The Clinical Application of New Generation Super-Mini Percutaneous Nephrolithotomy in the Treatment of ≥20 mm Renal Stones

Abstract

Purpose:

To evaluate the safety and efficacy of new generation super-mini percutaneous nephrolithotomy (New-SMP) in the treatment of ≥20 mm renal stones.

Methods:

We retrospectively analyzed the New-SMP procedures (14F) performed in the cases with ≥20 mm renal stones between April 1, 2016 and July 1, 2018. The cases with ipsilateral Double-J stent and/or nephrostomy tube preoperatively, ipsilateral ureteral stone, uncorrected coagulopathy, active urinary tract infection, congenital abnormalities, and urinary diversion were excluded.

Results:

Of totally 188 included cases, 8 (4.3%) were children (≤14 years) and 180 (95.7%) were adults. The cases had a mean age of 47.14 ± 15.13 years, a mean stone size of 31.57 ± 9.8 mm, and a mean S.T.O.N.E score of 7.02 ± 0.73. The New-SMP took a median operative time of 35 minutes (range 6–127). A total of 173 (92%) cases received single-access-tract procedure. The drop in hematocrit was 14.62 ± 8.36 g/L after the procedure. The serum white blood cells (WBCs) increased to 2.58 ± 2.89 × 10⁹/L. The mean hospital stay was 2.4 ± 1.5 days. New-SMP had a stone-free rate of 84% within 48 hours and 91.5% at 3 months postoperatively. Five (2.7%) cases required auxiliary procedures. The tubeless rate was 87.2%, including 44.1% cases with total tubeless, 41.5% cases with Double-J stent, and 1.6% cases with ureteral catheter. Totally 9.6% cases got fever (>38°C), 1.0% cases had urosepsis, and no cases developed shock. In addition, no transfusion and arterial embolization were required.

Conclusions:

New-SMP could be a safe and efficacious approach for the management of ≥20 mm renal stones. The novel modified technique and system may extend the indication of SMP to large renal stones. Further clinical studies and direct comparisons between New-SMP and other available modalities of percutaneous nephrolithotomy are required.

Introduction

Percutaneous nephrolithotomy (PCNL) is the first choice for the treatment of renal stones ≥20 mm.¹ However, PCNL has some severe complications such as bleeding.² The size of percutaneous access tract has been proven to be closely related to the risk of bleeding complications.³ Efforts to decrease the complications and increase the efficacy of PCNL were focused on access size and sheath design. In the past few years, Desai et al. developed microperc with an all-seeing needle and a 4.85 Fr tract without a working sheath.⁴ Then, Desai et al. introduced ultra-mini percutaneous nephrolithotomy (UMP) with a 11–13F metal sheath and a 6F mininephroscope to manage <20 mm renal stones with high stone-free rate (SFR) and low complication rates.⁵

Our group designed a miniature endoscopic system called “super-mini percutaneous nephrolithotomy” (SMP) that consists of a modified 10–14F access sheath with a suction–evacuation function and a 7F nephroscope with enhanced irrigation.⁶ The special suction–evacuation system of the SMP was designed and aimed to improve the efficiency of removing the fragments and lower the intrapelvic pressure. Of note, the access sheath of SMP was made of clear plastic material, which would cause sheath damaged easily. This first generation SMP was proved to be good at dealing with <25 mm renal stones.⁶

These novel-designed PCNLs with super-mini tracts to some extent compensated for disadvantages of PCNL. However, these novel PCNL instruments were just suitable for moderate-sized kidney stones. The reason was that super-mini access tracts might cause longer operating time for large-sized kidney stones, which could further lead to higher risk of getting infectious complications. Recently, our group introduced a modified SMP technique and system.⁷ The most remarkable feature of the new generation super-mini percutaneous nephrolithotomy (New-SMP) was a metal irrigation–suction sheath that made the new system have higher efficiency than the old system.⁸ Therefore, we tried to perform New-SMP on large-sized renal stones. In this study, we retrospectively evaluated the efficacy and safety of New-SMP in the treatment of ≥20 mm renal stones.

