A Prospective Evaluation of Aspheric (Tecnis),

Silicone, and Acrylic IOLs

on Retinal Image Contrast

and Functional Visual Performance

Robert M. Kershner, MD, FACS

Eye Laser Center

Suite 303, 1925 West Orange Grove Road

Tucson, Arizona 85704-1152 USA

Phone: (520) 797-2020  Fax:  (520) 797-2235

e-Mail: Kershner@EyeLaserCenter.com

©2003. Robert M. Kershner, M.D., F.A.C.S. All rights reserved.

No portion of this paper can be copied, reproduced in any way or distributed without the expressed written permission of the copyright holder.

  This paper was presented at the Symposium on Cataract, IOL and Refractive Surgery, April 12-16, 2003, San Francisco, California.  The Tecnis™ Z-9000 IOL is manufactured and distributed by Pharmacia Corporation, Peapack, NJ USA.  STAAR AA4207VF and ACRYSOF™ SA60AT are products of STAAR Surgical, Monrovia, CA and ALCON Surgical, Ft. Worth, TX, respectively. Dr. Kershner is the director of the Eye Laser Center in Tucson, Arizona, Clinical Professor of Ophthalmology at the Moran Eye Center, University of Utah School of Medicine, Salt Lake City, Utah, and the IK HO Visiting Professor of Ophthalmology at the Chinese University in Hong Kong, He has no proprietary or financial interest in the techniques or materials described in this article.  

This prospective, randomized study compares an aspheric IOL with conventional spherical silicone and acrylic lenses on retinal image contrast and functional visual performance. 221 eyes of 156 patients were randomly assigned to receive one of each of the three intraocular lenses with six months of follow-up. Measured parameters include visual acuity, fundus photographic retinal image contrast, and functional acuity contrast testing. The differences in the pre and postoperative spherical and astigmatic refractive error and preoperative best-corrected visual acuity between groups are not statistically significant. In the first postoperative month uncorrected visual acuity is best in the aspheric group. The aspheric IOL group exhibit up to a 47% increase in contrast for photopic, 38% in photopic with glare, 100% in mesopic and 100% in mesopic with glare functional acuity contrast testing. Acrylic IOLs show no increase in photopic, up to 38% increase in photopic with glare, 50% in mesopic and 50% in mesopic with glare. Spherical silicone IOLs show no increase in contrast testing when compared to cataract. Digital analysis of retinal imaging demonstrate increased threshold luminance levels in the aspheric group (range 116-208) and a four-fold increase in image contrast compared to the silicone and acrylic groups. The aspheric IOL (Tecnis) provides significant improvement in objective retinal image contrast and in visual performance as measured by visual acuity and functional acuity contrast testing. This improvement was most pronounced in night vision and night vision with glare contrast testing when compared with conventional spherical silicone and acrylic IOLs.

Abstract :

A Prospective Evaluation of Aspheric (Tecnis), Silicone and Acrylic IOLs on Retinal Image Contrast and Functional Visual Performance

Robert M. Kershner, M.D., F.A.C.S., A.B.E.S.

Purpose: Studies have shown that the loss of balance between the positive spherical aberration of the cornea and the crystalline lens seen with increasing age may be one cause of decreased contrast sensitivity in cataract patients. Conventional spherical intraocular lens (IOL) optics correct spherical error only and may add to the positive spherical aberration. The recent introduction of an anterior modified prolate, aspheric intraocular lens (Tecnis) may correct this aberration and result in an increase in contrast sensitivity in post cataract surgery patients. This study was designed to compare an aspheric IOL with conventional silicone and acrylic lenses on retinal imaging and functional visual performance.

Setting: Eye Laser Center, Tucson, Arizona USA

Methods: 221 eyes of 156 patients were randomly assigned to receive one of each of the three intraocular lenses. Visual acuity was measured preoperatively, at one day, one week, three weeks, one month, three months and six months postoperatively. Fundus photography and photopic and mesopic functional acuity contrast testing was performed preoperatively and at three months postoperatively.

Results: The differences in the pre and postoperative spherical and astigmatic refractive error and best-corrected visual acuity between groups were not statistically significant. Postoperative uncorrected visual acuity was best in the aspheric group in the first month. The aspheric IOL group had a 38-47% increase in photopic, 38% in photopic with glare, 43-100% increase in mesopic and 9-100% increase in mesopic with glare, functional acuity contrast testing. Acrylic IOLs showed no increase in photopic, 38% increase in photopic with glare, 25-50% increase in mesopic and 36-50% increase in mesopic with glare functional acuity contrast testing. Spherical silicone IOLs showed no increase in contrast testing when compared to cataract. Digital analysis of retinal imaging demonstrated increased threshold luminance levels in the aspheric group (range 116-208) and a four-fold increase in image contrast compared to preoperative cataract, silicone and acrylic groups.

Conclusions: All three intraocular lenses demonstrate improved visual acuity following cataract surgery. The aspheric IOL (Tecnis) provides significant improvement in retinal image contrast and in visual performance as measured by visual acuity and functional acuity contrast testing. This improved visual performance was greatest in the testing of night vision and night vision with glare when compared with conventional spherical silicone and acrylic IOLs.

