The comparison of totally ultrasound-guided versus fluoroscopy-guided shockwave lithotripsy in renal stone treatment: a systematic review and meta-analysis

Introduction

Extracorporeal shock wave lithotripsy (SWL) has been a well-known therapy for treating urinary stones since the early 1980s.^1,2 The stone disease may affect about 10% of the global population and remains an important cause of morbidity, which provides huge opportunities for healthcare costs and discomfort to patients worldwide. For the treatment of renal or pyelonephritis stones, there is a current trend toward using minimally invasive endoscopic methods, such as ureteroscopy and percutaneous nephrolithotomy. SWL remains one of the leading treatment choices for renal stones less than 20 mm despite this progression. SWL has a low incidence of complications and does not necessitate general anesthesia.^3–5

Ultrasonography (US) (B-scan ultrasound) or fluoroscopy (FS) must be used to appropriately visualize the stone to focus the shock waves as precisely as possible for SWL to be successful (X-rays). Radiopaque stones in the kidney calyces, renal pelvis, or ureteropelvic junction (UPJ) are frequently visible on US and FS. Although the energy source and coupling devices have changed little in recent years, advances in SWL technology have led to the ultrasonic (US) localization of stones. Real-time imaging without radiation exposure is a specialty of ultrasound, especially in the visualization of radiolucent stones or in pediatric and pregnant populations. Being also the modality with the best anatomical detail, fluoroscopy is, however, preferentially applied in localizing radiopaque stones, which makes it a standard imaging modality in many centers. The combination of US and FS has improved the success rate of SWL in a few studies. By alternating ultrasound and fluoroscopy, the lithotripter’s energy can be more concentrated on the target stone throughout the entire session, enhancing SWL efficiency. Unfortunately, few studies have examined the effectiveness of totally ultrasound-based and fluoroscopy-based lithotripters. It is difficult to compare the efficacy between different institutions due to variabilities in treatment procedures and operators.⁶ Although ultrasound-guided SWL is increasingly adopted due to its reduced radiation exposure, evidence comparing its effectiveness (stone-free rate) and safety (complication rate) with fluoroscopy-guided SWL remains inconclusive. This systematic review and meta-analysis aimed to compare both outcomes in the treatment of renal stones.

Methods

This study was conducted on the guideline of Preferred Reporting Items for Systematic Review and Meta-analyses (PRISMA) 2020. The study protocol was registered in the PROSPERO database (www.crd.york.ac.uk/prospero) under registration number CRD42023403319.

Results

We retrieved 272 hits on the five online databases ( Table 1). A total of 121 duplicate articles and 100 irrelevant studies were excluded from the analysis. After screening and selecting the articles, we included eight articles in this systematic review and quantitative analysis ( Figure 1). The articles included two randomized trials and six retrospective cohort studies.^7–14 The characteristics of the included studies are presented in Table 2. We extracted any available data from the studies, including subjects’ characteristics, stone size, stone density (in Hounsfield Unite - HU), stone location, SFR definition, SWL technique, and the outcomes. The quality of the study showed a low to moderate risk of bias based on the Rob Tools and ROBINS-I Tools ( Table 3, Figure 2). The oldest study was conducted in 2010, and the most recent study was conducted in 2023. The sample size varied from 40 to 495 patients and was divided into US-SWL and FS-SWL groups. Most patients presented renal stones from the imaging examination, however, a study by Motolova et al.¹⁰ included patients with proximal and distal ureters. Given the operator-dependent nature of ultrasound, all ultrasound-guided SWL procedures in the included studies were performed by urologists. We performed a quantitative analysis of seven studies, consisting of three of the children population and four of the adult population. Funnel plot analysis and Egger’s test could not be performed because the included studies were less than 10.

Figure 1. PRISMA flow diagram of study selection.

Figure 2. Risk of Bias (Rob) Tool for randomized interventional studies.

Stone free rate

The stone-free rate is an absence of residual stones in a follow-up period after the shockwave lithotripsy procedure. The stone-free rate definition varied across studies, which might be a confounding factor in the result. Eight studies comprised 1,255 patients; 609 (48,5%) underwent ultrasound shockwave lithotripsy. The stone-free rate in US-SWL varied from 52-93%, while in FL-SWL varied from 40-90.5%. In our metaregression analysis, there were significant differences observed in the case SFR between ultrasound-guided and fluoroscopy-guided RR 0.76(95% CI; 0.61-0.95, p=0.02) ( Figure 3). We also divided the patients into adult RR and children groups. There were significant differences within the adult RR 0.76(95% CI; 0.60-0.96) but not significant in children groups RR 0.68(95% CI; 0.24-1.88), respectively. Among the included studies, three studies; Arunagiri et al., Van Besien et al., Ozkaya et al., provided subgroup results based on stone location. These suggest that SFR tended to be lower for stones in the lower calyx compared to other sites. Similarly, other studies Periasamy et al. and Smith et al., reported higher SFR in stones <10 mm. However, variations in definitions and incomplete subgroup data limited formal comparison across studies.

