Ординатура / Офтальмология / Английские материалы / Phakic Intraocular Lenses_Hardten, Lindstrom, Davis_2004
.pdfFOREWORD
The French philosopher Voltaire once said that “we invent words to conceal truths.” In the context of this volume, he might have added that “we create texts to examine the truth.”
Phakic intraocular lenses (IOLs) are currently utilized to correct significant ametropia (ie, beyond the scope of LASIK and therefore are an accepted facet of the refractive surgeon’s armamentarium). However, the role of pseudophakia to correct aphakia is only a relatively recent event in the history of ophthalmic surgery. Less than three decades ago, lens implantation for aphakia was still a hostile concept; “a time bomb waiting to explode!” In spite of almost universal hostility, pseudophakia not only survived but blossomed to become the unquestioned answer for cataract-affected patients. The impetus of lens implantation could not simply be erased like a mistake in spelling. Unlike so many ideas that perished in their infancy, phakic lens implantation has also survived earlier negative exaggerations that attempted to condemn but ultimately served to market the proposition. Prescient leaders of refractive surgery, rather than associating themselves with the chilly disapprobation of the majority, generated their own frisson of excitement for what phakic lens implantation might achieve for those many disadvantaged ametropic patients. They were not to be deterred or so easily distracted from their intended purpose by the bleak insufficiency of a one-dimensional approach to refraction, whilst others were excited by the commercial opportunity born. Phakic implantation was not an idea condemned to perish in its infancy.
Technological advances and pseudophakic experience have each played their part to complement the pioneering efforts of George Baïkoff in France and Svyatoslov Fyoderov in what was then Soviet Union, amongst others. Their contributions were fundamental and have finally led to universal acceptance of the role of phakic implantation, which now complements pseudophakic surgery.
In an age where corneal refractive surgery has exploded on the back of increasingly sophisticated laser surgical technologies, so there has been a much slower but parallel development of phakic implantation. Phakic implant surgery is additive surgery and therefore potentially reversible, a safety aspect appealing to surgeons and patients alike. The optical quality of phakic IOL vision would seem to be superior to oblate corneal vision, so much so that the parameters for corneal laser surgery have receded whilst those for phakic implantation have advanced.
Phakic IOLs may be located in the anterior chamber (ie, angle supported). Arguably, this is the simplest of the surgical procedures but requires a substantial incision and therefore the skills to control the prospect of induced astigmatism, anathema to the refractive surgeon. Similarly, the iris-supported “lobster claw” Artisan-type phakic implant also requires not only a sizeable incision, but additional skills to fixate the IOL by subtle maneuvers. Prevention of induced astigmatism represents a further surgical challenge. A toric version adds to its potential value aiding the neutralization of pre-existing astigmatism. Further, a foldable version is imminent. Finally the posterior chamber, which always seemed to be the most unlikely location for phakic implants, has proved to have important advantages for their use. Small, selfsealing incisions avoid induced astigmatism, the IOL is invisible to the naked eye, and the optical quality is arguably superior because of its proximity to the crystalline lens.
Each type of phakic implant has its advocates. Surgeons, being creatures of habit, adopt one of the versions for their own particular reasons and then the added experience of continued usage invariably persuades them to become advocates for that particular process. This outcome confirms that all the systems are effective, the pros and cons offering the opportunity for continued animated debates amongst refractive surgeons as well as providing an opportunity for novice enlightenment.
I congratulate the authors of this volume for its timely production and the combined experience the book brings to its readership, many of whom will be contemplating adding phakic implant surgery to their repertoire of refractive surgical procedures. For the more experienced surgeons, this volume will allow reflection on their current practice and the probability of its variation with the desire of all refractive surgeons to customize the surgical process in accordance with a patient’s visual requirements.
Controversies that will continue are the place of phakic IOLs in presbyopic eyes, the comparisons and respective safety of refractive lens exchange versus phakic implantation, the value of multifocal phakic implants, phakic implant surgery versus corneal refractive surgery, and the choice of phakic implant, their design, and location.
