Clinical Comparison of Mini-Percutaneous Nephrolithotomy with Vacuum Cleaner Effect or with a Vacuum-Assisted Access Sheath: A Single-Center Experience

Abstract

Purpose:

To compare outcomes of two different miniaturized percutaneous nephrolithotomy (PCNL) techniques: minimally invasive PCNL (MIP) with the vacuum cleaner effect and vacuum-assisted mini-PCNL (vmPCNL).

Materials and Methods:

Data from 104 (66.7%) patients who underwent vmPCNL and 52 (33.3%) patients who underwent MIP at a single tertiary referral academic center between January 2016 and December 2019 were analyzed. Patient demographics and peri- and postoperative data were recorded, and propensity score matching was performed. Descriptive statistics and linear regression models were used to identify variables associated with operative time (OT) and patient effective dose. Logistic regression analyses were used to identify factors associated with infectious complications and stone-free (SF) status.

Results:

Patient demographics and stone characteristics were comparable between groups. vmPCNL was associated with shorter OT (p < 0.001), fluoroscopy time, and patient effective dose (4.2 mSv vs 7.9 mSv; p < 0.001). A higher rate of infectious complications was found in the MIP group (25.0% vs 7.7%, p < 0.01). Linear regression analysis showed that stone volume, multiple stones, and MIP procedure (all p values ≤0.02) were associated with longer OT. Similarly, OT and the MIP procedure (p ≤ 0.02) were associated with higher patient effective dose. Logistic regression analysis revealed that the stone volume, positive preoperative bladder urine culture, and MIP procedure (all p values ≤0.02) were associated with postoperative infectious complications. vmPCNL was not associated with the SF rate.

Conclusions:

Mini-PCNL performed with continuous active suction is associated with lower rates of infectious complications, shorter OT, and lower patient effective dose than MIP.

Introduction

Renal stones are an increasingly common urological pathology, affecting 10% of the population of developed countries and presenting an ascending trend in recent years.¹ The surgical treatment of kidney calculi varies according to the size of the stone and patient's characteristics. Current guidelines suggest percutaneous nephrolithotomy (PCNL) as the gold standard surgical technique for large kidney stones in adult patients.² Several studies have shown that PCNL is a highly effective procedure,³ but it can be associated with serious complications, including postoperative fever (10.8%), major bleeding with the need of blood transfusion (7%), pneumothorax (1.5%), and sepsis (0.5%).⁴

Recently, the use of miniaturized instruments in PCNL has gained popularity with the promise of decreasing treatment-related complications while maintaining good outcomes. Miniaturized PCNL was found to be as equally effective as standard PCNL in terms of stone clearance⁵ and associated with lower blood loss and transfusion rates.⁶ However, limitations associated with the use of smaller tract sizes are longer operative time (OT), decreased visibility,⁷ difficulty in stone retrieval, and increased intraoperative renal pressure (IRP), with consequent higher rates of infectious complications.⁸

To overcome these drawbacks, new technologies have been introduced with the aim of permitting simultaneous irrigation flow under low-pressure conditions. The minimally invasive PCNL (MIP) system (Karl Storz & Co. KG, Tuttlingen, Germany)⁹ was designed to be an open low-pressure irrigation system and proposes the innovative method of stone clearance by the vacuum cleaner effect.^10,11 Several studies have confirmed the efficacy of MIP in terms of the stone-free (SF) rate as well as its lower rate of major complications when compared with standard PCNL.^12,13

One of the most recent developments in the miniaturized PCNL armamentarium is the vacuum-assisted access sheath (ClearPetra; Welllead Medical, China). It consists of a 16F sheath externally plugged and equipped with a lateral oblique arm connected to an aspiration system, which allows continuous irrigation and lapaxy by aspirating the stone fragments together with the irrigation fluid during lithotripsy.

