[Experimental study of urinary calculi fragmentation with pulsed laser shockwave, based on their chemical composition].

OBJECTIVES

To attempt to classify the resistance of urinary calculi to shock waves according to the chemical composition of the calculi most frequently treated in clinical practice.

METHOD

Seventy four urinary calculi obtained after surgery were submitted to shock waves produced by an experimental neodyme glass phosphate pulsed laser, at a frequency of 532 nanometres, delivering impulses of 4.8 Joules lasting 25 nanoseconds with a recurrence of 1 second. The calculi (25 homogeneous: 90% of one constituent, 49 heterogeneous: > 45% < 90% of one constituent) had to be fragmented down to 2 mm fragments. The total fragmentation energy (TFE) required was the parameter adopted to classify calculi and to compare the fragmentation of various chemical compositions studied by infrared spectrophotometry. The TFE of 25 homogeneous calculi were compared to a microhardness study performed on calculi with the same chemical composition. The fragmentation thresholds energy (TFE) was used to compare the fragmentation thresholds of each type of calculi. Statistical analysis by ascending multiple regression was performed to classify the various factors likely to influence fragmentation.

RESULTS

Two groups were able to be significantly distinguished according to whether the TFE required to obtain 2 mm fragments was greater than 200 Joules (cystine, weddellite, brushite, uric acid) or less than 200 Joules (weddellite, carbapatite, struvite). Only cystine and whewellite significantly increased the fragmentation threshold. For the 125 homogeneous calculi, the correlation with microhardness demonstrated an inverse relationship between friability and increased microhardness. A crossover between microhardness and friability to shock waves was only observed for whewellite and cystine. For the 49 heterogeneous calculi, this study showed that when weddellite was the predominant component of a stone, the friability tended to increase. Struvite and whewellite significantly facilitated and decreased fragmentation of heterogenous calculi, respectively.

CONCLUSIONS

This study allowed urinary calculi to be classified into three groups: friable (weddellite, carbapatite, struvite), intermediary (brushite, uric acid, whewellite) and resistant (cystine). Although this laser cannot be used clinically, the constants were similar to those of pulsed lasers available in clinical practice and the results of the study corresponded to those already observed empirically by other teams or observed experimentally on a single stone corresponding to each type of chemical composition. The clinical application of this study would be to prospectively compare the results with those observed in clinical lithotripsy.