Laser Eye Surgery
We are Eva McElligott, Lila Farrag, and Teri Dombrowski. For our Physics 25 class with Professor Deardorff this semester, we had to select an area of technology and relate it to physics. Since we all have some sort of corrective eyeware, we decided to research something we'd all like to get done eventually--laser eye surgery!
Quick History:
Corrective eye surgery has been inpractice since the 1940s. Radialkeratotomy (RK) and photorefractive keratectomy (PRK) have had the greatestimpact on current methods of refractive eye surgery. The most common refractive surgery used todayis Laser in Situ Keratomileusis (LASIK).RK uses a diamond tipped knife to flatten the central cornea by making aseries of incisions radiating from the cornea.RK can only treat low degrees of myopia and astigmatism, unlikeLASIK. PRK uses a laser to correct therefractive area by removing corneal tissue.PRK can be discomforting or even painful because it removes nerve fibersthat run through the corneal epithelium.After this surgery, it can take up to six months for vision tostabilize. Immediately following thesurgery, vision is blurry until the corneal epithelium healed, and the bandagelens removed. PRK is similar to LASIKbecause it uses a laser to correct the corneal defects. LASIK has the same benefits, but with fewer,less-sever side effects than PRK. Also,LASIK is less painful, and recovery time is shortened because the cornealepithelium is not removed. The LASIKprocedure folds back the corneal epithelium and then a laser is used to reshapethe cornea.
The first lasers used were broad beamlasers (about 6 mm) that acted similar to a showerhead, splitting the mainlaser into smaller beams. Current lasertechnology implements the flying spot laser, named to describe the techniqueof creating a pattern with the laser, so that the full width of the flying spotbeam hits the cornea with each pulse.Advantages of this type of laser include a smoother corneal shape, andincreased accuracy and efficiency of the surgery.
Reference: Pictures taken from Bausch & Lomb http://www.bausch.com/us/vision/concerns/surgery/history.jsp
The Role of Physics in Laser Eye Surgery:
The deficiency of the eye can be examined using the physics of refraction. Refraction occurs mostly at the cornea due to the substantial difference between the index of refraction of the cornea, and that of air. The cornea and the lens combine to focus the light at the retina. The eye adjusts the curvature of the lens in order to alter the focal length, enabling it to focus objects at varying distances. A normal eye is able to converge light properly by changing the lens shape so that objects both near and far appear clearly.
In nearsightedness,the cornea is overly curved resulting in a greater index of refraction. The light converges, and the image forms infront of the retina. In order to correctthis problem with laser eye surgery the procedure begins with a surgeon temporarilypeeling back the epithelium. A laser isthen trained on the eye to remove small amounts of the cornea. The cornea is flattened in order to correctnearsightedness. The flattened corneahas a lower index of refraction than previously, focusing the image on theretina instead of in front of the retina.
Reference: Pictures taken from Physics at work http://kineticbooks.com/physics/17372/17405/sp.html
Another dysfunction of the eye is farsightedness. In farsightedness, the cornea is not curvedenough, and thus, the refractive index is lower. This causes the images focal point to belocated behind the retina. The surgeoncorrects this condition by using a laser to remove the outer edge of thecornea, thus steepening it. As a resultof this surgery, the eye is able to focus the image directly on the retina, dueto the greater index of refraction resulting from the curvature of the eyebeing increased.
Reference: Pictures taken from Physics at work http://kineticbooks.com/physics/17372/17405/sp.html
The type of laser used for these procedures is aultraviolet excimer laser, this type of laser usesand unstable inert gas with halogen, argon and fluorine to create an intense UVbeam. A list of approved lasers used forLASIK surgery is linked below.
http://www.lasikeyesurgery.com/approved-lasik.asp
Advantages, Disadvantages, and Risks:
Laser eye surgery is one of the fastest methods to correct visualimpairments. It has the ability to correct even severe refractive errors, butminimize the recovery time. Most patients are able to return to work within aweek! Many patients even return to normal activities the day after surgery. Thetreatment is permanent according to studies done so far, but as eyes changewith age re-treatment may be necessary over time. Eye drop medication isnecessary for the first week, but discomfort wears off after 5-6 hours. Themedication is used to prevent infection, not as a pain suppressant. The maindisadvantage of LASIK surgery is that it requires the creation of a flap in thecornea. This step takes less than a minute to complete, but there are risks ofcomplications.
As with any surgery, there are risks associated with theprocedure. The best way to avoid certain risks is to have a thoroughexamination with your doctor pre-surgery. Some of these risks of surgeryinclude:
Corneal infection (less than 0.1% withLASIK)
Undercorrection/overcorrection
Decrease in best corrected vision
Corneal haze, though risk is less withLASIK than PRK
Regression, can be re-treated to correct
Halo effect in dim light
Flap damage or loss, when hinged part ofthe cornea is removed completely
Distorted flap when the corneal flap healsirregularly
What to Expect:
The first step tohaving laser eye surgery is a medical examination with your doctor to determineyour prescription, look for any retinal problems or unusual intraocularpressure, and look for signs of eye disease. A corneal topographer is used tomake a map of your cornea to help the surgeon determine how to correct theirregularities. Before you go in forsurgery, be sure to not wear any contact lenses for a week because that changesthe shape of the cornea. Also dont use makeup or lotions around the eyes.
During surgery,which lasts approximately 30 minutes, you will lie on your back and numbingdrops are placed in your eye. A lid speculum holds the eye open as it iscleaned and high pressures are applied to create suction around the cornea. Thedoctor uses a microkeratome, a blade, to cut a flapinto the cornea. Then the suction ring is removed. The flap is folded back anda laser positioned over your eye. The doctor will ask you to fixate on a pointof light, then use a laser to remove the cornealtissue. Pulses are heard as the computer programmed laser removes a fixedamount of tissue determined by pre-operation measurements. A shield is placedover the eye to prevent damage to the eye or movement of the flap, as nostitches are used. The pressure from the eye will cause the flap to resealagainst the eye and it will heal naturally.
Reference: Eye Clinic, P.C. LASIK http://www.eyeclinicpc.com/lasik/lasik.htm
After surgery,there may be slight discomfort, which can typically be removed by a mild overthe counter pain reliever. These symptoms should disappear within the first fewdays. A follow up appointment is absolutely necessary the day after surgery toensure the flap has not been dislodged and is in the appropriate place. After that, check-ups on regular intervals for only six months.Vision may fluctuate during the months after surgery but should stabilizewithin three to six months.
Who should get laser eye surgery?:
Laser eye surgery is ideal for anyone who wants the freedom thatimproved eyesight can give. The hassles of glasses and corrective lenses can beremoved, often permanently, by the procedure. But, laser eye surgery is not foreveryone. Before undergoing surgery, all patients should have a completemedical examination with their doctor. People with persistent medicalconditions, have difficulties healing, or have vascular, autoimmune, or eyediseases may not besuitable for the procedure. Pregnant women should also refrain from surgery dueto changes in the eyes refraction during pregnancy.
Typical patientsfor eye surgery are at least over 18, though the age preference changes withthe type of laser used. Patients have mild to moderate nearsightedness,farsightedness, and astigmatisms, within the range of correction for thetreatment. Each laser has different ranges for the procedure performed.
Other References:
Bausch & Lomb http://www.bausch.com
Kinetic Books http://kineticbooks.com
Lasik Eye Surgery http://www.lasikeyesurgery.com
By: Eva McElligott, Lila Farrag& Teri Dombrowski
29 April 2005
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