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Do Stone Characteristics, Patient Factors, and the S.T.O.N.E Nephrolithometry Score Differ Between Metabolic and Infectious Stones?
Benjamin J. King, MD, Peter W. Callas, PhD, Kevan M. Sternberg, MD.
University of Vermont, Burlington, VT, USA.

BACKGROUND: The composition of large renal stone burdens has been shown to be changing from predominantly infectious to metabolic. Metabolic stones have been found to contain significant calcium phosphate contributions. While surgical outcomes have been investigated relating to stone composition, patient and stone factors associated with these have not been well described. We sought to define the differences between metabolic and infectious stones and between calcium phosphate and other metabolic stones using the S.T.O.N.E nephrolithometry score. We also examined how stone composition affected surgical outcomes in patients requiring percutaneous nephrolithotomy (PCNL). METHODS: We retrospectively reviewed all patients who underwent PCNL for large stone burdens at a single institution between February 2011 and March 2014. Stone analysis, stone characteristics, patient factors, and surgical outcomes were reviewed. These factors and the S.T.O.N.E. nephrolithometry score were compared in relation to stone composition. Stones were categorized as infectious if there was any component of struvite or carbonate apatite in the sample. Metabolic stones were classified by the largest component present greater than or equal to 50%. RESULTS: 50 kidneys underwent PCNL for large or complicated stone burdens. 40 (80%) were metabolic and 10 (20%) were infectious. Of the metabolic stones, 22 (44%) were predominantly calcium phosphate. Of these, 18 were apatite and 4 were brushite. The nephrolithometry score differed between infectious and metabolic stones, with a mean score of 9.4 (SD=1.8) for the infectious and 7.5 (1.3) for the metabolic stones (Wilcoxon rank sum test p=0.004). This was driven by significant differences in stone volume and number of involved calyces. There was a trend towards improved stone-free rates with the metabolic stones (60 % vs. 30%), but this was not statistically significant. Although the stone score was not significantly different when comparing apatite, brushite, and other metabolic stones, some score components did differ. The average hounsfield units differed with apatite (857), brushite (1392), and others (919), (Kruskal-Wallis test p=0.03). Tract length was also significant with apatite (11.8cm), brushite (7.8cm), and other (10.6cm), (p=0.04). We saw a trend towards improved stone-free rates with non-phosphate containing stones as well with 19 (68%) compared to 7 (39%) apatite and 1 (25%) brushite after the initial percutaneous procedure (Fisher’s exact test p=0.07). CONCLUSIONS: As described in other contemporary cohorts, we found the majority of our large and complex stones to have a metabolic make-up. Calcium phosphate was a major component of these stones. The reason for this change is unclear. The ability to differentiate between stone types is important for both the surgical and medical management of this patient population. Future research with larger cohorts may help better elucidate the differences and allow us to predict the make-up of stones requiring surgical intervention.


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