What are refractive errors or problems?

Light is focussed or bent (refracted) by the cornea, the clear front "window" of the eye, and the lens of the eye onto the back inner wall (retina) of the eye. The retina is the inner layer of the eye that senses light prior to its being sent to the brain. Your vision is clear if the cornea and lens combine to focus the image of an object exactly onto the retina. If this is not the case, you are said to have a refractive error in your eye.

• Nearsightedness (myopia)
  
  When the cornea and lens combination has too great a focussing power, they focus the image in front of the retina, within the eye, causing your vision to be blurry. This is known as myopia, or nearsightedness. A nearsighted individual typically sees blurry in the distance but can see clearly if the object is held close to the eye.
  
  
• Farsightedness (hyperopia)
  
  When the cornea and lens combination has too little focussing power, they direct the image behind the retina and eye, causing your vision to be blurry. This is known as hyperopia or farsightedness. A farsighted individual typically sees blurry both at distance and at near.
  
  
• Astigmatism
  
  If the shape of the surface of the cornea is not spherical or round but has flatter and steeper curvatures (more like the surface of an American football than a baseball) the image formed is clearer at some angles and more blurred at others. This is called astigmatism. An eye with astigmatism may also have nearsightedness or farsightedness.
  
  
• Presbyopia
  
  A special condition, sometimes confused with farsightedness, that appears in normal individuals about the age of forty whereby reading glasses become necessary to read. The distance vision remains unchanged.
  
  
  

Correction of refractive errors

The underlying theory in optical correction and refractive surgery is simple. If the curvature of the surface of the cornea is changed, the image will be focussed at a new location. Flattening the surface of the cornea moves the image back onto the retina, correcting nearsightedness. Making the cornea more curved, or steeper, brings the image forward and corrects farsightedness. The surgery is performed asymmetrically to correct astigmatism. Currently, the surgical correction of presbyopia (the need for reading glasses) is experimental and not yet available. Spectacles and contact lenses work by the same physical principle except that glass or plastic lenses, rather than corneal shape, are used to displace the image as desired.


What is refractive eye surgery?

If an individual has a refractive error, spectacles or contact lenses can be used to shift the image formed onto the retina, thus providing clear vision as long as the eye has no disease. Over the past 50 years, doctors around the world have conceived, developed and perfected various forms of eye surgery intended to reduce dependence on glasses and contact lenses. A pioneer in this area was Dr. J. I. Barraquer, of Colombia. He developed the basic procedures that are in principle still used today. Dr. Swinger studied with Dr. Barraquer in 1977 and brought his techniques to the USA for the first time. Since then, they have undergone further development, being combined with laser technology, and they have now become widely popular around the world.

Over the years, many refractive surgical techniques have been formed. Until this decade, they were all performed by mechanical means. A popular technique was radial keratotomy (RK), which makes a series of slit-like incisions in the cornea. Other techniques, known as lamellar surgery, included keratomileusis and ALK.


Laser refractive surgery

In laser refractive surgery the excimer laser performs the corneal modification, rather than mechanical means. The process of laser modification of the cornea is called ablation, or sculpting. The excimer laser achieves extremely precise removal of microscopic amounts of corneal tissue, at the indicated location, to achieve correction of the patient’s refractive error. A sophisticated computer program controls the laser beam. The laser beam may be applied directly to the front surface of the cornea (Photorefractive Keratectomy, or PRK) or onto an inner surface following creation of an anterior flap of corneal tissue using a device called a microkeratome (LASIK).



Comparison of PRK & LASIK

	PRK	LASIK
Range of Correction	Low to moderate	Low to severe
Wound Depth	Superficial	20% deep
Intraoperative Pain	None	None
Post-operative Discomfort	Minimal, 12-24 hours	Minimal, 12 hours
Postoperative Medications	2 to 4 months	4 days
Functional Vision Recovery	3 to 5 days	24 hours
Return to Work	1–2 days	1 day
Risk of Complications	Low (less surgeon dependent)	Low (more surgeon dependent)
Risk of Corneal Scarring	1 ~ 2 %	1 %
Patient Satisfaction	High	Very High