Cystine calculi: correlation of CT-visible structure, CT number, and stone morphology with fragmentation by shock wave lithotripsy

Cystine stones are often highly resistant to shock wave lithotripsy (SWL), but it has been reported that cystine stones of rough morphology are actually quite susceptible to SWL. Based on the observation that rough cystine stones contain void regions that are visible by helical computed tomographic (CT) imaging, we hypothesized that the internal structure of cystine stones would correlate with the susceptibility of stones to SWL. Cystine stones with average diameters between 4 and 7 mm were scanned using micro and helical CT, classified morphologically according to published criteria, and broken in a research electrohydraulic lithotripter, with fragments sieved through a 2 mm mesh every 50 SWs. Stones with regions of low X-ray attenuation visible on helical CT required only 650 +/- 312 SW/g for total comminution, while those that did not show CT-visible internal structure required 1,046 +/- 307 SW/g (mean +/- SD, P < 0.004). In addition, both average and minimum values for CT number (in Hounsfield units, HU) correlated with SW/g to comminution (P < 0.003 and P < 0.0003, respectively), and these relationships were independent of stone size. This study also confirmed the relationship between the morphological criteria of Bhatta et al. (J Urol 142:937-940, 1989) and cystine stone fragility: Rough stones required 609 +/- 244 SW/g (n = 11), smooth stones 1,109 +/- 308 SW/g (n = 8), and stones intermediate in morphology 869 +/- 384 SW/g (n = 7; rough different from smooth, P < 0.005). In conclusion, cystine stones that appeared homogeneous by helical CT required 61% more SWs for comminution than did stones showing regions of low X-ray attenuation. These findings demonstrate the feasibility of using helical CT to identify cystine stones that will be susceptible to SWL.