Abstract
Introduction
The aim of this study was to investigate the magnitude and quadrantal distribution of angle kappa in patients with cataracts and high myopia (axial length ≥ 26 mm).
Methods
A total of 2485 patients with cataracts and high myopia and 354 patients with cataracts without high myopia (axial length < 26 mm) were included in this study. The location and value of angle kappa were determined using an Oculus Pentacam HR. An angle kappa distance exceeding 0.5 mm was defined as a large angle kappa distance.
Results
In high myopic eyes, multivariable linear regression analysis revealed that a larger angle kappa distance was significantly associated with longer axial length (standardized regression coefficient [β] = 0.24), narrower white-to-white (β = −0.08), lower keratometry (β = −0.08), higher corneal astigmatism (β = 0.06), and older age (β = 0.06) (all P < 0.01). The axial length cutoff point with the maximum Youden Index indicating the presence of a large angle kappa distance was 30 mm (area under the curve = 0.64). In high myopic eyes, the most common regions of angle kappa relative to the pupillary axis were the temporal-superior (40%) and temporal-inferior (26%) quadrants. However, in non-high myopic eyes, the most frequent regions were the nasal-superior (31%) and nasal-inferior (31%) quadrants.
Conclusions
An axial length of ≥ 30 mm is a risk factor for the presence of an angle kappa distance greater than 0.5 mm. In high myopic eyes, the most common location of angle kappa is the temporal-superior quadrant relative to the pupil center.
Keywords: Angle kappa, High Myopia, Ocular Biometry
Key Summary Points
| Why carry out this study? |
| High myopia presents altered ocular anatomy that increases the risk of suboptimal outcomes with premium intraocular lenses. Accurate assessment of angle kappa is essential because a large angle kappa distance (> 0.5 mm) may compromise premium intraocular lens performance. |
| Existing evidence on the relationship between axial length and angle kappa in high myopia is conflicting, creating uncertainty in preoperative planning. |
| The study investigated whether angle kappa varies systematically with axial length in patients with cataracts, particularly whether longer axial length predicts a larger angle kappa distance. |
| What was learned from the study? |
| High myopia, especially when the axial length exceeds 30 mm, predisposes patients with cataracts to a large angle kappa distance. The most prevalent location of angle kappa in high myopes is the temporal-superior quadrant relative to the pupil center. |
| Our study enhances the understanding of ocular anatomy and provides insights into refractive cataract surgery for high myopic eyes. |
Introduction
Angle kappa is defined as the angular deviation between the pupillary axis and the visual axis. For patients with cataracts, preoperative measurement of angle kappa can effectively predict the postoperative separation between the visual axis and the geometric center of the intraocular lens (IOL). In the context of multifocal (MF) and extended depth-of-focus (EDOF) IOL implantations, precise alignment of the geometric IOL center with the visual axis is essential to ensure optimal light energy distribution [1]. A large angle kappa distance (> 0.5 mm) [2, 3] could potentially disrupt the intended light-splitting ratio of MFIOLs, thereby increasing the likelihood of photic phenomena such as glare and halos [3–5]. Furthermore, a large angle kappa distance may reduce the depth-of-focus (DOF) in patients with EDOF IOLs, potentially compromising their clinical efficacy [6]. Additionally, IOL orientation, tilt, and decentration may be influenced by angle kappa distance and could contribute to certain visual disturbances [7]. Therefore, accurate preoperative measurement of angle kappa is crucial to optimize the performance of the premium IOLs, particularly in patients with altered ocular anatomy or those seeking premium refractive outcomes.
High myopia (HM), characterized by excessive ocular elongation (axial length [AL] ≥ 26 mm) [8], is associated with extensive alterations in ocular anatomy [9]. In patients with cataracts and HM, ocular biometric changes not only complicate the accurate calculation of IOL power [10] but also heighten the risk of visual quality impairment following the implantation of premium IOLs [11]. Therefore, achieving optimal visual outcomes with premium IOLs in patients with HM necessitates meticulous preoperative evaluation and personalized IOL selection strategies. As angle kappa is a critical factor influencing the performance of MFIOLs and EDOF IOLs [4–6], analyzing the distribution characteristics of angle kappa in HM eyes may provide valuable insights into improving visual outcomes following the implantation of premium IOLs.
