Correcting Eye Defects With Lasers Biology Essay

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In today's world, many people are suffering from eye defects that restrict their ability to see. These errors include myopia (stort-sigthedness), hyperopia (long-sigthtedness) and astigmatism. It is estimated that the global prevalence of refractive errors is between 800 million to 2.3 billion. Eye defects affect the ability of people to see clearly. To a myopic, distant objects appear blurred and fuzzy while to a hyperopic, focussing on a near object is a real challenge.

A possible solution- Laser-assisted In Situ Keratomileusis (LASIK)

LASIK is a combination between laser and surgical procedure. The basic principle of LASIK is the modification of the cornea. Covering one-sixth of the outer layer of the eye, the cornea is the primary as well as the most powerful focusing structure in the eye. Since two-third of the total refractive power of the eye comes from the cornea (the remaining is accounted by the secondary focusing structure, the natural crystalline lens), altering the cornea changes the refractive status of the eye. Alteration varies according to the type of refractive defect. This is usually involving ablation on the inner surface of the stroma (substantia propria) by laser.

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The corneal layers

Figure 2-The structure of cornea

http://www.e-sunbear.com/anatomy_02.html

In treating myopia, the steep cornea is flattened to lessen the refractive power. To correct hyperopia, the steepness of the cornea is increased in order to escalate the refractive power. For astigmatism patients, a normal vision is restored by flattening the steeper curvature of the cornea. Consequently, the corneal curvatures in two axes of 90 degrees are equalised, enabling the eye to focus the light more efficiently.

LASIK: In depth

The patient will undergo a pre-surgery evaluation exam. Wavefront technology is used to map the cornea to calculate the amount of tissues to be removed. Antibiotic is prescribed to reduce the risk of infection.

www.wellenfront.info/ E:\lasik\pics-LASIK\untitled.bmp

www.wellenfront.info/

Figure 3-the Wavefront

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Flap creation

http://www.eyesurgeryeducation.com.

First, the eye is draped and anaesthetic eye drops are applied to numb the eye. Water soluble ink is used to mark the cornea in order to guide later replacement of the flap (Figure 4). E:\lasik\pics-LASIK\step1.bmp

Figure 4

Figure 5The ocular surface is rinsed and the conjunctiva is moistened. Excess solution, mucus and debris are removed to preventing them from getting under the flap. A suction ring is placed to put appropriate pressure, stabilise the cornea and to hold the eye in place (Figure 5). E:\lasik\pics-LASIK\Picture3.pngE:\lasik\pics-LASIK\Picture1.png

http://www.fda.gov/cdrh/lasik/animation.htm

The thin flap is created from 20% of the thickness of the outermost cornea using a device called 'microkeratome'. Approximately 250 µm thick of cornea is left untouched (called postablation residual stromal bed) to avoid weakening of the corneal structure and progressive ectasia.

E:\lasik\pics-LASIK\Picture4.pngE:\lasik\pics-LASIK\Picture5.png

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E:\lasik\pics-LASIK\Picture7.png

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http://www.fda.gov/cdrh/lasik/animation.htm

Figure 6

Corneal flap is created, lifted and folded back to reveal the stromal bed(Figure 7).

Remodelling of the cornea

http://www.dcmsonline.org/jax-medicine/1998journals/february98/excimerlaser.htm The corneal centre or the stroma is modified using a special laser called an 'excimer laser'. Excimer laser is a 'cold laser' made from combination of an inert gas and a reactive gas which emits ultraviolet light.

Figure 8-Human hair etched by excimer laser. (Photo courtesy of IBM)

Figure 7-Chiron Technolas 217 excimer laserOne unique property of excimer laser is that it can etch living tissues without heating up or causing any thermal damage to the surrounding tissues. This is so since UV light produced is readily absorbed by biological and organic materials. excimer laser machine.jpg

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In LASIK, the process is called 'photoablation'. The high-energy UV light of excimer laser breaks the molecular bonds linking the proteins of the stromal layer as each photon of light has energy of 6.4 electron volts whereas the energy required to hold the corneal protein molecules together is only 3 electron volts. The removed tissues would efficiently disintegrate into the surroundings. The corneal stroma consists of elongated Type I and Type V collagen assembled in a lamellar array. Since no heat is generated, shrinkage of collagen does not occur. Besides that, it can photoablate microscopic amount of tissues in a highly precise manner. Usually, lasers are equipped with a tracking mechanism that detects the movement of the eye and ensures that the laser locks on the pupil. In less than 60 seconds, the UV light emitted from the excimer laser beam reshapes the stroma with an accuracy of up to 0.25microns. One cell (10 microns) needs to be pulsed 40 times in order to be completely photoablated.

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Next, the corneal flap is returned back to its original position. The flap is allowed to form bonding with the main cornea. As the flap naturally adheres to the main cornea during healing process, stitches are not required. (Figure 9 and 10)

http://www.fda.gov/cdrh/lasik/animation.htm

E:\lasik\pics-LASIK\Picture10.png

Figure 10

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http://www.eyesurgeryeducation.com.

Figure 9

Antibiotic and anti-inflammatory eye drop are is administered.

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Benefits & Risks

On November 2006, data of a 10-year of study was presented at The American Academy of Ophthalmology meeting in Las Vegas. The study which involved 100 eyes corrected by LASIK showed that ten years after the surgery, patients experienced lasting visual improvement. The average is 25/20, close to the normal reading of 20/20.

Earlier on January 2008, Jorge L. Alio, MD, PhD. with his colleagues reported that patients who had their refractive errors corrected by LASIK showed a low rate of complications and a high rate long-term satisfaction on the procedure. Dubbed as one of the largest and longest follow-ups ever, the researchers followed various patients who had undergone LASIK for low, moderate and chronic myopia for ten years. The team reported that only 20% of the patients with low and moderate short-sightedness required retreatment. Another study reported that 90 percent of people who had LASIK managed to achieve 20/20 to 20/40 vision.

