

ORIGINAL ARTICLE 

Year : 2021  Volume
: 16
 Issue : 1  Page : 510 

A comparative study on the accuracy of SRK/T, holladay II, and barrett universal II formulas in intraocular lens power calculation of axial myopes undergoing cataract surgery
Pradeep Sune, Mona Sune, Ritica Mukherji
Department of Ophthalmology, Jawaharlal Nehru Medical College, Datta Meghe Institute of Medical Sciences (Deemed to be University), Wardha, Maharashtra, India
Date of Submission  21Oct2020 
Date of Decision  10Jan2021 
Date of Acceptance  25Jan2021 
Date of Web Publication  29Jul2021 
Correspondence Address: Dr. Ritica Mukherji T28 Shalinata P.G Girls Hostel, Datta Meghe Institute of Medical Sciences, Sawangi (M), Wardha  442 001, Maharashtra India
Source of Support: None, Conflict of Interest: None  Check 
DOI: 10.4103/jdmimsu.jdmimsu_374_20
Context: Calculation of intraocular lens (IOL) power for extremes of axial lengths (ALs), using classical formulae like SandersRetzlaffKraff/Theoretical (SRK/T), is unsatisfactory. With the advent of fourthgeneration formulae, surgeons are now aiming for better refractive outcomes for such patients postcataract extraction. Aims: The aim of this study was to assess and compare the accuracy of SRK/T, Holladay II, and Barrett Universal II formulae for IOL power calculation in high myopes with cataract by computing the mean absolute error (MAE). Settings and Design: This was a hospitalbased prospective comparative study conducted over a period of 1 year. Forty patients with AL ≥26 mm undergoing cataract extraction were included after taking inclusion and exclusion criteria into consideration. Subjects and Methods: After enrolling patients and obtaining informed consent, all patients underwent a comprehensive ophthalmic examination. The power of IOL to be implanted was calculated by SRK/T, Holladay II, and Barrett Universal II with the goal of achieving refraction within ± 1 D postoperatively. Phacoemulsification was performed for all patients and hydrophilic foldable IOLs were implanted. Patients were followed up for 2 months at the end of which MAE was calculated for all three formulae. Statistical Analysis: Statistical analysis was done by descriptive and inferential statistics using Kruskal–Wallis Chisquare test and Mann–Whitney Utest. Software used in the analysis was SPSS 24 version and P < 0.5 was considered as the level of significance. Results: Barrett Universal II formula had the lowest MAE with a mean of 0.06 ± 0.20 and a median absolute error of 0.02. This was followed by Holladay II and SRK/T. While Barrett Universal II outperformed for all powers of IOL, SRK/T had lower MAE for plus power IOLs and Holladay II was better for negativepower IOLs. Conclusions: Barrett Universal II formula is ideal for IOL power calculation in cataract patients with AL >26 mm.
Keywords: Barrett Universal II, high myopia, Holladay II, mean absolute error, SRK/T
How to cite this article: Sune P, Sune M, Mukherji R. A comparative study on the accuracy of SRK/T, holladay II, and barrett universal II formulas in intraocular lens power calculation of axial myopes undergoing cataract surgery. J Datta Meghe Inst Med Sci Univ 2021;16:510 
How to cite this URL: Sune P, Sune M, Mukherji R. A comparative study on the accuracy of SRK/T, holladay II, and barrett universal II formulas in intraocular lens power calculation of axial myopes undergoing cataract surgery. J Datta Meghe Inst Med Sci Univ [serial online] 2021 [cited 2022 Nov 29];16:510. Available from: http://www.journaldmims.com/text.asp?2021/16/1/5/322634 
Introduction   
High axial myopes with cataract pose multiple challenges for ophthalmologists in terms of preoperative considerations such as improper axial length (AL) calculations due to poor fixation or posterior staphylomas which affect the calculation of intraocular lens (IOL) power, intraoperative complications such as suprachoroidal hemorrhage, retinal tears or detachments, and posterior capsular rents.^{[1]} Postoperatively, surprise refractive outcomes, especially hypermetropia, are troublesome for the patients. While intraoperative deliberations vary with surgeons, postoperative refractive surprises can be countered by implanting appropriate lens power.
The prevalence of high myopia in central India has been found to be 0.53%.^{[2]} Multiple studies have shown conclusively that high myopia or pathological myopia is associated with faster progression to cataract.^{[3],[4],[5],[6],[7],[8]} This could be attributed to the longer ALs affecting nutrient delivery to the posterior aspect of the lens.
This study was conducted to evaluate the predictive accuracy of three IOL formulas belonging to the third and fourth generation in a subset of the Indian population of central India as there is very limited literature available on the same for this geographic area.
