What’s known on the subject? and What does the study add?

Stone diameter, volume and Hounsfield units values were significant predictors of laser energy level, ureterorenoscopy and laser time for distal ureteral stones. Also, use of a thinner probe decreased total laser time, laser energy level and laser energy/time ratio.

Introduction

Ureteroscopy (URS) procedure is an appropriate treatment modality for ureteral stones (1). The holmium:yttrium-aluminum-garnet (Ho:YAG) laser has been considered the standard lithotripsy modality for ureteroscopic lithotripsy (URSL) for the past 2 decades (2,3). With the development of semirigid and flexible URS technology and introduction of better instrumentation, including the Ho:YAG laser, URSL has improved in efficacy while sustaining a low morbidity and high success profile (4,5). In current studies, URSL stone-free (SF) rates were reported as 94.2%, 89.4%, and 84.5%, for distal, mid, and proximal ureteral stones, respectively (6).

Non-contrast computed tomography (NCCT) is the gold standard imaging modality to evaluate patients with urinary stones, providing predictive information for treatment planning (7,8). NCCT provides stone measurements (size and volume), stone location and stone density [Hounsfield units (HU) measurements]. Absolute HU inversely correlates with the effectiveness of extracorporeal shock wave lithotripsy (SWL) and has also been proposed to affect SF rates after URSL and percutaneous nephrolithotomy (PNL) (9-12). Stone fragmentation is an important factor for SF status, laser and operative times. Therefore, it is important to define the predictive factors affecting stone fragmentation and these factors may also be useful in predicting SF status, laser and operative times for URSL. In a recent study, URSL procedures were retrospectively analyzed and predictive factors for operative time and success were investigated and identified (13).

The purpose of our investigation was to prospectively evaluate the effect of preoperative NCCT findings and peroperative laser probe selection on treatment success rate, cumulative Ho:YAG laser time, URS time and laser energy level for single distal ureteral stones.

Materials and Methods

After approval was obtained from the Local Ethics Committee, we prospectively evaluated patients >18 years old who underwent URSL for single distal ureteral stone 5-25 mm in diameter from March 2015 to October 2016. The patients were evaluated with NCCT before URSL. The exclusion criteria were stones of <5 or >25 mm in diameter, obstructive and multiple stones, stones requiring emergency drainage (fever, Systemic Inflammatory Response syndrome, sepsis, urinary tract infection), patients with solitary kidney and patients with congenital urinary tract anomalies. Mid-proximal ureteral stones were also excluded. NCCT images using 2 mm sections with the liver’s dome as cranial border and the pubis joint as caudal border at 100 mA 120 kV (Brilliance 64, Philips®, Best, The Netherlands) were taken. All NCCT findings were evaluated by a radiologist. Stone diameters (longitudinal and transverse) were calculated in 2 different planes and the maximum diameters of the stones were taken into consideration. Stone volumes were measured using a computer programme. HU values were measured as previously described by Celik et al. (12) for the largest diameter of the stone (longitudinal or transverse) with bone window and large magnification. After serial measurements of the highest HU value (HUmax) and the lowest HU value (HUmin), HU value was calculated as the average of these two values (HUave). The difference in HU (HUdiff) was calculated as the difference between the HUmax and the HUmin values, as previously defined by Celik et al. (14,15) Hounsfield density (HD) was calculated as HUmax divided by stone size (16). All URSL treatments were performed with a 8/9.8 F rigid ureteroscope (Karl Storz®, Tuttlingen, Germany), 365 µm PercuFib or 550 µm RigiFib laser probes (LISA® laser products OHG, Katlenburg-Lindau, Germany) and a 80 watt Ho:YAG laser device (Sphinx®, Livermore, CA, USA). Laser probe was randomly selected during the the operation. Stone fragmentation was monitored using the Storz endovision system (Karl Storz®, Tuttlingen, Germany) throughout the procedure. All laser procedures were performed with 1.5 joules energy, 12 watts power and 8 hertz frequency. Usage of forceps and/or basket catheter and the data on obstruction (no obstruction/partial obstruction/complete obstruction) and impaction (no impaction/partial impaction/complete impaction) were noted according to URS observation during the procedure. The total laser energy level, laser time, URS time and operative time were noted at the end of the operation. Necessity of J stent placement was decided according to the operation findings. Status of stone impaction and obstruction and J stent placement were also noted. URSL success was defined as SF (absence of all of clinical significant and insignificant stones) status at the end of the operation.

Statistical Analysis

Success status was evaluated as SF and residual fragments (RF). Type of laser probe used was also evaluated and divided into two groups as 365 and 550 µm laser probe groups. Data were analyzed using the Statistical Package for Social Sciences, version 20.0 (SPSS, Chicago, Ill) software program. Nonparametric correlations between possible predictive parameters for URSL success were compared by using Pearson’s correlation coefficient. The Mann-Whitney U test and Pearson chi-square test were used to compare demographic and radiologic parameters between 365 and 550 µm groups. Data are given as mean ± standard deviation. However, the results of analysis are given as median data. Statistical significance was defined as p<0.05.