However, despite the clinical significance of angle kappa in refractive cataract surgery, previous studies conducted on Chinese (mean angle kappa distance: 0.23 ± 0.10 mm) [12], Israeli (mean angle kappa distance: 0.35 ± 0.20 mm) [13], and Mexican populations (mean angle kappa distance: 0.40 ± 0.18 mm) [14] have primarily focused on age-related cataracts without HM. The distribution of angle kappa in patients with cataracts and HM remains unclear. Additionally, the relationship between angle kappa and AL in high myopes remains ambiguous. Gharieb Ibrahim et al. [15] reported a trend for the angle kappa distance decreasing with increasing AL. Conversely, Neuman et al. [13] demonstrated a positive correlation between the angle kappa distance and AL in HM eyes. Given the clinical implications of angle kappa for refractive cataract surgery, we conducted this study to characterize the features of angle kappa in a large-scale sample with long ALs. Our results may help clarify the anatomical changes associated with ocular elongation and improve the clinical application of premium IOLs in patients with cataracts and HM [16].
Methods
Study Design
This was a retrospective hospital-based cross-sectional study examining the preoperative data of Han Chinese patients with cataracts. The study received approval from the Institutional Ethics Committee of the Eye and ENT Hospital of Fudan University (Shanghai, China; ethical permit no. 2013021). The research adhered to the tenets of the Declaration of Helsinki and its subsequent revisions. Prior to surgery, all patients provided written informed consent for study participation, data collection, and the use of anonymized data for publication.
Patients
We categorized the eyes into the following groups on the basis of their ALs [8, 16]: medium eyes (22 mm ≤ AL < 24 mm), medium-long eyes (24 mm ≤ AL < 26 mm), long eyes (26 mm ≤ AL < 28 mm), super-long eyes (28 mm ≤ AL < 30 mm), and extreme-long eyes (AL ≥ 30 mm).
Then, we consecutively enrolled patients with cataracts into each AL group. The inclusion criteria were as follows: (1) adult Han Chinese patients scheduled for cataract surgery at the Eye and ENT Hospital of Fudan University (Shanghai, China), (2) patients who demonstrated cooperative behavior during preoperative examinations, and (3) patients with complete records of ocular biometry.
We excluded patients with (1) a history of contact lens wear, (2) a history of ocular surface or intraocular surgeries, and (3) ocular comorbidities that could affect ocular biometric measurements, including strabismus, congenital anomalies in ocular development, pterygium, corneal abnormalities, uveitis, lens luxation or subluxation, glaucoma, and vitreoretinopathy.
In this study, all participants had bilateral cataracts. For statistical analysis, one eye from each participant was randomly selected using a random number method (1 = right eye; 0 = left eye), resulting in the inclusion of either the cataractous eye scheduled for surgery or the contralateral eye in the analysis.
Ocular Measurements
An Oculus Pentacam HR (software version 1.25r15) was employed to determine angle kappa [17]. The Oculus Pentacam HR reports angle kappa in a coordinate-based format, in which the corneal apex is defined as the origin (0, 0), representing the visual axis. The pupil center is subsequently located within this coordinate system as coordinates (x, y), reflecting the angular distance between the line of sight and the pupillary axis. The nasal, temporal, superior, and inferior directions were recorded according to each patient’s true anatomical orientation and were defined relative to the individual eye after accounting for mirror symmetry between right and left eyes. This approach ensured consistent anatomical interpretation across both eyes.
The magnitude of angle kappa distance is automatically calculated by the Oculus Pentacam HR. According to a 2025 study by Chen et al. [3], careful consideration should be given to the preoperative angle kappa distance, and a threshold of ≤ 0.5 mm appears to be a safe criterion for MFIOL implantation to achieve satisfactory postoperative visual quality. Therefore, an angle kappa distance > 0.5 mm was defined as a large angle kappa distance in this study.
We also measured the central corneal thickness (CCT) for each patient using the Oculus Pentacam HR. Additionally, biometric measurements, including AL, keratometry (K), corneal astigmatism (CA), white-to-white (WTW), and anterior chamber depth (ACD), were obtained for each patient using a Zeiss IOLMaster 700 (software version 1.88).
