Distribution of ocular biometric parameters and optimal model of anterior chamber depth regression in 28,709 adult cataract patients in China using swept‐source optical biometry

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 five studies of patients who underwent cataract surgery (Table 5) [8‐12]. To minimize selection bias, we compared the proportion of patients under 70 years of age in our study and with those of Cui’s [10] studies, which represented the cataract surgery scenario in China 10 years ago (July 2007 and June 2011). Both studies included cataract candidates scheduled for phacoemulsification, most of whom came from the urban area. The result, which indicated that there was a higher ratio of cataract patients under 70 years of age currently than there was 10 years ago, was statistically significant (χ² = 199.008, P < 0.001). Over the last 10 years, the development of phacoemulsification 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 finding 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 modifiable 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 confirmed 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 findings of other studies of cataract patients in different countries [21‐23]. Some studies showed that body height is positively correlated with AL and ACD; taller persons tended to have longer AL and deeper ACD than shorter people, and men are generally taller than women [9, 24, 25]. The sex-related differences in the AL 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 confirms 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 4). 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. 2). 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 confidence 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 AL (AL < 22 mm) was 6.4 %, which is similar to that of other studies [10, 28]; the proportion of eyes with shallow ACD (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 phacoemulsification 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. From the matrix of Spearman correlation of biometric parameters (Table 3) cataract patients with longer AL and wider WTW have deeper ACD. With increasing age and lens thickening ACD becomes shallower. The mean LT was 4.52 ± 0.45 mm and tended to be thicker in older patients and in shorter eyes. These findings are consistent with the findings of other studies [8, 9, 35]. These results confirm 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 44.26 ± 1.70 D and tended to be greater in shorter eyes, which is also in line with the mechanism of emmetropization.

Based on the standardized coefficients of ACD multivariate regression analysis, LT is the main factor that affects ACD, and is followed by AL (Table 4). Adjusted R² = 0.641 means that 64.1 % of the variance could be explained by this model. A study of an elderly Chinese population showed that approximately one in five 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. 3), and it tended to be shallower with age. Therefore, cataract patients with AL < 22 mm should be more concerned about their intraocular pressure when they are over 60 years old.

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 findings 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. This study provided reference values for AL, Km, CA, ACD, LT, CCT and WTW by using IOLMaster 700 for adult cataract patients in China. Cataract patients with longer AL and wider WTW have deeper ACD. With increasing age and lens thickening ACD becomes shallower. Based on the standardized coefficients of ACD multivariate regression analysis from the study, LT is the main factor that affects ACD, and is followed by AL.