Ⅰ. Introduction
According to the survey from United Nations in 2017, the population over the age of 50 is expected to account for 32.5% of the total population in 2050.1) The rapid aging of the population is producing various social problems, and the demand for visual acuity suitable for various lifestyles is also increasing. In particular, as the presbyopia population is getting younger and more feminine, it is necessary to improve the quality to meet the various demands for vision improvement.2) The symptoms of presbyopia, which make it difficult to see close-up, are caused by a decrease in the accommodative ability of the lens and are a natural aging process that occurs around the age of 40. One of the common ways to solve the problem of presbyopia is to wear multifocal contact lens (MFCL), and in their 40s, they often prefer contact lenses to glasses for an active lifestyle.
The MFCL is a aspheric design and the MFCL addition power is determined by the asphericity of the lens. The center-near MFCL have negative spherical aberration (SA) that the central area of the contact lens have maximum positive power for near vision and the power gradually decrease to the periphery of the optic zone for distance vision, resulting in a monotonic refractive power profile.3-7) On the contrary, the center-distance MFCL have minimum positive power for distance vision and gradually increase power to the periphery of the optic zone for near vision. Unlike progressive additional spectacle lenses, aspheric MFCL design allows simultaneous viewing without moving the eyes depending on the gaze distance. In the simultaneous vision, images through the far and near parts of the contact lens are formed on the retina simultaneously by adopting a clear image from the cerebral cortex to see clearly and suppressing the blurred image. However, the contrast sensitivity is lowered because clear and blur images are formed on retina simultaneously. In the eyes wearing MFCL, the focused and defocused retinal images are arrayed and transferred to the visual cortex, which can lead to decreased quality of vision,8,9) and decreased contrast sensitivity and ghosting.10)
The center-near MFCL is a aspheric design and have negative SA that it is reported the HOAs are increased after wearing MFCL, and induced HOAs have an influence on generating ocular aberrations.11) HOAs are expressed as a Zernike polynomial and closely related to the quality of visual acuity, such as contrast visual acuity and contrast sensitivity.12,13) Among the HOAs, SA and coma aberration affect the quality of vision. The SA is a phenomenon in which light passing through the periphery of the cornea is focused at a shorter distance than light passing through the center, the amount of SA is determined by the height of the incident ray parallel to the central optical axis. Coma aberration is known as a spreading phenomenon caused by rays incident obliquely to the optical axis at an arbitrary angle.
In order to control the HOAs of the eyeball, Dietze and Cox14) proposed a method of controlling the ocular SA with modified design of contact lens such as a aspheric soft contact lens (ASCL) design. It was reported in the eyes wearing ASCL and MFCL, SA can be reduced.15) In addition it has been shown that MFCL decentration on the cornea will cause changes in coma16) which, if increased much could further degrade image quality.17) Martin and Roorda18) have shown that visual quality with MFCL can be predicted based on CL-induced ocular aberrations. Also, the quality of visual acuity may decrease if HOA increases for any of the lens types, and especially in the MFCL the visual compromises increase with the add power.19,20)
However, little has been published on the associations between MFCL-induced aberrations and ocular aberration. In this paper, we investigate to analyze the effect of aspheric MFCL on ocular HOAs (total HOA, SA and coma) after wearing low and high add MFCL in young myopes.
Ⅱ. Subject and Method
1. Subject
To this end, this study screened and inspected 33 young adult (male: 12, female: 21) subjects in average age of 23.03±2.89 years old excluding those who have no ophthalmic disease and a refractive surgery before as well as those who have less than 0.8 in best corrected monocular acuity and 1.0 in best corrected binocular acuity. The average equivalent spherical refractive power of myopic eyes was –3.28±1.67 D, and the pupil size was 6.94±0.61 mm in scotopic condition (0 lx) and 4.94±0.68 mm in photopic condition (20 lx) (Table 1). Illuminance was expressed as the amount of light when the light source emitted from the pupillometer enters the eye when measuring the pupil, and scotopic and photopic condition were expressed as the luminance level provided by the company. All testing procedures and protocols have been approved by the Institutional Review Board (IRB, approval number: EU17-43), subjects participating in the study were informed about the purpose of the experiment and the measuring method by oral and written explanation and obtained the agreement.