Patients and Methods

This study was approved by the Institutional Ethics Committee. We retrospectively reviewed New-SMP procedures performed between April 1, 2016 and July 1, 2018. Patients were informed that this was a modified novel technique. The risks and benefits were explained and written informed consent was obtained from each of the participants or their legal guardians. Cases with renal stones ≥20 mm were included. The stone size was defined as the largest diameter of a single stone on plain abdominal radiograph of the kidney ureters and bladder (KUB) and noncontrast CT (NCCT). The abdominal windows in NCCT were used for largest diameter stone size measurement. In the case of multiple stones, the summation of the largest diameters of the stones was used. We excluded the cases with ipsilateral Double-J stent and nephrostomy tube, uncorrected coagulopathy, active urinary tract infection, ipsilateral ureteral stone, congenital abnormalities, and urinary diversion.

All the procedures were performed by the four experienced surgeons who perform >350 PCNL procedures per year in our center. All the SMP procedures were performed in prone position. The miniaturized nephroscope, irrigation–suction sheath, technique, and procedures of New-SMP have been well described in our published study.⁷ The basic components of the SMP system (Hawk, China) are an 8.0F miniaturized nephroscope with a newly designed irrigation–suction sheath (12 or 14F). The puncture was made with fluoroscopy, ultrasonography, or both, which depended on surgeons' propensity. A 550 μm laser was routinely used for stone fragmentation. A 0.8 mm pneumatic lithotripter probe was used only when there was impacted stone in the ureteropelvic junction (UPJ) or fragments fell down to the upper ureter to avoid damaging the ureter by laser. The irrigation fluid pressure was set as 200–250 mmHg. The suction pressure was controlled to a setting of 100–150 mmHg. Therefore, the pressure of irrigation fluid was a little higher than that of the suction, which maintained a sufficient intra-renal space for procedure. Of note, the irrigation channel and suction channel are separate. Fragments were sucked out using the irrigation–suction sheath. Baskets were used only when the fragments fell down to the middle or distal ureter. A 10F nephrostomy tube was placed when severe bleeding occurs or there were residual stones. A Double-J stent or ureteral catheter was placed only when there was the presence of impacted stone in the UPJ, tiny fragments in the ureter, minor/moderate bleeding or perforation of collecting system. The Double-J stent was routinely placed antegradely. The Double-J stent would be removed 2–4 weeks after surgery, whereas the ureteral catheter was removed at postoperative day 1 (POD 1). The operating time was defined as the time from the first puncture to remove the metal sheath or the placement of the nephrostomy tube. Stone-free status was defined as either the absence of any residual stone fragments or the presence of clinically insignificant residual stone fragments (CIRFs) in the kidney, which were defined as ≤4 mm, asymptomatic, nonobstructive, and noninfectious stone particles. KUB and renal ultrasound were used to assess for primary SFR within 48 hours postoperatively. If there was a discrepancy in imaging between the KUB and renal ultrasound, a non-contrast CT with 2 mm section thickness would be better to discover the presence of residual stones. The NCCT with 2 mm section thickness was performed to assess the final SFR at 3 months after surgery.

Results

Totally, 188 cases were included in this study. Table 1 lists patients' demographic data and stone characteristics. Eight (4.26%) cases were children (≤14 years) and 180 (95.74%) cases were adults. The patients had a mean age of 47.14 ± 15.13 years. The mean stone size was 31.57 ± 9.8 mm. The mean S.T.O.N.E score was 7.02 ± 0.73. There were 166 (88.3%) cases with 20–40 mm stone size (Group 1) and 22 (11.7%) cases with >40 mm stone size (Group 2).

Table 1.