Introduction

Materials and Methods

The study was designed as a six month, prospective, randomized evaluation of three commonly used though different IOLs. Informed consent, which met the standard for the Institutional Review Board where the study was performed, was obtained from all patients enrolled.  Each patient received a comprehensive ophthalmic examination which included tonometry, refraction, slit-lamp and dilated fundus examination. Patients with preexisting glaucoma, diabetes or macular pathology were excluded from the study.  Whenever possible, if the patient required cataract surgery on both eyes, they received the same IOL for the fellow eye. Data was collected from each eye, entered onto a Microsoft Excel spreadsheet (Microsoft Corporation, Redmond, Washington, USA) for each group and analyzed following the conclusion of the study. A total of 221 eyes from 156 patients were enrolled over three months and each followed for a total of six months. The average age was 71.6 years, and the ratio of females to males was 1.5:1. There was no loss of patient data due to attrition during the course of the study. Each patient was randomized to one of three groups for the lens to be implanted, a one-piece silicone, one-piece acrylic, or three-piece aspheric (Tecnis) IOL. Surgery was performed by a single surgeon experienced in microincision cataract surgery, (RMK), utilizing the same surgical methods for each lens implantation. Best corrected preoperative visual acuity, and uncorrected postoperative acuity was obtained in a controlled testing environment using a projected Snellen chart, calibrated illumination and room lighting and was measured by the same certified ophthalmic technician. One certified ophthalmic technician was assigned to obtain pre and postoperative functional contrast acuity testing, and one was assigned to obtain pre and postoperative fundus photography for all patients. All data was collected with the observers blinded as to type of IOL being tested. Patient's pupils were measured to conform to a range of 3-5mm without dilation for photography. Fundus photographs were obtained with a Canon CR4-45NM Dual non-mydriatic retinal camera preoperatively, at three months postoperatively, and analyzed using Adobe Photoshop 7.0 image software (Adobe Systems Incorporated, San Jose, California, USA) using the L*a*b color model ⁹. The L*a*b color model is based on the model proposed by the Commission Internationale d'Eclairage (CIE) in 1931 as an international standard for color measurement. In 1976, this model was refined and named CIE L*a*b.  L*a*b color is designed to be device independent, creating consistent color regardless of the device (such as a monitor, printer, computer, digital camera or scanner) used to create or output the image. L*a*b color consists of a luminance or lightness component (L) and two chromatic components: the ‘a’ component (from green to red) and the ‘b’ component (from blue to yellow). This software formula was applied to analyze the fundus photographs for luminance and contrast and display the data as a surface area chart where the ‘x’ axis is luminance, displayed over the ‘y’ axis which is the color spectrum. In Photoshop, the L*a*b mode has a lightness component (L) that can range from 0 to 100. The ‘a’ component (green-red axis) and the b component (blue-yellow axis) can range from +120 to -120. The threshold luminosity histogram can be displayed in numeric values from 0 to 250 representing the difference from white (low values) to black (high values). The digital data is represented numerically as the mean ± standard deviation (SD) tabulated for each group.

All patients received preoperative computed topography (Tomey Corporation, Computed Anatomy Tomography), and ultrasonic A-scan biometry with a Sonomed A-1500 A-Scan unit (Sonomed, Lake Success, NY USA). Keratometry was performed with a Bausch and Lomb keratometer (Bausch and Lomb, Rochester, NY). Calculations for IOL power selected for implantation were based upon a predicted of final correction of -0.25 to –0.50 D. The pooled data were applied from the average calculations obtained from the Binkhorst I, SRK II and Holladay IOL formulas.

Contrast Testing

Contrast refers to the difference in brightness levels from one part of an image to another.  The sine-wave grating method of testing is based upon the presentation of a basic target pattern of fuzzy gray bars. To provide a maximum of sensitivity, the bars are spaced closer together as higher contrast is tested. The image size can then be measured as a spatial frequency, the higher the number, the smaller the image size. Scotopic testing refers to levels of illumination less than 0.05 lux, (where essentially only rods are functioning and therefore is not tested in this study), mesopic testing between 0.05 lux and 50 lux and photopic testing greater than 50 lux¹⁰. The graphic representation of contrast sensitivity is recorded as a log function on the y axis and displayed for each of the five spatial frequency targets on the x axis (1.5, 3.0, 6.0 12.0 and 18.0 cycles per degree (CPD). A single log unit increase therefore would represent a ten-fold increase in contrast sensitivity.

Functional acuity contrast testing (F.A.C.T.) as developed by Arthur P. Ginsburg, Ph.D.¹¹ was performed with best spectacle correction for the target distance using a Stereo Optical Optec 3500 Vision Testing System (Stereo Optical Company, Chicago, IL USA), with sine-wave grating charts at levels of illumination for day (photopic) at 10 lux with a target illumination value of 85cd/m², and night (mesopic) at 1 lux with a target illumination value of 3 cd/m².  Glare luminance levels were set at 1 lux for night glare and at 10 lux for day glare.  The mean (± SD) of the log contrast sensitivity values are displayed in the graphs. The log base 10 of these values were used to plot the contrast sensitivity on the 'y' axis for each spatial threshold. The two-tailed t-test was used for comparing each group value. A difference of 0.15 log units is considered statistically significant.

The IOLs.

Figure 1-Diagrams of A) Aspheric TECNIS Z-9000, B) Silicone STAAR AA4207VF and C) Acrylic ACRYSOF SA60AT IOLS.

          One-piece silicone-105 eyes of 64 patients received the STAAR elastic AA4207-VF Silicone IOL, a 10.8mm overall length plate haptic IOL with 0⁰  haptic angulation and a 5.5 mm biconvex 1:1 ratio of the curvature of the optic (figure 1A). It is manufactured from an elastic RMX3-UV, ultraviolet absorbing silicone with an index of refraction of 1.413 and is designed to be implanted completely within the capsular bag following phacoemulsification cataract extraction. The IOL can be folded for insertion through an incision size of 3.5mm or smaller and is available in powers of +4.0 to +34.0 diopters. (STAAR Surgical Company, Monrovia, California, USA).

          One-piece acrylic-41 eyes of 27 patients received the ALCON SA60AT acrylic IOL, a 13.0 mm single piece overall length IOL with a 6.0 mm diameter, anterior asymmetric, biconvex optic, with 0⁰ haptic angulation (figure 1B).    It is manufactured from an ultraviolet-absorbing acrylate/methacrylate copolymer with an index of refraction of 1.55 in available powers of +6.0 to +34.0 diopters. The material is capable of being folded prior to insertion and is intended to be inserted into the capsular bag following cataract removal (ALCON Laboratories, Inc. Ft. Worth, Texas, USA).

