Nd:YAG fourth harmonic (266-nm) generation for corneal reshaping procedure: An ex-vivo experimental study

Abstract

Corneal reshaping is a common medical procedure utilized for the correction of different vision disorders relying on the ablation effect of the UV pulsed lasers, especially excimer lasers (ArF) at 193 nm. This wavelength is preferred in such medical procedures since laser radiation at 193 nm exhibits an optimum absorption by corneal tissue. However, it is also significantly absorbed by the water content of the cornea resulting in an unpredictability in the clinical results, as well as the high service and operation cost of the commercial ArF excimer laser device. Consequently, other types of solid-state UV pulsed lasers have been introduced. The present work investigates the ablation effect of solid-state laser at 266 nm in order to be utilized in corneal reshaping procedures. Different number of pulses has been applied to Polymethyl Methacrylate (PMMA) and ex-vivo rabbit cornea to evaluate the ablation effect of the produced laser radiation. PMMA target experienced ellipse-like ablated areas with a conical shape in the depth. The results revealed an almost constant ablation area regardless the number of laser pulses, which indicates the stability of the produced laser beam, whereas the ablation depth increases only with increasing the number of laser pulses. Examination of the ex-vivo cornea showed a significant tissue undulation, minimal thermal damage, and relatively smooth ablation surfaces. Accordingly, the obtained 266-nm laser specifications provide promising alternative to the traditional 193-nm excimer laser in corneal reshaping procedure.

Fig 1. (a) Schematic diagram and (b) lab photo of the implemented setup.

Fig 2. The designated globe holder.

Fig 3. Resultant ablation on PMMA.

(a) at different number of pulses, and (b) illustration of the cone-like shape in the depth direction.

Fig 4. (a) Ablation in PMMA at different number of pulses obtained with polarized microscope, (b) variation of the ellipse diameters with the number of applied pulses, and (c) change in the ablation depth with the number of applied pulses.

Fig 5. Ablation rate in PMMA at different fluence values.

Fig 6. Total ablation depth as a function of (a) normalized fluence and (b) total fluence, and (c) variation in the ablation rate with the fluence.

Fig 7. The resultant ablation in corneal tissue.

Fig 8. Histopathologic changes of corneal tissue after applying laser pulses.

(a) 50 pulses, and (b) 100 pulses.