Overall, both the MIP and vacuum-assisted mini PCNL (vmPCNL) have proved excellent safety and efficacy when compared with classic mini-PCNL,¹⁴ but no study has ever compared the two techniques. Therefore, the aim of this study was to compare the safety and efficacy of MIP with vmPCNL for treatment of kidney stones.

Materials and Methods

Study cohort

We conducted a retrospective analysis of all consecutive patients who underwent PCNL for renal stones in our tertiary referral academic center between January 2016 and December 2019. Mini-PCNL represented the technique of choice when PCNL was planned, except in case of complete or almost complete staghorn stones, for which standard PCNL (24F) was indicated. Between January 2016 and August 2017, all mini-PCNL procedures were performed with the MIP set, while from September 2017 to December 2019, all procedures were performed with the ClearPetra set by two experienced (performed >150 PCNL procedures) endourologists (E.M. and F.L.). A total of 206 mini-PCNL procedures were included.

Collected data included patients' anthropometrics and medical history. Comorbidities were scored with the Charlson comorbidity index (CCI).¹⁵ The diagnosis of urolithiasis was based on a preoperative urographic CT scan, which was used for estimation of stone density (HU) and stone location. The stone volume was calculated using the ellipsoid formula (length × width × height × π × 1/6).¹⁶ Preoperative bladder urine culture was required in each case. In case of negative culture, one-shot parenteral prophylaxis was administered. Patients with asymptomatic bacteriuria started a targeted therapy 48 and 72 hours before the intervention. In cases of leukocytosis, urinary symptoms, or fever, the surgery was postponed after a full antibiotic course and negative urine culture.

Surgical techniques

The surgical technique was standardized for both surgeons. All procedures were performed under general anesthesia with the patient in the supine Valdivia position. The surgical equipment included the MIP 16F metallic sheath and dilator and the 16F ClearPetra set (namely, vmPCNL), the 12F MIP nephroscope, and the holmium laser (VersaPulse PowerSuite 100W; Lumenis, Israel). The procedure started with retrograde pyelography to assess the pelvicaliceal anatomy and placement of a ureteral catheter in the renal pelvis to inject the contrast medium. Renal puncture was performed under combined fluoroscopic/ultrasonographic control. Tract dilatation was performed in one shot¹⁷ with the MIP 16F metallic dilator or with the ClearPetra sheath assembled with its stylet. Irrigation was performed with a saline gravity bag suspended 1.5 m above the kidney level. Stone fragmentation was performed with a 550-μm holmium:YAG laser fiber, with fragmentation settings according to surgical needs. Stone fragments were evacuated using the vacuum cleaner effect during MIP or through the aspiration-assisted sheath during vmPCNL. The aspiration pressure could be regulated throughout the procedure. A flexible ureteroscope (7.9F; Olympus URF-P6, Germany) and nitinol baskets were used through the percutaneous access when residual fragments could not be removed with the mentioned devices. An 8F nephrostomy tube was used for the exit strategy in all cases, while the ureteral catheter was either left in place or removed at the end of the procedure based on the surgeon's preference.

Intraoperative and postoperative data

The number of percutaneous tracts, litholapaxy modality, OT (defined as the time from placement of the ureteral catheter until its removal), and fluoroscopy time (FT) were recorded. Using the PCXMC software, patient effective doses and equivalent dose rates of various organs were calculated based on the International Commission on Radiological Protection Publication.^18,19 Collimation was adapted for every single patient by radiography technicians according to the as low as reasonably achievable (ALARA) principles, and effective doses were calculated based on each patient's exposed body area.

The evaluated postoperative data included hemoglobin drop and length of hospital stay. Postoperative complications were graded according to the PCNL-adjusted Clavien score.²⁰ Patients were evaluated within 3 months after surgery with abdominal ultrasound (US) or CT scan to identify residual stones. US was the preferred choice in patients with simple stones and when the procedure had been carried out without complications and with an effective intraoperative evaluation of SF status, according to the surgeon's opinion. CT, instead, was indicated after more complex cases. The SF rate was defined as the absence of residual fragments >4 mm in diameter. Patients with residual fragments were offered, according to stone dimension, observation or auxiliary procedures, including second-look PCNL, extracorporeal shockwave lithotripsy, or retrograde intrarenal surgery.

