Ординатура / Офтальмология / Английские материалы / LASIK and Beyond LASIK Wavefront Analysis and Customized Ablation_Boyd_2001

Chapter 35

Gaussian Beam

Advantage: provides a smooth surface. Disadvantage: creation of thermal effect due

to a reduction in the energy level below the threshold value required for cold ablation.

The Truncated Gaussian Beam Shape maximizes the smoothness and minimizes the thermal ef-

fect of cornea, resulting in reduction of ablation depth and enhancement rate. Also, this system increases visual acuity, treat larger optical zones to reduce “Halos”, and produce smother corneal surface to reduce “Glare”.

Figure 35-5: Technolas 217z combines the advantages of common beam shapes, the flap top and the Gaussian beam to produce a Truncate Gaussian Beam Shape.

Contents

Section 1

Section 2

Section 3

Section 4

Section 5

Section 6

Section 7

Subjects Index

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Conclusions

Zyoptix system is designed to enable surgeons to correct or reduce refractive errors with the possibility to improve the visual performance of the eye, reduce the potential for glare and halo by using wider optical zones. Also reduce invasive treatment depths and the number of re-treatments by possibly helping patients achieves their desired outcome with primary treatments.

Excimer laser custom ablation technology is not just another passing fashion; the technology could improve LASIK results, allowing for improved postoperative visual acuity and superior vision under low light environment. A lot of the inconsistency in outcome is caused by refraction measurement errors,

which wavefront sensors eliminate. This system could obtain 0.1 D of the intended refraction rather than within the standard 0.5 D. Therefore, the wavefront technology will progressively move the “bar up” for what is considered successful LASIK surgery.

Zyoptix Patient Case

Figures 35-6 and 35-7 show preoperative wavefront including all aberrations. The patient had a refraction of –6.75 –0.25 x 150 o and a BCVA of 0.8.

Figures 35-8 and 35-9 show preoperative wavefront including only High order aberrations on the same patient.

Chapter 35

Figures 35-10 and 35-11 shows a change in the high order aberrations by Zyoptix. Patient Postoperative refraction was 0.00 –0.25 x 150 o, UCVA improve to 1.2 and BCVA was 1.4.

Figures 35-12 and 35-13 show preoperative versus postoperative wavefront deformation.

Contents

Section 1

Section 2

Section 3

Section 4

Section 5

Section 6

Section 7

Subjects Index

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Introduction

Zyoptix is the new generation of excimer lasers used for the treatment of refractive disorders.

Nowadays, we are treating the refractive disorders with standard treatments, only having in mind the subjective refraction. However with such a technique, it is not always possible to achieve the previous best corrected visual acuity.

Zyoptix’s technique takes into account the patient’s subjective refraction, ocular optical aberrations and corneal topography, with the latter not only for the diagnosis, but also for the therapeutic

treatment, in order to design a personalized treatment based on the total structure of the eye. (Figures 36-1a and 36-1b)

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Chapter 36

Refractive Exam

First of all we have to get the patient’s visual acuity, uncorrected and corrected, as well as his subjective refraction, undilated and with cycloplejic.

Zywave Aberrometer

Optical Aberrations

All through the study of the human eye, optical aberrations have been attracting a continuous interest. In the beginning, some systems based on subjective ray tracing were developed, like the Foucault test and modified aberroscopes.(1-4) Further on, an objective wavefront sensor was developed: the Hartmann-Shack wavefront sensor.(5)

Zywave is an advanced wave-front sensor based on the Hartmann-Shack principle that provide us with a precise and fast test of the aberrations of the eye.

To understand how the Zywave works, we should know something about the optical aberrations of the eye and its influence on retinal image quality. Optical aberrations can be divided into chromatic aberrations and monochromatic aberrations.(6)

Chromatic Aberration: Lenses bring different colors of light to a focus at different points.

Monochromatic Aberrations

Spherical aberration: A spherical lens does not focus paralel rays to a point, but along a line. In this way, off-axis rays are brought to a focus closer to the lens than are on-axis rays. This is also applicable to spherical mirrors.

Astigmatism: A lens has different focal lenghts for rays of different orientations, resulting in a distortion of the image. Rays of light from the different meridians in a plane of the object are not focused to the same plane on the edges of the image.

Coma: Off-axis rays do not quite converge at the focal plane. Coma is positive when off-axis rays focus furthest from the axis, and negative when they are closest.

Distortion: The transverse magnification may be a function of the off-axis image distance. It can be positive (pin-cushion), or negative (barrel).

Field curvature (Petzval field curvature): It is caused because the focal plane is actually not planar, but spherical.

To know how these different aberrations have an effect on the eye one must have in mind two important terms: the Zernike polynomials and the Point Spread Function.