2. Measurements
1) Subjective refraction
The refractive error was measured with Wavefront Analyzer (KR-1W, Topcon, Japan) and Phoropter (VT-SE, Topcon, Japan).
2) Contact lenses
In the experiments that follow, single vision aspheric contact lenses (Biotrue, Bausch + Lomb, USA) and two types of MFCL (low add and high add) (Biotrue, Bausch + Lomb, USA) were used. The study contact lenses are outlined in Table 2.
3) Contact lenses fitting
The centration and movement of the contact lens can affect the HOA that the fitting evaluation was performed at 20 min after contact lens wearing. Fitting evaluation was confirmed through dynamic lens movement, centration, lag and push-up tests.
4) Pupil size
Pupil size was measured three times using Wavefront Analyzer (KR-1W, Topcon, Japan) at a pupil size of 4 mm under scotopic (0 lx) and photopic condition (20 lx) adjusted by the instrument in the examination room with 100 lx illuminance condition after adaptation.
5) HOA measurement
HOAs were measured in the scotopic (0 lx) and photopic condition (20 lx) of the instrument (adjusted state) using Hartmann Shark type (KR-1W, Topcon, Japan) aberrations analyzer. The ocular total HOA were used as root mean square (RMS) and the coma, SA were expressed as a zernike polynomial.
6) Data analysis
Statistical analysis was performed using SPSS version 21.0 (SPSS Inc., Chicago, IL, USA). Average ocular aberrations were compared using the ANOVA test. Bonferroni was used for post-hoc analysis. The empirical analysis of this study was validated at significance level p<0.050.
Ⅲ. Results
1. Total HOAs
Ocular total HOA before refractive correction was 0.108±0.038 ㎛ in the scotopic condition, and that of single vision contact lens (SVCL), the low and high add MFCL wearing were 0.118±0.030 ㎛, 0.130±0.038 ㎛, and 0.164±0.048 ㎛, respectively, Post-hoc analysis revealed total HOA was not statistically different in naked eye and SVCL wearing however, that was significantly different in naked eye and MFCL wearing eyes (p<0.001). Ocular total HOA before refractive correction was 0.104±0.034 ㎛ in the photopic condition and the SVCL, low and high add MFCL wearing were 0.114±0.036, 0.125±0.036, 0.161±0.039 ㎛, respectively. There was no significant difference between naked eye and SVCL wearing (p=0.733), however, significant differences in naked eye and MFCL wearing eyes (p<0.001). The ocular total HOAs in the photopic illuminance in naked eye, SVCL and MFCL (low and high add) wearing eyes were not different compared with those in scotopic condition (p=0.330, p=0.411, p=0.179, p=0.495, respectively) (Table 3).
2. Coma aberration
Ocular coma aberration before refractive correction was 0.045±0.019 ㎛ in the scotopic condition, and that of SVCL, low and high add MFCL wearing were 0.059±0.026 ㎛, 0.072±0.033 ㎛, and 0.099±0.036 ㎛, respectively, Post-hoc analysis revealed that there were significant increments in SVCL, low and high MFCL wearing eyes (p<0.001). The coma aberration before refractive correction in the photopic condition was 0.550±0.022 ㎛ and that of SVC low and high add MFCL wearing were 0.073±0.041 ㎛, 0.082±0.037 ㎛, and 0.108±0.040 ㎛, respectively and there were significant increments in SVCL, low and high MFCL wearing eyes (p<0.001). In the photopic illuminance, ocular coma aberrations decreased significantly in naked eye, SVCL and MFCL (low and high add) wearing eyes than those in the scotopic condition (p=0.002, p=0.001, p=0.001, p=0.045, respectively) (Table 4).
3. Spherical aberration
Ocular SA before refractive correction was 0.022±0.029 ㎛ in the scotopic condition, and that of SVCL, low and high add MFCL wearing were -0.012±0.037 ㎛, -0.012±0.037 ㎛, and – 0.018±0.045 ㎛, respectively and changes to the negative direction (p<0.001). In the MFCL (low and high add) wearing eyes, SA increased into more negative direction than SVCL wearing eyes (p<0.001). In the photopic illuminance, ocular SAs decreased significantly in naked eye, SVCL and MFCL (low and high add) wearing eyes than those in the scotopic condition (p<0.001, p=0.005, p=0.014, p=0.050, respectively) (Table 6).
