Evaluation of Anterior Chamber Depth in 28,709 Adult Cataract Patients in China using Swept-Source Optical Biometry

Background: To create an anterior chamber depth (ACD) regression model for adult cataract surgery candidates from China, and to evaluate the distribution of their ocular biometric parameters. Methods: The ocular biometric records of 28,709 right eyes of cataract surgery candidates who were treated at Aier Eye Hospitals in nine cities from 2018 to 2019 were retrospectively analyzed. All measurements were taken with IOLMaster 700. We included patients who were at least 40 years old and were diagnosed with cataract. Results: The mean age of the patients was 68.6 ± 11.0 years. The mean values recorded were as follows: axial length (AL), 24.17 ± 2.47 mm; mean keratometry (Km) value, 44.09 ± 3.25 D; corneal astigmatism (CA), 1.06 ± 0.98 D; ACD, 3.02 ± 0.45 mm; lens thickness (LT), 4.52 ± 0.45 mm; central corneal thickness (CCT), 0.534 ± 0.04 mm; and white to white (WTW) corneal diameter, 11.64 ± 0.46 mm. The proportion of patients with long axial length (AL >25 mm) decreased with age. ACD, LT, AL, Km, WTW, and age were correlated. In the multivariate regression analysis of ACD, which included LT, AL, WTW, sex, Km, CCT, and age, there was a reasonable prediction with adjusted R 2 = 0.629. Conclusions: The results show that high myopes are inclined to schedule cataract surgery at a younger age. LT and AL were found to be important factors that affect ACD. This study provides reference data for cataract patients from China.


Background
Modern cataract surgery has transformed from a rehabilitative procedure to a refractive operation. This can be attributed to the development of measuring devices, lens calculation formulas, surgical technology, and intraocular lenses. Accurate prediction of refractive outcomes after cataract surgery is crucial. When biometry is based on optical coherence interferometry (Zeiss IOLMaster), the error from anterior chamber depth (ACD) prediction amounts to approximately 42% of the total refractive prediction error [1]. Olsen [2] found that postoperative ACD is signi cantly predictable by a ve-variable regression method that incorporates preoperative axial length (AL), ACD, keratometry (K), lens thickness (LT), and refraction. Therefore, it is essential to acquire the biometric characteristics of cataractous eyes to enable the utilization of precise ocular biometric parameters for accurate intraocular lens calculation.
Previous research has con rmed that female sex, older age, shorter AL, shallow ACD, and smaller LT were risk factors for angle closure [3]. These risk factors are also closely associated with elderly cataract patients.
To the best of our knowledge, there is no study of a large series of biometry measurements acquired with the IOLMaster 700 device in the existing literature. The purpose of this study was to create an ACD regression model for cataract surgery candidates aged 40 years or older, and to review and evaluate their biometry parameters. The results of this study may provide a new reference for cataract patients based on a large group of patients.

Study Population
The medical records of 28,709 cataract surgery candidates over a year period (October 2018 to October 2019) (11,753 men [40.9%] and 16,956 women [59.1%]) from the Aier Eye Hospitals in nine provincial capitals in China were reviewed retrospectively. Due to the retrospective nature of the study, the need for informed consent was waived. The retrospective study was approved by the Ethics Committee of Aier Eye Hospital and was performed in accordance with the Declaration of Helsinki.
The ocular parameters were measured with a non-contact swept-source optical biometer (IOLMaster 700, Carl Zeiss Meditec AG, Jena, Germany). The inclusion criteria were as follows: 1) patients diagnosed with cataracts; 2) patients who were 40 years or older; and 3) 3) patients with good quality measurements with the IOLMaster 700 The exclusion criteria were as follows: 1) patients who had complications with other eye diseases including corneal diseases or in ammatory eye diseases; and 2) patients who had a history of trauma or ocular surgery.

