Professor Thomas Kohnen MD, PhD, FEBO
Repositioning misaligned toric IOLs can bring visual benefits in most eyes, even in some cases where the wrong power IOL was used, reports Professor Thomas Kohnen MD, PhD, FEBO, University Frankfurt, Germany.
“Always calculate the benefit of re-rotation even if the IOL is on axis,” Prof Kohnen told the 22nd ESCRS Winter Meeting in Belgrade, Serbia.
He reminded the audience that, if misaligned by 10 degrees, a toric IOL will have a 34% reduction in its anti-stigmatic effect, an IOL misaligned by 30 degrees off axis will have no anti-astigmatic effect and an IOL any further off axis will actually induce astigmatism.
He also noted that the posterior surface is a major contributor to miscalculation of toric IOL power. Although research has shown that the average refractive power of the posterior corneal surface is -0.33D, an eye with a sim-K of 0.1 can have a total corneal power of 1D of astigmatism.
ONLINE RE-ROTATION
Prof Kohnen emphasised that using calculators and formulas that take measurements of the posterior surface into account can help avoid misalignment.
There is now an online IOL calculator, available at
astigmatismfix.com,
designed specifically for eyes with residual astigmatism following toric IOL implantation. Developed by John Berdahl MD and David Hardten MD, the toric IOL rotation calculator compares the axis of the lens with the patient’s current manifest refraction and determines whether rotating the IOL can decrease residual astigmatism. It also indicates the ideal amount of IOL rotation and the expected residual error.
To illustrate the usefulness of the Berdahl-Hardten calculator, Prof Kohnen presented two examples of eyes with misaligned toric IOLs. In the first case, the axis of the IOL was at 65 degrees, as opposed to its intended axis of 92 degrees. The eye’s refraction was -3.0D sphere and +5.0 D cylinder at 120. The calculator showed that the ideal toric repositioning would be at 92 degrees, as originally intended, and that this would leave the eye with a postoperative refraction of -0.87D.
In the second case, the IOL was at its intended axis of 120 degrees, but the eye had a refraction of -2D sphere and +3D of cylinder at 40 degrees. Prof Kohnen noted that usually IOL exchange would be indicated in an eye with such a high residual refractive error. However, the online calculator showed that the ideal rotation would bring this patient close to emmetropia, with a myopia of -0.59 and a low astigmatism of 0.17D.
RE-ROTATION PEARLS AND PITFALLS
Prof Kohnen noted that the ideal time for re-rotation of a misaligned toric IOL is one-to-two weeks after the implantation of the lens. By that time, the capsular wound healing will improve rotational stability, compensating for the looser fit that can result from an eye having a capsular bag that is too large for the implant.
Changing the illumination is often helpful in eyes where IOL axis markings are difficult to see, he noted. Intraoperative optical biometry and aberrometry can be used to make axis markings intraoperatively on the cornea. The use of a viscoelastic helps protect intraocular structures and reduce the chance of sudden losses in intraocular volume. Haptic fixation may be necessary in IOLs misaligned late after capsular rupture or zonular dehiscence.
When repositioning the lens, it is important to ensure that the haptics are correctly positioned in the capsular bag. Pupil dilatation can dislocate a haptic, with the result that one haptic is in the capsule and the other is in the sulcus, he pointed out.
In cases where re-rotation of the IOL will not be useful, the IOL may be exchanged. The intervention is primarily indicated for cases of miscalculation or wrong IOLs. To remove the lens, the surgeon has the option of refolding it or bringing it into the anterior capsule and cutting it into pieces. Other options for patients with unsatisfactory outcomes include glasses or contact lenses, and corneal refractive surgery.
Thomas Kohnen:
kohnen@em.uni-frankfurt.de
