clinical 22 Optical aberrations and the eye – Part 3

In the final part of our series, Alejandro Cerviño and Dr Shehzad Naroo discuss the methods of correction required for low and high order wavefront aberrations

THE WAVEFRONT IS DEFINED as the encircling surface where a wave reaches in a particular moment during its propagation. A ray is an imaginary line perpendicular to the wavefront. Every alteration in the wavefront will cause the propagation to be less than ideal, and is called wavefront aberration. In other words, the variation on refractive power over different areas at the eye’s entrance could be called wavefront aberration. This wavefront can be described as a serial of polynomials, called Zernike polynomials, which are normalised within an unitary circle (and are recommended by the Optical Society of America as the standard method for specifying the eye’s wavefront).1 The ametropias of the human eye are FIGURE 2. Schematic diagram of the behaviour of the Hartmann-Shack principle-based wavefront wavefront aberrations affecting the visual analysis systems (courtesy of Bausch & Lomb) system, and even normal eyes have variations in the distribution of the optical systems, known as ‘adaptive optics’, was aberrations of the eye and, eventually, to power within the pupillary area, not developed in the field of astronomy, to aid a customised ablation. affecting much of the patient’s vision in compensate for atmospheric turbulence They all analyse the behaviour of the most of the cases. and to improve the imaging of bodies in light when passing through the optical They can be divided into two groups: outer space. system of the eye, but each of these the low order wavefront aberrations, which Adapting this technology, the correction technologies uses a different method of are the common spherocylindrical of wavefront aberrations is now being collecting data. ametropias and can be corrected with applied to the eye. spectacles, and the high order wavefront The Tscherning-based instruments, aberrations, ‘micro-aberrations’ which such as the Allegretto (Wavelight Laser TSCHERNING PRINCIPLE-BASED have more to do with the detail on the Technologies AG, Erlangen, Germany), This apparatus uses a entire grid of laser image, and which cannot be corrected with Hartmann-Shack technology, as used by spots. The image formed in the retina is traditional methods (Figure1). These the Zywave (Bausch & Lomb, Rochester, captured and defined through a very small topics are discussed in detail in Part 1 of New Jersey); the ray tracing with a aperture, and it will be more or less this series (OPTICIAN, April 25). scanning laser beam, as used by the Tracey deformed depending on the aberrations (Tracey Technologies, Houston, Texas); or produced by the examined eye. The spot the skiascopy principle-based OPD/ARK- deviation is measured and processed to MEASURING ABERRATIONS 1000 (Nidek, Gamagori, Japan) are the give the wavefront aberrations. Originally, the correction of wavefront current commercially available technolo- aberration produced by the optical gies used to measure the total wavefront HARTMANN-SHACK PRINCIPLEBASED This is the most popular technology for wavefront analysis. If a wavefront is divided spatially, then each one of those subdivisions can be approximated to a tilted perfect flat wave. The reconstruction of the tilts of every division will result in an approximation to the original wavefront. That is the principle on which the Hartmann-Shack- FIGURE 1. Simulation of the Snellen’s E, as seen by an eye with low order aberrations corrected based wavefront analysers rely. (myopia and astigmatism) and with mild higher order wavefront aberrations, mainly coma, (left), The sensor consists on a lenslet array and after the coma was removed (right). The simulation of the Snellen E was provided by the (like a compound insect eye) for wavefront WASCA(r) device (Wavefront Sciences Inc), a Hartmann-Shack based wavefront analyser decomposition and focusing of the