Emanual Rosen, MD
Visiting Professor, University of Manchester Institute of Science and Technology
Department of Optometry and Neurosciences
Manchester, UK
Director, Rosen Eye Clinics UK
Past President, International Intraocular Implant Club
Past President, European Society of Cataract & Refractive Surgeons
Coeditor, Journal of Cataract and Refractive Surgery
Chapter1
Demographics of Refractive Surgery:
The Role of Phakic Intraocular Lenses
David Rex Hamilton, MD, MS; David R. Hardten, MD, FACS; and Richard L. Lindstrom, MD
OVERVIEW
Refractive surgery is the most rapidly evolving discipline in ophthalmology today, with 1.3 million procedures performed in the United States in 2001 and 1.2 million projected for 2002 (Figure 1-1). Numerous surgical modalities and techniques exist to treat a wide range of refractive errors. The emergence of phakic intraocular lens (IOL) technology expands the capabilities of refractive surgery to address higher levels of myopia and hyperopia than were previously accessible. In this chapter, we will examine the demographics of refractive errors, how currently available surgical techniques address various ranges of refractive errors, and the role phakic IOLs will play in bringing effective solutions to patients who might otherwise be considered suboptimal refractive surgical candidates.
MYOPIA
Definitions
While there is some variability in terminology found in the literature, the following definitions will be used to stratify levels of myopia: low (less than -5.00 diopters [D]), moderate (-5.00 to -10.00 D), and high (greater than -10.00 D). Visually significant myopia is considered to be -1.00 D or greater. These specific stratifications are chosen because they are most representative of those used in the literature. In addition, the efficacy of different refractive techniques varies with attempted correction. In general,
photorefractive keratectomy (PRK) provides effective results for patients with low myopia while laser in-situ keratomileusis (LASIK) is effective for low and moderate myopia. Phakic IOLs will likely address high myopia.
Prevalence
The distribution of levels of myopia is highly variable among different races and ethnic groups. In general, the prevalence of myopia tends to be higher among more developed populations. The prevalence of myopia is estimated to be 70% to 90% in Asia, 30% to 40% in Europe and the Americas, and 10% to 20% in Africa.1 Among individuals aged 12 to 54 in the United States, the prevalence of myopia is approximately 25%.2 Among individuals over age 40 in the United States, 30.5 million have visually significant myopia.3 The most comprehensive, population-based study of myopia in the United States examined subjects ranging in ages from 4 to 74 years old and found 43% with low, 3.2% with moderate, and 0.3% with high myopia.4 Table 1-1 summarizes the relationship between levels of myopia among individuals wearing corrective lenses and age groups typically presenting for refractive surgery.5 Myopia is more prevalent in younger patients. More than three-fourths of those individuals aged 18 to 24 years that wear corrective lenses have low myopia. Of patients presenting for myopic refractive surgery, however, the numbers are skewed toward higher levels of correction. McCarty et al compared the stratified prevalence of myopia in the general population of Melbourne, Australia with that of individuals presenting for refractive surgery.3 The study found that while only
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Table 1-1 |
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PREVALENCE OF LOW, MODERATE, AND HIGH MYOPIC SPHERICAL |
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EQUIVALENT AMONG INDIVIDUALS WEARING CORRECTIVE LENSES IN |
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AGE GROUPS TYPICALLY PRESENTING FOR REFRACTIVE SURGERY |
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Level of Myopia/Age |
18 to 24 |
25 to 34 |
35 to 44 |
45 to 54 |
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Low (-0.01 to -5.0 D) |
77.0% |
71.4% |
62.0% |
37.6% |
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Moderate (-5.1 to -10.0 D) |
7.2% |
5.8 % |
4.6% |
2.4% |
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High (above -10.0 D) |
0.8% |
0.4% |
0.0% |
0.0% |
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Adapted from Roberts J, Rowland M. Refraction Status and Motility Defects of Persons 4-74 Years, United States, 1971-1972. National Center for Health Statistics. Vital Health Survey Series 11, Number 206. 1978.
