thus having the effect of steepening the cornea. For astigmatism, the curvature of the cornea is evened out by sculpting tissue along a specific area or axis. The corneal flap is then replaced and precisely aligned to its’ original position.
Because of the natural bonding process that occurs in the cornea, sutures are not required. Vision is usually much better immediately after surgery and gradually clears over the next 36 to 48 hours. The entire procedure usually takes about 10 to 15 minutes per eye, with a short recovery period after the surgery. Radial Keratectomy (RK) – is an incisional procedure used to correct mild to moderate amounts of myopia. A series of incisions are made with a diamond scalpel, preset to a precise measurement, starting near the center of the cornea and radiating out toward the periphery.
No incisions are made in the central cornea, thus allowing light to pass through a clear central window. Usually four to eight incisions are placed equally spaced around the cornea to a depth of 80% – 90% of the thickness of the cornea. The incisions allow the central cornea to flatten, thus reducing the amount of myopia. The number, depth, and length of the incisions determine the amount of correction that can be achieved with RK. With the advent of laser correction, RK is generally reserved for low levels of myopia or for people who for other reasons may not be candidates for laser surgery.
Astigmatic Keratectomy (AK) – is an incisional procedure used to correct mild to high amounts of regular astigmatism. Astigmatism occurs when the cornea is shaped like an oval, rather than a circle. The cornea with astigmatism typically has a steep curve in one quadrant and a flatter curve in the opposite quadrant. Incisions are made with a diamond scalpel, preset to a precise measurement, in the periphery of the cornea in the quadrant where the steepest curvature occurs. These incisions allow the cornea to relax and thus take on the shape of the flatter quadrant.
AK is often used in conjunction with both PRK and LASIK when a person has a high amount of astigmatism. The AK can be performed either prior to or after a laser procedure. Automated Lamellar Keratoplasty (ALK) – For those people with higher degrees of nearsightedness, there is another possible procedure for reshaping the cornea. In cases of moderate to severe nearsightedness, and even some cases of farsightedness, many leading ophthalmologists choose to perform Automated Lamellar Keratoplasty, or ALK. ALK is done with an instrument called a microkeratome.
The microkeratome is placed on the eye and the cap of the cornea is lifted and placed to the side. In nearsighted people, a microscopically thin section of the cornea is removed with the excimer laser. Then the cap of the cornea is placed back in position. It adheres quickly and seals itself without the use of stitches. In farsighted people the cap of the cornea is lifted, as it seals itself, the pressure within the eye pushed up on the cornea, steepening its curve. In either case, the result is a cornea that has been fine tuned to reduce the refractive error.