Laurence W. Grossman

Determining the imaging quality of intraocular lenses

Purpose: To validate the proposed optical requirements of a draft international standard for intraocular lenses (IOLs). Setting: Eight optical testing laboratories in the United States, Germany; Japan, and The Netherlands. Methods: The testing laboratories performed modulation transfer function (MTF) tests on various IOLs using a model eye and visual resolution tests in air. Each laboratory performed duplicate measurements on a set of 43 lenses that was circulated among the testing laboratories. Results: The interlaboratory tests showed that the MTF measurements using a model eye had better repeatability and reproducibility than the more common industry practice of resolution testing in air with parallel light and the United States Air Force three‐bar target. However, the two methods correlated well. The commonly applied criterion that an IOL resolve in air at least 60% of the Rayleigh cutoff spatial frequency corresponded to a minimum requirement of 0.43 MTF units at 100 mm−1 in a model eye. Conclusions: Either criterion may be applied in accordance with a proposed international standard for IOLs. The model eye method can be applied over a broader range of dioptric powers and is relevant for materials that interact with aqueous. Both tests appear to have a greater ability to detect unwanted surface aberrations than resolution testing of IOLs in a water cell using parallel light, a method described in the current American National Standards Institute standard.

Accuracy in determining intraocular lens dioptric power assessed by interlaboratory tests

Purpose: To describe a testing program conducted by a standards group as a guide for setting international tolerances for intraocular lens (IOL) dioptric power. Setting: Multicenter study. Methods: Seven biconvex, poly(methyl methacrylate) IOLs ranging in power from 10.00 through 30.00 diopters (D) were circulated among nine participating laboratories experienced in IOL optical measurements. Each laboratory performed repeated optical tests to determine dioptric power. These results were analyzed for repeatability and reproducibility in accordance with methods specified by the International Organization for Standardization. Results: Intralaboratory repeatability was less than 0.5% of the dioptric power, and interlaboratory reproducibility, when following a normalized procedure for correction and conversion, was less than 1.0% of the dioptric power. Conclusion: Tolerance limits of ±0.30 D in the range 0 to 15.00 D, ±0.40 D for 15.50 to 25.00 D, and ±0.50 D for 25.50 to 30.00 D have been proposed as an international standard for IOLs. The contribution of IOL power error within the limits of the standard are estimated to contribute less than 1.0% to the total error in postoperative refractive prediction.

Resolution testing of intraocular lenses

ABSTRACT A basic goal associated with the manufacture of intraocular lenses (IOLs) is that they not be vision limiting. Manufacturers must therefore test IOL resolution against some minimum requirement. Currently, American National Standards Institute voluntary standard Z80.7‐1984 addresses IOL optical requirements. One criterion of this standard is a minimum resolving power of 100 line‐pairs‐permillimeter. This report presents some results of lens testing that suggest this standard is inadequate because it may not control the performance of a large enough fraction of the optic aperture.

Minimum resolution specification of intraocular lens implants using the modulation transfer function

We investigated the use of modulation transfer function (MTF) measurements to provide a standard test of minimum optical quality of intraocular lenses. We used a water cell with plane entrance and exit windows. This geometry is independent of lens material but relatively simple to implement. We investigated the choice of aperture stop, and 3.0 mm was deemed a suitable choice of stop diameter. Minimum acceptable performance must be specified if this technique is to be adopted as a standard method. The MTF of an ideal lens defocused 1/2 wave is suggested as a possible reference. Strehl ratios, although desirable because they can be measured directly without determining MTFs, were found to be unsuitable. These ratios tend to emphasize the high-frequency response, and the observed ratios are typically too low to provide assurance that the low-frequency response is as high as desired. MTF integrals or contrast at spatial frequencies of particular interest were found to be useful benchmarks of acceptable optical quality.

Optical quality testing of monofocal intraocular lens implants with a 3 mm and a 4 mm aperture

ABSTRACT The optical quality of 81 monofocal poly(methyl methacrylate) intraocular lenses (IOLs) from eight U.S. firms was tested using a 3 mm aperture, as recommended by the American National Standards Institute Z80.7 standard for IOLs, and a 4 mm aperture. The use of the larger aperture had no effect on the measurements of refractive power and astigmatism. When examined with the 3 mm aperture, the average resolving power of the IOLs was 81% of the diffraction limit; when examined with the 4 mm aperture, the average resolving power was 67% of the diffraction limit. Use of the larger aperture would ensure that a larger area of the IOL had been examined. Incorporating the larger aperture into the ANSI minimum resolution requirement would not appreciably affect the rejection rate for currently manufactured monofocal lenses.

Survey of the optical quality of intraocular lens implants.

ABSTRACT Intraocular lenses (IOLs) from 15 U.S. firms were tested for conformance to the requirements of the American National Standards Institute Z80.7 standard for IOLs. A total of 162 IOLs were tested for resolution, astigmatism, and accuracy of labeled power. Average resolving power was 78% of the diffraction limit, much better than the ANSI minimum requirement which is typically equivalent to 30% of the diffraction limit. This suggests that the ANSI Z80.7 requirement could be significantly tightened with little effect on current production practices. Only one IOL exhibited astigmatism in excess of 0.25 diopter. Differences between measured and labeled power in excess of 0.50 diopter were found in 22 lenses, indicating that accuracy of refractive power may be the most commonly missed optical requirement of the standard.