Device And Measurement
The IOLMaster 700 has a swept source optical coherence tomography (SS-OCT) scanning capacity with 1,050 nm laser infrared light and measures AL, ACD, LT, and central corneal thickness (CCT). The device can obtain OCT images of the macular and visualizes the measurement of the AL of the eye. The IOL Master 700 uses telecentric keratometry for K measurements. Corneal power is measured in two meridians: the greatest and least radii of curvature (K 1 , K 2 ). Corneal astigmatism (CA) is calculated as the absolute difference between K 1 and K 2 values. Mean keratometry (Km) is the average of K 1 and K 2 . It provides keratometry measurements in the central 2.5-mm zone and uses a refractive index of 1.3375 for the biometry parameters. ACD is measured from the corneal epithelium to the anterior lens surface [4]. A single capturing process can provide multiple measurements for each parameter. We controlled the quality of the measurements according to the device instructions. All patients were tested by experienced examiners. We selected measurements with a standard deviation of less than 20 microns.

Statistical analysis
To avoid correlation between the right and left eyes, only the right eye of each subject was used for statistical analyses. Data were processed with the SPSS software version 19.0 (IBM, USA). We performed a four-step analysis. First, we strati ed the parameters according to sex and age groups. The study participants were classi ed into 5 groups based on age (40-49 years, 50-59 years, 60-69 years, 70-79 years, over 80 years). All eyes were divided into ve groups based on AL; they were also categorized into ve groups based on ACD. Second, the Mann-Whitney U Test was used to compare the ocular parameters according to sex and AL group. The chi-square test was used to compare the frequency distribution.
Third, we evaluated the association between age and other ocular parameters using Spearman's correlation. Finally, we performed a multivariate analysis with ACD as the dependent parameter and all other parameters that were signi cantly associated with ACD as independent variables.
Variation in ation factors were calculated to investigate multicollinearity. All P-values were two-sided and considered statistically signi cant when the values were less than 0.05.

Results
A total of 28,709 patients, including 11,753 men (40.9%) and 16,956 women (59.1%) diagnosed of cataract, were selected according to our inclusion criteria. The mean age of the patients was 68.6 ± 11.0 years (range, 40-101 years). The mean values measured were as follows: AL, 24.17 ± 2.47 mm; Km, 44.09 ± 3.25 D; CA, 1.06 ± 0.98 D; ACD, 3.02 ± 0.45 mm; LT, 4.52 ± 0.45 mm; CCT, 0.534 ± 0.04 mm; and white to white (WTW) corneal diameter, 11.64 ± 0.46 mm. The details of the ocular characteristics of the right eyes strati ed according to age groups are shown in Table 1. The subgroups and the distribution of AL and ACD are shown in Table 3. Figure 1 illustrates the percentage of patients in the AL groups strati ed according to age. The proportion of patients with long AL (AL > 25 mm) decreased with age.   In this study population, the AL was statistically signi cantly longer in male subjects than in female subjects (24.39 mm versus 24.02 mm, P < 0.001). The ACD was signi cantly deeper in men than in women (3.12 mm versus 2.96 mm, P < 0.001). Men had a similar LT as women (4.53 mm versus 4.52 mm), and the difference was not statistically signi cant (P = 0.178). The Km was signi cantly higher in men than in women (44.59 D versus 44.44 D, P < 0.001). The WTW was signi cantly wider in men than in women (11.78 mm versus 11.54 mm, P < 0.001).
We also compared the ACD of the three AL groups (< 22 mm, 22-25 mm and ≥ 25 mm) and found that the differences between them were statistically signi cant (P < 0.001). The ACD of the patients in the group with AL < 22 mm was the shallowest (Fig. 2).
Correlation analysis revealed that age, AL, ACD, LT, Km, and WTW were all signi cantly correlated with each other (P < 0.01), and that ACD was highly correlated with LT (r = -0.660, P < 0.001) and AL (r = 0.544, P < 0.001). The complete matrix of the correlation is presented in Table 4. The relationship between ACD and LT, AL, and age is shown in scattergrams (Figs. [3][4][5].  Km, sex, CCT, and age, there was a reasonable prediction with adjusted R 2 = 0.629 (Table 5).

