Dr. Sanket Bhatnagar,
B16754, Dr. Mathew Kurian, Dr. Luci Kaweri, Dr.
Rohit Shetty
Abstract
Purpose – To compare the calculated intraocular lens (IOL) cylindrical power using two different toric IOL calculation methods.
Study design – Prospectiveinterventional study.
Materials and methods – 73 eyes with astigmatism [34 with-the-rule (WTR) and 39 against the rule(ATR)]of 58 patients who underwent cataract surgerywith toric IOL implantation were analyzed. IOL power was calculated for each case using Abbott Medical Optics [AMO] Toric Calculator without and with Baylor’s nomogram adjustment and Barrett’s Online Toric calculator.
Results – Mean prediction error of residual cylinder power was 0.89 ± 0.68 and 0.94 ± 0.93 using the Barrett’s and AMO formula respectively for WTR astigmatism and 0.9 ± 0.6 and 0.78 ± 0.68 respectively for ATR astigmatism. In cases with WTR astigmatism 19 % with Barrett’s calculator and 17% with Baylor’s adjustment showed cylinder power same as AMO toric calculator. In ATR astigmatism cases this similarity was seen in 58% with Barrett’s online and 52% with Baylor’s adjustment.
Conclusion –Inclusion of the effect of posterior cornea in calculation of toric IOL alters the cylindrical power especially in cases with WTR astigmatism. Age related changes in anterior corneal astigmatism should be considered while calculating for toric IOLs.
Introduction
Prevalence of corneal astigmatism in patients undergoing cataract surgery is based on the type of population studied and varies demographically.[i],[ii]Study done by Ferrer-Blascoet al in 2009 suggests that around 20% to 30% of patients undergoing cataract surgery with standard monofocal intraocular lens have 1.25 diopters (D) or higher of uncorrected corneal astigmatism; of which 10% of patients have an astigmatism of 2.00 D or higher.[iii]Implantation of standard monofocal intraocular lens in all cases would correct the spherical refractive error but corneal astigmatic error will remain unadressed.[iv]
Toric IOLs are known to be safe and effective option for correction of astigmatism during cataract surgery.[v]The current available toric intraocular lenses (TIOLs) offer cylindrical correction over a varied range of astigmatism. For calculating the IOL power, various online calculators like the Abbott Medical Optics toric IOL calculator, Alcon Acrysoftoric IOL calculator, Hoya online calculators, Z calc by Carl Zeiss, Truligntoric IOL calculator by Bausch and Lomb are available. But all these are company specific and do not take into consideration the posterior corneal astigmatism. Importance of posterior cornea in astigmatism correction was first stressed by Koch et al in their study where they showed that posterior cornea has an astigmatism ranging from 0.26 D to 0.78 (mean -0.3D).[vi] Ignoring this posterior corneal astigmatism results in an under correction in patients having against-the-rule astigmatism (ATR) and overcorrection in patients having with-the-rule astigmatism (WTR). 6They introduced the Baylor’s toric nomogram that takes into account the effect of posterior cornea in the presence of WTR and ATR corneal astigmatism. It can also be used in conjunction with standard toric IOL calculators.
To make these calculations easy and user friendly, Barrett’s toric IOL calculatorwas made available online since 2013.It takes into consideration the posterior corneal astigmatism as well as effective lens position for each individual patient.[vii]The purpose of our study was to compare the calculated IOL cylindrical power obtained by Abbott Medical Optics (AMO) toric calculator without and with Baylor’s nomogram adjustment and by Barrett’s online toric calculator.
Materials and methods
The study had institutional ethical committee approval and conformed to the Declaration of Helsinki. It was a retrospective analysis of 73 eyes of 58 patients who underwent cataract surgery with implantation of toric IOL through a 2.2 mm clear corneal incision by a single surgeon at a tertiary eye care centre in Bangalore, India. Patients with any intraoperative complications or astigmatism secondary to corneal pathology like keratoconus were excluded from the study. A written informed consent was obtained for each patient.
Methods of Calculation: Corneal astigmatism was measured using Pentacam HR (Oculus, Inc, Wetzlar, Germany) and optical biometry was done using Lenstar LS 900(Haag-Streit AG, Switzerland). Using these values and taking surgically induced astigmatism (SIA) as 0.2D at the incision site (180* in the right eye and 20* in the left eye) toric IOL calculation was done using the online AMO toric calculator and the Barrett’s online toric calculator.
With-the-rule or Against-the-rule type of astigmatism was determined and accordingly addition or deletion of surgically induced astigmatism was done to the corneal astigmatism based on Baylor’s nomogram.6 Cylindrical power was then selected based on company classified IOL for that range of corneal astigmatism.
Results
There was a male (52.1%) preponderance in our study.34 eyes (46.6%) had with-the-rule (WTR) astigmatism and 39 eyes (53.4%) had against-the-rule (ATR) astigmatism. Mean prediction error of residual cylinder power for WTR astigmatism patients was 0.89 ± 0.68 using the Barrett’s calculator and 0.94 ± 0.93 using the AMO toric calculator whereas for ATR astigmatism, it was 0.9 ± 0.6 using the Barrett’s calculator and 0.78 ± 0.68 using the AMO calculator.
