BLOG: Breaking down IOL calculations in complex corneas, part 2
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The second part of this two-part blog looks at keratoconus, trauma, corneal dystrophies, lumps and bumps.
In patients with complex corneas, obtaining accurate IOL calculations is challenging. To make these calculations more precise and ensure hitting the refractive target, it helps to obtain more data points.
In normal corneas, we obtain corneal topography using a single topographer. Dr. Al-Mohtaseb’s practice utilizes the Galilei (Ziemer) while Dr. Farid favors the Pentacam (Oculus) tomographer, both of which are based on Scheimpflug imaging. We measure biometry with the Lenstar (Haag-Streit) and/or the IOLMaster 700 (Zeiss). The three main formulas we apply are Barrett (True-K for post-laser vision correction eyes and Universal II for all other eyes), Hill-RBF and Optimized Holladay 1.
For irregular eyes, however, the process changes as topographers and biometers are created for normal corneas. For more on this, see part one.
Keratoconus, trauma
There is more likely to be a hyperopic outcome in keratoconic eyes with steep corneas and higher posterior corneal astigmatism. There is higher variability when the anterior or posterior cornea is steeper. Therefore, for these patients, we want to target myopia.
For trauma and corneal scarring, we must determine if a corneal transplant is needed before cataract surgery. If there is peripheral scarring with irregular astigmatism, we typically select a monofocal lens, looking at the central 3-mm zone to choose the correct power IOL. This is another patient population of potential candidates for the IC-8 IOL (AcuFocus). If the patients have been in a rigid contact lens prior, however, then we always counsel them that they may need to continue their contact lens wear to achieve optimal clarity.
Corneal dystrophies, lumps and bumps
In the setting of corneal dystrophies, we can regularize the cornea by performing a superficial keratectomy. We then wait for 1 to 3 months for the cornea and topography to regularize before measuring biometry and finalizing the IOL power. We remove pathology before surgery and again wait for stabilization. We have seen situations in which patients with Salzmann nodules have toric implants and are very unhappy as the irregularity of the astigmatism was not identified before surgery and IOL planning.
The small aperture IC-8 IOL has a wavefront-filtering design that eliminates unfocused peripheral light rays, allowing only the central rays to 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. The IC-8 IOL may soon be an option in these patients and allow them to enjoy an increased range of vision after cataract surgery.
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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