Jean B, Bende T
Division Experimental Ophthalmic Surgery, University Eye Hospital, Dept I, Tübingen, FRG.
J Refract Corneal Surg. 1994 Jul-Aug;10(4):433-8.
Photoablation in the infrared (IR) is an option for future refractive and corneal surgery; its basic principles have not yet been investigated systematically. For the first time, the free electron laser allows the dynamic study of photoablation over a wide range of wavelengths with variable combinations of pulselength and energy. The goal of this study is to use the free electron laser as a tool to describe photoablation in the IR quantitatively. We studied the function of wavelength as it is related to target material spectroscopy and the effects of corneal hydration and the pulse repetition rate.
Surface absorption spectroscopy of the human cornea and of gelatin as a proven model of the cornea was performed between 2.7 and 6.7 microns. Gelatin probes of well-defined thickness (140 +/- 5 microns) and controlled hydration (wet/dry weight 1 to 4.5) served as target material. Photoablation was performed with the Vanderbilt University free electron laser (Nashville, Tenn) in September 1992 at a fluence of 1.27 J/cm2, and a macropulse of 4 microseconds, composed of 2 ps micropulses at a 2.9 GHz pulse repetition rate. Wavelength was tunable between 2.7 and 6.7 microns at stable beam profiles. Ablation experiments were performed as a function of energy, hydration, and pulse repetition rate. Ablation rates were assessed by a) perforation experiments, and b) direct measurements using confocal laser topometry (UBM, Ettlingen, FRG).
Ablation rate, assessed by perforation experiments and topometry, correlated well with the corresponding measured absorbencies of the target material: maximal ablation rate at maximal target absorption, around the 3- and 6-micrometer water absorption bands. The ablation threshold at 6.2 microns was 0.7 +/- 0.05 J/cm2 (perforation) and 0.55 +/- 0.08 J/cm2 for depth measurements. Ablation rate as a function of hydration increased to 2.3 (wet/dry weight) with a decrease for higher hydrations. Ablation rate as a function of the pulse repetition rate showed an increase of up to 20 Hz, where it was found to be 60% higher.
The first systematic use of free electron laser technology positively correlated ablation efficiency with target material absorption, thus identifying a "new" promising wavelength at around 6.2 microns for materials with a high water content such as corneal tissue.
红外光消融术是未来屈光和角膜手术的一种选择；其基本原理尚未得到系统研究。自由电子激光首次实现了在广泛波长范围内，通过可变的脉冲长度和能量组合对光消融进行动态研究。本研究的目的是利用自由电子激光作为工具，定量描述红外光消融。我们研究了波长与靶材料光谱的关系，以及角膜水化和脉冲重复率的影响。
在2.7至6.7微米之间对人角膜和作为角膜验证模型的明胶进行表面吸收光谱分析。厚度明确（140±5微米）且水化程度可控（湿/干重1至4.5）的明胶探针用作靶材料。1992年9月，使用范德比尔特大学自由电子激光（田纳西州纳什维尔）进行光消融，能量密度为1.27 J/cm²，宏脉冲为4微秒，由2皮秒微脉冲组成，脉冲重复率为2.9 GHz。波长在2.7至6.7微米之间可调，光束轮廓稳定。消融实验作为能量、水化程度和脉冲重复率的函数进行。消融率通过以下方式评估：a）穿孔实验，b）使用共焦激光角膜地形图仪（德国埃特林根的UBM）进行直接测量。
通过穿孔实验和角膜地形图仪评估的消融率与靶材料相应的测量吸光度密切相关：在最大靶吸收处，即3微米和6微米左右的水吸收带附近，消融率最高。6.2微米处的消融阈值为0.7±0.05 J/cm²（穿孔），深度测量为0.55±0.08 J/cm²。消融率随水化程度增加至2.3（湿/干重），更高水化程度时则降低。消融率作为脉冲重复率的函数，在高达每秒20次时显示增加，此时发现其高出60%。
自由电子激光技术的首次系统应用使消融效率与靶材料吸收呈正相关，从而为含水量高的材料（如角膜组织）确定了一个约6.2微米的“新”的有前景的波长。