During ocular biometry measurements, only successful measurements from the IOLMaster 700 and measurements graded as “OK” by the Pentacam HR were included in the analysis. The repeatability and reliability of IOLMaster 700 [18–23] and Pentacam HR [23–28] have been established in previous studies.
Statistical Analysis
First, we summarized the distribution of age, sex, angle kappa, AL, K, CA, CCT, WTW, and ACD across each AL group.
Second, we analyzed the intergroup differences in biometric parameters among the AL groups (Tables 1 and 2). Post hoc multiple comparisons with Bonferroni correction were subsequently performed to further examine the intergroup differences.
Table 1.
Biometric characteristics of each ocular axial length group
| Medium eyesa | Medium-long eyes | Long eyes | Super-long eyes | Extreme-long eyes | P-valueb | |
|---|---|---|---|---|---|---|
| Participant count | 136 | 218 | 925 | 690 | 870 | |
| Axial length (mm) | 23.1 ± 0.5 | 25.2 ± 0.6 | 27.0 ± 0.6 | 28.9 ± 0.6 | 31.8 ± 1.4 | < 0.001 |
| Corneal keratometry (diopter) | 44.3 ± 1.4 | 43.5 ± 1.6 | 43.2 ± 1.8 | 43.0 ± 2.1 | 43.1 ± 2.0 | < 0.001 |
| Corneal astigmatism (diopter) | 1.0 ± 0.7 | 1.0 ± 0.7 | 1.0 ± 0.7 | 1.1 ± 0.8 | 1.2 ± 0.8 | 0.001 |
| Central corneal thickness (μm) | 531 ± 31 | 539 ± 32 | 542 ± 35 | 539 ± 36 | 540 ± 36 | < 0.05 |
| White-to-White (mm) | 11.2 ± 0.4 | 11.4 ± 0.4 | 11.5 ± 0.4 | 11.4 ± 0.5 | 11.4 ± 0.4 | < 0.001 |
| Anterior chamber depth (mm) | 2.5 ± 0.4 | 2.8 ± 0.4 | 2.9 ± 0.5 | 3.0 ± 0.5 | 3.0 ± 0.5 | < 0.001 |
| Angle kappa value (mm) | 0.29 ± 0.34 | 0.22 ± 0.17 | 0.23 ± 0.18 | 0.26 ± 0.18 | 0.33 ± 0.24 | < 0.001 |
| Chord length > 0.5 mmc | 17 (13%) | 13 (6%) | 50 (5%) | 53 (8%) | 152 (17%) | < 0.001 |
| Quadrant location of angle kappad | < 0.001 | |||||
| Vertically superior | 2 (1%) | 2 (1%) | 13 (1%) | 4 (1%) | 9 (1%) | |
| Vertically inferior | 2 (1%) | 2 (1%) | 7 (1%) | 8 (1%) | 3 (1%) | |
| Horizontally temporal | 2 (1%) | 6 (3%) | 10 (1%) | 5 (1%) | 12 (1%) | |
| Horizontally nasal | 4 (3%) | 7 (3%) | 5 (1%) | 5 (1%) | 2 (1%) | |
| Temporal-superior | 12 (9%) | 52 (24%) | 293 (32%) | 289 (42%) | 421 (48%) | |
| Temporal-inferior | 9 (7%) | 38 (17%) | 235 (25%) | 181 (26%) | 219 (25%) | |
| Nasal-superior | 54 (40%) | 54 (25%) | 200 (22%) | 112 (16%) | 111 (13%) | |
| Nasal-inferior | 51 (38%) | 57 (26%) | 162 (17%) | 86 (12%) | 93 (10%) |
aEyes were categorized according to their axial lengths (AL) as follows: medium eyes: 22 mm ≤ AL < 24 mm; medium-long eyes: 24 mm ≤ AL < 26 mm; long eyes: 26 mm ≤ AL < 28 mm; super-long eyes: 28 mm ≤ AL < 30 mm; and extreme-long eyes: AL ≥ 30 mm
bP-values were calculated using one-way analysis of variance (ANOVA) for quantitative parameters and Pearson’s χ2 test or Fisher’s exact test for qualitative parameters
cAngle kappa values exceeding 0.5 mm are considered one of the contraindications for multifocal intraocular lens implantation during cataract surgery
dThe quadrant location of angle kappa refers to the position of the main corneal light reflex relative to the pupil center
Table 2.
