Breaking down IOL stones in complex corneas, part 1


March 09, 2022

3 minute read


Biography: Farid is director of cornea, cataract, and refractive surgery and vice chair of the faculty of ophthalmology at the Gavin Herbert Eye Institute at UC Irvine.


Disclosures: Al-Mohtaseb declares having financial interests with Alcon, Bausch + Lomb, Carl Zeiss, CorneaGen, Novartis and Ocular Therapeutix. Farid is a consultant for Allergan, Bausch + Lomb, Bio-Tissue, Carl Zeiss Meditec, CorneaGen, Dompé, Johnson & Johnson Vision, Kala, Novartis, Orasis, Sun and Tarsus.


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The first part of this two-part blog deals with eyes that have already had refractive surgery.

The precision of IOL calculations leads to targeted visual results in cataract surgery; however, in patients with complex corneas, these calculations are difficult. To improve the visual outcome and ensure hitting the refraction target in irregular eyes, it is helpful to get more data points.

Zaina Al-Mohtaseb

Marjan Farid

To calculate the IOL power in normal corneas, we first obtain the corneal topography using a single topographer. In Dr. Al-Mohtaseb’s practice, this is done with the Galilei (Ziemer), which measures anterior and posterior corneal astigmatism using Scheimpflug imaging and Placido’s ring topography to determine total corneal astigmatism. Dr. Farid favors the Pentacam tomograph (Oculus), which also assesses anterior and posterior curvature using Scheimpflug imaging. We are able to easily distinguish between anterior irregularity and posterior elevation maps. We then measure the biometrics with the Lenstar (Haag-Streit) and/or the IOLMaster 700 (Zeiss). The three main formulas we use and compare are Barrett’s formulas (True-K for post-laser vision correction eyes and Universal II for all other eyes), Hill-RBF and Optimized Holladay 1.

For post refractive surgery eyes, the process is different. Keeping in mind that topographers and biometers are created for normal corneas, we want to get more measurements to ensure IOL power accuracy.

LASIK, PRK

For post-LASIK eyes, Dr. Al-Mohtaseb adds keratometry readings from the Atlas 9000 (Zeiss) and compares them to those from the Galilei. One of the challenges for this group is that there is a change in corneal power measurements resulting from the surgery itself. Effective changes in lens position measurement can also lead to errors in IOL choices as well as higher order aberrations. For example, myopic LASIK patients have an ablation that results in a positive spherical aberration, while a farsighted ablation for LASIK or PRK usually results in a negative spherical aberration. This is important in terms of the type of lens we select. For post-hyperopic or PRK LASIK, a lens with zero spherical aberration will be chosen, such as the enVista IOL lens (Bausch + Lomb). The Galilei has a feature that allows us to examine higher order aberrations. The Barrett True-K suite on the IOLMaster 700 has been effective in reducing our error window in post-LASIK/PRK eyes as it takes corneal changes into consideration.

For a third set of corneal measurements, we use anterior segment OCT. Studies have shown that anterior segment OCT is helpful in obtaining more accurate results for post-myopic and post-hyperopic LASIK. We then enter all the data points into the ASCRS IOL calculator for post-myopic or hyperopic LASIK, which will calculate the power according to different formulas. Another data point is using the Intraoperative OPD ORA (Alcon), which uses real-time measurements intraoperatively to assess the total power of the eye. Of course, good data must be fed into the ORA system to get a good reading.

Finally, all patients who have received laser vision correction should be made aware of the unpredictability of the strength of their IOL. We advise all patients that a secondary procedure such as another laser vision correction enhancement may be required if the refraction target is not achieved.

The light-adjustable lens (RxSight) is an attractive option for post-LASIK/PRK patients who have good sights and an even anterior cornea. UV light can be used after the operation to modify the power of the IOL and reach the refraction target for those unpredictable eyes.

R.K.

It is even more difficult to hit the spherical target in eyes with anterior RK due to irregular astigmatism and the fact that the measurement can change slightly from morning to night (diurnal variation). In addition to obtaining additional measurements, we also use the prior RK option of the ASCRS calculator. For all patients who have had refractive surgery before, it is vitally important to counsel them on the possibility of an off-target outcome. RK patients should be advised of the potential need for IOL exchange and that refractive stability may take months after cataract surgery.

Emerging option

When the IC-8 Small Aperture IOL (AcuFocus) becomes available, it will be an excellent choice for these situations due to its large landing area. This IOL will have a larger refraction tolerance for the sphere and the cylinder. Even if we’re slightly off target, we can still get good visual results as the technology causes the aberrations to push to the periphery. We are especially excited to use the IC-8 IOL in RK eyes. These patients usually have high expectations for their results and are disappointed to learn that they are not candidates for multifocal lenses.

The wavefront filtering design of the IC-8 IOL eliminates unfocused peripheral light rays so that only the central rays focus on the retina. Studies have shown that the IC-8 IOL can provide up to 3D of extended depth of field and tolerate up to 1D of deviation from the spherical equivalent of target manifest refraction. The lens can attenuate up to 1.5 D of irregular astigmatism.

In the second part, we will discuss IOL calculations in patients with keratoconus and trauma as well as corneal dystrophies, bumps and bumps.

The references:

  • Ang RE. Wink Ophthalmol. 2018;doi:10.2147/OPTH.S172557.
  • Dick HB, et al. J Refractory cataract surgery. 2017; doi:10.1016/j.jcrs.2017.04.038.
  • Grabner G, et al. Am J Ophthalmol. 2015;doi:10.1016/d. add.2015.08.017.
  • TuckerJ, et al. Am J Optom Physiol Opt. 1975; doi:10.1097/00006324-197501000-00002.

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