Minimally Invasive Percutaneous Nephrolithotomy: Initial North American Experience

Abstract

Introduction and Objectives:

The goal of this study is to evaluate the outcomes in a cohort of patients who underwent minimally invasive percutaneous nephrolithotomy (MIP) at a single institution from 2017 to 2019.

Methods:

Sixty patients at a single institution with two different surgeons underwent MIP from 2017 to 2019. The MIP technique uses a proprietary nephroscope with a “vacuum” technique for stone evacuation. Patients were identified who had postoperative CT scan imaging available for direct review. A prospectively maintained database was queried along with retrospective chart review to evaluate the stone-free rate defined as no stones on CT imaging. Preoperative, intraoperative, and postoperative variables were analyzed including initial stone size, access type (fluoroscopic vs ultrasonic), access location, operative positioning (supine vs prone), operative time, and 60-day complications.

Results:

Forty-six of 60 patients had CT imaging postoperatively that were reviewable. Of these, 43% (n = 20) were stone free as defined by no identifiable fragments seen, 11% (n = 5) had residual fragments 0 to 2 mm, 7% (n = 3) had residual fragments 2 to 4 mm, and 39% (n = 18) had residual fragments >4 mm. Mean initial stone size was 21 mm (1.9–84 mm). Sixty percent (n = 28) of the patients were discharged the same day as surgery. Fifty-one percent (n = 24) of access was achieved through ultrasound alone. Seventeen percent of patients (n = 8) had a complication within 30 days of surgery. All complications were Clavien III or lower with unplanned return to operating room rate of 2% (n = 1).

Conclusions:

We present North America's first single institution analysis of MIP cases with acceptable outcomes comparable with both retrograde intrarenal surgery and standard percutaneous nephrolithotomy. The exact role of MIP in renal stone disease needs to be determined by future studies that critically assess their outcomes.

Introduction

Standard percutaneous nephrolithotomy (PCNL) utilizes a large-bore (24–34F) access sheath and has been a mainstay of treatment for patients with large stone burdens, typically >2 cm. In the past 10 years, mini-PCNL, which utilizes a smaller access sheath <24F, has become more widely adopted since its initial description in a pediatric population in 1998.¹ Some centers, particularly in Europe, have adopted mini-PCNL as the standard method for percutaneous stone surgery.² Criticisms of mini-PCNL compared with PCNL focus around the decreased sheath diameter that limits visibility, increases operative time, and lowers stone-free rates (SFRs).³ In contradistinction, two recent meta-analyses report mini-PCNL has higher SFRs than retrograde intrarenal surgery (RIRS) with comparable morbidity and thus the debate of utility of mini-PCNL has continued.^4,5

Ultimately, the goal of intervention for renal stones is to maximize stone removal and minimize morbidity; thus SFRs and perioperative events are critical surgical outcomes when comparing surgical modalities. Historically, SFRs have been determined with a myriad of imaging techniques each with a widely different sensitivity and specificity. Since CT scan remains the gold standard for SFR measurement, prior mini-PCNL literature has limited generalizability as CT is inconsistently utilized.^4,6

The mini-PCNL was further refined with the introduction of minimally invasive PCNL (MIP) in 2008. MIP utilizes a proprietary nephroscope that relies on “vacuum” irrigation to remove stones.⁶ Used widely in Europe, MIP was recently introduced to North America. The purpose of this analysis was to review the diffusion of the MIP technique to North America and provide a critical evaluation of the technology with a focus on “true” SFR and perioperative outcomes.