          Three-Piece Silicone, Aspheric- 75 eyes of 65 patients received the Pharmacia Tecnis Z-9000 Silicone IOL, a 12.0mm overall diameter, three-piece equibiconvex, 6.0mm optic with square edges and a 6⁰ haptic angulation (figure 1C). It is manufactured from an elastic, ultraviolet absorbing polysiloxane with a refractive index of 1.46. The anterior lens surface has a modified prolate, aspheric design which is thicker in the center and thinner in the periphery to minimize spherical aberration. The two haptics are configured into a capsular 'C' shape and are manufactured out of polyvinylidene fluoride (PVDF). The lens is designed to be implanted completely within the capsular bag following phacoemulsification cataract extraction. It is capable of being folded for insertion through an incision size of 3.5mm or smaller and is available in powers of +5.0 to +34.0 diopters. (Pharmacia Corporation, Peapack, New Jersey USA).

Surgical procedure

The operative eye was prepared for surgery with 1 drop of 1% cyclopentolate hydrochloride, and 2.5% phenylephrine hydrochloride (Cyclomydril, Alcon, Ft. Worth, Texas USA) starting 15 minutes prior to surgery with drops repeated once every five minutes for a total of three drops. Topical povidone iodine solution (Betadine 5%, Escalon, Lakewood, NJ USA) was instilled 10 minutes prior to surgery into the operative eye. The technique of topical anesthesia has been previously published.^12-15 Topical 0.5% tetracaine HCL (CibaVision, Atlanta, GA USA) was instilled at the start of the prep and before the procedure commenced. The eye and eyelid skin were prepared with Betadine solution and draped. A 2.85 mm trifaceted diamond keratome (Kershner Corneotome-Diamatrix, The Woodlands, Texas USA) for the silicone and aspheric IOLs or a 3.2 mm Becton Dickinson Clear Cornea Incision System slit blade (BD Ophthalmic Systems, Waltham, MA USA) for the acrylic IOLs, was used to create a clear cornea incision, just anterior to the limbus entering the anterior chamber parallel to the iris plane. There was no paracentesis and no attempt was made to correct preexisiting astigmatism with additional incisions for this study. The techniques of capsulorrhexis with the Kershner one-step capsulorrhexis forceps (Rhein Medical, Tampa, FL USA)^16,17 hydrodissection,¹⁸ single-incision, single-instrument intercapsular phacoemulsification,¹⁹ and intercapsular implantation of the injectable intraocular lens,^20,21 were then followed in order. Viscoelastic was used for the capsulorrhexis and filling of the capsular bag for IOL implantation (Healon, Healon GV and Healon-5, Pharmacia, Inc. Peapack, New Jersey USA). Phacoemulsification was performed with the Bausch and Lomb Millennium phacoemulsification unit (Bausch and Lomb Surgical, Rochester, NY USA) using linear surgeon control with a 30 degree phacoemulsification tip and custom-fitting teflon sleeve at maximum phaco powers of 20% - 30%. The lens bag was inflated with viscoelastic. A STAAR MSI-TR injector with disposable ST-45 cartridge for the STAAR silicone lens, the ALCON Monarch II injector with disposable Monarch cartridge for the acrylic IOL, and the Kershner-Mastel MicroInjector with disposable cartridge (Mastel Precision, Rapid City, South Dakota USA) for the aspheric IOL, was used to fold and insert the IOL through the unenlarged clear cornea incision into the capsular bag. All viscoelastic was removed with irrigation and aspiration. The incision was self-sealing and did not require hydration. No intracameral antibiotics or miotics were used. The total operative time for the procedure averaged four to six minutes. The eye was not patched at the conclusion of the procedure and the patient was allowed unrestricted activities following surgery.

Postoperative Procedure

Patients were started on topical tobramycin-dexamethasone solution (Tobradex-Alcon, Ft. Worth, Texas USA) one drop into the operative eye four times a day commencing the day of the procedure, and continuing for ten days. They were allowed to use artificial tear supplements as needed. No other postoperative drops or medications were used.  Patients were examined on the first postoperative day, at one-week, three weeks, one month, three months and six months following their surgery. All patients underwent measurement of uncorrected visual acuity, a full slit lamp examination, refraction, best-corrected acuity and applanation tonometry on each postoperative visit. Patients were provided with spectacle correction in the form of a reading glass or bifocal with their full refraction at the one-month postoperative visit. Funduscopic photography and contrast acuity testing with best correction was obtained at the three month postoperative examination during the study period.

Results

        There were no surgical or postoperative complications observed during the course of this study. Thirty-one patients (7%), unassociated with lens style, had postoperative pressure rises exceeding 24mm Hg on the first postoperative day that were relieved with releasing aqueous and the application of one drop of 0.5% Timolol (Merck and Co., Inc.). The YAG capsulotomy rate for the group at six months was 0.9% for the silicone, and 0% for the aspheric and acrylic IOLs.  

Figure 2-Preoperative best-corrected and postoperative uncorrected visual acuity at six months for each of the IOL groups

          The preoperative best-corrected visual acuity of 20/265±23 for the three groups {range 20/50-counting fingers} was not statistically significant between groups (Figure 2). Postoperatively, the best corrected visual acuity at one week for each of the three IOL groups, silicone 20/40±8, acrylic 20/40±11, and aspheric 20/38±5 were also not statistically significant. However, the data for uncorrected postoperative visual acuity (figure 2) is statistically significant. On postoperative day 1, the acuities for the silicone group were the poorest at 20/73±8, better with the acrylic group 20/59±11, with the best results seen in the aspheric group at 20/35±3. This trend was maintained until the one-month visit, when the acrylic IOLs matched the uncorrected acuity of the aspheric.  There was a statistically significant difference in uncorrected acuity that continued from the three to six month visit between the aspheric group (20/25±2) and the silicone (20/36±1) and acrylic (20/30±1) groups at six months.  This difference could not be explained on the basis of preexisting or postoperative refractive error as can be seen in figure 3.