* estimated
Figure 1-1. US refractive surgical procedures (adapted from Market Scope. Refractive Market Perspectives [serial online]. 2002;7(7). Available at: www.mktsc.com. Accessed June 12, 2003).
2% and 0.4% of the general population had moderate and high myopia, respectively, 42% and 13% of those patients presenting for refractive surgery had moderate and high myopia, respectively (Table 1-2). In other words, moderate myopes were ten times more likely to present for refractive surgery than low myopes, and high myopes were sixteen times more likely to present for refractive surgery than low myopes. A similar result reported by Ucakhan et al found that 17% of patients presenting for refractive surgery at a center in the United States had myopia of -6.00 D or greater compared to an estimated 2% prevalence in the general population.4
HYPEROPIA
Definitions
The following definitions will be used to stratify levels of hyperopia: low (less than +3.00 D) and high (greater than +3.00 D). Again, these stratifications are chosen because they are most representative of those used in the literature. In addition, the efficacy of different refractive techniques varies with attempted correction. In general PRK, LASIK, and conductive keratoplasty (CK) are effective for low hyperopia. Phakic IOLs will likely address high hyperopia.
Prevalence
In the adult population, low hyperopia remains largely silent clinically until the fifth decade due to the accommodative ability of the crystalline lens. In the United States, there are 12 million cases of high hyperopia among individuals over age 40.6 Table 1-3 summarizes the relationship between levels of hyperopia among individuals wearing corrective lenses and age groups typically presenting for refractive surgery, showing an increasing prevalence of low hyperopia with age. The incidence of high hyperopia does not appear to increase significantly with advancing age. Presbyopic changes to the accommodative apparatus of the eye clearly play a role in causing hyperopic individuals to seek refractive surgical corrections.
Demographics of Refractive Surgery: The Role of Phakic Intraocular Lenses |
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Table 1-2
PREVALENCE OF LOW, MODERATE, AND HIGH MYOPIA IN NONCATARACTOUS GENERAL AND MYOPIC POPULATIONS AND THOSE PRESENTING FOR MYOPIC REFRACTIVE SURGERY IN MELBOURNE, AUSTRALIA
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Population Presenting |
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for Myopic Refractive |
Level of Myopia |
General Population |
Myopic Population |
Surgery |
Low (-0.01 to -5.0 D) |
20% |
88% |
45% |
Moderate (-5.1 to -10.0 D) |
2% |
10% |
42% |
High (above -10.0 D) |
0.4% |
2% |
13% |
Adapted from McCarty CA, Livingston PM, Taylor HR. Prevalence of myopia in adults: implications for refractive surgeons. J Refract Surg. 1997;13:229-234.
Table 1-3
PREVALENCE OF LOW AND HIGH HYPEROPIC SPHERICAL EQUIVALENT AMONG INDIVIDUALS WEARING CORRECTIVE LENSES IN AGE GROUPS TYPICALLY PRESENTING FOR REFRACTIVE SURGERY
Level of Hyperopia/Age |
18 to 24 |
25 to 34 |
35 to 44 |
45 to 54 |
Low (+0.01 to +3.0 D) |
9.5% |
15.2% |
26.7% |
50.1% |
High (above +3.0 D) |
3.3% |
4.3% |
3.0% |
4.8% |
Adapted from Refraction Status and Motility Defects of Persons 4-74 Years, United States, 1971-1972. National Health Survey Series 11, Number 206.
REFRACTIVE SURGICAL
PROCEDURES FOR MYOPIA
Refractive surgical procedures available to treat myopia include LASIK, PRK, and its variants (laser subepithelial keratectomy, otherwise known as LASEK or Epi-LASEK), intracorneal ring segments, clear lens extraction, and phakic IOL implantation.
