dimitriya bozukova, phd, christophe pagnoulle, phd, christine j

LABORATORY SCIENCE

Biomechanical and opt
ical properties of 2 newhydrophobic platforms for intraocular lenses
Dimitriya Bozukova, PhD, Christophe Pagnoulle, PhD, Christine J�erome, PhD

Q

P

1404
2013 A
ublished

PURPOSE: To compare the biomechanical and optical properties of 2 new hydrophobic platformsand a series of commercially available foldable intraocular lenses (IOLs).

SETTING: Center for Education andResearch onMacromolecules, University of Li�ege, Li�ege, Belgium.

DESIGN: Experimental study.

METHODS: Eleven benchmark foldable IOLs (iPure, Podeye, Acrysof SN60WF, Envista MX60,Sensar AR40e, Tecnis ZCB00, Isert 251, AF-1 YA-60BB, Finevision, Acri.Tec 366D, and Ioflex)were tested by standard analytical methods for biomechanical, rheological, and opticalinvestigations under identical conditions.

RESULTS: With 1 exception, IOLs equilibrated in aqueous medium had a lower glass-transitiontemperature, higher deformability, lower injection forces, and complete recovery of their initialoptical properties after injection. Typical hydrophobic acrylic dry-packaged IOLs required higherinjection forces with high residual deformation and lost part of their initial optical quality afterinjection. Hydrophobic acrylic C-loop, double C-loop, and closed quadripod haptics appliedoptimum compression forces to the capsular bag with negligible optic axial displacement and tiltcompared with plate haptics and poly(methyl methacrylate) haptics.

CONCLUSIONS: The combination of the C-loop haptic and the bioadhesive glistening-free material,which absorbs a predetermined amount of water, allowed for a biomechanically stable IOL. Thesame material used in association with a double C-loop haptic design facilitated the perioperativemanipulation and placement of the IOL in a smaller capsular bag without impairing the otherbiomechanical properties of a single C-loop design.

Financial Disclosure: Dr. Pagnoulle has a proprietary interest in the GFmaterial. Drs. Pagnoulle andBozukova are employees of Physiol S.A. Dr. J�erome has no financial or proprietary interest in anymaterial or method mentioned.

J Cataract Refract Surg 2013; 39:1404–1414 Q 2013 ASCRS and ESCRS

Recently, the trends in cataract and refractive surgeryare toward diminishing corneoscleral incisions tolower the risk for postsurgical complications.1 There-fore, foldable intraocular lenses (IOLs) of acrylic orsilicone polymers are preferred for replacement ofthe opacified cataractous natural lens.

An IOL is considered to be compatible with mini-mally invasive cataract surgery when it (1) is injectablevia a sub-3.0 mm incision, (2) does not permanentlychange its optical andmechanical properties as a resultof IOL folding and compression during injection, (3)preserves its biocompatibility, (4) does not inducehigher posterior capsule opacification (PCO) rates,and (5) provides sufficient refractive stability.1

Sharp-edged IOL platforms have proved their effi-ciency in PCO prevention,2,3 although some authors

SCRS and ESCRS

by Elsevier Inc.

state that sharp-edge is not able to prevent PCO alone.4

In that aspect, the materials' properties,5–7 the IOLdesign,5,7 and the haptic shape, material, andstrength6–8 remain important factors. Previous studiesreport higher biocompatibility with a reduced risk forPCO formation,9,10 better biomechanical stability,7

and better refractive and rotational stability8 of hydro-phobic acrylic IOLs comparedwith their silicone plate-haptic counterparts.

For 1-piece IOLs, the material should be able to pro-vide the IOL with excellent optical properties as wellas sufficient rotational andmechanical stability, whichis usually obtained by selecting the proper haptic de-sign. It has been thought that haptics that apply stron-ger compression force to the capsular bag provide theIOLwith better biomechanical stability.11 However, in

0886-3350/$ - see front matter

http://dx.doi.org/10.1016/j.jcrs.2013.01.050

http://dx.doi.org/10.1016/j.jcrs.2013.01.050

1405LABORATORY SCIENCE: TWO NEW HYDROPHOBIC PLATFORMS FOR IOLS

experimental studies, Lane et al.7 and Pandey et al.6

found that the risk for posterior capsule striae, capsulestretch, and capsulorhexis ovaling was higher whensilicone or hydrophilic acrylic IOLs with poly(methylmethacrylate) (PMMA), polypropylene, or platehaptics were implanted than when hydrophobicacrylic 1-piece IOLs were used. Such deformations ofocular tissue may result in an increased risk for PCO,fibrosis, IOL decentration, and IOL tilt and may causezonular stress in a direction parallel to the IOLhaptics.6

Meanwhile, the mechanical resistance of the mate-rial of a typical hydrophobic acrylic IOL is animportant advantage but may also be a major limita-tion. Such material is usually slow to fold and unfold,which may cause it to become damaged duringinjection and lead to loss of optical quality afterimplantation as a result of IOL deformation.1 The pres-ence of a low, but controlled, quantity (!5 wt%) ofsmall molecules with a plasticizing effect, such aswater, may significantly improve this aspect.12 Itimparts the hydrophobic acrylic IOL with shapememory properties, improved foldability, andcontrolled unfolding while preserving its mechanicalstability and biocompatibily.13

The purpose of this study was to experimentallyevaluate the biomechanical and rheological propertiesof 11 benchmark IOLs of hydrophobic and hydrophilicacrylic materials with different optic designs (refrac-tive, diffractive) and haptic designs (C-loop, double-C-loop, plate, PMMA, closed quadripod). The resultsin this study should be considered in relation to resultsin clinical trials.

