contrast sensitivity after implantation of a spherical versus an aspherical intraocular lens in...

ARTICLE

Contrast sensitivity after implantationof a spherical versus an aspherical intraocularlens in biaxial microincision cataract surgerySabine Kurz, MD, Frank Krummenauer, PhD, Hagen Thieme, MD, H. Burkhard Dick, MD

PURPOSE: To determine whether implantation of a microincision intraocular lens (IOL) with a mod-ified anterior surface, designed to compensate for the positive spherical aberration of the cornea ineyes of cataract patients, results in improved pseudophakic quality of vision in pseudophakic eyesafter biaxial microincision phacoemulsification.

SETTING: Department of Ophthalmology, Johannes Gutenberg-University, Mainz, Germany.

METHODS: In a nonrandomized parallel cohort investigation, the visual performance of 52 eyes of52 patients unilaterally implanted with the aspherical Acri.Smart 36 A IOL (Acri.Tec) were comparedwith those of 25 eyes of 25 age-matched patients unilaterally implanted with the sphericalAcri.Smart 46 S IOL (Acri.Tec). Eight weeks after surgery, the following parameters were assessed:uncorrected visual acuity (UCVA), best corrected visual acuity (BCVA), pupil size under various il-lumination conditions, high-contrast and low-contrast visual acuities, photopic and mesopic con-trast sensitivities, capsulorhexis size, and wavefront aberration of the cornea and eye. Theprimary clinical endpoint of the comparison was defined as the area under the cycles per degree(cpd) curve of the contrast sensitivity profile.

RESULTS: The aspherical IOL group and the spherical IOL group did not differ in baseline charac-teristics. The median age was 71 years and 68% were women in the aspherical group versus69 years and 62% women in the spherical group. The preoperative median UCVA was 20/80 inboth groups. The UCVA, BCVA, pupil size, and capsulorhexis size were not statistically different be-tween the 2 groups. Furthermore, no clinically relevant or statistically significant between-group dif-ferences were observed in the primary clinical endpoint. The median postoperative low mesopiccontrast sensitivity without glare was 73 cpd in the aspherical group and 84 cpd in the sphericalgroup (P Z .624); a similar tendency was observed under high mesopic conditions (median 80cpd and 83 cpd, respectively) (P Z 1.000). Implantation of both IOL types resulted in a negativespherical aberration Z4

0, which was significantly different between the 2 groups (median�0.09 mm aspherical and �0.29 mm aspherical at a pupil size of 4.5 mm) (P<.001).

CONCLUSIONS: No clinically relevant postoperative differences in contrast sensitivity were ob-served between the aspherical microincision IOL and the spherical equivalent model. The develop-ment of microincision IOLs, which fit through corneal incisions smaller than 2.0 mm and improvenight-driving conditions (eg, reduction of glare), could optimize modern biaxial cataract surgery.

J Cataract Refract Surg 2007; 33:393–400 Q 2007 ASCRS and ESCRS

It is well known that some patients with good visualacuity still complain about the quality of their vision.However, with the recent development of wavefronttechnology and with functional vision tests, it is nowpossible to identify and assess the factors that affect vi-sual performance.

As we age, the optical quality of the crystallinelens changes, primarily as a result of a progressive in-crease in wavefront aberrations, particularly spherical

Q 2007 ASCRS and ESCRS

Published by Elsevier Inc.

aberration. Although the lens in young people hasa negative spherical aberration that can compensatefor the positive spherical aberration of the cornea,1,2

in older patients, the spherical aberration of the crys-talline lens progressively shifts toward positive values,increasing the total aberration of the eye and resultingin decreased visual function. Equally, all currentintraocular lenses (IOLs) with spherical surfaces havepositive spherical aberration, adding to the positive

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394 CONTRAST SENSITIVITY: SPHERICAL VERSUS ASPHERICAL IOL

spherical aberration of the cornea, leaving the patientwith visual function belowwhat it could be. In particular,even though standard IOLs provide higher contrastsensitivity than aphakia with spectacle correction,contrast sensitivity in pseudophakic eyes is significantlylower than in normal phakic eyes.3–5 Based on theseobservations, it has been postulated that an IOL able torestore the cornea–lens balance in the young eye wouldresult in better contrast sensitivity than a conventionalIOL.6 This can be achieved by modifying 1 or both sur-faces of the IOL toproduce a lens that introduces negativespherical aberration into the system.

