Articles

Clinical Update – LASIK Laser in-situ Keratomileusis

by Dr. Gary R. Tylock, MD
Dallas Medical Journal – January 2002

Ophthalmology is one of the fastest evolving fields in medicine. Never before have so many new techniques and scientific breakthroughs been brought before the public in such a short time. LASIK (laser in situ keratomileusis) has emerged as the procedure of choice to offer millions of people the opportunity to see without the use of glasses or contact lenses. In 2001 nearly 1 million people in the United States and millions worldwide had had LASIK surgery. Is this procedure safe? Why has it become so popular in such a short period of time?

Refractive Surgery

Refraction refers to the ability of the eye to refract (focus or bend) rays of light. In order to see clearly, light that enters the eye must be bent so it is focused on the retina. If the light is not bent properly and is focused instead in front of or behind the retina, then the eye will not see clearly, resulting in a refractive error. Refractive surgery is any surgical technique or procedure used to help correct such refractive error.

The front surface of the cornea is the main refractive structure of the eye. It behaves as a fixed focus lens, having approximately 75% of the refractive power of the eye. The slightest change in curvature of the cornea will cause dramatic changes in vision. Most LASIK surgeries remove less than the thickness of a human hair to achieve the end results.

Types of Refractive Errors

Nearsightedness (myopia) occurs when the curvature of the cornea is too steep for the length of the eye or the eye is functionally too long. As the light rays pass through the steep cornea, they are bent too sharply and come to a point of focus in front of the retina, resulting in a blurred image. Patients with nearsightedness are able to see near objects well, but have difficulty with distance vision.

Farsightedness (hyperopia) occurs when the eye is functionally too short or the curvature of the cornea is effectively too flat. The light rays that enter the eye are not bent sharply enough, causing them to come to a point of focus behind the retina. This produces a blurred image. Farsighted patients see distant objects more clearly than near objects, but often have difficulty with both, especially as they get older.

Astigmatism. Many people with nearsightedness or farsightedness have some degree of astigmatism. This means that the cornea, rather than being spherical like a basketball, is slightly oval and shaped more like half a football. People with astigmatism experience blurred vision and sometimes distortion or tilting of images because of unequal bending of the rays of light entering their eyes. Significant degrees of astigmatism will cause blurred vision for both distant and near objects. A high degree of astigmatism can be observed by looking at images distorted on the backside of a spoon.

History of Vision Correction

The ancient Chinese slept with sandbags on their eyes in attempts to correct their nearsightedness. Early in the 13th century, handheld magnifying lenses were used for reading; spectacles for correcting distance vision did not make an appearance until 1276 in Venice, Italy.

The first successful documented refractive surgery was performed in 1895. Faber, a Dutch ophthalmologist, performed a full-thickness transverse corneal incision (T 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.

In 1949, Barraquer described the principles of lamellar surgery. He changed the cornea’s shape by using an instrument called a microkeratome to remove the anterior cornea (the equivalent of today’s corneal flap), freezing it, and changing its shape with a mechanical cryolathe.

In the early 1970s, Fyodorov developed RK (radial keratotomy), where 16 or fewer radial corneal incisions were placed at approximately 90% depth from the anterior corneal surface. His system varied the number of incisions and the amount of central uncut clear cornea, permitting him to control the degree of visual correction.

An understudy, Leo Bores, introduced RK in the United States in 1978, and it soon became a subject of great interest and scientific scrutiny. In 1980, the National Institutes of Health sponsored the PERK (Prospective Evaluation of Radial Keratotomy) study that provided scientific data on RK performed in a standard manner in nine US centers. Although some controversy surrounding RK remained after the study was complete, it was the preferred procedure in the United States to correct moderate degrees of nearsightedness and astigmatism until 1995.

In 1987 Luis Ruiz, a protégé of Barraquer, modified the principles of microkeratome corneal resection using a modern automated form of the instrument to perform the operation in situ (directly on the eye). Ruiz developed a procedure called ALK (automated lamellar keratoplasty), which was used to correct higher levels of nearsightedness outside the range of RK. The corneal flap, attached by its hinge, now could be made with a much higher degree of safety and accuracy. After making the larger flap, a second pass of the microkeratome permanently removed a smaller diameter disc (refractive cut).

