Accuracy in determining intraocular lens dioptric power assessed by interlaboratory tests

Prediction of spectacle refraction uncertainties with discrete IOL power steps and manufacturing tolerances according to ISO using a Monte Carlo model

PURPOSE

The purpose of this study was to develop a concept for predicting the effects of both discrete intraocular lens (IOL) power steps (PS) and power labelling tolerances (LT) on the uncertainty of the refractive outcome (REFU).

DESIGN

Retrospective non-randomised cross-sectional Monte Carlo simulation study.

METHODS

We evaluated a dataset containing 16 669 IOLMaster 700 preoperative biometric measurements. The PS and the delivery range of two modern IOLs (Bausch and Lomb enVista and Alcon SA60AT) were considered for this Monte Carlo simulation. The uncertainties from PS or LT were assumed to be normally distributed according to ±½ the IOL PS or the ISO 11979 LT. REFU was recorded and analysed for all simulations.

RESULTS

With both lenses the REFU from discrete PS ranged from 0.11 to 0.12 dpt. Due to the larger PS for low/high power lenses with the enVista/SA60AT, REFU is more dominant in initially myopic/hyperopic eyes. REFU from LT ranged from 0.18 to 0.19 dpt for both lenses. Since LT increases stepwise with IOL power, REFU is more prevalent in initially hyperopic eyes requiring high IOL power values, and for lenses with a wide delivery range towards higher powers.

CONCLUSIONS

Since surgeons and patients are typically aware of the effect of discrete PS on REFU, these might be tolerated in cataract surgery. However, REFU resulting from LT is inevitable while the true measured IOL power is not reported on the package, leading to background noise in postoperative achieved refraction.

Intra-Observer and Inter-Observer Variability of Intraocular Lens Measurements Using an Interferometry Metrology Device

The NIMO TEMPO (Lambda-X, Nivelles, Belgium) is a novel, user-friendly and compact device designed for in vitro optical analysis of refractive and diffractive intraocular lenses (IOLs). This device analyzes the IOL wavefront and generates a synthetic eye model for numerical computation. The objective of this study was to evaluate the precision of this innovative device. Intra- and inter-observer variability were calculated using a two-way analysis of variance (ANOVA) after conducting ten measurements of eight different IOL models, with each measurement being repeated by three distinct operators (resulting in a total of 30 measurements for each IOL). The device demonstrated satisfactory intra- and inter-observer variability in evaluating IOL power and modulation transfer function (MTF) profiles, with values of 0.066 and 0.078 diopters for IOL power and 0.018 and 0.019 for MTF measurements, respectively. Furthermore, this hybrid optical and numerical in vitro IOL wavefront analyzer appears to have several advantages over conventional optical bench devices. It reduces the need for operator manipulation, and allows for numerical modeling of various optical environments, including cornea models and apertures. In conclusion, this novel metrology device designed for refractive and diffractive IOLs appears to provide a satisfactory precision, making it a promising tool in the field of IOL metrology.

Versatile optical setup customized to verify the quality of spherical and aspheric intraocular lenses

This paper proposes a customized and versatile optical setup to evaluate the optical performance of different commercially available intraocular lenses (IOLs). The setup was used to measure the wavefront error induced by different IOL models, verifying and analyzing the magnitude of low- and high-order aberrations induced by currently available IOLs. Independent analyses included non-toric spherical and aspheric IOLs from three different manufacturers. Three different dioptric powers were tested: 15, 20, and 25 D from each model. Dioptric power, induced cylinder, and spherical aberration were measured in each tested lens. All lenses had dioptric power within the tolerated margin of error. Induced cylinder was also within the current standards and clinically irrelevant. Spherical aberration varied according to the analyzed dioptric power and to the IOL model. We reported on the importance of the plane where the spherical aberration is reported, IOL or cornea. All analyzed IOLs were within current standards for the dioptric power and induced cylinder. Spherical IOLs had higher spherical aberration measured at the IOL plane.

Comparative evaluation of refractive outcomes after implantation of two types of intraocular lenses with different diopter intervals (0.25 diopter versus 0.50 diopter)

Background

Intraocular lenses (IOLs) with different diopter (D) intervals may have different tolerance, and may provide different accuracy of refractive outcome after cataract surgery. The aim of the study is to compare the accuracy of refractive outcome after implantation of IOLs with different D intervals after cataract surgery.

Methods

A total of 80 eyes from 40 patients who underwent phacoemulsification with implantation of a 0.50 D interval Akreos AO IOL in one eye and a 0.25 D interval Softec HD™ IOL in the other eye were enrolled. The percentages of eyes with refractive prediction error within ±0.50 D at one month after surgery were compared. To evaluate the effect of the dioptric errors of the IOL itself on refractive prediction error, the percentage of eyes with refractive prediction error within ±0.25 D of the IOL with a standard deviation (SD) of ±0.40 D was compared with that of the IOL with a SD of ±0.11 D through Monte Carlo simulations.