Distribution of demographic and biometric parameters in high myopic and non-high myopic eyes
| Total group | High myopic groupa | Non-high myopic group | |
|---|---|---|---|
| Participant count | 2839 | 2485 | 354 |
| Age (years) | 62 ± 11 | 61 ± 10 | 65 ± 10 |
| Sex (female %) | 1633 (58%) | 1396 (56%) | 237 (67%) |
| Axial length (mm) | 28.6 ± 2.7 | 29.2 ± 2.3 | 24.4 ± 1.2 |
| Keratometry (diopter) | 43.2 ± 1.9 | 43.1 ± 2.0 | 43.8 ± 1.6 |
| Corneal astigmatism (diopter) | 1.1 ± 0.8 | 1.1 ± 0.8 | 1.0 ± 0.7 |
| Central corneal thickness (μm) | 540 ± 35 | 540 ± 35 | 536 ± 32 |
| White-to-white (mm) | 11.4 ± 0.4 | 11.4 ± 0.4 | 11.3 ± 0.4 |
| Anterior chamber depth (mm) | 2.9 ± 0.5 | 3.0 ± 0.5 | 2.7 ± 0.5 |
| Angle kappa value (mm) | 0.27 ± 0.21 | 0.27 ± 0.21 | 0.25 ± 0.25 |
| Chord length > 0.5 mmb | 285 (10%) | 255 (10%) | 30 (8%) |
| Quadrant location of angle kappac | |||
| Vertically superior | 30 (1%) | 26 (1%) | 4 (1%) |
| Vertically inferior | 22 (1%) | 18 (1%) | 4 (1%) |
| Horizontally temporal | 35 (1%) | 27 (1%) | 8 (2%) |
| Horizontally nasal | 23 (1%) | 12 (1%) | 11 (3%) |
| Temporal-superior | 1067 (37%) | 1003 (40%) | 64 (18%) |
| Temporal-inferior | 682 (24%) | 635 (25%) | 47 (13%) |
| Nasal-superior | 531 (19%) | 423 (17%) | 108 (31%) |
| Nasal-inferior | 449 (16%) | 341 (14%) | 108 (31%) |
aEyes were categorized according to their axial lengths (AL) as follows: non-high myopic eyes: 22 mm ≤ AL < 26 mm; and high myopic eyes: AL ≥ 26 mm
bAngle kappa values exceeding 0.5 mm are considered one of the contraindications for multifocal intraocular lens implantation during cataract surgery
cThe quadrant location of angle kappa refers to the position of the main cornea light reflex relative to the center of the pupil
Third, on the basis of Pearson’s correlation analysis, we employed a multivariable regression model to investigate the associations of angle kappa distance (Table 3). The covariates included in the multivariable model were age, sex, AL, K, CA, WTW, CCT, and ACD. Both linear and logistic regression analyses were conducted to investigate the determinants of angle kappa distance.
Table 3.