Figure 3. Forrest plot of stone free rate.

Complication rate

We included all studies that prove complication rate data. Five studies reported complication rates between the two groups. The reported complications were pain, transient hematuria, fever, urinary tract infection, steinstrasse, and further interventions. However, we exclude one article¹² due to outlier values. Of the four studies, three studies reported the incidence in the Clavien-Dindo classification system (grade I - V). One study only reported the complication by the need for further interventions. None of the studies reported complications of Grade IV or V (threatening complications). In terms of specific complication types, only two studies (Arunagiri and Goren et al.) provided detailed breakdowns. Arunagiri et al. presented incidence rates for hematuria, fever, UTI, and steinstrasse, while Goren et al. reported low-grade infections and fever managed conservatively. Due to inconsistent reporting across studies, a subgroup meta-analysis by complication subtype could not be conducted. In the pooled analysis, the overall complication rate between the two groups did not differ by RR 0.91(95% CI; 0.35-2.39) ( Figure 4). The management of complications is mainly pharmacological, using anti-inflammatory drugs and antibiotics, and some need further interventions such as additional SWL sessions, secondary ureteroscopy, or percutaneous nephrolithotomy. A visual presentation of the funnel plot showed no potential sources of small study effects for complication rate ( Figure 4).

Figure 4. Forrest plot of complication rate.

Discussion

US-SWL and FS-SWL employ focused shockwaves to break kidney stones. However, the two techniques differ in their imaging modalities for localizing the stone during treatment.^15–17 Ultrasound is radiation-free, reducing the risk of radiation-induced complications for patients and healthcare professionals.¹⁸ Additionally, ultrasound is a real-time imaging modality, allowing for continuous monitoring and adjustment during the procedure and providing an accurate assessment of stone fragmentation and size. Focusing the shockwaves on the stone can be accomplished to achieve optimal fragmentation.^9,19 On the other hand, FS-SWL relies on fluoroscopy for stone localization.²⁰ Despite the exposure to ionizing radiation, fluoroscopy can better visualize certain stone types, particularly those with high radiopacity. This can lead to improved treatment outcomes in specific cases.²¹

This is the first meta-analysis comparing the effectiveness (SFR) and safety (complication rates) of US-SWL and FS-SWL. For the stone-free rate outcome, we divided the analysis based on the population, adults and children. In our study, we found that US-SWL is more effective than FS-SWL in terms of stone-free rate after the procedure. The stone-free rate ranged from 52 to 93% for the US-SWL group and 40 to 90.5% for the FS-SWL groups. Studies from Goren et al.,¹⁰ Arunagiri et al.,⁸ Smith et al.,¹³ and Van Beisen et al.¹⁴ showed a higher stone-free rate in the US-SWL group than the FS-SWL group. However, in the children population analysis, the stone-free rate does not differ. This might be due to the cooperativeness during the procedure.

Similar results were also demonstrated in the complication rate. Of the seven studies, only four studies provide the measurement of complication rates. The complication was relatively low, ranging from 0,1-32%. A study by Goren et al.¹⁰ reported significantly lower complications in the US-SWL group, while other studies reported no significant difference. None of the complications was life-threatening. There were no significant differences between both groups.

The reported complications were pain, lower urinary tract symptoms, transient hematuria, fever, urinary tract infection, steinstrasse, and further interventions. The management of the complications was mainly conservative with supported medication (anti-inflammatory, analgesic, antibiotics). Subjects with Clavien Dindo Grade 3 required further intervention for ureteric stenting or endourology procedures.

Generally, numerous factors can impact the ultimate stone-free rate (SFR) outcome of extracorporeal shock wave lithotripsy (SWL), regardless of the guidance method used. The factors were divided into stone characteristics (size, composition, and location), patient characteristics (age, BMI, anatomical factors), and technique-related factors (shockwave energy and frequency, number of treatment sessions, and operator experience). Stone size, density, and locations affect the outcome of SWL. The results of SWL for renal stones up to 10 mm in diameter are satisfactory regardless of their location in the kidney.^22–24 Stone density obtained from CT KUB was demonstrated as a predictor for the success rate of SWL.^25,26 Some stones (e.g., calcium oxalate monohydrate and cystine) are harder and more resistant to fragmentation, leading to a lower SFR. Gupta et al.²⁷ found that SWL outcomes were best when the mean stone density was 750 HU. In a separate prospective study involving 50 patients with urinary stones, the author determined that a stone density threshold of 970 HU is a precise and sensitive predictor of SWL outcome.²⁸ A study by El-Nahas et al.²⁹ discovered that stone density greater than 1000 HU significantly predicts SWL failure. El-Assmy et al.³⁰ concluded that an HU value of 600 HU and a stone length of 1.2 cm were significant independent predictors of SWL efficacy when treating urinary stones in children. Additionally, Stones located in the lower pole of the kidney or lower ureter tend to have a lower SFR due to challenges in clearing the fragments.¹⁶