Discussion
We reviewed and evaluated the biometry parameters of cataract surgery candidates and created an ACD regression model for predicting the refractive outcomes of cataract surgery. Optical biometry has been well accepted as the gold standard since the introduction of the IOLMaster optical biometer in 1999 [5]. As a newly available swept-source OCT-based optical biometry device, the IOLMaster 700 provides OCT imaging of the macula and visualizes the measurement of the AL of the eye. The repeatability and reproducibility of the swept-source optical biometer is excellent and its agreement with a standard biometer is very high; better lens penetration ability and more accurate AL measurements are obtained with the swept-source biometer than with the standard biometer [6]. The SS-OCT biometer showed better penetration in dense posterior subcapsular cataracts, measuring AL successfully in 96% of cases which possibly was a result of the reduced scattering and attenuation from ocular opacities by the 1055 nm light source used [7]. In the present study, ocular biometry was performed with the IOLMaster 700, which can capture all parameters in a single process. It is convenient for evaluating patients, especially elderly patients.
A gradual descending trend in average age was reported in ve studies of patients who underwent cataract surgery ( there was 10 years ago, was statistically signi cant (χ 2 = 199.008, P < 0.001). Over the last 10 years, the development of phacoemulsi cation and femtosecond laser-assisted cataract surgery [13,14] and the popularity of functional intraocular lenses [15,16] have advanced cataract surgery in China, leading to better visual outcomes. Urban Chinese cataract patients in the present study tended to choose surgical treatment earlier than cataract patients did 10 years ago, a decision which could improve their quality of life.
In present study, more female patients scheduled cataract surgery than male patients (59.1% versus 40.9%). This nding is almost identical to those of previous published studies [9,17] (60% versus 40%). Furthermore, female patients presented for surgery at a slightly older age than male patients (mean: 68.8 years versus 68.4 years, P = 0.005). This is similar to the results of Jivrajka's report (mean: 75 years versus 73 years), which considered that women had a longer life expectancy than men [9]. Moreover, some studies showed that women exhibit a higher prevalence of some types of cataract than men [18,19]. Freeman [20] suggested that a potentially modi able factor in cataractogenesis may be a woman's exposure to postmenopausal estrogen. This partially explains the sex-related differences in the rates of cataract surgery. Moreover, we believe that elderly women are more concerned about their eye health and are more likely to undergo surgery than elderly men. This needs to be con rmed by further research.
In the present study, female patients presented for surgery with a shorter average AL (mean 24.02 mm versus 24.39 mm, P < 0.001) and shallower ACD (mean 2.96 mm versus 3.12 mm, P < 0.001) than male patients. This echoes the ndings of other studies of cataract patients in different countries [21][22][23].
Some studies showed that body height is positively correlated with AL and ACD; taller persons tended to have longer ALs and deeper ACDs than shorter people, and men are generally taller than women [9,24,25]. The sex-related differences in the ALs and ACD measured in the present study may be attributed to the association between ocular dimensions and stature. Further, this is consistent with the higher incidence of primary angle closure (PAC) in women than in men [3,26]. Research con rms that the female sex, older age, shorter AL, shallow ACD, and larger LT are risk factors for angle closure [3,26]. These risk factors are closely associated with elderly cataract patients.
In the present study, the mean AL was 24.17 ± 2.47 mm; this result is similar to those of previous reports by Yu [27] (24.38 ± 2.47 mm) and Huang [28] (24.32 ± 2.42 mm). We found that AL was negatively correlated with age (r = -0.180 P < 0.001) ( Table 5). The longest mean AL was in the 40-49 years age group (26.63 ± 3.40 mm); this mean AL is longer than that of Cui's report (25.97 ± 3.53 mm) [10]. This may be because our study population had a higher proportion of eyes with a longer AL (AL > 26 mm) than the population in Cui's study [10] (14.7% versus 11.9%). This is especially apparent in the 40-49 years age group in our study, in which the proportion was as high as 52.1%. The proportion of eyes with AL > 25 mm decreased with age (Fig. 1). This corroborates Jivrajka's supposition that high myopia with increased AL predisposes to the development of cataract at a younger age [9]. On the contrary, newer lens formulas and adjustments/transformations are now available to reduce postoperative predictive error in high myopes [29,30]. Furthermore, the levels of satisfaction and spectacle independence experienced by high myopes were reasonably high after implantation of multifocal or trifocal intraocular lens [15,31]. Since these outcomes enhance the con dence of high myopes, they are more inclined to schedule cataract surgery at a younger age.