We excluded patients with congenital renal anomalies (N = 10); scheduled staged procedures for large stone burden (N = 23); concomitant additional procedures other than PCNL (N = 11); endoscopic combined intrarenal surgery procedures (N = 2); and stone fragmentation performed with ballistic, US, or combined modality (N = 21). A sample of 115 (65.7%) and 60 (34.3%) patients treated with vmPCNL and MIP, respectively, with complete perioperative and follow-up data was considered for statistical analyses.

Data collection adhered to the principles of the Declaration of Helsinki. All patients signed an informed consent agreeing to share their own anonymous information for future studies. This study was conducted retrospectively, collecting data obtained for clinical purposes, and all the procedures were performed as part of routine care. Consequently, our study did not need ethical approval.

Statistical methods

To control for measurable baseline differences among patients in the two groups, adjustment was performed using 1:2 propensity score matching (nearest-neighbor analyses using a caliper width of 0.2 of the standard deviation of the logit of the propensity score). Matching is a common technique used in observational studies to select control subjects who are matched with treated subjects on controlled background covariates to reduce biases to a minimum.²¹ Propensity scores were computed by modeling logistic regression, with the odds of being in the MIP group as the dependent variable and independent variables such as age, body mass index (BMI), CCI, stone volume, and stone location. After matching, 104 (66.7%) and 52 (33.3%) individuals in the vmPCNL and MIP groups, respectively, were considered for the final analysis.

Distribution of data was tested with the Shapiro–Wilk test. Data are presented as medians (interquartile ranges) or frequencies (proportions). A 95% confidence interval was estimated for the association of categorical parameters. The statistical significance of differences in medians and proportions was tested with the Mann–Whitney test and Fisher exact test, as indicated. After matching, descriptive statistics were used to assess potential differences in terms of clinical parameters and intraoperative and postoperative characteristics between the MIP and vmPCNL groups.

Univariable and multivariable linear regression models tested the association between clinical variables and OT and patient effective dose in the whole cohort. Similarly, logistic regression analyses were used to identify factors associated with stone-free status and infectious complications after PCNL. Statistical analyses were performed using SPSS, v.26 (IBM Corp., Armonk, NY). All tests were two-sided, and the statistical significance level was determined at p < 0.05.

Results

There was a significant difference in stone volume and stone location between the vmPCNL and MIP groups before matching (Table 1). After matching, patients and perioperative characteristics were evenly distributed.

Table 1.

Demographic Characteristics and Descriptive Statistics of Patients According to the Type of Surgery

