Definition of Lasik Vision Correction
Creating the Flap
If you wear glasses or contacts, you are either nearsighted, farsighted, or have astigmatism. This is called a refractive error. There are a number of different surgical procedures used to correct these refractive errors; these procedures are called refractive surgeries.
There are two types of refractive surgery: surgical procedures that involve actually cutting the cornea and the more common laser procedures which include:
- Laser Assisted in-Situ Keratomileusis, or LASIK/IntraLASIK
- Custom Wavefront LASIK
- Photorefractive Keratectomy, or PRK
- Laser Assisted sub Epithelial Keratomileusis, or LASEK
- Custom, Transepithelial, One-Step, No Touch Ablation, or cTEN
By far the most common laser refractive surgery procedure is LASIK, and there are several reasons for this:
- LASIK is the most predictable of the laser procedures because the excimer laser does most of the work.
- LASIK also has the best immediate and long term results.
- LASIK isn’t painful during or after the procedure.
- The cornea heals quickly.
There are several different variations of the original LASIK procedure, but the premise is the same. A portion of corneal tissue is removed so that light rays are focused directly onto the retina, correcting the patient’s need for glasses. In order to understand the LASIK procedure better, we need to know a bit about the anatomy of the cornea, which is the part of the eye on which LASIK is focused.
The cornea is, essentially, the eye’s window. It provides 70 percent of its focusing power and also prevents the eye from drying out by triggering tear production. The cornea is also very sensitive due to the high concentration of nerve endings in the tissue.
The cornea is comprised of five layers:
- The Epithelium – the first layer of the cornea is very thin and heals very quickly. This is the layer that is cut to make the flap during LASIK because of its ability to regenerate which prevents scarring.
- Bowman’s Membrane – the function of this second layer of the cornea is unclear. It is acellular and is made up of primarily collagen fibrils.
- The Stroma – the third layer of the cornea is also the thickest layer. The stroma gives the cornea its shape and stability. The stroma does not regenerate. For this reason, and because of its thickness, the Stroma is the target of LASIK procedures.
- Descemet’s Membrane – this fourth layer is very elastic because it is made up of type III collagen. If cut, this layer will retract, and it has no regenerative properties. It is the basement layer of epithelial cells for the cornea.
- The Endothelium – the last layer of the cornea maintains corneal deturgescence (relative dehydration that maintains corneal clarity) and is comprised of about 500,000 cells.
Only three of these five layers are manipulated during the LASIK procedure: the epithelium, Bowman’s membrane and the stroma.
The Layers of the Cornea
The LASIK Surgical Procedure
LASIK is actually a very simple procedure that typically only takes 10 to 15 minutes. The first part of the LASIK procedure involves cutting a “flap.” Using topical anesthesia, a surgical blade, known as a microkeratome, is used to cut through the epithelium, Bowman’s membrane and a portion of the stroma. A hinge is created by leaving a portion of the flap uncut.
After it is created, the physician lifts the flap and folds it over, exposing the stroma beneath. An excimer laser is then used to reshape this tissue in a specific pattern, which is determined by an algorithm. The patient’s refraction and corneal thickness is used as the variables in the algorithm and the laser does the rest.
Once the laser treatment is finished, the flap is folded back into place. Because of its ability to heal quickly, the corneal flap is generally sealed within 24 hours. However, a physician can still lift the flap without the use of the microkeratome for up to a year later. This is actually beneficial to the patient because it makes “touch-ups” much easier to perform.
Variations on the LASIK Procedure
There are several different variations of the LASIK procedure described above. These variations involve different ways of creating the flap; the laser portion of the procedure generally remains the same.
One of the most common of these is LASEK. Instead of using a microkeratome to cut the flap, an alcohol solution is used to loosen the epithelium which is then folded over creating the hinge.
Custom Wavefront LASIK is the one procedure that alters the pattern of the laser treatment itself. It is a more precise form of treatment that corrects not only the patients need for glasses, but also any higher order aberrations in the cornea that typically cause glare at night and other distortions.
Custom Wavefront Mapping
Another variation, commonly called Intralase or IntraLASIK, involves creating the flap with a femtosecond laser, which emits an optical pulse in extremely short bursts (approximately one-quadrillionth of a second). This prevents the transfer of heat and allows for an extremely precise incision.