In the present study, the proportion of eyes with short ALs (AL < 22 mm) was 6.4%, which is similar to that of other studies [10,28]; the proportion of eyes with shallow ACDs (ACD < 2.5 mm) was 11.8%. The refractive status of a patient with a shallow ACD and a short AL after cataract surgery would tend toward a myopic shift. Conversely, a patient with deep ACD and long AL would move toward a hyperopic shift, which was related to the postoperative ACD change [32]. The relative change in ACD after phacoemulsi cation is larger in short eyes than in normal and long eyes [33]. Melles found that the Barrett Universal Formula had the lowest mean absolute prediction error for eyes with short AL and shallow ACD [34]. This guided us toward choosing the suitable formula for these kinds of patients.
The mean ACD in the present study was 3.02 ± 0.45 mm. The ACD tended to be shallower in older patients and shorter eyes and was negatively correlated with LT. The mean LT was 4.52 ± 0.45 mm and tended to be larger in older patients and in shorter eyes. These ndings are consistent with the ndings of other studies [8, 9,35]. These results con rm Hoffer's [8] mechanism of emmetropization: the lens thins (or decreases in power) as the eye gets longer (myopic) and thickens (or increases in power) as the eye gets shorter (hyperopic). The Km in the present study was 44.09 ± 3.25 D and tended to be greater in shorter eyes, which is also in line with the mechanism of emmetropization.
Based on the standardized coe cients of ACD multivariate regression analysis, LT is the main factor that affects ACD, and is followed by AL (Table 6). Adjusted R 2 = 0.629 means that 62.9% of the variance could be explained by this model. A study of an elderly Chinese population showed that approximately one in ve people aged 50 years and over developed some form of angle closure over a 10-year period, and reported best cut-off values of 2.60, 4.72, and 22.92 mm for ACD, LT, and AL, respectively, in predicting incident PAC [26]. In the present study, the mean ACD of patients in the 60-69 age group was 2.55 mm when the AL was < 22 mm (Fig. 2), and it tended to be shallower with age. Therefore, combined with our ocular data, elderly cataract patients with AL < 22 mm are at a high risk of having PAC. Cataract surgery is effective for lowering the intraocular pressure of normal patients or glaucoma patients [36,37]. Therefore, we recommend early surgery for elderly cataract patients, especially those with short AL, to minimize the risk of PAC.
This study has certain limitations. First, the data from the nine hospitals do not completely represent the ocular parameters of the overall population in China. Second, we need to collect more information about anthropometric characteristics, education, occupation, income, and the type of intraocular lens, and evaluate the correlations among them. We will conduct more research to improve on the ndings of the analyses.
In summary, we collated data on the ocular biometric parameters of cataract surgery candidates in China, compared them, analyzed their correlations, and created an ACD regression model. We found that urban Chinese patients in this study tended to choose surgical treatment earlier than cataract patients did 10 years ago. We also found that high myopes are inclined to schedule cataract surgery at a younger age. These ndings con rm the development and improvement of cataract surgery in China. According to the ACD multivariate regression model, LT and AL are the main factors that affect ACD. Elderly cataract patients, especially those with AL < 22 mm, may be at a high risk of having PAC. We recommend that their cataract surgery be scheduled earlier.
Abbreviations ACD: anterior chamber depth; AL: axial length; K: keratometry; LT: lens thickness; SS-OCT: swept source optical coherence tomography; CCT: central corneal thickness; CA: corneal astigmatism; Km: mean keratometry; WTW: white to white Declarations Ethics approval and consent to participate The retrospective study was approved by the Ethics Committee of Aier Eye Hospital and was performed in accordance with the Declaration of Helsinki. Due to the retrospective nature of the study, the need for informed consent was waived.

Consent for publication
Not applicable.

Availability of data and materials
All the data supporting the conclusion of this article is included within the article.

Competing interests
The author declare that they have no competing interests.
Author's contributions QL participated in the collection of data, performed the statistical analysis and drafted the manuscript; YW participated in its design and critically revised the manuscript; HT participated in the collection of data and revised the manuscript; XF participated in the collection of data and revised the manuscript. All authors read and approved the nal manuscript.  Distribution of anterior chamber depth (ACD) across age groups with data markers strati ed according to three axial length (AL) groups. The ACD tended to be shallower in older patients and shorter eyes.   Scattergram showing the relationship between age and anterior chamber depth.