Associations of angle kappa value in high myopic and non-high myopic eyes
| Axial length groups | Pearson’s correlation coefficienta | Standardized regression coefficientb | Regression coefficient | 95% confidence interval |
|---|---|---|---|---|
| High myopic eyesc (n = 2485) | ||||
| Axial length (mm) | 0.24*** | 0.24*** | 0.02*** | 0.02, 0.03 |
| White-to-white (mm) | −0.06** | −0.08** | −0.04** | −0.06, −0.02 |
| Keratometry (diopter) | −0.04 | −0.08*** | −0.009*** | −0.013, −0.004 |
| Corneal astigmatism (diopter) | 0.07*** | 0.06** | 0.02** | 0.01, 0.03 |
| Age (years) | 0.02 | 0.06** | 0.001** | 0.001, 0.002 |
| Non-high myopic eyes (n = 354) | ||||
| Axial length (mm) | −0.15** | −0.27*** | −0.04*** | −0.06, −0.03 |
| Keratometry (diopter) | −0.11* | −0.25*** | −0.03*** | −0.05, −0.02 |
| Anterior chamber depth (mm) | 0.10 | 0.21*** | 0.09*** | 0.04, 0.14 |
| White-to-white (mm) | −0.07 | −0.20** | −0.09** | −0.15, −0.03 |
| Total group (n = 2839) | ||||
| Axial length (mm) | 0.19*** | 0.20*** | 0.02*** | 0.01, 0.02 |
| White-to-white (mm) | −0.05** | −0.09*** | −0.04*** | −0.06, −0.02 |
| Keratometry (diopter) | −0.05* | −0.08*** | −0.01*** | −0.02, −0.01 |
| Corneal astigmatism (diopter) | 0.07*** | 0.05** | 0.02** | 0.01, 0.03 |
| Age (years) | −0.01 | 0.04* | 0.01* | 0.01, 0.02 |
aPearson’s correlation coefficients were calculated to assess the associations of angle kappa distance (dependent variable) and the independent variables
bThe standardized regression coefficients were calculated using a multivariable linear regression model, with the angle kappa distance as the dependent variable and age, sex, axial length, anterior chamber depth, white-to-white, central corneal thickness, keratometry, and corneal astigmatism as independent variables. The independent variables are ranked in descending order according to the magnitude of their standardized regression coefficients with respect to the angle kappa distance
cHigh myopia was defined as an axial length exceeding 26 mm
***P < 0.001, ** P < 0.01; * P < 0.05
Fourth, we performed a diagnostic test to explore the diagnostic performance of AL in predicting the presence of a large angle kappa distance (> 0.5 mm). A receiver operating characteristic (ROC) curve was constructed. The cutoff value for AL predicting the presence of a large angle kappa distance was determined using the Youden index (YI) method.
Results
Study Population
A total of 2839 eyes were analyzed. The mean age of the participants was 62 ± 11 years; 58% were female (n = 1622), and 42% were male (n = 1206). There were 2485 patients with HM (AL ≥ 26 mm) and 354 patients without (AL < 26 mm). Among the 2485 high myopic eyes included in this study, 925 were classified as long eyes (26 mm ≤ AL < 28 mm), 690 as super-long eyes (28 mm ≤ AL < 30 mm), and 870 as extreme-long eyes (AL ≥ 30 mm). Table 1 presents the biometry data across different AL groups, and Table 2 presents the demographic and biometric characteristics of HM eyes (AL ≥ 26 mm) and non-HM eyes (AL < 26 mm).
Magnitude and Quadrantal Location of Angle Kappa
One-way ANOVA revealed significant differences among AL groups in the magnitude of angle kappa distances (P < 0.001). Eyes with extremely long ALs (AL ≥ 30 mm) exhibited significantly larger angle kappa distances compared with HM eyes with shorter ALs (26 mm ≤ AL < 30 mm) and non-HM eyes (AL < 26 mm) (all P < 0.05).
Pearson’s chi-squared test further revealed that extreme-long eyes (AL ≥ 30 mm) had a significantly higher proportion of large angle kappa distance (> 0.5 mm) than those with shorter ALs (all P < 0.05). Additionally, among HM eyes, super-long eyes (28 mm ≤ AL < 30 mm) demonstrated significantly greater angle kappa distances compared with long eyes (26 mm ≤ AL < 28 mm) (P < 0.05).
For the quadrantal distribution of angle kappa, Fisher’s exact test indicated significant differences associated with axial ocular dimension (P < 0.001). In HM eyes, the most frequent locations of angle kappa relative to the pupillary axis was in the temporal-superior (40%) and temporal-inferior (26%) quadrants. Conversely, in eyes with an AL shorter than 26 mm, the most frequent position of angle kappa relative to the pupillary axis was located in the nasal-superior (31%) and nasal-inferior (31%) quadrants.
