Toric intraocular lenses (IOLs) are increasingly used to correct corneal astigmatism at the time of cataract surgery. Several variables are known to affect the accuracy of toric IOLs, including IOL calculation formulas,1 IOL misalignment during surgery,2 and postoperative IOL rotation.3 In this report, we present two patients with unexplained residual refractive astigmatism following toric IOL implantation. In addition, we show how wavefront aberrometry may be used to find the source of residual refractive astigmatism following toric IOL implantation.
A 58-year-old woman was referred to our clinic with residual refractive astigmatism following cataract surgery with toric IOL implantation in her right eye. The IOL spherical power for emmetropia had been calculated using IOLMaster biometry (Carl Zeiss Meditec, Jena, Germany). The toric IOL cylinder power and alignment axis were calculated using a Web-based IOL calculator ( http://www.acrysoftoriccalculator.com) and based on automated keratometry values obtained with the IOLMaster (2.33 D @ 80°). The expected surgically induced astigmatism (SIA) was −0.25 diopters (D). Standard phacoemulsification was performed with a 2.2-mm limbal incision located at 95°. A SN60T5 (Alcon Laboratories Inc, Ft Worth, Texas) toric IOL with a spherical power of 29.50 D and a cylinder power of 3.00 D (2.06 D at the corneal plane) was implanted and aligned at 78°. Residual astigmatism of 0.01 D @ 78° was anticipated. No intraoperative complications occurred.
Four months postoperatively, she was referred to our clinic for analysis of residual refractive astigmatism. Uncorrected distance visual acuity (UDVA) in the right eye was 20/30 and corrected distance visual acuity (CDVA) was 20/22 with a subjective refraction of 0 −1.75 × 95, indicative of an astigmatic overcorrection. Slit-lamp examination showed the toric IOL appropriately aligned at 80°. Scheimpflug imaging (Pentacam; Oculus Optikgeräte GmbH, Wetzlar, Germany) demonstrated a regular bow-tie corneal pattern. Postoperative anterior chamber depth (ACD) was 2.34 mm and pachymetry 533 μm, with an estimated lens position of 2.87 mm. The effective cylinder power at the corneal plane, calculated using a vertex formula, was 2.55 D. Combined wavefront aberrometry and corneal topography (KR-1W Wavefront Analyzer; Topcon Corp, Tokyo, Japan) was performed to determine whether the correct IOL cylinder power had been implanted. The Figure shows the ocular, corneal, and internal astigmatism values for a 4- and 6-mm pupil. Internal astigmatism was −3.16 D @ 87° (6-mm zone), which corresponded with the intended toric IOL cylinder power (−3.00 D @ 78°). While performing aberrometry without mydriasis, we measured a photopic pupil diameter of 6.2 mm. Corneal astigmatism in the 6-mm zone was −1.40 D @ 174°, compared to −2.21 D @ 171° in the 4-mm zone.
Figure. Case 1. Postoperative combined wavefront aberrometry and corneal topography (KR-1W, Topcon Corp) following toric intraocular lens implantation. Corneal astigmatism, internal astigmatism, and ocular astigmatism (combined corneal and internal astigmatism) were measured for a 4- and 6-mm pupil.
Treatment options included IOL exchange with a lower cylinder power toric IOL (SN60T3 [Alcon Laboratories Inc], cylinder power 1.50 D), or corneal refractive surgery. However, the patient declined further treatment and used spectacles with a cylinder correction.
A 60-year-old man presented with decreased visual acuity in his right eye due to cataract. IOLMaster biometry was performed and demonstrated 1.51 D @ 173° of corneal astigmatism. Corneal topography showed regular corneal astigmatism. After discussing the available options, he wished to undergo cataract surgery with implantation of a multifocal toric IOL. The multifocal toric IOL cylinder power and alignment axis were calculated using a Web-based IOL calculator ( http://www.acrysoftoriccalculator.com). The expected SIA was −0.30 D. Standard phacoemulsification was performed with a 2.2-mm limbal incision located at 95°. A SND1T4 (Alcon Laboratories Inc) multifocal toric IOL with a spherical power of 22.50 D, an add power of 3.00 D, and a cylinder power of 2.25 D (1.50 D at the corneal plane) was implanted and aligned at 175°. Residual astigmatism of 0.24 D @ 175° was anticipated. No intraoperative complications occurred.
Two weeks postoperatively, UDVA was 20/50 and CDVA was 20/20 with a subjective refraction of +0.25 −1.00 × 102, indicating that an undercorrection of approximately 1.00 D had occurred. The IOL alignment axis was 170°. Postoperative ACD was 4.03 mm and pachymetry 527 μm (estimated lens position of 4.55 mm). The effective cylinder power at the corneal plane, calculated using a vertex formula, was 1.85 D. Combined wavefront aberrometry and corneal topography (KR-1W Wavefront Analyzer) was performed to determine whether the correct IOL cylinder power had been implanted. Internal astigmatism was −2.26 D @ 170° (4-mm zone), which corresponded with the intended toric IOL cylinder power (−2.25 D @ 175°). Photopic pupil diameter was 2.9 mm.
To treat the astigmatism undercorrection, IOL exchange with a higher cylinder power multifocal toric IOL (SN6AT5 [Alcon Laboratories Inc], cylinder power 3.00 D) was performed. Postoperatively, UDVA was 20/20.
This report presented two patients with residual refractive astigmatism following toric IOL implantation. We believe that a combination of factors was responsible for the over- and undercorrection of astigmatism that occurred in these patients, including the effect of the spherical power and ACD in toric IOL calculations,1 the effect of posterior corneal astigmatism,4 and the effect of a large pupil size.