Laser In-Situ Keratomileusis
There is little disagreement that LASIK affords the highest level of comfort, quickest recovery, and most stable, predictable outcome for low and moderate myopia.7,8 Efficacy and predictability decrease, however, when treating high myopia with LASIK.9,10 Table 1-4 reports the uncorrected visual acuity 1 month following LASIK as a function of
attempted correction in 1000 eyes treated at a single refractive center in the Midwest. The results demonstrate the significant decrease in efficacy at the 20/25 level for corrections above -8.00 D and at the 20/30 level for corrections above -10.00 D. In addition to a decrease in efficacy, visual function is more likely to be compromised with higher corrections. The incidence of decreased contrast sensitivity and increased glare and halos appears to be significantly higher with large myopic corrections.11 This decrease in visual function is likely related to alteration of the corneal asphericity following myopic laser ablation in which the natural prolate shape (ie, steep in the center, flat in the periphery) of the anterior surface is converted to an oblate shape (ie, flat in the center, steeper in the periphery).12 Oshika et al demonstrate a positive correlation between amounts of induced coma-like and spherical-like aberrations and increasing attempted correction.13 The study also
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Table 1-4 |
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MEAN UNCORRECTED VISUAL ACUITY 1 MONTH AFTER |
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PRIMARY MYOPIC LASIK AS A FUNCTION OF ATTEMPTED CORRECTION |
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Myopic Correction (D) |
>20/20 |
>20/25 |
>20/30 |
>20/40 |
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0 to 2 |
71% |
91% |
97% |
100% |
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2 to 4 |
85% |
93% |
96% |
100% |
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4 to 6 |
62% |
98% |
100% |
100% |
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6 to 8 |
51% |
71% |
91% |
100% |
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8 to 10 |
18% |
27% |
82% |
100% |
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10 to 12 |
33% |
33% |
50% |
83% |
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Adapted from analysis of 1000 eyes treated at Minnesota Eye Consultants, PA, Minneapolis, Minn.
shows a positive relation between eyes losing two or more lines of best corrected acuity and level of induced coma-like and spherical-like aberrations. Several studies report losses of two or more lines of best-corrected visual acuity in 3% to 5% of highly myopic eyes treated with LASIK.9,14 Several recent studies suggest best-corrected visual acuity may be better with the Artisan (Ophtec BV, Groningen, Netherlands) phakic IOL than LASIK in patients with high myopia.15 A large study by Maloney et al reported 6 month results on 84 eyes receiving the Artisan phakic IOL for myopic correction ranging from -5.5 to -22.5 D (mean: -13.0 D).16 At 6 months, 58% gained one or two lines of best-corrected visual acuity. This is likely attributable to the relative magnification achieved by elimination of spectacle correction in high myopia17 and the preservation of natural corneal asphericity.
Corneal thickness becomes a limiting factor with increasing correction, as there appears to be a correlation between risk of keratectasia and decreasing residual bed thickness following LASIK.18-20 While the etiology of keratectasia is not fully understood,21 it is generally accepted that ablating below a minimum residual bed thickness increases risk. The value of 250 m has been proposed as a threshold beyond which ablation should not proceed.22 Unfortunately, even this number may not be sufficient in every case, as evidenced by reports of keratectasia following shallow ablations with thicker residual beds.23 In some of these cases, forme fruste keratoconus as seen on topography may contribute to the development of keratectasia.24 In a recent study by Hori-Komai et al examining the reasons why patients presenting for refractive surgery did not undergo LASIK or PRK, 25% of 2784 consecutive patients did not undergo either procedure. Nearly 30% of the patients that did not have surgery had either high myopia (>-12.00
D) or had insufficient corneal thickness.25 The concern over adequate residual bed thickness, coupled with the lower predictability of LASIK refractive outcomes at higher corrections, leads to the additional issue of potential “nonenhanceability.” The high myope is more likely to require an enhancement than the low or moderate myope but also may not have enough tissue remaining to safely perform additional ablations. In these situations, a procedure that does not remove corneal tissue and does not alter the natural prolate shape of the anterior corneal surface, such as phakic IOL implantation, offers an attractive solution.
