Experimental Eye Research 140 (2015) 190e192
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Experimental Eye Research journal homepage: www.elsevier.com/locate/yexer
Methods in eye research
A rapid method for measuring intraocular lens power in vitro with a focimeter Mari Carmen García-Domene a, b, *, María Amparo Díez-Ajenjo a, b, María Artigas b Cristina Peris-Martínez a, Amparo Navea a, Jose a b
n Pío Baroja-General Aviles, s/n, 46015 Valencia, Spain FISABIO-Oftalmología M edica, Bifurcacio Universitat de Val encia, C/Dr. Moliner, 50, 46100 Burjassot, Valencia, Spain
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 May 2015 Received in revised form 11 September 2015 Accepted in revised form 15 September 2015 Available online 18 September 2015
In this paper we describe a new method for measuring the intraocular lens (IOL) power using a focimeter, a negative ophthalmic lens and a saline solution (0.9% NaCl). To test this we measured the power of 58 different IOLs and we compared them with the power stated by the manufacturer. Despite the limitations, the results show a good correlation. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Power Intraocular lens Method Focimeter Ophthalmic lens
1. Introduction The ISO 11979-2:2014 Annex A (normative) recommends some procedures to ascertain an IOL power which depend on the design of the IOL: monofocal, toric, or multifocal. These methods are: 1.1 Determining the radius over a 3 mm diameter using a radius meter, interferometer, or optical coherence tomography, measuring the lens thickness with a micrometer. This procedure can be used for monofocal and toric IOLs (measuring in the principal meridians). 1.2 Measuring back focal/effective focal length To obtain the back focal length (BFL), the distance from the back vertex of the IOL to the focal point is measured with a parallel light incident on-axis on the IOL. The effective focal length (EFL) is the distance from the second principal plane to the focal point with parallel light incident on-axis
dica, Bifurcacio n Pío Baroja* Corresponding author. FISABIO-Oftalmología Me General Aviles, s/n, 46015 Valencia, Spain. E-mail address:
[email protected] (M.C. García-Domene). http://dx.doi.org/10.1016/j.exer.2015.09.009 0014-4835/© 2015 Elsevier Ltd. All rights reserved.
on the IOL. The EFL can be measured with a nodal slide bench. These methods are better for conditions in the air and are suitable for all IOLs. 1.3. Measuring magnification Magnification can be determined by using an optical bench, a target with a measurable linear dimension, and an eye-piece that can measure the corresponding linear dimension on the image at the EFL position. This method can be used for all IOLs. The literature reports how to ascertain the IOL power by measuring the focal length with a confocal fiber-optic laser (Ilev, 2007) or the radius of the IOL surfaces using optical coherence tomography (Huang et al., 2011). In addition, there are some devices that can measure the IOL power automatically, for example the PMTF (Lambda-X, Nivelles, Belgium) or NIMO TR0815 (LambdaX, Nivelles, Belgium). Furthermore, we know that some authors have used a wet cell and a focimeter to determine the power of contact lenses (Pearson and Evans, 2012). For these reasons, in the present study, we describe a fast and inexpensive method for measuring the IOL power using a focimeter.
M.C. García-Domene et al. / Experimental Eye Research 140 (2015) 190e192
2. Materials and supplies
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Table 1 Distribution and power of the different measured IOLs.
The material used in our method is a focimeter (Magnon LM-350, Nidek Co. Aichi, Japan) with source of 546 nm, a negative ophthalmic lens (meniscus form, 10 D, with a refraction index of 1.7), and saline solution (NaCl 0.9% B.Braun, Melsungen AG, Germany). To validate this method, we measured the lens power of 58 different IOLs. Each IOL power was measured three times and by the same person.
Monofocal
Positive Negative
Toric
Multifocal
Number
Power (D)
Number
Power (D)
Number
Power (D)
44 3
þ20 ± 5 7 ± 1
2 1
19 ± 1 8D
8 0
þ22 ± 2
Statistical analysis to determine the concordance was made with a Passing-Bablock test since the distribution of the sample wasn't normal (Passing and Bablok, 1983).