As shown by Goggin et al,1 the IOL spherical power and estimated lens position (ACD plus pachymetry) determine the effective cylinder power of a toric IOL at the corneal plane. In the first patient, the calculated effective cylinder power at the corneal plane was 2.55 D, compared to an estimated power of 2.06 D used by the manufacturer. This effect may partly explain the over-correction of astigmatism in our first patient.
As shown by Koch,4 the posterior corneal surface also affects corneal astigmatism. The posterior corneal surface acts as a minus lens and is generally steep vertically. It therefore creates a plus power along the horizontal meridian and induces against-the-rule corneal astigmatism. In the first patient with with-the-rule corneal astigmatism, the posterior corneal surface already corrects part of the corneal astigmatism, thereby reducing overall corneal astigmatism. In the second patient, anterior surface corneal astigmatism showed an against-the-rule configuration. In this patient, the increase in against-the-rule corneal astigmatism may have resulted in the undercorrection of astigmatism. According to Koch,4 this effect may be accounted for in toric IOL calculations by decreasing corneal astigmatism by 0.50 D in patients with with-the-rule astigmatism and increasing corneal astigmatism by 0.30 D in patients with against-the-rule astigmatism.
The relatively large pupil diameter and the subsequent influence of the prolate or aspheric shape of the cornea may have also contributed to the overcorrection of astigmatism in the first patient. The normal aspheric shape of the cornea implies that the center of the cornea is steeper than the periphery. In this patient, corneal astigmatism in a 6-mm pupil zone was much lower compared to a 4-mm pupil zone. As a result, the strength of the toric IOL was overpowered by approximately 1.00 D. Various methods may be used to measure corneal astigmatism, including automated keratometry, manual keratometry, and corneal topography.5,6 However, these methods determine corneal astigmatism based on a central 2.0- to 3.0-mm zone of the cornea. This may be an effective measure of corneal astigmatism in the majority of patients, but it may be inadequate in younger patients with larger pupil diameters. We recommend measuring the pupil diameter in relatively young patients before implanting a toric IOL, as these patients generally have a larger pupil size. If the photopic pupil diameter is >4 mm, we recommend using corneal astigmatism values for a larger zone of the cornea.
Combined wavefront aberrometry and corneal topography can differentiate between aberrations caused by the cornea or internal ocular system7 and may help find the source of residual astigmatism. The advantage of the Topcon KR-1W is that corneal topography and aberrometry measurements are performed simultaneously, thereby minimizing potential errors that may be caused by realignment of the eye. A previous study showed that combined aberrometry and corneal topography may be used to determine the postoperative toric IOL alignment axis.8 In addition, in our report, we used aberrometry to determine the source of residual refractive astigmatism following toric IOL implantation.
Unexplained residual refractive astigmatism following toric IOL implantation was the result of multiple factors: the effect of the spherical power and ACD on toric IOL calculations, the effect of posterior corneal astigmatism, and the effect of a large pupil size. The first two issues may be compensated for by improving toric IOL calculations. In addition, we recommend performing pupillometry in relatively young patients who wish to undergo cataract surgery with toric IOL implantation. If the photopic pupil diameter is >4 mm, we recommend incorporating corneal astigmatism values for a larger zone of the cornea into the toric IOL calculation.
- Goggin M, Moore S, Esterman A. Outcome of toric intraocular lens implantation after adjusting for anterior chamber depth and intraocular lens sphere equivalent power effects. Arch Ophthalmol. 2011;129(8):998–1003. doi:10.1001/archophthalmol.2011.188 [CrossRef]
- Visser N, Berendschot TT, Bauer NJ, Jurich J, Kersting O, Nuijts RM. Accuracy of toric intraocular lens implantation in cataract and refractive surgery. J Cataract Refract Surg. 2011;37(8):1394–1402. doi:10.1016/j.jcrs.2011.02.024 [CrossRef]
- Shimizu K, Misawa A, Suzuki Y. Toric intraocular lenses: correcting astigmatism while controlling axis shift. J Cataract Refract Surg. 1994;20(5):523–526.
- Koch DD. Corneal optics for IOL selection: cracking the code. Presented at: American Society of Refractive and Cataract Surgery annual meeting. ; April 20–24, 2012. ; Chicago, IL. .
- Ahmed I, Rocha G, Slomovic AR, et al. Visual function and patient experience after bilateral implantation of toric intraocular lenses. J Cataract Refract Surg. 2010;36(4):609–616. doi:10.1016/j.jcrs.2009.10.044 [CrossRef]
- Holland E, Lane S, Horn JD, Ernest P, Arleo R, Miller KM. The AcrySof Toric intraocular lens in subjects with cataracts and corneal astigmatism: a randomized, subject-masked, parallel-group, 1-year study. Ophthalmology. 2010;117(11):2104–2111. doi:10.1016/j.ophtha.2010.07.033 [CrossRef]
- Visser N, Berendschot TT, Verbakel F, Tan AN, de Brabander J, Nuijts RM. Evaluation of the comparability and repeatability of four wavefront aberrometers. Invest Ophthalmol Vis Sci. 2010;52(3):1302–1311. doi:10.1167/iovs.10-5841 [CrossRef]
- Carey PJ, Leccisotti A, McGilligan VE, Goodall EA, Moore CB. Assessment of toric intraocular lens alignment by a refractive power/corneal analyzer system and slitlamp observation. J Cataract Refract Surg. 2010;36(2):222–229. doi:10.1016/j.jcrs.2009.08.033 [CrossRef]