Refractive Surgery
Patrick T. Yang MD
Louis E. Probst MD
Clara C. Chan MD, FRCSC, FACS
Refractive surgery has at least two unique characteristics when compared with the other subspecialties of ophthalmology. The preoperative and postoperative refractive evaluations allow calculations of efficacy, predictability, and stability that provide for detailed analysis and comparison. Refractive surgery is a purely elective procedure that the patients continuously evaluate visually for the rest of their lives. This means that the success of refractive surgery is entirely based on the results; patients pleased with the outcomes will refer others, whereas those displeased will not. Understandably, refractive surgeons and laser providers focus equally on the outcomes to achieve the expectations of the patients.
With the achievement of perfect uncorrected vision, success in refractive surgery has become an obsession for surgeons, equipment manufacturers, and patients. There are many studies and comparisons available to evaluate the various refractive procedures.
Options for Refractive Surgery
Before discussing the results of the various studies on refractive surgery, it is first necessary to provide a brief outline of each of the procedures available in the armamentarium of the refractive surgeon and the indications for each procedure (see Fig. 5.1). The reader should note that the indications for each refractive procedure are rapidly changing as new technologies become available and replace other procedures.
LASIK and photorefractive keratectomy (PRK) remain the mainstay of the armamentarium of refractive surgery. For PRK, the ideal treatment for maximum spherical myopia is -6.0 D (extended range up to -10.0 D). For LASIK, the ideal treatment for maximum myopia treatment is now -10.0 D (extended range -12.0 D). Other factors that influence the amount of correction include corneal thickness, flap thickness, pupil size, and the amount of ocular aberrations. Mitomycin-C (MMC) is used intraoperatively with higher myopic PRK,1 and after more than a decade, MMC has been found to be effective when used for the prevention and treatment of corneal haze.1,2 The limits of the hyperopic corrections have been reduced because of regression and disturbances in night vision associated with the smaller postoperative hyperopic optical zones noted with corrections over +3.0 D spherical equivalent.
Cochrane reviews of PRK versus LASIK for myopia and hyperopia have been conducted. For myopia, the effectiveness of these two procedures is comparable, but LASIK gives faster visual recovery than PRK.3 For hyperopia, no robust, reliable conclusions could be reached, but the nonrandomized trials reviewed resulted in comparable efficacy for either procedure.4 High-quality, well-planned open randomized control trials are needed in order to obtain a more robust base of clinical evidence.
Custom wavefront PRK and LASIK are now being performed for the same refractive range as conventional LASIK. Currently, wavefront treatments are available for up to -11.0 D of myopia and -4.0 D of astigmatism. Custom wavefront procedures have not consistently demonstrated superiority in terms of visual acuity and low aberration outcomes when
compared with conventional LASIK and PRK; however, the induction of higher order aberrations is reduced.5
compared with conventional LASIK and PRK; however, the induction of higher order aberrations is reduced.5
 FIGURE 5.1 The options for refractive surgery demonstrate the ideal and extended ranges for treatment of the various refractive options for myopia, hyperopia, and astigmatism. The indication for each of these procedures is constantly changing as more experience is gained and other options become available. |
Laser epithelial keratomileusis (LASEK) is a hybrid of PRK and LASIK. LASEK utilizes an epithelial flap created by exposing the cornea to ethanol. Proponents of LASEK believe that it reduces the risk of intraoperative flap complications and preserves posterior corneal stroma. Critics are concerned about the slow visual recovery and the risks of corneal haze. However, numerous reported studies have shown quicker visual recovery and reduced postoperative pain levels after LASEK than after PRK.6 A recent meta-analysis of studies involving LASEK versus PRK concluded that LASEK-treated eyes had no significant benefits over PRK-treated ones with regard to clinical outcomes, but there was less corneal haze observed with LASEK-treated eyes at 1 to 3 months after surgery.7
Epi-LASIK is a variation of LASEK. For epi-LASIK, the epithelial flap is created with a modified microkeratome or femtosecond laser. Proponents state that the flaps created with Epi-LASIK heal faster and the results are comparable to LASIK, although no randomized clinical trials have been conducted to confirm these claims.