The Zernike polynomials are a widely used method in optics to describe wavefront aberrations.(7) The aberration function ___n__m __is expanded as follows :

a-/c/even

where __abc refers to the maximal optical path difference of the lens mesured in units of wavelenght, while the factor Aac is determined by the object location (rocos _o, ro sin _o) and the orientation of the incident wavevector is given by the polar coordinates

(_nm , _nm).

We can descompose eye’s aberrations into Zernike polynomials up to tenth order.(7,8) The firstorder Zernike modes are the linear terms (corresponding to tilt). The second-order modes correspondes to the familiar aberrations, defocus and astigmatism. The third-order modes represents coma aberrations. The fourth-order contains spherical aberrations and other modes. The fifth to tenth-orders are the higherorder, irregular aberrations (they include trifoil, tetrafoil,...) (Fig. 36-2).

It has been proved that aberrations corresponding to fifth to tenth orders do not play a significant role in image quality, mainly for small pupils.(8)

Contents

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Section 2

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Section 4

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Section 6

Section 7

Subjects Index

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But, how can the retinal image quality be represented? How can we know the degree of degradation? Human eyes are not perfect optical systems. As a result when visual stimuli are passed through the optic elements of the eye, they suffer a certain degree of degradation.(8) If we have a very small dot of light and project it through a lens, the image of this point will be not the same as the original: the lens introduces a small amount of blur. The Point Spread Function (PSF) is the squared amplitude of the Fourier transform of the Generalized Pupil Function of the displayed wave aberration function. The ratio of the values of PSF is called Strehl ratio, and it

ZYOPTIX

is a tool that can provide information about “image quality”.

Basis and Usefulness

As we explained before, Zywave is based on a Hartmann-Shack wavefront sensor. It is composed by a laser beam, and a junction of lenses and some elements (collimators, diaphragms, lights) that focus the light on the retina. On the emerging path, it has an array of spherical lenslets and a CCD camera (Figure 36-3).

Zywave projects the laser beam into the eye and use the diffuse reflection from the retina (Figure 36-4). The reflected light passes through the eye and then passes through the array of spherical lenslets that divides the tested wavefront into a number of subapertures. The light through each subaperture is brought to a focus in the focal plane of the lens array.

If we do the test of an ideal plane wave it will result in a regular array of focus spots (Figure 36-5). Then, we use this pattern of a plane wave as a reference pattern. If a deformed wavefront is measured, the image spot at each subaperture shifts with respect to the corresponding point in the reference pattern.(5) The tested wavefront can be detected by measuring the shift of the focus spots.

When we measure the wavefront emerging from the eye we find a distorted wavefront, (Figures 36-6a and 36-6b) that is due to the ocular aberrations. The image of the wavefront is captured by the CCD camera (Figure 36-7a). The shift with respect to the reference pattern is analysed by the Zernike polynomials. Then Zywave shows in the computer screen the image spots, as well as diferent aberration maps. Ocular aberrations are represented in 2 maps, one of them shows us the astigmatic aberration and the other one the high-order aberrations. To allow an easier interpretation, the PSF is also showed. Therefore, you can know how much optical aberrations affect this eye. The program also makes a comparison between predicted PSF if we treat this eye with refractive surgery with a standard laser or if we do it with the Zyoptix laser. (Figure 36-8b)

Is important to know that for the same eye (as well as for any optical system), the mesured wavefront will change with the size of the pupil. The bigger the pupil is, the more distorted the

wavefront.(5,7,8) It is due to an increase of the aberrations in the peripheral zones of the optical systems. (Figure 36-7b).

Zywave Procedure

Phoropter Predicted Refraction (PPR)

This is the refraction given by the aberrometer. It is important to know that this refraction is not the one to be corrected, but just a tool to correlate subjective refraction got by the phoropter and the aberrometric refraction (obtained by the wave front) which is going to be used by the Zylink in order to calculate the more suitable treatment for each patient.

What we have to do is to seat the patient in front of the aberrometer (Figure 36-8a) and switch off any source of light in order to get a more trust-

able aberrations acquisition. So, we have to take three samples undilated, then three samples dilated just with fenilefrin drops in order to avoid any aberration induced by cycloplegic effect. Afterwards we take three samples with cycloplegic just to compare the results obtained by the three methods.

After this, we choose one of the measures obtained just with fenilefrin drops. If they are not similar we calculate the spherical equivalent choosing the more similar to the spherical equivalent of the phoropter refraction.

We do not use the cycloplegic PPR refraction because these drops can induce some new aberrations when dilating that would be taken into account in the treatment although they do not really exist.

Subjects Index

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