10 Toric Intraocular Lens Abstract Cataract surgeons should be prepared to deal with suboptimal refractive results after phacoemulsification with toric intraocular lens implantation. Tips for avoiding and dealing with suboptimal results are discussed in this chapter. Clinical examples are provided for illustration. Keywords: lenses, intraocular lens implantation, astigmatism, phacoemulsification, cataract, cataract extraction, refractive surgical procedures, cornea, pseudophakia Clinically significant corneal astigmatism is commonly found in a significant proportion of cataract surgery patients. Astigmatism correction at the time of cataract surgery can improve postoperative visual outcomes and reduce dependence on corrective lenses. In addition to other treatment options, including operating on the steep axis and peripheral corneal relaxing incisions (PCRIs), toric intraocular lens (IOL) implantation can achieve excellent astigmatic correction. Cataract surgeons should be prepared to deal with suboptimal refractive results after toric IOL implantation. Tips for avoiding and dealing with suboptimal results are discussed in this chapter. Clinical examples are provided for illustration. Preoperative planning for toric IOL implantation requires evaluation for preexisting corneal conditions, which may affect keratometry and refractive outcomes. A partial list includes dry eye, pterygium, Fuchs’ dystrophy, Salzmann’s nodular degeneration, keratoconus, prior corneal transplantation, prior scleral buckling, and epithelial basement membrane dystrophy. Identification and treatment of these conditions followed by adequate time for stabilization of the cornea and ocular surface are required before biometry and IOL selection are completed. Spherical IOL power calculation inaccuracy can negatively impact the refractive outcome of toric IOL implantation. Optical biometry using partial coherence interferometry has become increasingly accurate for axial length determination, but keratometric assumptions inherent in most devices are not always accurate for determining total corneal power. The accuracy of these devices is especially reduced for eyes that have undergone corneal refractive surgery. Keratoconus and other corneal ectasias will also impact the accuracy of biometry. Additionally, eyes with irregular astigmatism may not be amenable to treatment with toric IOLs. Corneal topography and tomography should be obtained to identify these conditions and act as an additional check on corneal power estimation by optical biometry. Manual keratometry may also be useful for comparison, looking for consistency across multiple measurements before completing calculations and IOL selection. As with keratometry, axial length measurement methods can introduce error. Optical biometry cannot penetrate dense cataracts and ultrasonography may occasionally be needed. Corneal compression during contact ultrasonic measurements will provide a falsely shorter axial length, which can be avoided with the immersion technique. Excessively long or short eyes additionally challenge the accurate calculation of IOL power. Long eyes (> 26 mm) need an adjustment factor when using “third-generation” formulas. Alternatively, formulas requiring measurements besides axial length and keratometry, such as anterior chamber depth, can increase accuracy. The Sanders–Retzlaff–Kraff (SRK)/T, Haigis, Barrett Universal II, Holladay 2, and Olsen formulas offer the best prediction of refractive results for axial length greater than 26 mm and IOL power 6.0 diopters or higher.1 Similarly, for short eyes (< 20 mm) the Hoffer Q appears to be more accurate for IOL calculation than SRK/T, but further study is needed. Kapamajian and Miller have proposed a correction for eyes that require negative power IOLs.2 Early toric IOL calculators have proven to be less accurate than previously thought. Some of the errors come from ignoring the vertex effect on toric IOL power calculation. Toric IOL calculation using the Barrett toric IOL calculator, or Abulafia–Koch adjustment added to the Alcon or Holladay calculator, using vector analysis, reportedly achieves a result of 77 to 78% within ± 0.50 diopter cylinder predicted.3 Without directly measuring posterior corneal astigmatism, it is common to overestimate with-the-rule (WTR) and underestimate against-the-rule (ATR) astigmatism.4 Even when compensating for the effect of the posterior cornea, we recommend undertreating WTR astigmatism and overtreating ATR when planning toric IOL implantation. A residual astigmatism of 0.3-diopter WTR postoperatively allows for some agerelated postoperative drift in astigmatism without negative consequences on uncorrected visual acuity. Accurately estimating surgically induced astigmatism (SIA) is important for calculations. We