By 1990, the Greek ophthalmologist Pallikaris took the moderately successful ALK procedure and added the unparalleled accuracy of the excimer laser, dramatically improving the results. This new procedure, which he named LASIK, combined the automated lamellar microkeratome to make a corneal flap, followed by excimer laser ablation of the exposed corneal bed. LASIK and PRK (photo refractive keratectomy) quickly supplanted RK after FDA approval of the excimer laser in 1995.

LASIK and its Advantages

Before surgery, patients undergo a thorough exam to determine their candidacy. A most valuable tool is computerized corneal topography. Two types are available: curvature corneal topography and elevation corneal topography. These technologies measure the surface contour of the cornea, which is responsible for about 75% of the refractive power of the eye. Color digital pictures generated from these measurements demonstrate a patient’s degree of a sphere, astigmatism, or irregularities measured at approximately 8,000 points across the cornea. These maps allow the surgeon to see the details of a patient’s astigmatism and whether the astigmatism is symmetric, asymmetric, or irregular. The maps also can demonstrate incipient corneal diseases that may not otherwise be clinically detectable, such as early keratoconus.

Computerized corneal topography also is most helpful with enhancement surgery when the surgeon assesses patient complaints, features of the prior ablation, and the degree of residual sphere and astigmatism. Using information from these maps, the experienced surgeon can customize the patient’s treatment.

The day before surgery, patients are treated with a topical fluoroquinolone and given specific instructions for lid hygiene the night before and the day of surgery. Preoperatively, topical antibiotic and nonsteroidal anti-inflammatory drops are administered in the office. Just before the procedure, topical Proparacaine and Marcaine are given for anesthesia. Betadine is used to clean around lids, lashes, and periorbital areas.
After the patient is positioned correctly, the surgeon uses a joystick to move the surgical table to make fine adjustments in the X, Y, and Z axis for positioning and focusing purposes. A lid retractor is inserted to allow adequate exposure and to keep the patient from blinking. The patient is instructed to look up at a red flashing fixation light. This helps ensure accurate placement of laser pulses and provides optimal eye position throughout the procedure.

I use the Moria Carriazo Barraquer keratome, which allows a great deal of flexibility in making the size of the flap and allows the hinge placement at any location 360 degrees. I typically prefer hinge placement superiorly at 12 o’clock.

Most often a 9 mm flap is made for nearsighted corrections and a 9.5 mm flap for farsighted corrections. Nearsighted ablations flatten the cornea by removing more tissue centrally treating out to 8 mm. Farsighted ablations steepen the cornea by removing more tissue peripherally treating out to 9 mm. The flap thickness is approximately 160 microns as compared to an average cornea thickness of 520 microns centrally, the thinnest section of the cornea. Thickness increases gradually, moving out from the center into the periphery of the human cornea.

After the flap is made, it is folded back, exposing the LASIK bed underneath (Figure 1). Once centered and activated, the VISX S3 ActiveTrack uses infrared cameras to track any eye movement in three dimensions. This ensures accurate placement of laser pulses despite any small sudden eye movements. If the eye moves outside 1.5 mm in the XY axis or 2.0 mm in the Z axis (up-down), the tracking device automatically will stop the laser. This happens infrequently, and the tracking device can be easily reset if needed.

The precision of the 193nm wavelength excimer laser makes it uniquely suited for refractive corneal surgery. Each pulse of the laser removes or vaporizes (ablation) 0.25 microns of tissue as it breaks carbon to carbon bonds. For example, it removes approximately 1/200 of the thickness of a human hair, 1/28 of a red blood cell, or 1/39 millionth of an inch in 4 billionths of a second. With little or no collateral tissue damage (confined to 0.25 microns), this laser is at least 50 to 1000 times more accurate than any other ophthalmic laser available (Figure 2). This allows the surgeon to sculpt the cornea, as a lens, into a more desirable shape, gently and precisely and allows rays of light to focus properly on the retina.

At the completion of ablation treatment, the flap is replaced close to its normal anatomic location. Balanced salt irrigation is used to float the flap back in position while cleaning the interface. A special technique is used to dry the flap back into position and test the adhesion at completion. Antibiotic and nonsteroidal anti-inflammatory medications are administered at the completion of surgery. In some patients, a bandage contact lens may be used overnight. The patient is checked using a slit lamp biomicroscopy before leaving the operating room. Patients are monitored in the office for 30 to 40 minutes, given postoperative instructions, and have their eyes examined again before discharge. All patients are seen one-day post-op, 3 to 6 days, then typically one month, 3 months, 6 months, and at one year.

