Distribution of corneal and ocular spherical aberrations in eyes with cataract in the Taiwanese population.

PURPOSE: To investigate the distribution of corneal and ocular spherical aberrations (SAs) in eyes with cataract in the Taiwanese population.
METHODS: Corneal and ocular SAs were measured in the central 6-mm optical zone using wavefront aberrometry. Axial length (AL) and keratometry (K) were also evaluated in each eye.
RESULTS: A total of 413 eyes in 234 patients were analyzed. The mean age of the patients was 66.8 ± 10.64 years. The mean AL and K values were 24.32 mm and 44.08 D, respectively. The mean corneal SA was 0.307 ± 0.135 μm and ocular SA was -0.042 ± 0.487 μm. Ocular and corneal SAs were significantly correlated (r2 = 0.04, p < 0.001). Corneal and ocular SAs were not significantly correlated with K (p = 0.096 and p = 0.634, respectively), but were significantly correlated with AL (p < 0.001). Multilinear regression showed that corneal SAs and age were the dependent variables that predicted ocular SAs (r2 = 0.143, F = 13.65, p < 0.01), especially in patients who were aged > 50 years, for whom a strongly significant positive correlation was found (r2 = 0.102, F = 11.10, p < 0.001).
CONCLUSION: Corneal and ocular SAs varied among cataract patients and correlated with AL. After 50 years of age, ocular SAs increased significantly because of an increase in internal (lenticular) SAs. Corneal SAs in Taiwanese patients were larger than those in Japanese patients and similar to those in Chinese and Malaysian populations. Preoperative measurement of wavefront aberrations is necessary to select which aspherical intraocular lenses are most suitable for achieving better postoperative visual quality.

1. Introduction

Optical aberrations of the eye can be divided into lower-order aberrations (LOAs) and higher-order aberrations (HOAs). Spherical aberration (SA) is a type of HOA and is caused by a difference in focus between central rays and peripheral rays that reach the retina at the same time. HOAs may interfere with visual quality by resulting in decreased contrast sensitivity, glare, and halos. Currently, the availability of wavefront-sensing devices in ophthalmic clinics has permitted a greater understanding of the impact of aberrations on vision. Advancements in cataract surgery and intraocular lens (IOL) design have made possible the customization of target refraction by appropriate IOL power selection. Moreover, aspherical IOLs have been designed to eliminate corneal SAs with substantial levels of success beyond visual acuity.12

The design of IOLs to reduce or eliminate SAs in pseudophakic eyes is based on compensating for the resulting corneal SA after removal of the crystalline lens.

Salmon and van de Pol3 established the population norms of ocular aberrations in a large cohort of 2560 eyes. However, only 134 eyes were from an Asian population (Japan). Shimozono et al4 reported the corneal SA values of a Japanese group, and Lim and Fam5 reported the distribution of SA values in a Singaporean-Malaysian population. There was a large difference in corneal SA values between these two ethnic groups. Furthermore, age may play a role in ocular HOAs. Fujikado et al6 reported that ocular HOAs increase significantly after the age of 50 years, but there was no correlation between age and corneal SAs. The aim of our study was to investigate corneal and ocular SAs (Zernike coefficient, Z04)in a Taiwanese population and to compare them with the findings of previously published studies.

2. Materials and methods

This retrospective study was comprised of patients with a diagnosis of senile or presenile cataract in Mackay Memorial Hospital from September 2011 to August 2012. All patients were Taiwanese, and they had cataracts that caused visual impairment with a best corrected visual acuity of <20/50. Patients with a history of corneal pathology or ocular surgery, but not of eyelid surgery, were excluded. Axial length (AL) and keratometry (K) were measured under regular room light conditions using partial coherence interferometry (IOL Master; Carl Zeiss Meditec AG, Jena, Germany), and all signal-to-noise ratios were ≥3. Eyes with previous ocular surgery and dense cataract that could not be measured using the IOL Master were also excluded.

Corneal and ocular SAs (Z04) were obtained using the Wavefront Aberrometer (NIDEK OPD-Scan II ARK-10000; Gamagori, Aichi, Japan). This scanner is a multifunction instrument that integrates Placido-based corneal topography with wavefront aberrometry of the entire eye. The wavefront measuring apparatus is based on the principle of retinoscopy, which uses an infrared slit of light to scan all 360° meridians over a 6-mm pupil. The time difference of the reflected light to stimulate an array of photodetectors is translated to a refractive wavefront map.7 Measurements were taken at the central 6-mm optic zone without mydriasis for all patients included in this study. The examination was performed by a single experienced technician.

