

ORIGINAL ARTICLE 

Year : 2022  Volume
: 15
 Issue : 3  Page : 290294 


Accuracy of Barrett versus thirdgeneration intraocular lens formula across all axial lengths
Raline Solomon, S Tamilarasi, Gitansha Sachdev, Ramamurthy Dandapani
Cataract and Refractive Services, The Eye Foundation Hospital, Coimbatore, Tamil Nadu, India
Date of Submission  17Jun2021 
Date of Decision  28Jul2021 
Date of Acceptance  18Dec2021 
Date of Web Publication  02Nov2022 
Correspondence Address: Raline Solomon The Eye Foundation, 582, Diwan Bahadur Rd, R S Puram West, Coimbatore  641 002, Tamil Nadu India
Source of Support: None, Conflict of Interest: None  Check 
DOI: 10.4103/ojo.ojo_188_21
Abstract   
PURPOSE: The purpose of this study is to evaluate and compare the accuracy of Barrett Universal II versus thirdgeneration formula for different intraocular lens (IOL) powers for Indian eyes with different axial lengths (ALs). DESIGN: This is a retrospective, nonrandomized consecutive case series. METHODS: This study reviewed 981 eyes from 825 patients who had uneventful cataract surgery and IOL implantation. The eyes were separated into subgroups based on AL as follows: short (<22.0 mm), medium (22.01–23.99 mm), and long (>24.0 mm). The predicted refractive outcome using formulas was calculated and compared with the actual refractive outcome to give the prediction error. The percentage of every refractive error absolute value for each formula was calculated at <±0.50D, 0.50D0.75D, and >±0.75D. RESULTS: In all, 981 eyes were analyzed. There were no significant differences in the median absolute error predicted by Barrett and the thirdgeneration formulae. The Barrett Universal II formula resulted in significantly lowest mean spherical equivalent in short eyes (P = 0.0047) as well as a higher percentage of eyes with prediction errors within <±0.50D, 0.50D0.75D, and >±0.75D. We found that the Barrett Universal II formula had the lowest predictive refraction error and mean absolute error across all ALs. CONCLUSION: The Barrett Universal II formula rendered the lowest predictive error compared with SRK/T, Holladay, and Hoffer Q formulas. Thus, the Barrett Universal II formula may be regarded as a more reliable formula for achieving emmetropia and reducing postoperative refractive surprises across all ALs.
Keywords: Barrett, intraocular lens formulae, thirdgeneration formulae
How to cite this article: Solomon R, Tamilarasi S, Sachdev G, Dandapani R. Accuracy of Barrett versus thirdgeneration intraocular lens formula across all axial lengths. Oman J Ophthalmol 2022;15:2904 
How to cite this URL: Solomon R, Tamilarasi S, Sachdev G, Dandapani R. Accuracy of Barrett versus thirdgeneration intraocular lens formula across all axial lengths. Oman J Ophthalmol [serial online] 2022 [cited 2022 Dec 4];15:2904. Available from: https://www.ojoonline.org/text.asp?2022/15/3/290/360399 
Introduction   
The prediction of refractive outcomes after cataract surgery has steadily improved, with more recent intraocular lens (IOL) power formulas generally outperforming those of prior generations.^{[1],[2]} Yet, there is still considerable debate about which formula provides the most accurate refractive prediction. Because no single formula has been shown to be highly accurate across a range of eye characteristics, some authors have suggested that cataract surgeons should use different formulas for eyes of varied ocular dimensions.^{[3],[4]} Popular thirdgeneration formulas (Hoffer Q, SRK/T, and Holladay 1) calculate effective lens position using the anterior chamber depth (ACD), axial length (AL), and keratometry (K). The Barrett Universal II formula uses a theoretical model eye in which ACD is related to AL and K. A relationship between the Aconstant and a “lens factor” is also used to determine ACD.^{[5]} The important difference between the Barrett formula and other formulas is that the location of the principal plane of refraction of the IOL is retained as a relevant variable in the formula. To our knowledge, there is no current published data on headon comparison of outcomes of the Barrett Universal II versus the thirdgeneration formulae across three different ALs. However, there are studies which have compared the refractive outcomes of each of these formulae separately or as a part of the analysis of outcomes of other formulae. There is now a trend for many surgeons in our institute and others moving toward the Barrett Universal II from the thirdgeneration formulae. This changing trend kindled our curiosity toward sketching this study.