Photorefractive Keratectomy
PRK, which has been performed for more than a decade, has proven extremely effective in treating low myopia, demonstrating high levels of safety, efficacy, stability, and predictability.26-28 PRK holds particular appeal for patients with corneas too thin for LASIK due to inadequate residual bed thickness. The absence of a flap typically adds 100 to 150 m to the treatable stromal bed in PRK. While LASIK became dominant in the late 1990s due to its faster recovery and improved patient comfort,29 PRK has experienced a resurgence of interest recently due to the introduction of wavefront-guided laser treatments30 and the notion that the microkeratome pass and flap healing in LASIK may introduce additional optical aberrations.31,32 PRK, even more so than LASIK, however, performs less impressively when treating moderate to high myopia.33 The likelihood of significant regression of treatment effect increases significantly with higher corrections, possibly due to epithelial hyperplasia.34 In addition, stromal wound healing can lead to subepithelial haze formation, with more severe haze developing with higher corrections.35 The haze, which can decrease visual acuity
Demographics of Refractive Surgery: The Role of Phakic Intraocular Lenses |
5 |
directly in its more advanced manifestation, is thought to closely relate to refractive regression.36 These two fac- tors—regression and subepithelial haze—dramatically reduce PRK efficacy in treating high myopia. One study found efficacy of only 30% for achieving 20/40 or better and 4% for 20/20 or better uncorrected acuity at 6 months in the high myope.37
Issues describing decreased visual function resulting from induced higher-order aberrations, which were discussed in the LASIK section, apply to high myopia PRK treatments as well. However, with the lack of a flap, PRK may introduce fewer aberrations if the healing process can be adequately controlled when treating large corrections. In particular, the use of mitomycin C (MMC) may be advantageous to reduce subepithelial haze formation in moderate and high myopic treatments using PRK.38,39 A recent prospective, randomized study compared the 6-month uncorrected visual acuity (UCVA) between eyes with moderate myopia treated with and without MMC. A statistically significant difference was found in UCVA at 6 months at both the 20/20 or better level (60% of the MMC group vs 30% of the control group) and the 20/40 or better level (100% of the MMC group vs 83% of the control group).40
LASEK or Epi-LASEK, which is a variant of PRK, creates an “epithelial flap” that is then repositioned after laser ablation in an attempt to improve the time course and outcome of the wound healing response. Recent studies suggest results are similar to those found with LASIK or PRK in treating low myopia.41-43 Some studies suggest that there may be some advantage to LASEK over PRK in treating moderate myopia due to a lower incidence of subepithelial haze44; however, this is controversial.
Intracorneal Ring Segments
Intracorneal ring segments (ICRs) address low myopia, typically correcting -3.50 D or less. A unique feature of ICRs is the ability to reverse or change the refractive effect by segment explantation or exchange.45 Efficacy, predictability, and visual function outcomes of ICRs are comparable to LASIK and PRK.46,47 There are conflicting reports, however, regarding a diurnal variation of refractive correction using ICRs. Some suggest that there is a myopic shift in the evening similar to that seen in post-radial keratotomy eyes, particularly in the early postoperative period.48,49 The surgical technique does not involve the central cornea, and it preserves the prolate shape internal to the ring segments. The narrow range of myopia for which intracorneal rings are effective, the limited sizes available within the range of correction, and the inability of this technology to deal with astigmatism, coupled with the tremendous success of LASIK and PRK in treating low myopia, have limited the use of this technology. A new application, however, is revitalizing interest in the tech-