LASIK avoids the potential haze, pain, and healing factors generally associated with PRK surface ablation. This result is achieved because the laser is applied only within the corneal substance, rather than removing a large area of an epithelium. When the epithelium is removed during PRK, the nerve endings are exposed. These exposed nerve endings are the most sensitive and cause pain during recovery. Additionally, more fibroblasts are underneath the epithelium, which contribute to scarring or haze formation. The manner in which the epithelium grows back, filling the epithelial defect created at the time of the PRK procedure, can vary among patients. These healing differences, and occasional environmental factors may affect the outcome and accuracy of the PRK procedure.

With LASIK, the epithelium remains almost entirely intact. As a result, the nerve endings stay covered and pain is minimal during recovery. With the epithelium intact and healed within 12 hours after the procedure, the risk of infection and hazing is much lower. LASIK overall is more accurate because it eliminates the surface healing variability of the PRK procedure.

Unlike PRK, treating under the flap with the excimer laser in LASIK is like working in a sanctuary. The eye does not recognize that anything happened to it. Eyes after LASIK are typically still, comfortable, and see well the next day.

Enhancement procedures to correct an over- or under response to LASIK can be performed when a patient’s vision is stable – generally 3 months or later after the original treatment. Specially designed instruments can be used to lift and release the original flap created during the LASIK procedure. The postoperative course is similar to the original procedure.

LASIK Candidates

Candidates must be at least 18 years old and have stable refraction. They should have no uncontrolled eye disease. Patients should not be pregnant or nursing. Patients with a significant cataract should not undergo LASIK vision correction. It is important that patients have reasonable expectations of the procedure and understand the eye’s natural limitations, such as presbyopia.

Various excimer lasers have different ranges of correction approved by the FDA. The VISX S3 laser has the widest overall treatment range approved by the FDA for nearsightedness, farsightedness, and astigmatism.

Nearsighted patients can have up to 12 diopters of spherical myopia and up to 6 diopters of myopic astigmatism corrected.

Farsighted patients can have up to 6 diopters of spherical hyperopia and up to 6 diopters of hyperopic astigmatism.

Presbyopia

Presbyopia is part of the normal aging process. It occurs as the protein composition of the lens changes, making it larger, harder, and less flexible. As the lens loses its ability to flex, it can no longer bend light rays as sharply, and the ability to focus on near objects is impaired. The onset of presbyopia typically occurs between age 40 and 50, and continues to worsen through age 60. When this occurs, people who already wear glasses may need bifocals, and those who have never worn glasses may require reading glasses.

The excimer laser has no effect on the eye’s focusing muscles or on the lens, so it cannot treat pure presbyopia. Thus, if a patient needs glasses for reading, refractive surgery usually is not likely to help. Additionally, if a patient is mildly nearsighted and in his 40s, he may notice that although he cannot read clearly with his glasses on, he can read well without them. One advantage of mild nearsightedness is the ability to remove the glasses after presbyopia sets in and still be able to read.

If a patient is nearsighted and obtains excellent distance vision after undergoing laser vision correction, he typically will become normal-sighted. This means that he will lose the ability to read near without glasses after presbyopia begins. Like other normal-sighted people, these patients will require reading glasses for small print, generally in their 40s.

Monovision

Refers to the “undercorrection” of one eye in a nearsighted patient or the “overcorrection” of one eye in a farsighted patient. The technique involves correcting the dominant eye for distance and the nondominant eye for near vision, thereby reducing the need for reading glasses. When both eyes are functioning together, the brain naturally “looks” through the eye that is clearer. So, for distance vision, the brain sees mostly with the dominant eye, and for near vision, it sees mostly with the nondominant eye.

Although monovision works well for some presbyopic patients, most patients older than 40 usually prefer to correct both eyes for distance. A trial of monovision using contact lenses often is quite helpful. A patient who has been successful with monovision using contact lenses also will continue to be so with LASIK. In the event a patient tried LASIK monovision and did not like it, additional laser correction can be performed to equalize both eyes.

Prevention and Management of Risks and Complications

Patients who are well-informed, have realistic expectations and take a preventive approach to manage their controllable risk factors before LASIK have the quickest recovery and are the most satisfied.

When considering LASIK vision correction surgery, it is important to understand the potential risks and side effects. The most risk in LASIK is related to the re-adhesion of the corneal flap and dealing with the temporary aggravation of dry eye side effects.