In cases with WTR astigmatism when toric IOL cylinder calculated by Barrett’s calculator and Baylor’s adjustment were compared to AMO toric calculator, the same cylinder power was selected in 19% and 17% cases only. Whereas in 75 % and 76% cases respectively, the Barrett’s calculator and Baylor’s adjustment gave an under correction when compared to AMO calculator. (Table 1)
1.Isyaku M, Ali SA, Hassan S. Preoperative corneal astigmatism among adult patients with cataract in Northern Nigeria. Indian Journal of Ophthalmology. 2014;62(11):1094-1095.
2.Yuan X, Song H, Peng G, Hua X, Tang X. Prevalence of Corneal Astigmatism in Patients before Cataract Surgery in Northern China. Journal of Ophthalmology. 2014;2014:536412.
3.Ferrer-Blasco T, Montes-Mico R, Peixoto-de-Matos SC,Gonzalez-Meijome JM, Cervino A. Prevalence of corneal astigmatism before cataract surgery. J Cataract Refract Surg 2009;35:70–75
4.Holland E, Lane S, Horn JD, Ernest P, Arleo R, Miller KM. The AcrySofToricintraocular lens in subjects with cataracts and corneal astigmatism: arandomized, subject-masked, parallel-group, 1-year study. Ophthalmology. 2010Nov;117(11):2104-11.
5.Visser N, Ruíz-Mesa R, Pastor F, Bauer NJ, Nuijts RM, Montés-Micó R. Cataract surgery with toric intraocular lens implantation in patients with high cornealastigmatism. J Cataract Refract Surg. 2011 Aug;37(8):1403-10.
6.Koch DD, Jenkins RB, Weikert MP, Yeu E, Wang L. Correcting astigmatism with toricintraocular lenses: effect of posterior corneal astigmatism. J Cataract Refract Surg. 2013 Dec;39(12):1803-9
7.Abulafia A, Barrett GD, Kleinmann G, Ofir S, Levy A, Marcovich AL, Michaeli A,Koch DD, Wang L, Assia EI. Prediction of refractive outcomes with toricintraocular lens implantation. J Cataract Refract Surg. 2015 May;41(5):936-44.
| Relationship between Toric IOL cylinder power selected
(Barrett v/s AMOtoric) |
Percentage
(No of patients) |
| Same | 19 (6) |
| Barrett lesser than AMO (Overall)
One step less Two step less |
75 (23)
52 (16) 23 (7) |
| Barrett more than AMO (Overall)
One step more Three steps more |
6 (2)
3 (1) 3 (1) |
| Relationship between Toric IOL cylinder power selected
(Baylor adjustment v/s AMO toric) |
Percentage
(No of pts) |
| Same | 17 (5) |
| Baylor lesser than AMO (Overall)
One step lower Two step lower Three step lower |
76 (22)
52 (15) 21 (6) 3 (1) |
| Baylor more than AMO (by two steps) | 7 (2) |
| Relationship between Toric IOL cylinder power selected
(Barrett v/s Baylor’s adjustment) |
Percentage
(No of pts) |
| Same | 53 (17) |
| Barrett lesser than Baylor (by one step) | 16 (5) |
| Barrett more than Baylor (Overall)
One step more Two step more |
31 (10)
25 (8) 6 (2) |
Table 2 shows relationship between toric IOL cylinder powers selected in cases with ATR astigmatism. Barrett’s online and Baylor’s adjustment showed same cylindrical power as AMO calculator in 58% and 52% cases respectively. Whereas in 54% cases, the Barrett’s calculation gave a cylinder power lesser than Baylor’s adjustment.
| Relationship between Toric IOL cylinder power selected (ATR)
(Barrett v/s AMO toric) |
Percentage
(No of patients) |
| Same | 58 (18) |
| Barrett lesser than AMO (One step less) | 4 (3) |
| Barrett more than AMO
One step more Three steps more (due to absence of corresponding cylinder power in Tecnis) |
38 (12)
35 (11) 3 (1) |
| Relationship between Toric IOL cylinder power selected
(Baylor’s adjustment v/s AMO toric) |
Percentage |
| Same | 52 (16) |
| Baylor lesser than AMO(by one step) | 45 (14) |
| Baylor more than AMO( by one step) | 3 (1) |
| Relationship between Toric IOL cylinder power selected
(Barrett v/s Baylor’s adjustment) |
Percentage |
| Same | 35 (11) |
| Barrett lesser than Baylor (Overall)
One step lesser Two step lesser Three step lesser |
54 (17)
39 (12) 9 (3) 6 (2) |
| Barrett more than Baylor (Overall)
One step more Two steps more Three steps more |
10 (3)
3.3 (1) 3.3 (1) 3.3 (1) |
Discussion
Toric IOLs are known to have an enhanced visual and refractive effect over opposite clear corneal incisions.[i]But the post-operative outcomes are not always the same as predicted by manufacturer’s guidelines.[ii]This could be due to the unaccounted effect of posterior corneal astigmatism towards the total corneal power.[iii]
The Baylor toricnomogram, introduced by Koch at el., takes into account the effect of the poste- rior cornea in the presence of with-the-rule (WTR) and against-the-rule (ATR) corneal astigmatism.6It can be used with various company specific online calculators, which do not include the effect of posterior corneal astigmatism in their calculation. However the calculation is cumbersome and confusing. Introduction of Barrett’s online calculator made it user-friendly and predicted post-operative residual astigmatism was an added advantage.