nique. Several recent studies indicate ICRs may be effective in reducing the corneal steepening and astigmatism associated with keratoconus and iatrogenic keratectasia following refractive surgery, thereby improving uncorrected and best-corrected visual acuity as well as contact lens fit, thus delaying the need for penetrating keratoplasty.50-53
Refractive Lens Exchange
Refractive lens exchange, or clear lens extraction, involves removal of the crystalline lens in a highly ametropic eye using a standard phacoemulsification technique followed by placement of a posterior chamber IOL. Interest in this technique has increased in recent years due to several factors:
1.Improvement in cataract extraction techniques using phacoemulsification, which provides a safer, less traumatic method for lens removal over previous extracapsular and intracapsular techniques
2.Advancements in posterior IOL designs, including the availability of low positive and negative power, foldable lenses54
3.Suboptimal efficacy and safety of corneal refractive procedures in the highly myopic or hyperopic eye
In addition, IOL surgery offers the potential for improved best-corrected vision by preserving corneal asphericity and providing relative magnification achieved with elimination of spectacle correction in high myopia.55,56 One significant concern with this technique is the risk of retinal detachment. This vision-threatening complication had a relatively high risk during the prephacoemulsification era, as high as 7% according to one large retrospective study.57 More recent studies provide conflicting evidence regarding the incidence of retinal detachment in patients following clear lens extraction using phacoemulsification. A study by Colin et al reports an 8% incidence of retinal detachment over a 7-year postoperative period in a series of 52 eyes.58 Other studies, however, indicate that the incidence may be significantly lower, especially following meticulous identification and prophylactic treatment of retinal pathology prior to surgery.56,59 There is insufficient evidence, however, to support 360degree prophylactic photocoagulation in these patients. Ripandelli et al reported on 41 eyes that sustained retinal detachments following clear lens extraction (mean preoperative refraction = -19.5 D).60 Sixty-three percent of these eyes had 360-degree prophylactic photocoagulation.60 Another concern is decreased accuracy of IOL power calculation in high axial myopia and the potential for a postsurgical refractive surprise that may require additional surgery. The availability of multifocal IOLs allows surgeons to address both distance and near vision using clear lens extraction.61,62 The loss of natural accommodation, however, is likely unacceptable to the young patient
6 Chapter 1
who is considering refractive surgery for high myopia. Phakic IOL technology, which addresses high myopia and preserves accommodation, may provide a more effective refractive surgical solution for young patients with high myopia.
Myopic Refractive Surgery:
Current Practice Patterns
A 2001 survey of 188 US members of the International Society of Refractive Surgery (ISRS) highlights the current trends among refractive surgeons in choosing among the various techniques for correction of a given refractive error.63 Ninety-five percent of surgeons would choose LASIK to treat a 30-year-old -3.00 D myope and 98% would choose LASIK to treat a 30-year-old -7.00 D myope. Fifty-three percent would treat a -12.00 D myope with LASIK, 26% would use a phakic IOL, and 10% would wait for a better surgical alternative. This survey was conducted in 2001, prior to US Food and Drug Administration (FDA) approval of any phakic IOL devices in the United States.
REFRACTIVE SURGICAL
PROCEDURES FOR HYPEROPIA
Refractive surgical procedures available to treat hyperopia include LASIK, PRK and its variants, thermal keratoplasty (CK and laser thermokeratoplasty [LTK]), clear lens extraction, and phakic IOL implantation.