Figure 3-A) Preoperative and B) postoperative spherical and astigmatic refractive error for each group  

           Preoperatively, the acrylic group had an average refractive error of [–0.9(±1.02)+1.36(±0.78) diopters [D]], the aspheric (-0.21(±1.24) +0.93(±0.45) D) and the silicone (-0.02 (±0.03) +0.87(±0.32) D). The preoperative spherical equivalents for each group were (-0.22 acrylic, -0.26 silicone, +0.46 aspheric). Postoperatively, the acrylic group had an average refractive error of (–1.05(±1.09)+1.04(±0.58) D), the silicone (-0.79 (±0.57)+0.65(±0.3) D) and the aspheric (-0.75(±0.78)+0.74(±0.43) D). The postoperative spherical equivalents for each group were not statistically significant  (-0.53 acrylic, -0.47 silicone, -0.38 aspheric).

          Figure 4 displays the mean (±SD) of the functional acuity contrast testing for each of the four groups under four different levels of luminance:  A) daylight (photopic), B) daylight with glare, C) night (mesopic), and D) night with glare, for each of the five spatial frequencies tested. 

Figure 4-A) Comparison of aspheric, silicone, acrylic IOLs and preoperative cataract on Functional Acuity Contrast Testing (F.A.C.T.) of day vision (Photopic)

B) Comparison of aspheric, silicone and acrylic IOLs on Functional Acuity Contrast Testing (F.A.C.T.) of day vision with glare

C) Comparison of aspheric, silicone, acrylic IOLs and preoperative cataract on Functional Acuity Contrast Testing (F.A.C.T.) of night vision (Mesopic)

D) Comparison of aspheric, silicone, acrylic IOLs and preoperative cataract on Functional Acuity Contrast Testing (F.A.C.T.) of night vision with glare  

In figure 4A are displayed the four curves for the contrast testing of patient’s individualeyes under daylight lighting (photopic). As would be expected based upon published data⁷,the contrast sensitivity of cataract patients is the lowest throughout all the spatial frequencies tested. There is improvement in the mean functional contrast acuity under photopic testing conditions, with each of the IOLs implanted. The differences between the preoperative cataract and the silicone and acrylic IOLs were not statistically significant. The aspheric IOL demonstrated increased contrast sensitivity for each spatial frequency tested under photopic conditions. The values for the aspheric group displayed in figure 4A represent statistically significant improved contrast testing of 38% at spatial frequencies of 1.5 to 6 cpd, and 47% at 12 cpd.

In figure 4B are displayed the four curves for the mean values (±SD) for contrast testing of patient’s individual eyes in daylight (photopic) lighting with glare. The levels for the preoperative cataract are the lowest. All three IOL groups show a 38% improvement in contrast over preoperative values at spatial frequencies of 1.5, 3, and 6 cpd. There was no statistically significant difference within the three IOL groups for photopic contrast testing with glare.

The differences between preoperative cataract and the postoperative IOL groups become most apparent with the data displayed in figure 4C for functional acuity contrast testing in dark (mesopic) conditions. Again, preoperative cataract demonstrates the lowest levels of contrast sensitivity tested. There is no statistically significant difference in contrast values between any of the groups at spatial frequencies of 1.5 or 3.0 cpd. Silicone IOLs show no improvement over the cataract group at any spatial frequency. At 6.0 cpd, the differences become significant. Acrylic IOLs show improved contrast values of 26% over the cataract, the aspheric IOL group demonstrates improved contrast of 43%.  At 12 cpd, the acrylic shows a 25% improvement over the cataract group, the aspheric 63%. At 18 cpd, or the smallest target patch presented to the patients, the acrylic IOL group had a 50% improvement in their contrast sensitivity over the preoperative levels, the aspheric IOL group had a 100%.

In figure 4D, the difference in contrast values for night (mesopic) levels with glare testing at 1.5, 3, 6 cpd, for the silicone, acrylic and preoperative cataract groups were not statistically significant. Silicone IOLs show no improvement over the cataract group at any spatial frequency. Aspheric IOLs show increased contrast of 9% over cataract at 1.5 cpd, 23% at 3 and 6 cpd, 43% at 12 cpd, and 100% at 18 cpd. Contrast testing in the acrylic IOL group was improved 36% over cataract at 12 cpd, and 50% at 18 cpd.

The fundus photographs from each group were individually analyzed for contrast using the Adobe Photoshop program creating luminosity histograms for each image and then analyzing the maps for the thresholds from the lightest to the darkest for each image (contrast).  This software analysis is displayed as a threshold luminance map. Three representative sets of photographs are shown in figure 5. 

Figure 5-A) Fundus photography of preoperative cataract (left image) and postoperative silicone IOL (right image) at 3 months with threshold luminance maps

B) Fundus photography of Preoperative Cataract (left image) and postoperative acrylic IOL (right image) at 3 months with threshold luminance maps

C) Fundus photography of preoperative cataract (left image) and postoperative aspheric IOL (right image) at 3 months with threshold luminance maps  

In set (A) the preoperative fundus photograph of an eye as imaged through the cataract is displayed on the left, the postoperative fundus photograph of the same eye as imaged through the AA4207VF Silicone IOL is displayed on the right.  Beneath each photograph is the threshold luminance map for that image. In set (B) the preoperative fundus photograph as imaged through the cataract of an eye is displayed on the left, the postoperative fundus photograph as imaged through the same eye through the SA60AT Acrylic IOL is displayed on the right. In set (C) the preoperative fundus photograph as imaged through the cataract of an eye is displayed on the left, the postoperative fundus photograph as imaged through the aspheric Tecnis Z-9000 IOL of the same eye is displayed on the right. When comparing the images on the left to those on the right, the clarity and contrast of all three images are markedly improved following removal of the cataract and implantation of an IOL.

