Urolithiasis is a very common condition in the United States, with an estimated prevalence of 11.7% by age 70. Furthermore, it has been associated with considerable patient morbidity and occasional mortality (Stamatelou, Francis et al. 2003). Direct costs associated with the treatment of renal calculi are enormous, as over 600,000 stone related medical procedures are performed annually in the United States (shock wave lithotripsy (SWL), ureteroscopy (URS), percutaneous nephrolithotomy (PERC), stone removal, ureteral stents, and stone basketing (source: Boston Scientific Corporation). Although the last two decades have seen considerable advances in less invasive techniques for the treatment of symptomatic stone episodes, as well as progress in mitigating the risks of new stone formation, our knowledge of the inciting lesion in human urolithiasis remains rudimentary and much debated.

Until recently, the sequence of events that leads to the formation of urinary calculi were poorly understood, most fundamentally due to the lack of appropriate in-vivo data. Earlier theories of calculogenesis proposed that stones could result from tubular epithelial injury due to oxalate toxicity, a lack of urinary inhibitors of crystal formation or crystal epitaxy on a pre-existing nidus (Khan, Finlayson et al. 1979). Theoretical work on free and fixed particle growth indicated that a transit time from the collecting duct to the bladder of only 10 minutes provided insufficient time for a crystal to grow to a clinically meaningful size (Jonassen, Cooney et al. 1999). Morphological classification of the directional growth of calculi supported the necessity for a fixed point of origin, in the absence of obstruction (Hinman 1979). These observations suggested that most stones must initiate from a fixed point or nidus in the collecting system or renal papilla.

One such nidus, first described more than sixty years ago by Alexander Randall, was proposed to be the originating lesion for the formation of calcium oxalate stones (Randall 1936; Randall 1937; Randall 1940). In microscopic studies of renal papilla obtained at necropsy, he demonstrated the presence of 2-3mm lesions in 19.6% of patients that were composed of calcium phosphate and devoid of evidence of inflammation. Adherent to this, in 65 of 1,514 pairs of kidneys, he identified nascent stones composed of calcium oxalate and calcium phosphate. When these stones reached sufficient size, he hypothesized that they would break free, taking with them the underlying plaque. In subsequent work, 256 voided or removed calculi were examined and 106 gave visible evidence of mural attachment (Randall 1940). Later microradiographic studies would confirm the presence of plaque in a papillary location that could be co-localized with stone (Carr 1954). Unfortunately, all earlier studies of stone pathogenesis have suffered from lack of definition of clear clinical stone-forming phenotypes.

There are intriguing reports to support Randall's, and our, hypothesis that stones originate from a fixed plaque composed of HA. Earlier microscopic studies of stone structure demonstrated the presence of concavities on small stones compatible with a point of mural attachment and indicated that apatite may be present at the attachment point (Rosenow 1940; Prien 1949). In an early x-ray diffraction and crystallographic study of 10,000 urinary calculi, Herring noted that HA was frequently found as the nucleus of calcium oxalate monohydrate, usually as a small discoid plaque which was felt to resemble Randall's plaque (Herring 1962). Later, Chambers performed an electron probe analysis of 115 small renal calculi. Of 92 predominantly calcium oxalate stones, he was able to identify small central areas of HA, usually 10-200 microns in diameter, in 70 (Chambers, Hodgkinson et al. 1972). Using scanning electron microscopy and x-ray dispersive energy, Cifuentes Delatte found that 63 of 87 passed calcium oxalate stones had evidence of plaque (Cifuentes Delatte, Minon-Cifuentes et al. 1985). Observation of uncalcified tubular lumens found in conjunction with these plaques suggested an interstitial papillary tip origin of this material (Cifuentes Delatte, Minon-Cifuentes et al. 1987).

We have noted that when endoscopically examining the renal papillae of patients undergoing PERC, oftentimes stones attached to renal papillae are encountered. We have collected three attached stones from three separate patients who were undergoing PERC. The stones were analyzed with a Micro CT device, and 3-D reconstruction with identification of mineral components was performed. The stones all showed multiple mineral components, including calcium oxalate, apatite, and probable regions of poorly mineralized matrix. Although the significance of these various components, in varying amounts, is not yet well understood, it is apparent that even at early stages of stone formation, multiple minerals in complex arrangement are present in papilla-attached stones. It will be only through a rigorous, prospectively designed protocol that the significance of attached renal calculi will be understood. By demonstrating that renal calculi in a population of common calcium oxalate stone formers originate from an HA plaque, we would link in a substantive way the origin of plaque with the subsequent development of stone.