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1Национальный медицинский университет им. А.А. Богомольца (Киев)
O.O. Bogomolets National medical university (Kyiv)
2Национальный технический университет Украины «Киевский политехнический институт»
National Technical University of Ukraine "Kyiv Polytechnic Institute"
The method of intraocular correction of aphakia has now gained world-wide recognition as the most important stage in medical and vocational rehabilitation of patients with cataract. One of the significant conditions for high optical results achieving is the intracapsular implantation of intraocular lens (IOL) with its maximal centering. However, violations of the IOL central position relative to the optical axis of the eye from the slight shift and decentration to subluxation and complete dislocation are known to occur even at perfectly performed operations, leveling the optical and structural advantages of modern IOLs [1, 4, 8, 10].
According to various authors, the frequency of IOL decentration varies widely – from 10 to 92% [1, 3, 6, 9, 11]. Consequently, IOL implanting in the capsular bag of removed crystalline lens can’t guarantee an exact match of its optical axis with the optical axis of the cornea. So the lens optical axis may be parallel to the cornea optical axis i.e. decentring laterally along the horizontal (axis OX), vertical (axis OY) or simultaneously along both these axes. IOL angular rotation (tilt) around one of the axis is another possible manifestation of decentration.
IOL decentration not only changes the attitude position of optical axis but also leads to marked increase in the aberrational mode amplitude of all degrees especially the lower ones, that have a significant impact on the patients’ visual functions and respectively their quality of life. In this regard, the study of the features of decentred IOL aberration properties is an important task that is essential, both theoretical and practical, since at the presence of a IOL model variety it is still an open question its optimal choice which is depended on individual characteristics of an eye and operation course [2, 5, 7, 8, 11]. Obtaining of objective data on the sensitivity of different IOL models to decentration which generate aberrations is an important factor at the final selection of the lens model for implantation.
The purpose of the study is to identify the changes in spectrum of aberration modes of different models of IOL caused by its decentration.
Materials and methods
The experimental model of the eye has been developed and manufactured at the Department optical and optoelectronic devices of National Technical University of Ukraine «Kyiv Polytechnic Institute» (NTUU «KPI») led by professor I.H. Chyzh.
Aberrometry has been conducted by the ray-tracing aberrometer TRACEY-VFA (USA, Tracey Technologies) with inserting of an IOL inside the physical model of an eye optical system. By means of special gears of IOL shifting along the horizontal axis OX and the angular rotation of IOL around vertical axis the lateral decentration within ±1 mm and rotary decentration within ±8 degrees have been set.
At first aberrometry of the eye model has been conducted without an IOL. The eye model was placed on the aberrometer and centered relative to the optical axis of the aberrometer by mirror images of centering infrared LEDs, produced by light reflection from front surface of the lens-cornea. Then investigated IOL was placed inside of the model and centered regarding to position of centering LEDs mirror images. The average value of the every aberration mode amplitude of the eye model without an IOL, found on the 10 sessions, was subtracted from the average value of the same aberration mode amplitude of the model with a centered IOL. Difference of values, obtained by means of statistical methods, indicated the value of the aberration mode amplitude brought into the model by the IOL itself.
IOL decentration relative to the axial light beams coming out of the lens-cornea, caused changes of the IOL aberration mode amplitudes. These changes were identified by comparing two average values of each aberration mode amplitude, one of which was obtained from the series of 10 sessions of aberrometry of the model with a decentered IOL, and the second – from the series of 10 sessions of aberrometry of the model with a centered IOL. Similarly it was defined the changes of IOL aberration mode amplitudes due to IOL rotation around the vertical axis.
IOL decentration along the axis OX was set to the value ±0.5 mm and ±1 mm, values of IOL angular rotation around the vertical axis were ±4° and ±8°. Aberrometry data of IOL centered relative to cornea are shown in the table 1.
Aberrometry data of IOL centered relative to lens-cornea
Analysis of the aberration mode spectrum of IOL SL-907 shows that small values of its RMS are the result of much smaller amplitudes of its aberration modes. In our opinion this shows clearly higher quality of this model manufacturing.
Table 2 shows the ranges of IOL decentration and angular rotation which do not lead to RMST values exceeding 0,05 µm (by Marechal) and 0.1 µm (RMS conditionally acceptable value). It is obvious that IOL SL-907 has widest tolerance due to lower sensitivity to decentration and rotation (tilt), slightly lower – IOLs TECNIS® and SN60WF. The IOL model SA60AT shows he lowest tolerance.
Valid values of IOL decentration and rotation
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