Lens Thickness-to-Anterior Chamber Depth Ratio: A Biometric Determinant of Chamber Angle Width in Han Chinese Cataract Patients

Abstract

Purpose: The purpose of this study was to individuate a novel ocular biometric index, the lens thickness-to-anterior chamber depth (LT/ACD) ratio, and identify its role in explaining anterior chamber angle width (ACAW) in Han Chinese patients with cataract.

Methods: We enrolled 400 patients with cataract with phakic eyes. According to the axial length (AL), the eyes were divided into short eyes (AL < 22 mm, n = 100), medium eyes (22 ≤ AL < 24 mm, n = 100), medium‒long eyes (24 ≤ AL < 26 mm, n = 100), and long eyes (AL ≥ 26 mm, n = 100). We used a Pentacam HR to determine ACAW and an IOLMaster 700 to measure ACD and LT. A narrow chamber angle was defined as an ACAW < 20 degrees.

Results: Using a multivariable linear model, which included the LT/ACD ratio, age, sex, AL, white-to-white, keratometry, central corneal thickness, and pupil diameter, the LT/ACD ratio was the strongest determinant of ACAW in the total group (standardized regression coefficient [β] = −0.71), short eyes (β = −0.71), medium eyes (β = −0.69), medium–long eyes (β = −0.72), and long eyes (β = −0.63). The LT/ACD ratio remained the strongest determinant of ACAW in eyes with an ACD < 3.0 mm (β = −0.51) or an LT ≥ 5.0 mm (β = −0.64). Multivariable logistic analysis revealed a significant association between a greater LT/ACD ratio and the presence of angle narrowness in the total group (odds ratio = 156.7, 95% confidence interval [CI] = 10.8–2270.4). An LT/ACD ratio of 1.8 could indicate angle narrowness (area under the curve = 0.94).

Conclusions: In Han Chinese patients with cataract, the LT/ACD ratio can be used as an ocular biometric determinant of ACAW. An LT/ACD ratio exceeding 1.8 could effectively identify the presence of angle narrowness.

The width of the anterior chamber angle (ACAW) is a pivotal factor in the pathogenesis of primary angle closure glaucoma (PACG), a leading cause of irreversible vision loss and blindness worldwide.¹ ACAW influences the outflow of aqueous humor into the trabecular meshwork, thereby playing an important role in the stabilization of intraocular pressure (IOP). Eyes with narrow ACAWs are susceptible to angle closure,² which further contributes to severe IOP elevation and subsequent glaucomatous optic neuropathy. Therefore, comprehending the determinants of ACAW is advantageous in screening for individuals who are predisposed to chamber angle narrowness and angle closure, thereby facilitating diagnostic and treatment activities for angle-closed glaucoma. 

Previous surveys on the conditions of the anterior chamber angle (ACA) have identified various demographic (female sex,³ old age,^2–6 etc.) and ocular biometric characteristics (thick central cornea,³ steep peripheral iris,^7,8 high lens vault [LV],^7,9 etc.) as risk factors for angle closure in phakic eyes. Prediction models^10,11 based on multiple ocular biometric indices performed well in predicting ACAWs. However, previous reports^2–9 have also indicated a relatively weak impact of individual parameters on ACAW. Moreover, despite the satisfactory performance exhibited by multiparameter algorithm models^10,11 in predicting ACAW, there is still a demand in clinical practice for enhanced simplicity to enable an effective deduction of chamber angle conditions. 

Lens thickness (LT) and anterior chamber depth (ACD) are two basic biometric indices that show the axial dimension of the anterior ocular segment in an intuitive way. A lower LT² and greater ACD^3,7 are associated with a wide ACA. According to a previous population-based survey,¹² LTs are not independently associated with ACDs. A thick lens can coexist with a deep anterior chamber, whereas a shallow anterior chamber may be accompanied by a thin lens.¹³ Considering the various possible combinations of ACD and LT in phakic eyes, incorporating both ACD and LT could provide an effective approach for elucidating ACAW. 