One well-studied IOL with a modified optic designis the Tecnis Z9000 (AMO).7–11 The optic of this IOL isof high-refraction-index silicone; the shape is equi-biconvex with a modified prolate anterior surface.The AcrySof IQ IOL (Alcon) is based on the sameconcept. Still, the design of these IOLs does not allowimplantation through a corneal incision smaller than2.0 mm during biaxial microincision cataract surgery.

The recent development of the aspherical hydro-philic Acri.Smart 36 A IOL (Acri.Tec) for microincisioncataract surgery allows implantation through an inci-sion smaller than 2.0 mm and introduces negative ab-erration into the optical system. The surfaces of thebiconvex 36 A IOL are designed so that an incidentplane wave on the (model) cornea is converted bythe system cornea plus the IOL into an essentially per-fect spherical wave; thus, the imaging of this system isessentially diffraction limited. The model cornea ex-hibits a surface power of 43.0 diopters and a topo-graphical asphericity of �0.26 (elliptically prolate).The apical radii of the refracting surfaces of the IOLare determined with the standard IOL formula (eg,ISO 1197912), which is valid for paraxial rays. Thenthe asphericity of the refracting surfaces of the IOL isvaried (trial and error) until both paraxial rays andaxis distant rays are refracted into the same focus. Atleast for positive IOL powers, the asphericities of therefracting surface of the optimized IOLs are always

Accepted for publication October 31, 2006.

From the Department of Ophthalmology (Kurz, Thieme), JohannesGutenberg-University, Mainz, the Clinical Epidemiology and HealthEconomy Unit (Krummenauer), Dresden University of Technology,Dresden, and the Department of Ophthalmology (Dick), Ruhr Uni-versity, Bochum, Germany.

No author has a financial or proprietary interest in any material ormethod mentioned.

Corresponding author: Sabine Kurz, MD, Department of Ophthal-mology, Johannes Gutenberg-University, Langenbeckstrasse 1,55131 Mainz, Germany. E-mail: [email protected].

smaller than �1 (eg, �5, �8,); that is, the meridians ofthe surfaces are always hyperbolas.

The goal of this prospective parallel cohort trial wasto compare the visual outcomes, contrast sensitivity,and glare disability under different illuminations in pa-tients who had biaxial microincision surgery in eyesimplanted with an aspherical Acri.Smart 36 A IOL orthe design-equivalent spherical Acri.Smart 46 S IOL.

PATIENTS AND METHODS

Seventy-seven eyes of 77 patients admitted for unilat-eral cataract surgery and suitable for IOL implantationwere recruited for a prospective epidemiologicalstudy. The studywas approved by the independent lo-cal ethics committee (Landesarztekammer Rheinland-Pfalz) in 2004. Only patients whomet inclusion criteria(healthy without ocular pathology) and who signedthe written informed consent were eligible for thestudy. Exclusion criteria included an expected visualacuity worse than 20/40, high lens opacity (LensOpacities Classification System III NO5–NO6, C4–C5, P4–P513), zonulysis or defective zonules, pseu-doexfoliation syndrome, use of topical or systemicsteroids or nonsteroidal antiinflammatory drugs, his-tory of uveitis, retinal disease such as diabetic retinop-athy, retinal vein occlusion, age-related maculardegeneration, amblyopia, axial length greater than25.0 mm, ocular ischemic syndrome, previous oculartrauma or intraocular surgery, pupil anomalies, youn-ger than 25 years, and coexisting ocular disease such asglaucoma, optic atrophy, or ocular tumors.

Intraocular lenses

The spherical IOL and aspherical IOL consist of hy-drophilic material with a hydrophobic surface, 6.0 mmoptic, standard haptic thickness of 0.25 mm, totaldiameter of 11.0 mm, and haptic angulation of0 degrees. The IOL optic is designed with rotationalsymmetry; that is, image quality is mainly indepen-dent of pupil size.

