Consistency evaluation of intraocular lens power calculation methods in eyes that have undergone small-incision lenticule extraction

To compare the differences in the consistency of intraocular lens (IOL) power calculations with four different formulas for cataract surgery after small-incision lenticule extraction (SMILE), and to select the most suitable formula to calculate the IOL power of cataract patients after SMILE.


Abstract Background
To compare the differences in the consistency of intraocular lens (IOL) power calculations with four different formulas for cataract surgery after small-incision lenticule extraction (SMILE), and to select the most suitable formula to calculate the IOL power of cataract patients after SMILE.
Methods 98 eyes of 51 patients who underwent small-incision lenticule extraction (preoperative spherical equivalents, -2.05 to -10.10D) between June 2019 to July 2019 in our department were included, all surgeries were performed by the same surgeon. IOLMaster 500 was used for biometric measurement and Haigis-L correction formula was calculated. AL-Scan was used to calculate Shammas-PL correction formula, Camellin-Calossi formula and Barret True K formula. We simulated the intraocular lens calculation before cataract surgery in all eyes with these four formulas. The consistency of the calculated IOL powers were evaluated by correlation analysis, one-way ANOVA, and Bland-Altman analysis.

Results
The average values of Barret True K formula, Shammas formula. The Camellin-Calossi formula and the Barret True K formula were signi cantly different from the Haigis-L correction formula. Bland-Altman analysis: for the difference-mean scatter plot and the ratiomean scatter plot: the maximum allowable error of the 95% limit is 9.61%, 7.45%, and 2.99%, respectively, indicating that the results are consistent.

Conclusions
Haigis-L and Shammas-PL formulas had good consistency when calculating the IOL power in eyes after SMILE.

Background
By 2050, the number of people (45-89 years of age) affected by cataract in China will reach 240.83 million [1]. As cataract surgery is also a refractive procedure now, patient expectations continue to rise, achieving a precise postoperative target refraction is an important goal in cataract surgery. Precise postoperative target refraction depends on the accurate calculation of IOL power. So, the accurate calculation of IOL power for these patients would be very important soon. Small incision lenticule extraction (SMILE) surgery was rstly reported by Sekundo [2] and Shah [3] in 2011, it is popular for excellent visual outcomes, superior biomechanics, faster recovery of dry eye and corneal nerve reinnervation [4]. Over 2 million patients have undergone SMILE, including 1 million Chinese patients. An unfortunate consequence of corneal refractive surgery is di culty in accurately calculating intraocular lens (IOL) power in eyes undergoing cataract surgery [5]. During the last few decades, IOL power calculation after keratorefractive surgery has been the subject of numerous studies and publications [6].
As far as we know, there is still no study on how to calculate the IOL power in patients receiving SMILE. In SMILE, an intrastromal lenticule is cut with a femtosecond laser, and subsequently removed through a minor incision. The variant lenticule extraction design makes SMILE differ from all the other corneal refractive surgeries in corneal biomechanical properties [7], which may have in uences on corneal refractive power estimation and creates di culties in accurately calculating IOL power. Haigis-L[8], Shammas-PL [9], Camellin-Calossi [9] and Barret True K [11] formula are commonly used formulas for IOL power calculation for cornea refractive surgery eyes [9,12]. The aim of our study was to evaluate the consistency of IOL power calculations in these four different formulas for cataract surgery in eyes after SMILE, and provide a basis for the calculation of the IOL power of cataract patients after SMILE.
Haigis-L algorithm within the partial coherence interferometer's software and Shammas-PL,Camellin-Calossi (medical history needed) and Barret True K(medical history needed) within the software of AL-Scan were evaluated. The devices' optimized lens constants were obtained from the User Group for Laser Interference Biometry (ULIB).

Statistical analysis
Correlation analysis, one-way ANOVA, and Bland-Altman analysis were used to assess the consistency of the prediction performance by different methods. Statistical analysis was performed using SPSS for Windows software (version 22.0, SPSS, Inc.). A p value less than 0.05 was considered statistically signi cant.

Results
The study comprised 98 eyes of 51 patients. Table 1 shows the mean values and standard deviation of arithmetic IOL prediction power calculated by the four formulas.   Table 2). Although the four calculation methods match well with one another, the correlation coe cients were highest for the Haigis-L between Shammas-PL and Barrett Ture K with Camellin-Calossi method. Bland-Altman analysis was used to assess the extent of agreement of the formulas. Figure 1 shows the Bland-Altman plot demonstrating differences in average IOL power calculations between the formulas.

Discussion
In our study, we evaluated the four most accurate methods of estimating the refractive power of IOL after corneal refractive surgeries, and compared the consistency of the calculations. There was no signi cant difference between the Shammas-PL correction formula and the Haigis-L correction formula. We also evaluated the correlations of the formulas, and it showed that the all the calculations matched well, but Shammas-PL is the best consistency with Haigis-L.
Postoperative spherical equivalent prediction errors or manifest refraction after cataract surgery are always used for the study on IOL predictability of formulas [13,14]. Since SMILE surgery was introduced in 2011, few patients have met the age-related cataract problem. So, we couldn't assess the accuracy of the formulas or adjust the calculation by compare the postoperative result. The best way for evaluation would be the consistency of different calculations.
The methods to correct or minimize prediction errors in the IOL power calculations for cataract surgery after corneal refractive surgery can be classi ed roughly as methods not requiring historical data, e.g., the Shammas-PL formula [12], the Haigis-L formula [15,16], with the advantage of being independent of potentially inaccurate historical data. And methods requiring historical clinical data [17], e.g. the Camellin-Calossi formula[18] and the Barrett Ture K formula [19], which is infeasible if previous clinical patient's data are not available. The consistency of Haigis-L and Shammas-PL in the four formulas is higher, indicating that the calculation of IOL can obtain relatively accurate values even without previous case data, which is more convenient for clinical calculation. And the high correlation of all the formulas demonstrated Camellin-Calossi formula and the Barret True K formula are available as a reference.
Haigis-L and Shammas-PL have been widely used and shown promising results which are based on a correction of the corneal power by adjusting the keratometric index [9,12]. The use of the adjusted keratometric index is a valid and easy method to estimate the central corneal power in corneas with previous keratorefractive surgery. The keratorefractive surgeries always require cutting of corneal lamellae that may reduce the biomechanical stability of the cornea. However, SMILE preserves the anterior corneal integrity and may, in theory, better preserve the corneal biomechanical strength than laser-assisted in situ keratomileusis(LASIK) and Photorefractive Keratectomy(PRK) after surgery [20]. Whether this would result in the difference in corneal power changes has not been investigated yet. Our results showed that Haigis-L and Shammas-PL could be better choices for IOL power calculations, which also helped to prove the minor differences of the corneal power changes between SMILE and LASIK/PRK. The results demonstrated the estimation of corneal power can be performed with the only requirement of measuring the postoperative corneal radius without in uences of the different surgery methods.

Conclusions
Our results show that for eyes with previous SMILE correction, the  and interpretation of data and in writing the manuscript.

Authors' contributions
Conceptualization and design, JL, SH; Methodology, BC; Data collection, JX, YHZ, SH; Formal analysis, JL, BC; Writing-original draft preparation, SH; writing-review and editing, JL, SH; We con rm that the manuscript has been read and approved by all named authors and that the order of authors listed in the manuscript has been approved by all of us.