STONE DISEASE

Variability of renal stone fragility in shock wave lithotripsy
Williams JC Jr, Saw KC, Paterson RF, Hatt EK, McAteer JA, Lingeman JE
Department of Anatomy and Cell Biology, Indiana University School of Medicine, Indianapolis 46202, USA
Urology 2003; 61: 1092-7

• Objectives: To measure, in an in vitro study, the number of shock waves to complete comminution for 195 human stones, representing six major stone types. Not all renal calculi are easily broken with shock wave lithotripsy. Different types of stones are thought to have characteristic fragilities, and suggestions have been made in published reports of variation in the fragility within some types of stones, but few quantitative data are available.
• Methods: Kidney stones classified by their dominant mineral content were broken in an unmodified Dornier HM3 lithotripter or in a research lithotripter modeled after the HM3, and the number of shock waves was counted for each stone until all fragments passed through a sieve (3-mm-round or 2-mm-square holes).
• Results: The mean +/- SD number of shock waves to complete comminution was 400 +/- 333 per gram (n = 39) for uric acid; 965 +/- 900 per gram (n = 75) for calcium oxalate monohydrate; 1134 +/- 770 per gram (n = 21) for hydroxyapatite; 1138 +/- 746 per gram (n = 13) for struvite; 1681 +/- 1363 per gram (n = 23) for brushite; and 5937 +/- 6190 per gram (n = 24) for cystine. The variation for these natural stones (83% +/- 15% coefficient of variation) was greater than that for artificial (eg, gypsum-based) stones (17% +/- 8%).
• Conclusions: The variability in stone fragility to shock waves is large, even within groups defined by mineral composition. Thus, knowing the major composition of a stone may not allow adequate prediction of its fragility in lithotripsy treatment. The variation in stone structure could underlie the variation in stone fragility within type, but testing of this hypothesis remains to be done.
  
• Editorial Comment
  A number of clinical series have attempted to retrospectively correlate stone composition with success of shock wave lithotripsy (SWL). However, the ability to predict stone composition preoperatively on the basis of density on plain radiographs or attenuation on CT has been disappointing. Likewise, inconsistency in stone fragmentation among stones of similar composition has further limited our ability to predict SWL outcomes.
  Williams and associates evaluated a series of human stones of different compositions as well as artificial stones to assess their susceptibility to and variability of fragmentation with SWL in vitro. Although uric acid and hydroxyapatite stones required the least and struvite and cystine stones the most shock waves to comminute, the variability within each group of stones with similar composition was remarkably high, suggesting that secondary factors, such as additional mineral components or variation in internal structure, also contribute to the overall susceptibility of a stone to SWL fragmentation.
  This relatively simple but important study suggests that recent attempts to determine stone composition on the basis of radiographic characteristics may provide less predictive information than previously hoped. Although some generalizations may be made about susceptibility of stones of certain compositions to SWL fragmentation, in any individual case the outcome is less certain due to the large variability in response of stones to shock waves. Thus, knowledge of stone composition in and of itself may provide insufficient evidence on which to base patient selection for SWL.

Dr. Margaret S. Pearle
Associate Professor of Urology
University of Texas Southwestern Med Ctr
Dallas, Texas, USA