Surgical procedure

All cataract operations were performed by the samesurgeon (H.B.D.), who was experienced in biaxial mi-croincision phacoemulsification and implantation ofmicroincision IOLs. Pupils were dilated before surgerywith 1 drop of tropicamide 0.5% and phenylephrine0.5% followed by 1 drop of diclofenac. Topical anes-thesia with lid block injections (bupivacaine 0.5%and lidocaine 2%) and tetracaine 1% on the conjunc-tiva was applied.

A clear corneal incision (width and length 1.5 mm)was made parallel to the limbus using the front ofa trapezoid steel keratome (Nanoedge, Geuder). The

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395CONTRAST SENSITIVITY: SPHERICAL VERSUS ASPHERICAL IOL

respective incision was located at the 11 o’clock posi-tion. A second incision 1.0 mm in width was locatedat the 2 o’clock position for the irrigation sleeve. Theanterior chamber was filled with an ophthalmic visco-surgical device (OVD) (sodium hyaluronate 1%[Healon]).

After a continuous curvilinear capsulorhexis wascreated with a 24-gauge self-bent needle, hydrodissec-tion and hydrodelineation were performed. A 20-gauge phaco needle (AMO) was then inserted throughthe 11 o’clock incision with the right hand. A 19-gaugeirrigating chopper (Dick, Geuder) with 2 irrigationports to the side was inserted into the 2 o’clock incisionwith the left hand. For grooving the nucleus, the stop-and-chop technique described by Koch14 was per-formed. The following settings were applied for bothprocedures using the Sovereign machine (WhiteStar,version 6.0 software, AMO): maximum phaco power55%, bottle height 57 cm, aspiration rate 29 cc/min,and maximum vacuum 50 mm Hg. For emulsificationand aspiration of the nucleus, the following settingswere used: maximum phaco power 40%, bottle height76 cm, aspiration rate 30 cc/min, and maximum vac-uum 300 mm Hg. Pulsed phacoemulsification energywas used with a duty cycle (time of active ultrasoundper second in percent) of 33%; the irrigation pressurewas 58 cc/min. Residual cortex removal and posteriorcapsule polishingwere performed using bimanual irri-gation/aspiration through both the nasal and tempo-ral incisions.

The anterior chamber and capsular bag were rein-flated with the OVD. The microincision was then en-larged from 1.5 to 1.7 mm to allow for implantation ofthe IOL using the A1-4205 Acri.Shooter injector (Acri.Tec). After IOL implantation, the OVD was removedand replacedwith a balanced salt solution. All incisionswere left sutureless. At the end of the procedure, allwounds were checked for leakage and found to be wa-tertight. Intraoperative complications and IOL powerwere recorded at the end of the surgery.

One day postoperatively, all patients received topi-cal prednisolone-21-acetate 4 times a day and gentami-cin drops 3 times a day.

Clinical examination

Preoperative and postoperative examinations weredone by the same investigator. Preoperatively, patientswere examined by slitlamp biomicroscopy. The exam-ination included anterior and posterior eye segments,determination of subjective and objective refraction,and intraocular pressure measurement. Eight weekspostoperatively, uncorrected visual acuity (UCVA)and best corrected visual acuity (BCVA) were mea-sured using Snellen charts. Contrast sensitivity was

measured with the CSV-1000 HGT testing instrument(Vector Vision Inc.), which presents a translucent chartdivided into 4 cycleswith spatial frequencies of 3, 6, 12,and 18 cycles per degree (cpd). The background illumi-nation of the translucent chart does not depend onroom lighting; it is provided by a fluorescent lumi-nance source of the instrument and is automaticallycalibrated to 85 candelas/m2. Each cycle contains 17round patches that are 1.5 inches in diameter. The firstpatch has a high-contrast grating andpresents the sam-ple. The test patches are arranged in 2 rows with 8levels of contrast. The levels decrease from left to rightalong the row in a logarithmic fashion in 0.17 log unitsfor steps 1 through 3 and 0.15 log units for steps 3through 8. Deviation of contrast level is equal in eachcycle and ranges between 0.7 and 2.08 log units, 0.91and 2.29 log units, 0.61 and 1.99 log units, and 0.17and 1.55 log units for 3 cpd, 6 cpd, 12 cpd, and18 cpd, respectively. The 2 patches from the upperand lower lines within the same frequency cycle com-pose a columnwith a similar contrast level; 1 path pres-ents a grating, and the other is blank. During theexamination a grating can appear at the top or bottomof the column. The patients are asked to identify whichpatch has a grating and to note whether both patchesseem blank. The last correct response shows the levelof contrast threshold. The examinations were per-formed with an undilated pupil and best spectacle-corrected visual acuity at 2.5 m.

