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We evaluated the efficacy, safety, and stability of femtosecond laser intrastromal refractive procedures in ex vivo and in vivo models. When compared with longer pulsewidth nanosecond or picosecond laser pulses, femtosecond laser-tissue interactions are characterized by significantly smaller and more deterministic photodisruptive energy thresholds, as well as reduced shock waves and smaller cavitation bubbles. We utilized a highly reliable, all-solid-state femtosecond laser system for all studies to demonstrate clinical practicality. Contiguous tissue effects were achieved by scanning a 5 μm focused laser spot below the corneal surface at pulse energies of approximately 2 - 4 microjoules. A variety of scanning patterns was used to perform three prototype procedures in animal eyes; corneal flap cutting, keratomileusis, and intrastromal vision correction. Superior dissection and surface quality results were obtained for lamellar procedures (corneal flap cutting and keratomileusis). Preliminary in vivo evaluation of intrastromal vision correction in a rabbit model revealed consistent and stable pachymetry changes, without significant inflammation or loss of corneal transparency. We conclude that femtosecond laser technology may be able to perform a variety of corneal refractive procedures with high precision, offering advantages over current mechanical and laser devices and techniques.

We investigated plasma-mediated surface ablation in corneal tissue using picosecond and femtosecond laser pulses in order to achieve high precision, non-thermal tissue removal with a non-ultraviolet laser source. Experiments utilized three laser systems, a regeneratively amplified Ti:sapphire laser, a synchronously amplified dye laser, and a regeneratively amplified picosecond Nd:YLF laser. Tissue ablation was performed by tightly focusing the laser beam on the tissue surface. Ablation thresholds were determined by monitoring the plasma spark, as well as the tissue surface. Tissue ablations were then analyzed by standard histologic methods and scanning electron microscopy. We observed a decrease in the ablation fluence threshold as the pulse duration is shortened from 200 ps to approximately 140 fs, in agreement with our theoretical predictions. Using identical pulse energies, the femtosecond laser pulses ablated tissue at higher efficiencies than the picosecond laser, with an approximately two-fold improvement in the etch depth curve. Histologic analysis reveal minimal adjacent tissue damage at either pulse duration. Femtosecond laser pulses may offer advantages that make them ideal tools for high precision tissue ablation.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We evaluated in vivo wound healing responses to plasma- mediated ablation in skin as a function of laser pulsewidth and energy. Experiments utilized a regeneratively amplified Ti:Sapphire laser operating at 800 nm with pulsewidths varied from 7 ns to 100 fs. Skin incisions were created in mice by tightly focusing the laser beam on the tissue surface. Incisions of equal depth were compared at time points ranging from 6 hours to 3 weeks using standard histologic methods. Incision depth was proportional to pulse energy at each pulsewidth. Fluence threshold dependence on laser pulsewidth agreed with those predicted by ex vivo testing. Histologic analysis revealed minimal adjacent tissue damage at pulsewidths less than a few picoseconds and energies near the fluence threshold. Longer pulsewidths and higher fluence levels were associated with more significant collateral effects. These in vivo results suggest collateral tissue damage and secondary effects may be minimized by controlling laser pulsewidth and energy.

We measured damage thresholds for corneal tissue over a range of pulse durations (from 150 fs - 7 ns) that could not be examined previously with a single laser source, using a Ti:sapphire chirped-pulse amplification (CPA) laser [1]. Our results depart from previous findings [2,3], which had to use multiple laser sources to investigate a similar pulsewidth range.