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R. Ackermann, R. Kammel, T. Thiele, D. Wagner, S. Nolte, A. Tünnermann; Optical and Mechanical Properties of Animal Lenses After Fs-Laser Treatment. Invest. Ophthalmol. Vis. Sci. 2008;49(13):3797.
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It is commonly accepted that one of the most contributing factors for presbyopia is the hardening of the nucleus of the crystalline lens. We show that the flexibility of the lens can be partially restored on a physiologically relevant scale by the application of ultrashort laser pulses.
Porcine and sheep eyes of different age were dissected within 48 hours post-mortem by extracting the lens within the ciliary body, which was then attached to the clamps of a lens stretching apparatus. It provides eight radial arranged arms in order to exert and measure forces to the ciliary body with a resolution of 1 mN. The lenses were kept in tap water or physiological saline to avoid drying. The laser effect was evaluated by comparing a laser structured with an untreated lens of the same individual.The laser system provides pulses with a duration of 400 fs with an energy of up to 5 µJ at 200 kHz. A downstream scanner system allows writing three dimensional structures with a focal spot size of about 7 µm. For a first appraisal of our method, the whole lens nucleus was filled with a simple cubic structure, neglecting issues of eye safety and remaining lens transmission.For the evaluation of the lens geometry a rotating Scheimpflug-camera was used that allowed to measure the lens shape as a function of the applied forces. Optical characterization is performed by measuring the transmitted wave-front during the simulated accommodation, using a common Shack-Hartmann sensor.
The analysis of the forces on the ciliary body shows a reproducible range with a linear force response, e.g. ranging to a maximum average force per clamp of 140 mN for young porcine eyes.Both untreated and laser structured porcine lenses show a maximum increase of the diameter which is less than 10 %. Yet, in the regime of linear force response this increase is significantly steeper for lasered eyes.This observation is also confirmed by the wave-front analysis. For the given setup, porcine lenses showed a refraction power in the range of 70 - 100 dpt in the unstretched state. In the corresponding regime of low forces the lasered eyes indicate an increased accommodation amplitude of about 10 dpt.
We have developed a setup that allows to characterize the effect of fs-laser structuring on the optical and geometrical properties of in-vitro lenses. This establishes the basis for further investigations that will focus on an optimization of laser structures with regard to laser safety and lens transmission.
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