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

The Wasca unit issues 785 data points, the other units show 16 and 216 data points respectively.

Auto-Refraction Procedure

The Autorefraction procedure provides an easy, reliable and very convenient tool. It allows performing quick measurements of most of the patients with normal accommodation behaviour.

Analysis Tools

The WASCA measure software is equipped with an easy-to-use 3D plot data representation tool. A nice 3D representation of the complete wavefront and of higher-orders only (Fig 3) can be shown.

Measurement

The measurement is extremely fast and there is no need to dilate the pupil. It is advisable to examine the pupil with the patient sitting up in a darkened room showing him a distant target in order to relax accommodation.

Image data are stored and all kind of analyzes can be done demonstrating the complete wavefront, the higher-orders only as well as all the different parameters (second order aberrations, cylinder, axis, measured pupil diameter analysis diameter of the pupil, optical path difference, and root mean square value).

The system is highly accurate. Three different measurements of the same person during the same day, demonstrated insignificant differences between measurements;Sph.eq.differences less than 0,1D,RMS values respectively 0,14/0,14/ and 0,16.

Interpretation

A wavefront interpretation is then done. Zernike polynomials allow disassembling the complex shape of the wavefront into a series of simple mathematical functions. Special attention is paid to third and fourth order aberrations like coma, Z 3 +1 and spherical aberrations Z 4, 0.

A customized high-order aberration correction is based on the following principles.

The Wasca work station diagnoses the highorder aberrations of the complete eye and the difference between measured wavefront and ideal wavefront is the ASAP Asclepion aberration smart ablation profile.

The first task of the refractive surgeon is to eliminate the aberrations induced by the surgery itself.

In this section we will discuss the results of ablation experiments in order to improve the wavefront guided ablation. In the experimental arrangement: an artificial eye is fixed on the head and processed with the WASCA analyzer (Fig.4). A PMMA plate can be inserted in the eye in such a way, that an artificially generated aberration pattern ablated on the anterior surface of this plate can be arranged precisely in the optical axis of this eye. Without this plate the artificial eye is nearly aberration free (RMS - HO = 0.05). A plane wave emitted by a lowest power (45 W), IR (850 nm) laser source is aligned in such a way that it forms a small spot on the back-side of the eye (yellow colored).This spot acts as a secondary point light source. The distortion of the wavefront emitted by this point source on the optical path back through the eye is investigated.

In case of the removed PPMA plate (corresponding to an ideal emmetropic, non-accomodated eye) the wavefront exiting the model eye is again a plane wave. With inserted PMMA plate containing aberrations the wavefront exiting the eye will deviate from a plane wave.

The artificial-eye-arrangement provides a method to check and to calibrate the correction of higher order aberrations by wavefront guided procedures. Without this optimization the correction of single Zernike coefficients is only partially (if at all) possible

APPLICATIONS OF WASCA TECHNOLOGY

We will discuss now the clinical and scientific applications.

In clinical settings the unit can be used as an autorefractor (a), as a primary or enhancement treatment tool (b), as a clinical diagnostic tool (c) and as a tool to study flap induced aberrations (d). Recently a new way of analysis "The Zonal Reconstruction" was introduced to improve on the resolution.

The Wasca Unit as an Autorefractor

T.Salmon, RW. West, W. Gasser and T. Kenmore compared accuracy, repeatability and instrument myopia of the ophthalmic ShackHartmann aberrometer, Wasca Unit with subjective

refraction and autorefraction.(1) Their conclusion was that accuracy, repeatability and instrument myopia with the Wasca unit is at least as good as clinical autorefraction and agreed well with subjective refraction.

