tilt and decentration of intraocular lenses in vivo from purkinje … and... · 2007-03-16 ·...

ARTICLE

Tilt and decentration of intraocularlenses in vivo from Purkinjeand Scheimpflug imaging

Validation study

Alberto de Castro, BSc, Patricia Rosales, MSc, Susana Marcos, PhD

PURPOSE: To measure tilt and decentration of intraocular lenses (IOLs) with Scheimpflug and Purkinjeimaging systems in physical model eyes with known amounts of tilt and decentration and patients.

SETTING: Instituto de Optica Daza de Valdes, Consejo Superior de Investigaciones Cientıficas,Madrid, Spain.

METHODS: Measurements of IOL tilt and decentration were obtained using a commercialScheimpflug system (Pentacam, Oculus), custom algorithms, and a custom-built Purkinjeimaging apparatus. Twenty-five Scheimpflug images of the anterior segment of the eye wereobtained at different meridians. Custom algorithms were used to process the images (correctionof geometrical distortion, edge detection, and curve fittings). Intraocular lens tilt and decentrationwere estimated by fitting sinusoidal functions to the projections of the pupillary axis and IOL axis ineach image. The Purkinje imaging system captures pupil images showing reflections of light fromthe anterior corneal surface and anterior and posterior lens surfaces. Custom algorithms were usedto detect the Purkinje image locations and estimate IOL tilt and decentration based on a linear sys-tem equation and computer eye models with individual biometry. Both methods were validated witha physical model eye in which IOL tilt and decentration can be set nominally. Twenty-one eyes of 12patients with IOLs were measured with both systems.

RESULTS: Measurements of the physical model eye showed an absolute discrepancy betweennominal and measured values of 0.279 degree (Purkinje) and 0.243 degree (Scheimpflug) for tiltand 0.094 mm (Purkinje) and 0.228 mm (Scheimpflug) for decentration. In patients, the meantilt was less than 2.6 degrees and the mean decentration less than 0.4 mm. Both techniques showedmirror symmetry between right eyes and left eyes for tilt around the vertical axis and for decentra-tion in the horizontal axis.

CONCLUSIONS: Both systems showed high reproducibility. Validation experiments on physicalmodel eyes showed slightly higher accuracy with the Purkinje method than the Scheimpflug imag-ing method. Horizontal measurements of patients with both techniques were highly correlated. TheIOLs tended to be tilted and decentered nasally in most patients.

J Cataract Refract Surg 2007; 33:418–429 Q 2007 ASCRS and ESCRS

In the past few years, cataract surgery has benefitedsignificantly from technological advances. While elim-inating the intraocular diffusion produced by the cata-ract is still the reason for the procedure, advances inoptical measurements and intraocular lens (IOL) de-sign provide good refractive outcomes and even aimat canceling the spherical aberration of the cornea. Inaddition to monofocal IOLs, advances have also beenmade in multifocal and pseudoaccommodating IOL

Q 2007 ASCRS and ESCRS

Published by Elsevier Inc.

418

design.1,2 Concern about surgically induced aberra-tions, initially limited to refractive surgery,3 is alsopresent in cataract procedures.4

Measurements of the ocular and total aberrationsafter cataract surgery show higher spherical aberra-tions in eyes with monofocal spherical IOLs.5 Recentdesigns with aspherical surfaces induce negativespherical aberration, with the aim of mimickingthat of the young crystalline lens.6 There are even

0886-3350/07/$dsee front matter

doi:10.1016/j.jcrs.2006.10.054

419IN VIVO MEASURING OF IOL TILT AND DECENTRATION

proposals for IOLs aiming to cancel other higher-order aberrations.7

The ultimate limitation of the customized IOL is pre-cision in its positioning. There have been theoreticalstudies of the impact of IOL tilt and decentration onoptical qualitywith newaspherical lenses.6,8 In a previ-ous study,4 we also assessed this question in vitro forspherical lenses and suggest that the effect of IOL tiltand decentration in the final optical quality dependsgreatly on the actual combination of tilt and decentra-tion in an eye. Therefore, individually measuring tiltand decentration 3-dimensionally is important to as-sess the optical degradation imposed by IOL position-ing and evaluate the benefits of specific designsimplanted in real eyes.

