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Modern-day cataract surgery: can we match growing expectations? M Rana, S Shah Cataract surgery has evolved from the ancient technique of ‘couching’ with suboptimal results to femtosecond laserassisted phacoemulsification with excellent visual results. It is the most frequently performed surgery in the world.1 The recent advances in surgical techniques have increased the safety and efficacy of this procedure. There have also been improvements in the techniques to measure ocular biometry and methods to calculate intraocular lens (IOL) power resulting in high expectations for increasingly better outcomes from surgeons and patients. The accuracy of the refractive end point depends on the potential errors from a multitude of factors, which are involved in the calculation of the IOL power, aside from the surgical aspects themselves. These variables include the axial length, corneal power measurements, assumed corneal refractive index, lens position and anterior chamber depth.2 Although these factors are closely related to each other, and a change in one may affect the other, they have been assumed to be relatively independent factors for the purposes of the mathematical formulae used to perform IOL power calculations. Hence, there is potential for errors in these calculations. Various mathematical formulae have been created to calculate the desired IOL power. Initially, the main variables used were axial length and corneal power and formed the ‘thin lens or theoretical formulae’.3–6 The next generation of regression formulae gave somewhat better results than the first generation.7–8 In the late 1980s, the third-generation formulae (Hoffer Q, Holladay 1 and SRK/T) were introduced and became widely used, and considered the effective lens position (ELP) in their calculation.8–11 However, there continued to be a variation in results, at least partly, due to the actual ELP compared to the calculated ELP.12 Preussner,13 in 2002, developed a raytracing method to calculate IOL power, and more recently, Olsen further steered Birmingham and Midland Eye Centre, City Hospital, Birmingham, West Midlands, UK Correspondence to Professor Sunil Shah, Birmingham and Midland Eye Centre, City Hospital, Birmingham B18 7QH, UK; [email protected]

the ray-tracing methods by taking every surface of cornea and lens into consideration in predicting the IOL power,14 However, despite these improvements, results remain somewhat variable. An error of 100 μm difference in AL measurement produces a refractive outcome of 0.3 D from the desired value.2 15 Ultrasound used for the measurement of AL has a mediocre longitudinal resolution of 200 μm and an accuracy of 100–120 μm,15 respectively. The emergence of partial coherence laser interferometry (PCLI) and optical low coherence reflectometry (OLCR), both non-contact techniques for optical biometry, have created a revolution in technology. PCLI and OLCR have a much higher resolution (about one-tenth of ultrasound).15–17 Many studies comparing PCLI against conventional ultrasound confirmed significantly better outcomes.15–16 However, despite this increased accuracy, the visual results remain variable. Knox Cartwright18 reported on a large multicentre audit of 55 567 procedures. A visual outcome of 6/6 or better was only seen in 51% of eyes with no other ocular pathology. Aristodemou12 similarly looked at refractive outcomes in their cohort of 8108 patients using three different formulae, and optimised A constants. Outcomes were found to be within ±0.25 D in 40%, ±0.50 D in 75% and ±1.00 D in 95% of patients. Although this may be considered acceptable to some, this still means that 5% of patients would have greater than 1 D refractive error and, hence, when considering premium IOLs, this would be a group that one may need to consider laser fine tuning. Norrby19 suggested that the three main sources of postoperative refractive errors were: preoperative AL measurement, preoperative estimation of postoperative IOL position and postoperative refractive determination; these factors made up for 80% of all errors. Of these, the largest cause was the prediction of IOL position (35%). Olsen has also highlighted this in his review of IOL power calculation.2 As choosing the correct IOL power remains problematic, there has been increasing interest in intraoperative measurements to achieve accurate results. Strategies that have evolved include intraoperative optical refractive biometry

(Lanchulev20) and wavefront aberrometry by the Orange (WaveTec Vision, Aliso Viejo, California, USA) using the Talbot moiré interferometry21 or the open field Aston aberrometer.22 They have shown to have predictive refractive errors within 0.25 D in 80% of cases. Hirnschall23 previously had reported on measurement of postoperative anterior chamber depth (ACD) using anterior segment time domain optical coherence tomography (OCT). The best indicator of the ACD value was the anterior capsule position after implantation of a capsular tension ring. The same authors have taken their previous study further, by configuring an intraoperative measured ACD partial least square regression (PLSR) formula to calculate the postoperative refractive outcome, and have compared the results to using conventional multiple regression formulae.24 The PLSR formulae proved to be superior to all the other formulae and the weighting of explanatory variables was reduced. Although the direct comparison and statistical analysis did not show a significant difference between the PLSR formula and other individual third-generation regression formulae, it still had relatively high prediction rates. This is potentially a major leap towards the more accurate prediction of ACD, a factor which has been proven, time and time again, to be responsible for poor refractive outcomes. The future looks bright with the potential incorporation of all these new techniques into our routine cataract practice leading to more consistent and accurate results for our patients. Contributors Both authors have been involved in the concept and writing of this article. Competing interests None. Provenance and peer review Commissioned; internally peer reviewed.

To cite Rana M, Shah S. Br J Ophthalmol 2014;98:1313–1314. Published Online First 17 July 2014


Br J Ophthalmol 2014;98:1313–1314. doi:10.1136/bjophthalmol-2014-304962

Rana M, et al. Br J Ophthalmol October 2014 Vol 98 No 10


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Rana M, et al. Br J Ophthalmol October 2014 Vol 98 No 10

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Modern-day cataract surgery: can we match growing expectations? M Rana and S Shah Br J Ophthalmol 2014 98: 1313-1314 originally published online July 17, 2014

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Modern-day cataract surgery: can we match growing expectations?

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