Correction of corneal astigmatism and stability of toric intraocular lenses

You don't understand anything until you learn it more than one way —Marvin Minsky Modern-day cataract surgery has evolved to refractive cataract surgery in which the surgeon can customize and tailor the refractive outcome for each individual patient. Advances in optical biometry and modern intraocular lens (IOL) calculation formulas have significantly reduced the incidence of postoperative refractive errors, whereas residual astigmatism is a concern for both surgeons and patients and can leave patients with symptomatic decreased visual function.1 Technological advancement has led to the use of partial coherence interferometry as a biometry technique that uses diode laser infrared light at a wavelength of 780 nm. Since the advent of the first commercial device in 2001 (IOLMaster, Carl Zeiss Meditec AG), noncontact optical biometry has become the technique of choice for ocular biometry. Over the past decade, several researchers investigated the prevalence of preexisting corneal astigmatism in patients undergoing cataract surgery. Although sex and race may play a role, the prevalence of preoperative corneal astigmatism of greater than 1.0 diopters (D) in patients undergoing cataract surgery is estimated to be between 30% and 37%.2–4 The prevalence of higher corneal astigmatism in the cataract population is much less. The prevalence is approximately 8% for corneal astigmatism greater than 2.0 D and between 1.9% and 2.6% greater than 3.0 D.3,4 Several surgical options are available to correct corneal astigmatism. This includes planning the clear corneal surgical incision on the steep meridian, opposite-site clear corneal incisions, peripheral corneal relaxing incisions, and the use of toric IOLs.5–8 Moreover, technologies such as excimer laser ablation (laser in situ keratomileusis and photorefractive keratectomy) and small-incision lenticule extraction can be used as a secondary intervention in the postoperative period to correct residual corneal astigmatism after cataract surgery. In the current era of refractive cataract surgery, toric IOLs have become an important surgical tool. In 1992, Shimizu et al. designed the first toric IOL to correct corneal astigmatism during cataract surgery.9 This 3-piece IOL was made of nonfoldable poly(methyl methacrylate) (PMMA) and required a 5.7 mm incision for implantation. The postoperative uncorrected distance visual acuity (UDVA) and residual refractive astigmatism outcomes were not described, but corrected distance visual acuity was 20/25 or better in 77% of eyes. However, approximately 20% of the IOLs rotated 30 degrees or more and 50% rotated more than 10 degrees. In 1994, the first foldable 1-piece toric IOL became available. This IOL was made of silicone material and could be implanted through a much smaller 3.2 mm incision.10 The first clinical results with this IOL were promising with 23% of patients achieving a UDVA of 20/25 or better compared with 4% of patients with a standard IOL.10 However, there was a high rate of postoperative rotation of more than 10 degrees in 20% to 30% of the eyes.11,12 Toric IOLs have been found to produce the most reliable results, with the lowest postoperative astigmatism for preoperative astigmatism between 1.0 D and 3.0 D.13 They also provide the best postoperative UDVA when compared with nontoric IOLs and limbal relaxing incisions. Furthermore, since toric IOLs do not require additional surgical wounds, the recovery time for patients is generally faster.14 However, the Achilles heel of the toric IOLs has been its rotational stability. Several factors come into play that affect the rotational stability of a toric IOL, including the lens design, lens material, size of the capsulorhexis, and surgical techniques. Toric IOLs are made of hydrophobic acrylic, hydrophilic acrylic, silicone, or PMMA biomaterial. The IOL biomaterial has a major influence on the postoperative rotation of the IOL. After implantation of a toric IOL in the capsular bag, the anterior and posterior capsules fuse with the IOL, preventing rotation.15 Strong IOL adhesions to the capsular bag are believed to prevent IOL rotation. Among the IOL materials, hydrophobic acrylic showed the highest adhesive properties, followed by hydrophilic acrylic and PMMA, and finally silicone.16 Extracellular matrix proteins, such as fibronectin, vitronectin, and type IV collagen found in the aqueous humor after cataract surgery may be involved in IOL adhesion to the capsular bag. Fibronectin, in particular, is believed to play a major role in IOL–capsular bag adhesion.15 Acrylic IOLs explanted from human autopsy eyes contained significantly more fibronectin than silicone or PMMA IOLs. These results indicate that acrylic IOLs form the strongest adhesions with the capsular bag.17 In addition to the biomaterial, design and size of the toric IOL play a role in long-term stability. Smaller size (10.8 mm) plate-haptic silicone IOLs were found to be more unstable compared with 11.2 mm plate-haptic silicone IOLs.18 Currently, toric IOLs are available in overall lengths ranging from 11.0 to 13.0 mm. Patel et al. in a randomized controlled trial comparing the postoperative rotation of plate-haptic and loop-haptic silicone IOLs found that the postoperative rotation was significantly higher with loop-haptic IOLs than with plate-haptic IOLs.19 However, there seems to be no statistically significant difference between a plate-haptic and a loop-haptic IOL made of acrylic material.20 In this issue, Yao et al. (page 1436) in a retrospective case series of 102 eyes evaluated the association between anterior segment parameters and the rotational stability of a plate-haptic acrylic toric IOL. In their cohort, plate-haptic toric IOLs rotated more in eyes with larger white-to-white distances and longer anterior segment length. This study provided relevant and interesting conclusions that a larger white-to-white distance and anterior segment length correlate with a greater risk for IOL rotation, presumably because of a larger volume and diameter of capsular bag. This would explain why some studies show a correlation with higher axial length and toric IOL rotation, but others do not. The study has several strengths: single surgeon, photographic measurement of misalignment, and use of digital image axis guidance. The sample size is not large but seems to be sufficient to observe a correlation with some measurements and not with others. I think the jury is still out to whether primary placement of a capsular tension ring in large eyes has any influence on the postoperative rotational stability of toric IOLs.