	Before propensity score matching		p^a	After propensity score matching		p^a
	vmPCNL	MIP	p^a	vmPCNL	MIP	p^a
No. of individuals (%)	115 (65.7)	60 (34.3)		104 (66.7)	52 (33.3)
Age, years			0.1			0.7
Median (IQR)	57.0 (49 to 76)	56.0 (48 to 76)		56.0 (49 to 75)	56.0 (50 to 75)
Range	23 to 82	23 to 81		23 to 81	23 to 81
Male, gender, n (%)	70 (60.8)	43 (71.6)	0.2	62 (59.6)	37 (71.2)	0.3
BMI, kg/m²			0.4			0.8
Median (IQR)	24.9 (22.1 to 27.9)	26.3 (24.1 to 29.7)		24.9 (22.5 to 27.7)	24.9 (23.2 to 28.0)
Range	17.9 to 41.1	19.2 to 42.8		18.5 to 41.1	19.2 to 39.6
CCI (score)			0.4			0.6
Median (IQR)	0.0 (0.0)	0.0 (0.0)		0.0 (0.0)	0.0 (0.0)
Mean (SD)	0.5 (0.2)	0.7 (0.5)		0.5 (0.3)	0.6 (0.4)
Range	0 to 5	0 to 5		0 to 5	0 to 5
CCI ≥1, n (%)	34 (29.5)	18 (30.0)	0.5	29 (27.9)	17 (32.7)	0.5
Laterality, n (%)			0.6			0.7
Right	53 (46.1)	26 (43.3)		47 (45.1)	22 (42.3)
Left	62 (53.9)	34 (56.7)		57 (54.9)	30 (57.7)
Stone volume, cm³			0.02			0.8
Median (IQR)	1.7 (1.3 to 3.6)	2.5 (1.8 to 3.1)		2.1 (1.0 to 3.5)	2.2 (1.0 to 3.5)
Range	0.5 to 19.0	0.4 to 20.1		0.5 to 19.0	0.4 to 18.9
Stone area, cm²			0.03			0.6
Median (IQR)	1.5 (1.1 to 2.8)	2.2 (1.5 to 2.8)		1.7 (1.4 to 3.1)	1.8 (1.4 to 3.2)
Range	0.5 to 7.7	0.4 to 7.8		0.5 to 7.7	0.4 to 7.8
Single stone, n (%)	44 (38.2)	29 (48.4)	0.04	41 (39.4)	21 (40.3)	0.8
Stone location			0.03			0.6
Upper pole calices	21 (18.2)	13 (21.6)		15 (14.4)	7 (13.4)
Central region calices	36 (31.4)	21 (35.2)		34 (32.6)	17 (32.7)
Lower pole calices	66 (57.7)	38 (63.4)		62 (59.6)	32 (61.5)
Pelvis	47 (40.8)	24 (40.0)		42 (40.5)	21 (40.3)
Staghorn stone, n (%)	33 (28.6)	20 (33.3)	0.03	33 (31.7)	16 (30.7)	0.5
Stone density, HU			0.07			0.3
Median (IQR)	1130 (876 to 1470)	1038 (754 to 1446)		1029 (869 to 1460)	1045 (743 to 1443)
Range	122 to 2286	476 to 2065		122 to 2286	476 to 2065
Positive urine culture, n (%)	21 (18.3)	13 (21.7)	0.6	19 (18.2)	11 (21.2)	0.6

p-Value according to the Mann–Whitney test and Fisher exact test, as indicated.

BMI = body mass index; CCI = Charlson comorbidity index; IQR = interquartile range; MIP = minimally invasive PCNL; PCNL = percutaneous nephrolithotomy; vmPCNL = vacuum-assisted mini-PCNL.

Table 2 details intraoperative and postoperative characteristics among groups after matching. Overall, OT was shorter in the vmPCNL group compared with the MIP group (p < 0.001). The use of flexible ureteroscopes and baskets to complete litholapaxy was more frequently reported during MIP than vmPCNL (all p values ≤0.02). Postoperative hemoglobin drop and length of hospital stay were similar between groups. FT and patient effective dose (p < 0.001) were lower in the vmPCNL group than the MIP group. The SF rate was higher for the vmPCNL group than the MIP group (89.4% vs 78.8%; p = 0.04). Among those who were not stone free, 2 (1.9%) patients in the vmPCNL group and 3 (5.7%) patients in the MIP group underwent second-look PCNL. RIRS was performed in 2 (1.9%) and 1 (1.9%) patients in the vmPCNL and MIP groups, respectively (Table 2).

Table 2.