Contrast can be mathematically defined as the difference between the brightness of the light edge (low threshold luminance [L]), subtracted from the dark edge (highest threshold luminance), divided by the sum of the two brightnesses (c=(L_max-L_min)/(L_max+L_min). This numerical value is displayed in decimal form as a percent of contrast. In each case, the threshold luminance map demonstrates that the image through the cataract contains the least difference between light and dark, or in other words, the least contrast to the image (A=124 to 174 threshold range of 50, contrast level 0.17), (B=121 to 186 threshold range of 65, contrast level 0.21), (C=115 to 190 threshold range of 75, contrast level 0.25). The histogram for each fundus image through the cataract shows tall and narrow peaks representing all the color data for that image crowded between the lightest (on the left of the histogram) to the darkest (on the right of the histogram). When compared to the postoperative fundus photographs for each group on the right, there is a marked improvement in contrast for each IOL from the baseline cataract image (A=120 to 229 threshold range of 109, contrast level 0.31, B=115 to 215 threshold range of 100, contrast of 0.30, C=89 to 212 threshold range of 127, contrast level 0.42). Postoperative fundus images show short and broad peaks over a larger area representing much more color data separating the lightest to the darkest. These flatter and broader luminance maps compare (A) silicone, AA4207VF to (B) acrylic, SA60AT to (C) aspheric Tecnis Z-9000 IOLs shows the greatest contrast over the broadest color range from red to green to blue (RGB). The greater the difference within each threshold luminance map, the greater the contrast in the image taken through the IOL. The mean contrast threshold is 109 for the silicone AA4207VF and 127 for the aspheric (Tecnis) IOL.  The lowest mean contrast threshold is for the acrylic SA60AT (100). The lowest threshold luminance for light is 89 for the aspheric Tecnis Z-9000 photograph. This represents the lowest light level imaging of the fundus. The image taken through the aspheric IOL demonstrates the greatest contrast level (percent contrast) at 0.42. The photographic data represented in figure 5 is corroborated by the pooled numerical data for the threshold luminance maps from all the fundus images obtained at three months for each group following surgery (figure 6). 

Figure 6-Threshold Luminance Ranges for preoperative cataract, silicone, acrylic and aspheric IOLs (with percent contrast for each group)

The mean range in threshold luminance for the preoperative cataract retinal images (contrast) from the lightest levels is 120±5 to the darkest levels of 183±8 (difference=63, contrast level 0.20). Analysis of the postoperative imaging demonstrates an improvement in range for the silicone group from lightest of 134±5 to darkest of 203±8 (difference=69, contrast level 0.20), for the acrylic group from lightest of 123±7 to darkest of 195±17 (difference=72, contrast level 0.23), and for the aspheric group from lightest of 116±13 to darkest of 207±10 (difference=91, contrast level 0.28). The aspheric IOL group demonstrated the greatest statistically significant difference in the range of threshold luminance from lowest to highest. The aspheric IOL group also demonstrated the greatest contrast level of the images of 0.28. As the threshold luminance graph illustrates, the lowest levels of the image contrast for the silicone and acrylic groups is greater than that for the cataract itself, demonstrating that the contrast in the low light range is poorer for the images taken through these IOLs as a group, than in the preoperative cataract group. By contrast, the aspheric group imaged the best in the low light contrast areas with the lowest threshold luminance of all four groups.  At the highest threshold luminance levels, the aspheric IOL group measured higher than the cataract, the silicone and the acrylic groups. These differences are statistically significant (P<0.001).

The contrast levels of the images obtained through the silicone IOLs (0.20) is the same as through the cataract (0.20).  The contrast level of the images obtained through the acrylic IOLs is (0.23). Comparatively, the contrast levels of the images obtained through the aspheric IOLs (0.28) represents a 40% increase in contrast of the postoperative images. These differences are significant (P<0.001).

Discussion

For over one hundred and thirty-eight years, ophthalmologists have been measuring visual acuity using a high contrast Snellen visual acuity chart.  Unfortunately, this is but one measure of a patient's true visual performance. Individuals are living longer, more independent lives, and need to meet the demands of daily living with better visual acuity. The technology and benefits of modern cataract surgery are accelerating at a tremendous pace, resulting in better visual outcomes and better uncorrected visual acuity for our patients¹. This places greater demands upon ophthalmic surgeons to deliver near-normal vision. Patients require more than just better acuity, they need better functional visual performance under all lighting conditions. The ability to compare and differentiate the differences between light and dark, especially under low light (mesopic) conditions is critical to such tasks as driving and reading signage. Combining the measurement of functional acuity contrast testing with visual acuity testing adds important information to our knowledge of visual performance. The potential visual disability that may come with reduced contrast sensitivity can interfere with a patient's ability to drive, especially at night. Owsley et al. performed a cross-sectional analysis on 274 older drivers and found that driver's with a history of crash involvement were eight times more likely to have a serious contrast sensitivity deficit in the worse eye than those who were crash free²². Contrast sensitivity decreases with advancing age^23-24.  It would be expected then that cataract patients would be most likely to be affected by a decrease in contrast sensitivity.