Afterward, the pupil diameter was determined witha dynamic pupillometer (P2000SA, Procyon Instru-ments) under scotopic (0.03 lux), low (0.82 lux), andhigh mesopic (6.4 lux) conditions. This pupillometerconsists of a binocular optical arrangement that pro-jects an infrared image of each pupil onto separate re-gions of an infrared-sensitive charge-coupled device(CCD) array. The optical arrangement provides infra-red illumination to highlight the pupil–iris border. Itprovides controllable illumination of the eye at visiblewavelengths to present an object for the patient to lookat. The CCD electronics provide a continuous videosignal output, which is sent to an external video framecapture device. The capture device is controlled froma personal computer.With this technique, 10 binocularvideo images are taken within 2 seconds. For evalua-tion, the pupillometer mathematically fits a circleon the border of the pupil. Then, the mean of the 10measurements is aggregated. In the next step, thehorizontal and vertical capsulorhexis diameters weremeasured after pharmacologic mydriasis using slit-lamp biomicroscopy.

Finally, spherical aberration (Z40) of the total eye

was measured using a WASCA wavefront analyzer(Zeiss Meditec) with a Hartmann-Shack sensor. Theeyes were illuminated by a plane infrared wave


emitted by a superluminescence diode (850 nm,45 mW). The light was focused on the retina by the re-fractive power of the cornea, and the lens generateda wavefront into the fovea, which was reflected back-ward. The subjectively determined refractive error ofthe eye was corrected using the focusing portion ofthe apparatus. Three images were recorded for eacheye, and the spherical aberration (Z4

0) was calculatedfor each image for a 4.5 mm pupil. The mean valueof all suitable images was determined for each eyewhen suitable measurements were available.

Statistical analysis

The primary endpoint of the investigation was thearea under the cpd curve of the contrast sensitivityprofile after IOL implantation. This endpoint was con-sidered continuous; its descriptive analysis was there-fore based on medians and quartiles in sub-samples.The IOL samples were contrasted using a 2-sampleWilcoxon test at a multiple 5% level for low mesopicconditions without glare and high mesopic lightingconditions with glare, respectively. The results ofthe 2 parallel Wilcoxon tests were summarized byBonferroni-adjusted P values.

The evaluation of secondary endpoints was per-formed according to their scale level: Data descriptionof continuous endpoints was based on medians andquartiles (graphically on nonparametric box plots, ac-cordingly) and description of categorical endpoints,on absolute and appropriate relative frequencies.Sub-sample comparisons were based on pair-wiseWilcoxon and Fisher tests for continuous and categor-ical endpoints, respectively; intraindividual changesin continuous endpoints were evaluated by pair-wisesign tests. Results of these significant tests were sum-marized by P values; because of the exploratory char-acter of these analyses, the latter were not formallyadjusted for multiple testing.

A multivariate comparison of the IOL samples wasbased on fitting multiple logistic regression modelsby forward selection; results of these modeling proce-dures were summarized by unadjusted likelihood ratiotest P values. All numerical and graphical analyseswere performed using SPSS (release 12.0 for Windows,SPSS, Inc.).

RESULTS

The data of 77 IOL implantations, 52 aspherical and25 spherical, were evaluated 8 weeks after surgery.Table 1 shows the distribution of age, sex, and preopera-tive UCVA in both IOL groups (Table 1). The groupsdid not differ in age (P Z .756, Wilcoxon test), sex(P Z .623, Fisher test), or preoperative UCVA(P Z .840, Wilcoxon test).

Both groups had a median gain in UCVA from pre-operatively (P Z .797) (Table 1) to 8 weeks postopera-tively (P Z .238) (Table 2). The best BCVA 8 weeksafter surgery did not differ significantly between the2 IOL groups (P Z .902) (Table 2).

