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Ablation Rate of Excimer Laser Ablation in Different Stromal Depth

1Genth U., 2Huebscher H.-J., 1Richter G., 1Pillunat L. E.,
1Universitätsklinikum Carl Gustav Carus, Klinik und Poliklinik für Augenheilkunde (Dresden)
2Helios-Klinikum Berlin-Buch, Augenklinik (Berlin)

Purpose: Corneal ablation using flying spot lasers with randomized ablation positions leads to local different ablation repetition rates. This is to determine the ablation rate comparing flying spot with large diameter ablations under different repetition rates in different stromal depths.
Methods: Excimer laser ablation (Schwind Keratom II, Kleinostheim, Germany) of the whole stroma was performed on 147 pig eyes, ablation diameter was 2, 6 and 8 mm. Each diameter was ablated using repetition rates from 5 Hz to 30 Hz in steps of 5 Hz. Each combination of ablation parameters was done 6 times. On-line thickness measurement was done during ablation with optical low-coherence reflectometry (Haag-Streit, Switzerland) with a resolution of 0.58 µm. Linear regression analysis on local stromal depths was done to determine the ablation rate.
Results: The overall ablation rate (mean 0.34 µm/pulse, SD 0.04 µm/pulse) of 121 eyes was correlated with the initial corneal thickness (mean 1085 µm, SD 201 µm) (Pearson: r=0.515), there was a difference (p=0.05) comparing the 5 Hz with the 30 Hz group (Scheffé and Duncan test), but no difference comparing 2 mm with 6 or 8 mm. In low frequencies (5, 10 Hz) a wave-like variation (up to 2 oscillations) of the ablation rate (up to 42%) according to the depth was calculated. A significant average increase in ablation rate in deeper stromal layers appeared in 42 of 108 cases, a lower ablation rate in deeper stroma in 6 cases.
Conclusions: In corneal stroma, uv-radiation is absorbed by collagen, water is a secondary chromophor. Ablation rate changes due to different hydration. Postulating a correlation of corneal hydration with initial thickness could explain the correlation of ablation rate with initial thickness. The variation of the ablation rate due to lower frequencies could depend on local inhomogeneity of hydration, caused by shock waves of the ablation process. Because of the considerable influence of the repetition rate the temporal distribution of the flying spot positions during PRK and Lasik should be homogeneous.