Nowadays UV-cross-linking is an established method for the treatment of keraectasia. Currently a standardized
protocol is used for the cross-linking treatment. We will now present a theoretical model which predicts the
number of induced crosslinks in the corneal tissue, in dependence of the Riboflavin concentration, the radiation
intensity, the pre-treatment time and the treatment time. The model is developed by merging the difussion
equation, the equation for the light distribution in dependence on the absorbers in the tissue and a rate equation
for the polymerization process. A higher concentration of Riboflavin solution as well as a higher irradiation
intensity will increase the number of induced crosslinks. However, performed stress-strain experiments which
support the model showed that higher Riboflavin concentrations (> 0.125%) do not result in a further increase in
stability of the corneal tissue. This is caused by the inhomogeneous distribution of induced crosslinks throughout
the cornea due to the uneven absorption of the UV-light. The new model offers the possibility to optimize the
treatment individually for every patient depending on their corneal thickness in terms of efficiency, saftey and
We present a high-speed photographic analysis of the interaction of cavitation bubbles generated in two spatially separated regions by femtosecond laser-induced optical breakdown in water. Depending on the relative energies of the femtosecond laser pulses and their spatial separation, different kinds of interactions, such as a flattening and deformation of the bubbles, asymmetric water flows, and jet formation were observed. The results presented have a strong impact on understanding and optimizing the cutting effect of modern femtosecond lasers with high repetition rates (>1 MHz).
The aim was to evaluate a method for visualizing fs laser pulse induced microincisions inside crystalline lens tissue. Porcine lenses were modified ex vivo by fs laser pulses to create defined planes at which lens fibers separate. Lens fiber orientation and fs laser-induced micro-incisions were examined using a confocal laser scanning microscope. Micro-incision visualization revealed different cutting effects depending on fs laser pulse energy, ranging from altered tissue scattering properties with all fibers intact to definite fiber separation with a wide gap. CLSM permits visualization and analysis and thereby control of fs laser pulse induced microincisions inside crystalline lens tissue.
A prominent laser based treatment in ophthalmology is the LASIK procedure which nowadays includes a cutting of the
corneal tissue based on ultra short pulses. Focusing an ultra short laser pulse below the surface of biological tissue an
optical breakdown is caused and hence a dissection is obtained. The laser energy of the laser pulses is absorbed by nonlinear
processes. As a result a cavitation bubble expands and ruptures the tissue. Hence positioning of several optical
breakdowns side by side generates an incision. Due to a reduction of the duration of the treatment the current
development of ultra short laser systems points to higher repetition rates in the range of hundreds of KHz or even MHz
instead of tens of kHz. This in turn results in a probable occurrence of interaction between different optical breakdowns
and respectively cavitation bubbles of adjacent optical breakdowns. While the interaction of one single laser pulse with
biological tissue is analyzed reasonably well experimentally and theoretically, the interaction of several spatial and
temporal following pulses is scarcely determined yet. Thus the aim of this study is to analyse the dynamic and interaction
of two cavitation bubbles by using high speed photography. The applied laser pulse energy, the energy ratio and the spot
distance between different cavitation bubbles were varied. Depending on a change of these parameters different kinds of
interactions such as a flattening and deformation of bubble shape or jet formation are observed. Based on these results a
further research seems to be inevitable to comprehend and optimize the cutting effect of ultra short pulse laser systems
with high (> 1 MHz) repetition rates.
Presbyopia is an age related effect which affects every human at the age of about 40 years. So far reading glasses
are the conventional treatment. According to Helmholtz' theory of accommodation one of the mayor reasons for
the development of presbyopia is the increasing sclerosis of the lens. In contrast to that the ciliary muscle and
the lens capsule remain mostly active and elastic the whole life. So a possible treatment could be the increase of
the flexibility of the lens by creating gliding planes with fs-laser pulses inside the lens tissue.
In former studies it was shown that fs-laser pulses were able to increase the flexibility of ex vivo porcine lenses
as well as ex vivo human donor lenses. Our current aim was to evaluate the effect of the fs-laser pulses on the
crystalline lens of living rabbit eyes due to the fs-lentotomy treatment. The main focus of the evaluation was
the exclusion of possible side effects of the treatment like cataract formation or retina damage. The treated eyes
were monitored using optical coherence tomography (OCT) and Scheimpflug imaging for localizing and studying
the tissue effects of the incisions. Furthermore histological sections of the lens and retina were prepared. The
rabbits were investigated pre operatively and up to six months post operatively.
