1. Excimer laser: Fundamentals
SCHRAEPEN P., TRAU R.,
University Hospital Antwerp

2. Refractive surgery
To correct anisometropia by modifying the refractive power of the eye
Cornea (2/3 of the refractive power)
Lens (1/3 of the refractive power)

3. Corneal refractive surgery
To modify the curvature of the anterior surface of the cornea
Relaxing incisions: RK
Ablation of tissue:
PRK, LASIK
Excimer
Myopia, astigmatism, hypermetropia
Compression of tissue:
Thermokeratoplasty
DTK, Holmium
Hyperopia
Implantation of biomaterial in the cornea (Intacs)

4. Photorefractive Keratectomy (PRK)
To modify the curvature of the anterior surface of the cornea
1D = 10µm = 40 pulses (optical zone = 6mm)
The purpose of photo refractive keratectomy is to modify the curvature of the anterior surface of the cornea, to correct any ametropia.
To correct one diopter, the excimer laser causes an ablation of 10 micron in the center, in approximately 40 pulses.
As an example I show you the corneal topography before and after PRK. You will notice the very large optical zone with smooth edges.

5. Excimer laser photo-ablation
LASER = Light Amplification by Stimulated Emission of Radiation
Monochromatic light : specific wave length (e.g. excimer: 193 nm)
Ray of light
photons
Carriers of energy

6. Excimer laser photo-ablation
Excimer = excited dimer
Ceramics cavity with 3 gases
Helium / neon (88% - 99%)
Rare gas (Argon, Xenon, Krypton)
Halogen (Fluoride, Chloride)

7. Excimer laser photo-ablation
Wave length: 193 nm
Maximal absorption of energy by corneal stroma
Minimal transmission of energy throuh the cornea
Minimal damage to surrounding tissue
Photochemical ablation
Energy: 6.4 eV / pulse
rupture of intermolecular bindings
vaporization of corneal tissue

8. Excimer laser photo-ablation
Vaporization  2000 m/sec  “plume”
 “central island”
Ejection of tissue fragments  typical crackling
Time: 10 – 20 nsec / pulse
Depth: 0.25 µm / pulse
 1D = 10 µm = 40 pulses (optical zone = 6 mm)

9. Vaporization of molecules

10. Gaussian Excimer Laser PRK
Delivery systems
Mechanical systems
Broad beam + diaphragm
Scanning systems
Physical system
Gaussian repartition of energy
The energy on the cornea can be delivered by two kinds of delivery systems: a mechanical and a physical. The easiest way is a broad beam of which the size is adjusted by a diaphragm.
A more sophisticated –and a very expensive mechanism is the flying spot, which encircles around the cornea. This system requires an eye tracking system.
The gaussian delivery system that we use, is a physical system and consists in an array of diffractive micro-lenses giving for every pulse a gaussian energy repartition on the corneal surface. This delivery system is cheap and efficient.

11. This first picture shows the gaussian energy repartition
This first picture shows the gaussian energy repartition. For every pulse, most of the energy reaches the center, an little gets the periphery.
This diagram shows the depth of the ablation, and as you can see it’ s a very smooth surface.
Here we see the micro plate with the lenses that generate the gaussian energy repartition.
This is a picture taken with an electron microscope of a corneal surface treated with a gaussian excimer.

13. Excimer laser: a “bad” laser
Unstable
frequent controls
calibration system
non homogeneous (hot / cold spots)
toxic for optics

14. Excimer laser: a “bad” laser
Non homogeneous:
beam homogenizers (turning prisms)
small beam (± 1 mm)
longer ablation time
loss of fixation
eye tracking devices
optical diffraction

15. What is the laser of the future?
Femtosecond laser ?
Solid
No toxic gases
More predictable
Intra-corneal ablation
Non-mechanical microkeratome

16. Gaussian Excimer Laser PRK: Results
P. Schraepen 1, E. Eskina 2, T. Vandorselaer 1, L. Gobin 1, R.Trau 1, M.-J. Tassignon 1
1. Dept of Ophthalmology, University Hospital Antwerp, Belgium
2. Sphere Eye Laser Surgery Clinic, Moscow, Russia

17. Gaussian Excimer Laser PRK
Purpose
To evaluate the refractive results of PRK with the InPro Gauss Excimer laser,
according to the Standard Graphs of G. Waring ( Journal of Refractive Surgery July 2000).
The purpose of this study is to evaluate our refractive results with the InPro Excimer laser. Therefore we used the guidelines of G Waring.