This study demonstrates that modern clear cornea cataract surgical techniques, with the implantation of a flexible and foldable IOL, results in a low incidence of complications and excellent visual outcomes. What this study set out to determine is if there is a difference in visual performance between conventional spherical optics of the flexible and foldable silicone and acrylic IOLs and the anterior surface, modified prolate, aspheric IOL. This study did not control for the type of lens material or the style of the implant, as each lens in the study was intentionally different. In a limited preliminary study, I compared the 911A a 6.0mm optic, three-piece IOL with PVDF haptics (Pharmacia Corporation, Peapack, NJ) with the Tecnis Z-9000 aspheric IOL (unpublished results-Kershner). The 911A is an identical style IOL with the same material, haptic design and shape. It did not demonstrate the added improvement in contrast acuity testing that was demonstrated by the identical lens (Tecnis) with the aspheric optic in this study. This data was also confirmed by a study by Holladay et al. in which they compared a spherically surfaced equi-biconvex lens of the same power with the anterior prolate IOL. Their results in improved contrast from the optical properties of the prolate lens were not shared by the same lens and design without the prolate surface²⁵.

It is difficult to postulate that haptic design or lens style would contribute to a difference in contrast sensitivity delivered through the optic and therefore no attempt was made to control these factors in this study. A study comparing the aspheric IOL optic with two commonly implanted though different lens materials and styles is but one approach to determine if the addition of an aspheric optic surface would impact contrast sensitivity and image contrast.

This study did not control for pupil size. It has been demonstrated that the anterior, modified prolate, aspheric optic of the Tecnis Z-9000 IOL demonstrates superior optical performance when compared to spherical IOLs for systems with 5-6mm pupils. It is postulated that the larger the pupil, the greater the contribution from the degrading effects of optical aberrations²⁵. Therefore, one would expect the optical performance of this aspheric IOL in this study to be superior under conditions where pupil size would be larger (mesopic and mesopic with glare testing). The data in this study confirms this hypothesis. It has also been shown with optical bench testing, that lens centration of less than 0.4mm and lens tilt of less than 7 degrees maximize the effects of the aspheric IOL²⁵.  This study made no effort to determine lens tilt or centration of any of the IOLs. With today’s advanced microincision cataract surgery techniques, it may be expected that these parameters are well within the tolerances for IOL implantation²⁶.

A review of the data on visual acuity, shows that uncorrected visual acuity in the first four weeks after surgery was better in the aspheric group when compared to silicone and acrylic IOLs (figure 2). The uncorrected visual acuities in the acrylic IOL group were better than the silicone but did not improve to the level of the aspheric IOL group until one month postoperatively. At six months there still was a difference in uncorrected acuity with the aspheric IOL group achieving the best visual levels. This difference cannot be explained on the basis of refractive error alone. The difference in spherical equivalent was not statistically significant following surgery between the groups. The differences may be attributed in part to the size of the incision, the acrylic IOL requiring a slightly larger incision than the silicone. However, the one-piece silicone IOL had uncorrected visual acuity that was not as good as the acrylic or aspheric IOLs during the course of this study and the incision sizes were essentially identical to the aspheric IOL group. There may have been slightly greater astigmatism or corneal edema in the postoperative course for these eyes, however this could not be determined from the data in this study. The best-corrected visual acuity was statistically similar in all three IOL groups, reflecting on average a six-line improvement over preoperative levels. The uncorrected visual acuity of the modified prolate aspheric IOL was superior to the other two groups. For the purposes of data collection and contrast testing for this study, only vision with best correction was utilized.

The data on functional acuity contrast testing in this study demonstrates that cataract patients exhibit the lowest levels of contrast sensitivity of all the groups tested in each of the four lighting levels. It can be expected when testing patients in bright light (photopic) conditions, that pupil size will be smaller and spherical aberrations induced by the lens, would be lessened. As this study demonstrates, there is little difference between the preoperative cataract, silicone and acrylic IOL groups in photopic conditions, with the aspheric IOL group demonstrating a 38% to 47% increase in contrast values especially at the highest spatial frequencies. The data for photopic testing with glare for all three IOL groups demonstrate improved contrast testing when compared to preoperative cataract levels.

The contributions from reduced spherical aberration contributed by the aspheric IOL should be maximized in mesopic conditions. The data clearly demonstrate that under mesopic conditions, the aspheric IOL performs the best. The silicone IOL group showed no increase in contrast sensitivity over preoperative cataract levels. At the smallest target sizes of 12 to 18 cpd, where it is most difficult for the patient to distinguish differences, the acrylic IOL group showed a 25% to 50% increase over preoperative cataract levels, the aspheric IOL group 50% to 100%. This pattern was even more apparent when viewing the data for mesopic testing with glare. There was no improvement in contrast testing with the silicone IOL group. The acrylic IOL group demonstrated an increase of 36% over preoperative cataract levels at 12 cpd and 50% at 18 cpd. The aspheric IOL group demonstrated improved contrast testing at every frequency tested from 9% at 1.5 cpd, 23% at 3 and 6 cpd, 43% at 12 cpd to 100% at 18 cpd. The data speak for themselves. The addition of an anterior, modified prolate optic to the IOL is associated with up to 100% increase in contrast sensitivity levels and that this improvement is demonstrable at all frequencies and most apparent at the highest frequencies, in lowest light levels and lowest light levels with glare. Based upon this data, one might expect this IOL to perform best for patients at night and when there is a glare source, such as oncoming headlights from an automobile.

What is the relevance of functional acuity contrast testing to actual real life visual demands and are the testing methods valid? Sine-wave gratings have been used for over three decades in visual psychophysics¹¹. Using computer generated sine-wave gratings in a view-in tester, as was applied in this study, eliminates the need to control room light levels, is accepted by the patient, and allows the observer to control the light source and present the target to one eye at a time. My previous work in contrast testing of bifocal and multifocal IOLs (1989-unpublished observations-Kershner) utilizing the Multivision Contrast Tester (MCT, Vistech Consultants, Dayton, OH USA) and in this study, with the Optec series (Stereo Optical) have confirmed that these instruments are an excellent standard by which test comparisons of contrast thresholds and spatial frequencies can be made. Contrast sensitivity testing has been shown to provide a consistently high degree of correlation with visual performance^22,25.