In 2005, it was estimated that nearly 70% of all LASIK patients in the United States chose to have the procedure performed with the femtosecond laser, if given the option.8 Early comparisons between the femtosecond laser and the microkeratome in LASIK flap creation showed that the femtosecond laser group had significantly more diffuse lamellar keratitis postoperatively and the microkeratome
group had significantly more epithelial defects intraoperatively.9 Earlier femtosecond lasers required higher total energy to cut a flap.10 Morphologic alterations in the corneal stroma produced by the currently available models of the IntraLase (Abbott Medical Optics Inc., Santa Ana, CA) laser are comparable to those produced by mechanical microkeratomes.10 Advances have resulted in a reduction in the total amount of energy delivered by the laser when it cuts the flap, and there is a decrease in the inflammatory response associated with femtosecond flap formation to the point that it is indistinguishable from the microkeratome at the cellular level.10 Current models of the femtosecond laser, the 150 kHz IntraLase machine, can create a flap in about 10 seconds. This is half the time that was required by the previous 60 kHz system.
group had significantly more epithelial defects intraoperatively.9 Earlier femtosecond lasers required higher total energy to cut a flap.10 Morphologic alterations in the corneal stroma produced by the currently available models of the IntraLase (Abbott Medical Optics Inc., Santa Ana, CA) laser are comparable to those produced by mechanical microkeratomes.10 Advances have resulted in a reduction in the total amount of energy delivered by the laser when it cuts the flap, and there is a decrease in the inflammatory response associated with femtosecond flap formation to the point that it is indistinguishable from the microkeratome at the cellular level.10 Current models of the femtosecond laser, the 150 kHz IntraLase machine, can create a flap in about 10 seconds. This is half the time that was required by the previous 60 kHz system.
The improved results of faster visual recovery and uncorrected visual acuity (UCVA) and enhanced safety profile, including less post-LASIK dry eye, have led to most surgeons choosing the femtosecond laser for flap creation.11,12,13 A systematic review concluded that while LASIK with the IntraLase femtosecond laser may offer limited benefit over LASIK with microkeratomes in regard to safety and efficacy, it has advantages in predictability of target refraction and flap thickness.14
Conductive keratoplasty (CK) has been approved by the U.S. Food and Drug Administration (FDA) for the treatment of hyperopia.15 The thermal corneal burns are applied with a radiofrequency probe down to about 90% of the corneal depth (500 µm). It is hoped that the deeper corneal penetration will help avoid the problems with regression associated with laser thermal keratoplasty (LTK).16,17 CK is no longer performed by most refractive surgeons; however, it is occasionally used to create “blended vision” in one eye for the correction of presbyopia.
Intracorneal rings (ICRs) or Intacs (Contact Addition Technology, Des Plaines, IL) are now rarely used to treat myopia but have been applied for specific situations to treat post-LASIK ectasia18 or keratoconus.19 While the initial ICR FDA studies were promising,20 the procedure never gained widespread acceptance because of the inability to treat astigmatism, difficulty in duplicating the initial FDA results, competition from LASIK, and the high explantation rate.
Phakic intraocular lenses (IOLs) and refractive lensectomy (RL) remain the main options for the correction of extreme ametropias. Advances in phakic IOLs include the Verisyse™ lens (Abbott Medical Optics Inc., Santa Ana, CA), also known as the Artisan® lens outside of the United States, that clips onto the iris stroma in the anterior chamber (Fig. 5.2). The U.S. FDA clinical trials have shown that the Verisyse phakic IOL provides excellent refractive outcomes, with endothelial cell loss within a mean of 5.0% over 3 years, or 1.8% per year, and few complications.21 Another phakic IOL that is currently awaiting FDA approval is the Alcon (Fort Worth, TX) angle-supported Acrysof CACHÉ lens (Fig. 5.3).22 The Visian ICL (implantable collamer lens, Staar Surgical, Monrovia, CA) is implanted behind the iris in the posterior chamber and was U.S. FDA approved in 2005 (Fig. 5.4).23,24
RL, also known as clear lens extraction (CLE) for myopia, has benefited from the availability of low diopter power IOLs; however, concerns about the increased risk of retinal detachment remain.25 RL for hyperopia has used pig-gyback IOLs for eyes requiring heavy corrections26; however, high-power customized foldable IOLs (e.g., up to 60.0 D CT Xtreme D, from Carl Zeiss Canada Ltd., Toronto, Ontario, not yet FDA approved) may make this less necessary in the future.
