Ординатура / Офтальмология / Английские материалы / Wavefront Analysis Aberrometers and Corneal Topography_Boyd_2003

optometers16,17 and video based systems18. Newly released video-based autorefractors provide measurements with temporal resolution up to 60 Hz19. Using these modalities, dynamics of accommodation have been intensely investigated in terms of defocus. However, little has been studied about the stability and precision of the higher order monochromatic aberrations, as well as their fluctuations during accommodation. Hofer et al.8 were the first to obtain time-resolved measurements of aberrations up to the 7th order. They examined three normal subjects for 5 seconds at 25.6 Hz with natural and paralyzed accommodation using a device based on HartmannShack sensor. They showed that fluctuations are present in all of eye’s wave aberrations, not just defocus. This initially supports the hypothesis that the fluctuations in aberrations are a direct consequence of accommodation microfluctuations: large changes in the accommodative state produce systematic changes in other aberrations in the eye. However, due to the frequency spectrum of these fluctuations being different from that of accommodation microfluctuations, authors suggested that these dynamic changes were not only due to change of the lens shape when accommodated but also to other causes such as tear film instability, eye movements or changes in the axial length of the eye caused by the heartbeat. With similar methodology, Artal et al.20 obtained time–resolved measurements of two subjects when changing fixation from infinity to a near target (vergence of 2.5 D) at 25 Hz. They found induced spherical aberrations as well as coma with increasing accommodation. Moreover, by simulating these data using a customized optical software, they produced

point spread functions showing that these aberrations could not be perfectly corrected for both near and distant vision.

In the University of Crete, using the commercially available Hartmann-Shack sensor (WASCA, Asclepion Meditec, Jena, Germany), which achieves a resolution frequency of 7.7 Hz, we initiated studies investigating the dynamic aspects (ie latency, reaction time, speed) of accommodation. Preliminary results (data under publication) are presented in figure 1, which shows temporal changes of Zernike polynomial coefficients representing spherical (Z40) and coma-like (Z31 and Z3-1) ocular aberrations for 3 subjects. It is evident, that the spherical aberration follows the classical change towards negative values when accommodating, for all subjects. Coma-like aberrations along y-axis do not show any particular trend among subjects. Coma-like along x- axis show a change towards positive values for two subjects. Linear regression analysis (see figure 2) shows that changes in spherical aberrations were systematically related to the accommodative response (ie spherical equivalent). In agreement with previous reports6,15, no correlation between changes in comalike aberrations and accommodative response was found. These erratic changes in coma could be attributed to the lateral displacement of the lens relative to the cornea occurring during the accommodation process6. Another possible explanation is the lack of rotational symmetry of the cornea and/or the lens that possibly induces asymmetric aberrations of variable magnitude upon the forward movement of the accommodating lens.

Fig 1: Temporal changes of third order coma-like along x axis (Z31) and y axis (Z8-1) and fourth order spherical-like (Z40)

upon accommodation and disaccommodation. Subjects were asked to change fixation between a distant target and a proximal target placed at the pre-determined (for each subject) near point of accommodation (ie different vergences for the near task were examined). Total time recording was about 30 sec (230 captured images).

Spherical Equivalent (D)

Fig 2: Correlation of spherical aberration (plotted on y axis) with spherical equivalent (plotted on x axis) for all time frames.

Regarding the retinal image quality we found variable changes in the MTF upon accommodation. In general, for young subjects, the MTF of the accommodating eyes was significantly impaired compared to the non-accommodative state for the whole range of spatial frequencies (figure 3a). However, no difference in the MTF was found in older subjects as it remained almost unaffected with the level of accommodation. It is likely that the dissimilar effect of the accommodation and induced aberrations on the quality of the retinal image could be due to the different magnitude of accommodative response within the subjects examined (Figure 3a-b).

Implications of Accommodation Dynamics on Wavefront-Guided Refractive Surgery

Since the application of excimer laser technology for the correction of eye's simple refractive errors (ie defocus and astigmatism), there has been a considerable debate concerning the visual impact of correcting the higher order monochromatic aberrations of the eye (eg spherical aberration, coma and secondary astigmatism), which also degrade retinal image quality10,11,21,22. Advances in the measurement of the eye's wave aberration and the ability to

accurately correct it in the laboratory with adaptive optics25 have initiated the effort in exploring the possibility of eliminating ocular aberrations to give an enhanced visual acuity23,24.

Even though, laboratory corrections of ocular aberrations with adaptive optics are likely to prove very useful in ophthalmic imaging systems (eg CCD fundus camera26 and Scanning laser Ophthalmoscopy27), it seems reasonable to be more sceptical about the potential of producing aberrationfree human eyes. There is currently a major ongoing effort to refine laser refractive surgery for higher order aberrations. The first groups, including VEIC, University of Crete, which have performed wave- front-guided customized ablation, have recently reported preliminary data that are encouraging but still tentative: an increase in the overall root-mean- square wavefront error is observed post-operative- ly28-31, which is, however, less pronounced compared with conventional LASIK29,32,33. Therefore, the target of the investigators should be first to minimise LASIK-induced higher order aberrations and then to reduce the pre-operative levels, especially in highly aberrated eyes.