Optician JULY 18, 2003 NO 5909 VOL 226 www.optometryonline.net clinical 23 outgoing wavefront from each subaper- FIGURE 3. Image showing ture, and a detector (Figure 2). the point of entrance of the different laser beams in the Tracey System RAY TRACING (Tracey Tec, Houston TX), This technique also uses a laser beam to as well as the order in measure the aberrations of the eye. This which they are shot laser beam is projected on to the retina, parallel to the visual axis, at several points. In a different way to the Tscherning devices, a laser beam is projected each time, making a rapid scan over several points, which give their corresponding projection (Figure 2). With the data obtained, a complete pattern is produced in just milliseconds. The system then generates maps showing the behaviour of the visual system. These systems have shown a high systems is the large number of points sensing technology is not capable of reproducibility,2 and have the advantage measured. detecting the details of irregular corneal of measuring relatively high aberrations, as The rapid development in the analysis astigmatism. the sequential projection of the laser beam of wavefront aberration can lead one to avoids the possibility of overlapping. think that corneal topography analysis will become obsolete, but such a thing cannot CORNEAL ABERROMETRY happen as both measurements are strongly From topography analysis, a reference SKIASCOPY PRINCIPLE-BASED linked. surface of known optical behaviour can These technologies use dynamic Knowledge of the corneal topographical be established so that the corneal skiascopy, similar to that used by standard profile is needed to alter the power of the topography can be expressed as elevation auto-refractors. An infrared slit scans anterior surface, and to differentiate an from that reference. This gives informa- through the pupil measuring the refractive increased wavefront aberration produced tion, not just about how different the power of the eye, generating auto-refrac- by the anterior surface of the cornea (as the corneal surface is from that surface of tion data, as well as calculating wavefront eye’s main refractive element) from those known shape, but also about the behaviour aberration from the difference of refractive produced in the other lenses of the visual of that surface when the light comes power between areas within the pupillary system. Also, as stated by Klyce,3 at the through. region. The main advantage of these present time, the spatial resolution of wave Also, there are some software applica- ▲

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through that optical The difference lies in the direction of system. There is the light analysed. For the second group, usually an application the direction has to be reversed, so that the available in clinical object plane is on the retina, and the light wavefront devices analysed comes out from the eye. offering a representa- tion of the retinal image of a point, given IMAGING OF THE INTERNAL by the PSF (Figure 5). STRUCTURES OF THE EYE The modulation This first group comprises all the optical transfer function improvement in terms of lens design and (MTF) is defined as system filtering, computing and so on, in the modulation of the order to get better resolution when image divided by the observing the inner structures of the eye. modulation of the In other words, it is the direct application stimulus (Figure 6). of adaptative optics to the eye. In a square wave Once the low and high order aberra- grating there are dark tions are measured, the next step is to FIGURE 4. Image showing the original topographic axial map from the bars and light bars. We compensate for them in order to get the Tomey TMS (v.2.1) videokeratoscope (upper left) and the axial map can measure the best image possible. (upper right), higher-order aberrations map (lower left) and the 3D amount of light Wavefront