43 Management of Astigmatism with Peripheral Corneal Relaxing Incisions and Toric Intraocular Lenses The goal of modern cataract surgery has been evolving over the past few years aided by significant improvements in intraocular lens (IOL) implant technology as well as advancements in preoperative and intraoperative surgical technology. Today, the majority of patients desire postoperative emmetropia, and the preoperative consideration of this goal has become an essential aspect of cataract surgery. With the advent of IOLs that correct astigmatism and presbyopia, in addition to the incisional methods of managing astigmatism, the tools necessary to fulfill patient expectations are now available. This chapter reviews two of the methods to achieve refractive neutrality—toric IOLs and peripheral corneal relaxing incisions (PCRIs; also referred to as limbal relaxing incisions [LRIs])—and discusses the preoperative, intraoperative, and postoperative considerations. The first step in the management of astigmatism is to perform a thorough preoperative ophthalmologic examination. A stable tear film, normal ocular surface, corneal epithelium, and basement membrane are critical in obtaining precise measurements. The amount of astigmatism, the location of the axis of cylinder, and the refractive status of the other eye should be determined during the examination, and the patient’s age is another factor to take into consideration. For patients to achieve spectacle independence, cataract surgeons should aim to treat preoperative corneal astigmatism > 0.50 diopters. There are multiple methods of measuring preoperative astigmatism, and advances in technology continue to provide additional techniques. It is important to measure both the astigmatic axis and the astigmatic power. Every evaluation should begin with a manifest refraction, which takes into account both the corneal and lenticular astigmatism. Because lenticular astigmatism is eliminated with removal of the cataractous lens, only the measured corneal astigmatism should be corrected. The corneal astigmatism can be measured with manual keratometry, automated keratometry, Placido-based corneal topography, Pentacam Scheimpflug elevation mapping (Oculus, Optikgeräte, Arlington, WA), Galilei Dual Scheimpflug analysis (Ziemer, Port, Switzerland; Alton, IL), partial coherence interferometry, and optical low-coherence reflectometry. A recent study compared manual keratometer, partial coherence interferometry, Pentacam, and auto keratometry and demonstrated that all methods were equally satisfactory in measuring preoperative corneal astigmatism.1 More advanced technology utilizing corneal elevation mapping, such as the Oculus Pentacam and Ziemer Galilei, evaluate both the anterior and posterior corneal surfaces. Recent studies have shown the importance of posterior corneal astigmatism in overall corneal toricity, and a nomogram has been developed at Baylor University (Houston, TX), which takes this into consideration (Fig. 43.1).2 In our experience, the best assessment of corneal astigmatic power comes from careful manual keratometry plus IOL Master interferometry (Carl Zeiss Meditec, Jena, Germany) or Lenstar reflectometry (Haag-Streit, Mason, OH), and the best assessment of astigmatic axis location comes from IOL Master interferometry or Lenstar reflectometry combined with qualitative guidance from corneal topography and corneal elevation mapping. Fig. 43.1 Baylor toric nomogram, with the rule (WTR) and against the rule (ATR), takes into account posterior corneal astigmatism. Fig. 43.2 (a) Preoperative patient is seated fixating at a distance. (b) The steep corneal axis is marked with gentian violet. Prior to the patient’s arriving in the operating room, the eye must be marked to correctly place the PCRIs or toric IOL on axis. The surgeon manually marks the 6 o’clock and/or 3 and 9 o’clock positions on the cornea while the patient is sitting up and looking straight ahead with both eyes open to avoid cyclotorsion (Fig. 43.2). Then, in the operating room, a marked fixation ring, astigmatic ruler, or arcuate axis marker is centered and oriented on the previously marked 6, 3, or 9 o’clock positions to identify and mark the steep corneal axis.3 Precise axis marking and alignment is critical for effective reduction of astigmatism. For toric IOLs, 1 degree of misalignment is equivalent to a 3.3% decrease in astigmatic correction. When a toric IOL rotates 30 degrees, there is no astigmatic correction induced, and when rotation is greater than 30 degrees there is an induction