Most calculi of the urinary system arise from common components of urine, such as calcium oxalate or phosphate or uric acid. Still other calculi may be promoted by anatomic or structural abnormalities associated with urinary diversion. An additional group of calculi includes those associated with caliceal, bladder, or urethral stones formed after ingestion of oral substances or medications capable of generating calculi. On some occasions, calculi arise from the presence of foreign bodies within the urinary tract. Some of these foreign-body stones result from iatrogenic causes, but a surprising number result from self-insertion of unusual objects or from migration into the urinary system from other organs of the abdominal contents.

One of the fortunate factors about these calculi is that limitation of the offending medication, removal of the foreign body and its associated stone, or correction of the anatomic abnormality related to the diversion results in “cure” of the calculous disease.

Even though these types of calculi are somewhat rare, their existence emphasizes the need to submit calculi or their fragments for stone analysis. Some of these calculi were discovered because “routine” stone analysis resulted in reports calling the stones “not consistent with any known composition or urinary calculus.”<sup>27 Sutherland S.E., Reigle M.D., Seftel A.D. , et al. Protease inhibitors and urolithiasis J Urol 1997 ;  158 : 31  [cross-ref]

Cliquez ici pour aller à la section Références</sup> This type of report should cause the physician to resubmit the stone for further analysis using known urinary profiles for medications. Another comment may state, for example, that “the nucleus of this calculus is composed of a hairpin.” This should prompt the urologist to ask the patient about possible self-manipulation and insertion of foreign bodies into the urinary system. If this is a common event for the patient, further counseling may be appropriate.

Another important consideration in these secondary types of calculi derives from their relative radiodensity or radiolucency. For example, radiolucent calculi of triamterene may be seen as radiodense stones on CT scanning and therefore confused with uric-acid calculi.<sup>10 Guevara A., Springmann K.E., Drach G.W. , et al. Triamterene stones and computerized axial tomography Urology 1986 ;  27 : 104

Cliquez ici pour aller à la section Références</sup> Attenuation values of calculi during CT scanning can be very useful for determining likely compositions of calculi.<sup>11 Hillman B.J., Drach G.W., Tracey P. , et al. Computed tomographic analysis of urinary calculi AJR Am J Roentgenol 1984 ;  142 : 549

Cliquez ici pour aller à la section Références</sup> Radiolucent uric-acid calculi have CT attenuation values of 70 to 400 Hounsfield units (HU). A xanthine stone demonstrates a value of 391 HU, whereas a cystine stone registers 586 HU. Stones composed of the medication indinavir are not radiodense on standard radiography or CT imaging.