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Рецензент: д.мед.н., проф. А.М. Петруня
ГУ «Институт глазных болезней и тканевой терапии им. В.П.Филатова» (Одесса)
SI “The Filatov Institute of Eye Diseases and Tissue Therapy of the National Academy of Medical Sciences of Ukraine” (Odessa)
Национальный медицинский университет им. А.А. Богомольца (Киев)
O.O. Bogomolets National medical university Kyiv
Trends in the status of modern cataract surgery are shifting the focus attention of researchers from quantitative to qualitative characteristics of visual functions. Its improvement is achieved by reducing the trauma of the surgery on the eyeball itself and through the optimization of the constructive peculiarities of implanted intraocular lenses (IOL). In this, the development and improvement of IOL optics that reduces or eliminates spherical aberration (SA) of the pseudophakic eyes take the special place [2, 3].
It is known that under normal conditions, the passage of light rays through the optical media of the eye induces spherical aberration. Positive SA are generated by the cornea and the negative – by the lens that in the state of norm leads to its compensation. With age due to structural pathologic changes of the lens the part of positive SA increases resulting in reduction of visual acuity and contrast sensitivity in lower illumination condition especially with wide pupil. A similar situation is developing in the postoperative pseudophakic eyes in which light rays passing through the peripheral zone of the IOL are refracted more compare to paraxial that induces positive SA. Furthermore, IOL with high diopter due to convex profile of its optical surfaces also generates positive SA [6, 7, 9].
According to opinion of many researchers, implantation of IOL with optical aspherical design can significantly reduce or even eliminate SA of the entire optical system of the pseudophakic eye, which is an important condition for improving the quality of visual functions in patients after cataract surgery. At the same time, other authors note that subjective indicators of vision quality such as absolute visual acuity (VA) and contrast sensitivity is not significantly different from those of spherical lenses implantation [1, 4, 5, 8].
Thus, despite the obvious advantages of aspheric IOLs in terms of theoretical optics, some surgeons don’t note at the clinic significant differences in terms of VA and spatial contrast sensitivity (SCS) compared to conventional spherical IOL. Consequently, although the large variety of aspheric IOL models is introduced into practice of ophthalmic surgery, it is still a very debatable question the relationship of the optical characteristics of the lens with their clinical efficacy, which formed the basis for the study on the quality of the visual functions depending on the sphericity of the optical surface of the implanted IOL.
The purpose of the study – to clarify the importance of spherical and aspherical component of the IOL optical design in achieving the quality vision after cataract phacoemulsification.
Materials and methods. It has been conducted the retrospective analysis of visual functions after phacoemulsification of cataract (FEC) with IOL implantation in patients with uncomplicated cataract. This study included patients with pseudophakia – 91 eyes (100.0%), among which spherical IOL SA60AT were implanted in 47 cases (51.6%) – group 1, aspheric lenses SN60WF were implanted in 44 cases (48.4% ) – group 2. All operations were performed by the same surgeon with a same technology.
Before surgery, all patients underwent the ophthalmologic examination that included biomicroscopy, visiometry, auto refractometry, tonometry, ultrasound and optical biometry, ophthalmoscopy. IOL calculation were made by SRK/T formula. Postoperative follow-up after 3 months in addition to the above also included the investigation of spatial contrast sensitivity, aberrometry and focus depth measuring.
Results and discussion. Characteristics of the eyes are shown in Table 1. From the data it is visible that the distribution of the studied parameters in the groups was comparable.
Characteristics of the studied patients groups (n=91)
In order to eliminate the influence of the refraction value on the depth of focus, eye refraction was equalized by correction of subjective indicators, for which the trial spectacle frames were used in condition of VA maximal correction for distance. Sequentially lens with variable optical power (in increments of ± 0,25 diopters) were inserted in a frame on the side of the test eye till the reduction of the maximum corrected VA for distance on the one line. Visual acuity for distance was measured with a maximum correction and VA for near and finite distance was measured without correction and with correction for distance (Table 2).
Indices of Visual Acuity in patients with spherical and aspheric IOLs implanted in 3 months after cataract surgery
According to the data there are no significant differences in the corrected VA for distance between the groups, at the same time it was established significant difference between the groups in indices of VA for near with correction for distance (CfD). The highest VA for near vision with CfD (0,24 ± 0,05) and without correction (0,41 ± 0,08) was registered in group 1. In the same group 1 VA for finite distance with CfD and without correction was 0,49 ± 0,03 and 0,70 ± 0,08 respectively (vs. 0,56 ± 0,06 and 0,6 ± 0,05 in group 2).
Thus, the data from our study showed that after cataract surgery regardless to the sphericity design of implanted IOL VA of patients significantly improved, that confirms the results of other authors, and it was not identified significant differences in visual acuity for distance between the two groups with spherical and aspheric IOLs. This fact showed that all IOLs have the same capability to rehabilitation of VA for distance.
Keratometry data showed that the noticeable interdependence between refraction and sphericity or asphericity of implanted IOL was not found out.
In studying of aberrometry data it was paid the particular attention to high-order aberrations (HOA) of the entire eye (Entire Eye Aberrations) to the fourth order inclusive (Z4), and point spread function (PSF). Data are presented in a Table 3.
Aberrometry data in 3 months after cataract surgery with IOL implantation
It had been revealed a marked difference between the groups in total HOA as well as in certain types of aberrations, in particular, total aberrations are higher in group 1compare to group 2. SA (Z4) in the group with implanted spherical IOL were significantly higher. Total coma had also higher values in group 1. A more detailed analysis of the coma showed that vertical coma (Z3-1) was significantly less in group 2. Values of the horizontal coma (Z3+1) did not differ between groups. In values of trefoil there was no significant differences between the groups. The difference in the value of the PSF (Strehl ratio) between the groups was not statistically significant.
The study established a significant difference in the depth of focus between the groups. In the group with a spherical IOL depth of focus was higher (1,79 ± 0,63), than the aspherical (1,39 ± 0,65). The depth of focus increasing in patients with implanted spherical IOL in our view can be explained by the fact that the spherical surface of the lens in the center and on the periphery of the optical part divides light stream into many separate focuses, that summing creates the effect of focus lengthening, however, due to this phenomenon image definition reduces, that subjectively noted by patients with spherical IOL implanted.
Correlation analysis revealed the key parameters of the wavefront affecting the VA for near and finite distance. In particular, we were able to establish that the total aberration, SA, HOA, coma, and the depth of focus are closely linked to VA for near and finite distance. It was determined a significant correlation between the total SA (r = 0,95, p <0,05), a simple coma (r = 94, p <0,05) and the depth of focus.
To assess visual function a study of contrast sensitivity was conducted, which was verified with a maximum spectacle correction in photopic and scotopic light conditions. These data confirmed that the aspherical component plays an important role in vision improving in low light conditions, whereas under normal conditions the difference between the two groups was not found.
Thus, the study of pseudophakic eye visual functions with implanted spherical or aspherical IOL showed that the quality of vision in each of these groups was sufficient high. Nevertheless, quantitative indices differences revealed inessential. It can be interpreted so that the final total effect depends on a large number of different factors, among which the importance of sphericity or asphericity of the IOL surface is masked, that makes it difficult to set clinical significance of the investigated phenomenon, especially that the impact of the corrective influence of brain visual centers is attached so it is not possible to isolate this due to lack of reliable methods for the psycho physiological effects studying. Though, it can be affirmed that there is a strong tendency towards improvement of visual quality produced by IOL aspherical design, that is evidenced by subjective sensations of patients with aspheric lenses implanted, which is difficult to express by specific numerical values.
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