MATERIALS AND METHODS

Tested Intraocular Lenses

The iPure (C-loop) and Podeye (double C-loop) IOLs(Physiol S.A.) are of a proprietary, patented hydrophobicacrylic glistening-free material (GF)14 and have obtainedConformit�e Europ�eenne certification. The other testedbenchmark IOLs were as follows: Finevision (Physiol S.A.),

Submitted: September 6, 2012.Final revision submitted: January 9, 2013.Accepted: January 27, 2013.

From the Research and Development Department (Bozukova,Pagnoulle), Physiol S.A., and the Center for Education andResearch on Macromolecules (J�erome), University of Li�ege,Li�ege, Belgium.

Supported by the R�egion Wallone (public Belgian support).

Corresponding author: Dimitriya Bozukova, PhD, Li�ege SciencePark, All�ee des Noisetiers, 4, B-4031 Li�ege, Belgium. E-mail:[email protected].

J CATARACT REFRACT SURG - V

Acri.Tec 366D (Carl Zeiss Meditec AG), Ioflex (MediphacosLtda.), Acrysof SN60WF (Alcon Laboratories, Inc.), EnvistaMX60 (Bausch&Lomb, Inc.), Tecnis ZCB00 and SensarAR40e (Abbott Medical Optics, Inc.), and Isert 251 andAF-1 YA-60BB (Hoya Surgical Optics GmbH) (Table 1).Systematically, new IOLs were used for each test. Whenappropriate, method validation was performed with a seriesof 5 samples of the Podeye IOL and the corresponding andsimilar variation in themeasuredvaluewasused for all testedIOL models.

Water Uptake

The quantity of water present in a material influences itsrigidity, opacity, sterilization ability, and biocompatibility.For determining the water uptake, IOLs (1 per IOL model)were incubated in water for 24 hours at 65�C to reach theirequilibrium water content. Their surface was then dried bya short air flush to eliminate water droplets, and the IOLswere weighed with a laboratory microbalance to obtain theirweight in the hydrated state (Wh). They were then dried at65�C for 24 hours and their weight in the dry state (Wd)was measured. The water uptake was determined accordingto the following equation:

WUwt% Z ½ðWh�WdÞ � 100�=Wd

The measurements were performed with a balance withprecision of 0.0001 g, which can induce 0.45% variation inthe measured value.

Glass-Transition Temperature

Glass-transition temperature (Tg) is the temperature atwhich an amorphous material passes from its rigid glassystate to its soft rubbery state.15 It is particularly importantin the case of IOLs because it determines their ability tofold upon implantation in the eye. Hence, the lower theglass-transition temperature (Tg) of the material, the morefoldable the IOL. This parameter was determined for alltest IOLs in their original packaging state with a differentialscanning calorimeter (Perkin-Elmer series 7, operating withPyris version 8 software). Specimens were hermeticallysealed in aluminum capsules, placed in the equipment,cooled to �60�C and then heated from �60�C to 100�Cwith a heating speed of 15�C/min. One measurement wasperformed per IOL model. The method was validated witha series of 5 samples of the Podeye IOL, and a value variationcoefficient of 10.7% was established.

Haptic Compression Force

The force applied by the haptics to the capsular bag is im-portant for IOL rotational and refractive stability and mayhave some effect on its resistance to PCO. The force wasdetermined with a compression force tester (MFC-1385-IOL, Applied Micro Circuits Corp.); the possible valuevariation was less than 0.2%. Before the measurement, theequipment was calibrated according to the standard manu-facturer's procedure. Intraocular lenses in their originalpackaging state (1 per IOL model) were placed betweenthe 2 jaws, and the compression force, in milligrams force,was measured for well diameters of 11.0 mm, 10.5 mm,10.0 mm, and 9.5 mm, corresponding to various sizes ofthe capsular bag. In all cases, measurements were taken

OL 39, SEPTEMBER 2013

mailto:[email protected]

Table 1. Experimental and manufacturers' data for the test and reference IOL models.