The fs-laser induced micro incisions were successfully applied to the left lens of each rabbit. The micro
incisions within the crystalline lens were detectable with OCT and Scheimpflug imaging up to six month. The
imaging within the lens showed a progressive fading of the incisional opacities generated by the femtosecond
laser during the six months and no indication of cataract formation was found. OCT and Scheimpflug images
emphasize themselves as necessary tools to monitor the micro incisions over time. Histopathological sections of
the lens tissue support the findings of the non invasive imaging techniques. Also the histopathological sections
of the retina show no thermal induced change due to the irradiation of the fs-pulses.
Up to now reading glasses are the conventional treatment of presbyopia, an age related effect for every human.
According to the Helmholtz theory the reason for the development of accommodative loss is a decreasing elasticity
of the lens due to the increasing sclerosis. Since the ciliary muscle and the lens capsule remain active and elastic
the whole life, a possible treatment could be the increase of the flexibility by creating gliding planes with fs-laser
flexibility of ex vivo porcine as well
as human donor lenses with a laboratory laser system. We will present new results with a compact 100 kHz
repetition rate turn key laser system which speeds up the treatment time by a factor of 10. This will offer the
opportunity for future clinical trials. Furthermore first in-vivo results on rabbits are presented.
According to Helmholtz' theory of accommodation one of the mayor reasons for the development of presbyopia is
the increasing sclerosis of the lens. One concept to overcome this hardening of the lens is to regain its flexibility
by inducing gliding planes inside the lens. Femtosecond laser pulses are a suitable tool for this treatment.
Showing in former work that we could increase the flexibility of enucleated porcine (ex vivo) lenses up to 25%,
we focused our recent work on human autopsy lenses. The age of the human donors ranged between 20 and
70 years. For an evaluation of the gain in flexibility the lens' thickness was measured undertaking the Fisher's
spinning test before and after laser treatment. Depending on the age and the quality of applied cutting pattern
the lens thickness increased after treatment up to 0.4 mm leading to an theoretical increase of several dioptres
of optical power. The flexibility could be increased up to 70 % compared to the measurements before treatment.
Since the age of the human donors had a broad range, leading to different degrees of lens hardening, the variance
of the measured flexibility changes was up to 30%. An addition the influence of the laser treatment onto the lens
on the accommodation amplitude will be shown in a three dimensional finite-element simulation.
Presbyopia is one age related effect every human is suffering beginning at the age of about 45 years. Reading
glasses are the conventional treatment so far. According to the Helmholtz theory the loss of accommodation in
age is due to the hardening and the resulting loss of elasticity of the crystalline lens. However the ciliary muscle
and the lens capsule stay active, respectively. Therefore a possible treatment concept is to regain the flexibility
by inducing gliding planes in form of microcuts inside the lens. The increase of flexibility in young porcine lenses
by different cutting patterns was shown by Ripken et al.1, 2 who verified the increase in flexibility by the spinning
test introduced by Fisher.3
We will present our first measurements of flexibility increase of human donor lenses. Furthermore the influence
of the laser cuts into the lens on the accommodation amplitude will be shown in a three dimensional finite-element simulation.
Rapid development of new laser technologies enabled the application of ultra short lasers in refractive surgery. Focused ultra short laser pulses in near-infrared spectral range can generate a laser induced breakdown (LIB) in the cornea, which will disrupt the tissue. Cutting depth and position can be established by varying the laser focus. The fs-LASIK technique allows both flap and lenticule to be formed by using fs-pulses without the presence of any mechanical impact. During the cutting process not all of the pulse energy is deposited into the cornea; approximately half of the remaining energy propagates through the eye and reaches the retina. Though defocused, the transmitted energy can still induce damage to the retina due to absorption by the retinal pigment epithelium and the transfer of thermal energy to surrounding tissue. The fs-LASIK process was simulated with two laser systems; one continous-wave and one in the fs-regime. For the simulation the exposure time and focusing numerical aperature which defines the retinal spot size were varied. The Damage thresholds of the laser beam exposed eyes were determined in terms of ophthalmoscopic and histopathologic observations.
Ultrashort laser pulses are increasingly used in refractive eye
surgery to cut inside transparent corneal tissue. This is
exploited by the fs-LASIK procedure which affords the opportunity
to correct ametropia without any mechanical effects. The cutting
process is caused by the optical breakdown occurring in the laser
focus. During this process only a certain amount of the pulse
energy is deposited into the tissue. The remaining pulse energy
propagates further through the eye and interacts with the retina
and the strong absorbing tissue layers behind. Therefore this
investigation shall clarify if the intensity of the remaining
laser pulse and the resulting temperature field can damage the
retina and the surrounding tissue. Threshold values of the retinal
tissue and theoretical calculations of the temperature field will