Laser In-Situ Keratomileusis
Hyperopic excimer laser techniques steepen the cornea by removing tissue differentially from the midperipheral cornea. Centration and the ability to treat larger optical zones are crucial factors. With the introduction of microkeratomes capable of cutting larger diameter flaps and scanning or “flying” spot lasers capable of larger optical zones equipped with active eye trackers that significantly improve optical zone alignment, the clinical results of LASIK for hyperopia have improved significantly.64,65 Table 1-5, comparing the results from the most recent studies on CK, PRK (see the next section), and LASIK for the treatment of low hyperopia, shows better than 90% efficacy for achieving 20/40 or better uncorrected acuity with all techniques. Recent investigations identify a threshold of hyperopic correction, above which predictability and efficacy of LASIK decreases. Cobo-Soriano et al identified a threshold of +3.00 D above which significant regression occurs, +4.00 D above which a significant decrease in predictability occurs, and + 6.00 D above which a significant decrease in safety occurs.66
Photorefractive Keratectomy
Early studies of PRK for treating hyperopia described poor predictability and efficacy for high corrections and a high sensitivity to optical zone decentration.67-69 A significant myopic shift or overcorrection was also noted, probably related to less predictable nomograms and small optical zones available with the broad-beam, erodible mask techniques used at that time. A large study of 482 eyes by Nagy et al reported UCVA 20/40 or better in 88.4% of patients at 12 months.70 Refractive stability was not achieved, however, as regression still occurred at 12 months.71 PRK, like LASIK, has benefited from recent technological improvements, providing larger flaps, expanded optical zones, and eye tracking systems. The most recent PRK studies have looked at treating low hyperopia with the latest laser technology, including one study using wavefront-driven ablation. These studies by Nagy et al72,73 are summarized in Table 1-5 along with LASIK and CK data for comparison. The results suggest hyperopic PRK and LASIK using the latest excimer laser technology may offer somewhat improved stability in treating low hyperopia when compared to CK. Studies examining PRK using the latest laser technology to treat high hyperopia have yet to be published.
Conductive Keratoplasty
The application of thermal energy to the corneal stroma to affect a change in curvature is known as thermal keratoplasty. This technique has been developed clinically to treat low hyperopia by steepening the corneal curvature. Unlike excimer laser ablative procedures, such as PRK and LASIK, the technique has the advantage of sparing the visual axis from surgical manipulation and preserving, rather than removing or ablating, corneal tissue. The CK technique, originally developed by Mendez et al,74 uses the electrical conductive and resistive properties of the cornea, rather than its thermal properties, to propagate energy through the stroma. The resistance of the stroma to current flow creates heat, causing collagen shrinkage. By applying spots in a circumferential pattern in the midperiphery, the collagen shrinkage causes central steepening, inducing the desired myopic shift. The technique currently treats hyperopia up to +3.00 D. Table 1-5 summarizes results from the FDA trial for CK75 and compares these results with studies on LASIK and PRK for the treatment of low hyperopia. While efficacy is comparable, LASIK and PRK appear to afford a higher level of stability than CK. LTK makes use of laser energy instead of radiofrequency energy to heat the stroma. LTK has been largely replaced by CK due to an unacceptable regression profile with LTK.
Demographics of Refractive Surgery: The Role of Phakic Intraocular Lenses |
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Table 1-5
COMPARISON OF CLINICAL DATA FROM STUDIES EXAMINING CK, PRK, WAVEFRONT PRK, AND LASIK TO TREAT HYPEROPIA
Paper |
McDonald75 |
Nagy72 |
Nagy73 |
Salz65 |
Instrument (Technique) |
Viewpoint |
Meditec MEL 70 |
Meditec MEL 70 |
LADARVision |
|
(CK) |
(PRK) |
(wavefront PRK) |
(LASIK) |
Mean age |
55 |
39 |
38 |
53‡ |
Mean preoperative MRSE (D) |
+1.80 |
+2.48 |
+2.90 |
NA† |
Preoperative MRSE range (D) |
+0.75 to +3.00 |
+1.00 to +3.50 |
+1.00 to +4.00 |
+0.88 to |
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|
+2.90† |
Follow-up (number of eyes) |
12 months |
12 months |
6 months |
12 months |
|
(344) |
(62) |
(40) |
(88†) |
UCVA >20/20 |
56% |
89% |
70% |
55%† |
UCVA >20/40 |
92% |
97% |
90% |
96%† |
SE within ± 0.50 D |
62% |
82% |
85% |
78%† |
SE within ± 1.00 D |
89% |
94% |
100% |
93%† |
Two line loss of BCVA |
2% |
2% |
13% |
5%† |
>1.0 D induced cylinder |
5% |
NA |
NA |
8%* |
>2.0 D induced cylinder |
0.3% |
NA |
NA |
1% |
Mean SE regression |
+0.23 D |
+0.00 D |
-0.25 D |
+0.05 D |
|
(6 to 12 m) |
(6 to 12 m) |
(3 to 6 m) |
(6 to 12 m)* |
MRSE = mean refractive spherical equivalent; SE = spherical equivalent; UCVA = uncorrected visual acuity; BCVA = best-corrected visual acuity
*For large spherical hyperopia cohort (mean SE = +2.68, range = +1.00 to +6.00 D)
‡For entire study cohort including spherical hyperopes, hyperopic astigmats, and mixed astigmats
†For low spherical hyperopia subcohort that most closely matches CK study
Note: The efficacy, predictability, and safety results are comparable with somewhat higher efficacy at the 20/20 level with PRK. The most significant difference occurs in the regression data in which LASIK and PRK appear to have better stability than CK, especially considering the LASIK data include corrections up to +6.00 D.
