Correlation of ocular biometry parameters with corneal astigmatism in cataract surgery subjects – A swept-source optical coherence tomography study

INTRODUCTION

An enormously popular ophthalmic intervention in the world is cataract surgery, which simultaneously removes the cataract and corrects refractive errors. Patients now expect very little refractive error following cataract surgery, which has metamorphosed the procedure from vision restoration to a refractive procedure.¹ To achieve a good refractive outcome, it is now important to measure corneal astigmatism accurately prior to surgery. The calculation of intraocular lens (IOL) power is equally significant, and it depends on accurate computation of anterior chamber depth (ACD) and axial length (AL).² Research has indicated that a primary factor contributing to decreased vision following surgery is corneal astigmatism.³

IOL Master 700 (Carl Zeiss Meditec AG, Jena, Germany, 2015), which uses swept-source Optical Coherence Tomography (OCT), is accepted as a novel optical biometer to measure AL and keratometry (K). It has admirable consistency and repeatability with Scheimpflug (Pentacam) and has outperformed both Lenstar LS 900 (Haag Streit AG, Switzerland, 2009) and IOL Master 500 (Carl Zeiss Meditec, Jena, Germany, 2010).⁴ There are multiple studies available on ocular biometry patterns in Asian populations using partial coherence interferometry,^5,6 including one from India.² We have previously reported the prevalence of preoperative astigmatism in Indian eyes using swept-source OCT.⁷ Herein, we intend to study the various biometry characteristics and explore their correlation in a larger study group using IOL Master 700 in Indian eyes.

MATERIAL AND METHODS

The ethics committee granted approval for the study to be conducted. A prospective hospital-based cross-sectional study was conducted at a tertiary eye care center in India. The study period spanned from March 2022 to December 2022. Participants in our study were patients recruited for cataract surgery, aged 21 years or older. Every patient received a thorough ophthalmic examination, which included measurements of optical biometry in phakic eyes only, dilated fundus examination, rebound tonometry, slit lamp examination, and visual acuity. Individuals having a history of prior intraocular surgery, pterygium, or corneal disease were excluded. The study excluded patients who had either aphakia or pseudophakia in one or both eyes. The principles of the Helsinki Declaration were followed in the research. All patients gave written informed consent.

IOL Master 700 (Carl Zeiss Meditec AG, Jena, Germany, 2015) is an optical biometer, which is non-contact and uses swept-source technology and a laser having a wavelength of 1055 nm. Using swept-source OCT, measurements are made of lens thickness (LT), AL, and ACD. With the help of light spots reflected from the cornea’s surface, telecentric K measures the curvature of the cornea.⁸

The central corneal 2.5 millimeters (mm) in two meridians—flat K called K1, and steep K called K2—are used to calculate the K values. The difference between the K2 and K1 readings was used to determine corneal astigmatism (ΔK). With-the-rule (WTR) corneal astigmatism was identified if the steep axis measured 90 ± 30 degrees. Against-the-rule (ATR) astigmatism is identified by the steep axis lying at 180 ± 30 degrees; oblique astigmatism is identified when the steep axis is neither WTR nor ATR. When K1 and K2 have the same value, astigmatism is considered absent.⁴ Two senior technicians used the IOL Master 700 to perform optical biometry in each of the cases.

RESULTS

500 eyes of 250 patients were examined. Study subjects consisted of 44.4% males (n = 111) and 55.6% females (n = 139). The average age of patients was 63.42 ± 9.572 years. Corneal astigmatism ranged from 0 to 4.27 D, with a mean being 0.920 ± 0.669 D. Astigmatism was absent in 1.6% (n = 8), 26% of eyes had WTR astigmatism (n = 130), 55.6% had ATR astigmatism (n = 278), and 16.8% had oblique astigmatism (n = 84). Astigmatism <1.00 D was found in 66.4% (n = 332), and astigmatism ≥1.00 D was found in 33.6% (n = 168).

All of our study participants ranged from 20.42 to 29.35 mm; the mean AL was 22.97 ± 0.988 mm. Females had shorter mean AL (22.77 ± 0.92 mm) than males (23.10 ± 1.00 mm), and the difference was statistically significant (p = 0.00001).