Methods

We received Institutional Review Board exemption from retrospective review of prospectively collected database from 2016 to 2019 (protocol #190883) and a waiver of individual authorization for use of Protected Health Information was granted for this study as stipulated by the HIPAA Privacy Rule, 45 CFR 164 section 512(I). Patients underwent MIP with the propriety Karl Storz™ MIP-M access sheath (Karl Storz, Tuttlingen, Germany) (17.5F outer diameter) and nephroscope described in prior studies.⁷ Patient positioning was either prone or in a modified Barts “flank-free” position.⁸ A ureteral occlusion balloon catheter or in some instances a flexible ureteroscope was placed retrograde before percutaneous access. Percutaneous access was achieved either through fluoroscopic, ultrasound, endoscopic intrarenal surgery, or combined technique. Single-step dilation with a 16F metal dilator followed by placement of the 17.5F access sheath was performed for all patients. A 12F nephroscope was used for renoscopy and either holmium:YAG laser or trilogy pneumatic ballistic lithotripsy (Boston Scientific, Marlborough, MA) was used for stone fragmentation. Fragments were evacuated using a continuous-flow hydrodynamic “vacuum” effect.⁶ Fragments refractory to vacuum removal were removed using a 1.9F Dakota™ Basket (Boston Scientific). At the conclusion of the case, a Double-J stent was placed and 5 cc of FloSeal™ (Baxter International, Deerfield, IL) was injected into the access tract. Operative time was calculated from the moment the “case” started: that is, the cystoscope is inserted or an incision is made in the flank, whichever was first. The end time is at the time of incision closure. Patients were generally discharged on the same day of surgery or on postoperative day 1. The patients followed up in clinic for stent removal (cystoscopically or by pulling string) 4 to 7 days postoperatively with subsequent imaging typically performed 4 to 8 weeks postoperatively.

We excluded from analysis subjects who did not have postoperative CT imaging. We performed chart review for the following information: preoperative stone burden, residual stone burden, operative time, patient positioning, percutaneous access technique, complications within 60 days, emergency department visits within 60 days, hospital readmission within 60 days, and unplanned procedures within 180 days.

Preoperative stone burden was grouped into three groups: (1) 0 to 10 mm, (2) >10 to 20 mm, and (3) >20 mm. We grouped residual stone size into four groups: (1) stone free that was defined as no visible fragments seen on postoperative CT, (2) total residual burden <2 mm, (3) total residual burden 2 to 4 mm, and (4) total residual burden >4 mm. Postoperative CTs were completed within postoperative day 1 and 3 months postoperation.

Complications were reviewed and categorized according to the Clavien–Dindo classification system modified for percutaneous procedures.⁹

Results

Upon review of electronic medical records, 60 patients underwent MIP from 2017 to 2019. Of those patients, 46 had postoperative CT imaging to determine SFRs. Preoperatively, the mean (standard deviation [SD], range) stone size was 21.3 mm (16.5, 1.9–84). Sixty-three percent of patients had stones 0 to 20 mm, whereas 37% had stones >20 mm. The majority (63%) of these stones was in the lower pole.

Mean operative time (SD, range) was 118 minutes (34, 57–192 minutes). Sixty-one percent of patients were discharged on the same day as surgery. Thirty-five percent of patients were discharged on postoperative day 1 and 7% were discharged beyond postoperative day 1 (Table 1). The median time to stent removal for these patients was 5 days.

Table 1.

Pre and Intra-Operative Variables

Total number	46
Mean age, years	60
Male, n (%)	23 (50)
Female, n (%)	23 (50)
Preoperative stone size
1–10 mm	6
10–20 mm	23
>20 mm	16
% 0–10 mm	13
% 10–20 mm	50
% >20 mm	35
Mean size	21.32
SD	16.75
Mean Hounsfield (range)	957 (421–1400)
Mean OR time (minutes)	117.8 (113–256)
Positioning
Prone, n (%)	36 (78)
Supine, n (%)	10 (22)
Access type
Fluoroscopic, n (%)	10 (22)
Ultrasound, n (%)	24 (52)
Combined, n (%)	12 (26)

OR = operating room; SD = standard deviation.