Model Design* Material*Hazardous-Light

Protection* Sterilization*Packaging

State* WU, %† Tg, �C† RI @ 23�C*

iPure Monofocal, 1-piece, C-loop

Hydrophobic acrylicGF materialA

UV/blue Steam Hydrated 4.9 9 1.52

Podeye Monofocal, 1-piece,double C-loop

Hydrophobic acrylicGF materialA

UV/blue Steam Hydrated 4.9 9 1.52

EnvistaMX60

Monofocal, 1-piece,double C-loop

Hydrophobic acrylic UV Gamma Hydrated 4.9 21 1.54

Acri.Tec366D

Bifocal, double-platehaptics

Hydrophilic acrylicwith hydrophobic

surface

UV Steam Hydrated 28.0 1 1.46

Finevision Trifocal, 1-piece,closed-hapticsquadripod

Hydrophilic acrylic UV/blue Steam hydrated 25.0 1 1.46

Ioflex Monofocal, 1-piece,C-loop

Hydrophilic acrylic UV Steam Hydrated 25.4 1 1.46

AcrysofSN60WF

Monofocal, 1-piece,C-loop

Hydrophobic acrylic UV/blue EO Dry 1.1 15 1.55

TecnisZCB00


Hydrophobic acrylic UV EO Dry 2.4 14 1.47

SensarAR40e


Hydrophobic acrylicwith PMMA haptics

UV EO Dry 1.4 16 1.47

AF-1YA-60BB



UV/blue EO Dry 0.0 18 1.52

Isert 251 Monofocal, pseudo1-piece, C-loop


UV/blue EO Dry 0.6 18 1.52

EO Z ethylene oxide; GF Z glistening free; RI Z refractive index; PMMA Z poly(methyl methacrylate); UV Z ultraviolet; WU Z water uptake*Data from the intraocular lens specification†Experimental data

1406 LABORATORY SCIENCE: TWO NEW HYDROPHOBIC PLATFORMS FOR IOLS

30 seconds after haptic compression to give the hapticmaterial time to relax.

Axial Displacement in Compression

On compression, the haptics of the IOLs may deformdifferently and, therefore, the IOL optic may remain at theinitial position or not, moving forward or backward alongthe optical axis. This is referred to as the effective lens posi-tion (ELP).

This displacement was measured in air at 20�C G 2� (SD)for all tested IOL models (1 per IOL model) in their originalpackaging state (hydrated or dried). The IOLswere placed inwells of 11.0 mm, 10.5 mm, 10.0 mm, and 9.5 mm. The axialdisplacement was determined using an optical comparatorwith a precision of G0.01 mm according to the proceduredescribed by International Organization for Standardization(ISO) 11979-9.16

Theoretical Maximum Optic Tilt

As a consequence of haptic compression and eventualoptic axial displacement, optic tilt can occur. This degree oftheoretical maximum optic tilt was calculated based onthe measured axial displacement according to the adaptedprotocol described in ISO 11979-916 (1 per IOL model).

The following equation was applied:


Tilt maximum �Z ½asinðAD=RÞ�where AD is the measured axial displacement for a certainwell diameter and R is the radius of the respective IOL optic.

Rheological Properties

The IOLs were tested for their rheological properties oncompression and relaxation with the aim of predicting theirbehavior during and after implantation, respectively. Theresults from the creep step of the rheological analyses givean idea of the IOL's mechanical resistance during foldingand injection and are compared with the forces requiredfor IOL injection in an in vitro model (see Intraocular LensInjection Force below).

The data obtained for the recovery refer to the shapememory of the IOL and its ability to recover its initial opticalproperties, which is monitored on an optical bench (seeOptical Bench Measurements below).

The rheological behavior of the optic portion of the IOLswas tested by dynamic mechanical analyses with the IOLsin their original packaging state (1 per IOL model; methodvalidation with 5 Podeye IOLs) with a dynamic mechanicalanalyzer (Perkin-Elmer series 7) in the creep/recoverymode. The IOLswere placed between the two 15.0mm circu-lar parallel plates, and the system was closed. Initially, theywere conditioned by applying 20 mN force for 3 minutes toreach the test temperature of 20�C, corresponding to the



temperature at which they are usually implanted in thesurgical room. In the first step, the creep step, a static forceof 1000 mN was applied for 60 seconds. This step evaluatesthe behavior of the IOL on compression, as in the case ofIOL folding during implantation. For the second step, therecovery step, the force was dropped to 20 mN and thematerial relaxation was observed for 300 seconds, which cor-responds to the IOL unfolding in the eye. Longer relaxationtimes were not tested because the water-absorbing materialsdehydrated and false results were obtained. The methodwas validated with 5 Podeye IOLs, and a value variationcoefficient of 10.5% was established.

Hence, a purely elastic material, such as rubber, deformsimmediately on force application, achieving its deformationequilibrium level instantly (Figure 1). Such material also rap-idly recovers its initial shape once the force is removed, withno residual deformation and with preserved molecularorganization.

In contrast, plastic or viscous materials, such as modelingclay, deform continuously on applied stress. Their moleculesirreversibly reorganize so that once the stress is removed,these materials are no longer able to recover their shapeand high residual deformation is observed.17

Intraocular Lens Injection Force

The force necessary to inject an IOL is an important factorbecause high injection forces may be related to an enhancedrisk for IOL damage (scratches, haptic or optic cracks) orinjection system damage (cartridge breaking).

Although the manufacturers of the IOLs in this study usu-ally recommend the use of particular injection systems, theforce required for IOL injection was tested each time withthe Accuject 2.2 -1P (Medicel AG) to allow comparison ofthe mechanical properties of the IOL materials (1 per IOLmodel). The injection system uses a standard cartridge thatis connected to the injector device. Sodium hyaluronate1.55% (Physiovisc Integral) was used as the ophthalmicviscosurgical device (OVD), and the measurements wereperformed at 21�C with a compression/traction mechanicalbench (FL Plus Lloyd Instruments, Ametek) with a possiblevalue variation of less than 0.05%, simulating surgicalmanipulation. The test equipment is supplied with a loadcell of 100 N and operates with Nexygen FM software(Chatillon, Ametek, Inc.). The typical injection procedureincludes the following traditional operational steps: (1) fill-ing the empty cartridge with OVD, (2) loading the IOL,

Figure 1. Relationship between the deformation level over time forviscous and elastic materials during and after compression.