Results

In this clinical study, the percentage of eyes with refractive prediction error within ±0.50 D by the Haigis formula in the Softec HD™ group (85.0%) was significantly greater than that in the Akreos AO group (57.5%; P = 0.027). In Monte Carlo simulations, all percentages of eyes with refractive prediction error within ±0.25 D by the Haigis and SRK/T formulas in the Softec HD™ group were significantly greater than those in the Akreos AO group.

Conclusions

The IOL with a 0.25 D interval was more accurate than the IOL with a 0.50 D interval in predicting refractive outcome after cataract surgery.

Trial registration

Current Controlled Trials KCT0002192, Retrospectively registered (Date of registration: 6 January 2017).

Determination of Personalized IOL-Constants for the Haigis Formula under Consideration of Measurement Precision

The capabilities of a weighted least squares approach for the optimization of the intraocular lens (IOL) constants for the Haigis formula are studied in comparison to an ordinary least squares approach. The weights are set to the inverse variances of the effective optical anterior chamber depth. The effect of random measurement noise is simulated 100000 times using data from N = 69 cataract patients and the measurement uncertainty of two different biometers. A second, independent data set (N = 33) is used to show the differences that can be expected between both methods. The weighted least squares formalism reduces the effect of measurement error on the final constants. In more than 64% it will result in a better approximation, if the measurement errors are estimated correctly. The IOL constants can be calculated with higher precision using the weighted least squares method.

Assessing the effect of laser beam width on quantitative evaluation of optical properties of intraocular lens implants

The design and manufacture of intraocular lenses (IOLs) depend upon the identification and quantitative preclinical evaluation of key optical properties and environmental parameters. The confocal laser method (CLM) is a new technique for measuring IOL optical properties, such as dioptric power, optical quality, refractive index, and geometrical parameters. In comparison to competing systems, the CLM utilizes a fiber-optic confocal laser design that significantly improves the resolution, accuracy, and repeatability of optical measurements. Here, we investigate the impact of changing the beam diameter on the CLM platform for the evaluation of IOL dioptric powers. Due to the Gaussian intensity profile of the CLM laser beam, the changes in focal length and dioptric power associated with changes in beam diameter are well within the tolerances specified in the ISO IOL standard. These results demonstrate some of the advanced potentials of the CLM toward more effectively and quantitatively evaluating IOL optical properties.

Noncontact common-path Fourier domain optical coherence tomography method for in vitro intraocular lens power measurement

We propose a novel common-path Fourier domain optical coherence tomography (CP-FD-OCT) method for noncontact, accurate, and objective in vitro measurement of the dioptric power of intraocular lenses (IOLs) implants. The CP-FD-OCT method principle of operation is based on simple two-dimensional scanning common-path Fourier domain optical coherence tomography. By reconstructing the anterior and posterior IOL surfaces, the radii of the two surfaces, and thus the IOL dioptric power are determined. The CP-FD-OCT design provides high accuracy of IOL surface reconstruction. The axial position detection accuracy is calibrated at 1.22 μm in balanced saline solution used for simulation of in situ conditions. The lateral sampling rate is controlled by the step size of linear scanning systems. IOL samples with labeled dioptric power in the low-power (5D), mid-power (20D and 22D), and high-power (36D) ranges under in situ conditions are tested. We obtained a mean power of 4.95/20.11/22.09/36.25 D with high levels of repeatability estimated by a standard deviation of 0.10/0.18/0.2/0.58 D and a relative error of 2/0.9/0.9/1.6%, based on five measurements for each IOL respectively. The new CP-FD-OCT method provides an independent source of IOL power measurement data as well as information for evaluating other optical properties of IOLs such as refractive index, central thickness, and aberrations.

A simple confocal fibre-optic laser method for intraocular lens power measurement

PURPOSE

To develop novel confocal fibre-optic laser method (CFOLM) for accurate and objective measuring of the dioptric power of both positive and negative intraocular lenses (IOLs).

METHODS

The CFOLM principle of operation is based on a simple apertureless single-mode fibre laser confocal design. The key element is a single-mode fibre coupler that serves simultaneously as a point light source (3-5 microm fibre diameter) used for the formation of a collimated Gaussian beam, and as a confocal point receiver that is highly sensitive to spatial displacements of the focused backreflectance laser emission. The basic CFOLM systems include IOL testing set-ups for the measurement of both positive and negative IOLs.

RESULTS

The CFOLM designs provide high accuracy (

CONCLUSIONS

The presented IOL power testing method offers a simple, accurate, objective, quick, and relatively inexpensive approach for dioptric power measurement of positive and negative IOLs. It provides an independent source of IOL power measurement data and information for evaluating the effectiveness and safety of novel IOL products.

