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General Explanation of Refractive Errors

The cornea and lens combine to focus visual images on the back of the eye. When the overall shape of the eye is incorrect or the curvature of the cornea is abnormal, visual images are out of focus. The cornea accounts for approximately 2/3 of the eye's focusing power. By surgically altering corneal curvature, most or all of the blur can be eliminated.

Laser In Situ Keratomileusis (LASIK), Laser Epithelial Keratomileusis (LASEK), and Photo-Refractive Keratectomy (LASIK) are techniques that utilize lasers to correct the curvature of the cornea.

    Refractive surgery refers to changing  of the course of the rays of light that enter the eye, and are then focused on the retina. Refraction simply means the "bending of light". The light rays are nearly parallel when entering the eye from a distance. Where  the light focuses in the eye (In front of the retina, behind the retina, or on the retina) determines what the refractive error is called:

Nearsightedness or Myopia

 If the light is focused in front of the retina, the refractive condition or error is called myopia or nearsightedness. This means that you can see things clearly up close, but are blurry in the distance

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Farsightedness or Hyperopia

        If the light could pass through the tissues of the retina, choroid, and sclera and be in focused behind the retina, it would be termed hyperopia or farsightedness. Farsightedness is not quite the opposite of nearsightedness. What it really means is that one has to change the shape of the lens in ones eye by thickening it to be able to have the image brought onto focus on the retina. When we are young, the lens is able to do that for us by just willing it to happen. When we get past the age of, roughly 40, the lenses in our eyes are becoming larger hard and less able to change the focus. That is when we need to start wearing glasses for distance and/or shortly thereafter, need a bifocal.

Astigmatism

Astigmatism is more complicated. It means that the light is not focused on the retina as a point or "stigma" in Latin, ("a" in Latin means "not" or "not a" and "astigmatism" ,therefore, means "not a point" focus) but is broken down into two focal lines rotated 90 degrees from each other and separated in space, one in front of the other. So, there are two focuses. Both focuses (foci to be perfectly correct) may be in front of the retina, one in front and one on the retina, one on the retina and one behind the retina, and both behind the retina, assuming the light could come to a focus behind the tissue of the eye which they obviously can't.  In glasses it is necessary to have the lens shaped like the curvatures of a football or teaspoon, rather than a symmetrically round surface like a tennis or base ball.

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Emmetropia

Emmetropia or normal vision means that the eye focuses the incoming light on the retina without the help of glasses or contact lenses.

What refractive surgery means is surgical (laser or conventional) configuring of the curvatures of the eye (cornea and/or lens) to allow the rays of light to be focused on the retina as a point. The means of doing this is by altering these surfaces.

 

The current methods of altering these surfaces are done by LASEK, LASIK, PRK, ICL ( Intraocular Contact Lens), Lens removal and/or replacement with an intraocular lens implant, and fine tuned with "Wave Front" technology. These procedures are explained by clicking on each for the link or just scroll down.

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In order to understand the difference between these two types of refractive surgery, one needs to know a little about the anatomy of the cornea. There are five layers to the cornea. On the outside is the epithelium, a soft five cells thick protective and lubricating layer. Below that is a clear layer called Bowman's membrane. Next is the stroma, an incredibly strong and firm, but clear layer. This is composed of strong protein cross linkages. Below that is another clear layer called Descemet's membrane. Finally, on the inside surface, the endothelium. This layer is responsible for keeping the cornea clear.

LASIK has its origin about thirty years ago and was originally developed to treat patients who had very poor vision due to corneal disease. It has evolved into a technique for the successful correction of refractive flaws. The current procedure, done on an outpatient basis, involves both the use of conventional and laser surgery to correct nearsightedness, farsightedness and astigmatism. The results are excellent: 95% of LASIK patients achieve 20/40 vision or better.