In our study, the mean prediction error of residual cylinder found in both WTR and ATR astigmatismwas relativelyhigher, which may be attributed to the presence of high astigmatism in few of our patients for which the required toric IOL cylinder power was not available and had to be under corrected. But the lower standard deviation with Barrett’s calculator suggests that the range of residual cylinder is much lessand therefore predictability is better with the Barrett’s calculator than the AMO toric calculator.
In our study we also found under correction with Barrett’s calculator and Baylor’s nomogram adjustment in comparison to AMO toric calculator in cases with WTR astigmatism. It is because both these formulae take into account the age related shift of WTR astigmatism of anterior cornea towards ATR..[iv]Both Barrett’s calculator and Baylor’s nomogram aim to leave eyes with small amounts of WTR refractive astigmatism.
In cases with ATR, approximately 50 % cases had same IOL cylinder as AMO calculator. This could be because of different parameters and formulas used by these calculators. AMO calculator uses Holladay 1 formula to calculate the cylinder power, while Barrett’s calculator uses Barrett’s universal II formula to calculate the toric IOL power at the corneal plane.
The disadvantage of these calculations including posterior corneal astigmatism is that they takeinto account the average posterior corneal astigmatism of the population as a whole and not individual patients. At present, we have crossed one more milestone in our quest for emmetropia. Further studies and long term post-operative results will make it possible for customisation of IOL for each patient and hence even better refractive outcomes.
References:
1.Isyaku M, Ali SA, Hassan S. Preoperative corneal astigmatism among adult patients with cataract in Northern Nigeria. Indian Journal of Ophthalmology. 2014;62(11):1094-1095.
2.Yuan X, Song H, Peng G, Hua X, Tang X. Prevalence of Corneal Astigmatism in Patients before Cataract Surgery in Northern China. Journal of Ophthalmology. 2014;2014:536412.
3.Ferrer-Blasco T, Montes-Mico R, Peixoto-de-Matos SC,Gonzalez-Meijome JM, Cervino A. Prevalence of corneal astigmatism before cataract surgery. J Cataract Refract Surg 2009;35:70–75
4.Holland E, Lane S, Horn JD, Ernest P, Arleo R, Miller KM. The AcrySofToricintraocular lens in subjects with cataracts and corneal astigmatism: arandomized, subject-masked, parallel-group, 1-year study. Ophthalmology. 2010Nov;117(11):2104-11.
5.Visser N, Ruíz-Mesa R, Pastor F, Bauer NJ, Nuijts RM, Montés-Micó R. Cataract surgery with toric intraocular lens implantation in patients with high cornealastigmatism. J Cataract Refract Surg. 2011 Aug;37(8):1403-10.
6.Koch DD, Jenkins RB, Weikert MP, Yeu E, Wang L. Correcting astigmatism with toricintraocular lenses: effect of posterior corneal astigmatism. J Cataract Refract Surg. 2013 Dec;39(12):1803-9
7.Abulafia A, Barrett GD, Kleinmann G, Ofir S, Levy A, Marcovich AL, Michaeli A,Koch DD, Wang L, Assia EI. Prediction of refractive outcomes with toricintraocular lens implantation. J Cataract Refract Surg. 2015 May;41(5):936-44.
8.Mendicute J, Irigoyen C, Ruiz M, Illarramendi I, Ferrer-Blasco T, Montés-MicóR. Toric intraocular lens versus opposite clear corneal incisions to correctastigmatism in eyes having cataract surgery. J Cataract Refract Surg. 2009Mar;35(3):451-8.
9.Goggin M, Moore S, Esterman A. Toric intraocular lens outcome using themanufacturer’s prediction of corneal plane equivalent intraocular lens cylinderpower. Arch Ophthalmol. 2011 Aug;129(8):1004-8.
10.Davison JA, Potvin R. Refractive cylinder outcomes after calculating toric intraocular lens cylinder power using total corneal refractive power. Clinical Ophthalmology (Auckland, NZ). 2015;9:1511-1517.
11.Ho JD, Liou SW, Tsai RJ, Tsai CY. Effects of aging on anterior and posteriorcorneal astigmatism. Cornea. 2010 Jun;29(6):632-7.