Results

A total of 72 patients with distal ureteral stones were included. Patient and stone characteristics are given in Table 1. Correlation between predictive radiologic parameters measured on NCCT images and peroperative URSL data was investigated in all the 72 patients. Correlation results are given in Table 2. In correlation analysis of 72 patients, laser time was strongly correlated with longitudinal and transverse diameters of stone, stone volume, HUmax, HUmin and HUave and negatively correlated with HD. URS time was found to positively correlate with longitudinal and transverse stone diameters and negatively correlate with HD. Laser energy was also positively correlated with only longitudinal stone diameter and negatively correlated with HD. However, energy/time ratio was not found to correlate with any predictive data. Stone diameters, volume and HU measurements were not significantly different between the groups with presence of obstruction and impaction. There were 17 patients in the 365 µm group and 55 patients in the 550 µm probe group. Patients and stone characteristics of the groups and comparison of statistical analysis results are given in Table 3. The rate of patients treated with SWL before the URSL procedure was found to be significantly higher in the 365 µm group compared to 550 µm group (p<0.05). There was no significant difference in URSL success rate and other predictive data between the groups. However, in the peroperative data, laser time, laser energy level and laser energy/time ratio were significantly lower in the 365 µm group compared to the 550 µm group (p<0.05). After exclusion of patients with previous SWL, 61 patients were evaluated (11 patients in the 365 µm group and 50 patients in the 550 µm group) and almost all parameters were similar between the groups. However, URS time was found to be shorter in the 365 µm group than in the 550 µm group (17.5 min vs 24.4 min, p=0.036).

Discussion

NCCT images provide more information about stone characteristics and stone density including HU measurements. HU measurements have been researched many times to date and an absolute inverse correlation has been found between high HU level and SF rates after SWL, URSL and PNL procedures. (9,10,11,12). Especially for URSL, stone fragmentation during the procedure is the most important factor for laser energy level, laser time and operative time. Therefore, defining the predictive factors affecting stone fragmentation is one of the major areas of research for URSL in recent years. In a recent study designed by Ito et al. (10), 219 patients who underwent URSL procedures for renal stones were retrospectively analyzed. There was a significant difference in the fragmentation efficiency and operative time in favor of the low HUave group (<1061 HU) compared to the high HUave group (≥1061 HU) in URSL procedures for <20.0 mm renal stones (10). However, we agreed with Ofude et al. (13) and we also evaluated both total laser time and URS time because laser time is more predictive than operative time. Ofude et al. (13) found that stone attenuations such as HUave and stone volume predicted laser energy level during URSL with Ho:YAG laser. Also, maximum stone diameter and severe hydronephrosis rate were found to be associated with laser energy amount. However, other radiological findings were not found to predict the laser energy levels, such as HD, whereas HD was found to be inversely correlated with stone volume. Therefore, the authors considered that HD was insufficient as an indicator of stone density. Also, Seitz et al. (17) retrospectively analyzed 543 patients with ureteral stones who underwent URS. The degree of hydronephrosis was found to be positively correlated with stone size (17). In contrast, stone diameter, volume and HU measurements were not found to be significantly different between the groups with presence of obstruction and presence of impaction in our study.

In an important study published by Molina et al. (18), stone volume, presence of renal stone and HD were found to be significantly positively correlated with laser energy level and stone volume and HD were significantly positively correlated with laser time. Also, size of laser probe and laser power setting were found to be positively correlated with laser energy level. In addition, another study reported that increasing laser settings that may increase laser energy level is associated with high intraluminal temperature, potentially causing ureteral tissue injury (19). When we look at our study; first, laser time was strongly correlated with stone diameter, stone volume, HUmax, HUmin and HUave measurements and negatively correlated with HD. Second, URS time was found to be positively correlated with stone diameter and negatively correlated with HD. Last, laser energy level was also positively correlated with only longitudinal stone diameter and negatively correlated with HD. In contrast, the energy/time ratio was not found to correlate with any predictive data.

When we look at previous literature on the type of laser probe, Vassar et al. (20) reported that higher energy level was generated by smaller laser fibers than by larger fibers. In our study, in the analysis of laser probe groups, laser time, laser energy level and laser energy/time ratio were found to be significantly lower in the 365 µm group compared to the 550 µm group (p<0.05). In addition, the 365 µm probe shortened URS time compared to the 550 µm probe, although not statistically significant. Interestingly, the rate of previous SWL was found to be higher in the 365 µm probe group. Although SWL may affect stones, stone characteristics were found to be similar between the groups in the study. Therefore, all parameters were evaluated after the exclusion of patients with previous SWL. Similar results were achieved and also URS time was statistically shorter in the 365 µm group.

Study Limitations

There are several limitations in this study. The most important limitation is the small number of patients in the study because of the different laser setting selection by different surgeons. Therefore, many patients who underwent URSL with different laser settings for distal ureteral stone were not included in the study. Other limitation is that previous SWL status was different between the groups. However, similar results were achieved after the exclusion of patients with previous SWL. Another limitation is that the distribution of patients according to laser probe selection was not similar between the groups because of the limited number of times the 365 µm probe was used during the study. Nonetheless, the results of this study showed that stone diameter, stone volume and HU parameters measured by NCCT are predictors of laser energy level and laser time. Also, the 365 µm laser probe decreases the laser energy level, laser time and laser energy/time ratio compared to the 550 µm probe.

Conclusion

In conclusion, for URSL procedures, it is important to acknowledge that NCCT measurements of the stone are effective predictive factors for operative time and laser energy level. Thinner probe usage decreases total laser time, laser energy level and laser energy/time ratio. In addition, thinner laser probe shortens URS time. Nevertheless, for clarification of these results further randomised, prospective studies with large series are warranted.

Ethics

Ethics Committee Approval:Dokuz Eylul University Ethics Committee, ethical protocol number: 1966 GOA, 2015/06-16.

Informed Consent: It was taken.

Peer-review: Externally peer-reviewed.

Authorship Contributions

Concept: S.Ç, O.B, I.B., Ö.G., Design: S.Ç, O.B, I.B., Ö.G., Data Collection and/or Processing: S.C, O.B, I.B., O.G., Analysis and/or Interpretation: Ö.D., M.S., A.E., Literature Research: S.Ç, O.B, I.B., Ö.G., Writing: S.Ç., O.B.

Conflict of Interest: No conflict of interest was declared by the authors.

Financial Disclosure: The author declared that this study has received no financial support.