How to improve IOL power calculations in keratoconus
One source of error in IOL power calculation is the use of the classical keratometric approach for the characterisation of the corneal optics. This approach is based on the assumption of only one corneal surface and a fictitious index of refraction (keratometric index, nk) for obtaining an estimation of the corneal power (Pk).
One source of error in IOL power calculation is the use of the classical keratometric approach for the characterisation of the corneal optics.1 This approach is based on the assumption of only one corneal surface and a fictitious index of refraction (keratometric index, nk) for obtaining an estimation of the corneal power (Pk).2
Specifically, the use of the classical value of nk of 1.3375 has been shown to overestimate the corneal power in healthy,3 post-laser refractive surgery4 and keratoconus eyes.5 Some algorithms that have been developed, for different types of IOL design in eyes with corneal problems or previous surgeries, minimise the impact of this keratometric error in IOL power calculations. There are also algorithms for optimising the estimation of the effective lens position (ELP).6,7
To this date, few studies have been conducted to investigate how to optimise IOL power calculation in keratoconus eyes. It should be considered that the posterior corneal curvature and thickness is abnormal in this type of eyes.
Park do and colleagues8 found, in patients with posterior keratoconus, that IOL power calculations from conventional keratometry may be inaccurate, and secondary piggyback IOL procedures might be needed after cataract surgery. Thebpatiphat and co-authors9 concluded in a retrospective cases series evaluating 12 keratoconus eyes undergoing cataract surgery that IOL calculation was more predictable in mild keratoconus than in moderate and severe disease.
It should be considered that the most significant increase in posterior corneal curvature and decrease in central corneal thickness are present in more severe keratoconus cases compared with the rest.10 We have recently conducted a simulation and clinical study to investigate the influence of the error in the calculation of corneal power due to the use of nk on IOL power calculation, as well as the potential benefit of using adjusted keratometric algorithms.
Our simulations
We have found that IOL power is underestimated if corneal power is overestimated, and vice versa. In our simulations, the use of the classical keratometric approach with a keratometric index of 1.3375 led to overestimations of IOL power up to -5.6 D and -6.2 D using Le Grand and Gullstrand eye models, respectively.
An adjusted keratometric approach was defined by our research group consisting of the use of a variable nk, which was dependent on the radius of curvature of the first corneal surface and the eye model used, as summarised in Table 1. With this adjusted keratometric approach, maximal error was within ±1.1 D, with most values ≤ ±0.6 D.
Considering that 1 D of variation of IOL power induces about 0.9 D of change in subjects’ refraction at the corneal vertex, this error can be considered as clinically acceptable, with most of cases not exceeding ± 0.60 D for most r1c-r2c combinations. Only the error was maximal for extreme values.
Preliminary clinical validation
A total of 13 eyes of 8 patients with keratoconus (with a mean age of 41.1 years ± 19.1 and ages ranging from 20 to 69 years) were evaluated in the Department of Ophthalmology (OFTALMAR) of the Vithas Medimar International Hospital (Alicante, Spain). In all cases, IOL power was calculated using the keratometric approach (equation 1)11 and the complete Gaussian formula, including posterior cornea and pachymetric data obtained with the Sirius system (CSO; Firenze, Italy).
The agreement between both IOL power calculation methods was studied. The adjusted keratometric IOL power was calculated.
In agreement with theoretical simulations, the IOL power calculated with the adjusted keratometric approach underestimated and overestimated the IOL power calculated using the Gaussian equation in a magnitude ranging from -1.1 to 0.4 D. No statistically significant differences between both IOL power calculations were found (p > 0.05) and a very strong and statistically significant correlation between them was observed (r = 0.99, p < 0.01).å
The Bland-Altman analysis revealed the presence of a mean difference between adjusted keratometric and Gaussian corneal power of -0.31 D, with limits of agreement of -1.34 and 0.72 D (see Figure 1).
Conclusions
The use of the classical keratometric approach with a single value of nk in keratoconus for the calculation of IOL power is inaccurate and may be the reason for some refractive surprises in this type of eyes after cataract surgery.
This inaccuracy can be minimised by using an adjusted keratometric approach based on the estimation of the keratometric corneal power, using a variable nk depending on the radius of curvature of the anterior corneal surface, with a maximum error in most of cases of approximately 0.6 D and over 1 D in very few cases.
We have conducted a successful preliminary clinical validation of this approach for IOL power calculation, and a clinical validation with a larger sample size including severe keratoconus cases is now necessary so that more consistent conclusions can be obtained.
References
1. Camps VJ, Pinero DP, de Fez D, Mateo V. Minimizing the IOL power error induced by keratometric power. Optom Vis Sci. 2013;90:639-649.
2. Camps VJ, Piñero Llorens DP, de Fez D, et al. Algorithm for correcting the keratometric estimation error in normal eyes. Optom Vis Sci. 2012;89:221-228.
3. Piñero DP, Camps VJ, Mateo V, Ruiz-Fortes P. Clinical validation of an algorithm to correct the error in the keratometric estimation of corneal power in normal eyes. J Cataract Refract Surg. 2012;38:1333-1338.
4. Camps VJ, Pinero DP, Mateo V, et al. Algorithm for correcting the keratometric error in the estimation of the corneal power in eyes with previous myopic laser refractive surgery. Cornea. 2013;32:1454-1459.
5. Camps VJ, Pinero DP, Caravaca-Arens E, et al. New approach for correction of error associated with keratometric estimation of corneal power in keratoconus. Cornea. 2014;33:960-967.
6. Piñero DP, Camps VJ, Ramón ML, et al. Positional accommodative intraocular lens power error induced by the estimation of the corneal power and the effective lens position. Indian J Ophthalmo. 2015;63:438-444.
7. Piñero DP, Camps VJ, Ramón ML, et al. Error induced by the estimation of the corneal power and the effective lens position with a rotationally asymmetric refractive multifocal intraocular lens. Int J Ophthalmol. 2015;8:501-507.
8. Park do Y, Lim DH, Chung TY, Chung ES. Intraocular lens power calculations in a patient with posterior keratoconus. Cornea. 2013;32:708-711.
9. Thebpatiphat N, Hammersmith KM, Rapuano CJ, Ayres BD, Cohen EJ. Cataract surgery in keratoconus. Eye Contact Lens 2007;33:244-246.
10. Pinero DP, Alio JL, Aleson A, et al. Corneal volume, pachymetry, and correlation of anterior and posterior corneal shape in subclinical and different stages of clinical keratoconus. J Cataract Refract Surg. 2008;36:814-825.
11. Camps VJ, Piñero DP, Caravaca E, de Fez D. Preliminary validation of an optimized algorithm for intraocular lens power calculation in keratoconus. Indian J Ophthalmol. 2017 (in press).
David P. Piñero, PhD
Dr Piñero is a lecturer and senior researcher at the Department of Optics, Pharmacology and Anatomy, University of Alicante, Spain. He has no financial interests in the subject matter.
Vicente J. Camps, PhD
Dr Camps is a senior researcher at the Department of Optics, Pharmacology and Anatomy, University of Alicante, Spain. He has no financial interests in the subject matter.
Esteban Caravaca-Arens
Dr Caravaca-Arens is a PhD student at the Department of Optics, Pharmacology and Anatomy, University of Alicante, Spain. He has no financial interests in the subject matter.