recommend using 0.4 diopter of SIA for an incision width of 2.4 to 2.75 mm, and reducing the estimate for a smaller incision. During surgery, compensation for cyclotorsion is achieved with a single preoperative limbal mark at the 6 o’clock position or two marks at the 3 and 9 o’clock positions, placed with the patient sitting upright. Intraoperative guidance systems (Alcon Laboratories Verion and ORA, Zeiss CALLISTO Eye, Clarity Medical Systems HOLOS IntraOp, and TrueVision Systems TrueGuide), which compare anatomic recognition to a reference image, are also useful adjuncts. Digital platforms have additional features such as intraoperative guides for PCRI and surgical incision placement. Some are capable of intraoperative aberrometry. When marking by hand, the target meridian for toric IOL alignment is confirmed and marked intraoperatively with the aid of a corneal gauge. Mean IOL alignment error using this method was found to be approximately 5% in one study.5 To minimize alignment error, care must be taken to avoid letting marks spread and to carefully align the visual axis of the eye with the surgical microscope view. After implantation and rotation of the IOL to within a few degrees of the target meridian, meticulous removal of viscoelastic material reduces the likelihood of IOL rotation postoperatively. The surgeon should not let the anterior chamber shallow postoperatively as this may cause unwanted IOL rotation. A final check of the IOL alignment with the target meridian should be done as the last step. When a pupil expansion device is removed, iris constriction can limit visualization of the IOL orientation. Thus, a small pupil may be considered a minor relative contraindication to implantation of a toric IOL. Patients should be cautioned against touching or rubbing the eye after surgery to avoid rotating the axis of the IOL. Capsule tension ring implantation may lessen the likelihood of toric IOL rotation in large myopic eyes with large capsular bags. The etiology of a suboptimal refractive outcome should be identified, if possible, prior to fellow eye surgery. This process begins with a thorough examination. Besides preexisting ocular comorbidity, common postoperative complications such as excessive inflammation and delayed wound healing should be ruled out. Posterior capsule opacification and cystoid macular edema should be identified and treated if present. Lenticular astigmatism may contribute to “whole-eye” astigmatism and be reflected in the preoperative manifest refraction. Preoperative astigmatism planning, however, considers only preoperative corneal astigmatism. Postoperatively, barring a decentered or tilted IOL, or toric IOL implantation, there should be no unwanted lenticular astigmatism. IOL manufacturers have an allowable spherical power error range of ±0.25 diopters, which, when combined with another error, could be visually significant. IOL tilt, decentration, and a lens position within the eye other than that anticipated by IOL calculations may impact the effective toric power of the IOL. A consistently round and centered capsulorrhexis, sized to cover the anterior edge of the IOL, is considered ideal and provides more reliable outcome data upon which to base a surgeon adjustment factor. As mentioned previously, using a vector analysis toric IOL calculator such as the Barrett online calculator is helpful. Assuming that the target meridian for toric IOL alignment is calculated and marked correctly, postoperative IOL rotation should be ruled out. This is done with a dilated pupil at the slit lamp biomicroscope, noting the orientation of the alignment marks on the toric IOL with the head perfectly straight. A reticule attached to the slit lamp, or aberrometry, can aid identification of the axis of the implant. If these methods are not available, using the clock hour of the implant alignment provides a fair estimate of the toric axis. Rotation in the early postoperative period was especially problematic with early plate haptic toric IOLs, especially those with a short haptic length. They were slippery and tended to find the long axis of the capsular bag. Rotations are less frequent with open loop haptic IOL designs. Opinions vary in the literature as to how much cylinder correction is lost from a misaligned toric IOL, but it is generally accepted that for every degree of misalignment the estimated reduction in astigmatic correction at the cornea plane is 3%. Nonsurgical treatment options include corrective spectacles or soft toric contact lenses. Rigid contact lenses are not recommended as they unmask lenticular astigmatism. IOL repositioning should be considered on a case-by-case basis. The magnitude of cylinder power is important. A toric IOL with a low cylinder correction at the corneal plane, when misaligned less