F. Deloison

Ultrashort pulse laser surgery of the cornea and the sclera

The strongly localized interaction process of ultrashort laser pulses with tissue makes femtosecond lasers a powerful tool for eye surgery. These lasers are now routinely used in refractive surgery and other forms of surgery of the anterior segment of the eye. Several clinical laser systems also offer options for corneal grafting and the potential use of ultrashort pulse lasers in glaucoma surgery has been the object of several recent studies which have shown promising results. While devices aimed for interventions in clear tissue may be based on available solid state or fibre laser technology, the development of tools for surgery in more strongly scattering tissue has to account for the compromised tissular transparency and requires the development of optimized laser sources. The present paper focuses on surgery of clear and pathological cornea as well as sclera. It aims to give an overview over typical medical indications for ultrashort pulse laser surgery, the optics of the tissues involved, the available laser technology, the laser–tissue interaction process, and possible future developments.

Effect of incident light wavelength and corneal edema on light scattering and penetration: laboratory study of human corneas.

PURPOSE The outcome of ultrashort pulse laser surgery of the cornea is strongly influenced by the light scattering properties of the tissue, for which little data are available. The purpose of the present study is to provide quantitative values for light scattering and its relation to the degree of edema. METHODS An experimental optical measuring setup based on confocal geometry was used to measure the unscattered and scattered fractions of light transmitted by eye bank corneas presenting various degrees of edema. From these measurements, the effective light penetration depth in the cornea was calculated as a function of wavelength. RESULTS Corneal transparency depends on the pathological state of the cornea and on wavelength. It may be predicted as a function of corneal thickness, ie, the degree of edema. In healthy and edematous cornea, the percentage of scattered light decreases with increasing wavelength. The total penetration depths at the wavelengths of ~1050 nm (which is used in typical clinical systems) and 1650 nm (which is recommended for future devices) are comparable; however, the former is limited by scattering, which degrades the laser beam quality, whereas the latter is only limited by optical absorption, which may be compensated for. CONCLUSIONS The use of longer wavelengths should help improve the surgical outcome in ultrashort pulse laser surgery of the cornea when working on pathological tissue. A wavelength of approximately 1650 nm appears to be a good compromise, as it allows for reduced light scattering while keeping optical absorption reasonably low.

Wavelength optimization in femtosecond laser corneal surgery.

PURPOSE To evaluate the influence of wavelength on penetration depth and quality of femtosecond laser corneal incisions in view of optimizing procedures in corneal surgery assisted by ultrashort pulse lasers. METHODS We performed penetrating and lamellar incisions on eye bank corneas using several ultrashort pulse laser sources. Several wavelengths within the near-infrared and shortwave-infrared wavelength range were used and the pulse energy was varied. The corneas were subsequently analyzed using light microscopy as well as transmission and scanning electron microscopy. RESULTS We found higher penetration depths and improved incision quality when using wavelengths close to λ = 1650 nm rather than the wavelength of λ = 1030 nm typical in current clinical systems. Optical micrographs show an improvement of the penetration depth by a factor of 2 to 3 while maintaining a good incision quality when using the longer wavelength. These results were confirmed with micrographs obtained with transmission and scanning electron microscopy. CONCLUSIONS A wavelength change from the standard 1030 nm to 1650 nm in corneal surgery assisted by ultrashort pulse laser considerably reduces light scattering within the tissue. This results in a better preservation of the laser beam quality in the volume of the tissue, particularly when working at depths required for deep lamellar or penetrating keratoplasty. Using this wavelength yields improved penetration depths into the tissue; it permits use of lower energies for any given depth and thus reduces unwanted side effects as thermal effects.

Histologic and ultrastructural characterization of corneal femtosecond laser trephination.

Purpose: The purpose of this study was to evaluate the quality of femtosecond laser corneal trephination in eye bank eyes by histologic and ultrastructural investigation. Methods: We performed Z-shaped, tophat-shaped, and mushroom-shaped trephinations of swelled corneas from eye bank eyes using an Intralase FS60 system. The corneoscleral discs were fixed immediately after the laser procedure without removing the buttons. Thin and ultrathin tissue sections were examined by light and transmission electron microscopy. Results: Optical micrographs of the corneal tissue revealed that the femtosecond laser was efficient in producing Z-shaped, tophat-shaped, and mushroom-shaped dissections with reproducible high cut regularity. Investigations by transmission electron microscopy demonstrated that cut edges were of good quality devoid of thermal or mechanical damage of the adjacent tissues. However, cellular and collagenous nanometric debris was created by the laser. In the anterior stroma, they formed a layer of several microns in thickness residing on the terminated disrupted collagen fibers, whereas in the posterior stroma, they formed a thinner pseudomembrane running along the edges of the incision. Conclusions: Corneal trephination performed by the femtosecond laser preserves the ultrastructure of the disrupted collagen fibers. In edematous corneas, a layer of cellular and collagenic debris thicker in the anterior stroma and thinner in the posterior stroma runs along the edges of the incision obtained at a constant laser energy density.

