Refractive Lens Exchange
R. Bruce Wallace III
Unlike laser vision correction of the cornea, refractive lens exchange (RLE) is still in relative infancy. However, many forces are at play that are sure to make RLE gain in popularity. A growing number of “baby boomers” are now past their mid-40s and are beginning to experience the inconvenience of presbyopia. Many will be satisfied with spectacle correction. But a certain percentage will be seeking surgical alternatives. And as laser-assisted in situ keratomileusis (LASIK) surgeons discover that aging lenses cause most of the aberrations found with wavefront measurements, the natural best procedure to reduce present and future aberrations would be a lens exchange, not corneal reshaping.
Fortunately, for these patients there have been an amazing array of improvements in cataract surgery that can help RLE surgeons refine their skills with cataract surgery before venturing into purely refractive lens procedures. More precise biometry, better anesthesia, safer lens removal, and newer presbyopia-correcting intraocular lenses (PC-IOLs) have combined to make RLE a natural evolution from cataract surgery. This chapter will explore the necessary ingredients that go into successful RLE:
Patient selection and patient counseling;
Preoperative biometry and intraocular lens (IOL) selection and power calculation;
Refractive lens surgery techniques, inflammation, and infection prophylaxis;
Postoperative patient management and maximizing results.
PATIENT SELECTION AND PATIENT COUNSELING
The best candidates for RLE are hyperopic presbyopes over the age of 50 who have a strong desire to reduce their dependency on spectacles. Like many LASIK surgeons, Kurt Buzard prefers RLE over LASIK for older hyperopes in order to achieve better quality and longer lasting multifocal vision.1 Certainly older myopes, especially those with greater than 3.00-D, can also benefit from RLE. However, the risk of postoperative retinal detachment is greater for high myopes.2 Fortunately, with improvements in phacoemulsification fluidics, allowing for more stable anterior and posterior segments, this is likely to become less of a concern. Patients with over 2.5-D of preoperative astigmatism can still achieve acceptable outcomes with astigmatic surgery, yet less predictably.
When discussing RLE, surgeons develop a sixth sense as to a patient’s level of understanding and his or her expectations. A demanding, unforgiving personality is not the best profile for an RLE candidate. Surgeons and staff members advise potential RLE patients of the benefits and limitations of various PC-IOLs. Brochures and educational videos can offer additional information to bring the patient to a proper level of realistic expectation. By emphasizing to patients that RLE requires adjusting to a new visual system and that they will need to allow time for this adjustment, most RLE patients will be tolerant in the first few months as visual cortical neuroadaptation yields better multifocality and unwanted visual sensations become less noticeable.
PREOPERATIVE BIOMETRY AND INTRAOCULAR LENS CALCULATION
Better biometry techniques combined with computerized fourth-generation theoretic formulas have led to a higher level of accuracy for IOL power calculation. Our goal is to be within 0.50-D of our spherical target for over 90% of eyes undergoing a cataract or refractive lens surgery. Inaccurate axial length measurements remain the most common cause for errors in IOL power selection.3 For most cataract surgeons, contact A-scan ultrasonography has been the chosen method to measure postoperative axial lengths. However, the lower level of accuracy of this technique compared to immersion A-scan and partial coherence interferometry makes contact ultrasound scans less suitable for RLE.4 Immersion A-scan, once mastered, is more accurate without necessarily requiring more time or effort.5,6 Partial coherence interferometry (Zeiss Humphrey IOLMaster) is especially well suited for RLE because this technology performs best when the crystalline lens is less opaque. For keratometry, many RLE surgeons rely on manual methods and on corneal topography in addition to automated keratometry.
Using the latest IOL formulas such as the Holladay II, Hoffer Q, and Haigis II is important to achieve the best refractive results.7,8 These formulas are available on computer programs with the ability to provide outcomes analysis and ongoing personalization of lens constants, a valuable asset to better IOL power calculation. Preferably, a high level of refractive accuracy should be established with these tools when implanting monofocal IOLs during cataract surgery before a surgeon can confidently enter the refractive lens arena.
A growing challenge is the ability to accurately measure the correct central corneal power for postcorneal refractive patients.9 New instrumentation, such as the Oculus Pentacam and contact lens fitting software available on the latest corneal topographers, may provide these frequently demanding patients more reliable keratometric measurements.
