The effect of volume and number on fragmentation of gallstones by lithotripsy

Several factors may affect the fragmentation of gallstones by lithotripsy. We hypothesized that stone volume is an important determinant of degree of fragmentation, while number of stones is less important. The volume of 30 single stones was measured in vitro by water immersion. Thirty sets of multiple stones were matched with single stones of similar volumes. The stones were subjected to 750 shock waves (18 kV) with the Dornier MPL 9000 lithotripter. Degree of fragmentation was assessed by passing fragments through sieves of 1, 2, and 4 mm and recording percentage of stone weight which passed through each sieve. The mean percentage of stone weight passing through sieves of 1, 2, and 4 mm for single stones was 38.2, 49.4, and 59.8. The corresponding values for multiple stones were 46.4, 61.3, and 75.4. These values were not significantly different at P less than or equal to 0.05. Thus, there was no significant difference in fragmentation between single and multiple stones when their volumes were similar. The mean volume of stones with 100% of fragments less than 4 mm (0.80 +/- 0.05 cc) was significantly less than that of stones with fragments greater than 4 mm (1.95 +/- 0.14 cc) (P less than 0.0001). There was a significant inverse linear correlation between stone volume and degree of fragmentation (r = -0.65). We conclude that stone volume is an important determinant of degree of fragmentation and may be the most important criterion in selecting patients for lithotripsy. In this in vitro setting, stone number alone had no effect on degree of stone fragmentation.