In this study, we individuated a novel ocular biometric index, the LT/ACD ratio, and demonstrated its significant predictive value in explaining ACAW in cataractous eyes with varying ocular dimensions. Our findings could contribute to the elucidation of the anatomic mechanisms underlying angle narrowness and potentially enhance the clinical utility of biometric indices in evaluating the risk of angle closure in phakic eyes. 

Methods

Study Design

This was a retrospective hospital–based cross–sectional study of the preoperative data of patients scheduled for cataract surgery. This study was approved by the Institute Ethics Committee of the Eye & ENT Hospital of Fudan University (Shanghai, China; ethical permit number: 2013021). The protocols of this research followed the tenets of the Declaration of Helsinki and its consequent revisions. Written informed consent was obtained from all participants. 

Patients

Four hundred phakic eyes, including 100 short eyes (axial length [AL] < 22 mm), 100 medium eyes (22 ≤ AL < 24 mm), 100 medium–long eyes (24 ≤ AL < 26 mm), and 100 long eyes (AL ≥ 26 mm), of 400 patients with cataract were included for analysis. Consecutive enrollment was performed in each AL group. The inclusion criteria for the study subjects were (1) adult Han Chinese patients scheduled for phacoemulsification with intraocular lens implantation at the Eye & ENT Hospital of Fudan University (Shanghai, China) from December 2023 to May 2024; (2) complete preoperative ocular measurements; and (3) satisfactory flatness of the peripheral iris identified by slit‒lamp examination and Pentacam HR Scheimpflug imaging. 

The exclusion criteria for this study included a history of mydriasis or myosis, previous ocular surface or intraocular surgery, eye trauma, contact lens wearing, pterygium, corneal abnormalities, uveitis, anterior and posterior synechia, plateau irises, anterior displacement of the ciliary body, lens luxation and subluxation, and aphakic and pseudophakic conditions. To avoid potential interference in ACAW measurements, we excluded patients with a history of angle closure or angle-closed glaucoma. Patients who could not maintain centric eye fixation during ocular examinations were also excluded. 

In this study, all participants had cataracts in both eyes. For statistical analysis, one eye from each subject was randomly selected. As a result, either the cataractous eye scheduled for surgery or the contralateral eye was eligible for inclusion in the analysis. 

Ocular Measurements

We used an IOLMaster 700 (software version 1.88) to determine the AL, keratometry (K), central corneal thickness (CCT), white-to-white (WTW), ACD, and LT for each patient. The ACD was defined as the distance along the visual axis from the anterior corneal surface to the anterior lens surface. The LT was determined on the visual axis as the distance between the anterior and posterior surfaces of the crystalline lens (Fig. 1). The LT/ACD ratio was calculated as the LT divided by the ACD. 

Figure 1.

 

Scheimpflug tomography image illustrating the primary outcome measures: anterior chamber angle width (ACAW), anterior chamber depth (ACD), and lens thickness (LT).

A Pentacam HR (software version 1.25r15) was used to determine the ACAW (see Fig. 1) and pupil diameter (PD). For each patient, the Pentacam HR captured and automatically delineated the ACA in the horizontal meridian, thereby quantifying the ACAWs at temporal and nasal locations. The Pentacam HR automatically displayed a smaller value between the nasal and temporal ACAWs as the proscenium ACA width reading. 

Statistical Analysis

First, we described the demographic and ocular status of the total group and each AL group. The characteristics of the different AL groups were compared via 1–way analysis of variance (ANOVA) for quantitative variables and Pearson's χ² test for qualitative variables. The least significant difference (LSD)–correction method was used following ANOVA for multiple intergroup comparisons. 

