Laser Refractive Surgery:
From One Medical Student to Another
2. Anatomy
In order to see clearly, the cornea and lens must bend (refract) light rays to focus an image on the retina. The cornea is responsible for roughly 2/3 of the eye's total 60 diopters of refractive power. Unlike the lens, which is able to change its refractive power through accommodation, the cornea's curvature is fixed. The cornea is composed of 5 layers that provide a clear window and refractive power to the eye. These layers are shown in Figure 1. The stroma provides the cornea with structural shape and thus its refractive power. Laser vision correction removes stromal tissue in order to change the shape of the cornea, making it flatter or steeper.
Figure 1: The cornea has an average central thickness of 540 micrometers. Layers from anterior to posterior: the epithelium, Bowman's layer, stroma, Descemet's membrane, and endothelium. 
Laser vision correction is most commonly used to treat myopia (nearsightedness), hyperopia (farsightedness), and astigmatism. Figures 2, 3, and 4 illustrate these refractive errors. Depending on the refractive error, the cornea is made flatter or steeper to focus light on the retina.
Figure 2: In myopia, the eye is too long or the cornea is too steep. Distant objects appear blurry because images are focused in front of the retina. 
Figure 3: In hyperopia, the eye is too short or the cornea is too flat. As a generalization, close objects appear blurry because images are focused behind the retina.
Figure 4: An astigmatic eye has an unevenly shaped cornea, like a football shape instead of a basketball, causing both close and distant objects to appear blurry.
In addition to corneal refractive errors, surgeons must account for presbyopia in patients who are middle aged or older. Presbyopia is the inability to focus on objects at near range due to loss of accommodation.Younger patients can accommodate, meaning they can make their relatively soft lens "thicker" to focus at near and "thinner" for distance. Presbyopia is a normal condition that typically occurs after the age of 40 and is caused by decreased lens compliance over time. With aging, the lens loses the ability to accommodate to see at near.Presbyopic patients have two options for laser vision correction: (1) Correct both eyes for distance and use reading glasses for intermediate/near work or (2) Correct one eye for distance and one eye for near (i.e., monovision) to decrease the dependence on glasses.