Two methods have been used to measure IOL tiltand decentration in vivo: Purkinje imaging andScheimpflug imaging. Purkinje images are reflectionsfrom the anterior (PI) and posterior (PII, usually notvisible) corneal surfaces and from the anterior (PIII)and posterior (PIV) lens surfaces. Since their descrip-tion by Purkinje in 1832, the images have been used tomeasure cornea and crystalline properties, particularlyphakometry. Clinical studies report the use of Purkinjeimages to obtain biometric data9,10 or to assess IOL tiltand decentration.11 A more systematic approach isthat proposed by Phillips et al.12 and then furtherused by Barry et al.13 These authors propose a set oflinear equations relating the position of PI, PIII, andPIV as a function of linear combinations of eye rota-tion, IOL tilt, and IOL decentration. In addition, theyincorporated a telecentric IOL to avoid parallax be-cause PIII lies on a different focal plane. Based on theseconcepts, our laboratory developed a compact systemto measure phakometry and crystalline lens tilt anddecentration.14 The system has been extensively vali-dated both computationally and experimentally.14,15

Accepted for publication October 29, 2006.

From the Instituto de Optica Daza de Valdes, Consejo Superior deInvestigaciones Cientıficas, Madrid, Spain.

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

Supported by grant #FIS2005-04382 from the Ministerio de Edu-cacion y Ciencia and an European Young Investigator Award(EURHORCs) to Dr. Marcos.

Sergio Ortiz, MSc, Jose Requejo-Isidro, PhD, and Ignacio Jimenez-Alfaro, MD, PhD, supplied technical assistance and helpfuldiscussions.

Corresponding author: Alberto de Castro, Instituto de Optica Dazade Valdes, CSIC, Serrano 121, Madrid-28006, Spain. E-mail:[email protected].

The Scheimpflug camera provides images of the an-terior chamber of the eye. Its configuration is such thatthe image, lens, and object plane intersect in 1 pointso that sections of the eye appear with large depthof focus. Conversely, Scheimpflug images sufferfrom geometric distortion (resulting from tilt of theobject, lens, and image planes) and optical distortion(because the different surfaces are viewed through an-terior refracting surfaces). Ray-tracing techniques aretherefore required to obtain reliable crystalline surfacegeometry.16,17 Scheimpflug research instruments18–20

have been used to study the shape of the crystallinelens and how it changes with accommodation or ag-ing.16,20 Phakometry data from Scheimpflug imaginghave been compared with data from other techniques.Koretz et al.21 compared the anterior and posterior ra-dii of curvature from the Scheimpflug technique withthose measured by magnetic resonance imaging in2 sets of subjects as a function of age. In a previousstudy,15 we compared anterior and posterior radii ofcurvature obtained with Scheimpflug and Purkinjeimaging in the same group of eyes, both unaccommo-dated and as a function of accommodation in a subsetof eyes. The clinical literature includes numerousreports of IOL tilt and decentration at different timesafter surgery or with different IOL types using com-mercial Scheimpflug instruments.22–28 With theseinstruments, the optical distortion is presumably notcorrected. Only a recent study of IOL tilt and decentra-tion in eyes with phakic lenses29 using a NidekScheimpflug system mentions that images were cor-rected using custom algorithms.

The availability of new Scheimpflug commercial in-struments may make the measurement of IOL tilt anddecentration more accessible. However, powerfuldata-processing routines, careful assessment of thelimitations of the technique, and experimental valida-tions are necessary before this information can be usedreliably.

In thismanuscript, we present measurements of IOLtilt and decentration of IOLs in a water-cell model eyeand in patients using a custom Purkinje imaging sys-tem and Pentacam Scheimpflug imaging with customalgorithms. To our knowledge, this is the first assess-ment of the accuracy of the techniques in measuringIOL tilt and decentration and the first cross-validationof Scheimpflug and Purkinje imaging to measure tiltand decentration.