Associations of Angle Kappa Distance
In HM eyes, a greater angle kappa distance was significantly correlated with longer AL (Pearson’s correlation coefficient [r] = 0.24), narrower WTW (r = −0.06), and larger CA (r = 0.07) (all P < 0.01). In non-HM eyes, a greater angle kappa distance was significantly associated with shorter AL (r = −0.15) and smaller CA (r = −0.11) (both P < 0.05). Neither age nor sex demonstrated significant correlations with angle kappa distance in either the HM group or the non-HM group (all P > 0.05).
A multivariable model was employed to investigate the associations of angle kappa distance in patients with and without HM (Table 3). In this model, the dependent variable was the angle kappa distance, while age, sex, AL, WTW, CCT, K, CA, and ACD served as independent variables.
For HM eyes, the multivariable model revealed that a greater angle kappa distance was significantly associated with longer AL (standardized regression coefficient [β] = 0.24), narrower WTW (β = −0.08), lower K (β = −0.08), higher CA (β = 0.06), and older age (β = 0.06) (all P < 0.01). In contrast, for patients without HM, factors significantly associated with a greater angle kappa distance included shorter AL (β = −0.27), lower K (β = −0.25), deeper ACD (β = 0.21), and narrower WTW (β = −0.20) (all P < 0.01). Figure 1 shows the correlations between angle kappa distance and biometric indices in HM eyes.
Fig. 1.
Scatter plots illustrating the correlations between the angle kappa distance and biometric parameters in patients with high myopia. β, standardized regression coefficient
Using multivariable logistic regression analysis, a larger angle kappa distance (> 0.5 mm) was significantly associated with longer AL (odds ratio [OR], 1.3; 95% confidence interval [CI], 1.2–1.4), lower K (OR, 0.9; 95% CI, 0.8–0.9), and higher CA (OR, 1.3; 95% CI, 1.2–1.6) in HM eyes (all P < 0.01). However, in non-HM eyes, a larger angle kappa distance was significantly associated with shorter AL (OR, 0.5; 95% CI, 0.3–0.8), narrower WTW (OR, 0.2; 95% CI, 0.1–0.6), deeper ACD (OR, 10.9; 95% CI, 3.9–30.8), and lower K (OR, 0.7; 95% CI, 0.5–0.9) (all P < 0.05).
Diagnostic Test for the Presence of Large Angle Kappa Distance
The association between AL and the presence of a large angle kappa distance (> 0.5 mm) indicates the diagnostic potential of AL in predicting large angle kappa distances. Figure 2 shows the performance of AL in predicting angle kappa distance in all 2839 cataractous eyes. The area under the curve (AUC) was 0.64 (95% CI, 0.60–0.68). The optimal cutoff point for AL, determined by the highest YI, was 30 mm. Using an AL threshold of ≥ 30 mm to predict the presence of a large angle kappa distance, the specificity and sensitivity were 73% and 53%, respectively.
Fig. 2.
Receiver operating characteristic curve demonstrating the diagnostic efficacy of axial length in identifying the presence of a large angle kappa distance (> 0.5 mm) in cataractous eyes
Discussion
In this survey, we found that an AL of ≥ 30 mm may serve as a potential risk factor for an angle kappa distance exceeding 0.5 mm. A larger angle kappa distance is associated with longer AL, narrower WTW, lower K, higher CA, and older age in patients with cataracts and HM. Although AL alone should not be considered an exclusion criterion for premium IOLs selection, it should be regarded as a risk indicator and interpreted in conjunction with direct measurements of angle kappa distance. Additionally, compared with non-HM eyes where a positive angle kappa (nasal corneal light reflex) is most prevalent, as AL increases, a negative angle kappa (temporal corneal light reflex) becomes more common in HM eyes.
In our study, the mean angle kappa distance in non-HM participants was 0.25 ± 0.25 mm, which is consistent with a previous study [12] also conducted on Chinese patients with cataracts (mean angle kappa distance = 0.23 ± 0.10 mm). However, according to existing literature, the angle kappa distances in Israeli patients (mean angle kappa distance = 0.35 ± 0.20 mm) [13] and Mexican patients (mean angle kappa distance = 0.40 ± 0.18 mm) [14] with age-related cataracts were greater than those in Han Chinese subjects [12]. Therefore, the variance attributable to ethnicity should be carefully considered in clinical practice when addressing angle kappa distances.