Laser parameters, focusing optics, and side effects in femtosecond laser corneal surgery

Nowadays, femtosecond lasers are routinely used in refractive eye surgery. Until recently, commercialised clinical systems were exclusively based on ytterbium or neodymium-doped solid state lasers emitting sub-picosecond pulses at a wavelength of about 1 μm and repetition rates of a few 10 kHz. These systems use pulse energies in the μJ range and focussing optics of NA = 0.3 to 0.5. Recent developments have provided a variety of alternative and equally viable approaches: systems are now available using nJ pulses at high numerical apertures and MHz repetition rates - an approach so far only used for femtosecond cell surgery - and fibre laser technology is now being used for femtosecond laser corneal surgery. Recent research has also provided more insight in side effects occurring in present systems: self focusing phenomena and so far unexplained periodical structures have been observed even at high numerical apertures (NA >> 0.5) and moderate pulse energies. The interaction of femtosecond laser pulses with strongly scattering tissue has been studied in view of extending the application of femtosecond lasers to keratoplasty for opaque corneas and to glaucoma surgery. The use of new laser wavelengths and adaptive optics has been proposed. Despite the reputation of femtosecond surgical systems for their precision, repeatability and the absence of secondary effects or complications, a closer examination reveals the presence of subtle phenomena which merit further investigation. We present three of these phenomena: the influence of optical aberration on the quality of the incision, the occurrence of filamentation effects, and the deposit of microscopic glass fragments when performing penetrating incisions.

Comparison of Picosecond and Femtosecond Laser Ablation for Surface Engraving of Metals and Semiconductors

Pico and femtosecond lasers present a growing interest for industrials applications such as surface structuring [1] or thin film selective ablation [2]. Indeed, they combine the unique capacity to process any type of material (dielectrics, semiconductors, metals) with an outstanding precision and a reduced affected zone. We report on results about surface engraving of metals (Al, Cu, Mo, Ni), semiconductor (Si) and polymer (PC) using a picosecond thin disk Yb:YAG-amplifier. The pulse duration of this source can be changed using two different configurations: direct amplification of a 34ps-oscillator on one hand, and 1ps-chirped pulse amplification (CPA) scheme on the other hand. The results obtained with this thin disk laser are compared to ones achieved with two commercial femtosecond lasers respectively based on Yb-doped crystals and fibers, and operating at similar output power levels (up to 15Watt).

New compact femtosecond laser source for penetrating keratoplasty at 1.65 µm

Femtosecond laser surgery in the volume of corneal tissue is typically performed wavelengths of about 1 μm, which gives excellent results on transparent corneas. However, the outcome is much worse in the case of oedematous or pathological corneas as the laser beam propagation is disturbed by optical scattering. Our studies suggest that this phenomenon can be greatly reduced by using a better suited laser wavelength. Best results are obtained at 1.65 μm. Currently, no compact femtosecond laser at this wavelength is commercially available. We have developed a new simple, compact and stable laser source consisting of a non linear crystal pumped by a compact commercial solid-state laser emitting at 1.03 μm in a configuration of an Optical Parametric Generation (OPG). The output wavelength of this system can be tuned in the spectral range of 1.45 - 1.8 μm. A series of ex vivo penetrating incisions using energies of the order of a few microjoules on corneal tissues have been performed while varying the wavelengths from 1.45 μm to 1.7 μm. The results have been compared to experiments performed at 0.8 μm and 1 μm. The use of longer infrared wavelengths around 1.65 μm for femtosecond laser keratoplasty significantly improves the quality and the penetration depth of incision in case of pathological tissues, without inducing any additional side effects.

Femtosecond Optical Parametric Amplification using β-BaB 2 O 4 and BiB 3 O 6 consecutively

We present a femtosecond Optical Parametric Amplificator (OPA) which was conceived to perform surgical interventions on eyes using eye-safe wavelengths. It is pumped by a Ti: sapphire laser emmiting 50 fs pulses at 820 nm and at a repetition rate of 1 kHz. A particular effort was made to obtain a very flat and efficient gain spectrum especially around degeneracy. An OPA scheme using consecutively β-BaB2O4 (BBO) crystal and BiB3O6 (BiBO) provides the required behaviour (figure 1). While most OPA are composed of identical crystals on all stages, the originality of our configuration consists in combining the advantages of two different crystals.

Ultrashort-pulse laser eye surgery uses fiber technology at 1.6 microns

New, compact, and wavelength-optimized ultrashort-pulse laser systems may facilitate corneal grafting and enable new forms of glaucoma surgery.