Introduction

The prevalence of urinary stones ranges from 1 to 20% [1]. Approximately 50% of patients have at least one lifetime recurrence [2, 3]. Urinary stones are commonly accompanied by several complications, including colic episodes, urinary tract infections, and even hydronephrosis, particularly in ureteral stones [4, 5]. Shock wave lithotripsy (SWL) and ureteroscopy (URS) are currently recommended as primary treatment modalities for ureteral stones [6]. The advantages of URS include direct visualization of fragmentation and a greater stone-free rate (SFR) but a higher complication rate [4]. SWL has the advantages of a low complication rate, no anesthesia requirement, and a high level of patient acceptance [7], but it may be associated with a lower SFR and higher retreatment rate [8]. Compared with distal ureteral stones, proximal ureteral stones are associated with lower success rates, which are ascribed to more difficult access, as well as proximal migration of the stone fragments during URS [9].

The COVID-19 pandemic had a significant impact on determining the intervention for ureteral stones. Procedures including general anesthesia and endotracheal intubation for URS pose a considerable risk of COVID-19 transmission [10, 11]. However, SWL is performed without general anesthesia and thereby may limit the risk [12]. The pandemic has also postponed large volumes of scheduled treatments to allow beds and staff to be available for patients seriously ill with COVID-19 [13].

There are only three prospective studies with small sample sizes that address this issue. Salem et al. [11] reported that the SFR was comparable between SWL and semirigid URS, while the mean costs were significantly higher in the SWL group; however, critical confounding factors, such as BMI, Hounsfield units of the stone, and hydronephrosis, were not adjusted in the study; Zhang et al. [14] found that the SFR following SWL was equivalent to semirigid ureteroscopy based on subgroup analysis, while SWL had a significant advantage with respect to the complication rate, charges, and period of hospitalization compared with the URS group. In another RCT, Ahmed et al. [15] reported that SWL is safe and effective compared to flexible URS; however, compared with the electrohydraulic lithotripter they used, the best results in stone fragmentation and fewer analgesia requirements occurred with the electromagnetic lithotripter [16]. The previous studies concerning this issue were limited and had a small number of participants. Furthermore, the conclusions were only suitable for radiopaque stones in the process of SWL.

The localization of stones can be guided by either ultrasound or fluoroscopy during SWL. Ultrasound localization not only provides real-time monitoring without X-ray exposure but is also suitable for both radiopaque and radiolucent stones. In addition, adding an early second SWL session significantly improved stone fragmentation, facilitated stone passage, and lower complication rate and cost [15, 17]. Therefore, based on a large prospective cohort using a propensity score matching (PSM) method, we compared the effectiveness, safety, and cost between electromagnetic SWL guided by ultrasound with the early second session protocol and URS in patients with proximal ureteral stones.

Methods

Study design

This prospective study was conducted on 1,230 patients of Shengjing Hospital from June 2010 to April 2022. Nine hundred and ninety-eight patients were treated with SWL, and 232 patients with URS, respectively (Figure S1). Ethical approval (No. 2020PS521K) was provided by the Ethics Committee of the Shengjing Hospital Affiliated China Medical University. Informed consent from all eligible subjects was obtained.

Inclusion and exclusion criteria

Inclusion criteria were: proximal ureteral stone with a size ≥ 6 mm, age ≥ 18 years, body mass index (BMI) < 30 kg/m2, ureter stone density < 1000 Hounsfield units (HU), and skin to stone distance (SSD) less than 11 cm, persistent obstruction, stone growth, infection.

Exclusion criteria were: pregnancy, coagulopathy, multiple or bilateral ureteral stones, anatomical obstruction distal to the stone or congenital genitourinary anomaly, transplanted kidney, solitary kidney, renal insufficiency.

Decision-making process: Patients were given the choice to undergo either URS or SWL after counseling them about the advantages and drawbacks of both procedures. After receiving the explanation, the patients selected the method they preferred.

The surgical technique of SWL

Extracorporeal shock wave lithotripsy was performed using a third-generation electromagnetic lithotripter (XYS.SUI-6B, Shenzhen, China). The focal depth was more than 110 mm; the focal area was ± 7 mm (radial) and ± 45–50 mm (axial). The procedure was performed in a supine or prone position without anesthesia or sedation requirements. The shock frequency used was 60–90 (shock waves/min). Stone localization and simultaneous real-time monitoring were performed by ultrasound (DC40, Shenzhen, China). All procedures in SWL group were outpatient procedures.

Follow-up KUB X-rays and urinary ultrasounds were taken one week after the first session, and patients underwent a second SWL session if there was stone fragmentation (≥ 4 mm) occurred. The preplanned SWL modality included two sessions of SWL within one week, as a package of intervention.

