Astigmatism occurs when the eye focuses light at 2 focal points because of a spherocylindrical shape of the cornea or lens. Advances in cataract surgery, such as limbal relaxing incisions (LRIs) and toric intraocular lenses (IOLs), have made spectacle independence a possibility for patients with moderate amounts of astigmatism.1 Astigmatism >3.00 diopters (D) is uncommon in cataract patients (2% to 3%), and prevalence of astigmatism greater than 6.00 D is rare (<0.2%).2,3 The detrimental effect of IOL malrotation and limited availability of toric IOL powers in the United States make correcting high astigmatism, >3.00 D, a challenge for surgeons. Eyes with a history of keratorefractive surgeries, such as radial keratotomy (RK), also impose additional challenges to surgeons when making IOL power calculations.4
Patient Consent Statement
Written informed consent was obtained from the patient to publish this case.
A 61-year-old woman with a history of automated lamellar keratoplasty, 4-incision RK, and arcuate incisions presented for cataract surgery. Manifest refraction (MR) was +2.25 –8.00 × 094 degrees in the left eye with corrected distance visual acuity (CDVA) of 20/50. Optical biometry (Argos, Alcon Laboratories, Inc.) showed keratometry of 37.85/46.86 D @ 10 degrees. Keratometry was 37.12/45.37 D @ 4 degrees on auto keratometry (Topcon KR-800, Topcon Medical Systems, Inc.). Simulated keratometry astigmatism on Scheimpflug topography (Galilei G6, Ziemer Ophthalmic Systems AG) was 7.52 D @ 4 degrees (Figure 1).
Barrett True K with a plano target was used for IOL calculations. The effective cylinder power was divided 50:50 between primary and piggyback IOLs. The patient underwent cataract surgery in the left eye with markings placed at 8 degrees for the astigmatism axis. A capsular tension ring (EyeJet type 10, Morcher GmbH) was implanted into the capsule bag, followed by a 13.5 D sphere and 6.00 D cylinder TECNIS Toric II (ZCU600) IOL through a 2.8 mm main incision. Next, an additional piggyback 6.00 D sphere and 5.75 D cylinder enVista toric (MX60T) IOL was implanted into the bag, and the optic was moved anterior to the capsulorhexis for reverse optic capture (Video 1). The ZCU600 was chosen for its rotational stability. A 6.00 D sphere MX60T was selected because this is the smallest spherical power, therefore the thinnest, available IOL of this model. The MX60T's large fenestrated haptics also aid in IOL manipulation. Cylinder powers at the corneal plane for primary and piggyback IOLs were 4.08 D and 3.91 D, respectively. Both lenses were aligned at the astigmatism axis, and toric marks overlaid well.
On postoperative day 1, the patient experienced symptomatic ocular hypertension, and IOP in the left eye was noted at 43 by applanation tonometry. This resolved with a paracentesis followed by a short course of medical management. A visually significant posterior capsular opacity was found postoperatively and treated with a YAG posterior capsulotomy without complication. After 2 months, some residual astigmatism was noted with an MR of −0.75 −2.00 × 74 degrees and a CDVA of 20/30 + 1. Vector analysis by the Alpins method at the spectacle plane using MR results from preoperative and 2-month visits revealed a target-induced astigmatism of 8.00 D × 94 degrees, surgically induced astigmatism of 6.59 D × 100 degrees, magnitude of error of −1.41 D, angle of error of 5.6 degrees, difference vector of 2.00 D × 164 degrees, and correction index of 0.82.5 The ASSORT Toric IOL Refractive Surprise Calculator (www.assort.com) was used with combined spherical and cylinder powers and averaged a constant of the 2 IOLs. The online calculator predicted rotating the IOL from its current position at 8 degrees to a new position at 4 degrees would produce an MR of −1.02 −1.45 × 93 degrees. Owing to this limited potential improvement, photorefractive keratectomy (PRK) refractive enhancement was performed with treatment of −0.61 −1.78 × 69 degrees with a total ablation depth of 20 microns (VISX Star S4 excimer laser, Abbott Medical Optics, Inc.). Lower lid punctal occlusion with a silicone plug was also required for dry eyes. Three months after laser ablation, uncorrected distance visual acuity (UDVA) was 20/25 + 2 in the left eye. There was good rotational stability 6 months after the cataract surgery (Figure 2).