Epi-LASIK involves cutting a flap from the epithelium (the first layer of the cornea) only. Epi-LASEK is essentially the same procedure, only instead of a microkeratome, the alcohol solution is used.
Full Length Femtosecond LASIK Procedure
Risks and Complications of LASIK
Like cataract surgery (or any eye surgery for that matter), LASIK has its risks and benefits. Although it is a very safe, routine procedure, there are some common complications that can occur including:
- Dry Eye Syndrome induced by the desensitization of the cornea caused by making the flap.
- Subconjunctival hemorrhage, or broken blood vessels on the sclera (the white part of the eye), are one of the more frequent complications of LASIK. They are the result of the suction used to stabilize the eye as the microkeratome cuts the flap. They do not affect vision, and are not painful. There is no treatment; the patient just has to wait for the body to reabsorb the blood which typically takes one to two weeks.
- Refractive errors may be over or under corrected resulting in the need for more surgery.
- Visual clarity will fluctuate for the first six months; however, there are instances when the patient’s vision will fluctuate constantly.
- Double vision or ghost images can be caused from an improperly positioned flap, an error in the measurements of the patient’s refractive error or decentered ablation of the underlying stroma.
- Halos or starbursts around lights, and glare at night that makes driving difficult can be a complication of the standard LASIK procedure; however, it is a rare occurrence with Custom Wavefront LASIK.
- Wrinkles in the flap, known as striae, can result from improper flap positioning. These wrinkles can cause visual distortions.
- Debris trapped under the flap, although rare, can cause visual distortion depending on where the debris is trapped. Debris in the periphery of the cornea is less likely to cause distortions than debris in the center which would be in the direct line of sight.
- Posterior vitreous detachment is a common complication. When the flap is cut, the intraocular pressure (pressure inside the eye) rises to above 80mmhg. This can cause the vitreous to detach from the retina, it can also cause a retinal detachment or macular hole.
- Dermatochalasis, or excess skin on the upper lid that hangs down over the front edge of the eyelid, can be caused by the lid speculum used to hold the eye open during the procedure. Although this is very rare, and often dependent on the patient’s age, it is a possibility.
- Recurrent corneal, or RCE, erosion can be caused an improperly cut flap. RCE is a disorder of the cornea that is caused by the upper lid sloughing off bits of the corneal epithelium with each blink. Often times, this occurs when the flap is too thin and essentially crumbles during cutting. This can also cause permanent distortions in the vision. The femtosecond laser and LASEK have made RCE a very rare complication.
Recurrent Corneal Erosion
Benefits of LASIK
For most people, the benefits of LASIK outweigh the possibility of complications. Many patients who undergo refractive surgery have been wearing glasses since childhood and the prospect of living without the need for corrective lenses is worth the risks.
Despite the possibility of complications, 98 percent of all LASIK procedures are successful in providing patients with useful vision (defined as 20/40 or better) without the need for glasses or contacts.
Is LASIK Right for You?
LASIK isn’t right for everyone. Over the past 20 years as a technician, I’ve found there is a certain type of patient that should avoid LASIK surgery, and that is the perfectionist. Typically, these people are meticulous in nature, such as engineers or biochemists.
Artists are also less likely to be happy with the results of their procedure. These are people who need their vision to be absolutely perfect, and there is no guarantee that LASIK or any other refractive procedure can provide perfect clarity, which makes them poor candidates for the procedure.
If you are a perfectionist by nature, you may want to think twice about LASIK or refractive surgery in general. On the other hand, if the ability to see at distance without glasses is more important than having crystal clear vision, you’re the perfect candidate.
Your prescription is also a determining factor. A patient with very little prescription really doesn’t need refractive surgery. However, someone who is legally blind without glasses would definitely benefit from these procedures.
Patients who are extremely nearsighted may not be able to have LASIK because their corneas may be too thin. Although with the advent of IntraLASIK, those with thinner corneas now have a better chance of having a successful procedure.
Your ophthalmologist will be able to tell you how likely you will be to achieve the outcome you desire based on the results of your pre-op testing. It’s important to talk to your ophthalmologist in depth to determine if LASIK or any other refractive procedure is right for you.
Questions & Answers
© 2013 Melissa Flagg OSC