(3) closing the cartridge and placing it on the mechanicalbench, (4) pushing the piston 20.0 mm at a speed of200.0 mm/min, (5) pushing the piston 18.0 mm more ata speed of 150.0 mm/min, (6) pulling the piston back5.0 mm with a speed of 200.0 mm/min, and (7) pushingthe piston 10.5 mm at a speed of 150.0 mm/min. The IOLwas typically expulsed during step 7, and the value of theforce maximum was taken into consideration.

A control assay was performed, where an injection simu-lation was performed with an empty cartridge (no IOLinside). The purpose of this test was to determine the forcegenerated by the friction of the silicone cushion within thecartridge.

Optical Bench Measurements for MonofocalIntraocular Lenses

A manufacturer of an IOL predicts the quality of theretinal image of an IOL by measuring a parameter calledthe modulation transfer function (MTF).1 For the monofocalIOLs in this study, the MTF was measured by an eye modelwith an optical bench (NIMO TR0815, Lambda X) accordingto ISO 11979-2.18 The IOLs (1 per IOL model) were posi-tioned in a quartz cuvette filled with buffered salt solution(0.9% sodium chloride, Baxter). The measurement wasperformed at a 3.0 mm aperture and spatial frequency of100 cycles/mm. The relative contrast sensitivity recoverywas calculated according to the formula:

Relative contrast sensitivity%Z�MTF � =MTF0�� 100

where MTF0 and MTF* are the MTF values measured beforeinjection and at each timepoint after injection, respectively.The relative contrast sensitivity is presented as the percent-age of contrast sensitivity recovered after injection atdifferent timepoints.

In a practical way, after IOL implantation, when 70% ofMTF recovery is achieved, the IOL is properly unfolded,the surgeon can manipulate it if necessary, and the surgerycan be achieved within a reasonable time.

Optical Bench Measurements for DiffractiveMultifocal Intraocular Lenses

The MTF of multifocal IOLs was measured by real-timeMTF and optical-power measuring using an eye modeloptical bench (PMTF, Lambda X) according to ISO11979-9.16 The IOLs (1 per IOL model) were positioned ina quartz cuvette filled with buffered salt solution (0.9%sodium chloride). The MTF value was determined for farvision, corresponding to the main optical lens power,3.0 mm aperture, and spatial frequency of 100 cycles/mm.The relative contrast sensitivity was calculated as describedabove. This equipment takes longer to measure thanthe NIMO TR0815; therefore, only the result 1 hour afterinjection was taken into consideration.

The optical properties of the IOLs after injection weredetermined by measuring their MTF before folding and upto 1 hour after injection with the model injecting systemand the OVD. The IOLs were divided into 4 MTF groupsaccording to the criterion of having recovered 70% of theirinitial MTF over a fixed time interval. These groups wereas follows: Group 1, recovered 70% in fewer than 30 seconds;Group 2, recovered 70% in fewer than 1 minute; Group 3,recovered 70% in fewer than 4 minutes; Group 4, recovered70% in 5 minutes or more.



RESULTS

General Intraocular Lens Characteristics, WaterUptake, and Glass-Transition Temperature

Table 1 and Figure 2 show the main IOL character-istics and platforms as they were experimentallyobtained or derived from the manufacturers' data-sheets. Except for the AF-1 YA-60BB, all IOLs absorbwater on heating. The hydrophobic acrylic IOLsiPure, Podeye, and Envista MX60 and the hydrophilicacrylic IOLs Finevision, Acri.Tec 366D, and Ioflex arepackaged and sterilized in saline buffer solution andachieve their equilibrium water content beforeimplantation.

The water present in the IOLs packaged in theirhydrated state imparted them with flexibility, andlower Tg values were measured for these models,which decreased with increasing water content. In


contrast, dry-packaged hydrophobic acrylic IOLs lefttheir glassy state at temperatures higher than 14�C,which was the case for the Acrysof SN60WF, TecnisZCB00, Sensar AR40e, AF-1 YA-60BB, and Isert251models. EnvistaMX60 IOLs, although conditionedin water, showed a relatively high Tg.

According to the refractive index values providedby the manufacturers, hydrophobic acrylic IOLs thatare said to comprise aromatic polymers in their for-mulations (Acrysof SN60WF, iPure, Podeye, EnvistaMX60, AF-1 YA-60BB, Isert 251) were distinguishedby their high refractive index (O1.5). Meanwhile, theother hydrophilic and nonaromatic hydrophobicIOLs presented a limited refractive index (range 1.46to 1.47). Therefore, at a constant dioptric power,thinner IOLs can be obtained from higher refractiveindex materials.

Figure 2. Designs of the test andreference IOLs derived from themanufacturers' datasheets.