The patient’s age, BMI, and anatomical abnormalities may also alter the SFR. Older patients may have a lower SR due to age-related factors, such as reduced renal function or altered anatomy.¹⁹ A higher BMI can decrease the effectiveness of SWL by increasing the distance between the shockwave source and the stone. Waqas et al. found that patients with BMI <30 kg/m2 have a higher SWL success rate than patients with BMI >30 kg/m2.³¹ Anatomy abnormalities, such as patients with skeletal anomalies, renal malformations, or strictures in the urinary tract, may have a lower SFR due to difficulty reaching the stones and unfavorable fragments passage.²¹

The SF can be affected by the frequency and energy of the shockwaves, with higher energy and lower frequency generally yield better results. Among the eight included studies, only Periasamy et al. reported detailed procedural parameters, including shock wave frequency (90–110 shocks/min), total number of shocks (3000 per session), and mean energy levels (64 J in the US group and 68 J in the FS group). Van Besien et al. used a standardized ramping protocol with a fixed frequency of 1 Hz, 2500 shocks per session, and a reported mean energy of 49–53 J. Goren et al. initiated treatment at 9.5 kV with stepwise escalation. A systematic review and meta-analysis by Kang et al.³² showed that low-frequency success rates (OR 2.2; 95% CI 1.5-2.6) and intermediate-frequency SWL (OR 2.5; 95% CI 1.3-4.6) were higher than high-frequency SWL.

Multiple sessions may be required to achieve a higher SFR. Data on the number of SWL sessions were available in Periasamy et al. (mean 2.6 sessions in the US group vs. 2.7 in the FS group) and Motolová et al. (mean 1.3 sessions in the US group vs. 1.5 in the FS group). However, the remaining studies did not report these procedural parameters, limiting our ability to assess their contribution to heterogeneity. According to Goren et al.,¹⁰ the median number of SWL sessions in the US-guided group was considerably smaller than in the FL-guided group. In a cohort study by Grabsky et al.³³ in the pediatric population, the SFR after only 1- session of SWL was 88.0% and increased to 91.7% after several sessions. Several other studies also reported the high SF in SWL achieved after several treatment sessions.^34,35

SWL treatment’s effectiveness also relies upon the operator’s level of expertise. The requirement of pinpoint imaging localization of the stone and proper acoustic coupling to the flank region of the patient is essential because these factors directly influence the quality of the outcome.¹⁴ In children, the US-SWL faces technical challenges. The probe used on the shockwave lithotripter was commonly convex and adult-sized. For focusing stones on infants and children, probes sized for adults can be challenging to use. In patients with a small abdominal volume, pressing the abdomen with an adult-sized probe may shift the kidney toward adjacent organs, resulting in the coaction of surrounding tissues by the shockwaves.¹⁰

Some studies reported lower stone-free rates in lower pole stones and larger calculi, for example, Arunagiri et al. and Smith et al., but these findings were not consistently observed, as most other studies did not report outcomes by stone size or location. Similarly, only a few studies, for example, Arunagiri and Goren, described specific complications such as hematuria or steinstrasse, while most reported only overall complication rates. These patterns should be interpreted with caution, and future studies should provide more detailed and standardized reporting for better subgroup comparisons.

Conclusions

The current study found significant differences in stone-free rates of renal stone between the US-SWL and FS-SWL in the adult group but not in the pediatric groups. There is no difference in terms of complication rates between the two imaging modalities. None of the studies reported any life-threatening complications. The US-SWL is more effective than FS-SWL in treating renal and ureteral stones, with a low incidence of complications, especially in the adult population. Subgroup comparisons based on stone characteristics or complication types could not be conclusively drawn due to inconsistent data reporting across studies, highlighting the need for future studies to address these specific variables. Further randomized controlled trials with larger populations are needed to explore the comparison more accurately. We also recommend evaluating the effectiveness of both modalities in ureteral stones in future studies.

Ethics and consent

No ethics and consent were required.