Optical Fibers in Medical Technology
Holmium and Thulium Lasers Make Keyhole Surgery Possible
PD Dr. Ronald Sroka is head of the Laser Research Laboratory at the University of Munich’s Großhadern Hospital.
Since the introduction of lasers into medicine and the development of fiber-optic technologies, new medical application fields have opened up in both diagnostics and therapy. These range from invasive and non-invasive treatments to endoscopic surgery to imaging diagnostics in so-called keyhole technologies. While low optical power is usually fed through optical fibers for diagnostic purposes, surgical procedures generally require the transmission of high power up to 200 W in cw operation. The medical certification of optical fibers represents a major challenge to clinical use.
Typical surgical laser applications include endoscopic vaporization, enucleation of the benign prostate, and fragmentation of kidney stones in urology; endoscopic destruction and removal of tumorous tissue in the bronchial branches in pulmonology; the endoluminal sclerosis of varicose veins; and tissue ablation in the nose and throat.
Endoscopes equipped with optical fibers are used for these types of operations: for the purpose of transmitting light into the hollow organ to see inside the organ, on the one hand, and transmitting image data using ordered fiber bundles from the hollow organ to the ocular and thus to the viewer, on the other hand.
Requirements for Optical Fibers in Medical Technology
In surgical procedures, optical fibers are used that are optimized for the transmission of high optical power in wavelengths from 500 nm to 2500 nm. Both pulsed and cw radiation are transmitted here. The optical fiber is fed through the working channel of an endoscope into an organ in order to transmit laser energy to tissue under visual control. In flexible endoscopes, it is important to make sure that the optical fiber only slightly affects the flexibility and bending capacity of the endoscope. For this reason, optical fibers with a small core diameter (200-400 µm) are preferred over more rigid fibers (600-800 µm). The outer diameter should not exceed 1000 µm. This allows extraction and rinsing to be carried out additionally through the occupied working channel. Based on the knowledge gained from the interaction between light and tissue, tissue effects that depend on the power density applied, and thus treatment effects, can be induced.
Laser Radiation at 2 µm for State-of-the-Art Surgery
The latest surgical procedures use laser light at a wavelength of approximately 2 µm. Destruction of kidney stones (lithotripsy) in urology is the domain of pulsed Ho:YAG laser radiation. The continuous and pulsed radiation of thulium lasers is used in the destruction of soft tissue to free the airways in the bronchial branches.
Destroying Kidney Stones with Laser Light
The introduction of Ho:YAG laser technology has revolutionized the treatment spectrum in urological stone destruction and is well established today [1,2]. In combination with the developments made in urological endoscopic instruments, this minimally-invasive method has replaced open surgery for such treatments.
Ho:YAG Laser Systems with Optical Fibers are Preferred
Ho:YAG laser systems with optical fibers from 356 µm to 600 µm are used in semi-rigid endoscopes; whereas, in flexible endoscopes, optical fibers with a core diameter of 220 µm are used to guarantee flexibility and rinsing during treatment. The complication rate in stone destruction supported by Ho:YAG lasers is low. Because all stone types can be fragmented via Ho:YAG laser radiation, Ho:YAG laser lithotripsy has become the preferred method of treatment, even over other methods such as via ultrasound, pneumatic destruction, and other pulsed laser types [3].
This technical advancement makes it possible today to carry out endoscopic stone treatments via Ho:YAG laser radiation into the renal caliceal group [2,4,5,6,7] (see Fig. 1: Optical fiber with stone).
Stone Destruction Mechanism
The mechanism of stone destruction is based in particular on the high absorption of the light of this wavelength in water. On the one hand, a cavitation bubble is created directly in front of the optical fiber: This clears the way for laser radiation to reach the stone. In addition, when the bubble collapses, it produces a pressure wave that can lead to the disruption of the stone.
The Ho:YAG laser radiation that advances to the stone is absorbed by the water in the stone, expands, and results in thermal fragmentation. As you can see in Figure 2, these mechanisms accompany the production of small fragments, which are flushed out of the urogenital tract by the flushing liquid or by urine itself [8,9,10].
Laser Technology in Respiratory Medicine
In pulmonology, Nd:YAG lasers (1064 nm) were primarily used in interventional endoscopic surgery [11,12,13,14].
Tissue Ablation
The Nd:YAG wavelength penetrates the tissue relatively deeply but is absorbed to a large extent by dark structures (e.g., blood and carbonization). This often leads to rapidly expanding steam pockets in the tissue that can tear in an uncontrolled manner and impair vision in the area of operation.
In contrast, the 2 µm radiation from thulium lasers in tissue water is absorbed, (relatively speaking) irrespective of the optical color, and thus has a low optical depth of penetration. This facilitates superficial, precise, and predictable tissue ablation that can be controlled by the user. [15,16,17,18]
Results of Study of Invasive Procedures in the Lungs with 2 µm Laser Light
The initial results of and first-hand experience reports from the study of interventional pulmonology using thulium lasers are available. The laser light of the wavelength 1940 nm was fed through the working channel of the flexible bronchoscope to the treatment site via a flexible optical fiber that has a core diameter of 365 µm. Due to the large amount of water absorption, very defined and precise laser effects in the form of coagulation and ablation in the tissue were achieved. The coagulation depths were 1-2 mm and, relatively speaking, independent of the power radiated, whereas Nd:YAG treatments produced uncontrolled and deep coagulation areas.
Clinical experiences showed that small superficial lesions were able to be completely removed. Deep coagulations were produced by penetrating the lumen-restricting tissue to be treated with the optical fiber end and were only located in the direct vicinity of the optical fiber end. The tissue that was coagulated (clotted) in this manner was able to be mechanically removed simply and without blood afterwards.
In the case of strictures, the laser incisions were able to be made in the growing tissue in a targeted manner and without drawing blood; this was carried out by repeatedly moving the optical fiber end back and forth across the cut site during laser emission. As seen in Figure 3, even tissue that grew into the lumen through the grid of the implanted stents was able to be removed without significant damage to the stent material. Thus, ingrown stents were able to be saved without any tissue damage.
These novel operation techniques are carried out under general anesthesia to additionally allow artificial respiration and optimal extraction. The use of flexible bronchoscopes makes it possible to precisely guide the optical fiber into the operational field [19].
Finally, it should be pointed out that laser treatments performed by specialized and trained medical personnel can only lead to the desired outcome if the know-how and experience of the user, including the safe and careful operative management regarding laser safety, are combined [20,21,22].
Treatments with 2-µm lasers in the surgical field have proven to be more promising and advantageous than conventional technologies. These types of minimally invasive treatment strategies can only be carried out because of the ongoing technological efforts concerning optimization in fiber technology taking into account biocompatibility, flexibility, and unbreakability.
The close, cross-disciplinary collaboration among research institutes, industrial partners, and medical users makes high-precision laser radiation applications in the operating room possible for improved patient treatment and betterment of the overall condition of the patient.
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PD Dr. Ronald Sroka, Laser Research Laboratory, LIFE Center, Großhadern Hospital
After obtaining his physics degree, PD Dr. Ronald Sroka became active in the research and development of fluorescence diagnostics, photodynamic therapy (PDT), and laser surgery in almost all medical disciplines. As head of the research group for clinical laser applications, he is responsible for the integration of novel laser treatments in everyday clinical life. This includes, for example, laser lithotripsy in urology or the PDT of prostate cancer. R. Sroka has headed LMU’s laser research laboratory at Großhadern Hospital since 2010.