Haptic Compression Force

For a test well diameter of 10.0 mm, correspondingto amean anatomic capsular bag size, higher compres-sion forces were measured for the IOLs with PMMAC-loop haptics (Sensar AR40E andAF-1 YA-60BB)and plate hydrophilic acrylic haptics, followed bythose with closed quadripod hydrophilic haptics(Table 2). Significantly lower compression forces innearly the same range were measured for all 1-piecehydrophobic C-loop and double C-loop IOLs andwere 1 order of magnitude lower than those in the first3 categories.

The general tendency, whatever the haptic designand IOLmaterial, was toward increasing compressionforce with a decreasing test well diameter. However,the slope of this increase was stronger for the PMMAC-loop and the plate-haptic IOLs. It was intermediatefor the closed quadripod models and lowest for theC-loop and double C-loop IOLs.

The Envista MX60 IOL behaved as a 1-piece hydro-phobic model for the higher well diameters (11.0 mmand 10.5mm) but showed elevated radial compressionfor the lower diameters (10.00 mm and 9.5 mm),comparable to 3-piece PMMA C-loop IOLs.

Axial Displacement in Compression and TheoreticalMaximum Optic Tilt

For a well diameter of 10.0 mm, the optical part ofthe plate-haptic (Acri.Tec 366D) and PMMA C-loop(Sensar AR40e, AF-1 YA-60BB) IOLs was prone to

Table 2. Compression forces applied by the IOL haptics to the capsular

ParameterIoflex21.0 D

AcrysofSN60WF19.5 D

TecnisZCB0022.0 D

Podeye20.0 D

iPure20.0 D

Haptic compressionforce (mg)

11.0 mm test well 15 15 24 5 1710.5 mm test well 28 22 29 22 3510.0 mm test well 34 34 39 38 509.5 mm test well 46 45 54 63 69

Axial displacement(mm)*

11.0 mm test well 0.000 0.000 0.000 0.000 0.00010.5 mm test well 0.027 0.000 0.024 0.011 0.01110.0 mm test well 0.046 0.000 0.034 0.047 0.0279.5 mm test well 0.032 0.054 0.084 0.091 0.065

Maximum tilt (mm)*11.0 mm test well 0.00 0.00 0.00 0.00 0.0010.5 mm test well 0.54 0.00 0.46 0.21 0.2110.0 mm test well 0.92 0.00 0.65 0.90 0.529.5 mm test well 0.64 1.03 1.60 1.74 1.24

*Absolute value


displace along the optic axis on haptic compression,which may result in severe tilt (up to 20 degrees)(Table 2). As expected, diminishing the test-welldiameter resulted in more severe shift and tilt.

The 1-piece C-loop (iPure, Acrysof SN60WF, TecnisZCB00, Isert 251), double C-loop (Podeye), and closedquadripod (Finevision) platforms showed lower axialdisplacement. Thus, the calculated theoretical tiltwas also low even with smaller well diameters.

This axial stability was also observed for the hydro-phobic acrylic Envista MX60 IOL with a well diameterup to 10.0. However, a 2.79-degree tilt was calculatedfor the 9.5 mm well diameter (Table 2).

Rheological Properties

Three groups of IOLmaterials may be distinguishedwith respect to their compressibility (Table 3 andFigure 3) and shape memory: (1) (hydrophilic) elasticlike, (2) (hydrophobic) viscoelastic, and (3) hydropho-bic plastic-like materials. The first group comprises thehydrophilic models Acri.Tec 366D, Finevision, andIoflex, which had higher equilibrium deformationlevels (15% to 16%) after 60 seconds of applied con-stant stress, which is in accordance with their highwater uptake and lowTg (1�C). These IOLs completelyrecovered their initial shape with less than 0.2% ofresidual deformation 300 seconds after the stress wasremoved as a result of the hydrogel elastic memory.

Among the hydrophobic acrylic models, soft visco-elastic and less deformable plastic-like materials may

bag.

Isert 25119.5 D

AF-1YA-60BB20.0 D

Finevision20.0 D

EnvistaMX6020.5 D

Acri.Tec366D20.0D

SensarAR40E20.0 D

11 77 3 19 100 17812 132 26 31 371 26528 222 195 126 318 36594 346 367 380 388 456

0.000 0.000 0.000 0.000 0.000 0.0000.007 0.106 0.021 0.053 0.297 0.2000.018 0.291 0.032 0.057 1.032 0.2000.063 0.436 0.045 0.146 1.025 1.135

0.00 0.00 0.00 0.00 0.00 0.000.13 2.02 0.39 1.01 5.68 3.820.34 5.57 0.60 1.09 20.12 3.821.20 8.36 0.84 2.79 19.98 22.23


Table 3. Residual optic deformation after 60 sec of compression and 300 seconds relaxation after compression.

