eMedicine Specialties > Ophthalmology > Refractive Disorders

Myopia, LASIK: Follow-up

Author: Michael Taravella, MD, Director of Cornea and Refractive Surgery, Rocky Mountain Lions Eye Institute; Professor, Department of Ophthalmology, University of Colorado School of Medicine
Coauthor(s): Timothy A Perozek, MD, Consulting Ophthalmologist, Private Practice, Perozek Professional Corporation and Westfield Eye Center; Scott A Thomas MD, Staff Physician, Department of Ophthalmology, University of Colorado, Rocky Mountains Lions Eye Institute
Contributor Information and Disclosures

Updated: May 11, 2009

Outcome and Prognosis

Analyzing and comparing outcomes from refractive procedures can be a complicated and frustrating process. Compounding the problem is lack of standardization in the way results are reported.

Clinicians need to be familiar and to look for certain parameters when outcomes are presented in journals or presentations. These parameters include the range of refractive error treated, the percentage of patients achieving 20/20 and 20/40 vision or better (efficacy), the percentage of patients within ±0.50 D and ±1.00 D of the target refraction (predictability), and the percentage of patients losing 2 or more lines of best-corrected visual acuity (safety).

In general, LASIK results are better for patients with low myopia (between 1-6 D) and low astigmatism (<1 D). Stability has been reported to be good with little or no change noted in most patients between 3 months and 1 year postoperative. Other factors that can affect results include the type of laser and microkeratome used and surgeon experience. Table 2 summarizes LASIK results for conventional myopic treatments; Table 3 summarizes LASIK results for custom myopic treatments. The author has elected to present outcomes from the FDA clinical trials that led to the approval of these procedures; clinical results outside of tightly controlled investigational trials have generally mirrored the outcomes obtained in these trials. Published outcomes are provided in the References section.^{39,40,41,42,43,44,45}

Open table in new window

*: Only includes patients whose preoperative BSCVA was 20/20 or better

Open table in new window

Future and Controversies

One area of significant controversy revolves around the issue of what method is best to create the corneal flap. Two technologies are available: the femtosecond laser and the more traditional microkeratome technology.⁴⁶ As of 2008, both devices are being used in almost equal frequency for LASIK flap creation, according to Richard J Duffey, in a presentation, entitled Trends in Refractive Surgery in the United States: The 2008 ISRS/AAO Survey, at the Refractive Subspecialty Day.

The femtosecond laser is a solid-state laser that uses an infrared frequency of 1053 µm to create 3 µm spots adjacent to one another (Intralase, AMO, Inc, Santa Anna, CA). A flap is created by delivering multiple laser shots to a predetermined depth of the cornea. Photodisruption essentially creates microscopic connected perforations in one layer of the cornea. Advantages appear to be a more predictable depth of treatment and an excellent safety profile.

Femtosecond flaps, unlike microkeratome flaps, tend to be uniform in thickness and not meniscus shaped (ie, thinner in the center). The edge of the flap is cut by the laser, is more vertical than that achieved with the microkeratome, and allows for a good fit of the flap over the stromal bed with minimal leeway for sliding. The laser also allows for precise customization of flap diameter and hinge location. This may be especially useful for hyperopic treatment, which generally requires larger optical zones than myopic treatments.^47,48

Several reports have shown improvement in the predictability of refractive outcome and less induction of higher order optical aberrations when the flap is made by a femtosecond laser.⁴⁹ However, this has not translated to improvement in uncorrected visual acuity. The flaps made with the laser are more difficult to lift initially, and occasional moderate inflammation and pain in the cornea were reported when the device was first introduced. This latter problem seems to have been addressed with changes in the amount of total energy delivered to the cornea and the pattern in which it is delivered (Raster pattern).

Other complications unique to femtosecond laser technology include epithelial gas breakthrough, transient light sensitivity (TLS), and opaque bubble layer (OBL). Epithelial gas breakthrough occurs when gas bubbles created during photodisruption break through the dissection plane into the subepithelial space.⁵⁰ It is thought to be related to thin flaps (<90 µm), improper docking, or focal breaks in the Bowman layer. Management includes aborting the procedure and rescheduling either for a PRK or for an attempt at creating a femtosecond flap deeper in the cornea. This is essentially the femtosecond equivalent of a buttonhole, but it is usually not associated with permanent corneal changes or scarring. Breakthrough of gas into the anterior chamber has also been reported. Again, no permanent corneal changes occur, and management includes aborting the procedure and rescheduling.

Microkeratome technology has also advanced considerably, improving the safety, precision, and ease of use of these devices as well. An example of such improvements can be seen with the Amadeus microkeratome (Zeimer, Inc) and the Moria one-use plus (Moria, Inc, Doylestown, PA). Both microkeratomes can be used with no on-eye assembly, making them easier to use for novice surgeons.

The Amadeus has a very sophisticated computerized interface so that the surgeon can vary the advance speed and the blade rpm frequency; important factors in determining flap thickness. The suction pump for the Amadeus also adjusts automatically for ambient air pressure, which may be important when working at elevation.

The Moria features a disposable head. This means a technician does not need to insert a blade, eliminating another potential source of error. A disposable head may decrease the risk of DLK since the flap interface will not be exposed to the cleaning chemical solutions used on disposable units prior to sterilization.

A suction break during flap creation has starkly different outcomes with the 2 devices.

With the femtosecond laser (Intralase), a docking ring is attached to the eye with relatively low suction. This docking ring couples the femtosecond laser to the cornea, ensuring proper depth and centration of the flap. If a suction break occurs, the device can usually be reattached and the flap completed with no adverse consequences if the laser pattern has not reached the visual axis. If the laser has reached the visual axis, the patient can be rescheduled; no permanent corneal changes appear to occur.

