Incorrect intraocular lens (IOL) power calculation resulting from incorrect measurements of the eye is the most likely cause of refractive errors after cataract surgery with IOL implantation.1 Furthermore, current standards regarding IOL power labeling allow a certain tolerance, and therefore the power on the label might not be the precise power of the IOL.2 All of these facts warrant the development of postoperative IOL adjustment technologies.1,2
Perfect Lens LLC has developed a femtosecond laser system (Perfector) for IOL power adjustment based on the concept of refractive-index shaping.3–5,A–D It uses green light (520 nm) and operates with energy levels that are below the threshold for ablation or cuts. The IOL power changes are obtained through laser-induced chemical reactions in the targeted area of the optic substance, with an increase in hydrophilicity and a decrease in the refractive index, while the laser builds a refractive-index-shaping lens within the treated area. Potential advantages over existing IOL power adjustable technologies are that the laser system can be applied to any commercially available hydrophobic or hydrophilic acrylic IOL because special IOL material is not necessary. The power adjustment is noninvasive and fast and can be performed under topical anesthesia. The IOL power can be increased or decreased to compensate for surgical errors, IOL tilt and decentration, or a change in the physical characteristics of the eye. Multiple adjustments can be performed because they change a very thin layer within the IOL optic substance. Premium functions (eg, multifocality) can be added to the IOL and removed if necessary.3–5,A–D
The precision of the power change obtained with this technology and its biocompatibility have been evaluated in previous in vitro studies using a relatively limited number of IOLs and in an in vivo study using the rabbit model.3–5,A–D The objective of the current study was to assess the impact of the power adjustment obtained by the femtosecond laser on the optical quality of a commercially available hydrophobic acrylic yellow (blue light–filtering) IOLs. Parameters such as light transmittance and light scattering were evaluated for the first time.
Materials and methods
Ten CT Lucia 601PY (commercially available single-piece yellow hydrophobic acrylic IOLs, Carl Zeiss Meditec AG) were used in this study. Light microscopy was performed on all IOLs. They were then placed in vials containing distilled water and allowed to hydrate at room temperature for at least 1 day before the measurements were obtained. All measurements described below were performed under hydration conditions before and after laser power adjustment with a target of −2.0 diopters (D). The IOLs were placed inside a purpose-designed IOL holder to keep the IOL stabilized in distilled water at all times during the laser treatment. Laser parameters and shaping had been automatically set by the system to match the desired diopter. The laser shaping time was 23 seconds per IOL. Light microscopy was again performed after laser adjustment/shaping.
Power and Modulation Transfer Function Measurements
All measurements were taken using a power and modulation transfer function device (Lambda-X S.A.), a power and modulation transfer function (MTF) measurement device designed for refractive and diffractive IOLs. It is International Organization for Standardization (ISO) 11979-2 compliant, has an ISO 11979-2 model eye,6 and uses a measurement wavelength of 546 nm.