analysis systems have a very representation of its Wavefront Error, WFE, (lower right), offered by coming from each. important role, as they will give informa- the CTView software (v.5.0) The maximum tion about the changes needed to make amount of light will the observation system perfect for imaging tions commercially available which allow come from the light and the minimum of an individual eye. In fact, Barry Thibos the practitioner to obtain wavefront from the dark bars. If the light is measured et al.5 have described the Hartmann-Shack analysis from previously obtained in terms of luminance (L) we can define aberrometer essentially as a modified topography exams. That is the case with modulation according to the following fundus camera which takes multiple the CTView (Sarver and Associates, equation: pictures of a single spot of light on the Florida), which offers graphical displays for retina. This description can be extended colour maps, spot diagrams, PSF, MTF, Modulation = (Lmax - Lmin ) / (Lmax + in some ways to all clinical wavefront WFE and so on from the topographies Lmin) analysers. obtained from a series of topography where Lmax is the maximum luminance The improvement of systems for instruments (Alcon EyeMap, CSO, of the grating and Lmin is the minimum. imaging the internal structures of the eye EyeSys, Humphrey, Keratron, Medmont, has a direct and very important clinical Nidek OPD, Orbscan and Tomey), When modulation is defined in terms relevance in the early detection of supporting also the wavefront data of light it is frequently referred to as a pathological and progressive conditions. obtained with the Nidek OPD, Tracey, Michelson contrast. Indeed, when one Changes such as oedema in the optic WASCA and Zywave wavefront analysers takes the ratio of the illumination from nerve head, or retinal spots in diabetic (Figure 4). the light and dark bars one is measuring retinopathy could be detected earlier, so It has been recently reported that the contrast. So, from this, we can say that the that the patients may have a better clinical repeatability of the videokerato- MTF represents the change in contrast prognosis. scope is enough to get reliable wavefront from the stimulus to the final image. error in measurements at a single session, but may require several examinations ANALYSIS AND CORRECTION OF and/or repeated independent measure- CLINICAL APPLICATIONS THE VISUAL PERFORMANCE ments at the same examination to The clinical applications of wavefront This second group comprises all the determine reliably some Zernike co- aberration analysis can be grouped into techniques developed to study and efficients. This has significant practical two wide fields: improve the visual performance of the implications for accurate customised human eye. This may influence several correction of aberrations.4 ◆ The imaging of the internal eye clinical areas. structures ◆ The analysis and correction of the RMS, PSF AND MTF – WHAT visual performance. Refractive surgery THESE ACRONYMS MEAN In fact, the preoperative wavefront The root mean square error (RMS) is used to describe two different concepts. The FIGURE 5. first quantifies errors in the wavefront Representation of the reconstruction. The second relates to how retinal image of a point the reconstructed wavefront differs from on the retina of an eye a perfect wave. In other words, the RMS suffering from higher- provides a general estimate of the variation order aberrations, of the wavefront from the ideal. mainly coma (left), and The point spread function (PSF) the improvement with represents the degradation suffered by the correction of the the stimulus when it reaches the image primary coma plane as a consequence of the wavefront aberrations (right), aberrations of the system. In other words, Images obtained using it represents how the image of a point the Zywave (B&L, source would be changed after going Rochester NY)

