Caroline Crotti

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.

Tiled-grating compression of multiterawatt laser pulses

High-energy petawatt lasers require large diffraction gratings for pulse compression. As an alternative to meter-sized gratings, we demonstrate the capability of a tiled-grating system to compress multiterawatt subpicosecond laser pulses. Using a 100 TW-class Nd:glass chirped-pulse amplification laser facility, we report on the performance of a two-grating mosaic to compress high-energy pulses to 2.5 J, 450 fs (5.5 TW) in air with a beam size of 50 mm and energy transmission of 63%. Stability of the grating mosaic alignment was realized by use of an accurate nanopositioning optomechanical system. The output Gaussian spectrum was preserved from grating-gap spectral clipping and was free of modulation.

Stromal striae: a new insight into corneal physiology and mechanics

We uncover the significance of a previously unappreciated structural feature in corneal stroma, important to its biomechanics. Vogt striae are a known clinical indicator of keratoconus, and consist of dark, vertical lines crossing the corneal depth. However we detected stromal striae in most corneas, not only keratoconus. We observed striae with multiple imaging modalities in 82% of 118 human corneas, with pathology-specific differences. Striae generally depart from anchor points at Descemet’s membrane in the posterior stroma obliquely in a V-shape, whereas in keratoconus, striae depart vertically from posterior toward anterior stroma. Optical coherence tomography shear wave elastography showed discontinuity of rigidity, and second harmonic generation and scanning electron microscopies showed undulation of lamellae at striae locations. Striae visibility decreased beyond physiological pressure and increased beyond physiological hydration. Immunohistology revealed striae to predominantly contain collagen VI, lumican and keratocan. The role of these regions of collagen VI linking sets of lamellae may be to absorb increases in intraocular pressure and external shocks.

Guiding glaucoma laser surgery using Fourier-domain optical coherence tomography at 1.3 μm

Glaucoma is a disease of the optic nerve that is usually associated with an increased internal pressure of the eye and can lead to a decreased vision and eventually blindness. It is the second leading cause of blindness worldwide with more than 80 million people affected and approximately 6 million blind. The standard clinical treatment for glaucoma, after unsuccessful administration of eyedrops and other treatments, is performing incisional surgery. However, due to post-surgical complications like scarring and wound healing, this conventional method has a global success rate of only about 60%. In comparison, as femtosecond laser surgery may be performed in volume and is a priori less invasive and less susceptible of causing scarring, glaucoma laser surgery could be a novel technique to supplement the conventional glaucoma surgery. We have been working on the development of a new tool for glaucoma treatment that uses an optimized femtosecond laser source centered at 1.65 μm wavelength for making the surgery and an imaging system based on optical coherence tomography (OCT) for guiding the laser surgery. In this proceeding, we present the results obtained so far on the development and utilization of Fourier-domain OCT imaging system working at 1.3 μm center wavelength for guiding the laser incision. Cross-sectional OCT image of pathological human cornea showing the Schlemms canal, where the surgery is intended to be done, is presented. By coupling OCT imaging system with the laser incision system, we also demonstrate real-time imaging of femtosecond laser incision of cornea.

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.

Study of light scattering and transparency in human edematous corneas and application to corneal grafts

The optical properties of the cornea have been a research subject of great interest for many years. Several early theories have been put forward to explain with more or less success the optical transparency of this tissue, but it was not until Maurice demonstrated in a very elegant way during the 50s that this optical transparency could be explained by the regular ultrastructure of the cornea. When becoming edematous, the corneas ultrastructure is perturbed and the tissue becomes a strongly scattering medium. With the emergence of ophthalmologic surgery by ultrashort pulse lasers in recent years, a regain of interest in the subject of corneal transparency arose. However, relatively little and no recent data of transparency spectra measurements covering a large wavelength range is available in the literature. The purpose of this study is to provide quantitative values for light scattering and its relation to the degree of edema by measuring the spectrum of transmitted light through corneas presenting different degrees of edema. This paper focus on the comparison of laboratory measurements published earlier with a new simple method we propose We also for eye banks to quantitatively measure the degree of transparency of corneal grafts by measuring the modulation transfer function of a Siemens star viewed through a corneal graft. Indeed, there is no current method to determine the transparency of corneal graft but the subjectivity of the laboratory technician or the ophthalmic surgeon.

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.