What contribution can the lens material and optic size contribute to the potential for improved contrast testing? This study compared three lens styles and two materials, and two optic sizes.  The one-piece silicone IOL demonstrated the least improvement on contrast testing and image contrast. The differences between this IOL and the aspheric IOL tested is the silicone lens is a one-piece, plate haptic IOL. The RMX-3 silicone material is similar to the polysiloxane silicone used in the aspheric IOL. It is possible that any of these factors may contribute to the optical performance of the IOL but in the case of the one-piece silicone IOL it is not likely to be the material. The acrylic IOL by comparison, is a one-piece IOL made of a different optic material, acrylate / methacrylate copolymer. It performed better than the one-piece silicone IOL but not as good as the aspheric for uncorrected visual acuity, image contrast and contrast sensitivity. It cannot be determined from this study with certainty, whether the difference in IOL performance is due to the material or the geometry of the optic. The optic sizes for the silicone and acrylic IOLs were 5.5mm, the aspheric was 6.0mm. One could postulate that the smaller optic sizes could be contributing to the loss of contrast sensititivity at larger pupil sizes. In an elderly population, such as in this study, it is uncommon to have naturally occurring pupil size greater than 5mm. That is one reason manufacturers choose 5.5 or 6.0mm for optic sizes. Therefore, it would be unlikely that optic size alone would contribute to the differences observed in this study. Packer’s studies, compared an acrylic IOL with a 6.0mm optic to the aspheric IOL tested in this study and failed to demonstrate a contributory difference from optic size or material²⁷. Another factor that could be postulated to affect the performance of the IOLs tested is the lens capsule. The contribution of the negative sphericity of the aspheric IOL versus the positive spherical aberration contributed from the spherical IOLs should be the primary factor in determining IOL performance with the testing parameters in this study. Patients were screened for capsular opacification and if identified and determined that it contributed to a decline in visual acuity from postoperative levels, then the patient underwent YAG laser capsulotomy for that eye prior to contrast testing. Only a small number of the one-piece silicone IOLs (0.9%) required this treatment during the course of the study. Defocus from incorrect IOL power or capsular opacification should not be significant contributers to contrast testing performance¹¹. Nonetheless, the results of this study cannot rule out other factors also contributing to visual performance of the IOLs.

 Retinal imaging is an area where cataract surgeons have not previously placed emphasis. With the early acceptance of IOLs, retinal surgeons were concerned that visualization would be impaired for such procedures as retinal detachment repairs or photocoagulation therapy. As cataract surgical outcomes have improved, this concern is not commonly voiced today. In the routine examination of the eye, ophthalmologists utilize an aspheric lens for visualizing the retinal fundus in indirect ophthalmoscopy. This aspheric handheld lens reduces spherical aberration allowing for clear undistorted imaging out to the retinal periphery. Applying similar logic, one could postulate that an aspheric lens implanted within the eye, may allow for better clarity and image contrast when visualizing the retinal fundus. How best to test this hypothesis? I used an innovative application of digital imaging to determine if the image taken under controlled conditions of the retina by a fundus camera, could be analyzed to determine if the image contrast was different depending upon the IOL through which the photograph was taken. Are these images subject to pupil size? In an effort to minimize pupil size as a factor, all patients were photographed through a pupil size of 3-5mm. The patients' own preoperative fundus photograph served as their control when comparing the images to avoid the variable due to the pigmentation of the patient's fundus. The threshold luminance maps represent the actual image data for color and contrast of each fundus photograph. This data can be displayed numerically. The difference from the lowest (lightest) values to the highest (darkest) is the range of luminance.  The threshold luminance is represented by the lowest and the highest values. Although the threshold luminance levels will vary from image to image, the difference from lightest to darkest (contrast) can be measured and compared between images and groups. Contrast is defined mathematically as the difference between brightness of the light edge, subtracted from the dark edge, divided by the sum of the two brightnesses. This numerical value can describe for us the percent contrast of an image. Using these values, the study reveals that the improvement in contrast of the retinal images were unchanged for the silicone IOL group compared to preoperative cataract, improved only 3% for the acrylic group but demonstrated 8% more contrast to the images obtained through the aspheric IOLs for an improvement of 40% compared to preoperative cataract.

During the training of fellows, I often use the expression, “what you see-in, is what the patient sees-out”.  If an intraocular lens provides better contrast sensitivity testing with the patient looking out, then this benefit should be apparent to the examiner when looking in. How do we objectively assess the appearance of increased contrast to a fundus image? The digital technology exists, although it is not generally applied in this manner.  To my knowledge this study for the first time applies the technology of digital photographic analysis to the image contrast of the retinal fundus. This study demonstrates that retinal image contrast data for each IOL mirrors the data obtained from completely different testing methods for functional acuity contrast.

Wavefront imaging has become the standard by which many refractive surgeons may soon be imaging the refractive power of the human eye. By passing light into the eye from a point source (the wavefront), the returned images can be recorded by sensors outside of the eye. The deviations of these images or the point spread function can be calculated and constitute the aberration or wavefront due to irregularities in the refracting surfaces of the cornea, lens and fundus. The modulation transfer function (MTF) describes the relationship between contrast of an object and its image. Guiaro et al. demonstrated (using the MTF) that the decline in visual performance in older individuals is due to changes in the crystalline lens which interferes with the optical performance of the eye²⁸. Unlike wavefront analysis, which depends upon these refractive changes, contrast image analysis is little affected by the focus of the image. In this study, the only variable we have changed is the optical aberrations contributed by the lens. The images displayed in figure 5 and the data in figure 6, represent imaging we see-in and therefore represent the quality of the image the patient sees-out.  The preoperative cataract retinal images showed the lowest difference in threshold luminance and the lowest percent contrast of the four groups tested. The silicone and acrylic IOLs showed a similar mean range in threshold luminance with no improvement in the percent contrast for the silicone IOLs and only a fifteen percent increase in contrast for the acrylic IOLs from the preoperative cataract group. By comparison, the aspheric group had the greatest difference in threshold luminance and the greatest percent contrast of the four groups tested exceeding the preoperative cataract group by forty percent. The fundus retinal image contrast analysis corroborates what was seen objectively in the patients with functional contrast acuity testing. The anterior surface, modified prolate, aspheric IOL (Tecnis Z-9000) is associated with more than a 100% increase in functional acuity contrast and a four-fold increase in image contrast of the patient's own retina imaging. The data demonstrates an even greater improvement in the contrast testing for night vision.

