With continuous improvements of ocular biometric measurements and innovation of cataract surgery technology, good outcome of a cataract surgery not merely implies good visual acuity, but also good vision quality. There were studies that reported postoperative visual complaints can be attributed to an elevated level of HOAs, especially, SA and coma5,6. The increased SA has been associated with glare and halos, coma has been correlated with double vision, and trefoil has been associated with starburst.7 In general, all Zernike coefficients are larger for corneal aberrations than for total wavefront aberrations, suggesting a compensatory effect from intraocular structures. For cataract patients, the lens is going to be replaced by an IOL. Thus, a new aberration balance between cornea and IOL plays a vital role in vision quality. Hence, a comprehensive knowledge of the distribution of corneal HOAs is meaningful for cataract surgery design and helpful for postoperative vision quality estimation. This work aimed to provide a comprehensive analysis of several types of corneal HOAs and their relationships with age and other ocular biometric parameters in senior Chinese patients with cataract.
1. Distribution of total HOA and SA
The mean RMS value of total corneal HOAs in this study, measured at an optical zone of 6 mm, was 0.65µm, consistent with the values reported in other studies of Chinese patients.8,9 At central 4 mm corneal zone, the mean RMS value of total corneal HOA was 0.20 µm, significantly lower that at central 6mm. And among them, 88.63% of patients had total HOA < 0.3µm, and 97% <0.5µm. Previous studies show that increased HOAs may deteriorate the visual outcomes of multifocal intraocular lens (MIOL).10,11 Therefore, we found that the majority of cataract patients are eligible for MIOL in HOAs, while there are still a fraction of people who are inappropriate for MIOL due to HOAs.
SA is a type of HOAs and is caused by a difference in focus between central rays and peripheral rays that reach the retina at the same time. In clinical practice, cataract surgeons may prefer aspheric IOLs to improve contrast sensitivity and visual quality.6,12−15 In our study, the mean RMS value of corneal SA was 0.30µm at central 6mm optic zone at the mean age of 63.52 ± 10.94 years, which was close to the result reported by Lai et al. in a Taiwanese population (0.307µm).16 Furthermore, our result was higher than that the Japanese populations17 (0.23µm), but lower than that in the Italian18 (0.328µm) population. According to another study based on Chinese population in Beijing,19 the mean corneal SA was 0.413 ± 0.161µm. Ruling out the race factor, we infer that the mean age of Beijing19 population (73 years) was older than our study (63 years). Besides, the different devices used to determine corneal aberrations may also account for this difference.
Previous studies have suggested that lowering postoperative SA can achieve an improvement in visual quality and contrast sensitivity.20,21 The mean corneal SA for our study was 0.30µm. Therefore, most commonly used aspheric IOL with SA of -0.27µm can better compensate for corneal SA and achieve moderate residual positive SA, which results in better contrast sensitivity and ideal visual quality.
2. Relationship between corneal HOAs and age
We investigated the relationship between age and total HOA, coma aberration, trefoil aberration, and SA. We found statistically differences in coma aberration, trefoil aberration and total HOA among different age groups, whereas no difference in SA. Wei, S. et al.9 reported similar findings, showing that coma and total HOA correlate with age, while SA does not. Fujikado et al22 reported that ocular total HOA, including ocular SA, increased significantly after 50 years, but there was no correlation between age and corneal SA. Lyall et al23 also found that ocular SA increase with age, while corneal SA does not, indicating that the increase of ocular SA mainly results from the increase of internal optical aberrations. These results suggest that total corneal HOA, coma, and trefoil increase with age, possibly induced by age-related changes of the anterior surface of cornea. Because coma aberration consists of tilt and asymmetry24, these results imply that corneas become less symmetric with age. On the contrary, SA consists of a central-to-peripheral balance of the corneal curvature.25 SA did not show any age-related changes, indicating that the central-to-peripheral balance of the corneal curvature is not significantly affected by age. In addition, the increase in total HOA with aging may be a factor that leads to a deterioration in their visual quality, which can help to explain why many early cataract patients often complain obvious discomfort like photophobia, glare, poor night vision, although still maintained a good daily vision.