The aim of this study was to investigate and compare the accuracy of the Barrett Universal II formula for all ALs versus the thirdgeneration formula: SRK/T for long eyes (AXL >24 mm), Holladay 1 for medium eyes (AXL = 22.00–23.99 mm), and Hoffer Q for short eyes (AXL ≤21.99 mm) in predicting refractive outcome for standard cataract surgery.
Methods   
 Study design: Retrospective, nonrandomized case series
 Setting: Tertiary Eye Care Hospital, South India
 Duration of data collection: January 2017 and December 2018 (18 months).
The study adhered to the tenets of the Declaration of Helsinki and approved by the institutions ethics committee. Informed consent was obtained from all the participants included in the study. Patients with agerelated cataracts undergoing uneventful cataract surgery were included in the study. Intraoperative complications, the presence of any corneal pathology, glaucoma, retinal pathology, postoperative corrected distance visual acuity (CDVA) worse than 20/40, patients with preoperative corneal astigmatism of >0.75D, eyes requiring additional surgical procedures at the time of cataract surgery (including peripheral corneal relaxing incisions), and previous intraocular surgery (including previous refractive corneal surgery) were excluded from the final cohort.
Ocular biometry was performed in all eyes using the IOLMaster 700 (Carl Zeiss Meditec AG, Jena, Germany) based on sweptsource optical coherence tomography technology. Patients were grouped into two groups, Group 1 – patients who had their IOL power calculated using the Barrett Universal II formula (Across all AL) and Group 2 – patients who had their IOL power calculated using the thirdgeneration IOL formulae (SRK/T for AXL ≥24 mm, what about, Holladay 1 for AXL = 22–23.99 mm and Hoffer Q for AXL ≤21.99 mm). IOL power with the first myopic target refraction was selected for implantation. All surgeries were performed by a single experienced surgeon using a 2.4 mm clear corneal incision and a standard phacoemulsification technique. All patients had implantation of an AcrySof SN60WF IOL (Alcon, Ft Worth, TX, USA). Preoperative examinations, operative details, postoperative findings, and refractive data were collected.
Statistical methods
Refractive prediction error (RPE) was considered primary outcome variable. Groups (Group 1 vs. Group 2) were considered as primary explanatory variable. All quantitative variables were checked for normal distribution within each category of explanatory variable using visual inspection of histograms and normality QQ plots. Shapiro–Wilk test was also conducted to assess normal distribution. Shapiro–Wilk test P > 0.05 was considered normal distribution. For normally distributed quantitative parameters, the mean values were compared between study groups using independent sample ttest (two groups). P < 0.05 was considered statistically significant. IBM Corp. Released 2013. IBM SPSS Statistics for Windows, Version 22.0. Armonk, NY was used for statistical analysis.^{[6]}
Statistical analysis
RPE was calculated as the difference between the postoperative refractive outcome expressed as spherical equivalent and the refraction predicted by each formula. A negative value indicates a myopic prediction error that shows a more myopic result than the predicted refraction. The mean numerical RPE for each formula, the mean absolute error (MAE) and median absolute error for each formula were calculated. The percentages of eyes within <±0.50 D, 0.50D0.75D, and >±0.75 D of the predicted refraction were calculated and analyzed.
Results   
The study composed of 981 eyes of 825 patients. The demographics of the patients are listed in [Table 1]. Keratometry, AXL, and ACD across all the study groups were comparable. There was almost no statistical difference on comparing postoperative uncorrected distance visual acuity, RPE, MAE, and CDVA across all the groups except a significant difference in mean refractive spherical equivalent in the group with short AL, as shown in [Table 2].
However, there was a good difference between the percentage prediction between the two groups, with the prediction error of the Barrett Universal II IOL formula to be far superior and much closer to emmetropia than the other thirdgeneration IOL formulae, as shown in [Table 3] and [Table 4]. There was no documented myopic or hyperopic surprise in any of the IOL formulae.
