| Background and object:The transitional infrared wavelength range refers to the wavelength band from 1.3 μm to 1.4 μm. With the increase of wavelength in this region, the most sensitive tissue changes gradually from the retina to the cornea. High transmission through the atmosphere and optical fibers made the NIR laser prevalent in military, medicine and telecommunication industry, and the person who research on or operate lasers operating at this wavelength region would face with severe risk of ocular injury. The investigations of ocular injury effects induced by transitional NIR lasers have revealed some unique characteristics. For instance, damage may be induced in one or more of ocular media and the ocular threshold lesions involve full thickness of that layer. Up to now, ocular damage thresholds for diverse exposure conditions have been determined, but the quantitative dependence of ocular injury on the exposure parameter has not been investigated systemically, especially for the nonlinear dependence regime(1 ms ~ 10 s). Therefore, this research aimed to investigate the ocular damage effect dependence on exposure parameters and the pathological characteristics of the reparative process.Method and results: 1. The corneal damage effects induced by transitional NIR laser. Method:(1)Build a 1338 nm corneal exposure set-up. Determine the corneal damage thresholds for corneal incident spot sizes from 0.14 to 3.7 mm at 5 ms exposure duration. The lesion/no lesion data were collected at 1h post-exposure and the ED50 s were analyzed with Bliss probit analysis method.(2)Build a 1319 nm corneal exposure set-up and determine the corneal damage thresholds for exposure duration for 75 ms to 10 s. The incident corneal irradiance diameter was fixed at 2 mm for all exposure conditions to avoid the influence of spot size on threshold. The dependence of ED50 on exposure duration was determined through regression analysis.(3)Animals were sacrificed for histological evaluation at 0~270d post-exposure. The corneas were dissected and processed for paraffin embedding. Serial sections(4 μm thick) were cut and stained with hematoxylin and eosin(H&E) for general morphology. Results:(1)The corneal ED50 thresholds given in terms of the corneal radiant exposure for incident spot diameter of 0.14 mm, 0.3 mm, 1.0 mm, 2.0 mm and 3.7 mm were 277.3 J/cm~2, 61.2 J/cm~2, 31.5 J/cm~2, 27.0 J/cm~2 and 27.9 J/cm~2 respectively.(2)The ED50 thresholds given in terms of the corneal radiant exposure for exposure durations of 5 ms, 75 ms, 0.35 s, 2 s and 10 s were 39.4 J/cm~2, 51.5 J/cm~2, 87.2 J/cm~2, 156.3 J/cm~2 and 311.1 J/cm~2 respectively. The 39.4 J/cm~2 was derived from the ED50 for 1338 nm(27.0 J/cm~2). The ED50 thresholds for exposure durations of 75 ms ~ 10 s were correlated by a power law equation, ED50=128.9t0.36 in J/cm~2 where t was input in the unit of second, with correlation coefficient(R) 0.997. The corneal damage threshold was independent on exposure duration when it below ~ 50 ms.(3)The corneal lesion induced by transitional NIR laser involved full thickness of the cornea. Under the slit lamp, at 1.5 times thresholds level, the corneal thickness of the damaged and adjacent regions increased remarkably at 6h and recovered at 3d post-exposure. But the corneal thickness of damage region was thiner than nomal level at at 7d~180d post-exposure and did not recover at 180 d. Under the microscope, the active division of cornal epithelial cells and the severe vacuolization of corneal stroma were observed at 3d~7d, which indicated that the corneal lesion begin to repair. The speed of reparation was quick at 14d~60d and slows at 90d~180d. The vacuolization of corneal stroma and the amount of epitheliam cell gradually decreased and the thickness of corneal lesion increased. The corneal damage at threshold level was recovered at 30d~60d while the corneal damge at suprathrshold level did not recover until 270 d. 2. The retinal damage effect induced by transitional NIR lasers. Method:(1)12 New Zeland and 12 Chinchilla grey rabbits(2~3 kg). Measured the axial lengths of 24 rabbits in 30d~370 d. In each specie, selected two group animals with axial length difference large than 1 mm.