Results of Wavefront Guided Laser

Surgery Using The Wasca Unit and

the Mel 70 Laser

A study was made in the department of ophthalmology of the INJ University Ilsan Paik hospital of South Korea under the direction of Professor

Do-Hyung Lee in 2001(2)

He studied induced wavefront aberrations of the eye after conventional LASIK surgery and compared these values with values of Wavefront Guided Lasik surgery. In the wavefront guided eyes the authors obtained visual acuity results which were better than conventional surgery in 50%, similar to conventional surgery in 29%, and worse than conventional surgery in 21%. Our general attitude is a follows; if after aberrometry we discover that the wavefront error is less than 0.2, we don’t carry out a Wavefront-Guided treatment because the patient

would have no great benefit from it. If the wavefront error is higher than 0.2, then we do it and this amount to about 20% of our cases? We treat another 10% to 15% with wavefront-guided treatment particularly if there are irregularities or decentrations from previous treatments. So about 65% of all patients still have standard laser treatment. Reviewing High Order aberrations of all our Wavefront Guided treated patients we noticed an improvement in HO aberrations in 75% of treated eyes.

Wavefront as a Diagnostic Tool

Wavefront is also a very important diagnostic tool. Fig 5 shows you a nice example of wavefront analysis of a congenital cataract comparing the slitlamp examination with the wavefront data (Pallikaris and Panagopoulou).The wavefront registration copies exactly the opacities in the lens.

Chapter 26: Wasca Wavefront Analyzer

Wavefront Study of Accommodation

Dynamic wavefront refractometry with the Asclepion aberrometer in different light conditions was done by Sophia I. Panagopoulou and Ioannis G. Pallikaris (6) using the Asclepion Wavefront analyzer for measuring the accommodation process. The procedure of accommodation was measured in full light, mesopic and scotopic conditions. The authors derived information for the lower and the higher order aberrations of 60 eyes in every stage of accommodation procedure.

From the evaluation of the results there was a statistical significant difference (p< 0.001) for some of the Zernike coefficients during accommodation and in different light conditions. For the first time ever accommodation could be measured in an objective way.

Dynamic wavefront refractometry with Asclepion aberrometer in the accommodation process (10 eyes) was studied by IG Pallikaris and SI Panagopoulou.(7) The accomodative eye was recorded continuously while the subject was focused on a moving target. They derived information for the lower and the higher order aberrations of each eye in every stage of accommodation procedure and for centre as well as for the periphery of the pupil measured. From the evaluation of the results there is a statistical difference (p< 0.001) for spherical and coma (always changing with the same way) during accommodation. There was continuous change in the sphere and high order aberrations with a periodicity and homogeneity during time of fixation. These phenomena were more intensive in the near target fixation. Time of stabilization from far to near and opposite, was also recorded and there was an influence from the age and the refraction of the eyes measured.

With theWasca Unit the time of accommodation could, for the first time ever, be measured

dynamically revealing very important information for the role and the contribution of the aberrations during this process.

Another study of dynamic vision and refractometry was done by Prof. Pallikaris in Crete.(8) He used this technology to analyze the speed of accommodation. He could separate between slow and fast accommodators demonstrating the difference in reaction time in younger and older people.

Accommodative IOL’s.

This study using WASCA was made by H. Burkhard Dick(9) at the department of ophthalmology at the Mainz Johannes Gutenberg University using the Hartmann-Shack aberrometer as a measuring device. Dr. Dick emphasized the fact that the WASCA unit allows continuous measurement of dynamic accommodation.For the first time ever the accommodation process could be objectively measured over time. The 1 CU accommodative IOL showed accommodating capabilities and a standard multifocal IOL demonstrated no accommodation. Besides that the WASCA system could monitor immediate changes in aberrations.

Zonal Reconstruction with the Wasca Unit: A New Analysis

Data, Processing and Presentation

The aim of this Multicentre investigation was to demonstrate the benefit of a new data analysis algorithm of the new WASCA software. The involved investigators are Dr. Frank Goes, Antwerp, Belgium, Prof. G nther Grabner, Salzburg, Dr. Burkhard Dick, and Mainz, Germany.(10)

The primary task of wavefront analysis is the accurate detection of wavefront shape and amplitude. To understand the relevance of sensor resolution for accurate wavefront reconstruction one must take a look at the different steps of sensing and processing the sensor information.