The Patients' Demographic and Preoperative Clinical Characteristics

Variable	Value
No. of patients	188
20–40 mm	166 (88.3%)
>40 mm	22 (11.7%)
Age
Children (≤14 years) (n, %)	8 (4.3%)
Adults (>14 years) (n, %)	180 (95.7%)
Mean (SD, range) age, years	47.14 (15.13, 1–69)
Gender
Male	115 (61.17%)
Female	73 (38.83%)
Mean BMI (SD, range), kg/m²	23.76 (3.79, 13.2–36.2)
Mean (SD, range) stone size, mm	31.57 (9.8, 20–72)
Mean S.T.O.N.E score (SD, range)	7.02 (0.73, 5–9)
Mean pre-Hb (SD, range), g/L	139.95 (16.65, 92–184)
Mean pre-WBC (SD, range), μM	6.93 (1.75, 3.52–13.94)
Mean pre-creatinine (SD, range), μM	86.08 (37.15, 20.1–291.6)
Laterality
Right	95 (50.5%)
Left	93 (49.5%)
Hydronephrosis
None	17 (9%)
Mild (G1/G2)	117 (62.2%)
Moderate (G3)	47 (25%)
Severe (G4)	7 (3.8%)
Urine culture
Positive	36 (19.1%)
Negative	152 (80.9%)
No. of stones
Single	61 (32.4%)
Multiple	127 (67.6%)
Comorbidities
Yes	50 (26.6%)
No	138 (73.4%)
Surgical history (URL/RIRS/PCNL)
Yes	70 (37.2%)
No	118 (62.8%)

WBC = white blood cell; URL = ureteroscopy lithotripsy; RIRS = retrograde intrarenal surgery; PCNL = percutaneous nephrolithotomy.

Tables 2 and 3 give intraoperative and postoperative information. The New-SMP took a median operative time of 35 minutes (range 6–127). The median operative time of Groups 1 and 2 was 34.5 minutes (range 10–127) and 45 minutes (range 12–121), respectively. A total of 173 (92%) cases received single-access-tract procedure. Four (2.1%) cases were punctured above the 11th rib and treated with tubeless. They recovered without any postoperative respiratory symptom such as respiratory difficulty and shoulder pain. The mean drop in hematocrit was 14.62 ± 8.36 g/L after the procedure. The increase of serum white blood cells (WBCs) was 2.58 ± 2.89 × 10⁹/L. The mean postoperative hospital stay was 2.4 ± 1.5 days. Group 1 stayed for 2.3 ± 1.42 days and Group 2 stayed for 3 ± 1.85 days. New-SMP had an SFR of 84% within 48 hours (primary SFR) including 2.7% CIRFs and 81.3% no residual stones, as well as 91.5% SFR at 3 months (final SFR), including 5.9% CIRFs and 85.6% no residual stones postoperatively. For the subgroup, Group 1 had a primary SFR of 86.7% and a final SFR of 92.8%. In Group 2, the primary SFR was 63.6% and the final SFR was 81.8%. Five (2.7%) cases required auxiliary procedures such as ureteroscopy lithotripsy, second-look SMP, and extracorporeal shock wave lithotripsy (SWL). The tubeless rate of New-SMP was 87.2%, including 44.1% cases with total tubeless, 41.5% cases with Double-J stent, and 1.6% cases with ureteral catheter. Moreover, Group 1 had a tubeless rate of 88% and Group 2 had a tubeless rate of 81.8%. The rate of infectious complications was 10.6%. Specifically, 18 (9.6%) cases got fever (>38°C), 2 (1.0%) cases had urosepsis, and no cases developed shock. In addition, no transfusion and arterial embolization was required. According to Clavien–Dindo grade system, there was no Clavien–Dindo grades III–IV. Twelve (6.4%) cases belonged to Clavien–Dindo grade I because of vomit, fever, and pain; 18 (9.6%) cases needed additional antibiotics and belonged to Clavien–Dindo II. The two most prevalent stone compositions were calcium oxalate accounting for 76.6% and uric acid accounting for 12.2%.

Table 2.

Patients' Intraoperative Information

Variable	Value
Median operative time (range), min	35 (6–127)
20–40 mm	34.5 (6–127)
>40 mm	45 (12–121)
Target calix
Lower	23 (12.2%)
Middle	96 (51.1%)
Upper	69 (36.7%)
Site of puncture
Above 11th rib	4 (2.1%)
Above 12th rib	103 (54.8%)
Under 12th rib	81 (43.1%)
Puncture under
C arm	138 (73.4%)
Ultrasound	50 (26.6%)
No. of accesses
Single	173 (92%)
Multiple	15 (8%)
Tubeless rate, n/N (%)	164 (87.2%)
(1)
20–40 mm	146/166 (88.0%)
>40 mm	18/22 (81.8%)
(2)
Total tubeless	83 (44.1%)
Double-J stent	78 (41.5%)
Ureteral catheter	3 (1.6%)
Transfusion	0
Intervention/embolization	0
Energy source
Holmium: YAG laser	177 (94.1%)
Pneumatic lithotripsy	6 (3.2%)
Mix	5 (2.7%)

Table 3.

