[Real-time monitoring of a femtosecond laser in Sub-Bowman Keratomileusis on human donor eyes using OCT]. / Echtzeitsteuerung einer Femtosekundenlaser Sub-Bowman-Keratomileusis an humanen Spenderaugen mittels optischer Kohärenztomografie.

PURPOSE: In LASIK (laser in situ keratomileusis) the thickness of the corneal flap is important since it is the residual corneal bed that determines corneal stability. The introduction of real-time OCT visualisation of the corneal layers during the fs-laser cut should enable the surgeon to control and monitor the position of the plane of laser-tissue-interaction during operation. To prove that optical coherence tomography (OCT) can be useful to guide femtosecond (fs)-laser in Sub-Bowman-Keratomileusis (SBK) an in-vitro experimental study was performed on human autopsy eyes in a research laboratory set-up. METHODS: Five human autopsy eyes, unsuitable for transplantation, received fs-laser keratomileusis (flap) cuts. The laser procedure was controlled in real-time with an OCT system (Thorlabs HL AG, Lübeck, Germany) to ensure that the cut was placed just underneath Bowman's layer. As a control all eyes were dissected histologically (H & E staining) and examined under the light microscope (LM). RESULTS: Videomonitoring of the laser process supported the feasibility of the concept to online monitor the fs-laser cutting process via OCT. A clear distinction of the corneal epithelium was possible in all eyes. Bowman's membrane was not identified in all autopsy eyes at the given resolution of the OCT used in this study. Still, LM sections confirmed that the online monitoring assured a positioning of the cutting plane at minimum distance underneath Bowman's membrane. CONCLUSION: It was proven that real-time OCT monitoring of fs-laser SBK on human eyes is in principle possible.

[fs-Lentotomy: presbyopia reversal by generating gliding planes inside the crystalline lens]. / Lentotomie mittels fs-Laserpulsen: Behandlung der Presbyopie durch Erzeugen von Gleitebenen in der Linse.

Based on the Helmholtz theory for accommodation, increasing sclerosis of the lens nucleus and cortex is the main cause for the development of presbyopia. Existing therapies, however, do not reverse the stiffness of the crystalline lens and thus do not regain real accommodation ability. A new approach to restore the flexibility of the lens has been realised by utilising the non-linear interaction of ultrafast laser pulses with transparent tissue, the so-called photodisruption. This process has been used to create micro-incisions which act as gliding planes inside the crystalline lens without opening the eye globe. This treatment method, known as fs-lentotomy, enables regeneration of real dynamic accommodation. For the first time, 3D structures for gliding planes were successfully generated in experiments with human donor lenses of different ages. An average increase in anterior-posterior lens thickness of 100 mum accompanied by a decrease of equatorial lens diameter was observed as a direct consequence of fs-lentotomy. This is attributed to the increased flexibility, as the force of the capsule bag moulds the lens tissue more spherically. Moreover, in vivo experiments on rabbit eye lenses did not induce an increasing opacification (cataract) over a six-month follow-up period. However, the incisions were still detectable using Scheimpflug imaging and histopathological techniques, although the visibility of the incisions was declining. Furthermore, no side effects were observed during the wound healing process and during a six-months follow-up period. Based on these findings fs-lentotomy might have the potential to become a procedure for the reversal of presbyopia.

[In vitro and in vivo investigations on the treatment of presbyopia using femtosecond lasers]. / In-vitro- und In-vivo-Untersuchungen zur Presbyopiebehandlung mit Femtosekundenlasern.

BACKGROUND: Ultrashort (femtosecond) laser pulses can generate precise cuts in biological tissue without damaging the surface. The application of femtosecond laser technology at the lens was evaluated with respect to a possible treatment of presbyopia. MATERIALS AND METHODS: Femtosecond laser lentotomy was performed on 150 pig lenses in vitro. Cutting geometry and laser settings were optimized to generate smooth cuts with a minimum of produced gas bubbles. Four rabbit lenses were treated afterwards in vivo and were controlled for 3 months post-treatment. The lenses were then extracted and evaluated. RESULTS: With suitable laser settings, light scattering due to residual gas bubbles could be almost completely avoided in pig lenses. A pulse energy of less than 1.2 microJ and a cutting geometry with spot separations of more than 5 microm are important. The rabbit lenses stayed macroscopically clear for 3 months in vivo. Only the cell structures directly adjacent to the laser focus were cut; structures 5-10 microm away appeared to be intact. No cataract formation occurred during this time. CONCLUSION: Femtosecond laser application allows precise and smooth cuts inside pig and rabbit lenses without damage to adjacent tissue.

Pulsed photothermal radiometry as a method for investigating blood vessel-like structures.

Pulsed photothermal radiometry (PPTR) is known to be suitable for in vivo investigations of tissue optical properties. As a noncontact, nondestructive method it is a very attractive candidate for on-line dosimetry of laser treatments that rely on thermal laser-tissue interaction. In this article, we extend the one-dimensional (1D) analytical formalism that has widely been used to describe PPTR signals to a two-dimensional treatment of a simplified model of a blood vessel. This approach leads to quantitative description of a PPTR signal that, unlike in an 1D treatment, not only shows changes in time, but also varies in space. Using this approach, we are able to gain instructive understanding on how target characteristics of a blood vessel-like structure influence such a spatiotemporal PPTR signal. Likewise, the ability of extracting target features from those measurements is evaluated. Subsequently, we present experimental realization of the idealized model of a blood vessel as used in our theory. Comparison of actual PPTR measurements with theoretical predictions allow vessel localization laterally and in depth. Using our setup, we furthermore demonstrate the influence of flow inside the vessel on the measured signal.

[Optoacoustic tissue alterations for optimizing laser cyclophotocoagulation. Transscleral detection of laser-induced optoacoustic pressure transients]. / Optoakustische Gewebsdifferenzierung zur Optimierung von Laserzyklophotokoagulation. Transsklerale Detektion laserinduzierter optoakustischer Drucktransienten.

BACKGROUND: Considerable problems occur in transscleral laser cyclophotocoagulation concerning energy dosage. We investigated the feasibility of localizing the ciliary body by the detection of thermoelastic pressure transients and of supervising on-line the degree of tissue damage during treatment. METHOD: We used a specially designed handpiece to apply short pulsed laser radiation with low energy levels to enucleated bulbs of rabbits. With an adjusted pressure transducer we examined acoustical transients generated in the area of absorption of the ciliary muscle or the pigmented epithelial layer and measured axial resolution of the method at various distances to the corneoscleral limbus. RESULTS: We detected acoustic transients that allowed rough localization of the target area. A marked change in signal was recorded with increasing level of ciliary destruction. CONCLUSION: This procedure can serve as an essential tool in the on-line supervision of the coagulation process. The laser parameters can thus be adjusted optimally to the progress of the treatment.