Since the introduction of phacoemulsification, cataract surgery has evolved remarkably. The use of premium intraocular lenses (IOLs) (aspheric, toric, multifocal), refractive lens exchange and patients after refractive surgery procedures require extremely precise clinical measurements and IOL calculation formulas to achieve desired postoperative refraction. For many years, ultrasound biometry has been the standard for measurement of ocular parameters. The introduction of optical biometry (fast and non-invasive) has replaced ultrasound methods and is now considered as the clinical standard for ocular biometry. Recently, several modern optical instruments have been commercially launched and there are new methods available, including the empirical, analytical, numerical or combined methods to determine IOL power. The aim of this review is to present current techniques of ocular biometry and IOL power calculation formulas, which will contribute to achieve highly accurate refractive outcomes.
Biometry, ocular biometry, optical biometry, optical biometry devices, intraocular lenses, IOLs, IOL power calculation, IOL power calculation formulas
Magdalena Turczynowska, Katarzyna Koźlik-Nowakowska, Magdalena Gaca-Wysocka and Andrzej Grzybowski have nothing to disclose in relation to this article. No funding was received in the publication of this article. This study involves a review of the literature and did not involve any studies with human or animal subjects performed by any of the authors.
Authorship:-All named authors meet the International Committee of Medical Journal Editors (ICMJE) criteria for authorship of this manuscript, take responsibility for the integrity of the work as a whole, and have given final approval to the version to be published.
August 16, 2016 Accepted
September 18, 2016
Magdalena Turczynowska, Department of Ophthalmology, Stefan Żeromski Specialist Municipal Hospital in Kraków, os. Na Skarpie 66, 31-913 Kraków, Poland. E: firstname.lastname@example.org
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.
An erratum to this article can be accessed by the link below.
Cataract surgery is currently the most frequently performed surgical technique worldwide. Since the introduction of phacoemulsification by Kelman in 1967, surgical technology and construction of implanted intraocular lenses (IOLs) have undergone considerable improvement. Small, sutureless incisions and the use of foldable intraocular lenses reduced the incidence of complications and surgically induced astigmatism.1,2 Furthermore, the use of premium intraocular lenses (aspheric, toric, multifocal or a combination) allows the patient to become fully spectacle-independent.3 The improvement of surgical treatment results in rising expectations of patients. The key issue is to achieve the desired refractive outcome. Essential for this purpose are precise measurements of the eye and selection of the optimal IOL calculation formula. The aim of this article is to present current techniques of ocular biometry and IOL power calculation formulas, which will contribute to achieve highly accurate refractive outcomes.
The first step to achieve satisfactory postoperative refractive outcome is accurate ocular biometry. Biometry enables the measurement of the various dimensions of the eye, including axial length (AL), anterior chamber depth (ACD), lens thickness (LT) or central corneal thickness (CCT). These values, together with the keratometry are essential for the IOL power calculation. Precision of measurements is crucial, as a 0.1 mm error in AL results in a refractive error of about 0.27 diopter (D).4
Ultrasound biometry For many years, the only way to measure the AL of the eye was with ultrasound biometry. This technique measures the distance from the surface of the corneal apex to the internal limiting membrane (ILM). Good alignment along the ocular axis is important and that requires patient cooperation (which can be difficult in children or patients with mental disorders). In cases where a probe has direct contact with the cornea, there is a risk of a corneal damage or infection. Therefore, a topical anaesthetic and proper disinfection of the probe are required. Occurring inter-individual differences are highly dependent on the pressure exerted on the eye by the ultrasound probe. High pressure results in corneal indentation and shortening of the AL. Immersion ultrasound minimises the indentation of the cornea as it uses a saline-filled shell between the probe and the eye. Clinical studies have shown that immersion biometry is more accurate and more reliable than ultrasound biometry performed in contact mode.5–8 A limitation of ultrasound biometry is low image resolution, as a consequence of using a long, low-resolution wavelength (10 MHz) to measure small dimensions. In addition, differences in retinal thickness near the fovea or the presence of other macular pathologies contribute to inconsistent measurements.9,10
by immersion ultrasound biometry,11 but this new method is fast, easy to reproduce by different examiners, non-invasive and non-contact. Repeatability and reproducibility of measurements obtained using this technique are high and the results are less dependent on operators’ skills. However, it is difficult to obtain a measurement in the presence of a dense cataract or other opacities such as corneal scar and vitreous haemorrhage. Optical biometry measures the distance from the corneal surface to the retinal pigment epithelium (RPE). It may be associated with overestimation of measurements of about 0.15–0.5 mm.12 Optical biometry can also be successfully performed in pseudophakic or silicone oil-filled eyes. Furthermore, in high myopic eyes, due to the presence of posterior staphyloma, it may give better results than conventional ultrasound techniques for measuring the AL.