Discussion   
CDVA has long been the principal outcome measure following cataract surgery; however, surgeons are now being judged more and more on refractive outcomes and the ability to achieve the desired refractive target and expected degree of spectacle independence.^{[7],[8]} Published results suggest that surgeons are, by and large, meeting expectations.^{[9],[10]} Refractive outcomes remain variable based on differences in surgeon technique and experience, preoperative diagnostic technology, and population cohort.^{[11],[12],[13],[14]} Proposed benchmark outcomes also vary. Based on a large subset of patients undergoing surgery across the National Health Service, Gale et al. have previously suggested that 55% of patients should achieve postoperative spherical equivalent of ± 0.5D of the intended target and 85% of patients within ± 1.0D.^{[15]} Subsequent papers, however, suggest that outcomes in excess of these figures may be feasible. Simon et al. achieved 67% of cases within ± 0.5D and 94% of cases within ± 1.0D in their own case series located at an academic teaching institution.^{[16]} Considering the combination of modern optical biometry, informed formula choice, and IOL constant optimization, Sheard had proposed that surgeons should be able to achieve 60% and 90% within ± 0.5D and ± 1.0D, respectively.^{[17]} To determine the effectiveness of the IOL formula in a relatively standard population, we calculated the theoretical performance of the Barrett Universal II formula in comparison with existing optimized formulas (Holladay I, SRK/T, and Hoffer Q).
In our study, the prediction error of <±0.50D using the Barrett Universal II formula across all ALs is given in [Table 3]. RPE of 96%, 92.3%, and 90.6% in patients with long, normal, and short ALs was seen. In those whose thirdgeneration formula was used, the prediction error of <±0.50 D was seen in 93.1%, 82.5%, and 75% in patients with long, normal, and short ALs [Table 4]. However, there was no statistical significance in prediction error of patients in extreme of axial length long eyes P = 0.4360, short eyes P = 0.0525. The percentage of prediction error of < ±0.50D in normal eyes between the Barrett and thirdgeneration formulae was statistically significant (P < 0.0001). This difference in statistical significance could also be due to large variation in the sample size across the three groups. In a study by Roberts et al., the percentage prediction of <±0.50D using SRK/T and Hoffer Q was 80%, Holladay –78%, and Barrett Universal II formula – 81%, compared to our study where the percentage prediction was SRK/T –93%, Hoffer –Q75%, and Holladay –82.5%, and Barrett Universal II formula was 96%, 92.3%, 90% across long, normal, and short eyes, respectively, although not statistically significant is much higher than the other formulae [Table 3] and [Table 4].^{[18]} A study by Gökce et al. comparing the prediction percentage outcomes of <±0.50D in short eyes using Hoffer Q was 64% and Barrett Universal II was 68.6% compared to our study which was 75% in short eyes using Hoffer Q and 90% using Barrett Universal II formula.^{[19]}
The MAE derived from using the Barrett Universal II was lower than those of the thirdgeneration formulae, across all ALs [Table 2]. The real challenge in giving the best postoperative refractive outcomes lies in selecting the IOL formulae that would give the lowest RPE, especially in eyes with extreme of ALs (AXL >24.00 mm and <22.00 mm). In our study, the Barrett Universal II formula had prediction error of 0.07 ± 0.31, 0.07 ± 0.49 versus 0.04 ± 0.35, −0.12 ± 0.13 using SRK = T, Hoffer Q in long and short eyes, respectively, the differences were not statistically significant and the results are almost comparable to those published by Zhou et al. and Gökce et al.^{[19],[20]} In terms of overall accuracy, the Barrett Universal II formula provided the equivalent or lowest variation within the data, and thereby smallest percentage of refractive surprises compared to other formulas for all cohorts. Our results, representative of a nontoric cataract population show that excellent results can be achieved combining optical biometry with consistent technique and latest IOL power calculation formulas. The advantages of the Barrett Universal II formula are that (i) it is independently available, (ii) it requires minimal additional manipulation to achieve excellent results across all ALs, and (iii) it does not require the calculation of surgically induced astigmatism. The limitation of our study remains the relatively small numbers in the short and long AL groups. Study inclusion was limited to the SN60WF IOL as this was one of the most commonly used IOL in this part of the world. Although it would be reasonable to expect that the formulas would produce similar outcomes for additional lenses, further investigation may be useful to confirm this.
Conclusion   
We found that excellent results can be obtained with a variety of IOL power calculation formulas for eyes with different ALs, especially extreme of ALs. The Barrett Universal II formula may provide additional benefits for patients by reducing possible refractive surprises and a very effective tool to reaching the goal of emmetropia which is a desirable goal for every cataract surgeon in the presentday world.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
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[Table 1], [Table 2], [Table 3], [Table 4]
