Laser light has become an essential part of medicine. It is successfully used to precisely process tissue and make hidden structures and processes visible. The precision and speed of a computer-controlled laser could never be achieved with human hands and instruments guided by them. In addition, laser light can work in places that cannot be reached with surgical instruments or only with major collateral damage, for example inside the eye.
The reason therefor is significantly related to the properties of laser light, which is always artificially generated light. This already indicates that THE LASER does not exist. A large number of energy sources and laser media, light colors, wavelengths and pulse durations distinguish lasers from each other and, due to their peculiarities, enable very different applications.
Compared to light from other light sources, laser light has properties that make it really interesting for medical (and other) applications. These properties make it possible to use laser light or the energy of light to destroy tissue structures so precisely that a fine cut can be made, even in deeper tissue layers, without having to impair the tissue structures above them.
The light of a common laser is monofrequency, i.e. it has only one wavelength/ light colour. The different light colours perceptible and not perceptible to the human eye (e.g.
infrared, ultraviolet, etc.) have different effects on the tissue coming into contact with them or penetrate into the tissue at different depths. With this knowledge, the light color/ wavelength
of the laser can now be selected in such a way that the desired effect is achieved during application.
Laser light can be focused very well. This allows a very high power intensity to be achieved on a small area (in focus). If the laser emits its light in continuous operation, modern laser devices generate a light output in the order of 100,000 W. If the laser beam is not emitted permanently but in pulses, peak powers of 1010 W are even achieved, which corresponds to a power of 100 million bulbs of 100 W each.
When light hits an obstacle - in our case human tissue - four different processes can occur, which can be described in simple terms. During reflection, the photons bounce off the tissue and continue their path outside the tissue in a different direction. If the photons can penetrate the tissue unhindered, we speak of transmission.
The processes relevant to the medical-surgical application of light are scattering and absorption. If the light penetrates the tissue, it can be scattered or refracted by various substances, i.e. it is deflected from its path within the tissue and then continues its path in another direction. For medical applications, this deflection must be considered in order to be able to bring the laser beam to where the absorption process is to take place. Here the tissue absorbs the energy of the light and converts it into heat. This leads to the destruction of the tissue - an incision is made.
Which reaction takes place at which point depends on the wavelength of the light and the nature of the tissue. The more precisely a laser system is adapted to a specific application, the more successful and precise the intervention can be.
Femtosecond lasers emit their light in unimaginably short pulses. A femtosecond (fs) is the millionth part of a billionth of a second (1 E -15 s). These ultra-short light pulses bring new usable properties to laser beams and open up new possibilities for microsurgery.
Femtolasers generate top performance. Within the extremely short period of time in which the laser emits its light beam, this light contains only little energy, but produces an
incredibly high power (comprehensible by the simple correlation power=energy/time). A single pulse of 100 femtoseconds in length, for example, contains just one millionth of a joules of energy,
but produces a peak power of 10 megawatts.
Short duration brings precision. If these pulses are focused on a few micrometers spot size, they ionize any form of tissue. The molecules, which are broken down into their components, are absorbed by the surrounding tissue. Due to the very low energy content of individual pulses, heat transfer and thus damage to the surrounding tissue doesn't matter.
Femtolasers are always on the right wavelength. When femtolasers are used, the color of the laser light - in contrast to conventional lasers - practically doesn't matter in
the tissue interaction process. In the eye, therefore, a wavelength is selected that lies in the near infrared range. On the one hand, the patient does not see it and is not irritated by it. On
the other hand, this area is transparent to the eye, so there is no absorption. The beam's focus, which is only a few micrometers in size, has an effect alone. The extremely strong
electromagnetic field of the laser pulse ionizes everything in its immediate vicinity (by the way, even diamond cannot withstand the fields). Thus a punctiform cutting process takes place. Many
millions of points in a row result in the desired cut.
Femtolaser systems are already well researched. After almost four decades of intensive work with femtosecond lasers, we now know what the optimal wavelength and almost optimal
pulse duration for microsurgical procedures must look like and can equip the systems accordingly. There is still room for improvement in terms of pulse energy. Fortunately, pulse energy can be
reduced not only by pulse duration, but also by stronger focusing. The trend is moving in this direction.
Although Femto laser systems are becoming cheaper and more compact, they are still a significant investment. First and foremost, all devices, whether already in use or still in development, are still highly complex systems that must be fine-tuned to the specific application in order to achieve an exact result. Multiple use for different areas of application is currently only possible to a very limited extent and is also fraught with pitfalls. High development, manufacturing and maintenance costs and a considerable space requirement only allow highly specialized clinics and practices to use such devices. In addition, despite successes in practice, some applications are still not recognised by the health insurance funds and must be privately financed by the patient.
Femtosecond laser systems are currently on the threshold of optimal usability for various applications. Some innovative manufacturers are already very far away, others less so. Even though these applications are still limited to refractive and therapeutic use on the cornea and crystalline lenses, numerous other applications are knocking on the door.
ROWIAK is working on a procedure to reverse presbyopia. Femtosecond laser pulses are used to create sliding planes in the aged lens, making it flexible again and increasing the amplitude of accommodation. Clinical studies are already in progress.
But surgical femto applications are also conceivable deeper in the back of the eye. Floaters can be resolved much more precisely and with fewer side effects. A posterior vitrectomy for the treatment of tractional vitreous attachments is also possible. One challenge, the distortion of the laser focus by optical aberrations of the vitreous body, can be overcome with so-called adaptive optics. The Laserzentrum Hannover (LZH) is working in this direction.
The fascinating thing about femtolaser pulses is the possibility not only to cut, but also to induce photochemical effects below the interference threshold. The group around Wayne Knox (Univ. of Rochester) has shown that the refractive index of corneal and eye lens tissue changes permanently when irradiated with fs pulses of a certain wavelength and intensity. It was possible to correct ametropia very elegantly without cutting or removing tissue. The same principle can be applied to already implanted intraocular lenses to correct or refine their refraction (light refraction). Companies such as Perfect Lens, Clerio Vision (both USA), Medicem (Czech Republic) and LicriEye/Merck (Germany) are working intensively on it.
But laser-induced chemistry does not only have the potential to change the refractive index. A group led by Chao Wang (Columbia Univ. NY, USA) showed how the collagen fibers of the cornea can be cross-linked with fs pulses through the formation of a low-density plasma, thus enhancing the mechanical properties (corneal cross-linking). At Glostrup Hospital (University of Copenhagen, Denmark), Line Kessel showed that the age-related yellowing of the human lens can be bleached by femtosecond laser photolysis. In this procedure, lens ageing can be delayed.
Many more application possibilities for femtolaser-based microsurgery are still waiting to be explored. We are working on it and look forward to the future.
apl. Prof. Dr. Holger Lubatschowski
Phone: +49 511 277 2954
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