Parameter

Optic Deformation on Compression (%)

Ioflex21.0D

AcrysofSN60WF19.5 D

TecnisZCB0022.0 D

Podeye20.0 D

iPure20.0 D

Isert 25119.5 D

AF-1YA-60BB20.0 D

Finevision20.0 D

EnvistaMX6020.5 D

Acri.Tec366D20.0 D

Sensar AR40E20.0 D

After 60 s compression 14.7 9.87 11.07 9.7 9.7 5.36 8.01 16.11 6.14 16.1 7.84After 300 s relaxation 0.18 1.57 2.4 0.7 0.7 1.71 2.46 0.11 2.3 0.20 2.33


be distinguished. The Acrysof SN60WF, iPure,Podeye, and Tecnis ZCB00 IOLs, belonging to theintermediate group of hydrophobic soft IOLs,deformed spontaneously on compression, reachingup to 11.07% deformation within 30 seconds. Theyrecovered their initial shape almost completely after1 hour of relaxation with 0.7% to 1.57% residualdeformation. Mechanically, the IOLs of theglistening-free hydrophobic acrylic material (Podeyeand iPure) were closer to the hydrophilic acrylicIOLs, with a low Tg (9�C) and lower residual defor-mation levels (0.7%) than the other soft hydrophobicIOLs.

The less deformable hydrophobic IOLs (SensarAR40e, AF-1 YA-60BB, Isert 251, and Envista MX60)after 60 seconds of compression achieved equilibriumdeformation levels of 5% to 8%, 2 to 3 times lowerthan the levels of their hydrophilic counterparts. Onrelaxation, they had a residual deformation of1.71% to 2.46%. These IOLs had a higher Tg (between16�C and 21�C).

The Tecnis ZCB00 IOL had a particular behavior,presenting relatively high residual deformation(2.4%) after 300 seconds, although it compressedsimilarly to the soft hydrophobic models and its Tgwas moderate (14�C).

Figure 3.Rheological properties of the test and reference IOLmodelstested at 20�C in their original state of packaging (dry or hydrated).


Intraocular Lens Injection Force

With the tested in vitro model simulating the surgi-cal intervention, lower injection forces (14 to 16 N)were measured for the hydrophilic acrylic models(Finevision and Ioflex) and were comparable to theforce of the control assay without IOL in the cartridge(Figure 4). A force of 19 Nwas required for injection ofthe Acri.Tec 366D IOL.

The hydrophobic IOLs from the less deformable,plastic-like category of the compression test (SensarAR40e, Isert 251, Envista MX60, and AF-1 YA-60BB)required higher injection forces (23 to 29 N; up to71 N for AF-1 YA-60BB with cartridge damage)compared with the 18 to 19 N required for the softhydrophobic viscoelastic IOLs (Podeye, iPure, AcrysofSN60WF, and Tecnis ZCB00).

These results are indicative only and should beconsidered with caution because the injectionforces may change when using injecting systems rec-ommended or provided by the suppliers for eachIOL model.

Optical Properties After Intraocular Lens Injection

Table 4 shows the loss of contrast sensitivity 1 hourafter IOL injection. Figure 5 shows the contrast

Figure 4. Force required for the injection of the IOLs.


Table 4. Loss of contrast sensitivity 1 hour after injection.

Parameter FinevisionAcri.Tec366D Ioflex iPure/ Podeye

AcrysofSN60WF Envista

TecnisZCB00 Isert 251

SensarAR40e

AF-1YA-60BB

Contrast sensitivity loss (%) 0.0 0.0 0.0 0.0 1.7 3.5 4.1 7.1 9.6 14.5


sensitivity of the IOLs before and after folding duringinjection.

The hydrophilic acrylic trifocal Finevision and bifo-cal Acri.Tec 366D IOLs were in MTF Group 1, recover-ing 70% of their initial MTF in less than 30 seconds.Within 1 minute, these IOLs had recovered 100% oftheir initial contrast sensitivity, unfolding more rap-idly than their hydrophilic acrylic counterparts Ioflex,iPure, and Podeye. These IOLs were in MTF Group 2,recovering 70% of their initial MTF in less than 1 min-ute. They recovered 100% of their MTF0 in 1 hour andhad 0.7% residual deformation after 5 minutes.

The hydrophobic acrylic Acrysof SN60WF, TecnisZCB00, and Sensar AR40e IOLs were in MTF Group3, unfolding quite slowly (up to 4 minutes for 70%MTF recovery). None of these models completelyrecovered their initial MTF value, losing 1.7%, 4.1%,and 9.6% of MTF0, respectively, even 1 hour after theinjection. These values are in good agreement withthe corresponding residual deformation levels of1.57%, 2.4%, and 2.33%, respectively, measured after5 minutes of relaxation.

The Envista MX60, Isert 251, and AF-1 YA-60BBIOLs were in MTF Group 4. They were the leastdeformable hydrophobic models according to therheological classification of the previous test, andthey required more than 5 minutes to unfold.

One hour after injection, the IOLs showed a loss of3.5% and 14.5% of their MTF0, respectively.

DISCUSSION

The water absorbed by some polymers for biomedicalapplication at a predetermined level means the

Figure 5. Contrast sensitivity of the IOLs before and after foldingduring injection (MTF Z modulation transfer function).


medical device can be preconditioned in aqueousmedium, allowing it to achieve its equilibrium watercontent before implantation. This is especially impor-tant for IOLs, which can develop glistenings afterimplantation, particularly if packaged in their drystate.19 In such cases, the devices are sterilized bysteam or gamma sterilization. According to Davis,20

steam sterilization is advantageous because of itsefficacy, speed, process simplicity, and lack of toxicresidues. In contrast, gamma sterilization may leadto partial oxidative degradation of the implant 20 orof its packaging.21 It may induce generation ofradicals, crosslinking, and chemical decomposition ofthe material or of some of its additives, such aschromophores.22–24

