Laser lithotripsy, as described in section 1.1, is the fragmentation of calculi using a laser beam, and the technique is used in the removal of urinary calculi and also billiary calculi in the gall bladder (e.g. Langhorst and Neuhaus, 2000; Rosin et al, 2000). The surgeon locates the calculus using a flexible endoscope attached to a viewing monitor and advances the tip of the optical fibre to touch the stone surface, or with a small separation. The laser energy is then imparted to the surface of the calculus to generate an action that will lead to the bulk disintegration of the calculus. Fragmentation is performed until either the calculus is dis-impacted, or until breakdown is complete, so that the fragments can be extracted using a wire basket, or left in situ to be passed naturally. The procedure is carried out under local anaesthesia. A schematic of the arrangement of the apparatus during the procedure is shown in Figure 1.3.
The advantage of using a laser is that the beam has an effect only in the region where it is absorbed. In some situations, such as blockage of the ureter caused by an impacted calculus, precise targeting afforded by a laser beam is necessary to avoid surrounding tissues. Compared to ultrasound and electrohydraulic techniques, laser lithotripsy has several important advantages. In particular, the small diameter of the flexible laser fibre within the optical scope overcomes difficulties encountered with rigid ultrasonic probes
and larger diameter electrohydraulic systems. Improved tissue safety has been reported to be another advantage with the laser (Coptcoat, 1987), minimising the risk of ureteral perforation compared to electrohydraulic lithotripsy. The practitioner has direct visualisation of the calculus, thus improving control and reducing the risk of accidental injury. Additionally, laser lithotripsy requires less energy to attain a similar fragmentation effect, generates less heat, and causes less calculus retro-propulsion (a ballistic effect seen when powerful shockwaves cause stones to recoil).
Flexible Ureteroscope
Ureter
Bladder
Collapsable Wire Basket Laser Fibre
Flexible Ureteroscope Kidney
Laser Fibre
Figure 1.3: Images showing the basic arrangement of apparatus for the delivery of laser energy to the calculus during laser lithotripsy procedure. (Images taken with permission;
Top: kidneystoneindia.com 01.07.09; Bottom: sosromandie.ch 01.07.09).
The principle of laser destruction of calculi is by direct effect of laser radiation, or by shockwaves produced when the laser is absorbed in a thin layer of liquid near the fibre end-face. Although a full description is complex and not yet fully understood, in general, when the fibre touches the stone direct ablation is dominant and a slight pressing of the fibre against the stone results in a drilling action. If the distance between the fibre end-face and the stone surface is greater than the thickness of the water layer that can be penetrated by the laser radiation, the laser energy is absorbed in the water and the destruction runs due to the shock waves. For certain values of the distance the two mechanisms are simultaneous and a combined action is possible.
The action of laser radiation on calculi depends on several laser light properties, i.e.
wavelength, duration of exposure, delivered energy, and intensity (rate of delivery of energy over the focussed area of the target). The effect of laser wavelength is very important as it determines the amount of absorption by the calculus and intervening liquid, and therefore the following energy transfer/ dissipation dynamics. However, the wavelength of the laser light must be from the region of the spectrum where flexible fibres having good transmission are available.
The duration of the laser pulse or the exposure time (in the case of CW lasers) plays a vital role in the energy balance dynamics of optical interaction with calculi. Depending on the extent of duration three identifiable different processes take place. These are categorised as:
• Continuous wave (CW) lithotripsy
• Long pulse lithotripsy (pulse duration: μs to ms)
• Short pulse lithotripsy (pulse duration: ns)
Continuous wave laser action results in a temperature rise leading to melting and vaporisation of the material. After several holes have been drilled into the calculus bulk it may break apart. As this effect is relatively slow, some of the heat is conducted into the surroundings and collateral damage can result. The necessary energy to minimise this is too high to be afforded by most commercial lasers, and therefore the method is
not practical for in vivo application. Pulsed laser energy allows the application of high instantaneous pulse power i.e. higher rate of energy delivery, to fracture the calculus.
Application of these pulses with a low duty cycle reduces the average power needed for calculus fracture. Therefore, stone fragmentation can be done without harmful thermal effects to the surrounding tissues.
The mechanical action on the calculus must be strong and most importantly of short duration, so that the inertia becomes important and an overall motion of the calculus is prevented. Therefore, conservation of momentum is only done with a small part of the calculus and if by this action the breaking point of the calculus material is exceeded, fragmentation starts. Mechanical action can be generated by pressure pulses arising from pulsed laser radiation. Long pulse lithotripsy makes use of the optical absorption properties of the calculus to produce mechanical stress transients within the bulk material, and, therefore, this method is dependent on calculus composition. In short pulse lithotripsy, the laser pulses are absorbed by plasma and shockwaves are released, which then act on the calculus. This method is independent of any optical properties of the stone.
Laser light energy and intensity also determine the fragmentation rate. Besides laser parameters the optical and mechanical properties of the calculi are also important for the conversion efficiency of optical energy into mechanical stress. Calculi may vary widely in contents and composition and thus show different hardness and absorption coefficients.