Study of an Extracorporeal Lithotripter Using a Synthetic Stone Model*

Efficiency and efficacy of different intracorporeal ultrasonic lithotripsy units on a synthetic stone model

PURPOSE

The efficiency and efficacy of the available intracorporeal ultrasonic lithotripters were compared in a stone model experiment.

MATERIALS AND METHODS

Plaster of Paris (POP) stone phantoms having ratios of 1:1, 1.5:1, and 2:1 with water were fabricated into cubes of various hardnesses weighing an average of 24.6 g. The stones were immersed in water in a plastic container, and continuous irrigation through a rigid nephroscope was used. Ultrasonic lithotripters from ACMI, Olympus, Storz, and Wolf manufacturers were evaluated for efficacy in breaking up the three POP concentrations. Time to complete stone fragmentation, occurrence of probe or tubing occlusion, and probe overheating were evaluated.

RESULTS

Efficiency of fragmentation and time to fragmentation of the Storz lithotripter were significantly different from those of the Wolf (p = 0.01 and p = 0.02, respectively) and ACMI (p = 0.001 and p = 0.02, respectively) lithotripters. Comparison of the efficiency of fragmentation and time to fragmentation of the ACMI and Wolf lithotripters showed significant differences (p = 0.005 and p = 0.03, respectively) in favor of the Wolf device. The Olympus lithotriptor resulted in incomplete fragmentation of phantoms of all POP concentrations.

CONCLUSION

The Storz ultrasonic lithotriptor was found to have the lowest fragmentation time and highest efficiency in the fragmentation of phantom stones.

Production of artificial urinary stones from natural materials and their physical properties. First results

Considered from a clinical point of view, all artificial stones known so far are inappropriate for experimental investigations, because they do not correspond to natural urinary stones in shape, physical properties and chemical composition. A special technique has now been developed for the production of biometric artificial stone models. Standard artificial carbonate apatite stones were produced by a coating technique where several layers of a suspension consisting of pure substance were applied around a core. Density and crushing strength of these ball-shaped artificial stones corresponded to natural apatite stones. Experimental chemolitholysis investigations showed comparable results in respect to the dissolution of artificial apatite stones and of natural stones of the same chemical composition. The disintegration behaviour was studied in ESWL tests which revealed a correlation between generator voltage and number of shock waves necessary for complete disintegration. For the first time, an artificial urinary stone has now been produced which is comparable to a natural stone of the same chemical composition not only in shape but also in its physical properties and lysis behaviour. These stones are suitable for use in systemic scientific investigations on disintegration and chemolitholysis.

Comparison of the performance of two pulsed dye lasers using a synthetic stone model

The performance of two pulsed dye laser lithotripters was compared in vitro using a synthetic calculus with gross physical properties similar to natural urinary tract calculi. Synthetic calculi were subjected to 500 shocks at nominal pulse energies of 40, 60, 80, 100 and 120 mJ using a 320 micron. fiber. Three calculi were treated at each energy level on the Technomed Pulsolith and two at each level on the Candela MDL 2000. The pattern of fragmentation was similar with both lithotripters. Increasing the energy level resulted in a linear increase in fragmentation (Pulsolith r = 0.95, MDL 2000 r = 0.99), but fragmentation was greater with the MDL 2000 than with the Pulsolith (6.01 mg/mJ versus 5.00 mg./mJ, p < 0.001). Both lasers fragmented the calculi satisfactorily. We recommend that all users of such lasers acquire a laser meter to allow monitoring of the energy delivered by the fiber tip.

Mechanisms responsible for diminished fragmentation of ureteral calculi: an experimental and clinical study

We molded 24 synthetic stones (mean weight 680 mg., range 641 to 715) from a commercial mixture of gypsum, silica, cellulose and polyvinyl acetate. Each stone was subjected to 400 shocks on a Wolf 2300 Piezolith and groups of 6 stones were treated in 4 different modes. Mean amounts fragmented were 243 +/- 18 mg. in a free environment, 62 +/- 18 mg. confined loosely in a latex tube, 22 +/- 8 mg. impacted in the tube and 30 +/- 8 mg. impacted alongside a 7F stent. During a 30-month period 118 patients received in situ extracorporeal shock wave lithotripsy for ureteral calculi using the same lithotriptor. The mean stone burden was 11.4 mm. (range 4 to 29). Success was greater for patients with calculi 10 mm. or less than for those with stones greater than 10 mm. (71% versus 51%, p less than 0.05), despite the former group receiving less shocks (5,404 versus 7,491). The influence of size was then excluded by studying the number of shocks delivered per mm. of calculus. Patients receiving 500 to 699 shocks per mm. showed a higher success rate than those receiving a smaller number of shocks per mm. Treatment with a greater number of shocks per mm. did not improve success rate. The experimental study demonstrated that confinement and impaction significantly diminish the rate of fragmentation of calculi. However, the clinical study suggested that there may be an optimum number of shocks per mm. that should be delivered. Treatment beyond this point fails to improve results. The 28% failure rate even in those receiving the highest number of shocks per mm. suggests that large, impacted calculi are unsuitable for treatment with in situ shock wave lithotripsy on this machine.

A comparison of stone damage caused by different modes of shock wave generation

A standard stone phantom was used to compare stone damage after extracorporeal shock wave administration from electrohydraulic, electromagnetic and piezoelectric lithotripters. For each machine, a low and high shock wave intensity setting was chosen: 18 & 24 kV for electrohydraulic; 16 & 19 kV for electromagnetic; power levels 1 and 4 for piezoelectric. The shock wave was focused either at the front (surface facing the wave source) or back surface of the stone and 50, 100, 200 or 400 shocks were delivered to different stone groups. Effects of varying physical properties in the stone phantom were also investigated. Stone damage was described in terms of volume loss and both depth and width of the resulting damage crater. At the lower intensity settings, all three machines produced stone volume loss which was linearly related to the number of shock delivered. At higher intensity settings, volume loss increased rapidly as the number of shocks increased. With the same number of shocks, stone volume loss was greatest with the electrohydraulic machine, followed by electromagnetic and piezoelectric lithotripters for both low and high intensity settings. Damage craters from the piezoelectric device were narrow and deep; those from the electromagnetic machine were of the shape of a right angle circular cone; whereas those from the electrohydraulic lithotripter were shallow and wide. At the high intensity settings, damage from the piezoelectric and electrohydraulic lithotripters appeared to depend upon the position of the focal point with a higher volume loss when the shock waves were targeted at the front surface of the stone. For the electromagnetic device, a higher volume loss was found when we positioned the focal point at the back surface of the stone phantom. Stone phantoms with lower mechanical strength and acoustic impedance were more easily damaged than those with higher values. Finally, a computer regression model was developed to express volume loss in terms of the intensity setting, focal position and number of shocks for each lithotripter.