Second, we calculated Pearson's correlation coefficients (rs) to preliminarily investigate the relationships between ACAW and demographic and other ocular parameters. On the basis of the correlation analysis findings, we further performed multivariable regression analysis to identify the determinants of ACAW. The multivariable model included the LT/ACD ratio, age, sex, AL, WTW, K, CCT, and PD as the independent variables and ACAW as the dependent variable. The collinearity of the model was assessed via variance inflation factors (VIFs). Any individual independent variable with a VIF greater than 3.0 was excluded from the model. Linear regression coefficients (Bs), 95% confidence intervals (CIs) of Bs, and standardized Bs (βs) were calculated via the multivariable linear regression model. The contribution of each independent variable to ACAW was estimated according to the magnitude of βs. The R² statistic indicates variation in ACAW that can be explained via the model's independent variables. A similar methodology was employed to identify the determinants of ACAW in different groups stratified by ocular biometric indices (AL, ACD, and LT). 

Third, a receiver operating characteristic (ROC) curve was constructed to assess the relationship between the LT/ACD ratio and ACAW. The area under the curve (AUC) was used as an indicator to evaluate the diagnostic performance of the LT/ACD ratio in reflecting ACAW. We calculated the Youden indices (YIs) of the LT/ACD ratio via the following formula: YI = sensitivity + specificity − 1. The cutoff point of the LT/ACD ratio to indicate chamber angle narrowness (ACAW < 20 degrees) was determined on the basis of the maximum YI. 

Using the Shaffer grading system,^14,15 we defined a narrow chamber angle as an ACAW < 20 degrees, which corresponds to possible angle closure. The shallow anterior chamber and thick lens were defined as having an ACD < 3.0 mm¹⁶ and an LT ≥ 5.0 mm,¹⁷ respectively. All P values in the statistical analysis were calculated via 2–sided tests, and a probability value less than 0.05 was considered statistically significant. We used SPSS Statistics (version 26.0; IBM Corporation, SPSS Inc., Chicago, IL, USA) for statistics and GraphPad Prism (version 9.3.1; Dotmatics, Boston, MA, USA) and R programming languages (version 4.2.0; R Core Team, Vienna, Austria) for graphics. 

Results

Study Population

Among the 400 individuals enrolled in this study, the mean age was 66 ± 12 years (range = 30–94 years), and 66.0% were women (n = 264). The AL ranged from 16.8 mm to 35.7 mm in the total group, with a mean value of 24.6 ± 3.0 mm. According to the results of the Pearson's χ² tests and 1–way ANOVA, there were no differences in age or sex constituent ratios among the four AL groups (both P > 0.05). There were significant differences in the ACAWs between the AL groups (P < 0.001). The ACAWs in the medium–long and long eyes were significantly greater than those in the short and medium eyes (all P < 0.05). The ACAWs in medium eyes were also significantly greater than those in short eyes (P < 0.001). Table 1 presents the demographic data and ocular status of the total group and each AL group. 

Table 1.

 

Demographic Data and Ocular Status of 400 Han Chinese Patients With Cataract

 

Determinants of Anterior Chamber Angle Width

Pearson's correlation analysis of the total group revealed that ACAW was significantly associated with younger age (r =−0.1), male sex (r = −0.2), longer AL (r = 0.3), wider WTW (r = 0.3), flatter K (r = −0.2), thinner LT (r = −0.6), and deeper ACD (r = 0.7, all P < 0.01). The LT/ACD ratio was inversely correlated with wider ACAW in the total group (r = −0.7, P < 0.001), short eyes (r = −0.7, P < 0.001), medium eyes (r = −0.7, P < 0.001), medium–long eyes (r = −0.7, P < 0.001), and long eyes (r = −0.6, P < 0.001; Fig. 2). In addition, ACAW was associated with a thinner CCT (r = −0.2, P < 0.05) in short eyes. 

Figure 2.

 

Scatter plots showing the relationships between the LT/ACD ratio and the ACAW in short eyes (A), medium eyes (B), medium–long eyes (C), and long eyes (D). ACAW, width of the anterior chamber angle; LT/ACD ratio, lens thickness–to–anterior chamber depth ratio; β, standardized linear regression coefficient.