Intraoperative and Postoperative Characteristics of the Whole Cohort After Propensity Score Matching (N = 156)

	vmPCNL (N = 104)	MIP (N = 52)	p^a
Number of access tracts, n (%)			0.5
Single	97 (93.3)	47 (90.4)
Multiple	7 (6.7)	5 (9.6)
Operative time, minutes			<0.001
Median (IQR)	90.0 (75 to 125)	120.0 (110 to 162)
Range	36 to 210	60 to 257
Litholapaxy with basket, n (%)	40 (38.5)	48 (94.1)	<0.001
Use of flexible ureteroscope, n (%)	42 (40.4)	31 (59.6)	0.02
Exit strategy, n (%)			0.07
Nephrostomy only	82 (78.8)	33 (63.4)
Nephrostomy+ureteral catheter	22 (21.2)	19 (36.6)
Hospitalization time, days			0.07
Median (IQR)	4.0 (3.0 to 5.0)	5.0 (4.0 to 7.5)
Range	2.0 to 12.0	2.0 to 14.0
Hemoglobin drop, g/dL			0.1
Median (IQR)	−1.5 (−2.0 to −0.8)	−1.6 (−2.9 to −0.7)
Range	−5.1 to −0.1	−6.0 to −0.2
Fluoroscopy time, seconds			0.03
Median (IQR)	463.5 (303 to 664)	562.5 (360 to 836)
Range	103 to 965	165 to 1723
Patient effective dose, mSv			<0.001
Median (IQR)	4.2 (2.8 to 7.8)	7.9 (5.4 to 11.8)
Range	1.1 to 21.3	1.1 to 28.5
Dose area product, mGy × cm²			<0.01
Median (IQR)	20,298 (12,213 to 31,523)	30,684 (18,572 to 44,160)
Range	5699 to 85,680	8337 to 101,860
Follow-up imaging, n (%)			0.5
Ultrasound	63 (60.6)	34 (65.4)
CT scan	41 (39.4)	18 (34.6)
Stone-free rate, n (%)	93 (89.4)	41 (78.8)	0.04
Auxiliary procedures, n (%)			0.09
No treatment	7 (6.7)	7 (13.4)	0.07
RIRS	2 (1.9)	1 (1.9)	0.9
Second-look PCNL	2 (1.9)	3 (5.7)	0.1
Stone composition			0.01
CaOx monohydrate	46 (44.2)	7 (13.5)	0.01
CaOx dihydrate	11 (10.6)	4 (7.6)	0.7
Ca carbonate	14 (13.4)	0 (0.0)	0.1
Ca phosphate	6 (5.7)	28 (53.8)	<0.001
Uric acid	17 (16.3)	11 (21.1)	0.6
Cysteine	10 (9.8)	2 (4.2)	0.5

p-Value according to the Mann–Whitney test and Fisher exact test, as indicated.

Ca = calcium; CaOx = calcium oxalate; RIRS = retrograde intrarenal surgery.

Table 3 details postoperative complications in the whole cohort. Overall, rates of postoperative complications were similar between groups (38.5% for MIP vs 24.0% for vmPCNL; p = 0.09). Clavien-Dindo grade >II complications were found in 7 (6.7%) and 3 (5.8%) patients after vmPCNL and MIP (p = 0.1), respectively. According to the modified Clavien-Dindo classification,²⁰ a higher rate of infectious complications was found after MIP compared with vmPCNL (p < 0.01) (Table 3).

Table 3.

Postoperative Complications in the Whole Cohort After Propensity Score Matching (N = 156)

	vmPCNL (N = 104)	MIP (N = 52)	p^a
Overall complications, n (%)	25 (24.0)	20 (38.5)	0.09
Highest Clavien-Dindo grade, n (%)	0.1
I–II	18 (17.3)	17 (32.7)
IIIa–IIIb	7 (6.7)	3 (5.8)
Type of complication (highest Clavien-Dindo grade), n (%)			0.02
Pain	7 (6.7)	3 (5.8)	0.81
Bleeding	5 (4.8)	5 (9.6)	0.31
Infectious	7 (6.7)	12 (23.1)	0.003
Drainage	5 (4.8)	0 (0.0)	0.19
Other	1 (1.0)	0 (0.0)	0.47
Infectious complications
Any Clavien-Dindo grade, n (%)	8 (7.7)	13 (25.0)	<0.01

p-value according to the Fisher exact test.

