Stone-retrieval Basket with Integrated Optical Fiber for Laser Lithotripsy
A German manufacturer of stone-retrieval baskets works with a U.S. manufacturer of specialty optical fiber. The result is a basket that, thanks to its coaxially integrated optical fiber, can simplify and shorten minimally invasive urological surgery.
The treatment of kidney stones has changed dramatically over the years. Instead of open surgery, today minimally invasive endoscopic-based procedures can be used. Once a stone is found, it can usually be removed using a nitinol basket. If the stone is too far up the urinary tract, fragmentation using laser energy is used to pulverize it. Pulverization is achieved by the introduction of an optical fiber to deliver the laser energy. This procedure is called intracorporeal lithotripsy.
Pulverization using laser energy may vary. Combining a long pulse duration with low pulse energy and high pulse frequency will blast the stone into dust. The small dust particles are eliminated, naturally. But the high pulse energy will cause the ambient temperature to rise and may cause damage to surrounding tissue. An alternative to pulverization is fragmentation. Fragmentation uses laser energy with a short pulse duration, high pulse energy and low pulse frequency. The resulting fragments can then be captured using a stone-retrieval basket.
Usually the stone is fragmented prior to the pieces being captured by the basket. But sometimes, depending on the location of the stone, the reverse order is necessary. In these cases, where the stone is captured and then fragmented, there is the risk of the laser energy damaging the stone-retrieval basket as well as the surrounding tissue.
The next logical development in intracorporeal lithotripsy is an instrument that coaxially integrates optical fiber with the stone-retrieval basket. This improved instrument enables positioning of the basked and the optical fiber at the same time. The stone is safely trapped and fragmented without damaging surrounding tissue or the basket. Surgery time is shortened since only one instrument is needed.
This new device was developed by Endosmart GmbH in Stutensee, Germany together with OFS, a U.S. designer and manufacturer of specialty optical fiber.
A typical laser system for lithotripsy is based on Ho:YAG (Holmium:Yttrium-Aluminum-Garnet) laser which uses at a wavelength of 2123 nm with an average power of 30 W. Pulse duration, peak power and frequency are adjusted according to the individual treatment. For example, the laser pulse could be up to 18 kW peak power or 3.5 J pulse energy. To enable orientation of the instrument, the system delivers a visible red or green pilot light.
Light is guided even under extreme bending
The step-index multimode optical fiber used to guide the laser can have a pure silica core and a fluorine-doped glass cladding or a Germanium-doped core with a pure silica cladding. The different refractive indices of core and cladding enable the laser to propagate longitudinally in the fiber core. For guiding the light under extreme bending, an additional UV cured fluoroacrylate coating is applied. The fluoroacrylate coating has a lower refractive index than either of the glass claddings and acts as a secondary cladding for guiding the light. The optical fiber that is used with the nitinol basket described above has a core diameter of 272 µm and a silica cladding diameter of 299 µm. Around that, a 330 µm UV cured fluoropolymer coating is applied acting as a second optical cladding and finally, an ETFE buffer of 400 µm is applied.
Glass fibers are also used for medical diagnostics. Current developments are focused on simultaneous diagnosis and treatment.