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

Elevation Topography (Orbscan) (Figure 36-9)

What we should focus on in the orbscan is: -Anterior corneal shape (to leave aside

keratoconus and other pathology) -Posterior corneal shape (to leave aside

frustre keratoconus ) -Paquimetry

-Kappa angle. This can suggest us

to decentre a little bit the eyetracker when performing the laser session.

-Others: -Thinnest point.

-Keratometric map.

-Pupillometry

Other Exams we Perform Rutinely are:

-Curvature topography. -Ultrasonic paquimetry.

-Pupilar scotopic diameter (colvard) -Biomicroscopy

-Intraocular pressure -Complete retinal exam

Figure 36-9: Elevation map performed by the Orbscan topography unit.

Figure 36-10: Integral refractive workstation

Zylink (Figure 36-11)

Once we have collected all the data we need, we introduce them in an special software called Zylink which is used to calculate the final treatment we will order the laser to execute.

Zylink requires the PPR and the orbscan data, and then it automatically, after mixing both, gives you the exact refraction to perform.

There are two parameters you can vary according to the patient characteristics which are the optical zone and the ablation. The Zylink recommends an optical zone and a depth of ablation, but you can enlarge the optical zone as much as you want according to the scotopic pupil diameter so as to reduce to the maximum the risk of halos and glare post lasik.

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ZYOPTIX

Figure 36-11: Zylink software used to calculate the final treatment.

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The bigger the optical zone, the higher ablation it will do, so we must take care of the paquimetry, not to leave a residual stroma thinner than 225 microns, if so the risk of ectasias is so high (Figure 36-12).

Another interesting and useful aspect of the Zylink is that it shows you the Point Spread Function (PSF). This is in a few words the image projected on the retina of a single remote point (e.g. a star in the sky). The perfect PSF would be a single and sharp point. What Zylink shows you is the actual PSF without correction in the right of the screen,

the predicted PSF if we use a Planoscan laser in the middle, and the predicted PSF if performing a Zyoptix (refer to figure 36-8b). This way you can compare the predicted results, and it helps you decide whether Zyoptix would really be useful for the selected patient or not. If there is a large difference between the PSF with Planoscan and the PSF with Zyoptics in favour of it (smaller PSF), we can conclude that the latter is worth doing. The other way round we may choose the technique we feel more comfortable. (Figure 36-13).

Figure 36-13: High order aberrations with astigmatic component. Comparison between Zyoptix and standard laser treatment.

ZYOPTIX

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PREPARING THE LASER

The next step is transferring the final refraction given by the Zylink to the laser (Keracor 217 Z). We do so by a diskette of 3.5”. After saving all the data from the Zylink in it, we introduce it to the laser. (Figure 36-10).

Another aspect that makes Zyoptics differ-

ent is the need of a special card to be introduced in- Help ? side the laser for any eye we treat. Without this card

you cannot perform the laser because the laser beam has to pass through it to be effective. Every card has three holes, two of them are of 2 mm diameter, one for the truncated gaussian beam which is used to do the treatment, and the other one is for the flat top beam which is used for the fluence test. The third

hole is 1 mm diameter and it is also for a truncated gaussian beam, not to treat, but to smooth the surface (Figure 36-14). The problem of the flat top beam is that ablates more than necessary and that cannot make uniform and smooth ablations. Otherwise, the gaussian beam defect is that the periphery of the beam has just a thermal effect due to its low energy level.

Its ablation does not have any refractive effect, so it is unnecessary. (Figures 36-15 and 36-16)

What we have achieved with the truncated gaussian beam is to ablate just what has a real refractive effect, with no thermal effect, and succeed in having a smooth and uniform ablation with an ideal transition area.

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CLINICAL CASES

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Chapter 36

Figure 36-17a: Preoperative comparative topography in a myopic patient.

Figure 36-17b: Postoperative topography 1 day after laser ablation.

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Figure 36-17c: Postoperative topography 1 week after laser ablation.

Figure 36-17d: Comparative results in the same patient between PlanoScan and Zyoptix.

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REFERENCES

1.W.M. Rosenblum and J.L. Christensen, “Objective and subjective spherical aberration mesurement of the human eye”, in Progress in Optics, E.Wolf, ed.(NorthHolland, Amsterdam, 1976), Vol. 13, pp. 69-91.

2.M.Campbell,E.Harrison,and P. Simonet”, Psychophysical measurement of blur on the retina due to optical aberrations of the eye”, Vision Res. 30, 1587-1602 (1990).

3.H. Howland and B.Howland “A subjective method for the measurement of monochromatic aberrations of the eye” J. Opt. Soc. Am 67, 1508-1518 (1977)

4.G. Walsh, W. M. Charman, and H. Howland “Objective technology for the determonation of monochromatic aberrations of the human eye” J. Opt. Soc. Am A1, 987-992 (1984)

5.J.Liang, B.Grimm,S.Goelz,and J.F. Bille “ Objective measurement of wave aberrations of the human eye with the use of a Hartmann-Shack wave-front sen-

sor” J. Opt. Soc. Am 11, 1949 - 1957 (1994).

6.E. Hecht, A. Zajac, ”Optics” ed. Addison-Wesley Iberoamericana, (1986).

7.J. Y. Wang and D.E. Silva, “Wave-front Interpretation with Zernike polynomials. Applied Opt., vol.19, No 9 (1980).

8.J.Liang and D.R. Williams “ Aberrations and retinal image quality of the normal human eye” J. Opt. Soc. Am. vol 14, No 11, 2873-2883 (1997).

Andreu Coret, M.D.

Medical Director

Instituto Oftalmológico de Barcelona

Barcelona, Spain

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A B

Figure 37-1 AB: A) Schematic illustration of smoothing agent effect on a corneal surface having irregularities. B) Testing PALM gel ablation rate: Pig eyes received 60-microns of ablation at the presence of a stainless steel mesh in order to produce a periodic grid-like pattern on the corneal surface. A PALM lenticule was formed over this irregularity and half of the cornea was irradiated until gel was totally ablated.

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