MATERIALS AND METHODS

Purkinje imaging system

A system for phakometry and lens tilt and decentra-tion measurements based on phakometry and

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420 IN VIVO MEASURING OF IOL TILT AND DECENTRATION

implemented at the Instituto de Optica, ConsejoSuperior de Investigaciones Cientıficas, was used inthe study. The optical setup, processing algorithms,and validations have been described.14 In brief, thesystem consists of (1) 2 illuminating channels(for measurements in right eyes and left eyes) withcollimated light from infrared (IR) light–emittingdiodes (LEDs) at an angle of 12 degrees horizontally;(2) an imaging channel with an IR-enhanced charge-coupled device (CCD) camera with a telecentric lensmounted in front of the eye and conjugate with theeye’s pupil; (3) a fixation channel with a minidisplay,collimating lens, and Badal system that allowsprojection of visual stimuli foveally and at differenteccentricities, as well as correction of refraction. Thesystem sits on a 500 mm � 400 mm optical table.Software written in Visual Basic (Microsoft VisualStudio 6.0) automatically controls the image acquisi-tion, LED switching, and stimulus display. Dataprocessing is performed using Matlab (Mathworks,Inc.) and the Zemax optical design program (FocusSoftware). The detection of Purkinje images PI, PII,and PIII and the pupil center in the pupil images isperformed using Gaussian fitting with routineswritten in Matlab. The processing routines assumethat P1, P3, and P4 (positions of PI, PII and PIII,respectively, relative to the center of the pupil) arelinearly related to eye rotation (b), lens tilt (a), andlens decentration (d).

P1 Z Eb

P3 Z FbþAaþCd

P4 Z GbþBaþDd

These equations are applied to both horizontal andvertical coordinates. Coefficients A through G are ob-tained using a computer model eye (simulated usingbiometric data available for each eye). Intraocularlens decentration is referred to the pupil center, andIOL tilt is referred to the pupillary axis. Computer sim-ulations using realistic eye models and the actual spec-ifications of the experimental setup showedmaximumdiscrepancies between nominal and experimentalvalues of 0.1 degree for eye rotation, 0.25 degree forIOL tilt, and 0.013 mm for IOL decentration.

Scheimpflug imaging

A commercial Scheimpflug imaging System (Penta-cam, Oculus) was used to image sections of the ante-rior segment of the eye at different meridians (25) byprojecting a slit (blue light). The systems’ software cor-rects geometric distortion but shows uncorrected im-ages. Because work was performed directly on theimages captured, a routine was implemented to

correct this distortion. The commercial software pro-vides quantitative information about the anterior andposterior corneal surfaces, but not the crystalline lensor IOL. In addition, the edge detection routines usuallyfail to detect the edges of certain types of IOLs (ie,acrylic) because their scattering properties are differ-ent from those of the crystalline lens. Algorithms thatwork directly on the raw images and calculate IOLtilt and decentration were developed. These includethe following:

1. Correction of the geometrical distortion of the images.The appropriate correction was found with previ-ous calibrations using a reticule.

2. Routines to find the edges of the cornea and IOLs. Twoapproaches are used depending on the diffusingproperties of the IOLs (threshold filtering andedge detection of binary images for the crystallinelens and more diffusing lenses and detection ofmaximum values at the edge of the lens for the leastdiffusing lenses).

3. Routines fitting the edges of the pupil and lens to find thepupil center, IOL center, IOL tilt, and eye rotation. Thepupil center is calculated as the midpoint betweenthe 2 visible pupil segments. The IOL center is calcu-lated as the midpoint of the intersection of the 2 cir-cumferences that fit the anterior and posterior edgesof the IOL. The reference axis is calculated as the linepassing through the anterior corneal center of cur-vature and the center of the pupil, known as the pu-pillary axis.30 The IOL axis (L) is calculated as theline joining the centers of curvature of the anteriorand posterior lens edges. These axes are referredto a vertical axis in each image.

4. Application of these procedures to each of the 25 sec-tions obtained at each meridian. For each meridian,the calculated parameters are projected to the hor-izontal and vertical axes. These projections (asa function of meridian angle) are fitted to a sinusoi-dal function. The horizontal and vertical compo-nents of pupil center, IOL center, IOL axis, andpupillary axis are then computed, evaluating thefitted sinusoidal functions at 90 degrees and180 degrees.