In the first step, the wavefront reconstruction algorithm calculates the wavefront slope for each lenslet. In the next step this slope information is lined-up to reconstruct the original wavefront shape. The resolution limit is the sensor resolution itself.

Fig. 6 demmonstrates that with insufficient resolution (a), the resulting wavefront shape is not only less detailed but also wrong. The condition improves with a higher resolution (b), but the reconstruction of wavefront shape and amplitude is still only an approximation. Only a high resolution reconstructs

the original wavefront shape with a good accuracy in respect of shape and amplitude (c).

SH-wavefront analysis is based on the assumption that the elementary incident wavefront above each lenslet is plano. The higher the changes in local wavefront geometry, the higher are the probability of a strong change of wavefront slope within a single lenslet area. This problem is related to the resolution of the used Shack-Hartmann Sensor.

If there is a strong change in wavefront slope above a single lenslet, i.e. the wavefront is not plano, the resulting spot will be blurry. (Fig 7) and it will become impossible to determine the spot location. A high resolution sensor will still be able to break down this wavefront portion in plano parts and assure accurate spot localization.

With a low resolution the change of wavefront slope from one lenslet to its neighbour might be so big that their focal spots cross. As a result the spot will be associated with the wrong lenslet. With a high resolution sensor the change of wavefront slope from one lenslet to its neighbour is only minimal. As a result focal spots move in groups and the chance of spot overlapping is minimized. Fig.8

To obtain the benefit of the theoretical advantages of the sensor of WASCA, Carl Zeiss Asclepion implemented a new algorithm of data processing and presentation. Almost every paper on wavefront analysis in ophthalmology presents data in form of Zernike coefficients.

Although Zernike coefficients are a well established standard now they are a rather abstract way of data presentation. The pure numbers are fairly meaningless unless you got deeper into the topic of wavefront analysis. The computation time rises dramatically to calculate also higher order aberrations and beyond the 4th order single coefficients are not related to specific optical errors anymore. A reconstruction with limited order also limits the resolution.

Because of this limitation the new WASCA software offers an alternative way of data analysis that utilizes the full sensor resolution for the analysis of smallest details.

The result is a wavefront image that is only limited by the sensor resolution, not by the Zernike order. The so-called zonal reconstruction is not based on Zernike polynomials. The comparison of 3D wavefront maps after Zernike reconstruction (Fig.9 left) and zonal reconstruction (Fig 9 right) demonstrates the more detailed result of the zonal reconstruction algorithm.

In computer simulations the zonal reconstruction with the WASCA (1.452 lenslet sensor) equals a 24th order Zernike reconstruction.

most common higher-order aberrations, namely spherical aberration and coma.(11)

An ideal customization can best be created with a Gaussian beam with optimized spot overlap. Eye tracking becomes also more critical for the smaller laser spots required for correcting higher order aberrations. In order to adequately follow and track saccadic eye movement during laser vision correction, a very fast sampling rate tracker (> 200Hz) is required. The new launched Mel 80 laser fulfils these criteria.

ZEISS MEL 80 AND HIGH ORDER ABERRATIONS

Introduction

We had the privilege to use the new Carl Zeiss Meditec MEL 80 laser in our praxis since July 2002 (Fig 10).

Scanning lasers with spot sizes greater than 1-mm in diameter are not able to adequately treat the

CONCLUSION

Marketing claims have made some patients naively believe that any ablation that is not wave- front-guided and any visual acuity result worse than 20/10 is bad. In the past 2 years, excimer laser and ophthalmic diagnostic manufacturers have promoted wavefront diagnosis as the coming technology for a golden age of refractive surgery. But will the technology really be all that it has been promoted to be?

Wavefrontguided corneal ablation may be used to correct not only the traditional spherical and cylindrical errors of the eye, but also to reduce the higher-order aberrations. Such corrections, when perfected and refined in the future, could result in patients acquiring better than normal vision (15)

.Because optical aberrations play a significant role with increasing pupil size, mesopic vision may benefit most from a wavefront -guided ablation.