Optician JULY 18, 2003 NO 5909 VOL 226 www.optometryonline.net clinical 25 aberrometry examination should help to FIGURE 6. decide whether a traditional refractive Image showing the surgery procedure would solve the visual bidimensional problems of the patient, or if an ablation representation of the profile customised by the wavefront MTF corresponding to aberrations is indicated. Traditional refrac- the topography map tive surgery corrects the main components shown in Figure 4. The of the wavefront aberrations (defocus and representation was astigmatism) but it has been widely obtained through recognised by several investigators as CTView resulting in an increase in the third order aberrations,6,7 which do not usually affect the optical performance of the eye substantially. These aberrations can leave a healthy eye which has been treated by traditional surgery with success (so no residual myopia, hyperopia or astigmatism is left), unable to perform as well as expected, by contact lenses seem to be relatively has been shown to be negative in nuclear especially with low contrast targets or in a predictable, depending on the material cataract and positive in cortical scotopic environment.7-9 and parameters of the contact lens.14,15 cataract.18,19 However, it has been reported that the Vision has shown improvement when the Wavefront aberration analysis systems change is small and within the normal lens design is modified according to the may also help in other conditions affecting range of contrast sensitivity.10 Also, it is wavefront aberration (especially spherical the most anterior optical element of the critical to be able to quantify these aberra- aberration). eye, namely the tear film. It is known that tions in patients with irregular astigma- Nevertheless, the rotation of the lens the tear film has an important role in the tism due to keratoplasty, topographically on the eye reduces the possibility of smoothing of the optical quality of the asymmetrical astigmatism, congenital aberration correction.16 As the contact lens anterior cornea, so it is easy to understand corneal irregularities and so on. seeks to correct the lower and higher order that alterations in the tear film will affect In irregular corneas, or irregularities in aberration, the translational and rotational wavefront integrity. internal lenses (posterior corneal surface movement of the lens should be more Thibos et al demonstrated that the good and crystalline lens), traditional refractive accurate and predictable. state of the tear film has great importance surgery does not solve the visual problem, As it is widely known, contact lenses when trying to image the inner structures or at least not completely, and most of the have several advantages over surgery, of the eye,5 and it has been also referred times the surgery itself is contra-indicated. mainly based on reversibility. Also, the as a possible application of the Hartmann- Currently, the customised corneal possibility of compensating wavefront Shack aberrometers as an aid for ablation is becoming popular, being aberrations of the eye with contact lenses evaluating ocular dryness (Figure 7).20 applied not just to specific cases of irregu- benefits from adjustments in their parame- Right now, there are studies going on larities in the visual system, but also in ters when the desired performance is not trying to determine what is the contribu- the traditional refractive surgery candidate, achieved by the patient because of tion of the tear film to minimise those giving results that in most of cases leave changes in the aberrations of the eye aberrations. better acuities than 1.0 (6/6).11 (effect of ageing,17 surgeries, trauma and In recent years, an increase in devices so on), so the optical performance can be used to quantify wavefront aberrations has maintained. Also, in cases where refractive IOL lens selection occurred, and a more in depth study of the surgery is contra-indicated, the contact It has been recently demonstrated that ablation itself is beginning to be lens is the only solution and great aberra- the implantation of an intraocular lens developed for each patient, due to the tions may be compensated. (IOL) can produce more wavefront aberra- previously reported relation between the tions than Lasik, increasing mainly the corneal parameters, the total wavefront spherical aberration.21 IOL design has aberrations of the eye, the ablation size Ocular pathology experienced many improvements in the and depth, and the possible complica- Besides all the benefits that adaptative last few years, making possible an tions12,13 prior to surgery. optics in imaging systems would give in accommodative IOL, and improving the Furthermore, an evaluation of the post- retinal pathology detection, wavefront optical performance. By designing a more operative corneal aberration can be analysis may help in other diagnosis and prolate anterior surface, spherical aberra- performed, so, in case an enhancement is evaluation of some ocular anomalies. tion is significantly reduced.22 needed, the correction over the first There are many pathologies which The refractive surgery technique ablation can be customised to make the produce a change in wavefront aberrations known as ‘bioptics’, could soon be a best compensation of the total aberration of of the eye. Anterior pole pathologies, such beneficiary of the advance in wavefront the eye and get an optically close-to-perfect as those which occur with morphomet- technology. It can be expected that, with ocular system12. Complete compensation is rical alterations of the cornea (keratoconus, the increasing development of wavefront not possible, however, as the aberrations are marginal degeneration, or ectasias in analysis systems and lasers, in patients not constant but vary with accommoda- general) produce a noticeable increase in with high ametropias, an intraocular lens tion, age, tear film and so on. the third order wavefront aberrations, and can be inserted to correct the largest part result in irregular astigmatism that cannot of the ametropia leaving the reshaping of be corrected with spectacles. the cornea for a later adjustment and total Contact lens practice Also, pathologies affecting the inner wavefront aberrations correction. The use of contact lenses for correcting lenses of the eye produce these increases This would increase the chances of a wavefront aberrations of the eye is a in wavefront aberrations. It has been stated better-than-6/6 vision in patients over 10D developing area of research. that cataracts increase the wavefront without the need for deep ablation (which