5. Intraocular lens decentration is obtained from the dis-tance between the IOL center and the pupillary axis. Ascale of 0.02 mm/pixel in the lateral dimensionwas used. This factor was obtained in the calibrationprocess described in number 1 above after the geo-metric distortion was corrected using the same reti-cule. Because there were no optical surfaces in theobject, optical distortion does not influence thisscale factor. The angles between axes are obtainedby means of scalar products. The angle betweenthe pupillary axis P and the line of sight (l) is


calculated directly from the components of the pu-pillary axis as follows:

coslx ZPz �Dzffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

P 2y þ P 2

z

q�

ffiffiffiffiffiffiffiffiDz2p

cosly ZPz �Dzffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

P 2x þ P 2

z

q�

ffiffiffiffiffiffiffiffiDz2p

The angle between the lens axis L and the referenceaxis P (a) is obtained by means of the scalar productof the 2 director vectors.

cosax ZPy � Iy þDz�DzffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiP 2y þDz2

q ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiI 2y þDz2

q

cosay ZPx � Ix þDz�DzffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiP 2x þDz2

q ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiI 2x þDz2

q

where lx and ax or ly and ay represent the tilts aroundx-axis or y-axis of the pupillary axis from vertical line,l, or of the IOL axis frompupillary axis, a. For each im-age, a positive angle indicates that the correspondingaxis has a positive slope and vice versa for negative an-gles. The coordinate system is right-handed with thez-axis being the direction of propagation from the fix-ation target into the eye. Because the fixation target isfoveal, it is assumed that the corresponding projectionof the line of sight for each of the Scheimpflug imagesis a vertical line (ie, z-axis). Positive horizontal coordi-nates stand for nasal in the right eye and temporal inthe left eye. Positive vertical coordinates stand for su-perior decentrations and negative, for inferior.

The use of 25 images from all orientations providesa more robust estimation of the IOL tilt and decentra-tion than using only 2 images (captured at horizontaland vertical meridians). The uncertainty in tilt and de-centration measurements was estimated, assuming re-alistic errors in edge detection and function fitting, and

an error propagation analysis was performed. Thesecalculations predicted an accuracy of 0.2 degree forIOL tilt measurements and 0.01 mm for decentration,provided that there were no optical distortions in theoriginal images and no additional source of error.

Physical model eye

A physical model eye in which nominal values ofIOL tilt and decentration can be set was built for thestudy. Figure 1 shows a photograph (A) and schematicdiagram (B) of the model eye. It consists of apoly(methyl methacrylate) (PMMA) water-cell modelwith a PMMA contact lens simulating the cornea andIOLs on a XYZ micrometer stage and rotational stage.The cornea was built by a contact lens manufacturer(AR3 Vision) with parameters similar to those of theGullstrand eye model (corneal diameter 11.20 mm, an-terior corneal radius 7.80 mm, posterior corneal radius6.48 mm, central thickness 500 mm). Different IOLswith spherical or aspherical designs from differentmanufacturers (Pharmacia, Alcon, Advanced MedicalOptics) and powers of 19.00 diopters (D), 22.00 D, and26.00 D were used in place of the crystalline lens.Decentration was achieved in the horizontal direction,with a precision of 0.1 mm. Tilt of the IOL wasachieved in the horizontal direction, with a precisionof 0.01 degree. The anterior chamber depth could bevaried, but was kept constant at 5.0 mm in this study.

Patients

Twenty-one eyes of 12 patients (mean age 72 yearsG 8 [SD]) with IOLs were measured. Time after sur-gery was at least 6 months. The IOLs had asphericaldesigns. All protocols adhered to the declaration ofHelsinki and followed protocols approved by institu-tional review boards. All patients signed informed

Figure 1. A: Photograph. B: Schematic dia-gram of the physical model eye developedfor this study. A PMMAcontact lens simulatesthe cornea (R1 Z 7.80 mm; R2 Z 6.48 mm;TZ 500mm). The IOLs are positionedwith anXYZ micrometer stage and rotational stage.


consents after receiving an explanation of the purposesof the study.