The ACD of our study participants ranged from 1.9 to 4.65 mm, with a mean ACD of 3.17 ± 0.40 mm. Females had a shallower mean ACD (3.12 ± 0.42 mm) than their male counterparts (3.20 ± 0.37 mm); this difference was statistically significant (p = 0.008).

LT in our study subjects ranged from 1.37 to 5.79 mm; the mean LT was 4.29 ± 0.50 mm. Mean LT was less in females (4.22 ± 0.48 mm) than in males (4.34 ± 0.51 mm); the difference was statistically significant (p = 0.01).

The WTW of our study population ranged from 10.7 to 13.2 mm, with a mean WTW of 11.90 ± 0.40 mm. Like other parameters, the mean WTW in males (11.95 ± 0.40 mm) was wider than in females (11.82 ± 0.38 mm); the difference was statistically significant (p = 0.0005).

Analysis of our study reveals several notable positive correlations. Notably, age exhibits positive correlations with LT (r = 0.25), K2 (r = 0.03), and ΔK (r = 0.17). Additionally, AL demonstrates positive associations with ACD (r = 0.33), WTW (r = 0.35), and ΔK (r = 0.10). Positive correlations also exist between WTW and ACD (r = 0.25).

Table 1 displays the biometric profile distribution of our study population by age group and sex. Table 2 displays the distribution of age among the study subjects along with their average astigmatism. Table 3 displays the magnitude of corneal astigmatism in the study participants. Table 4 displays the univariate regression analysis of the different variables amongst each other; their correlation coefficient (r) and p-values are shown. Figure 1 shows the age-wise distribution of astigmatism. Figure 2 shows the average preoperative astigmatism for the various age groups. Figure 3 shows the correlation of various parameters examined in our study with the age of the participant. Figures 4 and 5 illustrate the correlation between various parameters examined in our study with AL and WTW, respectively.

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Figure 1:

Age wise distribution of astigmatism. ATR: Against-the-rule, WTR: With-the-rule.

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Figure 2:

Average preoperative astigmatism for the various age groups.

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Figure 3:

Correlation of various parameters examined in our study with the age of the participant. AL: Axial length, ACD: Anterior chamber depth, LT: Lens thickness, WTW: White to white, K1: Flat K, K2: Steep K, Delta K: Difference between K1 and K2.

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Figure 4:

Correlation of various parameters examined in our study with the AL of the participant. AL: Axial length, ACD: Anterior chamber depth, LT: Lens thickness, WTW: White to white, K1: Flat K, K2: Steep K, Delta K: Difference between K1 and K2.

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Figure 5:

Correlation of various parameters examined in our study with WTW of the participant. WTW: White to white, AL: Axial length, ACD: Anterior chamber depth, LT: Lens thickness, K1: Flat K, K2: Steep K, Delta K: Difference between K1 and K2.

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DISCUSSION

Using swept-source OCT, the current study evaluated the various biometry characteristics and explored their correlation in participants scheduled for cataract operation. Participants consisted of Indians living in the country’s northeastern part, including Assam and neighboring states (Manipur, Meghalaya, Mizoram, and Tripura). Related studies that used swept-source OCT to report the biometry parameters in the Indian population have not been found using a PubMed search. Nonetheless, numerous studies have evaluated the accuracy and repeatability of swept-source OCT (IOL Master 700) in comparison to Scheimpflug-Placido disc-based tomography,⁹ Eyestar 900,¹⁰ Anterion,¹¹ and Argos,¹² as well as optical low coherence reflectometry (IOL Master 500).¹³

Previous studies have reported a prevalence of ≥1.00 D preexisting astigmatism in participants undergoing cataract operation, ranging from 30.8 to 66.9%. Our study had astigmatism of ≥1.00 D in 33.6% of participants. This was substantially lower than other research conducted in Northern India (40.49%),¹⁴ North East India (45.55%),² China (51.5%),⁴ Nigeria (66.9%),¹⁵ and the United Kingdom (42%)¹⁶; but it is nearly comparable to that found by Joshi RS et al. (32.5%)¹⁷ and Rashid MA et al. (30.4%).¹⁸

