In standard cataract surgery, one of the major goals is to reach target refraction. Based on keratometry measurements, axial length and anterior chamber depth, most of the intraocular lens calculation formulae are suitable to achieve this aim. Further evaluation of corneal refractive parameters like anterior and posterior corneal surface by Scheimpflug devices led to a significant enhancement of precision in astigmatic and post-refractive surgery cases.
Cataract surgery, intraocular lens (IOL), imaging, corneal refractive power analysis
Fritz H Hengerer, Gerd U Auffarth and Ina Conrad-Hengerer declare no financial interests in the topic mentioned in this article
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.
October 27, 2017
December 08, 2017
Ina Conrad-Hengerer, Department of Ophthalmology, Ruprecht-Karls-University Heidelberg, Im Neuenheimer Feld 400, 69120 Heidelberg, Germany. E: firstname.lastname@example.org
The publication of this article was supported by Oculus.
Cataract surgery had undergone major improvements in different areas over the last 10 years. New intraocular lens (IOL) designs and fourth generation IOL formulae are available, allowing spectacle independence for many patients. Femtosecond laser-assisted cataract surgery (FLACS) has been introduced, and the options seen on television and web-based sources have increased patients’ understanding and raised their expectations. Cataract surgeons, as well as the manufacturers of optical biometers and diagnostic equipment, recognized this and consider the corneal optical conditions and evaluate possible ocular surface diseases.
In naïve eyes with senile cataract measurement of basic parameters like axial length (AL), keratometry and anterior chamber depth (ACD) were used to calculate the IOL power prior cataract surgery. Many cataract surgeons recognise these needs today while performing FLACS by: using new aspheric, toric and multifocal IOL designs; minimising incision size; and taking advantage of the new fourth generation IOL calculation formulae. Whilst the aim of this article is not to compare the precision and benefits of new IOL power formulae, this review will look beyond this, while pointing out other sources which may influence the quality of patients’ vision.
In Table 1, a summary of fourth generation to standard formulae and their applications are listed.
These formulae reduce mean absolute error (MAE), meaning more patients achieve final results within 0.5 D, 0.75 D and 1 D of the expected target refraction. But is this still enough to satisfy all patients’ expectations today?
Beyond intensive and individual patient consultation with regard to premium IOLs, such as multifocal or multifocal toric IOLs, intensive pre-operative assessment of corneal and retinal conditions is indispensable.
In recent years, many different optical biometers have been launched to provide, besides the basic necessary parameters like AL and anterior keratometry (ant K’s), additional information such as ACD, posterior keratometry (post K’s), total corneal power (TCP) and total corneal refractive power (TCRP), lens thickness (LT), horizontal-white-to-white (HWTW), for IOL power calculation. For an enhanced evaluation of the corneal shape some devices provide topography and tomography. Using this additional information, it is now possible to produce a more precise IOL power calculation and an enhanced preop assessment before performing premium cataract surgery. Table 2 lists the currently available optic biometers.
In our university eye clinic, we are using the latest, to date, optical coherence tomography (OCT), the IOL Master 700® (Zeiss, Oberkochen, Germany) and the new Pentacam® AXL (Oculus, Wetzlar, Germany). The Pentacam AXL is a Scheimpflug-based anterior segment tomographer with a built-in optical biometer. The Pentacam has proven to provide precise keratometry of the anterior and posterior corneal surface, which is the key-parameter for accurate IOL power calculation.1–7 The latest studies published, demonstrated a perfect correlation of AL measurements performed with IOL Master 500, IOL Master 700 and Pentacam AXL, as well as a high precision of AL, ACD and corneal curvature.8,9
The main part of our pre-cataract screening routine is focused on objective assessment of the ocular surface, the cornea, anterior chamber and crystal lens conditions. Modern tomographers, like the Pentacam, support us in detecting forme fruste keratoconus (FFKC), past refractive surgery and corneal diseases such as Fuchs endothelial dystrophy, or signs of dry eye prior to cataract surgery.10–17 Assessing the crystaline lens density helps in optimising the settings for femtosecond lasers,18–20 in order to reduce the total amount of laser and ultrasonic energy, to reduce the stress for the corneal endothelium, and the surgery time.
For our premium IOL patients we pay high attention to possible ocular surface diseases prior cataract surgery. For many years Schirmer test I and II were used to quantitatively evaluate the amount of tear film. Today this is no longer sufficient. The Schirmer test gives no information about the quality of the tear film, and can be painful for the patients, too. There is common consent that quality of vision is strongly related to ocular surface quality.21–23 Sufficient non-invasive tear film break-up time (NIBUT), as well as active Meibomian glands, provide the basis for an excellent visual quality outcome after cataract surgery. Moreover, an intuitive summary of the different measurements is key in busy clinical and operative settings. The JENVIS Dry Eye Report, based on the measurements performed with the Keratograph 5K (Oculus, Wetzlar, Germany), is an excellent example on how to present measured values relative to normal data in a clear and easy style. Figures 1 and 2 show a patient with severe ocular surface diseases before and after treatment with dexamethasone eye drops and antibiotic eye drops, as well as Acular® (Allergan, Dublin, Republic of Ireland).
The next challenge is selection of the best-suited IOL for every patient. Therefore, intensive patient consultation helps us understand their habits, their way of living, future plans and, most important, their individual expectations. But can we always satisfy all these?
