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Impressum
On-Line Measurement of the excimer laser ablation of corneal stroma
1Genth U., 2Huebscher H.-J., 3Seiler T.
Department of Ophthalmology, 1University Eye Clinic Dresden, Germany; 2Eye Clinic Berlin-Buch, Germany; 3University Eye Clinic Zurich, Switzerland
Purpose: Using the same algorithm to calculate excimer ablations
for Lasik could result in overcorrections comparing to PRK. Changes of
the ablation rate in different stromal depths could be responsible for
this effect. Former investigations showed that the ablation rate is not
constant in different depths of ablation, especially in Bowman´s
layer.
Methods: PTK ablation using an excimer laser (Schwind Keratom II)
was performed on 126 whole pig eyes with variation of corneal hydration
(580 to 1530 µm thickness), the repetition rate (5, 10, 15, 20,
25 and 30 Hz) and the ablation diameter (2, 6 and 8 mm). The laser output
energy was set within 220 to 260 mJ to provide a constant fluence. The
experiments were done on 6 days, varying all parameters on each day to
prevent systematic errors. The corneal thickness was measured during ablation
using optical low-coherence reflectometry (Haag-Streit). This device is
based on the michelson interferometry principle, splitting the light into
two arms. One arm, colinear with the excimer beam, detects changes of
the refractive index of the cornea. Measurements were taken 15 times per
second. The theoretical solution was 0.58 µm.
Results: The ablation rate does not change due to the stromal depth
in all corneas. It varies in different corneas within a range of 0.27
µm to 0.44 µm/pulse. A decrease of the ablation rate due to
corneal dehydration on the surface during the ablation using high repetition
rates could be measured in a few cases. There is no significant relation
regarding to the repetition rate, the diameter or the number of the experiment
(day).
Conclusions: At room temperature collagen absorbs uv-radiation,
water is secundary absorbing at higher temperature. Ablation rate changes
due to different hydration, but the distribution of different collagen
types in different stromal depths does not alter the ablation rate. Overcorrections
during Lasik could not be explained due to different depths of the ablation
zone. Changes of the corneal hydration during the procedure are probably
responsible for residual refractive errors after Lasik.