Multivariable linear regression analysis was performed to investigate the determinants of ACAW in the total group. The multivariable model, which included the LT/ACD ratio, age, sex, AL, WTW, K, CCT, and PD as the independent variables and ACAW as the dependent variable, explained 57% of the variability in ACAW. The strongest determinant of ACAW was the LT/ACD ratio (β = −0.71, P < 0.001), followed by the CCT (β = −0.12, P = 0.001) and K (β = 0.11, P < 0.05). 

When the participants were stratified by AL, the LT/ACD ratio was the only determinant of ACAW in medium eyes and medium–long eyes (β = −0.69, P < 0.001 and β = −0.72, P < 0.001, respectively). In short eyes and long eyes, the LT/ACD ratio remained the strongest determinant of ACAW (β = −0.71, P < 0.001 and β = −0.63, P < 0.001, respectively). In short eyes, ACAW was also associated with male sex (β = −0.16, P < 0.05) and lower CCT (β = −0.27, P < 0.001), whereas younger age was another significant determinant of ACAW in long eyes (β = −0.21, P < 0.05; Table 2). The significant association between K and ACAW in the total group was not significant in any individual AL group (all P > 0.05). 

Table 2.

 

Association of Anterior Chamber Angle Width with the Lens Thickness−to−Anterior Chamber Depth Ratio in 400 Han Chinese Cataract Patients (Multivariable Linear Regression Analysis)^*

 

The multivariable model was then used for eyes with an ACD < 3.0 mm (n = 177, range of ACD = 1.8–2.9 mm and mean ACD = 2.6 ± 0.3 mm). In eyes with shallow anterior chambers, the LT/ACD ratio remained the strongest determinant of ACAW (β = −0.51, P < 0.001). This pattern was consistent in eyes with an LT greater than 5.0 mm (n = 66, mean LT = 5.2 ± 0.2 mm and range of LT = 5.0–5.7 mm). The LT/ACD ratio continued to be the strongest determinant of ACAW in eyes with thick lenses (β = −0.64, P < 0.001). 

Diagnostic Performance of the Lens Thickness–to–Anterior Chamber Depth Ratio

Multivariable logistic regression analyses were used to determine the link between the LT/ACD ratio and chamber angle narrowness (ACAW < 20 degrees). In the total group, after adjusting for age, sex, AL, WTW, K, CCT, and PD, a greater LT/ACD ratio was significantly associated with the presence of angle narrowness (odds ratio [OR] = 156.7, 95% CI = 10.8–2270.4, P < 0.001). 

Then, we constructed an ROC curve to identify the diagnostic performance of the LT/ACD ratio in reflecting ACAW (Fig. 3). The AUC was 0.94 (95% CI = 0.91–0.98; P < 0.001). The maximum YI value on the ROC curve was 0.76, which corresponded to an LT/ACD ratio of 1.8. The sensitivity and specificity of utilizing an LT/ACD ratio of 1.8 as a standalone measure for determining chamber angle narrowness (ACAW < 20 degrees) were 95% and 81%, respectively. 

Figure 3.

 

Receiver operating characteristic (ROC) curve showing the diagnostic performance of the lens thickness–to–anterior chamber depth (LT/ACD) ratio in indicating the width of the anterior chamber angle. TPR, true positive ratio; FPR, false positive ratio.

Discussion

In this study, we individuated a novel ocular biometric index, the LT/ACD ratio, and demonstrated its strong predictive role in explaining ACAW among Han Chinese patients with cataract. An LT/ACD ratio exceeding 1.8 could effectively identify the presence of chamber angle narrowness (ACAW < 20 degrees). The LT/ACD ratio was the most influential factor in determining ACAW across the entire AL spectrum. In cases of excessively thick lenses (LT ≥ 5.0 mm) or shallow anterior chambers (ACD < 3.0 mm), the LT/ACD ratio remained robust in its ability to reflect ACAW. 