The surgical technique of URS

All procedures were performed under general or spinal anesthesia. An 8Fr/9.5Fr rigid ureteroscopy (Richard Wolf, Knittlingen, Germany) was employed. The fragmentation source was pneumatic and ultrasonic lithoclast systems (Electro Medical Systems, Nyon, Switzerland). Fragments were extracted by forceps or nitinol baskets (Ngage, Cook Incorporated, USA). Special anti-migration tools (stone cone, Boston Scientific, USA) were applied to prevent stone migration into the kidney. A 6 Fr JJ stent was placed after URS.

Propensity score matching

We used the PSM method to adjust baseline confounding variables between the URS and SWL group in an effort to derive more accurate conclusions. Multivariate logistic regression analysis was used to determine propensity scores for each patient based on age, gender, BMI, smoking status, comorbidities, previous history of ureteral stone, and stone data (all baseline variables in Table 1). The URS and SWL groups were matched 1:4 using a caliper width 0.2 of the standard deviation of the logit of the propensity score through the nearest neighbor matching (Figure S2).

Table 1 Demographics and clinical data in this cohort according to the surgery type before and after PSM
Full size table

Statistical analysis

Data were analyzed by SPSS 22.0 software. Continuous variables were presented as the median (interquartile range). Categorical variables were reported as the number (percentage). Before PSM, independent samples Student’s t test was used to compare the mean of two continuous normally distributed variables and the Mann–Whitney U test was used to compare the mean of two continuous non-normally distributed variables, The χ2 test or Fisher’s exact test was used for categorical variables. After PSM, the categorical variables were compared by binary conditional logistic regression. Paired samples Student’s t test was used to compare the mean of two continuous normally distributed variables, and the Wilcoxon test was used to compare the mean of two continuous non-normally distributed variables in univariate analysis. A p value of less than 0.05 was considered statistically significant.

Results

A total of 1230 patients were included in the final analysis and divided into two groups. We achieved a balance of baseline variables between the two groups after PSM; 998 patients treated with SWL and 232 patients treated with URS were balanced into 774 patients treated with SWL and 220 patients treated with URS. In the matched group, the median age of the patients in the SWL and URS groups was 55 and 54 years, respectively (P = 0.265). Most patients were males (64.7 vs. 61.4%). The BMI was 25.22 and 25.39 kg/m2 (P = 0.594); see details in Table 1.

The procedural duration was comparable between SWL and URS (39 vs. 41 min, P = 0.145). The rate of stone migration into the kidney in the URS group was 16.40%. The SFR at one month (87.7% vs. 83.6%, P = 0.114) and three months (96.8% vs. 98.2%, P = 0.272) was equivalent between the two groups. Of the postoperative data, there were a little lower secondary treatment rates in the SWL group than that in the URS group (10.10% vs. 12.70%, P = 0.261). The incidence of minor complications (Clavien 1–2), major complications (Clavien 3–4), SIRS, and sepsis was comparable between the two groups. The mortality (Clavien 5) of both groups was 0%. The rate of steinstrasse in the SWL group was 1.7%, which was a little higher than that in the URS group (0.00%, P = 0.084). One percent of patients in the SWL group developed renal colic (analgesic requirement), and no patients suffered from renal hematoma in both groups. No patient in the SWL group had a ureteral injury (stenosis or perforation), but the ureteral injury rate in the URS group was higher (1.4%, P = 0.011). The length of hospital stay (one day vs. two days, P < 0.001) and cost (2,000 RMB vs. 25,053 RMB, P < 0.001) in the SWL group were significantly lower than that in the URS group (Table 2).

Table 2 Treatment outcome and postoperative data in this cohort according to the surgery type before and after PSM
Full size table

Discussion

SWL and URS are the most commonly used treatment modalities for proximal ureteral stones; however, previous studies with a prospective study design comparing these two techniques were limited with a small sample size, and the conclusions were only suitable for radiopaque stones. The present study aimed to compare the effectiveness, safety, and cost between ultrasound-guided SWL with the early second session protocol and URS in patients with proximal ureteral stones. Furthermore, this prospective PSM cohort provided a fair comparison. SWL had equivalent effectiveness and increased safety and less cost compared with URS, whether the stones were radiopaque or radiolucent.