Patients' Postoperative Information

Variable	Value
Drop in hematocrit (SD, range), g/L	14.62 (8.36, 1–39)
20–40 mm	14.19 (7.92, 1–38)
>40 mm	17.82 (10.83, 1–39)
Increased serum WBC (SD, range),10⁹/L	2.58 (2.89, −5.19–12.32)
20–40 mm	2.57 (2.58, −5.19–12.32)
>40 mm	2.61 (3.21, −1.77–8.61)
Mean postoperative hospital stay (SD, range), days	2.4 (1.5, 0.5–9.0)
20–40 mm	2.3 (1.42, 0.5–8)
>40 mm	3 (1.85, 1–9)
Primary SFR within 48 hours postoperatively, n/N (%)	158 (84%)
CIRFs	5/188 (2.7%)
No residual stones	153/188 (81.3%)
20–40 mm	144/166 (86.7%)
>40 mm	14/22 (63.6%)
Final SFR at 3 months postoperatively, n/N (%)	172 (91.5%)
CIRFs	11/188 (5.9%)
No residual stones	161/188 (85.6%)
20–40 mm	154/166 (92.8%)
>40 mm	18/22 (81.8%)
Requiring auxiliary procedure, n/N (%)	5 (2.7%)
URL	2 (1.1%)
Second-look SMP	2 (1.1%)
SWL	1 (0.5%)
Clavien–Dindo grade
Clavien–Dindo grade I
Antiemetics: 1, antipyretics: 2, and analgesics: 9	12 (6.4%)
Clavien–Dindo grade II
Additional antibiotics for fever and/or urosepsis	18 (9.6%)
Infectious complications	20 (10.6%)
Fever (>38%)	18 (9.6%)
Urosepsis	2 (1.0%)
Uroseptic shock	0
Stone composition
Calcium oxalate	144 (76.6%)
Uric acid	23 (12.2%)
Carbonate apatite	10 (5.4%)
Struvite stones	4 (2.1%)
Cystine	4 (2.1%)
Calcium phosphate	2 (1.1%)
Ammonium urate	1 (0.5%)

CIRFs = clinically insignificant residual stone fragments.

Discussion

Although conventional PCNL has a high efficiency to treat ≥20 mm renal stones, with >90% SFR, it also causes some severe complications such as bleeding.^1,3 Severe bleeding needs arterial embolization, which might impair the renal function.⁹ The size of percutaneous access tract has been proven to be closely related to the risk of bleeding complications.³ Mini-PCNL was first introduced by Jackman in 1998 to treat pediatric renal stone with the aim to lower the morbidity.¹⁰ Mini-PCNL was gradually applied to adults.¹ In recent years, urologists have been still attempting to reduce size of the sheath and modify the sheath.^4,5 They aimed to reduce the risk of bleeding. However, the limitations of these PCNLs with super-mini/micro access sheath could not manage larger stones because of low efficiency.

In 2014, our group introduced first generation SMP consisting of a 10–14F access sheath with a suction–evacuation function and a 7F nephroscope with enhanced irrigation.⁶ The special suction–evacuation system of SMP improved the efficiency of removing the fragments and lowered the intrapelvic pressure. The first generation SMP was proved to be a safe and effective procedure for <2.5 cm renal stones. The first generation SMP was then compared with retrograde intrarenal surgery for the treatment of 1–2 cm lower pole stones. The results demonstrated that SMP had a better SFR and lesser auxiliary rate but more postoperative pain.¹¹ The first generation SMP was also a safe treatment option for pediatric renal stone, with high efficacy, less times of surgery, and general anesthesia.¹² The first generation SMP was a revolutionary and useful technique; however, there were two main deficiencies in this system. One is that the access sheath of SMP was made of clear plastic material, which would cause sheath damaged easily. Thus, it was difficult for the plastic sheath to access the target calix with an acute angle. The other deficiency is when sucking fragments out of the sheath using the negative pressure system, the irrigation fluid from the nephroscope might push the fragments back to the collecting system, which lowers the efficiency of removing the fragments.