Table 4 depicts linear regression models testing the association between clinical variables and either OT or patient effective dose. Multivariable linear regression analysis revealed that stone volume, multiple stone locations, and MIP surgery were significantly associated with longer OT after adjusting for BMI and stone density (all p values ≤0.02). Similarly, OT, use of multiple access tracts, and MIP surgery were significantly associated with patient effective dose (all p values ≤0.02) after adjusting for stone volume and BMI (Table 4).

Table 4.

Linear Regression Models Predicting Operative Time and Patient Effective Dose in the Whole Cohort After Matching

	UVA model, β; p-value [95% CI]	MVA model, β; p-value [95% CI]	UVA model, β; p-value [95% CI]	MVA model, β; p-value [95% CI]
Operative time			Effective dose
Age	0.4; 0.1 [−0.58 to 0.94]		0.11; 0.07 [−0.01 to 0.12]
BMI	0.25; 0.75 [−1.34 to 1.84]	0.23; 0.9 [−1.77 to 1.75]	1.02; 0.12 [−0.7 to 2.12]	1.01; 0.21 [−0.2 to 1.45]
CCI ≥1	1.01; 0.47 [−0.96 to 1.07]		1.17; 0.21 [−0.66 to 3.05]
Female, gender vs male	0.99; 0.87 [−0.98 to 1.05]		−0.43; 0.62 [−0.21 to 1.29]
Stone volume	2.76; 0.01 [1.21 to 5.34]	2.61; 0.02 [0.34 to 4.97]	0.13; 0.23 [−0.27 to 0.45]	0.13; 0.31 [−0.41 to 0.13]
Stone density, HU	1.03; 0.11 [−0.1 to 1.76]	1.03; 0.2 [−0.01 to 1.45]	1.11; 0.19 [−0.96 to 1.43]
Multiple stones vs single stone	1.65; 0.01 [1.03 to 3.45]	1.55; 0.01 [1.1 to 3.74]	1.17; 0.17 [−0.49 to 2.85]
Operative time			1.37; <0.01 [1.01 to 4.45]	1.21; 0.01 [1.02 to 2.34]
Number of access tracts			4.92; <0.01 [1.75 to 8.25]	3.59; 0.02 [1.39 to 6.81]
ClearPetra vs MIP	−4.34; <0.001 [−8.72 to −1.12]	−3.52; <0.001 [−5.72 to −1.24]	−2.97; 0.001 [−4.77 to −1.23]	−2.10; 0.01 [−3.96 to −0.26]

CI = confidence interval; MVA = multivariate model; UVA = univariate model.

Multivariable logistic regression analysis revealed that stone volume (OR 1.22, p < 0.01), positive preoperative bladder urine culture (OR 3.66, p = 0.02), and the MIP procedure (OR 3.57, p < 0.01) were independently associated with postoperative infectious complications (Table 5). Smaller stone volume (OR 0.81; p ≤ 0.001) and the absence of staghorn stones (OR 1.72; p ≤ 0.001) were found to be predictors of the SF rate after accounting for BMI, stone density, the number of involved calices, and the type of access sheath (MIP vs vmPCNL) (Table 5).

Table 5.

Logistic Regression Models Predicting Postoperative Infectious Complications and Stone-Free Status in the Whole Cohort After Matching