(2)Build a 1319 nm retinal exposure set-up and determine the retinal damage thresholds at 0.1 s exposure duration and 2 mm incident spot diameter for four groups of subjects.(3)The in-vivo retinal damage thresholds(ED50s) were determined in chinchilla grey rabbits for 1319 nm laser radiation for exposure durations from 0.1 s to 10 s. The incident corneal irradiance diameter was fixed at 5 mm.The dependence of ED50 on exposure duration was determined through regression analysis. Results:(1)The retinal ED50s(TIE) for two groups of Chinchila grey rabbits with axial lengths of 16.04±0.33 mm and 17.25±0.22 mm were 1.05 J and 1.73 J respectively. The retinal ED50 s for Chinchila grey rabbits and New Zeland rabbit with axial lengths of 17.06±0.29 mm and 16.95±0.22 mm were 3.6 J and 4.4 J respectively. The thresholds for rabbits with shorter axial were significantly smaller than those for rabbits with longer axial length.(2)The ED50 s given in terms of the total intraocular energy(TIE) for exposure durations of 0.1 s, 1 s and 10 s were 1.36 J, 6.33 J and 28.6 J respectively. The ED50 s were correlated by a power law equation, ED50=6.31t0.66 in J where t was input in the unit of second, with correlation coefficient R=0.9999. 3. Thoretical calculation on the intraocular propagation of transitional NIR laser. Method: Build the four-surface schematic eye models for human, rhesus and rabbit and calculate the retinal spot size and the relative energy arriving the retina. According to the principle of the same dose cause the same injury effect, the retinal ED50 s for Chinchila grey rabbit were extrapolated to those for rhesus and human. Result: For 1338 nm laser, the radius of retinal spot in eye model of human, rhesus and rabbit were 173.4 μm, 106.2 μm and 57.2 μm respectively at 1 mrad divergence and 5 mm incident spot diameter. For human, rhesus and rabbit, the transmission of the ocular refraction system were 1.4%, 3.2% and 6.7% respectively. The intraocular energy required to induce injury in human retina was 3.9 times of that in rabbit retina. For given beam divergence, the relative retinal radiant exposure decrease with the incident spot diameter, while for given Incident spot diameter, the retinal radiant exposure increase with beam divergence inversely.Conclusion:(1)For 1338 nm at 5 ms exposure duration, the corneal ED50 s varied with the incident spot diameter inversely when it was smaller than 1 mm and independent on the incident spot diameter when it was larger than 1 mm. For 1338 nm laser ar 5 ms exposure duration, the cornea was more susceptible when the incident spot diameter was smaller than 2 mm and the retina was more susceptible when the incident spot diameter was larger than 2 mm.(2)For exposure duration from 75 ms to 10 s, the corneal ED50 thresholds for 1319 nm were correlated by ED50=128.9t0.36(R=0.997)in J/cm~2, which was slightly smaller than those of medium or far-IR.(3)The ocular lesion induced by transitional NIR lasers involved full thickness. The repair process of corneal lesion showed that active division of epithelium cell and the vacuolization of substantia propria layer. The corneal damage at threshold level was recovered at 30d~60d while the corneal damge at suprathrshold level did not recover until 270 d.(4)The influence of axial length on the retinal damage threshold(TIE) was greater than that of retinal pigmentation. The retinal damage threshold increased with axial length.(5) For exposure duration from 0.1s to 10 s, the retinal ED50 thresholds(TIE) for 1319 nm were correlated by ED50=6.31t0.66 in J. The retinal threshold dependence on exposure duration for transitional NIR laser was not significantly deviate that for other visible and NIR lasers since there existed thermal lensing effect.(6)For 1319 nm laser, the retina was more susceptible for exposure duration shorter than 1 s while the cornea was more susceptible for exposure duration longer than 1 s. In transitional NIR wavelength region,there were adequate safe regions existed between the ocular damage thresholds and the MPEs in current laser safety. |
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