MONOVISION AND PRESBYOPIA-CORRECTING INTRAOCULAR LENSES
A number of IOLs are available for the surgical correction of presbyopia. Blended vision or monovision can be created with standard monofocal IOLs. Similar to contact lens monovision, not all patients will be happy with this choice, especially if they were unsuccessful with monovision contact lens trials. However, for the successful monovision contact lens wearer who has become contact lens intolerant, monovision RLE makes sense. We usually target plano to –0.50-D for the dominant distance eye and –1.75-D for the nondominant near eye. This combination can result in surprisingly good uncorrected near vision without sacrificing intermediate vision. Success with lower levels of myopia in the near eye after RLE compared to contact lens fitting may be due to a pseudoaccommodative effect of monofocal IOLs.10
Most RLE surgeons, however, are choosing a PC-IOL over monovision monofocal IOLs. Even though more costly, this method maintains binocularity and stereopsis.11,12 Multifocal and accommodative IOLs are the types of PC-IOLs available today. Similar to the early days of monofocal IOLs, multifocal IOLs have experienced a relatively slow acceptance. Clinical investigation for almost two decades has shown significantly better uncorrected near vision with multifocal IOLs compared to monofocal IOLs, yet unwanted visual sensations delayed their popularity.13,14,15,16
The first multifocal IOL to be granted FDA approval in the United States was the Allergan Array IOL. The Array is a zonal progressive refractive IOL with five blended power zones of alternating distance and near that provide distance (50%), near (37%), and intermediate (13%). Many clinical studies demonstrated the refractive benefit of the Array IOL over monofocal IOL controls.17,18,19 Fortunately, most patients learn to ignore halos and glare after a period of visual cortical adaptation.20 Another concern about multifocal IOLs has been the potential for loss of contrast sensitivity. Even though some measurable loss of contrast has been detected in clinical studies, patients have not found contrast sensitivity loss with multifocal IOLs, like the Array, to be problematic.21
Recently the Array IOL was replaced with the AMO ReZoom refractive IOL. The ReZoom is a second-generation zonal refractive IOL manufactured on the acrylic three-piece AR-40 Sensar monofocal platform. This lens is the result of extensive study of the optical changes necessary to reduce halos and glare occasionally experienced after Array IOL implantation. By altering zone diameters, there was less of an incidence of unwanted visual sensations. Another advantage includes a round, square “Optiedge” to reduce posterior capsular opacification while at the same time avoiding dysphotopsia associated with peripheral retinal reflections from squared edges. Because of its three-piece design, the ReZoom can be implanted in the ciliary sulcus if a posterior capsular tear is encountered. Yet a power adjustment will be necessary due to relative anterior insertion compared to capsular bag placement. Because it has a refractive optic, all light is transmitted, which is an advantage over diffractive multifocal IOLs. Early experience with the ReZoom has been encouraging with a large majority of patients never or almost never needing glasses after surgery. Compared to the Array, the ReZoom appears to offer better near vision and less halo and glare.22
Another multifocal IOL available today is the Alcon ReStor. Originally designed by 3M (Minnesota Mining and Manufacturing) with a posterior diffractive surface, the ReStor’s diffractive component is on the central anterior surface of the IOL. The apodized diffractive-refractive Alcon ReStor IOL has become increasingly popular. The anterior optical surface of the monofocal Acrysof was modified by adding diffractive rings to the anterior central 3.6 mm of the 6.0-mm optic of the Acrysof IOL, which varies in step height and spacing in order to maximize multifocality and, at the same time, reduce halo and glare. As the pupil enlarges in scotopic conditions, there is more light for the distance vision and less light for near. This special modification of a diffractive optic has been termed apodization. Half of the 82% of light transmission is for distance and half for near with the remaining 18% lost to higher orders. This one-piece acrylic IOL with frosted square edges is also available in a three-piece version if sulcus implantation is indicated. FDA submitted data show that 80% of bilaterally implanted ReStor patients never returned to wearing glasses after surgery.23
Another category of PC-IOLs includes accommodating IOLs. These IOLs attempt to mimic natural accommodation of the crystalline lens.24 The current accommodative IOLs include the eyeonics crystalens and the HumanOptics 1CU. The only accommodative IOL currently available in the United States is the eyeonics crystalens. This single piece silicone IOL has hinged haptics to allow for posterior vaulting and anterior-posterior movement of the 4.5-mm optic (Fig. 1). FDA trials demonstrated impressive results with this lens design.25 A recent study has shown that some patients’ vision appears to improve during the first 3 years after implantation.26