3. Detailed methods Previous to the measurements, the focimeter was calibrated measuring 8 ophthalmic lenses of different power (±14D ±10D ±5D and ±0.5D). For the experimental procedure the focimeter was placed in a vertical position (Fig. 1a) with the negative lens with its concave surface facing upward and the saline solution inside the lens to make a “wet cell” where we placed the IOL, we have centered the target of the focimeter as accurately as possible (Fig. 1 b). Traditional focimeters can measure powers from þ25 D to 25 D in steps of 0.25 D. An IOL power is usually more than þ25 D, so using a negative lens of 10 D plus saline solution we added þ4.5 D to the power range, since, if we focused on the focimeter target with the negative lens immersed in saline solution, a power reading of 4.5 D was obtained instead of zero. The usual measurement procedure for a focimeter was used. First we have to center the target just with the divergent lens plus saline solution, then we introduce the IOL and we center again the target by moving the IOL, this way lenses are aligned. The real IOL power is the result of subtracting 4.5 D from the focimeter reading. In order to check this method, we measured the power of 58 IOLs of different designs and materials e spherical, aspheric, toric and multifocal, phakic and aphakic (see Table 1). The mean IOL power was þ19 ± 9 D and we compared the results with the IOL power stated by the manufacturer. We used the mean power for far and near focus for multifocal IOLs and the mean power of the two principal meridians for toric IOLs. 4. Results The mean of the experimental power was þ19 ± 9 D and the
Fig. 2. Linear fit between the measured IOL power and the labeled power stated by the manufacturer.
mean difference from the stated power was þ0.3 ± 0.3D. The error made in the three measurements of the power of each IOL was smaller than the minimum step in the lensmeter scale (0.25 D). We obtained the same value in all cases for the three measures except in the near focus of one multifocal lens since to having two focus (one out defocused) the measurement was more difficult with an standard deviation of 0.14 D. So the repeatability of the measurement was practically 100%. Fig. 2 shows the result of comparing both powers (measured and labeled). As we can observe, there was a good linear correlation (r ¼ 0.99) as well as a slope of nearly 1. The sample was normal, so we carried out a T-test. Statistics showed that the mean was
Fig. 1. Experimental device: focimeter in a vertical position (a) and negative lens with saline solution (b).
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M.C. García-Domene et al. / Experimental Eye Research 140 (2015) 190e192
A previous study uses a focimeter to measure the power of the contact lens (Pearson and Evans, 2012), and here in this study we present a modified method to measure an IOL power. With an ophthalmic lens of 10 D and saline solution we can increase the range power by þ4.5 D, which is necessary for high IOL powers. However, with a more powerful ophthalmic lens we can extend this range. The results show a good linear fit between the measured and labeled IOL powers assuming that both are similar since the slope is practically 1 (Fig. 2). We have also a good concordance between the measured and labeled power. Although they are statistically different (p < 0.05), the ISO normative regarding IOL power allows an error ranging between 0.3 D (in IOLs with a power of under 15 D) and 1 D (in IOLs of over 30 D). The error in our measurements is þ0.3 ± 0.3 D, so we can conclude that our method is sufficiently accurate since the power stated is not exact. In any case, these results are promising.
Fig. 3. Result for Passing-Bablock test to analyze the concordance between the labeled and measured IOL power.
significantly different (p ¼ 0.04). In order to evaluate the concordance we have used the PassingBablock test. In Fig. 3 we can see the result, we have a good concordance since the ideal and regression lines are inside the confidence limits. As we can observe for high powers the error is higher than for the rest. By other hand we have some negative powers outside the limits, that could be due because we have a few lens with negative power. 5. Potential pitfalls and trouble shooting In this paper we describe a new, accurate, inexpensive, fast, and easy-to-carry-out IOL power measurement method for any type of intraocular lens using a focimeter. With this method, it is easy to find the principal meridians in an astigmatic IOL, or a far and near focus in a multifocal IOL, of an either symmetric or asymmetric design.
Financial disclosure The authors have no relevant financial interests in this study. Acknowledgment We wish to thank the C atedra Alcon-Universitat de Valencia for their support. References Huang, Y., Zhang, K., Kang, J.U., Calogero, D., James, R.H., Ilev, I.K., 2011. Noncontact common-path Fourier domain optical coherence tomography method for in vitro intraocular lens power measurement. J. Biomed. Opt. 16, 126005. Ilev, I.K., 2007. A simple confocal fibre-optic laser method for intraocular lens power measurement. Eye 21, 819e823. Pearson, R.M., Evans, B.J., 2012. A comparison of in-air and in-saline focimeter measurement of the back vertex power of spherical soft contact lenses. Ophthalmic Physiol. Opt. 32, 508e517. Passing, H., Bablok, W., 1983. A new biometrical procedure for testing the equality of measurements from two different analytical methods. Application of linear regression procedures for method comparison studies in Clinical Chemistry, Part I. J. Clin. Chem. Clin. Biochem. 21, 709e720.