Thirdgeneration formulas such as SRK/T have been found to give accurate results for mediumsized eyeballs.^{[9]} This generation makes use of constants to find effective lens position (ELP), i.e., the distance between the anterior corneal plane and the IOL plane. Other formulas in this generation are Hoffer Q and Holladay I. Fourthgeneration formulas include Holladay II, Barrett Universal formula II, Olsen formula, Haigis, and Hoffer H. They have a separate set of constants and also consider anterior chamber depth (ACD) for better assessment of ELP.^{[10]}
The aim of this study was to gauge the accuracy of SRK/T, Holladay II, and Barrett Universal II formula in calculating IOL power to achieve a postoperative refraction in the range ± 1 D in high myopes (AL > 26 mm) undergoing cataract surgery. The primary objective was to review and compare the mean absolute error (MAE) for all three formulae, while the secondary objective was to compare the same in the three patient groups based on the power of IOL implanted.
Subjects and Methods   
Study design
The design of this study was prospective comparative.
Study location
The study was conducted at Department of Ophthalmology in a Teaching Tertiary Care Hospital in Wardha, Maharashtra.
Study duration
The study was conducted over a period of 1 year from April 2019 to April 2020.
Sample size
The sample size was 50 eyes of 40 patients.
Subjects and selection methods
A total of 40 patients were evaluated. The patients with AL >26 mm were sequentially selected after taking inclusion and exclusion criteria into consideration. Of 40 patients, 10 patients underwent cataract extraction in both eyes which had AL >26 mm. Therefore, a total of 50 eyes were assessed.
Inclusion criteria
 Patients with AL ≥26 mm
 Patients with uncomplicated senile cataract.
Exclusion criteria
 Patients not giving consent
 Patients where AL could not be determined with absolute clarity
 Patients lost to followup (up to 2 months postoperatively)
 Patients with preexisting anterior and posterior segment ocular pathology that could affect refraction postoperatively such as corneal dystrophies, keratoconus or keratoglobus, corneal scars, retinal detachment, or macular edema
 Patients with intraoperative complications that could affect refractive status postoperatively like loss of capsular support requiring sulcus or anterior chamber placement of IOL, vitreous loss, etc.
 Patients with postoperative ocular pathologies affecting refraction
 Patients with systemic diseases such as uncontrolled diabetes mellitus and hypertension, who could not be taken for cataract surgery
 Uncooperative patients or patients with psychiatric illnesses.
The study was approved by the Institutional Ethics Committee (IEC) of the University (Ref. DMIMS (DU)/IEC/2019/7975) and was carried out per the Declaration of Helsinki.
After obtaining informed consent, all subjects underwent a thorough ophthalmic evaluation including bestcorrected visual acuity, intraocular pressure measurement, slitlamp biomicroscop, y and fundus examination.
All patients underwent keratometry using Topcon KR8000 auto keratometer. ACD, lens thickness (LT), and AL measurement was done using Ascan applanation ultrasonography (PacScan Plus by Sonomed, Model 300A+).
On fundus examination, three subjects were diagnosed to have an eccentric posterior staphyloma. For these patients as well as patients with highly dense cataract where direct visualization of fundus was not possible, AL measurement was done using both A scan and B scan. The visualization of the fovea allowed retinal peak of A scan to coincide with it and give a true measurement of AL.
Calculation of IOL power was done using SRK/T formula software available on the Ascan machine as well as backcalculation using Holladay II formula (https://www.hicsoap.com/calc/) and Barrett Universal formula II (https://calc.apacrs.org/barrett_universal2105/) software which was freely available online. AL adjustment was not done for the latter formulae. Whitetowhite distance was left unspecified. The goal of the surgeon was to achieve a postoperative refraction within the range of ± 1 D spherical equivalent (SE) which was the primary deciding factor in selecting of IOL power to be implanted. If there was a difference of more than 0.5 D in between the formulas, the IOL power given by SRK/T formula was implanted Furthermore, the surgeon preferred a myopic predictive outcome over a hypermetropic outcome whenever possible.
All patients underwent an uncomplicated 3–3.5 mm sutureless phacoemulsification within the bag IOL implantation and were given standard postoperative care. Patients were followed up for a period of 2 months and refraction was done using the same Topcon KR8000 auto keratometer at the end of 2 months.
The MAE was calculated and compared for all three formulae to determine accuracy.^{[11],[12]} Furthermore, patients were divided into three groups based on the power of IOL implanted, i.e., plus, minus, or zero power. The MAE of all three formulae was compared for all three groups.