than 5 degrees, will yield limited potential benefit if repositioned. Surgical reintervention must be weighed against the risks of the intraocular procedure. For higher power toric IOLs, even a small misalignment can reduce visual outcome significantly, and may warrant repositioning. Surgical options for correction of suboptimal refractive results after toric IOL implantation include corneal refractive surgery such as relaxing incisions, laser in situ keratomileusis (LASIK), and photorefractive keratectomy (PRK); IOL repositioning; IOL exchange; and piggyback IOL implantation. The status of the lens capsule is important when considering surgical correction of residual astigmatism. Corneal refractive surgery does not depend on an intact capsule. Small residual refractive errors are effectively corrected with laser refractive surgery. Before proceeding, we recommend waiting 3 months for refractive stability. PCRIs can correct mixed astigmatism up to 2.5-diopter cylinder and can be done with manual blades or a femtosecond laser. Preplaced femtosecond stromal incisions can be selectively opened during or after surgery to titrate astigmatic correction.6 Larger refractive errors may be treated by IOL repositioning or exchange. This approach requires an intact capsular bag and is best performed early in the postoperative period to avoid capsular fibrosis. If anterior capsule polishing is performed at the time of surgery, it will result in less fibrosis and easier IOL manipulation. Toric IOL repositioning can be considered if the residual spherical equivalent (SE) refractive error is less than 0.5 diopters. One should attempt to reopen the original incision. The goal is to align the toric IOL with the steep axis of postoperative corneal cylinder, not the original target meridian from preoperative calculations. Toric IOL exchange is reserved for SE errors greater than 0.5 diopters or when there is a problem with the lens itself. A piggyback IOL can be considered for treating simple myopic or hyperopic refractive errors greater than 1 diopter. The procedure carries less risk of incorrect IOL power, capsule rupture, vitreous loss, and cystoid macular edema as opposed to IOL exchange, balanced against risks including pigment dispersion. We recommend a three-piece, rounded-edge silicone IOL such as the Staar AQ5010V implanted in the ciliary sulcus to avoid interlenticular fibrosis and late hyperopic shift. Piggyback IOL implantation can reduce the effective power of the primary IOL if the zonules are loose, resulting in a surprise hyperopic shift. To wrap up this chapter, we present three patients who had postoperative problems following toric IOL implantation. Each presented with a unique set of circumstances and each required a different management approach. This patient had a history of myopic LASIK, followed by cataract extraction with toric IOL implantation, in both eyes. Neodymium:yttrium aluminum garnet (Nd:YAG) laser capsulotomies had been performed several months before initial evaluation. At the time of presentation, the patient complained of blurred vision in both eyes. Examination revealed small central posterior capsule openings in both eyes, opacified peripheral capsules, and a few pits on the lens in the right eye. Laser capsulotomy enlargements were performed in both eyes and resulted in much improved glare sensitivity and nighttime vision. At a follow-up visit, the patient expressed an interest in undergoing a refractive enhancement to improve the uncorrected distance visual acuity (UDVA). Examination revealed UDVA of 20/30 in the right eye and 20/25 in the left eye. The manifest refractive error of the right eye was – 1.75 + 0.50 × 144, resulting in corrected distance visual acuity (CDVA) of 20/15 −1. Manifest refraction of the left eye was −1.75 + 0.50 × 116, resulting in CDVA of 20/20 + 2. Pachymetry measurements were 440 and 486 μm, respectively. Preoperative Pentacam corneal topography is shown in
10.1 Introduction
10.2 Preoperative Planning
10.3 Toric Power Calculation
10.4 Surgical Considerations
10.5 Suboptimal Outcomes
10.6 Managing Residual Refractive Error
10.7 Clinical Examples
10.7.1 Case 1: Photorefractive Keratectomy Enhancement
Fig. 10.1. After discussing the options, we decided to proceed with PRK enhancement of the left eye to improve UDVA. If the patient could not adjust to the resulting monovision, we would perform the same procedure on the right eye sometime afterward to balance the two eyes. One month after PRK, the patient was satisfied with the outcome and comfortable with the resulting monovision. UDVA was 20/40 in the right eye and 20/15 in the left eye. The manifest refraction of the left eye was −0.25 + 0.50 × 030, resulting in unchanged CDVA of 20/15.
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