The aberrations were tabulated in Excel 2007 (Microsoft, Inc., Redmond, WA, USA), and the analysis was performed using Graph-Pad Instat software (GraphPad Software, Inc., La Jolla, CA, USA). All parameters were normally distributed (passing the Kolmogorov–Smirnov test of normality); thus, linear regression and Pearson correlation coefficients (r) were used to compare groups and values. A multilinear regression model, consisting of ocular parameters associated with ocular SA, was tested. These parameters were age, K, corneal SA, and AL. A p value < 0.01 was considered statistically significant.

3. Results

We analyzed 413 eyes (OD eyes: 207; OS eyes: 206) in 234 patients (98 men; 136 women) with a diagnosis of cataract from September 2011 to August 2012. The mean [± standard deviation (SD)] age of the patients was 66.80 ± 10.64 years (range, 38–97 years). The mean AL was 24.32 mm [95% confidence interval (CI), 24.136–24.512 mm], and the mean K was 44.08 D (Table 1).

Table 1

Statistical results of 413 eyes in 234 patients.

	Mean ± SD	Range
Age (y)	66.80 ± 10.64	38.00 – 97.00
Axial length (mm)	24.32 ± 1.94	21.06 – 32.67
Keratometry (D)	44.08 ± 1.51	40.30 – 48.32
Corneal SA (μm)	0.307 ± 0.14	–0.200 – 0.840
Ocular SA (μm)	–0.042 ± 0.49	–3.100 – 2.180

SA = spherical aberration; SD = standard deviation.

The mean corneal and ocular SAs were 0.307 ± 0.135 μm (range, −0.200–0.840 μm) and −0.042 ± 0.487 μm (range, −3.100–2.180 μm), respectively. The distribution of corneal SAs is shown in Fig. 1. Three eyes had negative corneal SAs, from −0.2 μm to −0.11 μm; all of the patients denied trauma or previous ocular surgeries, and no corneal deformities were found at slit lamp examination. The relationship between ocular and corneal SAs was statistically significant (r2 = 0.04, p < 0.001). Corneal and ocular SAs were not significantly correlated with K (p = 0.096 and p = 0.634, respectively), but were significantly correlated with AL (p < 0.001).

Fig. 1

Distribution of corneal spherical aberrations (Z04) was compatible with normality distribution, despite three negative values. The mean Z04 value of 0.307 ± 0.135 μm is represented by the thick solid line.

Although corneal SAs were not significantly correlated with age (p = 0.895), ocular SAs were strongly correlated with age (p < 0.001). Linear regression analysis in patients aged > 50 years showed a strong correlation between ocular SAs and age (r = 0.256, p < 0.001); such a correlation was not observed between corneal SAs and age (r = 0.049, p = 0.338; Fig. 2). Corneal and ocular SAs were not significantly correlated with age in patients aged < 50 years (r =-0.458, p = 0.032). Fig. 3 shows the mean ocular and corneal SAs in the different age groups: 30–39 years (n = 3), 40–49 years (n = 19), 50–59 years (n = 81), 60–69 years (n = 142), 70–79 years (n = 117), and ≥80 years (n = 51).

Fig. 2

The relationship between age and spherical aberrations (SAs) in patients aged >50 years. Solid line: linear regression between age and corneal SAs (r = 0.049, p = 0.338); dotted line: linear regression between age and ocular SAs (r = 0.256, p < 0.001).

Fig. 3

Mean ocular and corneal spherical aberrations (SAs) in the different age groups: 30–39 years, 40–49 years, 50–59 years, 60–69 years, 70–79 years, and ≥ 80 years. There was a significant intergroup difference in ocular SAs (ANOVA: F = 12.131, p < 0.001) but not in corneal SAs (ANOVA: F = 1.866, p = 0.099). ANOVA = analysis of variance.

Multilinear regression analysis showed that corneal SAs and age were significant correlates for ocular SAs (r2 = 0.143, F = 13.65, p < 0.01; Table 2), especially in the patients who were aged > 50 years (r2 = 0.102, F = 11.10, p < 0.001).

Table 2

Linear model of ocular spherical aberrations and associated factors.

Variable	Regression coefficient	95% CI	p
Corneal SA (μm)	0.508	0.174 – 0.841	0.003
Axial length (mm)	–0.030	–0.058 to –0.004	0.024
Average K (D)	0.015	–0.016 – 0.046	0.348
Age (y)	0.010	0.001 – 0.016	<0.001

CI = confidence interval; K = keratometry; SA = spherical aberration.

