Abstract
Purpose: :
Despite the fact that many ocular tissues are more sensitive to ultraviolet (UV) damage, the eye is remarkably well–protected from direct UV exposure by geometrical factors, which, unfortunately, cannot be well–simulated in a laboratory environment. Boettner and Wolter (1962) and others have measured direct and total (direct and scattered) light transmittance of the cornea, aqueous humor, lens, and vitreous humor by spectrophotometry. The total transmittance of ocular tissues was found to be slightly higher than the direct transmittance due to scattering. This study evaluated the diffuse transmission of ocular structures, such as the sclera and iris, which may also contribute significantly to intraocular exposure, considering their direct exposure to UV. Pigmented and non–pigmented rabbits were used as an animal model to more fully evaluate the transmission of UV radiant energy through tissues of interest.
Methods: :
Spectral transmission of ocular tissue from 6 New Zealand White (NZW) and six Dutch–Belted rabbit eyes were evaluated in this study. Sclera, cornea, iris, and crystalline lens material were dissected carefully within 3 hours of enucleation of the rabbit globes. Each tissue specimen was mounted at the entrance aperture of an integrating sphere(RSA–BE–65, LabSphere, North Sutton, NH) to measure total diffuse spectral transmittance with a Beckman DU–800 spectrophotometer at wavelengths between 200nm and 1100nm.
Results: :
Light transmission of sclera and iris tissue, to our knowledge, has not been previously presented in the literature. The most significant difference in spectral transmission, as large as 4–fold, was seen in the iris tissue due to pigmentation from 400–1100nm. There was no detectable difference between the spectral transmission of the cornea, sclera and lens material between the NZW and Dutch–Belted rabbit eyes. Less than approximately, 10% of diffuse UV radiant energy was transmitted through the sclera and iris tissues in the rabbit model.
Conclusions: :
Little UV radiant energy is transmitted through the iris and sclera. The pigmented iris is an effective shield of the germinative region of the lens for direct solar radiation. The sclera does transmit a very small amount, but due to the very high scattering coefficient of this tissue, surprisingly little is transmitted to the lens and ciliary body. The anatomical factors, including lid geometry and pupil diameter, in varying light conditions, have a significant role in protecting the ocular tissues from UV exposure. These results may provide insight in determining necessary UV protection in biomaterials for ocular applications.
Keywords: radiation damage: light/UV • iris • protective mechanisms