Statistical analysis used
Statistical analysis was done by descriptive and inferential statistics using Kruskal–Wallis Chisquare test and Mann–Whitney Utest and software used in the analysis was SPSS 24.0 version (Chicago, Illinois, USA) and Graph Pad Prism 7.0 version (GraphPad Software, San Diego, CA, USA) and P < 0.05 is considered as the level of significance.
Ethical clearance
The Institutional Ethics Committee of DMIMSDU has approved the Research work proposed to be carried out at Jawaharlal Nehru Medical College, Sawangi(M), Wardha. Date: 5^{th} April 2019 with Reference no DMIMS (DU)/IEC/2019/7975.
Results   
The maximum age of the patients in the study was in the range of 60–69 years, i.e., 45%. Only one patient was seen in the age group of 10–19 years. Both eyes of this patient were included in the study accounting for 3% of the sample size. The mean age was 54.35 ± 13.07 years and ages ranged from 10 to 76 years [Table 1] and [Figure 1].
About 32.5% of the patients were male and 67.5% were female with a maletofemale ratio of 0.48 [Table 2] and [Figure 2].
The mean corneal power (K) of eyes was 45.16 ± 2.91 and values ranged from 40.00 to 53.00. The mean ACD for all eyes was 4.00 ± 0.42 mm with a minimum ACD of 3.20 mm and a maximum ACD of 4.97 mm. The average AL for 50 eyes was 28.39 ± 2.08 with a range from 26.06 to 34.67. The mean preoperative spherical dioptric power recorded for all patients was − 10.14 DS and values ranged from − 4.00 DS to −20.00 DS [Table 3] and [Figure 3].
The MAE for each of the three formulae was calculated as the difference between the preoperative predictive and postoperative actual SE. SE was computed by adding half the cylindrical power to the spherical power of the patient. Kruskal–Wallis Chisquare test was employed to compare the above. Barrett Universal II formula showed the lowest MAE as compared to the other two formulae with a mean of 0.06 ± 0.20 and a median absolute error of 0.02. The values of absolute error for this formula ranged from −0.51 to 0.41.
This was followed by Holladay II with an average of 0.14 ± 0.03. The median absolute error was found to be 0.21 and the difference between the true and predicted preoperative refraction ranged from −0.90 to 0.82.
SRK/T disclosed an MAE of 0.28 ± 0.24 with a median value of 0.32 and a range between −0.92 and 0.76 [Table 4] and [Figure 4].
The betweengroup difference for the three formula was statistically significant with P = 0.0001. Thus, to further clarify the relationship between the three formulas employed, the nonparametric Mann–Whitney Utest was used to compare the formulae with each other independently.
Barrett Universal II when compared with Holladay II and SRK/T individually also showed statistical significance with P = 0.0001 in the Barrett Universal II–SRK/T and in the Barrett Universal II–Holladay II group [Table 5]. Furthermore, there was a statistically significant betweengroup difference in the SRK/THolladay II group with P = 0.049 [Table 5].
Patients were further grouped according to the power of IOL which was ultimately implanted. All three formulae and their MAE were analyzed according to these groups.
Of 50 eyes, 36 eyes were implanted with a plus power IOL, 11 eyes with minus power, and 3 eyes with zero power IOLs. The IOL powers implanted ranged from −14 D to +16.5 D.
SRK/T had a mean of 0.24 ± 0.29 for values of MAE in patients implanted with plus power IOLs. The MAE ranged from −0.92 to 0.73 for these patients. Similarly, the mean for minus power IOLs and zero power IOLs was 0.31 ± 0.27 and 0.62 ± 0.12, respectively, whereas the ranges lay between −0.24 to 0.72 and 0.52 to 0.76, respectively. The difference between the three groups was statistically significant with P = 0.038.
Holladay II had the lowest mean for minus power IOLs, i.e., 0.05 ± 0.49 followed by plus power IOLs with a mean of 0.13 ± 0.32 and zero power IOLs with an average of 0.52 ± 0.02. The difference for all three groups was statistically significant with P = 0.044. This formula generated MAE in the range between −0.77 to 0.68 and −0.90 to 0.82 for plus and minus power IOLs, respectively. The range for zero power IOLs was 0.50–0.55.
Barrett Universal II formula yielded the lowest average among the three formulae for all powers of IOL implanted with a mean of −0.09 ± 0.19, −0.02 ± 0.17, and 0.29 ± 0.10 for plus, minus, and Plano IOLs, respectively. The range of MAE was found to be −0.51 to 0.20 for plus power, −0.25–0.38 for minus power, and 0.22 to 0.41 for Plano IOLs [Table 6] and [Figure 5].