The iPure and Podeye IOLs are made of a proprie-tary, patented hydrophobic acrylic glistening-freematerial incorporated with both ultraviolet and blue-light protection filters covalently copolymerized withthe main polymer network. The IOLs are steam steril-ized. Despite the small amount of absorbed water, theglistening-free material shows bioadhesiveness com-parable to that of typical hydrophobic acrylic IOLmaterials, and it is plasticized by this water content.13

This bioadhesiveness and the 360-degree squarededge of the 2 IOLs minimize PCO rates.7

Any water molecules present in the material wouldhave a plasticizing effect, facilitating the chain mobil-itywithin the polymer network. This would contributeto a decrease in the Tg of the IOL material, making theIOL softer, even at lower temperatures; more elastic;and easily injectable. It would also give the IOL goodshape memory. However, for a 1-piece IOL made ofsuch material, the IOL design should be preciselyselected to compensate for the material deformabilityand ensure appropriate haptic compression.

According to Krag and Andreassen25 and Fisher,26

the ultimate load resistance of the posterior capsulefor persons between 40 years old and 60 years old isin the range of 400 to 800 mg force. This means thatfor an average capsular bag size of 10.0 mm, the hap-tics of the Sensar AR40e IOL and Acri.Tec 366D IOLapply forces that are close to the mechanical resistanceof the posterior capsular bag.

On the other hand, excessive force applied tothe capsular bag by the IOL can result in decentrationand tilt, which would affect visual quality.7

Indeed, this was the case for the Acri.Tec 366D, AF-1



YA-60BB, Sensar AR40e, and Envista MX60 IOLs,which shifted significantly along the optical axis. Insuch cases, the ELP would not be easily predictableand refractive errors due to inappropriate IOL powercalculation would be significant.

In contrast, the haptics of iPure, Podeye, AcrysofSN60WF, Tecnis ZCB00, Isert 251, and Ioflex IOLshave a C-loop or double C-loop 1-piece design. TheseIOLs applied a relatively constant force, always lowerthan 94 mg, whatever the capsular bag size. C-loophaptics have high bending deformation capacity toallow the IOL to adapt to all capsule sizes with lowstress to the capsule. This finding implies that theC-loop haptic design in a 1-piece IOL prevents capsu-lar bag damage, which is in accordance with previousclinical11 and experimental6,7 results. Such biomechan-ical behavior helps prevent capsular bag striae.Although reversible in some cases, these striae caninitiate lens epithelial cell migration and eventuallyPCO formation and fibrosis. Moreover, the axialdisplacement and tilt tests showed that whatever thetest-well diameter, the optical part of the C-loop anddouble C-loop IOLs remained in a stable position.Clinically, the C-loop haptic design combined with a1-piece hydrophobic acrylic platform and the applica-tion of compression forces in the measured magnitudeprovides rotational stability, which is necessary, espe-cially for the astigmatism-correcting toric IOLs.7,8

In the case of the Finevision IOL, the deformation ofthe 4 closed haptics with the reduction in the capsulesize yielded higher compression force to the capsularbag than C-loop IOLs, but without impairing theIOL's position stability. The 4 closed haptics act as anti-vaulting deformable loops and can deform in sucha way that the optic remains in nearly the same planeas the haptics. This is especially important when thediffractive trifocal optic of the Finevision is usedbecause its optical properties can be altered by anyoptic shift or tilt.

In this study, the Acri.Tec 366D IOL showed a con-siderable shift with reduction of the test-well diameter,and that shift might significantly affect the IOL's clin-ical optical performance. Indeed, plate haptics intrinsi-cally have limited deformation capacity in the IOLplane.

The haptic compression forces measured in thisstudy for the 1-piece C-loop and the PMMA hapticsare in agreement with the results reported by Laneet al.7 for similar models. The authors suggested that1-piece hydrophobic acrylic C-loop IOLs provide sta-ble postoperative refraction, maintain their initial posi-tion in the capsular bag, and ensure long-termrefractive stability.4 Assia et al.27 reached a similarconclusion. The combination of a C-loop or a dou-ble C-loop haptic design that applies moderate


compression forces and a bioadhesive hydrophobicacrylic material is a prerequisite for IOL rotationaland refractive stability.5,8,10 The haptic compressiontest simulates capsular bag compression on symmetricfibrosis and contraction of the capsular bag. Theresults should be considered in relation to actualclinical cases.

The optic of a hydrophilic acrylic IOL, such as theFinevision, Acri.Tec 366D, and Ioflex, has a relativelyhigh water content; relatively high compressibility,elasticity, deformability; and good shape memory.These characteristics provide easy IOL folding, moder-ate injection force, entire recovery of the initial IOLshape, and recovery of optical quality within a reason-able time after injection of the IOL. Here, the Acri.Tec366D IOL was an exception, with a slightly higherinjection force than that measured for the other 2 hy-drophilic IOLs (Finevision and Ioflex). This may bebecause the IOL's hydrophobic surface affects itsfriction and increases the injection force.

The diffractive IOLs (trifocal Finevision and bifocalAcri.Tec 366D) unfolded more rapidly than theirmonofocal counterpart, Ioflex, probably because ofthe reinforcing effect of the diffractive optic patternin accordance with the rheological behavior of theseIOLs during the recovery step.