Gills first reported using 2 toric IOLs for the correction of high astigmatism.6,7 Originally, this was accomplished with 2 STAAR Surgical (AA4023 TF) silicone plate IOLs placed in the capsular bag. This method achieved 5.38 D of surgically induced astigmatism, reducing MR from +6.50 −4.75 × 160 degrees to +0.50 −1.00 × 110 degrees with preop CDVA and postop UDVA of 20/50 and 20/30, respectively.6 Later, Gills modified the technique by suturing the IOL haptics together to prevent opposite IOL rotation and the unknown effect this may have on patient vision. The IOLs were oriented back-to-back to prevent optic dimpling and implanted through a 6.0 mm scleral tunnel.7 This achieved similar results as before, but LRIs were required to account for astigmatism induced by the scleral incision. Similarly, further refractive treatments were required to achieve desired results in our case. The ideal cylindrical powers for the primary and piggyback IOLs are currently unknown. Further study could improve preoperative planning and accuracy. Toric implantable collamer lenses (ICLs) are another options. However, ICLs available in the United States, such as the recently approved STAAR EVO Visian ICL, only reach 4 D of cylinder power at the spectacle plane.
More common strategies for astigmatism correction during cataract surgery include LRIs, toric IOLs, and strategic placement of the primary incision used alone or in combination. Creating the primary incision on the steep axis combined with LRIs can flatten corneal curvature and achieve 1.5 to 2.0 D of surgically induced astigmatism. However, the presence of RK incisions compromises corneal biomechanical properties, making LRI less predictable.1 RK incisions also limit LRI placement because overlying incisions will compromise corneal stability. Customizable IOLs with up to 12.00 D of cylinder power exist, but cylinder powers greater than 6.00 D are currently not approved for use in the United States.8 When attempting correction of high cylinders, residual astigmatism after toric IOL implantation is also common. Hoffman et al. found >0.50 D of residual astigmatism was present in >50% of patients with preoperative astigmatism >2.50 D.3
Given our patient's history of keratorefractive surgeries, PRK was the best option and attained desired results. LASIK and PRK have been successfully used in post-RK eyes, but PRK offers the advantage of avoiding further corneal structural weakening, flap complications, or ectasia. Studies on post-PRK haze development show risk factors that include early postoperative UV-light exposure, higher degrees of myopic and astigmatic correction, hyperopic correction, and previous corneal surgeries.9,10 Although there is no consensus on its use with some studies showing nonsignificant effects, intraoperative mitomycin-C is often used for haze prophylaxis but was not used in our case.11 Some have posited an increased risk of endothelial toxicity in post-RK eyes, but mitomycin-C has been safely used in post-RK eyes.12
One cause of residual refractive error is malrotation of the toric lens, and with increasing cylinder powers, the more pronounced the effects of malrotation on refractive error.1 IOL rotational stability is essential for single toric IOLs, and perhaps even more important with 2.6 The use of a capsular tension ring aids in intraoperative centration and may improve toric rotational stability by lessening capsular bag shrinkage, which occurs over 3 months postoperatively.13,14 The frosted haptics found on the TECNIS Toric II also provides better rotational stability than its predecessor, although current literature lacks long-term data.
One-piece acrylic IOLs with reverse optic capture have been used in cases of posterior capsular bag tears.15,16 IOLs placed in the sulcus have an effectively greater power that requires compensation to avoid a myopic shift. However, Jones et al. reported that reverse optic capture produces minimal myopic shifting (−0.32 D) compared with sulcus IOLs (−0.8 D).15 The capsular haptic placement keeps the optic in a more posterior position, so power calculation compensation is unnecessary. In this case, the Barrett True K IOL power calculation was chosen for its high accuracy in eyes with RK incisions. Since RK flattens both the anterior and posterior corneal surfaces, other formulas fail to accommodate this without underestimating the IOL power.4 In addition, the Barrett True K formula may be used with or without entering surgically induced change in refraction because it uses an internal regression formula to estimate changes in MR, and it also includes extra parameters to increase formula stability.17
Interlenticular opacification (ILO) is a known complication of piggyback IOLs, particularly when both lenses are a hydrophobic acrylic material and placed in the bag.18 Placing the piggyback IOL in the ciliary sulcus reduces the chances of ILO, but sulcus placement of a single-piece acrylic IOL is not recommended because the bulky, sharp-edged haptics may contact the iris and cause pigment dispersion or uveitis–glaucoma–hyphema syndrome.18,19 With the haptics in the capsular bag and optic anterior to the capsulorhexis, reverse optic capture of the single-piece acrylic piggyback IOL minimizes these risks while providing a high astigmatic correction. If available, using an IOL designed for the sulcus is the ideal piggyback approach to avoid the development of ILO.
WHAT WAS KNOWN
- Limbal relaxing incisions, toric IOLs, and strategic placement of the primary incision may achieve spectacle-free vision for most cataract surgery patients with mild astigmatism.
- Astigmatism greater than 3.00 D is rare and poses a challenge for cataract surgeons, especially in patients with a history of extensive refractive surgery.
WHAT THIS PAPER ADDS
- The use of double toric IOLs, primary and piggyback single-piece acrylic toric IOLs with reverse optic capture, seems to be a viable option for treating high astigmatism in patients with extensive prior keratorefractive surgery.
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