Discussion   
With the advent of newer IOL power calculation formulae, it has become comparatively easier to the postoperative refraction of the patient and modifies the type and power of IOL to be inserted based on the desired refractive outcome. This is especially important in the current scenario where patients undergoing cataract surgery expect and demand perfect vision post surgery. The process becomes more challenging in patients with extremes of ALs. Multiple studies in the west have compared the different generation of formulas to understand how these formulae behave with increasing and decreasing ALs.
Abulafia et al. conducted a retrospective case series analysis on 106 eyes with AL >26 mm and concluded that in patients implanted with IOL of power ≥ 6 D, thirdgeneration formulae SRK/T, Hoffer Q, Barrett Universal II, Olsen II, Haigis, and Holladay II were performed at par with each other with a prediction error of ±1 D in 93% eyes, whereas in eyes implanted with IOL of power <6 D, performance of Barrett Universal II, Holladay I, and Haigis formula using ALadjusted method was superior.^{[13]} Wang et al. in 2018 demonstrated through their metaanalysis involving 4047 eyes that Barrett Universal II gave the maximum number of patients with prediction error within ±0.5D in patients with ALs longer than 26 mm.^{[14]} Stopyra W and associates concluded from their study, involving 81 eyes of 70 patients with axial lengths >25mm, that Barrett Universal II gave the lowest average and median of absolute error of prediction as compared to all three third generation formulae, Holladay II and Haigis formula.^{[15]} Similarly, Zhang et al. found Barrett Universal II to have the least predictive error compared to thirdgeneration formulae and Haigis formula.^{[16]}
These are in contrast to the studies conducted by Bang et al.^{[17]} and Terzi et al.^{[18]} who stated that Haigis formula is most accurate in calculating IOL power in long eyes as compared to Holladay II and SRK/T as well as the studies conducted by Tang et al.^{[19]} and Wan et al.^{[20]} who found no significant betweengroup difference in calculating IOL power for high myopes by Hill RBF, Holladay II, and Barrett Universal II formulae.
In the present study as well, Barrett Universal II formula had a lower average MAE as compared to SRK/T and Holladay II formula. When compared with each formula independently, the difference in MAE was still statistically significant and found to be lower than the other two formulae and for all powers of IOLs implanted. This could be attributed to the use of AC depth and LT in the formula which allows a more precise calculation of power. This resounds well with other studies which also prove Barrett Universal formula to be superior to others in IOL power calculation of high myopes with cataract.^{[21],[22],[23],[24]}
The performance of Holladay II was found to be better for minus power IOLs in our study and almost comparable to the performance of Barrett Universal II. It performed better than SRK/T in terms of MAE as well as had lower predictive errors for all powers of IOLs implanted. This correlates well with the findings of Melles et al.^{[24]} who also found that the Holladay II formula was better than SRK/T for predictive accuracy in higher myopes with cataract.
Zaldivar et al.^{[25]} found that 92% of patients had a postoperative refraction in the range of ±1 D in case of plus power IOLs when SRK/T formula was used and 41% of patients had the same in case of minus power IOLs when Holladay II was used, which was also seen in this study. Ghanem and ElSayed^{[26]} also concluded that Holladay II had a better performance than SRK/T for longer ALs.
Of the three formulae evaluated, SRK/T had the highest MAE for all powers of IOL and among the other two formulae. Anand et al.^{[9]} had put forward that SRK/T was the most accurate formula for calculation of power in patients with ALs >24.5; however, the study had not taken into account fourthgeneration formulae like Holladay II or Barrett Universal formula II. Doshi et al.^{[27]} also found predictability of SRK/T, Hoffer Q, Holladay I, and Haigis to be comparable for ALs >24 mm.
One of the main limitations of our study was that the MAE for the formulae was not correlated to the ALs. It can be argued that most patients receiving minus power IOLs are found to have longer ALs^{[25]} as compared to those who receive plus power IOLs. Zhou et al.^{[22]} found that SRK/T and Barrett Universal II formula had better predictive outcomes for pluspowered IOLs than minus power IOLs, but Barrett Universal II still had an advantage over other formulas for such eyes, which is also reflected in our study. Another limitation was the low number of patients implanted with negative or zero power IOLs, but this reflects the actual patient demographic encountered with high myopes even though the surgeon made a conscious effort to aim for myopia rather than hyperopia whenever possible.
We recommend further largescale studies comparing different constants, for example, ULIB constants and with different AL adjustments, such as those seen in the Holladay II software, in very long eyes for more accurate results. Fifthgeneration formulae like HillRBF formula should also be evaluated as the literature for the same is very limited in the AsianIndian population.
Conclusions   
Barrett Universal II formula had the highest predictive accuracy in patients with high myopia and cataract. Holladay II has better outcomes for minus power IOLs and may be used for very long ALs.^{[28],[29], [30,[31],[32]}
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]