Light Transmittance and Backlight Scattering
Light-transmittance measurements were performed with a Lambda 35 UV-VIS spectrophotometer (PerkinElmer, Inc.) operated in a single-beam configuration with an RSA PE-20 integrating sphere (Labsphere, Inc.).7–11 Each IOL was fitted to a plastic custom insert with a 5.0 mm diameter aperture for the optic; the insert was designed to hold a 6.0 mm diameter optic. The insert containing the IOL was then mounted on a standard rectangular quartz cuvette filled with distilled water. Care was taken to prevent the presence of air bubbles inside the cuvette. The assembly was then placed directly in front of the opening of the integrating sphere so that the anterior surface of the IOL was facing the light source. Before the measurements, a background correction was performed with the empty inserter immersed in distilled water inside the quartz cuvette. Background transmittance spectra were checked to ensure that 100% ± 0.5% (SD) transmittance was achieved. The IOL spectra were then collected at room temperature with the following settings: wavelength range 850 to 300 nm, slit width 2 nm, scan speed 120 nm/min, and data interval 1 nm. Background transmittance was checked every other sample to ensure that it did not shift during measurements. Results were expressed as the percentage of light transmittance in the visible light spectrum (400 to 700 nm).7–11
Backlight scattering was also measured as described in previous studies.7–11 A custom 3-piece dark eye model with a poly(methyl methacrylate) cornea was used to hold the IOLs under immersion in distilled water. Care was taken to prevent the presence of air bubbles inside the eye model during loading and assembly. The distilled water–filled model containing the IOL was then placed in front of a Nidek EAS-1000 Scheimpflug camera (cornea facing the device), and the room lights were turned off. A cross-sectional image of the IOL inside the model was then obtained (settings: flash level 200 W, slit length 10.0 mm, meridian angle 0) and analyzed using the densitometry peak function. Backlight scattering was measured at the center of the IOL optic substance within the laser treated area (after treatment), along the axis of a line that crossed perpendicularly through the center of the IOL optic. Results were expressed in computer-compatible tape (CCT) units. This is a measure of brightness or intensity of reflected (scattered) light on a scale of 0 (black) to 255 (white).7–11
Figure 1 shows light photomicrographs of 1 of the IOLs included in this study before and after laser treatment. Surface contaminants, such as small fibers and dust-like deposits, were observed on the surface of some IOLs. Their presence was the result of the study being performed in a laboratory setting under nonsterile conditions. Light microscopy of the IOLs after laser treatment showed the phase-wrapped structure created by the laser in all treated IOLs within their optic substance. The phase-wrapped structure was centered in all cases. In the treated area, the yellow color of the IOL optic was slightly darker. None of the IOLs showed damage, deformation, pitting, or marks.
Table 1 shows power and MTF results from the 10 IOLs used in the study, measured before and after laser treatment. The mean change in power after laser treatment was −2.037 ± 0.047 D, which was associated with a mean change in the MTF of −0.064 ± 0.053.
Table 2 shows the light-transmittance and backlight-scattering results from the 10 IOLs used in the study, measured before and after laser treatment. The mean change in light transmittance was −1.46% ± 0.98%, and the mean change in backlight scattering was +56.8 ± 14.7 CCT units.
Figure 2 shows the light-transmittance curves of a representative IOL before and after laser treatment. The change in light transmittance after laser treatment was −1.16% in this IOL. The graphs show that the majority of the light-transmittance change occurred between the 420 nm and 560 nm range, with an increase between 420 nm and 460 nm (violet–blue range; light transmittance from 55.00% ± 5.33% to 57.88% ± 5.23%) and a decrease between 470 nm and 560 nm (blue–cyan–green range; light transmittance from 90.35% ± 7.17% to 85.73% ± 7.8%7). Figure 3 shows Scheimpflug photographs of a representative IOL before and after laser treatment. The increase in backlight scattering within the optic substance of the IOL after laser treatment appeared to correspond to the area of increased hydrophilicity within the substance of the IOLs created by the laser shaping.