To overcome these two defects of first generation SMP, our group modified the access sheath system by using a metal straight sheath and “handle.”⁷ The straight sheath had a two-layered metal structure as a channel for irrigation. The straight sheath had side holes at the distal tip, which allow egress of irrigation. Without irrigation from the nephroscope, this new system had one-direction suction, which improved evacuation of fragments. Another critical point was that the working channel of the 3.3F nephroscope could be used only for instruments, therefore, 550 μm laser fiber or 1.0 mm pneumatic lithotripter probe can be used, which to a large extent increases efficiency of fragmentation and shortens operating time. The system of New-SMP and first generation SMP were compared.⁸ The results showed that New-SMP had a shorter operating time and postoperative hospital stay. With this new generation SMP system, we attempted to perform New-SMP in the treatment of ≥20 mm renal stones in the past 2 years. In this study, we retrospectively analyzed 188 cases and summarized our experience. The results indicated that New-SMP took a median operative time of 34.5 minutes (range 10–127) for 20–40 mm size stones and 45 minutes (range 12–121) for >40 mm size renal stones, which was similar to mini-PCNL.¹³ We decreased the size of access sheath for the purpose of lowering the risk of severe bleeding, the result showed that no transfusion and arterial embolization was required. Moreover, the patients who received New-SMP treatment recovered quickly with a 2.4-day postoperative hospital stay. The reason could be that New-SMP had a high tubeless rate of 87.2%. Lower bleeding, shorter hospital stay, and high tubeless rate were attributed to smaller nephrostomy tracts. However, high renal pelvic pressure and increased risk of getting urosepsis would be a concern for most urologists when using small tracts. Our previous study indicated that the renal pelvic pressure in the New-SMP system remained <30 mmHg.¹⁴ The data of this study revealed that 9.6% cases got fever (>38°C), 1.06% cases had urosepsis, and no cases developed shock. This result was similar to the reported infectious complications of PCNL.² In addition, there were four cases whose puncture sites was above the 11th rib probably suffering from pleural damage.¹⁵ However, they were treated with tubeless and recovered without any postoperative respiratory symptom such as respiratory difficulty and shoulder pain. A possible explanation is that the sheath size of SMP was small (diameter is up to 4 mm) and the continuous negative pressure of the irrigation–suction system, which would maximally relieve the symptoms. Fourteen (63.6%) cases with >40 mm renal stones had no residual stone within 48 hours postoperatively. However, there were 18 (81.8%) cases treated with tubeless. The reason why small residual stones in four tubeless cases could not be detected intraoperatively was that the primary composition of these cases was uric acid. In these four cases Double-J stent was placed and 2 mm NCCT scan indicated no residual stones 2 months after removal of Double-J stent. For large renal stones (>40 mm), New-SMP had an 81.8% final SFR and a mean 3-day postoperative hospital stay, which implied that the indication of New-SMP could be extended to larger renal stones. However, the number of cases with large renal stones in this study was limited. In addition, a randomized controlled trial could be carried out to compared the clinical application between New-SMP and mini-PCNL or standard-PCNL for the treatment of large renal stones in the next study.

Some other limitations in this study are as follows. This study was a retrospective study. Important information such as visual analogue score was missed. We are planning to conduct a multicenter prospective cohort study comparing New-SMP, minimally invasive PCNL, and standard PCNL techniques.

Conclusions

New-SMP could be a safe and efficacious approach for the management of ≥20 mm renal stones. The novel modified technique and system could extend the indication of SMP to large renal stones. Further clinical studies and direct comparisons between New-SMP and other available modalities of PCNL are required.

Footnotes

Acknowledgments

This work was financed by grants from the National Natural Science Foundation of China (Grant Nos. 81602541, 81670643, and 81402430) and Natural Science Foundation of Guangdong Province (Grant No. 2016A030310276).

Author Disclosure Statement

No competing financial interests exist.

Abbreviations Used

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