Sometimes yes, sometimes no. During cataract surgery the crystal lens is removed leaving the cornea as the main optical and refractive part. The assessment of corneal optical quality is one key factor in customised IOL selection. Clinicians and surgeons like and adhere to clear routines. The Cataract Pre-OP Display, developed by Naoyuki Maeda from Japan, is a good example for this. He suggests a four-step screening routine, prior to premium cataract surgery (see Table 3).24
IOL calculators should be designed to be intuitive and avoid potential misinterpretation and false entries by the user. Having one IOL calculator providing IOL power calculation formulae for every single imaginable case, is still the dream for cataract surgeons. But manufacturers for such devices have made progress.
Online calculators provided by the manufacturers of toric IOLs are different. Some still use a fixed ratio for the cylinder power at the cornea, and the IOL plane of 1.46, for example. Newer calculators use algorithms to estimate the individual effective lens position, others allow entering the data of the posterior cornea and some use nomograms to estimate the net corneal astigmatism like for example the Barrett toric calculator or the Abdulafi-Koch formula.25,26 Another growing group of patients are those who have previously undergone refractive surgery such as myopic or hyperopic laser-assisted in situ keratomileusis (LASIK) or photorefractive keratectomy (PRK). For the majority of these patients no data prior to refractive surgery is available. With thanks to Dr Douglas Koch, Dr Warren Hill and Dr Li Wang, the ASCRS online calculator was created and further developed (http://iolcalc.ascrs.org). Using it properly, it offers a solution to many of these patients, including those who underwent radial keratotomy (RK) presenting with highly irregular corneal shape.
The Pentacam AXL includes an IOL calculator that covers all needs in our university clinic, as we have to deal with all kinds of eyes. The standard third generation formulae such as Hoffer Q, Holladay 1, SRK/T and Haigis, and the Barrett Universal 2 are useful for monofocal and multifocal IOL calculations. Since every formula except the Barrett Universal 2 have shown limitations in correct prediction of the expected post-op refraction with regard to AL,27 we should take advantage of the recent ones and compare our standard method and formulae for IOL power calculations, to the latest available formulae.
IOL power calculation for post-refractive patients requires special formulae which are also included in the latest software release of the Pentacam. The known double-K method developed by Aramberri,28 and the Barrett True K,29 requires the keratometry prior to surgery, which also requires the spherical equivalent (SEQ) prior surgery, support our daily work if historical data is available. For the majority we are using no-history formulae, such as the PotvinShammasHill,30 which is the modified Shammas formula for post-myopic LASIK patients. Although rare, we still encounter patients who have previously undergone RK. Due to loss of any pre-operative refractive data, followed by pure measurement of these highly abnormal corneas, all standard biometers and topographers will often fail. Scheimpflug technology therefore has its benefit. The PotvinHill31 formula is a no-history formula and can be used for these patients. Even if the mean error appears relatively low, we still have to deal with outliers with +/-0.5 D or +/-1 D, limiting the patient’s expectations.
The hottest topic today is toric IOL power calculation. Studies from Fityo et al.32 and Koch et al.33 have shown the influence of the posterior astigmatism with regard to the total corneal astigmatism.32,33 The Savini toric calculator is based on the TCRP, which considers the individually measured posterior cornea. First studies have shown promising outcomes, but further clinical investigation is needed.34,35 In large sample studies,34,35 the latest formulae such as Barrett toric,25 have shown the smallest MAE, and most patients are within the expected post-operative refraction. However, there are still outliers where posterior corneal surface has an influence not present in nomograms.
Figure 3 shows the calculation with the Savini toric calculator for a patient having an astigmatism with the rule. The IOL implanted was an TECNIS ZCT300 (Abbott Medical Optics, Santa Ana, CA, US), with an SEQ of 16.5 D. In this particular case the expected post-operative refraction was 0.18 D with -0.07 D at 168°. The patient’s subjective refraction was plano post-operative.
Pre-operative refractive data for our patients should be available in the operating theatre, ideally paperless. The IOL Calculator provides a readable pdf file for all electronic medical systems (EMR) systems. Its network compatibility allows last-minute calculations as well as the intuitive entry of used IOL data directly after cataract surgery. Moreover, Pentacam AXL can be linked to Leica microscopes (Leica Microsystems GmbH, Wetzlar, Germany) and True Vision software (TrueVision® Systems Inc., Santa Barbara, CA, US) allowing a superimposition of the implantation axis into the ocular of the microscope, and an eye-tracking system based on iris structures or blood vessel recognition. In combination with an eye-tracking system, a more precise toric IOL positioning can be achieved.
We usually see our patients 1 and 4 weeks after surgery. Study patients have to attend more often to have a close follow-up. Careful refraction of our patients is key to track and improve our outcomes. Usually subjective refraction is performed using trial frames or phoropters; however, only evaluation of visual acuity values lacks information about optical quality. The subjective impression is the only parameter, which matters for the patient. During pre-operative conversation patients´ expectations can be evaluated and adjusted; however, there may be still some complaints from the patient. To objectively quantify them we exam those patients to evaluate the tear film conditions, and we test contrast sensitivity with and without glare. We found a strong tendency between the tear film quality and contrast sensitivity related to the subjective impression of the patients. Studies are running to understand these relations better.
In conclusion, we would state that advanced pre- and post-operative diagnostic for modern cataract surgery has to go beyond standard biometry, standard tests for dry eye, and subjective refraction. Objective parameters and clinical routines have to be established in order to better meet patients’ expectation and provide the best possible way of care for them.