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Key Words

• intraocular lens dioptric power
• confocal fibre-optic laser method
• single-mode fibre coupler

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MESH Headings

• Diagnostic Techniques, Ophthalmological/instrumentation
• Equipment Design
• Fiber Optic Technology/methods
• Humans
• Lasers
• Lenses, Intraocular
• Optics and Photonics

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Grants

• FD999999 Intramural FDA HHS

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Affiliation(s)

I K Ilev Division of Physics, Office of Science and Engineering Laboratories, Center for Devices and Radiological Health, US Food and Drug Administration, Rockville, MD 20857, USA.

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Using the lens haptic plane concept and thick-lens ray tracing to calculate intraocular lens power

PURPOSE

To develop a methodology for intraocular lens (IOL) power calculation in which the task of predicting the postoperative position of the IOL is separated from the calculation itself.

SETTING

Pharmacia, Groningen, The Netherlands.

METHODS

The minimum biometry input needed for IOL power calculation is the mean anterior corneal radius and axial length of the eye. The lens haptic plane (LHP) is the plane where the IOL haptics make contact with eye tissue. It is an anatomical site (eg, the equator of the capsular bag) and is independent of the IOL model. The position of the IOL optic in relation to the LHP is determined from the exact design of the IOL. Gullstrand's eye model is adopted to obtain the posterior corneal radius, thickness of the cornea, and refractive indices of the eye media. Thick-lens ray tracing in the paraxial limit is used for the optical calculation.

RESULTS

A spreadsheet is given for the calculation.

CONCLUSIONS

The methodology developed allows for IOL power calculation from first principles (ie, using true physical distances, radii, and refractive indices as input for the optical calculation).

Multicenter biometry study of 1 pair of eyes

PURPOSE

To assess the accuracy of biometry for intraocular lens (IOL) power calculation.

SETTING

Six ophthalmic surgery centers in Europe and 1 in the United States.

METHODS

Biometry was done as if in preparation for cataract surgery in 2 eyes of the same person with 10 combinations of operator and instrument. Data analysis followed a standardized procedure to assess repeatability (within-center variability) and reproducibility (between-center variability) of the test methods.

RESULTS

The reproducibility of the corneal radius measurement was 0.06 mm. The use of different keratometric indices made conversion to K-values less reliable. The repeatability of the axial length (AL) measurement of 0.30 mm and the reproducibility of 0.66 mm converted to a calculated IOL power of 0.75 diopter (D) and 1.65 D, respectively. Thus, this potential patient runs the risk of a refractive surprise of up to 1.10 D purely as a result of a measurement error within a center and up to 2.30 D if the patient goes to the worst-case center.

CONCLUSIONS

The biometry results show that the measured corneal radius can be used with confidence. Reproducibility errors in AL determination require personalization of formula constants or correction at the source by proper calibration.

Design and evaluation of a null lens for testing the optical performance of silicone intraocular lenses

A two-element null lens, optimized for a 20-diopter silicone intraocular lens (IOL), was designed and tested. The performance of the null lens was examined for silicone IOL's with in situ powers from 16 to 24 diopters. Improvements in resolution, resolution efficiency, and modulation transfer function were obtained over the tested power range.

Evaluation of an unused 1952 Ridley intraocular lens

PURPOSE

To evaluate an unused 1952 historic Ridley intraocular lens (IOL) brought to Bombay, India, in 1952 from an Oxford Ophthalmologic Conference in England and given to 1 of the authors during his residency.

SETTING

Alcon Laboratories, Fort Worth, Texas, USA.

METHODS

The Ridley IOL was evaluated at Alcon Laboratories, Inc., using the established procedures of its Intraocular R&D Laboratories. Various optical and physical aspects of the Ridley lens were evaluated including (1) dimensions, (2) weight, (3) power, (4) resolution efficiency and modulation transfer function (MTF), (5) surface sphericity by interferometry, (6) ultraviolet (UV)-visible transmission characteristic, (7) attenuated total reflectance (ATR)-Fourier transform infrared reflectance spectrum, and (8) cosmetics by visual inspection using light microscopy.

RESULTS

This 8.5 mm diameter, 2.4 mm thick, 23 diopter biconvex IOL weighed 108 mg. The ATR spectrum, UV-visible transmission, and refractive index confirmed its poly-(methyl methacrylate) material. The 0.56 MTF value at 100 line pairs/mm, per the International Standards Organization--IOL Optics Standard, and 93% resolution efficiency in water, per the American National Standard Institute IOL Optics Standard, revealed the IOL's excellent optics. This was confirmed by 0.278 wave root mean square surface figure as measured by Zygo interferometer using a 633 nm wavelength. Visual inspection revealed rough edges with sharp corners and some surface scratches. Early clinical experience with Ridley IOLs in Bombay, India, is briefly given.

CONCLUSION

The Ridley IOL had excellent optical quality, meeting the requirements of current IOL optics standards. The selection of its dimensions was guided by the human crystalline lens, and the Ridley IOL was half as bulky. Although its clinical results were mixed, successful cases inspired subsequent improvements, leading to modern, highly satisfactory IOLs. This IOL represented a revolutionary innovation in ophthalmology.