Although patient satisfaction with LASIK seems typically high, it is important to note that in LASIK industry, success is measured as the uncorrected visual acuity under bright light. In fact, a huge number of satisfied patients reported having visual disturbances under low illumination and other symptoms of loss of quality of vision.

The possibility of developing flap complications is 3 in every 1000 patients who underwent LASIK with mechanical microkeratome. There were reports on infectious keratitis following LASIK. Some patients cannot undergo the second stage of LASIK due to flaps that are too small, irregular or with button-holes. Microkeratome blades are also blamed responsible for epithelium damage. To solve this, a new technology of creating corneal flap using femtosecond laser (Intralase) was promoted as a safer alternative. Intralase produces smoother, more even surface with smaller deviation from planned thickness compared to conventional microkeratome. Some studies illustrated that this method delivers better results. However, this technique is questionable as Intralase has been related to photosensitivity. Furthermore, flaps created by femtosecond laser are much more difficult to be lifted and this leads to higher chance of torn flaps. (342 words)

One of the most common side-effects of LASIK is eye-dryness. It is due to damage on the corneal nerves that involved tear production when the cornea is cut and ablated. A study by Hovanesian, Shah, and Maloney in 2001 discovered that 48% of LASIK patients complaint experiencing symptoms of dryness at least six months after the surgery. Following that, a study published by Mayo Clinic in 2004 by Calvillo et al demonstrated that after three years of LASIK, the corneal nerves density reduced to less than 60%. In 2006, De Paiva et al did a randomized clinical trial on 35 adult myopic patients, aged 24 to 54 years to study the incidence of dry eye and its risk factors after LASIK. They concluded that it is common for individuals with no history of dry eye to acquire the symptoms after LASIK surgery.

The cornea is not capable of healing completely after LASIK. In 2005, researchers from Emory University examined post-LASIK corneas and reported of finding permanent phatologic changes in all of them. The changes include undulation of Bowman's layer, spatial separation of the LASIK flap from the stromal bed, epithelial thickening over the wound margin, interface debris, and severed and severely disordered collagen fibrils. ( Kramer et al, 2005)

Figure 11-Photo of a post-LASIK eye taken using slit-lamp(bio-microscope) after staining with fluorescein dye. The circle is the flap edges which is stained green-yellow due to the fact that the interface is still open as the wound does not heal completely. Courtesy of Dr. Edward Boshnickdryeye3.jpg

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Ethical, Economical and Social Impacts

A study shows that most Internet Websites offering LASIK service do not provide sufficient information especially in terms of possible risks of complications. Of the 21 Web sites, 5(24%) had no information on complications. Of the 16 sites that had information on complications the author of the information was clearly identified in 5 (31%), the content was only referenced in 2 (12.5%), and evidence of the information having been updated was only seen in 2 (12.5%). (Fahey et al 2003) The quantity of information is generally minimal and the information itself is generally difficult to understand and locate. This, I believe is a violation of consumer's right as the public are being mislead into believing that LASIK is a risk-free and many may undergo the surgery without being made well-aware of the possible adverse effects that the procedure might inflict on them. Nevertheless, I am concerned that the number of sample is quite low and that the result of this study can be more statistically accurate if observation were made on larger number of websites.

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Individuals with post-LASIK complications find it hard to continue with their daily routines. FDA trial showed that six months after LASIK, 17.5% of patients report halos, 19.7% report glare (starbursts), 19.3% have night-driving problems and 21% complain of eye dryness. (Bailey et al, 2007). In most cases, the painful condition lasts for months, some even permanent. Patients may be financially affected as they are forced to purchase expensive eye-drops to alleviate the pain. These conditions, I concede, could potentially lower the patients' quality of life.

Alternatives to LASIK

Eye Glasses

Wearing spectacles is the most common way to correct refractive errors. Aside from being inexpensive, it does not pose any negative side effects as the eyes remain untouched. However, some people are concerned that glasses bring discomfort, limit the visual field and affect their physical appearance. Apart from that, it is impractical for certain activities such as swimming, mountain climbing and other rigorous sports activities. The lenses can become greasy or trap vapour when eating hot food, swimming, walking in rain or rapid temperature changes (such as walking into a warm building from cold temperatures outside), reducing visibility significantly. (Wikipedia, 2009)

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Contact lenses

Basically, contact lenses are similar to glasses; however, they are light and virtually invisible. However, since they are worn directly on the eyes, regular cleaning and care are important in order to protect the eyes. Improper management and excessive wearing of contact lenses can lead to damage to the eyes such as dry-eyes, irritation, light sensitivity and infections on the cornea and the eyelids. A study by Zuguo Liu et al. in 2000 concludes that "Long-term contact lens wear appears to decrease the entire corneal thickness and increase the corneal curvature and surface irregularity."

Evaluation of source materials

1. Article `LASIK de-Mythified' by W.H. Chua in Medical Grapevine.

Dr. Chua is a refractive surgery expert from Singapore National Eye Centre, a leading eye care centre in the Asia-Pacific. He is also an Adjunct Research Scientist at Singapore Eye Research Institute and a clinical teacher in Yong Loo Lin School of Medicine, National University of Singapore. The article is also still new (2008) and is published on a renowned medical magazine.

2. American Academy of Ophthalmology official website

AAO is the world's largest association of eye physicians and surgeons. It is an agency dedicated that conduct large-scale studies in ophthalmological field. Therefore I believe that information from this source is valid.

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Total: 2016 words