Refractive properties of an IOL can be customized after implantation using a femtosecond laser through construction of a refractive-index-shaping lens within the implanted IOL with micrometer precision. A recent study4 assessed the chemical basis for the alteration in the refractive properties of an acrylic IOL with a femtosecond laser. To determine the nature of the changes created in the material by exposure to the femtosecond laser, various hydrophilic and hydrophobic materials were tested with the following 3 microscope setups: laser-induced fluorescence microscopy, Raman microscopy, and coherent anti-stokes Raman scattering microscopy. The authors found an increase in hydrophilicity in targeted areas within the lens caused by photo-induced hydrolysis of polymeric material in the aqueous media, creating hydrophilic functional groups. Water will subsequently diffuse into the areas with increased hydrophilicity to form hydrogen bonds with the functional groups. As a result, the polymeric material becomes more hydrophilic within the targeted areas, which creates a negative refractive-index change. The surface and untreated area within the IOL remain unchanged. Standard leachable tests have been performed on laser-treated IOLs, and no leachables were found.4
The targeted area within the IOL optic is treated with a specific shape, a phase-wrapped structure, which contains the entire curvature of a traditional convex or concave lens in 1 thin collapsed layer (50 to 200 μm). A refractive-index change (Δn) of 0.01 in a 6.0 mm conventional IOL will produce a change of 0.4 D, while a 0.01 Δn in a phase-wrapped lens with the same optic diameter creates a change of 3.3 D. Therefore, using this structure, significant dioptric changes can be obtained by treating a very thin layer of the optic substance.3,4
Other in vitro studies of this technology3,4,A,B have been recently published or presented, showing changes in power within 0.1 D of the target. These results are consistent with our current findings. Each of the 10 IOLs came within ±0.1 D of the initial target of −2.0 D, further affirming the accuracy and repeatability of this process. In addition, the MTF values were not significantly affected, resulting in a mean change value of −0.064. Other studiesC,D also showed that this technology could be used to create multifocality in a monofocal IOL and to cancel the diffractive multifocal addition of a traditional multifocal IOL.
We further analyzed 2 other parameters for the first time; that is, light transmittance and backlight scattering after laser treatment. The mean change in the overall light transmission in these yellow IOLs was small (from 83.28% to 81.82%). Further inspection of the individual light-transmittance curves showed that the majority of the changes occurred between 420 nm and 560 nm. The treated area became darker in color, almost orange, which explains the change in the light-transmission curve. A prospective randomized double-masked study with intraindividual comparison12 evaluated the clinical relevance of in vitro data by comparing the visual performance of patients with a yellow-tinted IOL in 1 eye (filters blue light) and an orange-tinted IOL (filters blue–green light) in the contralateral eye. The authors found no statistically significant difference in corrected distance visual acuity, contrast sensitivity at different contrast levels, or illuminations or color perception between the eyes. Another clinical study of 56 eyes found that orange or yellow blue light–filtering IOLs were comparable to a clear IOL in terms of photopic and mesopic contrast sensitivity or color discrimination.13
The increase in backlight scattering in our current study appeared to correspond to the area of increased hydrophilicity within the substance of the IOLs created by the laser shaping. The levels observed are not expected to be clinically significant according to previous studies using Scheimpflug photography.8,10,11 Light scattered forward to the retina (straylight) has the potential to degrade image quality by creating a roughly uniform veil over the true image. The change in MTF in our study was minimal, indicating that straylight after laser shaping was not significant.11
This technology was evaluated in vivo in a recent study performed in our laboratory using the rabbit model.5 The postoperative outcomes in terms of uveal and capsule biocompatibility were similar between treated IOLs and untreated IOLs, as seen on clinical examination and by complete histopathology. The laser power adjustment procedure did not induce inflammatory reactions in the eye or damage to the IOL optic. Even though an eye interface had to be designed for this study, which was also the first done in vivo, the change in power obtained was consistent between the treated eyes.5 The precision of the power change obtained with this technology, without a significant change in the optical quality characteristics, as well as the lack of toxicity shown in the in vivo preclinical studies, warrant further evaluation with clinical assessment.
What Was Known
- The refractive properties of a commercially available hydrophobic or hydrophilic acrylic IOL can be customized after implantation using a femtosecond laser through construction of a refractive-index-shaping lens within the implanted IOL with micrometer precision.
What This Paper Adds
- Evaluation of IOL power, MTF, light transmission, and light scattering of commercially available blue light–filtering IOLs before and after power adjustment of −2.0 D by a femtosecond laser showed accurate changes in dioptric power while not significantly affecting the quality of the IOL optic.
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Mr. Enright, Mr. Alley, and Ms. Sahler are employees of Perfect Lens LLC. None of the other authors has a financial or proprietary interest in any material or method mentioned.
Other cited material
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