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

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)

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)

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.

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

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

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

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)

Vaporization of molecules

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.

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.

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

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

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

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

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.

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.

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.

Gaussian Excimer Laser PRK: Results Refraction pre-op post-op ± SD 183  -3 D - 0.13 D ± 0.15 D 540 > - 3 D  - 6 D - 0.17 D ± 0.25 D 203 > - 6 D  - 9 D - 1.65 D ± 0.39 D 109 > - 9 D  - 20 D - 1.73 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.

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.

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.

Gaussian Excimer Laser PRK: Results Defocus Equivalent Refraction (DER) = sphere + cylinder/2 without sign Example : -1 +2 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.

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

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.

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, 18.25 sec for 10D. Besides we had only few complications. Maintenance costs are very low with this system.