	UVA model, OR; p-value [95% CI]	MVA model, OR; p-value [95% CI]	UVA model, OR; p-value [95% CI]	MVA model, OR; p-value [95% CI]
Infectious complications			Stone-free status
Age	1.1; 0.72 [0.97 to 1.04]		0.95; 0.87 [0.76 to 1.34]
BMI	1.06; 0.28 [0.95 to 1.17]		0.96; 0.65 [0.72 to 2.45]	0.96; 0.76 [0.71 to 3.14]
CCI ≥1	1.23; 0.67 [0.46 to 3.28]
Female, gender vs male	0.65; 0.41 [0.24 to 1.81]
Stone volume	1.24; <0.01 [1.06 to 1.48]	1.22; <0.01 [1.05 to 1.41]	0.86; <0.001 [0.81 to 0.89]	0.81; <0.001 [0.76 to 0.89]
Stone density, HU	1.01; 0.75 [0.87 to 1.01]		0.87; 0.02 [0.79 to 0.91]	0.88; 0.31 [0.75 to 1.85]
No. of involved calices, single vs multiple			1.14; 0.07 [0.96 to 1.76]	1.12; 0.26 [0.91 to 1.27]
Staghorn stone, no vs yes			1.78; <0.001 [1.34 to 4.13]	1.72; <0.001 [1.15 to 5.01]
Positive preoperative	3.34; 0.01 [1.23 to 9.04]	3.66; 0.02 [1.24 to 7.83]
Urine culture
Operative time	1.12; 0.01 [1.03 to 1.34]
No. of access tracts	1.31; 0.12 [0.28 to 6.47]
ClearPetra vs MIP	0.25; <0.001 [0.09 to 0.65]	0.28; 0.01 [0.1 to 0.77]	1.13; 0.04 [1.02 to 2.13]	1.01; 0.23 [0.87 to 2.39]

OR = odds ratio.

Discussion

The recent tendency toward miniaturization in PCNL has raised some questions about the prolonged OT and the increased chance of infectious complications related to higher IRP compared with standard procedures with larger tracts.⁸ Conversely, several studies have shown that miniaturized PCNL is a safe and effective treatment modality for large kidney stones.⁵

In this study, we specifically compared two mini-PCNL procedures: (i) MIP, characterized by continuous low-pressure irrigation and the suction-like vacuum cleaner effect^9
–11 for litholapaxy; and (ii) vmPCNL with its continuous irrigation–aspiration system (semiclosed circuit).^14,22 To our knowledge, this is the first study that specifically compared clinical and procedural outcomes of the two techniques. We showed that PCNL performed with active aspiration was associated with lower rates of infectious complications, shorter OT, and reduced patient effective dose compared with MIP procedures.

Previous studies have shown the safety and efficacy of MIP. Nagele and colleagues first described a series of 29 patients with lower pole stones treated with MIP and reported an SF rate of 96% with excellent safety profile.²³ Similarly, Abdelhafez and colleagues²⁴ described a series of 197 MIP procedures comparing outcomes between small and large renal stones (maximum diameter cutoff of 2 cm). Authors found that hemoglobin drop and OT were lower in patients with small renal stones, but the rate of postoperative complications was similar between groups. SF status was achieved in 83.8% of patients, and retreatment was needed in 13.1% of cases.²⁴ Outcomes from our series of MIP are similar to those from previous publications in terms of OT, mean hemoglobin drop, and SF rate.²⁴ Of note, we reported a 25% rate of infectious complications in our MIP series, in line with the published literature from the general PCNL population (21%–39.8%).²⁵

Recently, patented, negative pressure aspiration systems have been described in PCNL with promising results.²² Lai and colleagues compared 75 patients treated with vmPCNL and 75 individuals who underwent PCNL with a peel-away access sheath.¹⁴ Authors showed that vmPCNL was associated with a higher SF rate, but shorter OT and fewer infectious complications, than standard procedures.

In our study, the shorter OT of vmPCNL could be related to the efficacy of active negative pressure suction in fragment retrieval when compared with the vacuum cleaner effect. Moreover, the vmPCNL group was characterized by lower rates of flexible ureteroscope and grasper/basket employment to complete litholapaxy. These aspects are of primary clinical relevance in view of reducing OT and PCNL-related costs. In this study, the SF rate was higher for the vmPCNL group than the MIP group (89.4% vs 78.8%) and the aspiration-assisted sheath was univariably associated with SF status. However, this association was not confirmed in multivariable logistic regression analyses. In multivariable analyses, the type of sheath may have a lower magnitude of effects when compared with standard prognosticators (such as stone volume, location, and staghorn stones).

