NOV 12, 2011
The authors of this study published in October in the Journal of Cataract & Refractive Surgery used two methods to analyze changes in the eye's refractive properties when a toric IOL rotates. They found that these methods provide equivalent results and indicate that toric IOL rotations of less than 10 degrees change the eye's refraction less than 0.50 diopter. The study's conclusion that small rotations do not significantly interfere with outcomes is encouraging.
In this experimental study, the authors compared the matrix definition of astigmatism with another vector representation. The methods they compared were: (1) The cylinder, C, resulting from the addition of two cylinders C1 and C2,whose axes form an angle a, is obtained by the addition of two vectors of values C1 and C2 forming an angle 2a; and (2) the power matrix, F, of a thin astigmatic dioptric system that decomposes naturally into three orthogonal components: the purely spherical part Fnes, the ortho-astigmatism For and oblique astigmatism Fob.
The authors found that the residual cylinder was one-third of the corneal astigmatism when a toric IOL rotated ± 10 degrees when the cylinder values for the cornea (C1) and IOL (C2) were equal. However, in most cases C1 is greater than C2, which means that the residual astigmatism did not change noticeably with small rotations. The angle of rotation, b, which annuls the astigmatism correction, could be obtained from the following: cos (π + 2b) = -r/2, with r being the ratio between the IOL and cornea cylinders.
The authors conclude that when the IOL cylinder has a different value than that of the corneal astigmatism, a lower, rather than higher, cylinder value is a better choice to reduce residual astigmatism. They say the results indicate that small axis rotations are not an obstacle to satisfactory astigmatism correction with toric IOLs.