4. Discussion

Cataract surgery is one of the most common surgeries in the world, and the goal of cataract surgery is to restore youthful vision. Improvements in surgical techniques and IOL designs have resulted in the possibility of both better visual acuity and better visual quality. SA plays an important role in HOAs, and it is associated with visual quality and performance. Total ocular SAs include corneal SAs and intraocular SAs, and most intraocular SAs are derived from lens factors. A positive corneal SA is commonly seen in the general population, with lower ocular SAs in younger age groups due to compensation by negative intraocular SAs.

We reported that the mean (± SD) corneal SA of the Taiwanese population was 0.307 ± 0.135 μm (95% CI, 0.294–0.320 μm), which was close to the result reported by Lim and Fam5 among a Chinese population in Malaysia (0.312 ± 0.114 μm). Furthermore, our result was higher than that for the Japanese4 (0.203 ± 0.1 μm) and Canadian8 (0.27 μm) populations, but lower than that in the Italian study9 (0.328 ± 0.132 μm). Our result was similar to that found in a Malaysian population (0.307 μm vs. 0.312 μm) but was lower than that found in a Chinese population in Beijing10 (0.413 ± 0.161 μm). The Taiwanese population is considered to be of the same race as the Chinese population; however, we observed a large difference in corneal SAs between our study and the Beijing study. It is unclear whether there is a true difference in corneal SAs among different ethnic groups. Many different subethnic groups exist in mainland China, Taiwan, and Malaysia; therefore, miscegenation among ethnic groups must be considered. Specific subethnic groups are not mentioned in current literature, and most studies comprise of patients by nation or residence. Population selection by ethnicity, even subethnicity, and not by nation is necessary to determine whether ethnic variation significantly affects corneal SAs. In addition, the mean age of the Malaysian5 population (31.44 years) was less, as the patients were recruited from the refractive surgery clinics. Our study and the Beijing10 study recruited older patients with a diagnosis of cataract.

According to Fujikado et al6 ocular total HOAs, including ocular SAs, increased significantly after 50 years of age, but there was no correlation between age and corneal SAs. Amano et al11 and Lyall et al12 also reported that ocular SAs (but not corneal SAs) increase with age, mainly because of internal optical aberrations.

We showed the same relationship between aging and SAs in the current study, evident by the linear regression line shown in Fig. 2 (between age and ocular SA: r = 0.256, p < 0.001) However, Yuan and Bao10 reported a positive correlation between corneal SAs and age in the Beijing study; the correlation between total ocular SAs and age was not mentioned. The increase in ocular SAs with age is believed to be due to an increase in lenticular SAs,61112 but it could not be proven because there was no equipment available to measure lenticular SAs. Aging may decrease corneal endothelial cell number, however, it does not usually change cornea size or curvature. Compared with the cornea, lens aberrations (anterior and posterior lens radius and curvature, lens thickness, and refractive index) are more significantly associated with aging.13 Thus, age-associated corneal changes have less of an effect on ocular SAs.

In the current study, we found that corneal SAs correlated with AL and that K were not the dependent variable that predicted corneal SAs. Shimozono et al4 found a significantly negative correlation between AL and corneal SAs (r = −0.135). Beiko et al8 reported a positive correlation between K and corneal SAs (r = 0.241–0.295), but K readings were not a predictive variable for corneal SAs. Although a significant correlation was found between corneal SAs and AL, it was not strong enough to predict corneal SAs. There is wide individual variation in corneal SAs, and accurate preoperative measurement of corneal SAs for IOL selection is still needed. Previous studies have suggested that lowering postoperative ocular SAs can improve visual quality, especially contrast sensitivity.14 The mean (± SD) corneal SA for the Taiwanese group was 0.307 ± 0.135 μm; therefore, aspherical IOLs with a negative SA design–such as Tecnis ZCB00 (Abbott Medical Optics, Santa Ana, CA, USA) and Alcon SN60WF (Alcon Laboratories Inc., Fort Worth, TX, USA)–may better compensate for positive corneal SAs. Optimal and complete customization is difficult to achieve, therefore we select the asphericity of the IOL that is most appropriate for each patient.

In our study, corneal and ocular SAs varied among patients with cataract and correlated with AL. Ocular SAs increased significantly after 50 years of age, mainly due to increases in internal (lenticular) HOAs. Corneal SAs were larger in the Taiwanese population than in the Japanese population, but were similar to those of the Chinese and Malaysian populations. For wide variance in corneal SAs, pre-operative measurement of wavefront aberrations using corneal topography is necessary to select which aspherical IOLs are most suitable for individual patients.