Experimental protocols

The artificial eye is fixated in a translational XYZand then aligned with the system. The main differencewith respect to measurements in patients is the opticalaxis of the model eye is collinear with the optical axisof the instrument (as opposed to the line of sight).

Measurements were done for horizontal decentra-tions ranging from 0 to 2.0 mm (every 1.0 mm) andhorizontal tilts (around the vertical axis) rangingfrom 0 to 4 degrees (every 1 degree). Because the IOLdoes not rotate around its own axis, decentrationinduced by tilt was compensated for when necessary.Alternate measurements with both the Purkinje andScheimpflug systems were taken for each conditionof tilt and decentration.

Measurements in patients were performed with pu-pils dilatedwith tropicamide 1%. In the Purkinje appa-ratus, patients were aligned with respect to the line ofsight while they foveally viewed a fixation target pre-sented in the minidisplay. Stabilization was achievedwith a dental impression. Series of images were cap-tured for different fixation angles (with fixation stimulipresented from �3.5 to 3.5 degrees horizontally andfrom �2.5 to 2.5 degrees vertically). Although onlya snapshot is necessary to obtain IOL tilt and decentra-tion, different eccentricities were tested to avoid over-lapping the Purkinje images. To assess measurementreproducibility, the entire procedure was repeated 3times. For Scheimpflug imaging, the patient foveallyfixated on a fixation target. Three series of 25 imageswere obtained per eye.

Data processing of Purkinje imaging data requiresseveral individual ocular biometry data to obtain coef-ficients A through G in the Phillips equations. For theartificial eye, these were taken from nominal values. Inpatients, anterior corneal radius and anterior chamberdepthweremeasured by optical biometry (IOLMaster,Zeiss). The radii of curvature of the anterior and poste-rior IOL surfaces were measured using the phakome-try mode of a previously described Purkinje imagingsystem14 if the geometry of the lens was not known.

RESULTS

Purkinje imaging and Scheimpflug raw data

Figure 2 shows typical images for the artificial eyecaptured with the Purkinje imaging system (top) andScheimpflug camera (bottom), respectively. Left im-ages correspond to an eye with a 2.0 mm decenteredsilicone IOL and right images to an eye with a 3.0 de-gree tilted acrylic IOL. Nominal decentration and tiltwere set with the micrometer stages in the artificialeye. Figure 3 shows typical examples of Purkinjeimages (A) and Scheimpflug images (B) for 1 realeye. Differences in the diffusing properties of the realcornea and PMMA cornea of the artificial eye can beobserved.

The relative positions of PI, PIII, and PIV with re-spect to the center of the pupil are detected from im-ages such as those shown in Figures 2 and 3, anddata are processed as explained above to obtain Pur-kinje tilt and decentrations. The centers of curvatureof the corneal and lens surfaces and pupil center arecomputed from each of the 25 Scheimpflug sections,as those shown in Figures 2 and 3.

Figure 2. Raw images obtained using the Pur-kinje imaging system (A and B) and Scheimp-flug system (C andD) from themodel eye. Theexamples correspond to a tilted silicone IOL(B and D) and a decentered acrylic IOL (Aand C). The PI, PIII, and PIV surfaces aremarked on the image from the Purkinje sys-tem. The fitted curves and calculated axesare superimposed on the Scheimpflug image.


Figure 3. Raw images from the Purkinje (A)and Scheimpflug (B) systems in a real eye.As in Figure 2, the Purkinje image locationsand fitted curves and the calculated axis aresuperimposed.

Figure 4 shows the projections of the pupillary axis,IOL axis, and decentration in 1 eye for each of the 25meridians (with 180-degree slit rotation, from 47 to220 degrees in right eyes and from 139 to 312 degreesin left eyes). Data are fitted to sinusoidal functions. Thevalue of the function at 180 degrees is the x-coordinateof the IOL axis or horizontal decentration in the x-axis,and the value at 270 is the y-coordinate of the IOL axisor vertical decentration.