The study population’s average age was 63.42 ± 9.57 years, more than that found in a study from North India (59.12 ± 15.19 years),¹⁴ but comparable to a study from northeast India (64.04 ± 10.81 years),² Bangladesh (61.9 ± 8.1 years),¹⁸ and Nigeria (60.8 ± 12.7 years).¹⁵ The average age of the subjects in a study from the UK seems to have been higher (79 ± 7 years).¹⁶

The current study’s mean preoperative corneal astigmatism was 0.920 ± 0.669 D. It was lower than the 1.17 ± 0.75, 1.16, and 1.17 ± 1.15 D reported by Anuj Sharma et al.¹⁴, Isyaku M et al.¹⁵, and Rashid MA et al.¹⁸, respectively. Tan Y et al.⁴ found that individuals with high myopia had a mean astigmatism of 1.20 ± 0.83 D. However, their reported control group mean astigmatism was 0.93 ± 0.69 D,⁴ which is comparable to the current study.

The mean AL in our study was 22.97 ± 0.988 mm, with females having a statistically significant lower AL (22.77 ± 0.922) compared to males (23.10 ± 1.00). It is lower than that found by Tanie Natung et al. (23.34 ± 1.12 mm)² and the control group of Tan Y et al. (23.46 ± 0.72 mm)⁴, though it is similar to a study from Central India (22.6±0.91).¹⁹

The mean ACD in our study population was 3.17 ± 0.40 mm, with females having a shallower ACD (3.12 ± 0.42 mm) than males (3.20 ± 0.37 mm). ACD is found deeper in this study as compared to Chinese⁶ and Whites²⁰, while shallower than Caucasians.²¹

The mean LT in our participants was 4.29 ± 0.50 mm. Mean LT was less in females (4.22 ± 0.48 mm) than in males (4.34 ± 0.51 mm). LT was not reported in other related studies, so we are unable to compare it with existing literature.

The mean WTW in our study group was 11.90 ± 0.40 mm, with females (11.82 ± 0.38) having values lower than males (11.95 ± 0.40). This was similar to another study from northeast India (11.92 ± 0.54 mm).²

In the current study, ATR astigmatism was more common (55.6%) compared to WTR astigmatism (26%). Comparable results were found in studies from Western India (ATR 44.6% & WTR 20.7%),¹⁸ UK (ATR 51.7% & WTR 29.83%),¹⁷ and North India (ATR 51.7% & WTR 29.83%).¹⁵ Age-wise distribution of types of astigmatism indicated that in the 41–50-year age group, WTR astigmatism was more than ATR astigmatism (16 vs. 9). However, ATR astigmatism was common than WTR astigmatism in all other age groups [Figure 2]. The Tan Y et al. study on patients with high myopia revealed that 63.1% of the study population had WTR astigmatism, despite the patients being younger (less than 60 years old).⁴ Our study revealed oblique astigmatism was 16.8%, which is less than the study from Western India (32%), ¹⁸ but comparable to the study from northeast India (18.4%)² and North India (15.59%).¹⁵

The strength of our study is that we have used swept-source OCT (IOL-Master 700) for the first time to describe the different biometry characteristics in Indian patients scheduled for cataract surgery. We also report here the prevalence of preoperative corneal astigmatism in the study group. Prior research in India was conducted using the LensStar LS9000¹⁵ and IOL Master 500 devices.² Additionally, we have evaluated data from each patient’s both eyes, something that hasn’t always been done in published literature.¹⁷ Data from both eyes provides a more comprehensive understanding of the population’s biometric parameters.

A limiting factor in our study is that it is restricted to hospital-based research from Assam, Northeast India, which might not be representative of the general population.

CONCLUSION

This study provides an in-depth analysis of the biometry parameters of subjects from the Indian subcontinent. A normative database is a prerequisite for any interventional procedure, and this study intends to serve that purpose. We also report that about two-third of subjects undergoing cataract operation had corneal astigmatism <1.0 D and do not require toric lens implantation. This study also highlights the variations in biometric characteristics across different regions of India.