Transcript Excimer laser: Fundamentals - new

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)
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
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•
•
•
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
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).
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%)
Gaussian Excimer Laser PRK
Surgical procedure
• Topical anesthesia: oxybuprocaine
• Ablation
• Therapeutic contact lens
• Non-steroid anti-inflammatory (ketorolac)
and antibiotic drops
Gaussian Excimer Laser PRK: Results
N
183
Refraction
pre-op
 -3 D
Refraction
post-op ± SD
- 0.13 D ± 0.15 D
540
>-3D-6D
- 0.17 D ± 0.25 D
203
>-6D-9D
- 1.65 D ± 0.39 D
109
> - 9 D  - 20 D - 1.73 D ± 0.35 D
Achieved Spherical Equivalent Refraction (D)
Gaussian Excimer Laser PRK: Results
10
9
8
7
R2 = 0.97
6
5
4
3
2
1
0
0
2
4
6
8
Attempted Spherical Equivalent Refraction (D)
10
Gaussian Excimer Laser PRK: Results
90.0
81.6
80.0
70.0
% of Eyes
60.0
50.0
40.0
30.0
20.0
10.0
5.8
1.0
0.0
-3
-2
9.7
1.0
1.0
0.0
0.0
0.5
1
2
3
0.0
-1
-0.5
0
Pos toperative Spherical Equivalent Refraction (D)
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
Gaussian Excimer Laser PRK: Results
97.1
100
81.6
99.0
99.0
100.0
86.4
% of Eyes
80
60
40
< 0.5 D 86.4 %
< 1.0 D 97.1 %
20
0
0
0.5
1
2
3
Postoperative Defocus Equivalent Refraction (D)
4
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
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