Tilt and decentration in the physical model eye

Figure 5 shows measured tilt from Purkinje imagingand from Scheimpflug imaging as a function of nomi-nal tilt in the artificial eye for 3 IOLs. The solid linerepresents the ideal results. There was good corre-spondence between nominal and measured valuesfor Purkinje imaging (mean slope 1.088; mean absolutediscrepancy 0.279 degrees) and Scheimpflug imaging(mean slope 0.902; mean absolute discrepancy 0.243degrees). Error bars represent the standard deviationof repeated measurements. Figure 5 also shows mea-sured decentration from Purkinje imaging and fromScheimpflug imaging as a function of nominal decen-tration in the artificial eye for 3 IOLs. There was goodcorrespondence between nominal and measuredvalues for Purkinje imaging (mean slope 0.961; meanabsolute discrepancy 0.094mm) and a higher disagree-ment for Scheimpflug imaging (mean slope 1.216;mean absolute discrepancy 0.228 mm) when therewas consistent overestimation for 2 of the measuredlenses.

Tilt and decentration in patients’ eyes

Figure 6 shows tilt and decentration of IOLs in righteyes and left eyes. Positive tilts around the x-axis indi-cate that the superior edge of the IOL is moved for-ward and vice versa for negative tilts around thex-axis. Positive tilts around the y-axis stand for nasaltilt and indicate that the nasal edge of the IOL ismoved backward) and vice versa for a negative tilt

around the y-axis in right eyes. A positive tilt aroundthe y-axis stands for temporal tilt (nasal edge of theIOL moves forward) in left eyes. A positive horizontaldecentration stands for a nasal decentration in righteyes and temporal in left eyes, and vice versa forvertical decentration. There was clear mirror symme-try in tilt (measured with both techniques) and a lesssystematic trend for decentration in this group ofeyes. The mean absolute tilts around the x-axiswere 1.89 G 1.00 degrees (Purkinje) and 1.17 G 0.75degrees (Scheimpflug), the means absolute tiltsaround the y-axis were 2.34 G 0.97 degrees (Purkinje)and 1.56 G 0.82 degrees (Scheimpflug), the meanabsolute horizontal decentration was 0.34 G 0.19 mm(Purkinje) and 0.23 G 0.19 mm (Scheimpflug),and the mean absolute vertical decentration was0.17 G 0.23 mm (Purkinje) and 0.19 G 0.20 mm(Scheimpflug). Figure 7 compares tilt and decentrationmeasured with Scheimpflug and Purkinje imaging.The results with both techniques were highly signifi-cantly (P!.001) correlated for horizontal decentration(r Z 0.764) and tilt around the y-axis (r Z 0.762)(ie, horizontal displacements of the IOL). Forvertical decentration and tilt around the x-axis, the cor-relations were not significant and the estimated valueswere close to the measurement error and methodaccuracy.

Repeatability of both the Purkinje and Scheimpflugmethods was high: The mean standard deviation ofrepeated measurements was 0.61 degrees and 0.20degrees for tilt and 0.05 mm and 0.09mm for decentra-tion for Purkinje and Scheimpflug, respectively. Ananalysis of variance for repeated measures to testwhether the mean value (for each type of measure-ment) was representative of data found this to be thecase in all conditions.

After this analysis, intraclass correlation coeffi-cients (ICCs) were calculated to assess the reliabilityof the methods because the intraclass is sensitive torandom error and systematic bias. The analysisshowed that the methods were reliable for tilt around


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Figure 4. Pupillary axis, IOL axis location,and decentration obtained for each of the25 images captured with the slit orientated atdifferent meridians (indicated in the x-axis).Distances in the y-axis are indicated in pixels.For the pupillary and IOL axes, they refer tothe projection of each axis to a horizontalline (difference of horizontal values between2 arbitrary Z positions; here Z Z 0 pixels andZ Z 20 pixels). For decentration, they refer tothe distance between IOL center and pupil-lary axis. The data (ie, projections for eachorientation) are fit to a sinusoidal function,shown by a solid line. The fit of the decentra-tion is slightly noisier because of the tilt of thereference axis. The horizontal and verticalcoordinates of each axis, IOL tilt (degrees),and the decentration (mm) are calculatedfrom those data, as explained in the text.

the y-axis (ICC 0.830) and decentration in the x-axis(ICC 0.836).