Multifocal, accommodating, and diffractive IOLs such as the ReZoom, previously Array (AMO, Santa Ana, CA), Crystalens, previously AT-45 (Eyeonics, Aliso Viejo, CA), and the ReSTOR IOL (Alcon Laboratories, Fort Worth, TX) have been under development for over two decades. Several modifications have been made to improve distance, intermediate, and near vision compared with their predecessors. Unfortunately, these modifications have also resulted in unwanted side effects such as glare and halos, decreased contrast sensitivity in multifocal lenses, and inconsistent nearvision results in accommodating IOLs. Careful patient selection is crucial for successful results.27
 FIGURE 5.2 Verisyse Artisan® (A) and Artiflex® (B) and (C) anterior chamber iris-claw lens (permission for figure reproduction granted from OPHTEC Inc.). |
Femtosecond laser cataract surgery has recently arisen in the forefront of refractive cataract surgery. There are four platforms approved by the FDA at the time of preparing this manuscript, including the LensAR (LensAR Inc., Winter Park, FL), LenSx (Alcon Laboratories, Fort Worth, TX), Catalys Precision Laser System (OptiMedica Corp., Sunnyvale, CA), and the VICTUS Laser System (Bausch & Lomb/Technolas
Perfect Vision, Munich, Germany). Various functions “depending on the machine” have received U.S. FDA approval (corneal incisions, astigmatic keratotomies, capsulorhexis, nucleus fragmentation or softening). The LenSx laser was the first to receive FDA approval and in initial studies, femtosecond laser lens fragmentation on grade 3 and 4 cataracts resulted in a 43% reduction in phacoemulsification power and a 51% decrease in effective phacoemulsification time.28 In the initial series of procedures performed on human eyes, femtosecond laser capsulotomy and lens fragmentation was complete with no operative complications.28 When compared with manual capsulorhexis for IOL implantation, femtosecond laser capsulotomy formation improved the predictability of the effective lens position.29 Much additional study is underway to determine if femtosecond laser-assisted cataract surgery will significantly improve the refractive outcomes of cataract surgery.
Perfect Vision, Munich, Germany). Various functions “depending on the machine” have received U.S. FDA approval (corneal incisions, astigmatic keratotomies, capsulorhexis, nucleus fragmentation or softening). The LenSx laser was the first to receive FDA approval and in initial studies, femtosecond laser lens fragmentation on grade 3 and 4 cataracts resulted in a 43% reduction in phacoemulsification power and a 51% decrease in effective phacoemulsification time.28 In the initial series of procedures performed on human eyes, femtosecond laser capsulotomy and lens fragmentation was complete with no operative complications.28 When compared with manual capsulorhexis for IOL implantation, femtosecond laser capsulotomy formation improved the predictability of the effective lens position.29 Much additional study is underway to determine if femtosecond laser-assisted cataract surgery will significantly improve the refractive outcomes of cataract surgery.
 FIGURE 5.3 Acrysof® Cachet™ angle-supported phakic intraocular lens (permission for figure reproduction granted from Alcon Inc.). |
 FIGURE 5.4 STAAR® Visian implantable collamer lens (permission to reproduce figure granted from STAAR surgical). |
Radial keratotomy (RK) is no longer performed,30 as other refractive procedures offer a more predictable and stable outcome. For RK, a diamond knife was used to create radial incisions in the cornea.
LTK is no longer performed for low hyperopia, and the bankruptcy of the laser manufacturer officially ended its tenure. For LTK, peripheral thermal burns were applied to the peripheral cornea for the correction of small degrees of hyperopia.