In LASIK, an eye drop anesthetic is used to numb the eye. The surgeon then uses an instrument to cut into and behind a layer of the cornea. A portion of the cornea is peeled back to create a flap and expose the inner portions of corneal tissue. The eye is then positioned under an Excimer laser which has been programmed to remove microscopic amounts of internal corneal tissue. Removal of the tissue changes the curvature of the cornea. If the patient is nearsighted, tissue closer to the central part of the cornea is removed to decrease the curvature or flatten the cornea. If a patient is farsighted, tissue in the periphery of the cornea is removed to increase curvature. To correct for astigmatism, selected tissue at certain angles is removed to insure that the cornea curves equally in all directions. After the laser has been used, the flap is returned to its original position. The cornea has extraordinary natural bonding qualities that allow the flap to stick down within 3 minutes. It, however, never heals or regains its original strength completely

 

The LASIK Keratome for cutting the Cornea, laser procedure, and flap replacement

     

Since only local anesthetic is used, patients remain awake during the procedure. The entire procedure takes only a few minutes. Improved vision is often possible on the day following the surgery. Eye drops and night protection are necessary for designated periods of time.

The Advantages Of LASIK Include:

The Disadvantages Of LASIK Include:

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In order to understand the difference between these two types of refractive surgery, one needs to know a little about the anatomy of the cornea. There are five layers to the cornea. On the outside is the epithelium, a soft five cells thick protective and lubricating layer. Below that is a clear layer called Bowman's membrane. Next is the stroma, an incredibly strong and firm, but clear layer. This is composed of strong protein cross linkages. Below that is another clear layer called Descemet's membrane. Finally, on the inside surface, the endothelium. This layer is responsible for keeping the cornea clear.

Photo-Refractive Keratectomy, or PRK, is another method of surgically reshaping the cornea using an Excimer laser. The difference between LASIK and PRK is that a deep corneal flap is not created in PRK. That is, the outer layer of the cornea remains in place and the laser removes tissue directly from this outer layer. PRK is used for low, moderate, and moderately high  amounts of nearsightedness.

It is a better treatment for hyperopia than LASIK.

Just as in LASIK, the laser treatment requires less than a minute. Unlike LASIK, the healing period is longer. Corneal curvature is modified so that the visual images are properly focused on the retina.

 

  

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In order to understand the difference between these two types of refractive surgery, one needs to know a little about the anatomy of the cornea. There are five layers to the cornea. On the outside is the epithelium, a soft five cells thick protective and lubricating layer. Below that is a clear layer called Bowman's membrane. Next is the stroma, an incredibly strong and firm, but clear layer. This is composed of strong protein cross linkages. Below that is another clear layer called Descemet's membrane. Finally, on the inside surface, the endothelium. This layer is responsible for keeping the cornea clear.

Photo-Refractive Keratectomy, or PRK,  and LASEK are other methods of surgically reshaping the cornea using an Excimer laser. The difference between LASEK and PRK is that a corneal stromal flap is not created. That is, the outer layer of the cornea, the epithelium, is removed (PRK) or slid back (LASEK) and the laser removes tissue directly from the surface of the stroma. PRK is used for low, moderate. and moderately high amounts of nearsightedness. In LASEK, the epithelial flap is reposited or replaced over the cornea.

Just as in LASIK, the laser treatment requires less than a minute. Unlike LASIK, the healing period is longer. The correction for nearsightedness, farsightedness and astigmatism is the same as in LASIK. Corneal curvature is modified so that the visual images are properly focused on the retina.

It is our opinion at this time (12/15/04) that LASEK or PRK is the procedure of choice for the best long term results.

 

        

The Advantages Of LASEK Include:

         No stromal flap - much fewer serious potential complications (Disseminated Lamellar Keratopathy, Interface infections, buttonholes, etc.)

         Minimal or no induction of optical aberrations

         Quicker recovery of corneal sensation and the blink reflex  (4 months vs. 1 1/2 years with LASIK)

         More corneal strength retained

         Less subject to trauma or injury

         Higher amounts of nearsightedness can be corrected

         Better WaveFront results - no damping from a stromal flap

         Better vision in the long term

         Allowed in Army Special Forces, Navy Seals, and Air Force Pilots

The Disadvantages Of LASEK Include:

         More discomfort

         Variable time, two to seven days before useful vision returns and sometimes longer.

Click to go to Wave Front Technology

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Dangerous Symptoms |  Laser Surgery |  Site Map | Meet Our Doctors  |  Map  | Forms | Photos 

 Home | Patient Education | Patient Instructions | Laser FAQ | Cataract FAQ | About Our Doctors
Pfoff Laser and Eye, 6881 South Yosemite Street | Centennial, Colorado 80112 | 303-588-7900|