On postoperative imaging (mean 78 days), 43% of patients were stone free with no identifiable fragments. Fifty-four percent of patients had residual stone burden <2 mm and 61% of patients had residual stone burden <4 mm (Table 2). For patients with stone burden 0 to 10 mm, the SFR (no visible fragment) was 57%, and 71% of patients had total residual burden ≤4 mm. For patients with stones 10 to 20 mm, SFR (no visible fragment) was 52%, and 74% of patients had total residual burden <4 mm. For patients with stones >20 mm, SFR was 25%, and 37% of patients had residual total burden <4 mm.

Table 2.

Stone Free Rates for Different Stone Sizes as well as using Variable Definitions of SFR

Postoperative stone burden
Total	46
Stone free	20
<2 mm	5
2–4 mm	3
>4 mm	18
% stone free	43.5
% 2 mm or less	54.3
% 4 mm or less	60.9
% >4 mm	39.1
Stones 1–10 mm
Total	7
Stone free	4
<2 mm	1
2–4 mm	0
>4 mm	2
% stone free	0.571
% 2 mm or less	71.4
% 4 mm or less	71.4
% >4 mm	28.6
Stones 10–20 mm
Total	23
Stone free (no fragments)	12
< 2 mm	2
2–4 mm	3
>4 mm	6
% stone free	52.2
% 2 mm or less	60.9
% 4 mm or less	73.9
% >4 mm	26.1
Stones >20 mm
Total	16
Stone free	4
<2 mm	2
2–4 mm	0
>4 mm	10
% stone free	25.0
% 2 mm or less	37.5
% 4 mm or less	37.5
% >4 mm	62.5

Within 30 days, 10 patients (21.7%) presented to the emergency department for evaluation and 18% (8) were readmitted. Eight patients (17%) had a complication within 30 days. Within 60 days six more patients presented to the emergency department for evaluation and 8% (4) were readmitted.

Within 30 days, 75% of these complications were Clavien I or Clavien II (Table 3). One patient had urinary retention after surgery requiring temporary catheterization, one patient had obstruction of suprapubic tube on postoperative day 4 from gross hematuria that resolved with irrigation, one patient had diabetic ketoacidosis admitted on postoperative day 7, one patient had postoperative rash on postoperative day 7 that self-resolved, two patients had postoperative flank pain treated with analgesics on postoperative days 7 and 14, respectively, 25% were Clavien IIIb that included a pleural effusion, which resolved after thoracentesis, and another patient had stent removed on postoperative day 1 and returned with flank pain on postoperative day 2 with imaging showing obstructing ureteral stone, which required ureteral stenting and then ureteroscopy for stone removal later. There were no Clavien IV or Clavien V complications (Table 3). Within 60 days, another 77% of these complications were Clavien I or Clavien II. Twenty-three percent were Clavien IIIb that involved an obstructing ureteral stone, which required return to OR. Again, there were no Clavien IV or Clavien V complications.

Table 3.

both Unplanned Emergency Department Visits and Complications for both 30 Days and 60 Days Post-Operatively

Complications	ED within 30 days n = 10 (21%)		ED within 60 days n = 6 (13%)		n
Complications	Clavien score	n	Complications	Clavien score	n
Ureteral stone requiring stent	IIIb	1	Ureteral obstruction	IIIb	2
Pleural effusion	IIIa	1	UTI	II	2
Diabetic ketoacidosis	II	1	Urosepsis	II	1
Hematuria	I	1	Dysuria	I	1
Pain	I	2	Pain	I	3
Rash	I	1
Urinary retention	I	1

Secondary procedures included the following: within 30 days, one patient underwent cystoscopy with ureteral stent for obstructing stone. Within 60 days, one patient underwent cystoscopy with ureteral stent for obstructing stone and then had elective ureteroscopy/lithotripsy. Another patient underwent elective ureteroscopy/lithotripsy for obstructing stone. Within 180 days, one patient underwent diagnostic ureteroscopy for possible ureteral stricture (none seen). Another patient underwent another PCNL for encrusted stent removal. A third patient underwent ureteroscopy for dilation of ureteral stricture/ureteral calculi removal.

