As traditional IOL power calculations in individuals who have keratoconus (KCN) lead to inaccurate postoperative outcomes proportional to disease severity,^1,2,3,4 implementation of the latest methods to approach this challenge is essential to maximize these patients’ visual rehabilitation.⁴

Here, we discuss the latest methods.

New IOL Power Formulas
Available measurements in KCN eyes tend to overestimate the corneal power, leading to unexpected hyperopic outcomes. Traditionally, the SRK/T formula has demonstrated good performance in KC eyes,¹ notably with the long-axial length Wang-Koch adjustment.⁵ This can be attributed to SRK/T’s tendency to overestimate IOL power in eyes that have steep corneas, and its accuracy in eyes with long axial lengths (AL).^6,7 Newly developed IOL formulas aim to better estimate the total corneal power and minimize the hyperopic error. They are the Barrett True-K KC formula, and the Kane KC formula.

The Barrett True-K KC formula incorporates the central corneal thickness and posterior corneal astigmatism, as predicted, or as measured by the IOLMaster 700 (Carl Zeiss Meditec AG) biometer or the Pentacam (Oculus Surgical, Inc.) topographer.

The Kane KC modifies the corneal power measurement and reduces the effect of corneal power on effective IOL position prediction.⁸

The extra time to manually input biometric data into these formulas is worth the surgeon’s efforts, especially in eyes with steeper Ks. Although there is advantage in all K ranges, the advantage of these formulas is most prominent in eyes with Ks >50 D.^5,9

Ideally, to minimize typographical errors, these formulas would be incorporated in the biometer’s display. However, if logistics prevent the use of online calculators, the SRK/T with the Wang-Koch AL adjustment¹⁰ for AL ≥ 25.20 mm (adjusted AL = AL × 0.8544 + 3.7222) is an option for KCN eyes.⁵

IOLs for KCN
When selecting an IOL for ectatic corneas, we recommend aberration-free and light-filtering models. Additionally, we address toricity considerations.

• Aberration-free IOLs. KCN eyes have prominent higher-order aberrations (HOAs), notably coma. By aiming to minimize visual disturbances associated with irregular corneas and their HOAs, aberration-free IOLs provide a tailored approach. These IOLs: Akreos AO (model AO60, Bausch + Lomb); Akreos AO Microincision lens (Akreos MICS; model MI60, Bausch + Lomb); enVista MX60E (Bausch + Lomb); enVista MX60ET, Bausch + Lomb); RayOne Aspheric (RA0600C, Rayner), and the SofPort Advanced Optics Aspheric Lens (Bausch + Lomb).

• Light-filtering IOLs. The Light-Adjustable Lens (LAL) IOL (RxSight) allows adjustment of residual spherical and cylindrical error postoperatively from -2.0 D to +2.0 D and up to
2.0 D, respectively. (For more information on this, see “Implementing the Light Adjustable Lens in Practice,” at bit.ly/48CjQaR.)

The IC8-Aphthera IOL (Bausch + Lomb) offers a small aperture mechanism that improves vision in eyes with KCN by reducing the impact of the corneal HOAs.¹¹ Its pinhole effect also offers the benefit of extending the depth of focus, while minimizing the risk of halos, glare, and starbursts. Additionally, the IC-8 can address lower-order aberrations: It tolerates up to 1.0 D of defocus and 1.50 D of astigmatism.

• Toric IOLs. Another consideration when selecting an IOL in KCN cataractous eyes is astigmatism correction. Ideal candidates for toric lenses are KCN patients who are wearing a manifest refraction with astigmatism correction. In cases where the keratoconic patient’s manifest cylinder and axis align similarly with corneal topography, an astigmatism-correcting lens can improve visual outcomes.

Of note: As a patient’s astigmatism is sometimes larger than the greatest magnitude available in toric lenses, anecdotally, even reducing a portion of the patient’s pre-operative astigmatism can yield favorable results. This can be true even in cases in which the patient will be re-fit with a scleral contact lens after cataract surgery, as it can improve uncorrected visual acuity.

Additional Considerations
In addition to implementing the aforementioned techniques to approach IOL power calculations, we recommend the following preoperative steps to increase the likelihood of an accurate postoperative outcome in KCN patients: optimizing the ocular surface (e.g., discontinuing contact lenses prior to surgery, and managing any dry eye or atopic lid disease); documenting topographic disease stability and comparing different sets of measurements from different biometers, if available; and managing patient expectations (e.g., discussing the variability of outcomes despite maximal preparation). CP 

Karim Kozhaya, MD is a research post-doctoral fellow in the ophthalmology department of the Baylor College of Medicine., in Houston, Texas. He has no disclosures.

Allison J. Chen, MD, MPH is an assistant professor in the ophthalmology department at the Baylor College of Medicine. She specializes in keratoconus, Fuchs’ dystrophy, cataract surgery, LASIK/PRK, refractive correction, and astigmatism correction.  She has no disclosures.

References:

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2. Bozorg S, Pineda R. Cataract and keratoconus: minimizing complications in intraocular lens calculations. Semin Ophthalmol. 2014;29(5-6):376-379. doi: 10.3109/08820538.2014.959193.

3. Kamiya K, Iijima K, Nobuyuki S, et al. Predictability of intraocular lens power calculation for cataract with keratoconus: a multicenter study. Sci Rep. 2018;8(1):1312. doi: 10.1038/s41598-018-20040-w.

4. Ghiasian L, Abolfathzadeh N, Manafi N, Hadavandkhani A. Intraocular lens power calculation in keratoconus: a review of literature. J Curr Ophthalmol. 2019;31(2):127-134. doi: 10.1016/j.joco.2019.01.011.

5. Kozhaya K, Chen AJ, Joshi M, et al. Comparison of keratoconus specific to standard IOL formulas in patients with keratoconus undergoing cataract surgery. J Refract Surg. 2023;39(4):242-248. doi: 10.3928/1081597X-20230124-01.

6. Hoffer KJ. The Hoffer Q formula: a comparison of theoretic and regression formulas. J Cataract Refract Surg. 1993;19(6):700-712. doi: 10.1016/s0886-3350(13)80338-0.

7. Melles RB, Holladay JT, Chang WJ. Accuracy of intraocular lens calculation formulas. Ophthalmology. 2018;125(2):169-178. doi: 10.1016/j.ophtha.2017.08.027.

8. Kane JX, Connell B, Yip H, et al. Accuracy of intraocular lens power formulas modified for patients with keratoconus. Ophthalmology. 2020;127(8):1037-1042. doi: 10.1016/j.ophtha.2020.02.008.

9. Heath MT, Mulpuri L, Kimiagarov E, et al. Intraocular lens power calculations in keratoconus eyes comparing keratometry, total keratometry, and newer formulae. Am J Ophthalmol. 2023;253:206-214. doi: 10.1016/j.ajo.2023.03.037.

10. Wang L, Shirayama M, Ma XJ, Kohnen T, Koch DD. Optimizing intraocular lens power calculations in eyes with axial lengths above 25.0 mm. J Cataract Refract Surg. 2011;37(11):2018-2027. doi: 10.1016/j.jcrs.2011.05.042.

11. Shajari M, Mackert MJ, Langer J, et al. Safety and efficacy of a small-aperture capsular bag-fixated intraocular lens in eyes with severe corneal irregularities. J Cataract Refract Surg. 2020;46(2):188-192. doi: 10.1097/j.jcrs.0000000000000045.