HM exhibits a high prevalence among the adult population in East Asia [29]. Individuals with HM often experience an earlier onset of cataracts [30]. The distinct anatomical structure and refractive characteristics of high myopic eyes present significant challenges for cataract surgery [9]. Previous studies have indicated that high myopes are at higher risk for myopic maculopathy [29], making them less suitable candidates for premium IOLs, such as MF or EDOF IOLs. However, our study demonstrated a significant association between ocular elongation and an increased angle kappa distance in high myopes. Therefore, the rationale for limiting the use of premium IOLs in patients with high myopia should extend beyond the presence of retinopathy to include the compromised visual quality caused by a large angle kappa distance. Consequently, preoperative evaluation of angle kappa is particularly critical when planning refractive cataract surgery for patients with cataracts and HM.
Additionally, according to a study by Tutchenko et al. [31], postoperative changes in the spatial relationship between the first Purkinje image and the pupil center may contribute to unexpected refractive astigmatic changes after phacoemulsification. In addition, eyes with a larger angle kappa distance in the early postoperative period were more likely to exhibit subsequent angle kappa relocation. This study highlights the importance of postoperative angle kappa in refractive outcomes following cataract surgery. However, the relationship between preoperative and postoperative angle kappa remains incompletely understood. Further investigation is therefore warranted to better elucidate the role of angle kappa in refractive cataract surgery.
The magnitude and quadrantal distributions of angle kappa in different AL groups are summarized in Table 1. The magnitude of the angle kappa distance exhibits a V-shaped trend as AL increases. Specifically, from the medium-eye group (AL: 22–24 mm) to the medium-long-eye group (AL: 24–26 mm), the angle kappa distance decreases from 0.29 ± 0.34 mm to 0.22 ± 0.17 mm. Subsequently, as AL increases from the medium-long-eye group to the extreme-long-eye group (AL: ≥ 30 mm), the angle kappa distance progressively increases and reaches its peak in eyes with an AL exceeding 30 mm. Furthermore, the V-shaped trend in the change of the angle kappa distance is accompanied with variations in the location of the corneal light reflex during angle kappa measurements. The predominant locations of the corneal light reflex in medium and extreme-long eyes were the nasal region (78%) and the temporal region (73%), respectively. Therefore, across the AL spectrum, ocular elongation is associated with the gradual shift in the location of angle kappa from the nasal to the temporal region.
The association between ocular dimension and variation in angle kappa distribution may reflect asymmetric globe deformation during myopia development (Fig. 3). With ocular elongation, the sclera at the temporal region may extend more than that on the nasal side, thereby causing the fovea to gradually shift from the temporal to the nasal side relative to the pupillary axis. This pattern not only leads to changes in the quadrantal distribution of angle kappa during ocular elongation but also results in a V-shaped change in angle kappa distance as AL increases. As AL increases, the fovea gradually shifts toward the pupillary axis (Fig. 3A), overlaps with it (Fig. 3B), and subsequently deviates to the opposite side (Fig. 3C). Concurrently, a V-shaped pattern in the change of angle kappa distance can be observed during ocular elongation.
Fig. 3.
Schematic diagram illustrating angle kappa in different axial length (AL) groups. A Medium-eye group (22 mm ≤ AL < 24 mm): the fovea is predominantly located on the temporal side of the pupillary axis (dotted black line), resulting in a positive angle kappa with the corneal light reflex positioned nasally. B Medium-long-eye group (24 mm ≤ AL < 26 mm): the visual axis (solid red line) exhibits optimal alignment with the pupillary axis, yielding the smallest angle kappa distance within this AL range. C High myopia group (AL ≥ 26 mm): angle kappa is predominantly situated on the nasal side of the pupillary axis, causing a negative angle kappa with the corneal light reflex appearing temporally
Previous studies [32, 33] have shown that the majority of retinal breaks are located on the temporal side. The pattern of globe deformation, as described above, may provide an explanation for this phenomenon: The sclera at the temporal side may undergo greater extension than at other locations during ocular elongation, leading to increased mechanical stretch on the retina and making the temporal retina a retinopathy-prone region.