Refractive Lens Exchange
Most studies investigating refractive lens exchange for the treatment of hyperopia do not examine patients younger than ages 35 to 40, primarily because clinically significant hyperopia is rare below the prepresbyopic age, with the exception of hyperopic children with strabismus and/or amblyopia. The risk of retinal detachment is significantly lower among hyperopes than high myopes. This fact, coupled with the greatly reduced accommodative amplitudes of presbyopic individuals, makes clear lens extraction a much more attractive option for the treatment of high hyperopia than for the treatment of high myopia,
excluding nanophthalmic eyes. Recent studies indicate good efficacy and predictability in treating high hyperopia with this technique.76,77 The development of posterior capsular opacity appears to be higher than that of conventional cataract extraction, as high as 54% in one study.76 This may be due to the more vigorous inflammatory response seen in younger patients. One of the difficulties of clear lens extraction is that high refractive power IOLs are required to achieve emmetropia. Even though IOL manufacturers make higher power lenses, the lens thickness often makes foldable insertion impossible. As a result, several investigators advocate piggyback lens insertion for
8 Chapter 1
high hyperopic clear lens extraction.78,79 Additionally, IOL power calculations are relatively inaccurate in high hyperopic eyes. The Holladay II formula appears to provide improved accuracy in this population.80
Hyperopic Refractive Surgery:
Current Practice Patterns
The 2001 ISRS survey of US refractive surgeons63 found that 70% would choose LASIK to treat a 45-year-old +1.00 D hyperope while 17% would choose CK/LTK. Eightyfour percent would choose LASIK to treat a +3.00 D hyperope. Only 29% would use LASIK to treat a +5.00 D hyperope while 39% would choose clear lens extraction and 30% would wait for a better surgical alternative. The survey was conducted in 2001 prior to US FDA approval of any phakic IOL devices and, in fact, did not include phakic IOLs as a surgical option for treatment of hyperopia.
SUMMARY
Excimer laser procedures afford the most effective treatment for low and moderate myopia and low hyperopia. Conductive keratoplasty is also effective in treating low hyperopia. The advantages of phakic IOLs over other refractive surgical techniques are significant for highly ametropic eyes. Efficacy, predictability, and safety of excimer laser procedures in the treatment of high myopia are suboptimal. The maintenance of natural corneal prolate asphericity and resultant preservation of contrast sensitivity and visual function represents a significant advantage over techniques that remove and/or reshape corneal tissue (eg, excimer laser and thermal keratoplasty techniques). This advantage may also allow for improvements in best-corrected visual acuity not available with other keratorefractive techniques. The preservation of accommodation is a distinct advantage of phakic IOLs over clear lens extraction. The ability to avoid the violation of the crystalline lens/capsule complex and the minimal mechanical trauma associated with the phakic IOL surgical technique may offer additional advantages over clear lens extraction by preventing vitreoretinal complications in the highly myopic eye. Astigmatism in these patients may be addressed through “bioptics”81-83 (LASIK flap + phakic IOL implantation + laser ablation to correct residual ametropia) and/or toric phakic IOLs. See Chapter 18 for a discussion of bioptics and Chapter 21 for a discussion of toric phakic IOLs.84
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