We individuated the LT/ACD ratio on the basis of the results of the correlation analysis. ACAW may be associated with multiple factors, such as age,^2–6 sex,³ ACD,^3,7 LT,² etc. In this study, we started with a Pearson's correlation examination of these parameters to investigate their relationship with ACAW in Han Chinese subjects. Pearson's correlation analyses revealed that ACD and LT were the two parameters that had the strongest links with ACAW. As we presented in our Results section, the ACD and LT were positively (r > 0) and negatively (r < 0) correlated with ACAW, respectively. On the basis of the above findings of correlation analyses, we algorithmically individuated the LT/ACD ratio to converge the influences of both ACD and LT on ACAW, thereby providing a novel possible solution to ACAW predictions in phakic eyes. 

β in linear regression analysis expresses the average change in standard deviation (SD) of an outcome variable associated with a one–SD change in a predictor variable. Therefore, in the total group, an absolute β of 0.71 between the LT/ACD ratio and ACAW indicates that a one–SD change in LT/ACD ratio would result in a 0.7–SD change in ACAW. The βs of different independent variables can be directly compared to determine their strengths of associations with the dependent variable. In our study, among the independent variables included in the multivariable model, the LT/ACD ratio outperformed the other arguments in β. Thus, the LT/ACD ratio was the strongest determinant of ACAW in the phakic eyes of Han Chinese individuals. 

The structure of the human anterior eye segment underlies the significant association between a greater LT/ACD ratio and a lower ACAW (Fig. 4). As the anterior chamber serves as the spatial foundation for the chamber angle, greater ACDs promote larger chamber volumes and wider chamber angles.^3,7 In the peripheral anterior chamber, the chamber angle is formed between the iris and cornea. For phakic eyes, the convex anterior lens surface pushes the iris toward the cornea, resulting in a narrower chamber angle as the LT increases. In addition, the biometric characteristics of the cornea may influence ACAW. Anatomically, steep corneal curvature and thin corneal thickness at the chamber angle may provide more space for accommodating a wide angle. In the total group, we observed greater ACAWs in eyes with thinner CCTs and steeper Ks, which can be explained by the anatomic relationship between corneal biometry and ACAW. 

Figure 4.

 

Scheimpflug tomography images illustrating the anterior ocular segment of an eye with a normal angle width (A) and an eye with angle narrowness (B), defined as an angle width of less than 20 degrees. ACAW, anterior chamber angle width; ACD, anterior chamber depth; LT, lens thickness.

We identified the LT/ACD ratio as a reliable indicator that effectively reflects ACAW, thereby providing a possible novel and efficient index to screen for angle narrowness (ACAW < 20 degrees) in patients with cataract. Previous studies on non-cataract phakic patients of Chinese⁷ and Caucasian¹⁸ ethnicities identified ACD (β = 0.64),⁷ anterior chamber volume (ACV),¹⁸ LV (β = −0.46), and iris curvature (β = −0.26)⁷ as significant determinants of ACAW. However, the β values of these individual biometric indices were relatively low, indicating weaker associations with ACAW, compared to the LT/ACD ratio (β = −0.71). Prior studies^19,20 on patients with glaucoma also identified ACD and LV as potential parameters for predicting angle closure, with both achieving AUC values exceeding 0.9. In our study, the LT/ACD ratio demonstrated comparable predictive performance (AUC = 0.94) for detecting angle narrowness. Furthermore, considering the ACD and LT in conjunction offers a significant advantage in predicting angle closure in eyes with extreme anterior segment biometry. Therefore, the LT/ACD ratio, as a standalone measure, can be considered a possible solution to ACAW prediction in phakic eyes. 

