Patel PM, Kandabarow AM, Chen VC et al: Axial diameter is superior to volumetric measurement in predicting ureteral stone passage. Urol Pract 2021; 8: 571.

Stone size and location are considered among the strongest predictors of spontaneous stone passage, and these are typically determined by noncontrast computerized tomography imaging. While assessment of stone burden in this fashion is expeditious, this method does not take into account the variable and irregular geometric shapes of calculi, and inter-rater variability in the measurement of stone size remains problematic.^1–3 This has led to interest in volumetric measurements using computerized tomography imaging as a more accurate and reproducible method of determining stone burden.^4–6

Unfortunately, 3D modeling can be expensive, time-consuming, and cumbersome, thus signifying the importance of research evaluating the clinical impact of these models. Our group has been working to understand whether and how volumetric measurement can be used clinically, and has previously demonstrated its underwhelming role in predicting outcomes after percutaneous nephrolithotomy.⁷ The purpose of our study is to further investigate the utility of volumetric measurement in predicting the passage of symptomatic ureteral stones when compared to standard linear measurement.⁸

We used the iPlan® CMF Planning Software (Brainlab Technology, Munich, Germany) to reconstruct 3-dimensional (3D) stone models using axial slices of the stone. In brief, this time-intensive process requires that the user select each slice of interest within the entire computerized tomography stack and then draw a curvilinear line highlighting the entirety of the visible stone in each slice; once this is performed on all available slices, the software organizes it all into a 3D rendering (see figure). A total of 70 patients were included in the volumetric analysis, 37 (53%) of whom passed their stones.

Figure 1. Representation of a patient’s helical CT images and the associated three-dimensional render.

We demonstrated that 2-dimensional linear measurement in the axial dimension was positively and significantly correlated with 3D stone volume, as has been previously reported.⁷ We found that if the stone’s axial size and location in the ureter are known, additional knowledge of stone volume does not improve the ability to predict stone passage. The converse, however, is not true, ie starting with the stone volume and location in the ureter does not make the stone’s axial diameter obsolete.

There were several limitations to this study. First, despite the availability of a large cohort of patients, only a representative sample could be analyzed with the volumetric software. Second, retrospective analysis means the information gathered from the volumetric software was not available to the provider during counseling. Prospective validation of our findings is required. Lastly, while multiple volumetric tools now exist, only 1 tool was explored in this study and it was used only in ureteral stones.

We believe that there is minimal, if any, utility for 3D measurement in the setting of symptomatic ureterolithiasis, as axial diameter offers a stronger predictive model of stone passage. Clinically, this suggests that providers continue to rely simply on a stone’s axial diameter when counseling patients with symptomatic ureteral stones regarding their probability of spontaneous passage.