18. Gaussian Excimer Laser PRK
Materials & Methods
Retrospective study
Excimer with gaussian delivery system
1035 eyes
3-year follow-up
Myopia from –1.50 D to –20 D
Astigmatism from 0.75 D to 3 D (combination: 29%)
For this study we’ve cooperated with the Sphere Eye Clinic in Moscow since they use exactly the same laser as we do in Antwerp. Together we have treated 1035 eyes in a 3-year follow-up period.
Myopia ranged from –1.50 D to –20 D and astigmatism ranging from 0.75 D to 3 D. In 29% of all cases we did a combined treatment of myopia and astigmatism.

19. Gaussian Excimer Laser PRK
Surgical procedure
Topical anesthesia: oxybuprocaine
Ablation
Therapeutic contact lens
Non-steroid anti-inflammatory (ketorolac) and antibiotic drops
For each procedure we used topical anesthesia; we did a manual debridement of the epithelium. After the ablation always a therapeutic contact lens, anti-inflammatory and antibiotic drops.

20. Gaussian Excimer Laser PRK: Results
Refraction
pre-op
post-op ± SD
183
 -3 D
D ± 0.15 D
540
> - 3 D  - 6 D
D ± 0.25 D
203
> - 6 D  - 9 D
D ± 0.39 D
109
> - 9 D  - 20 D
D ± 0.35 D
We’ve split our cases in 4 groups: low myopia to –3D, moderate myopia to –6D, high myopia to –9D and very high myopia to –20D.
The spherical equivalent at the end of the follow-up period ranges from –0.13D in the low myopia group to –1.73D in the 4th group.

21. Gaussian Excimer Laser PRK: Results
We’ve compared the attempted with the achieved spherical equivalent of the first 100 consecutive cases. As you will notice the correlation is up to 0.97.

22. Gaussian Excimer Laser PRK: Results
In 81.6% we’ve achieved a postoperative spherical equivalent of 0 D. 9.7% was –0.5D and 5.8% was -1D. This means that 97.1% ended between – 1 and 0 D. This diagram includes only myopia cases up to –6D.

23. Gaussian Excimer Laser PRK: Results
Defocus Equivalent Refraction (DER)
= sphere + cylinder/2 without sign
Example : à 90°
SER=0 DER=2
81,6 % of eyes had a DER of 0
86,4 % of eyes had a DER < 0,50
A frequently used parameter is the defocus equivalent refraction. This is an interesting parameter since it gives a more realistic idea of the astigmatism than the spherical equivalent. For example: sphere –1, cylinder +2 results in a spherical equivalent of 0, but a defocus equivalent of 2.

24. Gaussian Excimer Laser PRK: Results
In our study, 81.6% of eyes had a defocus equivalent of 0D.
86.4% of eyes had a defocus equivalent of less than 0.5D

25. Gaussian Excimer Laser PRK
Complications
10 eyes haze (+, ++)
22 eyes haze and refractive regression
12 eyes subjective complaints linked to aberrations; only in the high myopia group
We didn’t experience any major complications. There was a development of haze in 10 eyes, haze with regression in 22 eyes.
To compare the haze, we used a 0 to 4 scale. All eyes with haze were between one and two on this scale; and all were temporary.
Only in the high myopia group there were 12 eyes with aberration linked subjective complaints.

26. Gaussian Excimer Laser PRK
Conclusion
Good refractive results
Smooth ablation surface
Large ablation diameter
Short ablation time (3D = 9 sec; 10D = 18 sec)
Few complications
Low maintenance costs
To conclude I think that we can say that we’ve achieved very good refractive results. The gaussian delivery system allows a smooth ablation surface and edges with an optical zone of app. 8 mm. The treatment is based on the active participation of the patient to fixate, which results in well centered treatments.
The ablation time is rather small: 9 sec to correct 3D, sec for 10D. Besides we had only few complications.
Maintenance costs are very low with this system.