The theory behind an aspheric IOL is that it compensates for the average spherical aberration of the human cornea in a cataract population and therefore should offer superior optical performance when compared to a spherical IOL when testing with pupil sizes of 5-6mm. At larger pupils sizes (as is seen in mesopic testing) the degrading effects of any optical aberrations become more significant²⁵. Indeed the data reported for night vision and night vision with glare is better for this IOL. Packer et al. performed a prospective randomized trial of the aspheric lens and found that the Tecnis Z-9000 IOL provided significantly better contrast sensitivity at 1.5 and 3 cpd under mesopic conditions and at 6, 12, and 18 cpd under photopic conditions²⁸. Their group further compared the aspheric IOL with an acrylic IOL, the AR40e IOL (Opti-Edge^TM, Advanced Medical Optics, Santa Ana, CA, USA). Their results demonstrated that at peak contrast sensitivity, the Tecnis IOL group had 38.5% greater contrast under photopic conditions and 77.9% greater under mesopic conditions²⁸.

Why does this lens work the way it does? It is now well established that the contribution from the cornea (primarily it’s anterior surface) and the crystalline lens, each play a compensatory role in neutralizing the spherical aberration within the human eye^29-30. The designers of the anterior modified prolate aspheric IOL, estimated the total ocular spherical aberration from a pool of patients and the rotationally symmetric aberration contributed from the IOL to create an IOL, that in most patients, would neutralize or reduce the average spherical contribution from the cornea²⁵. The positive spherical aberration of the human cornea (approximately one-half of a sphere or –0.52) may compensate for the negative spherical aberration of the human crystalline lens seen in youth⁶. The positive spherical aberration of the cataract in the aging eye may add to this aberration and make the quality of vision worse in elderly patients. Replacing the crystalline lens with a spherical IOL that has an inherent positive spherical aberration may fail to correct the error created by the cataract. The theory behind the development of an aspheric optic IOL is based upon research that an IOL that mimics the negative spherical aberration of the youthful lens, may reduce the ocular aberration of the aging eye and therefore improve the optical quality of the pseudophakic eye²⁵.

Our patients today benefit from our success with modern cataract surgery. Today’s procedures are quicker, safer and provide better uncorrected visual acuity than ever before possible with surgery. Along with these improved outcomes however, comes the burden that surgeons must continue to strive for better functional visual performance for our patients with the least disability from our procedures. At one time, simply removing the cataract was sufficient, banishing the patient to a lifetime of visual disability with aphakic spectacles. More modern approaches to the removal of cataracts have allowed millions of people worldwide to see better with spectacles and an intraocular lens. With today’s high technology IOLs and microminiaturized surgical techniques, many people are now spectacle free following cataract surgery for most tasks¹. In the not too distant future, an IOL that restores accommodation and eliminates the need for reading glasses may become available. Improving visual acuity for all distances of visual need may just not be enough. We as ophthalmic surgeons, entrusted by our patients with their most precious gift of vision, must strive to provide a better quality of vision. Improved contrast sensitivity may be just what the patient needs. The ability of our patients to perceive images under a variety of different lighting conditions, maximizing their perception of image contrast, especially at night, may bring us closer to the ultimate goal of super vision-better visual performance than has ever before been possible.  The ability to improve contrast sensitivity in our cataract patients is no longer just a theory, it is today a scientifically, demonstrable reality. A new standard of care awaits us as cataract surgeons, we must now rise to meet the challenge.

Conclusions

         This prospective, randomized study compares an aspheric IOL with conventional spherical silicone and acrylic lenses on retinal image contrast and functional visual performance. The results demonstrate that postoperative uncorrected visual acuity is improved in all three IOLs tested with the greatest improvement in the aspheric group. An improvement in functional acuity contrast testing cannot be expected with cataract extraction and IOL implantation alone. Patients implanted with the one-piece silicone IOL failed to demonstrate improved contrast testing even though there was an improvement in visual acuity. The one-piece acrylic IOL demonstrated improved contrast testing under some lighting levels when compared to preoperative cataract, but the increased values were significantly less than that seen with the aspheric IOLs. Although the clarity of the retinal imaging was improved in all groups over the preoperative cataract images, the retinal image contrast with the silicone and acrylic IOLs was virtually the same as that measured for the preoperative cataract images. The aspheric IOL (Tecnis Z-9000) provides statistically significant improvement in objective retinal image contrast and in visual performance as measured by visual acuity and functional acuity contrast testing. This improvement, from the preoperative cataract, was greater than that measured for conventional spherical silicone and acrylic IOLs. The improved contrast sensitivity values were greatest for night vision and night vision with glare. This study therefore supports the contention that implantation of an anterior, modified prolate, aspheric IOL (Tecnis Z-9000) is associated with improved retinal image contrast and functional visual performance.

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©2003. Robert M. Kershner, M.D., F.A.C.S. All rights reserved.

Robert M. Kershner, MD, FACS-Eye Laser Center-Suite 303-1925 West Orange Grove Road-Tucson, AZ 85704-1152 Phone 520-797-2020, Fax 520-797-2235 e-mail: Kershner@EyeLaserCenter.com

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