Limitations of using synthetic blood clots for measuring in vitro clot capture efficiency of inferior vena cava filters

The purpose of this study was first to evaluate the clot capture efficiency and capture location of six currently-marketed vena cava filters in a physiological venous flow loop, using synthetic polyacrylamide hydrogel clots, which were intended to simulate actual blood clots. After observing a measured anomaly for one of the test filters, we redirected the focus of the study to identify the cause of poor clot capture performance for large synthetic hydrogel clots. We hypothesized that the uncharacteristic low clot capture efficiency observed when testing the outlying filter can be attributed to the inadvertent use of dense, stiff synthetic hydrogel clots, and not as a result of the filter design or filter orientation. To study this issue, sheep blood clots and polyacrylamide (PA) synthetic clots were injected into a mock venous flow loop containing a clinical inferior vena cava (IVC) filter, and their captures were observed. Testing was performed with clots of various diameters (3.2, 4.8, and 6.4 mm), length-to-diameter ratios (1:1, 3:1, 10:1), and stiffness. By adjusting the chemical formulation, PA clots were fabricated to be soft, moderately stiff, or stiff with elastic moduli of 805 ± 2, 1696 ± 10 and 3295 ± 37 Pa, respectively. In comparison, the elastic moduli for freshly prepared sheep blood clots were 1690 ± 360 Pa. The outlying filter had a design that was characterized by peripheral gaps (up to 14 mm) between its wire struts. While a low clot capture rate was observed using large, stiff synthetic clots, the filter effectively captured similarly sized sheep blood clots and soft PA clots. Because the stiffer synthetic clots remained straight when approaching the filter in the IVC model flow loop, they were more likely to pass between the peripheral filter struts, while the softer, physiological clots tended to fold and were captured by the filter. These experiments demonstrated that if synthetic clots are used as a surrogate for animal or human blood clots for in vitro evaluation of vena cava filters, the material properties (eg, elastic modulus) and dynamic behavior of the surrogate should first be assessed to ensure that they accurately mimic an actual blood clot within the body.

On the closing sounds of a mechanical heart valve.

In the 1994 Replacement Heart Valve Guidance of the U.S. Food and Drug Administration (FDA), in-vitro testing is required to evaluate the potential for cavitation damage of a mechanical heart valve (MHV). To fulfill this requirement, the stroboscopic high-speed imaging method is commonly used to visualize cavitation bubbles at the instant of valve closure. The procedure is expensive; it is also limited because not every cavitation event is detected, thus leaving the possibility of missing the whole cavitation process. As an alternative, some researchers have suggested an acoustic cavitation-detection method, based on the observation that cavitation noise has a broadband spectrum. In practice, however, it is difficult to differentiate between cavitation noise and the valve closing sound, which may also contain high-frequency components. In the present study, the frequency characteristics of the closing sound in air of a Björk-Shiley Convexo-Concave (BSCC) valve are investigated. The occluder closing speed is used as a control parameter, which is measured via a laser sweeping technique. It is found that for the BSCC valve tested, the distribution of the sound energy over its frequency domain changes at different valve closing speeds, but the cut-off frequency remains unchanged at 123.32± 6.12 kHz. The resonant frequencies of the occluder are also identified from the valve closing sound.

Optical performance of decentered monofocal intraocular lenses

ABSTRACT A model eye water cell was used to evaluate the optical performance of biconvex, meniscus, and piano‐convex (spheric and aspheric) monofocal poly(methyl methacrylate) intraocular lenses when the lenses were centered and when they were decentered 1 mm and 2 mm. Resolution, induced astigmatism, and modulation transfer function measurements were performed for all lenses with the more convex surface of the lens oriented toward the incident light. The same measurements were performed for the piano‐convex and meniscus lenses in the reverse orientation. The lens shapes least affected by decentration were the biconvex and spheric piano‐convex with the convex surface oriented toward the incident light. When centered, the aspheric piano‐convex lenses had the best overall contrast performance based on the modulation transfer function measurements. However, once decentered the performance of the aspheric lenses approached that of the meniscus lenses in the reverse orientation, the lens shape which had the worst performance.

Performance Testing of Huber Needles for Coring of Port Septa

The Food and Drug Administration received complaints of Huber needles creating cores in the septa of ports of gastric banding devices. One of these complaints represented a cluster of similar events, even though no deviations from design specifications or recommended practices were subsequently identified by the manufacturer. The authors conducted this comparative investigation of off-the-shelf Huber needles and ports from several manufacturers to determine if engineering parameters could be identified that could account for the coring complaints. Huber needles from ten manufacturers were evaluated for coring using intravascular access ports from five manufacturers. A detailed optical analysis was also performed to identify needle features that would possibly account for coring. The majority of the tested needles performed as they should, i.e., they perforated the port septa without creating cores. However, needles that did produce cores were found to have sharp edges at the heel edge of the needle lumen, the edge of the ground bevel opposite from the needle tip that opens to the inner surface of the cannula tube. Manufacturing processes, which dulled or rounded the sharp heel of the bevel after bevel grinding, prevented coring. As a result of this investigation one manufacturer voluntarily recalled their product and another manufacturer implemented coring testing as part of their quality control. To prevent coring needles from entering the market as a result of manufacturing flaws, optical inspection of the heel edge and coring testing should be performed as part of routine quality control.