There’s a lot of marketing hype surrounding laser eye surgery these days. We feel this does patients a disservice and makes it difficult not only to choose the right surgeon, but also the right procedure. In this 2-part blog series we’ll clarify some concepts and leave out the hype, in an effort to clear up the confusion.
In order to understand the procedures, we first need to have a general understanding of the different types of eye problems. The easiest way to understand these basic eye problems is to learn a bit about optics, which is essentially the study of the behavior of light rays. Therefore, that’s the main topic of this blog post, the first in a 2-part series.
Optics 101 for Patients:
When you look at an object, light rays from that object enter your eye. In a normal sighted eye, those light rays come to a point focus on the retina which creates a sharp image.
A nearsighted eye is typically anatomically longer than a normal sighted eye. Therefore, when a nearsighted eye looks at that same object, the light rays from the object fall short of the retina producing a blurry image. This condition is corrected with minus (concave) lenses. There is no way to actually shorten the eye, so instead the excimer laser is used to remove microscopic amounts of tissue centrally, which flattens the cornea so that the light rays have a shorter distance to travel.
A farsighted eye is typically anatomically shorter than a normal sighted eye. Therefore, when a farsighted eye looks at that same object, the light rays from the object come to a point focus beyond (behind) the retina producing a blurry image. This condition is corrected with plus (convex) lenses. There is no way to actually elongate the eye so instead, the excimer laser is used to remove microscopic amounts of tissue in the periphery, which steepens the cornea so that the light rays have a longer distance to travel.
An eye with astigmatism has a cornea that is not uniform. Instead, there are steeper and flatter parts in certain areas, known as meridians, which are usually 90 degrees apart. The easiest way to conceptualize this is by utilizing an exaggerated analogy: Imagine an eye without astigmatism as a basketball – completely spherical, and imagine an eye with astigmatism as a football. When an astigmatic eye looks at that same object, light rays from the object come to a line of focus (instead of a point focus as in nearsightedness and farsightedness). This can produce an image that may appear elongated or blurry. This condition is corrected with cylindrical lenses in glasses and toric or hard contact lenses. The excimer laser is used to remove microscopic amounts of corneal tissue to create a more uniformly spherical corneal surface.
An eye with presbyopia is unable to accommodate to view near objects clearly. This usually occurs in patients beginning in their mid 40’s. Prior to age 40, when viewing close objects, the natural lens of the eye changes shape which allows for accommodation during close work. After age 40, the lens of the eye begins to lose its elasticity and the eye muscles become weaker, until eventually accommodation can no longer be accomplished, making close objects blurry. This condition is corrected with reading glasses, which are plus (convex) lenses. If you put a plus lens in front of a normal sighted eye, you are basically making that eye nearsighted. This is the concept behind monovision, which may be an option for some presbyopic patients. Read more about monovision laser vision correction in our Monovision blog.
In all of the above instances, when the cornea is precisely re-profiled during laser eye surgery, light rays are brought to a corrected focus on the retina. This produces a clearer image without the need for glasses or contact lenses.
Each patient has a unique visual system. There are several factors that determine which laser vision procedure is recommended for that particular visual aberration. The amount of prescription, type of corneal curvature, thickness of the cornea and medical health of the eye all play a role in this decision. Another factor is patient lifestyle. Therefore, the type of procedure should be chosen after a full medical eye exam and discussion between surgeon and patient.
Whether or not this procedure is recommended largely depends on the shape and thickness of the cornea. This is determined during the preliminary diagnostic tests done at the time of our free laser vision surgery evaluation. The topography (curvature) of the eye is determined with a corneal topographer and is again evaluated during retinoscopy. The thickness of the eye is determined either bypachymetry or using pentacam technology.
During LASIK, corneal tissue underneath the flap is reprofiled with the precision of the excimer laser, to the quarter micron. The average human cornea is approximately 490 – 600 microns thick. There is a certain amount of tissue that needs to be left on the corneal bed. The recommended amount is approximately 250 microns. For each diopter of power, approximately 12.5 microns of tissue is removed for nearsighted patients. Therefore, the higher the prescription the more microns of corneal tissue need to be removed. Therefore, thickness plays a big role in determining which procedure is best for the patient.
Post-operative discomfort differs between procedures. In LASIK, PRK and LASEK, the same excimer laser is used to re-profile the cornea. The difference between these procedures lies in the steps that precede the excimer laser. We will explore these differences, in detail, and other laser eye surgery methods in blog 2 of this 2-part series.