Although these ideas are simple in principal, the visual benefit we could receive from attempts to correct higher order aberrations depends on a number of parameters that reduce retinal image quality and limit the finest detail we can see. Beside photoreceptor sampling34,35 and cortical magnification36 that cannot be regulated and will always limit visual resolution, other factors include chromatic aberration37 pupil size and eye’s aberrations (see figure 4).

Finally, optical monochromatic performance will only be achieved if the eye is precisely focused. Even small errors of focus, characteristic of the accommodation system, markedly degrade retinal image quality with larger pupils, for which the potential benefits of aberration correction appear to be the

Section IV: Aberrations and Aberrometer Systems

Fig 4: Limits of visual performance as set by diffraction (Rayleigh criterion), the optical aberrations of the eye38 and photoreceptor spacing35. It is obvious that the visual benefit of correcting ocular aberrations is greatest for the largest pupil diameters.

greatest. Although, is apparent that any small changes in accommodation will be of vast importance in wavefront-guided customised refractive surgery, the exact trend of different terms of high order aberrations upon accommodation is not yet fully understood. However, since the aberrations of the eye change with the level of accommodation6,14,15,20 the associated retinal image quality is also modified (see figure 3). Hence, the "correction" of the aberrations can only be effective at one object distance, and will also become less efficient as the patient ages.

Discussion

It is plausible that static corrections as customized ablations would not provide optimal results for both near and distant visual performance, but the clinical importance of this implication remains unclear. In general, it seems likely that any young individual, who has undergone "aberration correction" will at best only obtain minor benefits under

natural conditions, where the pupil and accommodation system are active. It may, however, be that a minority of individuals who need optimal vision at distance for some specific purpose might be prepared to tolerate aberration correction combined with cycloplegia and dilated pupil to obtain the maximal benefit from the diffraction-limited performance. Therefore, it is important to keep on using current technology (adaptive optics, wavefront mapping) in order to further our understanding on the dynamics of the visual system.

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20.Artal P, Fernandez EJ, Manzanera S (2002). Are optical aberrations during accommodation a significant problem for refractive surgery? Journal of Refractive Surgery, 18: 563-566.

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28.MacRae S, Schwiegerling J and Snyder R (2000). Customized corneal ablations and super vision. Journal of Refractive Surgery. 16:230-235.

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30.Mrochen M, Kaemmerer M, and Seiler T (2001). Clinical results of wavefront-guided laser in situ keratomileusis 3 months after surgery Journal of Cataract and Refractive Surgery, 27:201-207.

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Section IV: Aberrations and Aberrometer Systems

Sotiris Plainis, PhD,

Vision Scientist

Vikentia J Katsanevaki, MD,

Ophthalmologist

Sophia I Panagopoulou, BSc,

Physicist

Harilaos S Ginis, BSc,

Physicist

Ioannis G Pallikaris, MD, PhD.

Professor in Ophthalmology,

Department of Ophthalmology

School of Medicine

University of Crete

PO Box 1352

Heraklion, Crete

Greece

Amar Agarwal, MD, MS,FRCS, FRCOpth Nilesh Kanjiani DO, MD, FERC

Soosan Jacob, MD, MS, DNB, FERC Athiya Agarwal, MD, FRSH, DO

Sunita Agarwal, MD, MS, FSVH,FRSH,DO Tahira Agarwal, MD, F.I.C.S., DO, F.O.R.C.E.,

Introduction

The next evolution to come onto the visual science scene in refractive ocular imaging is the aberrometer, the Orbscan and wavefront analysis. This technology is based on astrophysical principles, which astronomers use to perfect the images impinging on their telescopes. Dr. Bille, Director of the Institute of Applied Physics at the University of Heidelberg first began work in this field while developing this specific technology for astronomy applications in the mid-1970’s. For perfect imaging, astrophysicists have to be able to measure and correct the imperfect higher-order aberrations or wavefront distortions that enter their telescopic lens system from the galaxy. To achieve this purpose, adaptive optics are used wherein deformable mirrors reform the distorted wavefront to allow clear visualization of celestial objects. Extrapolating these same principles to the human eye, it was thought that removal of the wavefront aberrations of the eye might finally yield the long awaited and much desired ultimate goal of "super vision".

So far, the only parameters that could be modified to obtain the optical correction for a given patients refractive error were the sphere, cylinder and axis even though this does not give the ideal optical

correction many a times. This is because the current modes for correcting the optical aberrations of the eye do not reduce the higher order aberrations. The ideal optical system should be able to correct the optical aberrations in such a way that the spatial resolving ability of the eye is limited only by the limits imposed by the neural retina i.e. receptor diameter and receptor packing.