Optical biometry devices
New optical biometry devices provide measurements not only of AL but also other important variables, such as: keratometry, ACD, LT, CCT, pupil size (PS) or white-to-white distance (WTW). To measure the AL of the eye, currently available devices use different technologies. IOLMaster 500 (Carl Zeiss Meditec, Jena, Germany), AL-Scan (Nidek, Aichi, Japan) and Pentacam AXL (Oculus, Menlo Park, California, US) use partial coherence interferometry (PCI) technology. Lenstar LS 900 (Haag-Streit, Koeniz, Switzerland), Aladdin (Topcon, Tokyo, Japan), Galilei G6 (Ziemer, Port, Switzerland) and OA-2000 (Tomey GmbH, Nürnberg, Germany) use optical low-coherence interferometry (OLCR). Swept source OCT (ss-OCT), used by the IOLMaster 700 (Carl Zeiss Meditec, Jena, Germany) and ARGOS (Movu, Santa Clara, California, US) devices, is the newest technology to be implemented in biometry.
The IOLMaster 500 was the first optical biometer and was introduced in autumn 1999. The device is based on the PCI principle and measures AL using infrared light (λ=780 nm) of short coherence emitted by semiconductor laser diode. Furthermore, it measures keratometry, analysing the anterior corneal curvature at six reference points at approximately 2.3 mm optical zone. The ACD is measured using slit-lamp illumination and is defined as a distance from the corneal epithelium and to the anterior lens surface. WTW is obtained by analysing the image of the iris using an infrared light source (wavelength 880 nm). All measurements are performed simultaneously. IOLMaster 500 is currently considered as a gold-standard biometer.13–15 Its repeatability and reproducibility have been assessed in several studies.16–19
AL-Scan uses an 830 nm infrared laser diode for AL measurement with PCI. It also measures keratometry (K) at 36 measurement points in two circles with diameters of 2.4 mm and 3.3 mm, reflected from the corneal surface. WTW and PS are obtained by analysing the image of the iris and fitting the best circle with the lowest error square to the detected edge. ACD and CCT are measured with an incorporated Scheimpflug camera with a 470 nm monochromatic light. The device was introduced for clinical practice in Europe in 2012. Srivannaboon et al.20 compared the repeatability and reproducibility of ocular biometry and IOL power obtained with AL-Scan and IOLMaster 500. AL-Scan provided excellent repeatability and reproducibility for all measured parameters (AL, K, ACD and WTW). Agreement with the IOLMaster 500 was good except for the WTW. This can be caused by different algorithms used by these devices for edge detection around iris image. Furthermore, the light source used for WTW measurements is different: AL-Scan uses a green light source (wavelength 525 nm) and IOLMaster uses an infrared light source (wavelength 880 nm). Kaswin et al.21 evaluated the agreement in AL, K, ACD measurements and IOL power calculations with AL-Scan and IOLMaster 500. They reported excellent correlation in AL measurements and K readings as well as good agreement in ACD measurements between these two biometers. The IOL power calculations were also highly comparable between these devices.
The Pentacam AXL device consists of a Scheimpflug camera which rotates around the eye and a PCI-based optical biometer. It was introduced in autumn 2015. In addition to anterior segment tomography, ACD, CCT and WTW measurements, corneal topography, anterior and posterior corneal surface and spherical aberrations, it also has integrated AL measurement. Calculation of toric IOLs is based on the total corneal refractive power and it takes into account the influence of the posterior corneal surface. To our knowledge, no study has yet evaluated the repeatability, reproducibility and accuracy of biometry measurements obtained using this device.
The Lenstar LS 900 biometer is based on OLCR. Using a 820 μm superluminescent diode as light source, it allows the measurement of the AL, CCT, LT and ACD. The retinal thickness can also be determined from the scans, but this requires subjective alignment of a cursor. It also uses 950 μm light to assess by image analysis central corneal curvature using two rings of diameter 1.65 mm and 2.30 mm of 16 light spot each. WTW and PS are obtained by fitting the best circle with the lowest error square to the detected edge. Optional T-cone module complements this device with a Placido topography of the central 6 mm corneal zone. Several studies confirmed Lenstar’s repeatability, reproducibility and agreement with other biometry devices. Generally, Lenstar provided results that correlated very well with those of the IOLMaster. Excellent agreement has been shown between the AL measurements taken by Lenstar and IOLMaster,22–25 but only good22 or moderate24 agreement
Optical biometry The introduction of optical biometry has steadily replaced ultrasound methods and is now considered the clinical standard for ocular biometry. The results are comparable to those achieved between these two devices in ACD measurements. In some cases, small but statistically significant differences in K and ACD measurements were reported.24 However, in a few studies, the AL measurements taken by Lenstar were slightly higher than the IOLMaster measurements, but the differences were not clinically significant.22,23 The Lenstar was unable to take measurements due to lens opacities in a similar number of patients to the IOLMaster.23
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Biometry, ocular biometry, optical biometry, optical biometry devices, intraocular lenses, IOLs, IOL power calculation, IOL power calculation formulas