Eyeworld

47 EW RESIDENTS February 2016 by Tomas Moreno, MD, Mark Breazzano, MD, Niraj Nathan, MD, and Laura Wayman, MD, Vanderbilt Eye Institute, Vanderbilt University Medical Center can be reduced by appropriate con- sideration of the posterior corneal astigmatism. Study summary Reitblat and colleagues present a retrospective study evaluating 115 eyes from 92 patients that received toric IOL implantation accompanied by preop measurements with optical biometry. The LENSTAR-LS900 (Haag-Streit, Koniz, Switzerland) and Scheimpflug camera (Pentacam, Oculus, Arlington, Wash.) were used to gather measurements, and 5 methods of toric IOL calculation were compared: 1) anterior corneal astigmatism using the LENSTAR device, 2) application of Baylor toric nomogram, 3) posterior tomography combined with anterior corneal measurements using vector summation, 4) Pentacam's true net power (TNP) using radii of anterior and posterior corneal curvatures, and 5) total corneal refractive power (TCRP) using ray tracing by Pentacam. Exclusion criteria were any previous ocular pathology, perioperative complications, incom- plete data, or insufficient quality of measurements by Hill validation criteria. The main outcome measure was the median simulated residual astigmatism (SRA). Compared with calculations using anterior corneal measurements only, Baylor nomogram, TNP, and TCRP measurements, lower SRA Review of "The effect of posterior corneal astigmatism on power calculation and alignment of toric intraocular lens: a comparison between different methodologies" What is the best way to factor the contribution of posterior corneal astigmatism when implanting toric IOLs? I asked the Vanderbilt residents to review this study in the February issue of JCRS. –David F. Chang, MD, EyeWorld journal club editor Laura Wayman, MD, director of resident education, Vanderbilt Eye Institute, Vanderbilt University Medical Center, Nashville, Tenn. continued on page 48 EyeWorld journal club Introduction Accurate measurements and calcu- lation in modern cataract surgery are imperative to minimize postop refractive error to reduce the need for spectacle wear after cataract surgery. Corneal astigmatism has posed a challenge to minimizing this need. A systematic review of the literature showed that the implantation of toric IOLs does improve uncorrected best visual acuity, reduce the need for spectacle wear, and decrease overall refractive astigmatism. 1 However, residual refractive astigmatism was still seen in all studies that reported postop refraction. In 3 recent studies, the average postop refractive astigma- tism was 0.95±0.66 D, –0.62±0.41 D, and 0.28±0.21 D. 2–4 Traditionally, posterior corneal astigmatism has not been incor- porated in toric IOL calculations. However, a large study analyzing 715 corneas showed that the mean magnitude of posterior corneal astig- matism was –0.30, and ignoring the posterior corneal astigmatism may yield incorrect estimation of total corneal astigmatism. 5 In a recent study, Koch et al. used the Galilei Placido dual Scheimpflug analyzer (Ziemer, Port, Switzerland) to mea- sure both the anterior and posterior corneal astigmatism and compare it to traditional methods that only measured anterior corneal astigma- tism in estimating the actual corneal astigmatism of patients that received toric IOLs. 6 The study concluded that corneal astigmatism was overes- timated in with-the-rule astigmatism by all devices and underestimated in against-the-rule astigmatism by all except the Placido dual Scheimpflug analyzer; the authors proposed a new toric IOL nomogram. 6 In their study, Reitblat et al. compare 4 methods of calculation that consider posterior corneal astigmatism in patients who under- went toric IOL implantation, one of which is a method proposed by the authors of combining anterior corneal measurements and posterior corneal tomography using vector analysis as previously described by Holladay et al. 7–8 The study con- cludes that residual astigmatism following toric IOL implantation The effect of posterior corneal astigmatism on power calculation and alignment of toric intraocular lens: a comparison between different methodologies Olga Reitblat, MD, Adi Levy, BSc, Guy Kleinmann, MD, Adi Abulafia, MD, Ehud I. Assia, MD; J Cataract Refract Surg (Feb) 2015;42. Article in press Purpose: To compare the accuracy of different methods that consider posterior corneal curvature in toric intraocular lens (IOL) calculations. Setting: Ein-Tal Eye Center, Tel-Aviv, Israel. Design: Comparative case series. Methods: Consecutive cases of toric IOL implantation and preoperative measurements by optical biometry with the Lenstar-LS900 and Scheimpflug camera (Pentacam, Oculus) were retrospectively reviewed. Five methods of toric IOL calculation were compared: (1) anterior corneal astigmatism using the Lenstar device, (2) application of the Baylor nomogram, (3) posterior tomography combined with anterior corneal measurements using vector summation, (4) Scheimpflug camera's true net power (TNP), and (5) total corneal refractive power (TCRP). Toric IOL astigmatic power and axis, aiming for the lowest residual astigmatism, were selected according to these methods. Simulated residual refraction was calculated for each method based on manifest refraction and measured IOL alignment >3 weeks following surgery. Results: The study included 115 eyes of 92 patients. The median simulated residual astigmatism was lower when based on vector summation of anterior and posterior astigmatisms compared with calculations based on anterior corneal measurements only, application of the Baylor nomogram, TNP and TCRP readings (0.49 diopters [D] vs. 0.70 D, 0.60 D, 0.64 D, and 0.76 D, respectively, P<.001). Conclusions: Residual astigmatism following toric IOL implantation can be reduced by appropriate consideration of the posterior corneal astigmatism. We suggest using methods that take into account the effect of the posterior cornea in toric IOL calculations.

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