Thus, there may be a large group of patients whose best corrected visual acuity (BCVA) may actually improve significantly on removal of the optical aberrations. These optical aberrations are contributed to by the eye’s entire optical system i.e. the cornea, the lens, the vitreous and the retina. This study was conducted to determine the existence of a unidentified entity which we identify as "aberropia" wherein patients with best corrected visual acuity of < 6/9 (0.63), corneal topography not accounting for the lack of improvement in BCVA and with no other known cause for decreased vision improved by ≥ two Snellen lines after refractive correction of their wavefront aberration.

Materials and Methods

16 eyes of 10 patients were included in this retrospective study carried out at the Dr. Agarwal's

Eye Institute, India between May to December 2002. Only patients who had visual acuity less than 6/9 (0.63) prior to the procedure and whose visual acuity improved by more than or equal to two lines after the procedure were included in the study. None of these patients had any other known cause for decreased vision and their corneal topography did not account for the lack of improvement in BCVA. The routine patient evaluation including uncorrected (UCVA) and best corrected (BCVA), slit lamp examination, applanation tonometry, manifest and cycloplegic refractions, Orbscan, aberrometry, corneal pachymetry, corneal diameter, Schirmer test and indirect ophthalmoscopy had been performed for all the patients. Patients wearing contact lenses had been asked to discontinue soft lenses for a minimum of 1 week and rigid gas permeable lenses for a minimum of 2 weeks before the preoperative examination and surgery. Informed consent was obtained form all patients after a thorough explanation of the procedure and its potential benefits and risks.

The Zyoptix procedure was then performed using the Bausch and Lomb Technolas 217 Z machine. The parameters used were: wavelength 193 nm, fluence 130 mJ/cm2 and ablation zone diameters between 4.8mm and 6 mm. The Hansatome (Bausch and Lomb) was used in all the eyes. Either the 180-µm or the 160-µm plate was used in all the eyes. The aberrometer and the Orbscan, which checks the corneal topography, are linked and a zylink created. An appropriate software file is created which is then used to generate the laser treatment file.

Postoperatively, the patients underwent complete examination including UCVA, BCVA, slit lamp examination, Orbscan and aberrometry. The mean follow up was 37.5 days.

For statistical analysis, the Snellen acuity was converted to the decimal notation. Continuous variables were described with mean, standard deviation, minimum and maximum values.

Results: 16 eyes of 10 patients satisfied the inclusion criteria. The mean age of the patients was 29.43 years (range 22 to 35 years). 6 patients were females and 4 were males. The mean preoperative pupil diameter measured on aberrometer was 4.69 mm and mean postoperative pupil diameter measured on aberrometer was 4.53 mm.

The mean pre-operative spherical equivalent was – 4.94 D (range –12.50 to –1.5 D). The mean spherical equivalent at 1 month post-operative period was –0.16 ± 0.68 D (range –1.0 to 1.5) . Mean preoperative sphere was –4.95 D (range-12.50 to –0.75 D) and the mean postoperative sphere was –0.13 ± 0.68 D (range-1 to 1.5) at 1 month. The mean preoperative cylinder was –1.34 D (range 0 to –3.50). The mean postoperative cylinder was –0.08 ± 0.24 D (range 0 to –0.75 D) at one month. Postoperatively, at the end of first month, 70% of the patients were within ± 0.5D and 90% were within ± 1D of emmetropia (Fig 1). Preoperatively mean RMS (Root Mean square) values (Fig 2) were : Z 200 Defocus –9.22, Z 221 Astigmatism 0.12, Z 220 Astigmatism 1.02, Z 311 Coma –0.041, Z 310 Coma –0.04, Z 331 Trefoil 0.23, Z 330 Trefoil 0.016, Z 400 Spherical aberration –0.054, Z 420 Secondary astigmatism 0.103, Z 421 Secondary astigmatism 0.029, Z 440 Quadrafoil –0.103, Z 441 Quadrafoil –0.021, Z 510 Secondary coma 0.025, Z 511 Secondary coma –0.015, Z 530 Secondary trefoil 0.0049, Z 531 Secondary trefoil -0.00219, Z 550 Pentafoil 0.023, Z 551 Pentafoil 0.046. Postoperative mean RMS values were : Z 200 Defocus –0.429, Z 221 Astigmatism 0.07, Z 220 Astigmatism –0.07, Z 311 Coma 0.149, Z 310 Coma –0.079, Z 331 Trefoil –0.102, Z 330 Trefoil –0.004, Z 400 Spherical aberration -0.179, Z 420 Secondary astigmatism 0.015, Z 421 Secondary astigmatism 0.031, Z 440 Quadrafoil 0.019, Z 441 Quadrafoil –0.069, Z 510 Secondary coma –0.008, Z 511 Secondary coma 0.008, Z 530 Secondary Trefoil -0.002, Z 531 Secondary Trefoil –0.014, Z 550 Pentafoil 0.006, Z 551 Pentafoil 0.026.