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Cataract and Refractive Surgery Intraocular Lens Power Calculation – Still Searching for the Holy Grail Nino Hirnschall 1 and Oliver Findl 2 1. Resident; 2. Chief of the Institute and Chief of the Department, Vienna Institute for Research in Ocular Surgery (VIROS), Department of Ophthalmology, Hanusch Hospital, Vienna, Austria Abstract Since the introduction of optical biometry and modern intraocular lens (IOL) power calculation formulae, the refractive outcome after cataract surgery improved significantly. This is necessary, as patient demand for spectacle independence is increasing. However, especially when it comes to short and long eyes, all formulae have their difficulties in predicting the effective lens position – and, therefore, the post-operative refractive outcome. This review summarises the development of IOL power calculation formulae, explains their basics and presents some alternative calculation methods. Keywords Intraocular lens power calculation, cataract surgery, refractive outcome Disclosure: Nino Hirnschall and Oliver Findl have no conflicts of interest to declare. No funding was received in the publication of this article. Open Access: This article is published under the Creative Commons Attribution Noncommercial License, which permits any non-commercial use, distribution, adaptation and reproduction provided the original author(s) and source are given appropriate credit. Received: 23 May 2015 Accepted: 26 July 2015 Citation: European Ophthalmic Review, 2015;9(1):13–6 Correspondence: Nino Hirnschall, VIROS, Department of Ophthalmology, Hanusch Krankenhaus, Heinrich-Collin Straße 30, 1140 Vienna, Austria. E: [email protected] Intraocular lens (IOL) power calculation is probably not one of the most attractive topics in ophthalmology. As a consequence, the results from the optical biometry device are often trusted blindly. However, every cataract surgeon should be aware of the fact that the calculation of the accurate IOL power is crucial for the post-operative patient satisfaction. Even with modern measurement techniques, such as optical biometry 1 and using the latest generation of IOL power calculation formulae, there is a relevant unpredictability of the post-operative manifest refractive outcome. Approximately 20 % of patients, depending on their axial length, require a refractive correction of more than ±0.5 diopter (D) spherical equivalent post-operatively. 2 However, in short eyes this percentage increases to 30 % to 40 % in eyes below 22.0 mm, or above 26.0 mm of axial eye length (AL). These refractive surprises result in poor unaided visual acuity and a reduced patient satisfaction. This problem becomes more and more relevant as patients’ expectations are rising. Whereas for first IOL implantations some decades ago only one IOL power was available to restore better post- operative corrections compared with aphakia, today patients often want to be completely spectacle independent for distance vision or even at all distances. For a patient with an AL of 20.5 mm who wants a multifocal IOL, the chance of a refractive surprise of more than 1.0 D is approximately 20–30 %. Why Do We have Refractive Surprises after Cataract Surgery? Norrby investigated the error distribution of different factors on the post- operative manifest refraction. Many different parameters were shown to 3 Tou ch MEd ica l MEdia have an influence, such as AL, corneal anterior apical radius (mm) and corneal posterior/anterior radius ratio, corneal anterior and posterior asphericity, corneal thickness and the refractive indices of aqueous and vitreous, as well as pupil size (mm), the error of the post-operative manifest subjective refraction and, most importantly, the prediction of the post-operative anterior chamber depth (ACD). Since the introduction of optical biometry 4 (IOLMaster, Carl Zeiss Meditec AG, Jena, Germany), which was shown to be a reliable, operator-independent, 5 non-contact measurement of AL, measurements became much more accurate. Relatively new software algorithms allow the use of a composite scan, which means that several scans are averaged to improve the signal- to-noise ratio. This software improvement led to a better success rate in dense cataracts, especially dense posterior subcapsular cataracts. 6 New technologies, such as swept source optical coherence tomography (OCT) imaging of the entire eye, use an additional fixation check to assure measurement of the patient’s visual axis. However, two factors still have a significant impact on the post-operative refractive error, namely the prediction of the post-operative IOL position and the post-operative refraction itself. The latter is probably difficult to understand, because it basically means that the outcome (post-operative manifest refraction) is a source of error, but this can be explained due to the weak reproducibility of subjective refraction. 3 Not taking this factor into account, the error in the prediction of the post-operative IOL position has by far the most relevant impact on the refractive outcome. Which Eyes have a Higher Likelihood of a Refractive Surprise? IOL power calculation is especially difficult for short and long eyes. 2 In short eyes this error is even more relevant, as short eyes usually need a high IOL power; therefore, the prediction of the position is even 13