An incorrect corneal power will cause errors in the various parts of IOL power calculation. Corneal power accounts for about 2/3 of the total dioptric power of ocular refractive system. The IOL power calculation requires three basic factors: corneal power measurement, effective lens positioning (ELP) and axial length measurements that in these patients due to increase in eye length, irregular corneal surface and advanced astigmatism, an accurate IOL power with high reliability is not being calculated 7.Īs discussed earlier, one of the factors of IOL power calculation is to determine the corneal refractive power. For these people, the accurate IOL power calculation is a challenge, while determining the correct IOL power plays an important role in achieving acceptable visual results and optimal refractive and also in patients satisfaction after cataract surgery. Patients with advanced keratoconus may be candidates for keratoplasty with cataract extraction and intraocular lens implantation (IOL) but patients who have had a well-corrected vision with contact lenses or glasses before cataract may not require corneal surgery 6. Therefore, it is necessary for ophthalmologists to prepare for a growing population of patients with keratoconus or PMD that require cataract surgery. On the other hand, keratoconus patients are more likely to get cataract than non-keratoconus patients 5. With regards to the population growth rate of people aged above 65 years old, which is reported to be 15.1%, the number of patients with cataract will increase 4. Although there is no evidence of the incidence or prevalence of PMD, studies have shown that its outbreak is less than keratoconus 3. These results were fivefold to tenfold higher than previously reported 2. A study on more than 4 million patients aged 10–40 years old showed that the incidence of keratoconus was 1:7500 in the relevant age category and its estimated prevalence in the general population was 1:375. Causes of these problems, stopping the progress of the disease and improving the vision of patients consider as a challenging task. These changes result in irregular Astigmatism and reduced vision. Keratoconus (KCN) and Pellucid marginal degeneration (PMD) are non-inflammatory and almost progressive corneal diseases known by ectasia, central, paracentral and peripheral corneal thinning 1. It seems that using the total corneal power along with the Barrett formula can prevent postoperative hyperopic shift, especially in eyes with advanced ectatic disorders. The corneal power in the 4 mm region and the Barrett formula resulted the prediction error of six eyes within ± 1 diopter. Then, by the use of preoperative corneal images, corneal power was calculated by considering the anterior and posterior parts of the cornea and refractive index of 1.376 and the customized corneal refractive index in different regions and finally it was entered into the IOL power calculation formulas. The ray tracing analysis was performed on the customized eye models, and the corneal refractive index was determined by minimizing the difference between the measured aberrations from the device and resulted aberrations from the simulation. The implanted IOLs were designed by Zemax software. All the measured points of the posterior and anterior parts of the cornea converted to points cloud and surface by using the MATLAB and Solidworks software. At least three months after cataract surgery, axial length, cornea, IOL thickness and the distance between IOL from cornea, and aberrometry were measured. Seven eyes with moderate and severe corneal ectatic disorders, which had been under cataract surgery, were included. The aim of this study is to determine the customized refractive index of ectatic corneas and also propose a method for determining the corneal and IOL power in these eyes.
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