Our findings also justify lens removal in patients with phakia with angle-closed glaucoma. According to a population-based study of 6640 individuals,²¹ IOP increases with a narrower ACA. Considering the link between a narrow ACA and a high LT/ACD ratio, reducing the LT/ACD ratio is a possible solution for IOP elevation in phakic eyes. As lens removal can decrease LT and increase the ACD at the same time, it can be considered an effective treatment for patients whose IOP is elevated due to narrow or closed ACAWs. The curative effectiveness of lens removal in angle-closed glaucoma has been identified in the literature.^22,23 

Our study had several limitations. First, our study utilized a cross-sectional observational design. A longitudinal study tracking cataract progression would offer deeper insights into the predictive value of the LT/ACD ratio for ACAW. Second, we did not explore the relationship between the LT/ACD ratio and ACAW in non-cataract phakic eyes. Thus, a population-based study is needed to validate our findings and ensure their applicability to the general population. Third, some novel indices were not incorporated into our relativity analysis. Previous studies on the associations with ACAW have identified several iridal characteristics, such as iris curvature,⁷ iris thickness,⁸ and the iris area,⁹ as potential determinants of angle width. Despite the relatively low βs values (all absolute βs < 0.4)^7–9 that were observed between ACAW and these iridal indices, integrating characteristics of the peripheral iris in multivariable analysis may contribute to elucidating the determinants of ACAW. In addition, several specialized studies^24–26 on glaucoma pathogenesis have reported the use of several novel parameters, such as the light-room angle recess area,²⁴ trabecular-iris space area,²⁵ and cumulative gonioscopy score,²⁶ for predicting the progression of angle-closure glaucoma. Owing to the limited scope of this paper, these new indices were not included in the current study but will be investigated in our future research. 

Conclusions

In conclusion, in Han Chinese patients with cataract, the LT/ACD ratio can be used as an ocular biometric determinant of ACAW. An LT/ACD ratio exceeding 1.8 can independently identify the presence of chamber angle narrowness (ACAW < 20 degrees). The ability of the LT/ACD ratio to predict ACAW was consistent across short (AL < 22.0 mm), medium, and long eyes (AL ≥ 26.0 mm) and eyes with excessively thick lenses (LT ≥ 5.0 mm) or shallow anterior chambers (ACD < 3.0 mm). This report presents an effective and straightforward biometric index for evaluating ACAW, offering valuable insights into the pathogenesis and diagnosis of angle narrowness and angle closure in phakic eyes. 

Acknowledgments

The authors thank Guan, Kwan, Zhao, and Xie for their invaluable assistance throughout the research process. 

Supported by the Natural Science Foundation of Shanghai (Research Project of Science and Technology Commission of Shanghai Municipality; grant numbers 22ZR1410400 and 19ZR1408600); the National Natural Science Foundation of China (grant numbers 81300747 and 82101103); the “Young Clinical Scientist Training Program” (Shanghai Medical College, Fudan University; grant number 2023, DGF828019-2/038); the Excellent Young Doctor Training Program of Shanghai (2015–2018), Shanghai Municipal Health Commission; the Scientific Research Program of Shanghai Pudong New Area Health Commission (the Joint Research and Development Program); and the “Medicine + X” Interdisciplinary Research Project of Tongji University (2025). 

Author Contributions: A.M. was responsible for the conception and design of the work, the acquisition, analysis, and interpretation of data, the creation of new software used in the work, and drafted the work. F.Y. was responsible for the analysis and interpretation of data, the creation of new software used in the work, and drafted the work. T.H.C. was responsible for the acquisition and interpretation of data, the creation of new software used in the work, and drafted the work. D.J.Q. was responsible for the analysis and interpretation of data and the creation of new software used in the work. Y.X.J. was responsible for the analysis and interpretation of data and the creation of new software used in the work. J.X. was responsible for the conception of the work, the interpretation of data, and substantive revision of the work. T.Y.Z. was responsible for the design of the work, the interpretation of the data, and substantive revision of the work. All the authors have read and approved the final manuscript. 

Consent for Publication: Written informed consent for publication was obtained from all participants. 

Availability of Data and Materials: The datasets used and/or analyzed during the current study are available from the corresponding author upon reasonable request. 

Disclosure: A. Miao, None; F. Yang, None; T. Chen, None; D. Qian, None; Y. Jiang, None; J. Xu, None; T. Zheng, None