The samples differed gradually in pupil sizes, irre-spective of the lighting conditions during assessment.Table 2 shows median pupil size under scotopic, lowmesopic, high mesopic conditions. No statistically sig-nificant differences were found between the 2 groupsin the horizontal capsulorhexis size and vertical capsu-lorhexis size (P Z .952 and P Z .891, respectively).

There were no clinically relevant or statistically sig-nificant differences between groups in the primaryclinical endpoint. Table 2 shows the median low mes-opic contrast sensitivity and high mesopic conditionswithout glare (P Z .624, Bonferroni adjusted Wil-coxon) and with glare (P Z 1.000, Bonferroni adjustedWilcoxon). Stratification by patient age showed no dif-ferences (Figures 1 and 2).

The spherical aberration determined by WASCAshowed a statistically significant difference betweenthe 2 groups. The median spherical aberration Z4

0 fora 4.5 mm pupil was �0.09 mm in the aspherical IOLgroup and �0.29 mm in the spherical group (P!.001,Wilcoxon) Figure 3.

A multivariate comparison using multiple logisticregression modeling showed that spherical aberrationwas the only patient characteristic that statistically dif-fered between the 2 groups (P!.001, likelihood ratio).The remaining variables did not showmultivariate sig-nificant differences (scotopic pupil size, P Z .271; lowand high mesopic contrast profiles, P Z .804 andP Z .309, respectively). No eye had intraoperative orpostoperative complications such as posterior capsulerupture, zonulolysis, vitreous prolapse, postopera-tive fibrin formation, hypotony, or wound dehiscence.

Table 1. Medians and quartiles for the distribution of age andpreoperative UCVA in the aspherical and spherical groups.

Treatment Group

Parameter Aspherical IOL Spherical IOL P Value

Sex (%) .623Female 68 62Male 32 38

Age (y) .756Median 71 69IQ range 66–77 64–79

UCVA .840Median 20/80 20/80IQ range 20/200–20/50 20/200–20/40

IOL Z intraocular lens; IQ Z interquartile; UCVA Z uncorrected visualacuity


DISCUSSION

The goal of this study was to evaluate the performanceof the aberration-correcting aspherical Acri.Smart 36 AIOL, designed to provide cataract patientswith a betterquality of vision, and to compare the results with thoseof the corresponding spherical IOL model. To ourknowledge, this is the first standardized comparisonbetween microincision aspherical and spherical IOLs.Eight weeks after implantation, UCVA, BCVA, pupil

Table 2. Medians and quartiles for the distribution of postoper-ative patient data in the aspherical compared with the sphericalIOL treatment group.

Treatment Group

ParameterAspherical

IOLSpherical

IOLP

Value*

UCVA .238Median 20/33 20/33IQ range 20/40 to 20/25 20/40 to 20/25

BCVA .902Median 25/25 20/25IQ range 20/25 to 25/25 20/25 to 25/25

Pupil size (mm) .611Scotopic

Median 4.64 4.37IQ range 4.15 to 5.11 4.09 to 4.96

Low mesopic .312Median 3.70 3.55IQ range 3.33 to 4.11 3.22 to 4.02

High mesopic .603Median 2.99 2.88IQ range 2.64 to 3.35 2.69 to 3.22

Contrast sensitivityWithout glare .624

Median 73 84IQ range 62 to 84 63 to 93

With glare 1.000†

Median 80 83IQ range 63 to 89 75 to 90

Capsulorhexis diameter (mm)Horizontal .952

Median 4.9 4.9IQ range 4.5 to 5.2 4.6 to 5.2

Vertical .891Median 4.8 4.7IQ range 4.4 to 5.1 4.4 to 5.2

Spherical aberration !.001Median �0.09 �0.29IQ range �0.15 to 0.02 �0.36 to �0.22

BCVA Z best corrected visual acuity; IOL Z intraocular lens; IQ Z inter-quartile; UCVA Z uncorrected visual acuity*Wilcoxon test P values between IOL groups (P values for the primaryclinical endpoint Bonferroni corrected for multiplicity)

†Corrected for multiplicity

size under various illumination conditions, and thevertical and horizontal capsulorhexis diameters weresimilar in the 2 groups. In addition, no clinically rele-vant or statistically significant sample differenceswere observed in the investigation’s primary clinicalendpoint (contrast sensitivity under low and highmesopic conditions). However, spherical aberrationdiffered significantly between the 2 groups, witha larger, but negative, aberration in patients with thespherical IOL.