Furthermore, in the present study, a significant positive correlation between corneal astigmatism and angle kappa distance was observed in the HM group, whereas no such association was found in non-HM eyes. This finding suggests that myopia-related ocular deformation [34] may simultaneously affect multiple anatomical structures of the eye. Progressive axial elongation [35] and globe remodeling in HM have been shown to influence not only the posterior segment [36–39] but also anterior segment geometry [9, 40, 41], potentially leading to concurrent changes in corneal shape and visual axis alignment. Therefore, in patients with cataracts and HM, marked corneal astigmatism may serve as a clinical indicator of a larger angle kappa distance. Recognition of this association may aid preoperative assessment and surgical planning, particularly in cases requiring precise optical centration.
Several limitations of this study should be acknowledged. First, the retrospective design may introduce selection bias and limits the ability to establish causal relationships. Second, as a single-center study, it may have limited generalizability to other populations or clinical settings. Third, angle kappa measurements were obtained using a specific device and definition, which may affect comparability with studies employing different measurement methods or reference axes. Therefore, future prospective multicenter studies are warranted to validate and extend these findings.
Conclusions
In this hospital-based cross-sectional study involving 2839 participants, a larger angle kappa distance was significantly associated with longer AL in high myopic eyes. An AL of ≥ 30 mm may be considered a potential risk factor for an angle kappa distance exceeding 0.5 mm. Among patients with cataracts and HM, the most common location of angle kappa is the temporal-superior quadrant relative to the pupillary axis. Our findings may provide insights into the anatomical changes in myopic eyes, assist in optimizing the refractive outcomes of modern cataract surgeries for high myopes, and offer guidance in selecting candidates for premium IOLs.
Acknowledgments
We extend our sincere appreciation to Wenbo Nie for his invaluable assistance throughout the research process. We also thank the participants of the study.
Author Contributions
Ao Miao: conception and design of the work; acquisition, analysis, and interpretation of data; creation of new software used in the work; and drafting of the study; Dongjin Qian: analysis and interpretation of data; creation of new software used in the work; and drafting of the study; Jiajia Huang: acquisition and interpretation of data; creation of new software used in the study; and drafting of the work; Tianhui Chen: acquisition and interpretation of data and creation of new software used in the work; Junjie Xie: analysis and interpretation of data and creation of new software used in the work; Likun Lin: analysis and interpretation of data and creation of new software used in the study; Jie Xu: conception of the study; analysis and interpretation of data and creation of new software used in the work; and substantive revision of the work; Yongxiang Jiang: design of the study; interpretation of data; and substantive revision of the work. All the authors have read and approved the final manuscript.
Funding
This work was supported by the National Natural Science Foundation of China (grant nos. 82101103 and 82571187) and open research fund of Shanghai Key Laboratory of Gene Editing and Cell Therapy for Rare Diseases (grant no. gect-2025-Q13). The Rapid Service Fee was paid for by the authors.
Data Availability
The datasets used and/or analyzed during the current study are available from the corresponding author upon reasonable request.
Declarations
Conflict of Interest
Ao Miao, Dongjin Qian, Jiajia Huang, Tianhui Chen, Junjie Xie, Likun Lin, Jie Xu, and Yongxiang Jiang have nothing to disclose.
Ethical Approval
The study received approval from the Institutional Ethics Committee of the Eye and ENT Hospital of Fudan University (Shanghai, China; ethical permit no. 2013021). The research adhered to the tenets of the Declaration of Helsinki and its subsequent revisions. Prior to surgery, all patients provided written informed consent for study participation, data collection, and the use of anonymized data for publication.
Footnotes
Jie Xu and Yongxiang Jiang are joint corresponding authors.
Prior Presentation: This study was presented at the 29th Congress of the Chinese Ophthalmological Society, held in Hangzhou, Zhejiang Province, China, from 4 September to 7 September 2025.
Ao Miao, Dongjin Qian, and Jiajia Huang contributed equally as co-first authors.
Contributor Information
Jie Xu, Email: 16111260008@fudan.edu.cn.
Yongxiang Jiang, Email: yongxiang_jiang@163.com.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Data Availability Statement
The datasets used and/or analyzed during the current study are available from the corresponding author upon reasonable request.