BLOG: Breaking down IOL calculations in complex corneas, part 1
The first part of this two-part blog addresses eyes with previous refractive surgery.
The accuracy of IOL calculations drives on-target visual results in cataract surgery; however, in patients with complex corneas, these calculations are challenging. To improve the visual outcome and ensure hitting the refractive target in irregular eyes, it helps to obtain more data points.
For calculating IOL power in normal corneas, we first obtain 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 ring topography to determine total corneal astigmatism. Dr. Farid favors the Pentacam (Oculus) tomographer, which also assesses anterior and posterior curvature using Scheimpflug imaging. We are able to easily distinguish anterior irregularity and posterior elevation maps. We then measure biometry with the Lenstar (Haag-Streit) and/or the IOLMaster 700 (Zeiss). The three main formulas we use and compare are the Barrett 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 accuracy of the IOL power.
LASIK, PRK
For post-LASIK eyes, Dr. Al-Mohtaseb adds keratometry readings from the Atlas 9000 (Zeiss) and compares them to those of the Galilei. One of the challenges in this group is that there is a change in corneal power measurements stemming from the surgery itself. Effective lens position measurement changes can also lead to errors in IOL choices plus higher-order aberrations. For example, myopic LASIK patients have an ablation that results in a positive spherical aberration, while a hyperopic ablation for LASIK or PRK typically results in negative spherical aberration. This is important in terms of the type of lens that we select. For post-hyperopic LASIK or PRK, we will choose a zero spherical aberration lens, such as the enVista IOL (Bausch + Lomb). The Galilei has a feature that allows us to look at higher-order aberrations. The True-K Barrett suite on the IOLMaster 700 has been effective at narrowing our window of error in post-LASIK/PRK eyes as it takes into consideration the corneal changes.
For a third set of corneal measurements, we use anterior segment OCT. Studies have shown that anterior segment OCT is helpful in getting more accurate results for post-myopic and post-hyperopic LASIK. We then input all of the datapoints into the ASCRS IOL calculator for post-myopic or hyperopic LASIK, which will calculate the power according to different formulas. Another data point is to use ORA intraoperative aberrometry (Alcon), which uses real-time measurements intraoperatively to assess the total power of the eye. Of course, good data must be put into the ORA system to get a good readout.
Finally, all post-laser vision correction patients have to be educated as to the unpredictability of their IOL power. We let all patients know that a secondary procedure such as another laser vision correction enhancement may need to be done if the refractive target is not met.
The Light Adjustable Lens (RxSight) is an exciting option for post-LASIK/PRK patients who have good mires and a regular anterior cornea. UV light can be used postoperatively to shift the IOL power and achieve the refractive target for these unpredictable eyes.
RK
It is even more difficult to hit the spherical target in eyes with previous RK due to the irregular astigmatism and the fact that the measurement can change slightly from the morning to the evening (diurnal variation). In addition to obtaining extra measurements, we also use the ASCRS calculator’s prior RK option. For all previous refractive surgery patients, it is vitally important to counsel them to the possibility of an off-target outcome. RK patients must be informed of the potential need for an IOL exchange and that refractive stability may take months after cataract surgery.
Emerging option
When the small-aperture IC-8 IOL (AcuFocus) becomes available, it will be a great choice for these situations due to its large landing zone. This IOL will have more refractive forgiveness for sphere and cylinder. Even if we are slightly off target, we can still get good visual outcomes because the technology causes aberrations to be pushed out into the periphery. We are particularly excited to use the IC-8 IOL in RK eyes. These patients typically have high expectations for their outcomes and are disappointed to learn 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 the IC-8 IOL can provide up to 3 D of extended depth of focus and tolerate up to 1 D deviation from the target manifest refraction spherical equivalent. The lens can mitigate up to 1.5 D irregular astigmatism.
In part two, we will discuss IOL calculations in patients with keratoconus and trauma as well as corneal dystrophies, lumps and bumps.
References:
- Ang RE. Clin Ophthalmol. 2018;doi:10.2147/OPTH.S172557.
- Dick HB, et al. J Cataract Refract Surg. 2017;doi:10.1016/j.jcrs.2017.04.038.
- Grabner G, et al. Am J Ophthalmol. 2015;doi:10.1016/j. ajo.2015.08.017.
- Tucker J, et al. Am J Optom Physiol Opt. 1975;doi:10.1097/00006324-197501000-00002.
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