There have been several reports of contrast sensitiv-ity and glare disability in patients with another aspher-ical IOL, the Tecnis Z9000. The contrast sensitivitywith this silicone IOL, which has a modified anteriorsurface, has been compared to that with several con-ventional spherical IOLs. Mester et al.7 compared theTecnis IOLwith a biconvex 10Lwith spherical surfaces(SI-40, Allergan) in an intraindividually randomizedstudy of 45 patients. They examined high-contrastand low-contrast visual acuity, pupil size, photopicand mesopic contrast sensitivity, and wavefront aber-ration 1 and 3 months postoperatively. In their study,the Z9000 IOL significantly improved quality of visionwhen low-contrast visual acuity and contrast

aspheric (36A) spheric (46s)

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Figure 1. Nonparametric box plots for the distribution of low mes-opic lighting contrast profiles (area under the cpd curve) stratifiedfor IOL group and age.


sensitivity were assessed. The authors conclude thatthis aspherical IOL compensates for the positive spher-ical aberration in older eyes, which leads to a signifi-cant improvement, particularly in contrast sensitivityand mesopic visual acuity. Mester et al. performedtheir measurements with the VSRC CST 1500 view intester (Ginsburg box). In our studywe used a compara-ble device, the CSV 1000 HGT, to measure contrastsensitivity. This device identifies contrast sensitivityat identical spatial frequencies (3, 6, 12, and 18 cpd).Mester et al. used the same aberrometer (WASCA),calculating the Zernike coefficient (Z) up to the 5th or-der for a 4.0 mm pupil. In our study, the Zernike coef-ficient was calculated similarly, but for a 4.5mmpupil.The wavefront measurements of Mester et al. did notshow significant spherical aberration in eyes witha Tecnis Z9000 IOL but did show significantly positivespherical aberration in eyes with the spherical IOL.Our measurements showed negative spherical aberra-tions with both the aspherical IOL and spherical IOL.

In general, the WASCA analyzer expresses ocularaberrations in terms of the optical path difference(OPD) and thus displays the opposite sign of sphericalaberration from most references in the ophthalmic


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Figure 2. Nonparametric box plots for the distribution of high mes-opic lighting contrast profiles with glare (area under the cpd curve)stratified for IOL group and age.

literature (which define WF Z �OPD). In WASCA-measured data, the predominant sign of spherical ab-erration in phakic eyes is therefore negative.15 Mesteret al.7 observed a mean pupil of 2.3 mm under phot-opic lightning conditions and 3.5 mm under mesopiclightning conditions 3 months after surgery. Thesevalues are comparable to our results of 3.7 mm in theaspherical IOL group and 3.6 mm in the sphericalIOL under low mesopic lightning conditions and3.0 mm and 2.9 mm, respectively, under high mesopicconditions. Mester et al. did not correlate pupil sizewith contrast sensitivity. In our study, no associationwas observed between contrast sensitivity and pupilsize whether it was assessed under scotopic, low, orhigh mesopic conditions.

The benefit of aspherical IOLs depends on the pupildiameter. As the pupil diameter of younger patients isoften larger than 5.0 mm,Werner and Roth16 concludethat at the least, these patients may gain improvedmesopic visual function from aspherical IOLs.

Improvement in low-contrast visual acuity and con-trast sensitivity could influence the quality of visionpositively, particularly for car drivers.17–19 This im-provement becomes increasingly important with largerpupils.20,21 Holladay et al.22 and Mrochen and Seiler23

found significant higher order aberrations in 98% of pa-tients with a 4.5 mmpupil diameter, leading to a reduc-tion in the quality of the retinal image.

Bellucci et al.8 compared visual acuity and contrastsensitivity between the Tecnis IOL and a foldable hy-drophobic IOL (AcrySof SA60AT, Alcon) interindivid-ually. Contrast sensitivity wasmeasured by sinusoidal


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Figure 3. Nonparametric box plots for the distribution of sphericalaberration Z4

0 (mm) stratified for IOL group.


grading charts for photopic and mesopic luminancelevels and assessed at spatial frequencies of 1.5, 3, 6,12, and 18 cpd. Eyes with the aspherical IOL had sig-nificantly better contrast sensitivity at all spatial fre-quencies in photopic and mesopic conditions, andclinically relevant differences were found at higherspatial frequencies.