In order to further investigate the association between age and corneal HOAs, restricted cubic splines was used to flexibly model and visualize the non-linear relation of total HOA and age. And a ‘U’ shape was observed: total HOA decreased with age until 60 years and then started to increase aferwards. Interestingly, another study investigated the relationship of corneal astigmatism and age among Chinese senile cataract population older than 50, and also found a ‘U-like’ shape between age and corneal astigmatism: the turning point was at 65 years old. This indicated both corneal low-order and higher-order aberrations change with age, not in a linear correlation, but in a ‘U’ shaped non-linear correlation.
To futher investige the potential correlation between low-order aberrations (like astigmatism) and HOAs, we made a subgroup analysis according to astigmatism. We observed that total HOA, coma and trefoil increase with astigmatism. In a study conducted by Chenge et al. 26 astigmatic eyes tend to have total HOA larger than non-astigmatism eyes. This suggests that patients with higher astigmatism tend to have higher level of HOAs. Hence, a comprehensive corneal aberration examination was recommended for high astigmatism patients, which can help surgeons to select appropriate IOL type and make a preoperative visual quality assessment.
3. Relationship between corneal HOAs and ocular biometric parameters
In addition to age, the relationship between HOAs and other factors have also been explored. Philip et al27 studied the total HOA and corneal topography of myopic, emmetropic, and hyperopic eyes of 675 adolescents and found no difference for anterior corneal SA among different refractive status. However, Llorente et al28 found that the average corneal SA was significantly higher in the hyperopic group than in the myopic one.
Accommodation can influence the corneal curvature, and especially, change corneal HOAs.29,30 Mohamed et al31 observed that neither CCT nor peripheral corneal thickness (PCT) was significantly associated with corneal aberrations. Besides, Arba Mosquera and de Ortueta32 demonstrated that corneal SA correlate well with Q values.
In the current study, we found that SA, coma aberration and total HOA differs among different AL groups. The emmetropic eyes had the lowest level of aberration values, while hyperopes had the highest level of SA. In this regarding, there are contrasting results. Shimozono et al17 found a significantly negative correlation between AL and corneal SA (r=-0.135). Kirwan et al33 demonstrated that third-order aberrations were higher in myopia33. Llorente et al28 reported that third-order aberrations (coma and trefoil) and SA were higher in young hyperopic eyes than myopic eyes, while intraocular SA was not significantly different between these two groups. For hyperopic eyes, anterior segment is relatively narrow, and thus lead to corneal deformation, which may be related with the increase of HOAs. Furthermore, total RMS coma constituted a larger proportion of HOAs in hyperopes. It is known that angle kappa is larger in hyperopes compared to myopes and emmetropes, and a larger displacement of the pupillary axis from the visual axis is responsible for higher levels of coma among hyperopes. Remarkably, although we did observe differences among different AL groups, AL was not stronger enough to predict corneal HOAs in a multiple regression model.
In the multiple linear regression model of corneal HOAs, MK and CCT play the most important role in predicting corneal HOAs, which are negatively correlated with total HOA, SA and coma. Nonetheless, the accuracy for HOAs prediction based on ocular biometric parameters is not good enough. Due to the various influencing factors and individual difference, it is hard to establish an ideal prediction model under the current conditions based on a small sample size. In addition, lack of equal pupil size in the measured eyes, different age distributions, various measuring devices in each study necessitates comparisons between studies be made with caution. Hence, more efforts need to be attributed to standardize the measurement method and a personalized measurement of wavefront aberration is required at present.
Limitations of this study include the fact that we studied only a limited sample size with all the subjects enrolled at the same study center. In addition, although human parameter measurements typically exhibit a normal distribution, we obtained non normal distribution for the ophthalmic parameter values assessed. A possible explanation might be the age of patients, who were elderly (63 ± 10.94 years).
In summary, approximately 90% of total aberrations are caused by the cornea;17 therefore, corneal wavefront analysis is an important tool for evaluating vision quality. The mean value of total corneal HOA and SA at an optical zone of 6 mm was 0.60µm and 0.30µm, respectively. With increasing age, the value of total HOA decreased first and started increasing at 60 years. Total HOA, coma and trefoil aberration increased with astigmatism. Among different AL groups, emmetropes and the lowest level of total HOA, SA and coma aberration, while hyperopes had the highest values. HOAs prediction based on ocular biometric parameters is not accurate and personalized measurements of wavefront aberration are required.