Relaxation of micro indentations in calcium oxalate urinary stones

Purpose: We define energy requirements for stone micro indentation as a quantifiable event equivalent to in vivo energy delivery and investigate the change in indentation characteristics with time. Materials and Methods: The 7 stones extracted from 7 patients were cut, embedded in resin and polished. Multiple micro indentations were performed on each stone section using a diamond Vickers micro indentor with a 500× light microscope and video system. The resulting indentations were observed by optical and scanning electron microscopy as a function of time. Organic matrix content was determined by dissolving stones in ethylenediaminetetraacetic acid solution. Results: The energy requirement for stone indentation varies among stones (median range 43.6 to 109.9 kg./mm.²) and at different locations in the same stone. Indentations relaxed by 10 to 70% during the first 2 weeks after indentation. Stones with a high organic matrix content were ductile and the phenomenon of indentation relaxation was pronounced. Brittle, low matrix stones relaxed to a lesser extent. Conclusions: The relaxation phenomena may have a practical implementation when considering repeat shock wave lithotripsy. A significant fraction of the energy invested in a stone which did not cause fracture or critical cracks is lost within 1 to 2 weeks after the procedure, particularly in elastic stones with a high organic matrix content. We suggest that the preferred interval for repeat shock wave lithotripsy be less than 2 weeks.