In a 2003 prospective evaluation, Kershner9 com-pared the Tecnis IOL with the AcrySof SA60AT IOLand the AA4207VF foldable silicone IOL (Staar Surgi-cal). The IOLs were compared in functional visual per-formance interindividually. Photopic and mesopicfunctional acuity contrast testing using a Ginsburgsystem (Optec 3500 Vision Testing System) at 1.5, 5,3, 6, 12, and 18 cpd was performed preoperativelyand 3 months postoperatively. Better UCVA in theaspherical group was reported; however, no statisti-cally significant difference in BCVA was observed be-tween the groups. The aspherical IOL provideda significant improvement in functional acuity con-trast testing, mostly in night vision and night visionwith glare, compared with the other IOLs.

In a parallel cohort study, Packer et al.10,24 founda difference in contrast sensitivity between the TecnisIOL and another hydrophobic acrylate IOL (AR40,AMO); they found adifference at the peak contrast sen-sitivity (3 cpd) of 78% under mesopic conditions. Inboth studies, eyeswith theTecnis IOLperformedbetterthan eyes with the conventional IOL. Other studies in-dicate that each IOL produces a different modulationtransfer function (relationship between the contrast ofan object and its image) when implanted in eyes, givingrise to differences in contrast sensitivity.25–27 Further-more, apart from the optic design, the underlying opticmaterial and refraction index could play a role in theamount of induced aberration.25

Centration of an aspherical IOL could also play animportant role in the quality of vision. Preussneret al.28 used ray-tracing calculations and found im-proved optical quality with aspherical IOLs, but onlyin cases in which there was good centration. The corre-sponding effect was low as soon as the IOLwas decen-tered 0.5 mm or more.22,28 The single-piece designused in our study may be expected to be independentfrom tilt, but the latter proposition must be proved infurther studies.

No significant differences in contrast sensitivitywere found between the aspherical IOL and sphericalIOL 8 weeks after surgery. One possible explanationis that the relatively older patients (median: 71 yearsaspherical group and 69 years spherical group) hadsmaller pupil diameters than younger individuals(Table 2). The effect of the aspherical IOL on largerpupil diameters must be evaluated in further studies.Another possible explanation is the postoperative

deflection of the IOL by capsular bag shrinkage anda corresponding loss of the aspherical effect. A furtheraspect could be found in the centration of the IOL,which was not measured in our study. Furthermore,the investigation is limited because of its interindivid-ual approach; an intraindividually randomized com-parison between the eyes of 1 patient by implantationof an aspherical IOL in 1 eye and a spherical IOL inthe other eye could improve the detection of IOLdiffer-ences. On the other hand, the asphericity of the Acri.Smart IOL may not be sufficient to the ‘‘normal’’ cor-nea; that is, intraindividual randomization would be-come difficult (if nor impossible) in most patients.

In conclusion, our investigations did not show clini-cally relevant improvement in contrast sensitivity bythe implantation of an aspherical microincision IOLover implantation of a spherical IOL with an equiva-lent design. A negative spherical aberration was ob-served in both IOL groups. Along with improvedoutcomes, however, comes the surgeon’s responsibil-ity to continue to strive for better functional visual per-formance while incurring the least disability from theprocedure. High-technology IOLs and microminiatur-ized surgical techniques can leave patients spectaclefree after cataract surgery29 andprovide results compa-rable to those achieved by refractive surgery in case ofmoderate myopia.30 In particular, cataract is a diseasewith increasing demographic and economic relevance;thus, surgeons must continue performing research toprovide better quality vision. As cataract surgery ispredominantly performed in the elder patient, an opti-mized IOL should fit through a microincision and si-multaneously counteract spherical aberration (Z4

0) witha surface modifying Z (Z4

0) as a contributing factor inreduced contrast sensibility and glare.31 The develop-ment of microincision IOLs that fit through cornealincisions smaller than 2.0 mm and improve night-driving conditions is challenging and could optimizemodern biaxial cataract surgery.

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