The wavefront aberrations produced aberration co-efficients, but the polarity would require really thick original corneas ▲ www.optometryonline.net JULY 18, 2003 NO 5910 VOL 226 Optician clinical 26

and involves more risks) and the possibility of performing an easier and quicker customised ablation that would be really hard to achieve if full correction is performed over the cornea.

WHAT IS NEXT? With the technology available at this time, an in-depth knowledge of the corneal morphometry can be achieved for various applications: refractive surgery, contact lens fitting, orthokeratology, IOL and so on, but wavefront technology will be able to provide information of every change to the optical properties of the ocular system. Customised refractive surgery for monovision, where the distance eye is corrected for the wavefront aberrations in distance, and the eye for near is corrected for its aberrations in close vision, or monovision with contact lenses, or aberra- tion free accommodative IOL lenses and so on may all be achievable. In the field of ocular imaging, systems which include real time wavefront analysis and adaptative optics will be available for ophthalmoscopes, retinographs, and any other system for internal imaging, providing a wonderful tool for the early diagnosis of retinal pathology. At the current rate of development, it is to to be expected that corneal wavefront sensing systems will allow a point by point analysis of the wavefront aberrations of the eye, getting information of the optical behaviour of the eye over as many points as corneal topographers do.

REFERENCES FIGURE 7. Suggested standards for SH aberrometry patterns in dryness evaluation: (a) Grade 0: 1 Thibos LN. Wavefront data reporting and Flat grid, parallel lines, without distortion; (b) Grade 1: Trace distortion of the lines, trace terminology. J Refract Surg. 2001; 17: s578-s583. peripheral ‘curvature effect’; (c) Grade 2: More evident distortion and ‘curvature effect’, 1-2 2 Molebny VV, Panagopoulou SI, Molebny SV, et blank spots, or grid ‘breaks’; (d) Grade 3: Severe distortion and ‘curvature effect’, more blank al. Principles of ray tracing aberrometry. J Refract spots; Grade 4: The grid is highly distorted Surg. 2000; 16(5): s572-s575. 3 Klyce SD. Corneal topography and the new wave. Cornea. 2000; 19(5): 723-729. 11 Arbelaez MC. Super Vision: Dream or Reality. 18 Kuroda T, Fujikado T, Maeda N, et al. 4 Gobbe M, Guillon M, Maissa C. J Refract Surg. J Refract Surg. 2001; 17(suppl): s211-s218. Wavefront analysis in eyes with nuclear or cortical 2002;18(5):S567-71. 12 Endl MJ, Martinez CE, Klyce SD, et al. Effect cataract. Am J Ophthalmol 2002;134(1):1-9. 5 Thibos LN, Hong X. Clinical applications of the of larger ablation zone and transition zone on 19 Kuroda T, Fujikado T, Maeda N, et al. Shack-Hartmann aberrometer. Optom Vis Sci. corneal optical aberrations after photorefractive Wavefront analysis of higher-order aberrations in 1999;76(12):817-25. keratectomy. Arch Ophthalmol. 2001; 119: 1159- patients with cataract. J Cataract Refract Surg 2002; 6 Marcos S, Barbero S, Llorente L, et al. Optical 1164. 28(3):438-44. response to LASIK surgery for myopia from total 13 Argento C, Cosentino MJ, Tytiun A, et al. 20 Cervi§o A, McDonald MB, Klyce SD. Shack- and corneal aberration measurements. Invest Corneal ectasia after in situ keratomileusis. J Hartmann pattern as an aid in the diagnosis and Ophthalmol Vis Sci. 2001; 42 : 3349-3356. Cataract Refract Surg. 2001; 27: 1440-1448. evaluation of dry eye: A retrospective study. 7 Marcos S. Aberrations and visual performance 14 Patel S, Fakhry M, Alio JL. Objective assess- Invest Ophthalmol Vis Sci. 2002; 43: E-Abstract following standard laser vision correction. J Refract ment of aberrations induced by multifocal contact 2029. Surg. 2001; 17(5): s596-s601. lenses in vivo. CLAO J. 2002; 28(4); 196-201. 21 Miller JM, Anwaruddin R, Straub J, et al. 8 Applegate RA, Howland HC. Refractive 15 Hong X, Himebaugh N, Thibos LN. On-eye Higher order aberrations in normal, dilated, surgery, optical aberrations and visual perfor- evaluation of optical performance of rigid and intraocular lens, and laser in situ keratomileusis mance. J Refract Surg. 1997; 13: 295-299. soft contact lenses. Optom Vis Sci. 2001 ; 78(12) ; corneas. J Refract Surg. 2002; 18(5): s579-s583 9 Nakamura K, Bissen-Miyajima H, Toda I, et al. 872-880. 22 Holladay JT, Piers PA, Koranyi G, et al. A new Effect of laser in situ keratomileusis correction on 16 Lopez-Gil N, Castejon-Mochon JF, Benito A, intraocular lens design to reduce spherical aberra- contrast visual acuity. J Cataract Refract Surg. 2001; et al. Aberration generation by contact lenses tion of pseudophakic eyes. J Refract Surg 27: 357-361. with aspheric and asymmetric surfaces. J Refract 2002;18(6): 683-691. 10 Mutyala S, McDonald M, Scheinblum K, et al. Surg. 2002; 18(5): s603-s609. Contrast sensitivity evaluation after laser in situ 17 Kuroda T, Fujikado T, Ninomiya S, et al. ◆ Alejandro Cerviño is a postgraduate at keratomileusis. Ophthalmology. 2000; 107: 1864- Effect of ageing on ocular scatter and higher order Aston University, where Dr Shehzad Naroo 1867. aberrations. J Refract Surg. 2002; 18: s598-s602. is a lecturer

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