The topic of radiation exposure has become increasingly important in management of renal stones.²⁶ The reduction of FT and patient effective dose observed in the vmPCNL group can be explained by several reasons. First, radiation exposure usually increases along with OT, which in turn rises along with the use of a flexible ureteroscope for fragment retrieval, all of which were advantageous for the vmPCNL group, compared with the MIP group. Second, the patient effective dose in vmPCNL might be reduced because the procedural radiation dose delivered by the beam source with a metallic set is likely to be higher compared with the procedure performed with a plastic set. Indeed, a radiopaque object placed between the beam source and the automatic exposure control sensors attenuates the beam, causing an increase in tube output to reach its air kerma threshold.^27,28 Hemoglobin drop and the rate of overall complications were similar between vmPCNL and MIP, which is consistent with other studies asserting the feasibility and safety of mini-PCNL.⁶ Of clinical importance, infectious complications were fewer in the vmPCNL group than in the MIP group. Infectious complications during PCNL have been strictly connected to IRP fluctuation.²⁹ Nagele and colleagues described that in vitro MIP contributed to lower IRP compared with a conventional 18F sealed nephroscope sheath¹⁰ and the same group has later studied how active suction of the irrigation fluid can further decrease IRP during miniaturized procedures with the MIP system.³⁰

Recently, Lai and colleagues¹⁴ showed that vmPCNL was characterized by lower IRP than nonvacuum-assisted procedures, contributing to the lower rate of infectious complications observed in this group. We can similarly speculate that in our study, the lower rate of infectious complications observed in the vmPCNL group might be related to reduced IRP¹⁴ together with shorter OT, which has been previously identified as an independent factor associated with infections after PCNL.³¹ Furthermore, we should also consider the higher prevalence of calcium phosphate stones in the MIP group than the vmPCNL group, which is usually associated with higher urine pH possibly favoring UTI and consequently infectious complications.

This study is innovative because it specifically compares mini-PCNL with a suction-like irrigation system (MIP) with vmPCNL, which are among the most innovative tools in the armamentarium of miniaturized PCNL. The second strength of the study is that we have analyzed a homogeneous cohort of patients with a thorough clinical and perioperative evaluation. In particular, most previous studies have used FT to report patient radiation exposure, which does not provide information on the actual amount of radiation exposure.³² To address this limitation in the current literature, we analyzed the patient effective dose to strengthen our results and for greater collocation in the clinical scenario.

Our study is not devoid of limitations. Although propensity score matching analysis is a valid method to reduce the selection bias of a retrospective study, the lack of randomization may limit the conclusions that can be drawn. In addition, this was a single-center-based study, which raises the possibility of selection bias; therefore, larger studies across different centers and cohorts are needed to externally validate our findings. In addition, MIP procedures were performed earlier in time than vmPCNL, therefore the surgeon's learning curve might favor the latter group. However, all PCNL procedures were performed by surgeons who were experts (performed >150 PCNL procedures) even before the beginning of this study. Last, the study lacks a standardized protocol for patients' follow-up. Although CT scan is the preferred imaging modality for assessing the SF status, in our institution, we usually weigh the need to submit the patient to additional radiation doses after surgery if not strictly necessary.

Conclusions

In conclusion, the results of this study revealed that both the MIP and vmPCNL systems are valid alternatives for the surgical treatment of kidney stones. PCNL performed with active suction was associated with lower rates of infectious complications, shorter OT, and lower patient effective dose than MIP procedures. In the future, large randomized studies are needed to validate our findings.

Footnotes

Author Disclosure Statement

No competing financial interests exist.

Funding Information

No funding was received for this article.

Abbreviations Used

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