DISCUSSION

Limitations of the techniques

The Purkinje imaging system has been extensivelyvalidated computationally and experimentally in pre-vious studies.14,15 Computer simulations using eyemodels and the actual optical configuration of thesystem show that deviations from spherical modeleyes resulting from corneal asphericity or corneal ir-regularities and anterior and posterior lens surfaces

asphericities did not significantly affect the results ofIOL tilt and decentration.

Scheimpflug images from the Pentacam system arenot corrected for optical distortion, and the CCD im-ages also suffer from geometric distortion. The formeris corrected by software at the corneal level, but not atthe crystalline lens or IOL levels. The second can becompensated for using calibration grids. This is imple-mented in the commercial software to provide cor-rected biometry values, and we developed a routineto compensate for the raw images. The presence of un-corrected distortions induces errors that must be stud-ied. We compared the measured values of tilt and


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Figure 5. Experimental IOL tilt and decentra-tion for the model eye for 3 different lenses(plotted with different symbols) as a functionof nominal values of tilt and decentration.Nominal tilt ranged from 0 to 4 degrees andnominal decentration ranged from 0 to2 mm. The ideal X Z Y line has also beenplotted.

decentration in the physical model eye with and with-out compensation for geometric distortion. When pro-cessing the data of Figure 5 without correctinggeometrical distortion, we found a constant underesti-mation of tilt for all IOLs (mean slope 0.866; mean dis-crepancy 0.280 degrees) and higher overestimation ofdecentration (mean slope 1.233; mean discrepancy0.247 mm).

In addition, we performed control experiments toevaluate the possible effect of optical distortion ontilt and decentration measurements with the Scheimp-flug system. Using the same physical model eye, weperformed measurements in which the spherical cor-nea was replaced by flat surfaces as well as experi-ments with and without water in the cell. Thesedifferent configurations necessarily change the contri-butions of the optical distortion. Figure 8, shows nom-inal versus measured tilts (as in Figure 5, now for thedifferent physical model eye settings). In general, therefractive powers in front of the lens did not affecttilt measurement. Correlations of nominal versus

experimental values show slopes of 0.849 for thespherical cornea with water, 1.015 for the sphericalcornea without water, 0.883 for the flat cornea withoutwater, and 0.968 for the lens alone without a cornea.We conducted similar control experiments for decen-tered lenses. We found slopes of 1.0530, 0.888, 0.927,and 1.025, respectively. This shows that optical distor-tions have only a moderate influence on the measure-ment of tilt and decentration.

Our measurements with both techniques showedhigh reproducibility. The Purkinje imaging systemhas limitations when lenses are very flat, for whichPIII is quite large. The system also relies on the appro-priate measurement of the anterior and posterior lensradii of curvature. Scheimpflug imaging requires suffi-cient pupil dilation to visualize the posterior lenssurface and collaboration from the patients to fixatefor 1.5 seconds without moving while illuminatedwith a blue light (versus 30 exposure time and IRillumination with the Purkinje imaging system).Scheimpflug imaging also poses some challenges


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Figure 6. Tilt and decentration in right eyesand left eyes of patients using Purkinje (opensymbols) and Scheimpflug (solid symbols) im-aging. Refer to the text for details on sign con-ventions. The sign of the tilt around x-axiswas changed to allow a more graphic repre-sentation of lens positioning, assuming a fron-tal view of the patients’ eyes.

with low-diffusing IOLs. Optical and geometricaldistortion of the Scheimpflug images (obtaineddirectlyfrom the CCD) produce slight discrepancies of themeasured tilt and decentration, which improve withthe correction of the geometrical distortion.

In real eyes, in general, both techniques agreed wellfor horizontal IOL tilt and decentration. The larger dis-crepancies, particularly for vertical decentration, maybe attributed to small magnitudes found close to thenominal accuracy of the techniques.

Comparisons with previous studiesand implications

In this study, we present experimental validation ofa previously presented Purkinje imaging system tomeasure IOL tilt and decentration using a physicalmodel eye. We also proposed a new robust methodto calculate IOL tilt and decentration using a commer-cial Scheimpflug imaging system, which is validatedusing the same physical model eye. A comparison of

tilt and decentration measurements in real eyes usingboth techniques is also presented.

