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.