Dry eye, common among preoperative LASIK patients, is the most frequent risk factor patients have that can protract and interfere with recovery after the LASIK procedure. The drops used during surgery, as well as the surgery itself, have a temporary drying effect on the eye. This emphasizes the need to identify dry eye patients and to have this condition under control before surgery.

As people age, their eyes become drier regardless of whether they have had LASIK. By far the most common reason for failure in contact lenses is dry eyes, and often these people look to LASIK as an alternative. But a significant number of patients are unaware their eyes are dry.

Physicians should routinely assess the quality of the ocular surface and the degree of dryness in each patient. When needed, a specific plan to control patients’ dryness preoperatively is established. Some patients may need to postpone their LASIK procedure while they get their dry eyes under control.

If dryness exists before surgery, or if dry eye symptoms persist after surgery despite the frequent use of artificial tears, blocking the lower nasal lacrimal drainage ducts with punctual plugs may be recommended. This brief procedure prevents the natural tears from draining away so quickly and results in improved lubrication of the surface of the eye.

It is common for patients to experience “grittiness” in the eye after LASIK. This condition tends to resolve itself over the first one to 3 months. In the meantime, adequate application of lubricating eye drops often will alleviate the symptoms.

Dry eye symptoms can be particularly noticeable if patients use the computer frequently, read for long periods of time, or drive extended distances. These activities exacerbate dry eyes because the concentration required naturally causes one to stare and not blink as often, regardless of whether they have had surgery. If patients’ eyes are dry, it is important that they make a conscious effort to blink more while engaging in activities that require concentration, because the blink response is temporarily suppressed after LASIK surgery.

The most critical healing period for the flap is approximately the first 16 to 24 hours after LASIK surgery. Flap adhesion is still building during these early postoperative hours, and it may be susceptible to dislocation. A dry eye having less lubrication and greater friction is at greater risk of its flap shifting. Trauma or rubbing the eye in this immediate postoperative period also can cause the flap to shift The chance of a patient’s flap shifting postoperatively is about 1% but may vary depending on techniques used, postoperative protocol, patient compliance, and other factors. Ninety-nine percent of flap adherence or shifting problems are evident early the next day, fewer than 24 hours after the LASIK procedure. If the flap is in good position the first postoperative day, it has little chance of becoming dislodged later under normal conditions, even in dry eye patients.

If the flap shifts slightly, wrinkles (striae) may result. Striae present in the center of the cornea may distort vision and require smoothing. This is done by lifting the original flap, hydrating it, and smoothing out wrinkles. Striae are best handled when treated early.

Another complication that may occur is diffuse lamellar keratitis, a rare inflammatory reaction that leaves small white deposits underneath the corneal flap after LASIK. The cause of this reaction is not well understood. Depending on the amount of inflammation, there may be no symptoms, or some haziness may be noted in vision. Prevention requires maintaining a clean corneal interface. This is done by meticulous surgical detail and by thoroughly irrigating under the flap during the procedure, followed by examining the flap interface after the surgery. When present, most cases of DLK respond to treatment by steroid drops. More severe cases may require that the surgeon lift the corneal flap, then irrigate and clean beneath it.

A reduction in best-corrected vision can occur when patients develop diffuse lamellar keratitis, or persistent striae. These complications are rare and can lead to a reduction of two or more lines of vision in fewer than 1% of patients.

Although infection is the most feared complication, it is extremely rare. As with any surgery, it is avoided through sterile surgical technique and antibiotics. Patients can help prevent infection by making sure their eyes are clean and free of makeup before surgery. If a patient has an acute infection on the day of scheduled surgery, it should be postponed until the patient has recovered.

Conclusion

LASIK has propelled the realm of refractive surgery into the 21st century by providing patients with nearly instantaneous, virtually pain-free vision correction. LASIK brings unsurpassed outcomes and stability to refractive surgery. The results, although not guaranteed, are certain enough that the procedure has gained worldwide acceptance in the fields of ophthalmology and optometry, as well as with the general public.

In the United States, LASIK has been approved for treatment of nearsightedness, farsightedness, and astigmatism. The issue of presbyopia is looming, and anyone older than 40 knows the frustration of losing his near vision. New research may restore the near vision in the majority of aging patients. Methods to restore the focusing muscles of the eye are promising; one day, even reading glasses may be obsolete.