The direction of SA in scotopic condition, 54 eyes (81.8%) have positive SA (0.032±0.020 ㎛) in the naked eye, and the 12 eyes (18.1%) with negative SA (-0.020±0.013 ㎛) in the naked eye (Fig. 1). The eyes have positive SA, the positive SA was compensated by the MFCL having negative SA and were changed to negative values (-0.003±0.015 ㎛, -0.012±0.037 ㎛) after wearing MFCL (low and high add), respectively. The eyes with negative SA showed more negative values (-0.050±0.019 ㎛, -0.063±0.014 ㎛) after wearing MFCL (low and high add), respectively (Table 6).
The direction of SA in photopic condition, 61 eyes (92.4%) have positive SA (0.018±0.020 ㎛) in the naked eye, and the 12 eyes (8.6%) with negative SA (-0.016±0.005 ㎛) in the naked eye (Fig. 2). The eyes have positive SA, the positive SA was compensated by the MFCL having negative SA and were changed to negative values (-0.003±0.023, -0.009±0.028 ㎛) after wearing MFCL (low and high add), respectively. The eyes with negative SA showed more negative values (-0.030±0.014, -0.056±0.013 ㎛) after wearing MFCL (low and high add), respectively (Table 7).
Ⅳ. Discussion
In this study we investigate the HOA of naked eye, and wearing SVCL, MFCL (low and high add) in scotopic conditions using Hartmann Shark type KR-1W aberration analyzer. Aberrations that affect the quality of vision are known as coma, trefoil, and SA.20-22) Huang et al.23) reported HOAs, especially SA and coma aberration were increased in MFCL wearing eyes compared with SVCL wearing eyes. The coma aberration occurs when the visual axis of the optical elements does not pass through the geometric center of the pupil, it may be related to the lens movement and rotation. Compared with the spherical design of contact lenses, the aspheric MFCL made a greater impact on coma aberration.24) A previous study by Fedtke et al.25) also supported the finding that coma aberrations were more serious in MFCL than SVCL. Accordingly, the results of this study were also consistent with previous studies as coma aberration increased when wearing MFCL compared to SVCL.
HOAs of the eye vary according to pupil size, corneal refractive power, and age. The studies on HOA by age are insignificant, and different researchers report different results. Farid et al.26) reported that SA increases in the negative direction with increasing age, Radhakrshnan et al.27) reported that SA increases in a positive direction with increasing age. This study was conducted on people in their twenties, and 81% of the subjects had positive SA in the naked eye state. In subjects having positive SA, SA was changed to the negative direction after wearing low and high add MFCL compared to the naked eye state, and in subjects having negative SA, it was increased to more negative direction. Hammer et al.28) reported that the amount of SA occurring after wearing ASCL was decreased, and similar results were found in this study. Kim et al.29) reported that SA was decreased after wearing ASCL, but there was no difference in visual acuity. However, when a subject having negative SA wearing ASCL, SA was increased to the more negative direction, and the quality of vision may decrease, it is considered that further research on this is necessary.
Peyre et al.11) analyzed the HOA and Zernike coefficient induced before and after wearing MFCL. As a result, the Zernike coefficient measured after wearing MFCL significantly increased, It has been reported that the near addition region of the MFCL influences on generating aberrations on the eye. Kim et al.30) analyzed the aberration before and after wearing the MFCL and reported that HOA increased, which increased much as the addition power increased.
As mentioned before, there are several factors that influence the HOA. The most influencing factors are age, refractive error, pupil size, pupil center shift, corneal refractive power, and corneal astigmatism. Many researchers reported different results,31,32) that it is thought that further studies will be needed.
In the subject who have positive ocular SA, the ocular SA may compensate the negative SA of the aspheric MFCL that could reduce the addition effect of the MFCL. In that case could occur in the presbyopia correction, the near vision may different depending on the direction of the wearer's SA that it is considered that further researches are necessary for the presbyopia.
Ⅴ. Conclusion
After wearing aspherical MFCL, the total HOA and coma aberration increased, and the SA increased in the negative direction. The addition of MFCL is generated using negative SA in front of the lens that subjects having positive SA in the naked eye could reduce the addition effect by reducing the negative SA of MFCL. In subjects with negative SA, it is estimated that distance vision blur may occur because SA increases to a more negative value, and the quality of vision may decrease due to an increase in coma aberration.