Another important role for custom ablation may be to improve the vision of those patients with suboptimal outcome from previous surgery, such

as decentred ablations or patients with significant night-vision complaints. Deterioration of vision under scotopic lighting conditions is the most frequent complaint of patients who undergo keratorefractive procedures, like RK, PRK or LASIK. In the beginning, increased glare and halos were attributed to scars (RK) and haze (PRK). Recently it has become more evident that the surgically-induced increase in optical aberrations is the main cause of this sequelae.

We noticed an important improvement and practical absence of laser induced HO aberrations using the Mel 80 Laser.

We also appreciated the diagnostic possibilities of the Wasca unit and the many scientific applications of the system allowing detailed analysis of induced aberrations leading to amelioration of ablation profiles as well as objective measurement of accommodation.

Currently, wavefront-guided LASIK and PRK show promise in the correction of ametropia. Once the efficacy and safety have been established through larger clinical trials, it may possibly be applied to emmetropic eyes, aiming for the general improvement of visual acuity, especially under scotopic conditions.

The aberrometer is nowadays a measurement tool for the refractive surgeon. It will become also a diagnostic tool for the refractive surgeon as well as an indispensable tool in the armamentarium of every ophthalmologist. We are extremely satisfied with the WASCA Wavefront analyzer and are exploring new applications.

REFERENCES

1.Salmon TS, West RW, Gasser W, Kenmore T. Accuracy, repeatability and instrument myopia with the COAS Shack-Hartmann aberrometer. Interlaken, 2002, February 15-17,3rd; International Congress of Wavefront Sensing and Aberration-Free Refractive Correction

2.Lee DH. Symposium Wavefront Seoul, Jan

2002

3.Nagy Z. Wavefront aberrations after hyperopic Lasik. In press.

4.Dausch D. Symposium Wavefront, Seoul , Jan

2002

5.Goes F, Reinstein D. Can we foresee flap induced HO aberrations? In press.

6.Panagopoulou SI, Pallikaris IG. Dynamic wavefront refractometry with Asclepion aberrometer in

different light conditions. Interlaken, 2002, February 1517,3rd; International Congress of Wavefront Sensing and Aberration-Free Refractive Correction

7.Pallikaris IG, Panagopoulou SI. Dynamic wavefront refractometry with Asclepion aberrometer in the accommodation process. Interlaken, 2002, February 15-17,3rd; International Congress of Wavefront Sensing and Aberration-Free Refractive Correction

8.Pallikaris IG. Dynamic vision and refractometry.ASCRS Philadelphia 2002

9.Dick B,Kaiser S;Dynamischer Aberrometrie w hrend der Akkomodation phaker Augen sowie Augen mit potentiell akkomodatover Intraokularlinse.In press

10.Goes F, Grabner G, Dick B, Schr der E. Zonal reconstruction with the WASCA unit. ASCRS Philadelphia 2002.

11.Kaemmerer M, Mrochen M. Limitations of wavefront corrections with a scanning spot laser system. Interlaken, 2002, February 15-17,3rd; International Congress of Wavefront Sensing and Aberration-Free Refractive Correction

12.Waring G. Standard graphs for reporting refractive surgery. J Refract Surg 2000; 16: 459-466

13.Goes F. MEL 80 Multicentre study in myopia. ISRS Orlando, 2002

14.Ditzen K. Orlando. MEL 80Hyperopia results. ISRS Orlando, 2002

15.Seiler T, Kaemmerer M, Mierdel P, Krinke HE. Ocular optical aberrations after photorefractive keratectomy for myopia and myopic astigmatism. Arch Ophthalmol 2000; 118: 17-21

____________________

Frank Goes MD

W.Klooslaan,6,

B2050 Antwerp Belgium

Tel : 32.3.2193925

Fax : 32.3.2196667

Email frank@goes.be

Web site www.goes.be