IOLs with more complex optical designs are now available. It may be possible to offset the positive spherical aberration of the cornea in pseudophakic patients by implanting an IOL with the appropriate negative asphericity on its anterior surface. IOLs with a toric surface may be used to correct astigmatism. Rotational stability may be of greater concern when plate-haptic toric lenses are implanted in the vertical axis than when they are implanted in the horizontal axis. As a toric lens rotates from the optimal desired angular orientation, the benefit of the toric correction diminishes. Misalignment of a toric lens may occur because of excyclotorsion or incyclotorsion of the eye as the patient moves from a vertical position to a recumbent position during surgery. Therefore, it is important to mark the eye for purposes of orientation while the patient is standing or sitting up. For the same reason, optical registration systems obtain their orientation data while the patient is sitting up. A misalignment of a properly powered toric IOL of only 10° reduces its efficacy by 30% and a misalignment of more than 30° off-axis increases the residual astigmatism of an eye; if it is 90° off-axis, the residual astigmatism doubles. Fortunately, some benefit remains even with lesser degrees of axis error, although the axis of residual cylinder changes. Newer designs are more stable than earlier ones.
Toric IOLs do not correct lenticular astigmatism and correct only that portion of corneal astigmatism that is regular. Although toric IOLs may hold benefit for patients with irregular, nonorthogonal, asymmetric, or unstable astigmatism, as may occur with keratoconus, caution should be exercised with the degree of astigmatic correction to be attempted.
Investigators have developed an IOL in which the optical power can be altered by laser after lens implantation. Similarly, a laser system in development may alter the optical power of a conventional acrylic lens after implantation. These technologies would be useful for correcting both IOL power calculation errors and residual astigmatism.
Mester U, Dillinger P, Anterist N. Impact of a modified optic design on visual function: clinical comparative study. J Cataract Refract Surg. 2003;29(4):652–660.
Sahler R, Bille JF, Enright S, Chhoeung S, Chan K. Creation of a refractive lens within an existing intraocular lens using a femtosecond laser. J Cataract Refract Surg. 2016;42(8): 1207–1215.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.