In this study, we found that the SFR was equivalent between the SWL and URS groups at one (88.7% vs. 83.6%, P = 0.114) and three months (96.8% vs. 98.2%, P = 0.272). In agreement with this finding, in a RCT that enrolled 108 patients with radiopaque proximal ureteral stones (< 1.5 cm in size and ≤ 1,000 Hounsfield units), Ahmed et al. [12] found that there were no statistical differences between the early second session SWL (within 48–72 h of the first session) and flexible-URS SFR (92.6% vs. 96.2%, P = 0.418) and an auxiliary procedure (5.6% vs. 3.7%, P = 0.640). Therefore, they concluded that SWL is safe and effective compared to flexible URS. In another RCT that enrolled 200 patients with radiopaque proximal ureteral stones ≤ 20 mm in size, Salem et al. [11] demonstrated that the SFR was comparable between SWL and semirigid URS for stone size ≥ 1 cm (60% vs. 88%, P > 0.05) or < 1 cm in size (80% vs. 100%, P > 0.05). They suggested SWL should be the first-line therapy for proximal ureteral stones < 1 cm in size because SWL is less invasive, while URS is an acceptable treatment modality for proximal ureteral stones, especially in stones ≥ 1 cm in size.

In a retrospective study including 400 patients with proximal ureteral stones < 20 mm in size, Iqbal et al. [18] found that the SFR for SWL was inferior to rigid URS with pneumatic lithotripsy after the first (62.5% vs. 84%, P = 0.001) and second session (86% vs. 94%, P = 0.01). The main reasons that accounted for these discrepancies were that the patients were treated with an early second session SWL protocol under real-time ultrasound localization. This protocol significantly improved the effectiveness of fragmentation and subsequently enhanced the SFR [17]. In addition, the retrospective design and small sample size also limited validation of the results.

In the present study, the secondary treatment morbidity was comparable between the SWL and URS groups (10.10% vs. 12.70%, P = 0.261). In a meta-analysis that included 11 studies focusing on secondary procedures, 880 patients with proximal ureteral stones were treated primarily with SWL and 787 were treated primarily with URS. The secondary treatment rate varied greatly between studies, ranging from 2.6 to 45% in the primary SWL group and 4%-50% in the primary URS group [7]. The relatively low rate in the present study may be attributed to the early second session SWL protocol, which had advantages in achieving SFR and reducing complications.

The assessment of complications is essential for further comparisons of both interventions. In this study, morbidity was low and comparable between the SWL and URS groups, except for ureteric injuries. There was no ureteric injury observed in the SWL group, whereas 3 of 220 patients treated with URS had ureteric injuries (0.0 vs. 1.4%, P = 0.011), which is reported to be the most severe complication during the URS process. In agreement with this finding, Castro et al. [19] reported a 1.1% ureteral perforation rate in the treatment of proximal ureteral stones based on a prospective study enrolled in > 9600 patients from multicenters. The mechanism may be that the proximal ureter had lower levels of tensile strength, which were more likely to cause ureteral perforations [20]. In addition, the invasiveness of ureteroscopy may lead to mucosal edema or injury. The patients with ureteric injuries were treated successfully with the placement of a stent. Although the incidence of steinstrasse and renal colic was a little higher in the SWL group than in the URS group, the incidence was low and there was no statistical difference between the two groups. In addition, these complications were immediately and successfully treated by the early second session SWL protocol.

This study demonstrated that SWL was less cost than URS, with a significantly shorter length of hospital stay and a lower mean cost. Similar results have been described by Zhang [14], who reported that the SWL requires lower costs (USD 445 vs. 1327, P < 0.001) and a shorter period of hospitalization (5.4 vs. 6.6 days, P < 0.001) compared with the URL in an RCT that recruited 155 patients with proximal ureteral stones. The addition of special instruments developed for URS, such as tipless nitinol baskets, also increases the cost. In addition, the requirement for expensive repairs, especially with respect to the learning curve of the URS, should also be kept in mind in the management of these patients. SWL has the benefit of avoiding double “J” stent fixation and removal, general anesthesia, and the associated complications and cost. Therefore, the medical costs were also dramatically reduced. In addition, all patients in the SWL group were treated as day surgery patients in this study and could resume normal activities as soon as possible.

There were several limitations in the present study. First, this was a non-randomized study from a single situation with inherent selection bias; however, the known confounders were controlled by PSM in this study. Second, no available data were found about stone composition; however, we adjusted the computed tomography (CT) value to obtain reliable results. Third, the stone-free status was evaluated by a combination KUB X-ray and ultrasound instead of CT, which might result in an overestimation of the SFR. Fourth, we were not able to report long-term complications, such as the incidence of ureteral stenosis. Nevertheless, this is the first large-scale PSM cohort study to explore the effectiveness, safety, and cost between URS and short-interval SWL for proximal ureteral stones. In addition, SWL is preferred for treating proximal ureteral stones during the COVID-19 pandemic because SWL is performed by an outpatient procedure with fewer medical resources consumed. Further multicenter RCTs are needed to accurately compare these two treatment modalities.

Conclusion

This prospective PSM cohort demonstrated that ultrasound-guided SWL with an early second session protocol had equivalent effectiveness but better safety and lower cost compared with URS in the treatment of patients with proximal ureteral stones, whether the stones were radiopaque or radiolucent. These results will facilitate treatment decisions for proximal ureteral stones.