When a suction break occurs during a microkeratome pass, an incomplete or thin and irregular flap can occur. Management usually consists of replacing the flap as best as possible and aborting the procedure. Corneal scarring can result. Depending on how the cornea heals, the flap may be recut in 3-6 months instead of scheduling a transepithelial PRK with mitomycin C within several weeks in the event of irregular surface healing. Back-up suction pumps on the Moria and the Amadeus microkeratome are examples of technological improvements in microkeratomes that help prevent loss of suction when compared to earlier devices.

The differences in LASIK outcomes between the 2 types of devices appear to be small. The laser is expensive, and the cost versus complication profile may ultimately determine whether or not this technology becomes widely adapted.

Another important technological development has been the use of tracking devices to follow eye movements during surgery. Important considerations in tracking include the speed of saccadic and nonvoluntary eye movements that occur during fixation versus the speed of response of the tracking device used. The response time requires 2 components: recognition that the target being tracked has moved (the eye) with a subsequent shift of the targeting mechanism of the laser to follow the movement. Two types of trackers are available for current laser platforms: video tracking and laser radar. Video tracking is based on real-time video images of the pupil.

The VISX, Technolas, and Wavelight Allegretto system use this type of tracking technology. Laser radar is used exclusively by the Alcon LADARVision excimer laser platform and requires pupil dilation to work properly. This method relies on tracking the edge of a dilated pupil. Four beams of light are projected onto the eye at the pupil margin. Eye movement is detected from changes in reflected light.

Both methods have limitations. Video tracking of the center of the pupil generally works well; however, the center of the pupil will change as the pupil dilates or constricts, introducing one source of error. There is a lag time between video detection of movement of the pupil, computer processing of the images of the pupil, and movement of the targeting mirrors to adjust for the new target location. This lag time is variable and is dependent also on the repetition rate of the laser; the faster the shot pattern is delivered the faster the response of the system to target movement and acquisition must be.

Tracking the dilated pupil margin also has some limitation. Occasionally, pupils will not dilate enough to allow for the tracker to engage, and other circular objects, such as the edge of an intraocular lens, can be mistaken for the boundary of the iris by the tracker. Custom wavefront guided procedures require even greater precision of tracking eye movements and must include monitoring of cyclotorsion.

At present, both systems appear to work well clinically for conventional and wavefront guided laser ablations.

Surgery may be performed bilaterally or unilaterally. Advantages of unilateral surgery include the potential for increased safety, and, perhaps, better predictability because the surgery algorithm can be adjusted for the second eye based on the results of the first eye. Advantages of bilateral surgery are mostly economic and include convenience for the patient and the surgeon in terms of time off of work, scheduling surgery, and postoperative visits.

To date, several studies addressing this issue have not shown increased risk of serious complications associated with bilateral surgery.^51,52 In addition, unilateral surgery is associated with a minimal increase in predictability. Most surgeons performing LASIK today offer their patients the option of bilateral surgery.

Long-term effects of LASIK on the cornea may occur. Because this procedure is relatively new, the long-term effects cannot be determined satisfactorily.

Of particular concern is the ability to identify patients at risk of developing progressive ectasia and central corneal thinning (see Complications). In an attempt to prevent the biomechanical weakening of the cornea and to combine some of the best features of PRK and LASIK, much attention has been given to the sub-Bowman keratomileusis (SBK) procedure. In this procedure, the LASIK flap is purposely made very thin, on the order of 100 µm, to avoid cutting and ablating into the deeper layers of the cornea. Unlike traditional PRK, visual recovery is rapid. The risk of haze appears minimal.

Evidence suggests that if the flap and the ablation depth can be limited to the anterior one third of the cornea, improved biomechanical properties of the cornea can be maintained, similar to those seen with PRK.^53,54 Evidence also suggests that less induction of higher order optical aberrations may occur compared to thicker flaps and deeper ablations.⁵⁵ SBK can be performed either with a femtosecond laser or with special microkeratomes designed for thin-flap LASIK.

Determination of the better method is under research, and studies of this technique have only just begun. Its disadvantages may include the difficulty in handling thin flaps, the difficulty in lifting these flaps for subsequent enhancement, and, in the author’s opinion, the observation that thin flaps may be more prone to striae. At present, long-term stability (>1 y) of standard LASIK appears to be good. The late development of ectasia is still a concern, and patients who have progressive myopic changes following LASIK must be evaluated for this possibility with serial topography and pachymetry. Topographers capable of mapping the posterior corneal surface (Orbscan) have proven useful in detecting this problem.⁵⁶

LASIK may affect not only the quantity but also the quality of vision. Contrast sensitivity and glare testing studies comparing preoperative refraction, postoperative corneal curvature, and scotopic pupil size will be helpful in defining patient selection criteria and improving outcomes and patient satisfaction.

• Search for CME/CE on This Topic »

• Myopia, Intracorneal Rings (Ophthalmology)
• Myopia, Phakic IOL (Ophthalmology)
• Myopia, Clear Lens Extraction (Ophthalmology)
• Astigmatism, LASIK (Ophthalmology)

• Vision Correction Surgery Introduction
• Eye and Vision Center
• Vision Correction Surgery Preparation

• Implantable Lens Provides Predictable, Stable Correction of Moderate to High Myopia for 4 Years
• Long-Term Study Shows Laser Surface Ablation for Myopia Is Safe in Thin Corneas
• FDA Launches Study on Quality-of-Life Outcomes After LASIK Surgery

• Management of Choroidal Neovascularization in Myopic Macular Degeneration
• Intravitreous Bevacizumab to Treat Subfoveal Choroidal Neovascularization in Highly Myopic Eyes: Short-Term Results
• Intracorneal Ring Segments for Keratoconus

• Cataract and Refractive Surgery Resource Center