of increased astigmatism.4 Newer methods have been developed to further increase the accuracy of astigmatic axis marking. These techniques involve preoperative digital imaging that captures keratometry values and iris, limbal, and conjunctival vessel patterns, and uses these landmarks to map the steep axis of astigmatism. One example of this is the VERION™ system (Alcon, Fort Worth, TX) that links preoperative diagnostic imaging data to the operative procedure to orient the positive axis of astigmatism without the use of manual markers. A reference unit in the office captures images and data such as keratometry and limbus and pupil location and transfers this data to the VERION digital marker in the operating room via a USB key (Fig. 43.3). Real-time eye registration and tracking is generated in the surgeon’s microscope view and on a flat screen monitor to provide intraoperative astigmatic guidance for the surgeon. Callisto (Zeiss) has a similar intraoperative guidance system, as does the Cassini Truevision 3D system (Leica Microsystems, Buffalo Grove, IL). Peripheral corneal relaxing incisions continue to be a viable method of intraoperative astigmatic correction even with advances in toric IOL technology. The procedure is cost-effective and can correct one to three diopters of corneal astigmatism. The advent of femtosecond laser technology has provided another platform for utilizing intraoperative corneal relaxing incisions. PCRIs can also be used in conjunction with toric IOLs to correct larger amounts of astigmatism than corrected by toric lens implants alone. The correction of astigmatism with multifocal and accommodating IOLs has traditionally been accomplished with the addition of PCRI incisions, with toric presbyopic IOLs only recently becoming available in the United States. However, the addition of PCRIs can still be helpful to correct high amounts of astigmatism in patients with astigmatic presbyopia correcting lenses. In addition, patients who have low degrees of astigmatism of +0.75 to +1.00 D do well with PCRIs instead of toric IOLs, as there is a risk of overcorrection with even the lowest powered lenses. Lastly, intraoperative PCRIs can be used to correct astigmatism when a lack of zonular or capsular integrity precludes the use of a toric IOL. Most cataract surgeons are more comfortable correcting astigmatism with a toric IOL versus corneal relaxing incisions. The techniques for the use of a toric IOL is not significantly different from that used for conventional one-piece foldable lens implant. The first toric lens was a plate haptic one-piece silicone Staar toric IOL (Staar Surgical, Monrovia, CA) that was approved by the Food and Drug Administration (FDA) in 1998 (Fig. 43.4a). Wide adaptation of this IOL was hindered because its toric position was not always stable, and the lens was found to occasionally rotate off the axis.5 In 2005, the one-piece acrylic AcrySof toric IOL (Alcon) was approved by the FDA and was noted to have excellent rotational stability, with less than 4 degrees of rotation at 1 year6 (Fig. 43.4b). More recently, Abbott Medical Optics (AMO, Santa Ana, CA) has released its Tecnis toric IOL (Fig. 43.4c). This acrylic IOL also shows excellent rotational stability. Bausch and Lomb (Rochester, NY) has introduced the Trulign toric IOL. This lens is based on the same platform as the Crystalens (Bausch and Lomb) accommodating lens implant. Currently, in the United States, toric IOLs can correct 1.00 to 4.11 D of corneal astigmatism. It is important to note that toric IOLs cannot correct irregular astigmatism and higher order aberrations. Also, they should not be used in patients with zonular instability, lack of capsular integrity, or significant pupil irregularity, and should never be implanted in the ciliary sulcus. Furthermore, caution should be taken in patients with very high axial myopia with a large capsular bag because of a higher risk of postoperative lens rotation. There are situations in which patients have greater than 4.5 D of astigmatism, and the use of PCRIs in addition to a toric IOL should be considered because this enables correction of up to 7 diopters of corneal astigmatism. Gills et al7 found that implanting a toric IOL in combination with PCRIs allowed for the correction of high astigmatism while avoiding the induction of corneal irregularities. Fig. 43.4 (a) The Staar toric intraocular lens (IOL) is the first FDA-approved toric IOL. It has a silicone plate haptic design. As with all plate lenses, it should not be implanted in a compromised