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Effect of Stone Composition on Operative Time and Rate of Stone Clearance During Percutaneous Nephrolithotomy
Johann P. Ingimarsson, MD, Vernon M. Pais, Jr., MD.
Dartmouth-Hitchcock Medical Center, Lebanon, NH, USA.

BACKGROUND:
The efficacy with which a renal stone is fragmented by a pneumatic, laser, or shock wave lithotripter is affected by the stone’s chemical composition. However, only limited and conflicting data exist on how chemical composition affects operative time, rate of stone clearance, and stone free rates in percutaneous nephrolithotomy (PCNL).
METHODS:
Charts of consecutive patients who underwent PCNL by a single surgeon form 2008-2012 were reviewed. Those with documented operative time, extracted stone weight, stone composition and a computed tomography or plain abdominal x-ray on post-operative day 1 were included. Those with stones that were without a dominant component (>50%) were excluded. Stone free state was defined as no residual fragment ≥ 2 mm on imaging on post-operative day 1. Rate of stone clearance was calculated as mg of stone extracted per minute of operation.
RESULTS:
156 patient met inclusion criteria. The distribution of stone composition was: calcium oxalate 43%, calcium phosphate (CaP) 32%, uric acid 14%, struvite 6% and cystine 5%. Average stone weight was 2470±5482 mg (median 1187 mg) and operative time 190±71 min (median 188 min). Struvite stones were larger (8349 mg, p=0.001) and with longer operative times (253 min, p<0.001), than other stones. CaP stones were significantly larger than other non-struvite stones (3147 mg, p=0.03), but did not have longer operative times. Average rate of stone clearance was 12.0 mg/minute (median 6.8). Struvite and CaP stones were cleared faster than other stones, at 33.8 (p=0.03) and 14.8 (p=0.04) mg/minute, respectively. Additionally, increasing stone size correlated with faster rate of stone clearance, such that for every 1000 mg increase in stone weight, rate increased by 3.1 mg/minute.
When accounting for this in a multivariate logistic regression, size - but not stone composition -affected rate of stone clearance.
Average stone free rates were 72% with no significant difference between stone compositions (range 66-76%).
CONCLUSIONS:
Chemical composition does not affect rate of stone clearance and operative time during PCNL. Rather, we have noted that these are driven by stone size alone. As such, presumed composition does not need to be factored into the allotment of scheduled operative time.


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