Understanding Refractive Disorders
------[ Your Prescription in Diopters ]
The History of Vision Correction Procedures
LASIK Laser
NEW! INTRALASE™ FS Laser
---and IntraLASIK® Software
The New VISX Star S4 ActiveTrak™ Laser
PRK - Photorefractive Keratectomy
Your Doctor's Role
Results - PRK vs. LASIK
Potential Risks





Understanding Refractive Disorders
Your Prescription in Diopters

Refractive disorders for myopia (nearsightedness), hyperopia (farsightedness) and astigmatism are measured in units called diopters. Diopters represent the amount of correction needed to provide 20/20, or normal vision. The greater the degree of nearsighted or farsighted vision, the higher your prescription in diopters. Measurements in negative diopters refers to nearsighted vision while measurements in positive diopters refers to farsighted vision.
Also see
Presbyopia

Diopter measurements are often referred to with the letter "D". An example, -5.00 D means the degree of nearsighted correction is 5.00 diopters of correction to obtain 20/20 vision.

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MYOPIA:

(Nearsightedness)

Nearly 70 million people (25% of the population) in North America are myopic. With myopia, the cornea or optical schematic is too steep and light rays are focused in front of the retina instead of directly on the retina, creating blurred vision. People who are nearsighted can see images clearly with near vision, but distance vision is blurred.

Nearly 90% of all myopic patients have corrections of less than -6.00 D, which is in the mild to moderate range. The severity of myopia is demonstrated below:


Mild
Moderate
Severe
Extreme
< -6.00 D
-6.00 D to -10.00 D
-10.00 D to -15.00 D
> 15.00 D

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HYPEROPIA:

(Farsightedness)

Another 62 million people (22% of the population) in North America are farsighted. With hyperopia, the cornea/optical schematic is too short and light rays entering the eye focus behind the retina. Farsighted people can see better at a distance, but have increased blurred vision at close range.

Mild
Moderate
Severe
Extreme
+1.00 D to +2.00 D
+2.00 D to +4.00 D
+4.00 D to +5.00 D
> +5.00 D

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ASTIGMATISM

Regular astigmatism occurs when light is focused at two focal points, both in front and behind the retina. This happens because the cornea is shaped more like a football than a basketball. People with astigmatism have difficulty seeing clearly with both near and far vision.

Astigmatism is also measured in diopters. Over 50% of myopic patients have mild astigmatism as well.


Mild
Moderate
Severe
Extreme
<1.00 D
1.00 to 2.00 D
2.00 to 3.00 D
> 3.00 D

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PRESBYOPIA

Presbyopia is a normal process of aging. This happens when the fibers attached to the lens lose power and then lose elasticity and flexibility to change from distance vision to near vision. Most people begin experiencing presbyopia after they turn 40 years of age. This problem is treated with reading glasses or bifocals. Presbyopia cannot be treated with laser or any other form of vision correction surgery.

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The History of Vision Correction Procedures

There has been interest in vision correction procedures dating back as early as the sixteenth century when Leonardo da Vinci contemplated the possible source of visual disturbances. A little later, in 1619, Scheiner measured the anterior surface of the cornea. His discoveries are still used by ophthalmologists today who recognize that refractive surgery often depends on changing the cornea's anterior contour. Even lens removal as a means of correcting high degrees of myopia was discussed by Boerhaave in 1746.

Johannes E. Purkinje observed in 1823 that images form on optical surfaces when they reflect external light. His observations led to the development of the Purkinje principles and the four Purkinje images. From these developments our modern-day understanding of keratometry (measuring the curvature of the cornea) and theories of visual accommodation began to grow.

Several decades later came the advent of topical anesthesia, which led to cataract surgery after the civil war. In 1867, with the development of the keratometer (instrument to measure curvature of the cornea), surgeons could measure astigmatism following cataract surgery. In 1869, Snellen (after whom vision charts of today are named) proposed using incision across the steep meridian of the cornea to flatten it and treat astigmatism. However, it wasn't until 1890 before Galezowski would actually attempt to flatten the corneal contour.

In 1895, Faber performed a full thickness corneal incision to decrease naturally occurring astigmatism in a 19 year-old patient, enabling him to pass his vision test for entrance into the Royal Military Academy. But all these efforts were focused on astigmatism, not myopia or hyperopia.

At this time, a Dutch physician, Leendert Jan Lans began to systematically study and define the principles of keratotomy (the making of incisions in the cornea) from 1895 through 1939. By varying the number, direction and shape of incisions on the cornea, Lans could manipulate the effects and tailor the visual correction. This period was considered the first trial and error for refractive surgery.

In Japan in 1936, Dr. Sato observed the flattening of the cornea in patients who had sustained traumatic injury to the eyes. His work led assistants to establish the value of radial keratotomy, built upon the principles outlined by Lans. Sato brought anterior and posterior keratotomy to clinical practices in hundreds of patients and reported the results in the 1940's.

Around 1949, Jose Barraquer in Columbia developed the idea of lamellar (pancake and flap based) corneal surgery to alter the shape of the cornea. He discovered that lamellar keratoplasty could flatten the cone of a keratoconus patient, significantly reducing myopia. He changed the cornea's shape by removing the anterior cornea (similar to today's corneal flap used in LASIK) with an instrument called a microkeratome. In 1987, a protege of Barraquer, Luis Ruiz of Columbia, modified the principles of microkeratome corneal resection by using an automated form of the instrument to perform the operation directly on the eye. This procedure, called automated lamellar keratoplasty (ALK) was used to correct high levels of myopia and was the first real breakthrough in the treatment of hyperopia.

By the mid-1970's, Russia's Dr. Fyodorov had determined that most of the radial keratotomy flattening effect could be obtained with sixteen or fewer incisions placed on the anterior cornea. He developed a system, that by varying the number of incisions and the amount of uncut clear central zones, permitted him to carefully control the degree of vision correction. It was he who convinced the world that RK surgery could indeed reduce or eliminate myopia.

RK surgery was introduced into the United States in 1978 by Leo Bores, one of Dr. Beitman's mentors. Since its introduction, over 2 million people have had RK surgery in the U.S. alone.

Limitations of RK prompted further research and development, leading to today's modern laser techniques. In the early 1980's, Bechman and Peyman and their associates used a carbon dioxide laser to create thermal shrinkage of the cornea in order to change corneal contour. In 1981, John Taboada reported that the argon-fluoride Excimer laser had the ability to indent eye tissue. Work was then proceeded on ablation (removal) of corneal tissue to flatten the cornea. Further evaluation was performed by Steve Trokel.

The first use of the Excimer laser on blind human eyes took place in 1985 by Seiler in Germany, followed by L'Esperance of the United States. The procedure was called photorefractive keratectomy or PRK. In 1991, Michelson and a group of elite ophthalmologists became the first five clinical investigators of the Excimer laser in the United States, utilizing the laser manufactured by Summit Technology.

By 1991, Pallikaris and Buratto and their associates had combined lamellar splitting (using the blade of a microkeratome to make a corneal flap) with Excimer laser ablation. It was Pallikaris who coined the term LASIK (laser insitu keratomileusis). The initial clinical trials of LASIK in the United States began in 1996. These clinical investigations culminated in the approval by the FDA of the LASIK procedure in 1999.

When compared to PRK, the results of LASIK appear to be better and healing is significantly faster, with fewer complications. As a result, LASIK has become the procedure of choice by vision correction surgeons throughout the world.

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