capsular bag. It is available in two models with different lengths. Model AA4203-TF (10.8 mm in length) comes in spherical powers of 24.0 to 28.5 D. Model AA4203-TL (11.2 mm) comes in spherical powers of 9.5 to 23.5 D. This lens comes in two cylindrical corrective powers: +2.0 D is intended for patients having 1.5 to 2.25 D of preexisting corneal cylinder, whereas the 3.5 D toric IOL is intended for patients having greater than 2.25 D of preexisting corneal astigmatism. The axis of toric power is marked with two hash marks at the optic periphery. (Courtesy of the Staar Surgical Company, Monrovia, CA.) (b) AcrySof Toric IOL. (c) AMO Tecnis Toric IOL. There are multiple PCRI and toric IOL nomograms available in the literature as well as on the Internet. Widely used PCRI nomograms include those by Doug Koch and Skip Nichamin (Figs. 43.5 and 43.6). Online nomograms are available from Alcon, AMO, and PalmScan AP2000 (Micro Medical Devices, Calabasas, CA). These nomograms take into account the amount of astigmatism, the patient’s age, and the location of the steep axis. The online software available at LRIcalculator.com enables the physician to choose between the DONO (Donnenfeld) and the NAPA (Nichamin age and pachymetry adjusted) nomograms depending on physician preference. Surgeons should try multiple nomograms, record their postoperative results, and then modify the chosen nomogram to fit their surgical technique. It is thought that nomograms that plan incisions based on degrees of arc are more accurate than those based on millimeters of arc.8 The number of incisions can be single or double, depending on the amount of astigmatism that needs correction. In addition, the incision should be tailored to correspond with the astigmatic “bow tie” that is seen on corneal topography. For example, in asymmetric astigmatism, the longer PCRI should be in the larger arm of the bow tie and the shorter incision in the smaller arm. The PCRI incisions are created at the beginning of cataract surgery on the steep corneal axis in the peripheral clear cornea using a preset 550- to 600-µm double-cutting diamond knife (Fig. 43.7). Some nomograms require a pachymetry-adjusted blade thickness using intraoperative assessment of the corneal thickness at the site of the incisions. With the introduction of femtosecond laser–assisted cataract surgery, PCRIs can be placed very precisely using the laser at the beginning of the procedure just after the capsulorrhexis and lens softening. These incisions can be opened at the time of surgery, or the incisions can be opened postoperatively to enable adequate astigmatism correction.9 There are currently four femtosecond laser platforms available in the United States to treat astigmatism at the time of cataract surgery. The incision axis location, peripheral location, depth, and length can be planned preoperatively, and this information can then be programmed into the laser’s computer program. The incisions are created in a posterior to anterior direction and can be programmed to either break through the epithelium or remain intra-stromal. Nomograms for femto incisions are currently being created with preliminary nomograms available from Skip Nichamin and Julian Stevens. To avoid intraoperative complications, it is recommended that the PCRIs be performed prior to beginning cataract surgery. In addition, it is important to avoid intersecting the corneal incisions; for example, if an PCRI incision is coincident with the temporal clear corneal cataract incision, which occurs with against-the-rule astigmatism, the PCRI should be limited to the length of the cataract incision (for example, 2.4 mm) and only lengthened after the IOL is placed. If the PCRI incision is near the paracentesis incision, as occurs in with-the-rule astigmatism, the paracentesis should be made peripheral to the PCRI to avoid intersecting incisions. Once the PCRI incision is made, intraoperative assessment of the effectiveness of the PCRIs can be assessed using intraoperative wavefront aberrometry. This information can be used to titrate the length of the incision.10 After the PCRI incisions are created, the phacoemulsification cataract surgery proceeds in a routine fashion. The spherical power of the lens implant does not need to be altered if PCRIs are used secondary to corneal coupling, which entails no net change in overall corneal power.11
Preoperative Assessment
Intraoperative Marking
Choosing the Appropriate Surgical Correction
Technique
Limbal Relaxing Incisions
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