LASIK, PRK, LASEK, and epi-LASIK are the main methods for the treatment of astigmatism. Astigmatic keratotomy and limbal relaxing incisions are generally used in conjunction with
other intraocular procedures to partially reduce astigmatism. The limits for the treatment of astigmatism by PRK, LASEK, or LASIK have been expanded by utilizing the cross-cylinder ablation or bitoric ablation technique originally proposed by Vinciguerra et al.31 Toric pseudophakic and phakic IOLs are also available for the treatment of astigmatism associated with lens implantation.32
other intraocular procedures to partially reduce astigmatism. The limits for the treatment of astigmatism by PRK, LASEK, or LASIK have been expanded by utilizing the cross-cylinder ablation or bitoric ablation technique originally proposed by Vinciguerra et al.31 Toric pseudophakic and phakic IOLs are also available for the treatment of astigmatism associated with lens implantation.32
U.S. Food and Drug Administration Studies: Advantages and Challenges
Apart from the Prospective Evaluation of Radial Keratotomy (PERK) study of RK,33 there have been no large-scale multicenter trials to evaluate the different techniques and technologies of refractive surgery as compared with the comprehensive studies performed for the other ophthalmic subspecialties. However, refractive procedures involve the use of new devices and therefore require the submission of detailed studies to the FDA, which are available on the FDA web site shortly after approval (http://www.fda.gov). The FDA submission criteria require that the data be submitted in a standardized format so that the results of different lasers, procedures, and devices can be compared.
While there are obvious advantages to using the FDA data for comparisons, in practice, there are some limitations also. First, excellent results in an FDA study do not always correlate with those in general practice. The most notable example of this discrepancy was with Intacs. The result of the FDA study for Intacs was outstanding; however, the results in the hands of most surgeons were disappointing, which led to the failure of Intacs as a viable option for the correction of myopia. Second, FDA studies are generally sponsored by the company seeking FDA approval and performed by physicians with close relationships with those companies and so at least some degree of bias could be involved. Finally, in some cases, FDA studies have been submitted years apart, so it is inappropriate to compare the results from one study submitted years before with another that used different and probably inferior technology. Despite these limitations, the FDA approval data provide an excellent comparison of the results of refractive procedures (see Table 5.1) as well as a good sample of the complications. This chapter includes not only a detailed analysis and comparison of the FDA data but also other independent studies in the literature to provide a balanced and more updated view of the results of the procedures.
The Evaluation of Refractive Surgery Results
The results of refractive surgery are generally reported as the percentage of eyes achieving 20/20 and 20/40 vision (efficacy) and the percentage of eyes achieving within +0.5 D of emmetropia and ±1.0 D of emmetropia (predictability). The overall reduction in the degree of myopia and the stability of this number over the length of follow-up in the study are also reported, as is the percentage of eyes with complications.
The indices of efficacy and safety may provide the best assessment of visual improvement using the standard methods of visual assessment.34 The efficacy index is the ratio of the preoperative best-corrected visual acuity (BCVA) divided by the postoperative UCVA, with both numbers in the decimal visual form. This value represents the result that patients truly wish to achieve—uncorrected vision at least as good as the corrected vision with their glasses or contact lenses. The safety index is the ratio of the preoperative BCVA divided by the postoperative BCVA, with both numbers in the decimal visual form. This provides an overall assessment of the changes in BCVA that allows an excellent evaluation of safety using standard vision testing. Unfortunately, these reporting methods have not been widely accepted; so, the efficacy and predictability indices will not be reported in this chapter.
Photorefractive Keratectomy
Myopia. The efficacy and the predictability of the FDA results for the various excimer lasers for PRK are found in Table 5.1. After 2000, the FDA submissions were made for LASIK results rather than for PRK results. It can be seen that the early results for PRK
were modest, with only 40% to 60% of eyes achieving 20/20 UCVA. The high degree of loss of BCVA of two or more Snellen lines is of particular interest, ranging from 1% to 7%. There are few reports of the results of the use of modern excimer lasers and techniques for conventional PRK as most reports now focus on custom LASIK; however, the results have markedly improved, with 20/20 rates for conventional PRK as high as 92%.35
were modest, with only 40% to 60% of eyes achieving 20/20 UCVA. The high degree of loss of BCVA of two or more Snellen lines is of particular interest, ranging from 1% to 7%. There are few reports of the results of the use of modern excimer lasers and techniques for conventional PRK as most reports now focus on custom LASIK; however, the results have markedly improved, with 20/20 rates for conventional PRK as high as 92%.35
TABLE 5.1 FDA Data. FDA Results of the Various FDA Studies Reported on the FDA Web Site. Since All New Devices Require an FDA Study Review, a Tremendous Amount of Comparative Data can be Gathered | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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