The low predetermined water uptake of theglistening-free hydrophobic acrylic material resultedin relatively low injection forces for the Podeye andiPure IOLs. They unfolded within approximately 1minute, as shown by the relaxation and contrast sensi-tivity recovery results. Thus, they unfolded moreslowly than their hydrophilic acrylic counterparts,which may help prevent ocular tissue damage.25,26

There was no residual deformation 1 hour after injec-tion, which indicates these IOLs have predictablevisual quality.

The Envista MX60 IOL, although containing thesame amount ofwater at the equilibrium as the Podeyeand iPure IOLs, showed less deformable behavior.This IOL had a Tg of 21�C, which is higher than thatof the glistening-free material of the iPure and PodeyeIOLs (Tg 9�C). Thus, the Envista MX60 IOL was in-jectedwith higher force and required longer relaxationtimes. The plasticizing effect of water was limited inthis case, and high residual deformation was mea-sured, resulting in significant loss of contrast sensitiv-ity at least 1 hour after injection. This IOL wouldrestore its initial shape slowly in a real clinical case,and the surgeon would require a longer time to locateit properly in the capsular bag. A possible explanationfor this is the excessively high Tg of the initial polymermatrix before water uptake or crosslinking reactionswithin the polymer material during gammasterilization, which would increase the IOL's rigidity.



Rheologically, typical hydrophobic acrylic IOLs(Acrysof SN60WF, Sensar AR40e, Isert 251, and AF-1YA-60BB) were more plastic due to the lack of waterin their composition. The slow and low deformationmeasured during the creep step explains the relativelyhigh injection forces. High residual deformation wasestablished in these cases and could account for theshort-term decrease in optical quality after IOLinjection.

This corroborates that the injection is driven mainlyby the optic deformation rather than by surfacefriction. Thinner IOLs are expected when the materialrefractive index increases. However, this was notfound to significantly influence the injection forces(compare Acrysof SN60WF, which has a refractiveindex of 1.55, and the Tecnis ZCB00,which has a refrac-tive index of 1.48).

Over the long-term, the relaxation processes areexpected to continue, and the IOLs may recover theiroptical properties, which would benefit the patient'svision. However, testing this process for longerperiods would be difficult with conventional opticalbenches because the IOL position in the experimentalsetup may be affected, leading to biased measure-ments. However, the purpose of the present test wasto evaluate the rheological behavior of the IOL duringimplantation, which takes a few minutes. Moreover,testing period of 1 hour is long enough to give a clearidea about the material shape recovery capacity.

In conclusion, this experimental study assessed thebiomechanical properties of 11 commercially availableIOLs to compare their compressibility, deformability,injectability, and optical quality before and after injec-tion. It was found that the double C-loop and theclosed quadripod 1-piece IOLs provided moderatehaptic compression force, which contributes to anIOL's positional and refractive stability. In contrast,plate haptics and PMMAhaptics had a lower deforma-tion capacity and therefore applied higher radialcompression forces to the capsular bag. In extremelysmall capsules, the IOLs can shift along the opticalaxis or tilt, affecting the refractive stability.

The lower deformability and longer material relaxa-tion time of the optic of typical hydrophobic acrylicdry-packaged IOLs (Acrysof SN60WF, TecnisZCB00, Isert 251, Sensar AR40e, and AF-1 YA-60BB)resulted in short-term loss of visual quality andcontrast sensitivity with relatively high residual defor-mation levels. The same phenomena were observedfor the recently developed hydrophobic acrylicEnvista MX60 IOL, which is packaged in aqueoussolution. These parameters are also responsible forthe high injection forces required for Envista MX60,Sensar AR40e, and AF-1 YA-60BB IOLs with themodel injection devices under the test conditions.


Hence, cartridge damage was observed with theAF-1 YA-60BB IOL.

Meanwhile, a predetermined amount of waterabsorbed by the glistening-free material plasticizedit, allowing iPure and Podeye IOLs to completely re-cover their initial shape and optical properties withina reasonable time after injection while preserving anadvantageous bioadhesivity13 that is comparable tothat of typical hydrophobic acrylic materials. TheseIOLs are equilibrated and steam sterilized in physio-logical solution before implantation, thus avoidingthe risk for glistening formation.

This experimental study may help surgeons makeproper IOL selections but should be considered inrelation to clinical results. Several clinical studies arebeing performed with the iPure and Podeye IOLs,and the results will be reported in following publica-tions to reinforce the knowledge and comprehensionof these IOLs and of the glistening-free material.

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WHAT WAS KNOWN

� Hydrophobic IOLs can lose their optical properties due toresidual deformation after implantation. This is in contrastto hydrophilic IOLs, which are more elastic and havebetter shape memory.

WHAT THIS PAPER ADDS

� The new hydrophobic glistening-free material with lowand controlled amount of absorbed water completelyrecovered its initial shape and optical properties afterinjection.

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39, SEPTEMBER 2013
First author:Dimitriya Bozukova, PhD

Research and DevelopmentDepartment, Physiol S.A.,Li�ege, Belgium










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dimitriya bozukova, phd, christophe pagnoulle, phd, christine j

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