To evaluate whether preoperative computed tomography (CT) findings could predict the presence of an impacted stone. Preoperative identification of an impacted ureteral stone may influence patient preparation and operative decisions. Factors predicting ureteral stone impaction have not been clearly identified.

We identified all patients from June 2014 to July 2016 that underwent ureteroscopic treatment of an impacted ureteral stone. Patients that had ureteral prestenting or previous treatment for their stone were excluded. Noncontrast CT images were reviewed to calculate stone size, stone volume, degree of hydronephrosis (0-3), and Hounsfield units (HU) of the stone as well as the ureter distal and proximal to the stone. These were compared with a control group of patients that had nonimpacted stones.

Patients with impacted stones had a greater stone size, volume, HU of the ureter under the stone, HU under/above ratio, and degree of hydronephrosis on univariate analysis. Multivariate analysis demonstrated that HU under the stone was a significant predictor of ureteral stone impaction (odds ratio 1.17; 95% confidence interval 1.11-1.25). Distal ureteral density above 27 HU demonstrated a sensitivity of 85%, specificity of 85%, positive predictive value of 89%, and negative predictive value of 81% for ureteral stone impaction.

Impacted stones are associated with ureteral density cut-off value of 27 HU or greater. Measuring this value on preoperative noncontrast CT may help predict which patients are more likely to have impacted stones.