Purkinje and Scheimpflug methods have been usedbefore to measure IOL tilt and decentration. To ourknowledge, only the reports of Barry et al.13 and Ro-sales and Marcos14 were based on thoroughly vali-dated Purkinje imaging methods. Several studiesreport tilt and decentration measured with Scheimp-flug imaging, in most cases from 2 sections of the ante-rior segment captured at perpendicular meridians.Coopens et al.,29 working with a modified Nidek sys-tem, used 2 or more images to create a redundant dataset to check the procedure. In the present study, weused combined information from 25 meridians. Toour knowledge, only Coopens et al. corrected theScheimpflug images for geometric and optical distor-tion to measure IOL tilt and decentration. We havefound that not correcting for geometric distortioncauses slightly discrepancies in the measured values.

We found mean Scheimpflug and Purkinje valuesof 0.21 G 0.28 mm horizontally and 0.03 G 0.38 mm


Right eye Left eye

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Figure 7. Comparison of the horizontal andvertical components of IOL tilt and decentra-tion between Scheimpflug and Purkinje tech-niques. The ideal X Z Y line is also shown.

vertically for decentration and �0.26 G 2.63 degreesaround the x-axis and 1.54 G 1.50 around the y-axisfor tilt. Decentrations in x-axis and tilts around they-axis were nasal in both eyes. In general, the amounts

of tilt and decentration we report are lower than thoseof the earliest reports in the literature. For example,Phillips et al.12 report a mean decentration of 0.7 G0.3 mm and mean tilt of 7.3 G 3.0 degrees without

Spherical cornea with water

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Figure 8. Nominal versus experimental tiltand decentration in the physical model eyefor a set of conditions, aiming at assessingthe influence of optical distortion on the tiltand decentration estimated from Scheimpflugimages. Each symbol represents a differentcondition (flat or curved ‘‘cornea,’’ with orwithout water, isolated lens). Tilt and decen-tration in these control experiments were setas in the experiments in Figure 5.


specifying the direction of tilt or decentration. Onereport of IOL position after transscleral implantationfound systematically high amounts of tilt and decen-tration (mean 5.97 G 3.68 degrees and 0.63 G 0.43degrees, respectively), which the authors attribute tothe implantation technique.31 Our results are morecomparable with those in more recent studies, report-ing lower tilt and decentration values, and are proba-bly associated with an improvement in surgicalprocedures. Previous results with our Purkinje imag-ing system of measurement of IOL tilt and decentra-tion14 showed a mean tilt of 0.87 G 2.16 degreesaround the x-axis and 2.3 G 2.33 degrees around they-axis; mean horizontal decentration was 0.25 G 0.28mmandmean vertical decentration,�0.41G 0.39mm.

Other studies using noncorrected Scheimpflug im-ages report mean tilts between 2.61 G 0.84 degrees(3-piece acrylic IOLs22) and 3.4 G 2.02 degrees (sili-cone IOLs27) and mean decentrations of 0.29 G0.26 mm to 0.37 G 0.19.22 To our knowledge, only 1study mentions interocular mirror symmetry for tiltand decentration with phakic IOLs.29 Although moststudies and techniques provide similar mean valuesthat agree well with the mean values reported hereusing both Purkinje and Scheimpflug, we have shownthat in individual patients, some discrepancies acrosstechniques may be found. This is particularly impor-tant when using Scheimpflug images that have notbeen corrected for geometric and optical distortion,as in the raw images provided by commercially avail-able instruments such as the Pentacam.

Finally, the amounts of tilt and decentration foundin patients were in general low, and particularly forthe decentration were of the order of the resolutionof the techniques in many patients. The clinical rele-vance of tilt and decentration of these amounts is likelylimited, although there are case reports in the literaturein which they resulted in decreased visual function(Rosales P, et al. IOVS 2006; 47:ARVO E-Abstract313).32

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First author:Alberto de Castro, BSc

tilt and decentration of intraocular lenses in vivo from purkinje … and... · 2007-03-16 ·...

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