Discussion

This is the first study performed in North America that evaluates a large cohort of patients undergoing MIP in which we provide a critical appraisal of perioperative outcomes. Prior studies have reported “stone free” with different definitions (stone free defined anywhere from 0 to 4 mm) and utilizing various imaging with widely different accuracies.^4,10,11 Our SFR (defined as no visible fragments on CT) was lower than commonly reported for both standard and mini-PCNL at 44% but in line with more contemporary series that report low SFR in endourology when strict criteria of no fragments on CT are used to define “stone free.” Emmott et al.¹² utilized a similarly strict SFR definition and reported a SFR of 55% on 658 PCNL patients. Steinberg et al. also reported low SFR of 43% after ureteroscopy when applying a similar strict CT SFR definition.¹³ These reports may be more accurate representations of our true stone surgery outcomes.

Furthermore, the use of the term “clinically insignificant residual fragments” is likely a misnomer given recent reports of <4 mm fragments causing significant complication rates.¹⁴ In all, the use of non-CT postoperative imaging and “clinically significant residual fragments” has blurred the efficacy of surgical stone treatment. Arguably this is not only a disservice to patients but also an impediment to advancing stone surgery to achieve higher SFRs.¹⁵

With respect to our primary outcome, SFR, the literature suggests that mini-PCNL is superior to RIRS. A meta-analysis by De et al. reported that OR SFR was 70% (p = 0.03) higher for mini-PCNL vs RIRS.³ Another meta-analysis also demonstrated similar superiority of mini-PCNL over RIRS OR 1.6, p = 0.005 of mini-PCNL vs RIRS.⁴ And yet it is the increased morbidity of mini-PCNL vs RIRS that detracts from the value of mini-PCNL. We report MIP has an acceptable comparable safety profile with 15% complication rate and majority of patients being discharged on day of surgery. Moreover, the degree of complications is in line with those reported for ureteroscopy (Clavien I–III). Of note, we had zero patients with postoperative infectious complications or sepsis, which is more in line with URS than reported for PCNL. In addition, there has been some concern regarding increased risk of blood loss or transfusion with mini-PCNL vs RIRS and studies have often shown increased drop in postoperative hemoglobin in mini-PCNL patients vs RIRS.¹⁶ None of the patients in our study required transfusion, although a larger sample size might permit a more accurate transfusion rate.

Given our findings and prior literature, we suggest that MIP may be a valuable alternative to RIRS for situations that challenge RIRS in terms of operative time or efficacy such as larger more durable stones and stones in unfavorable anatomy (e.g., acutely angulated lower pole). Moreover, we report that the majority of our patients (61%) were able to go home 1 to 2 hours after surgery—no different than standard the RIRS procedure. The limitation of this outcome is that 17% of patients were readmitted. And yet readmission rates for RIRS vary widely (likely because of morbidity of the particular surgery) and are not negligible.¹⁴ Therefore, we suggest that MIP can be considered for RIRS cases that otherwise would be challenging and time consuming as already noted.

Whether mini-PCNL is superior to standard PCNL remains a debatable subject in the literature. A recent meta-analysis suggested that mini-PCNL SFR outcomes were superior with regard to SFR and morbidity, although operative time was longer.^10,11 But other well-cited meta-analyses challenge this notion.⁴ Taken together, we suggest that mini-PCNL is unlikely a superior technology with respect to SFR, but instead its benefits should be seen in light of its lower morbidity (blood loss, length of stay, etc.). Moreover, mini-PCNL might be better reserved for smaller stones given prolonged operative time for mini-PCNL.

Several limitations of our study merit noting. As already stated, this study is observational—not comparative—and does not attempt to directly compare our MIP outcomes with PCNL or RIRS outcomes. As such, we cannot control for key variables (stone size, location, etc.) needed to effectively contrast two different operative modalities. Comparisons already noted are merely extrapolations from outcomes in the literature. Another limitation is that the majority of our patients were discharged on the day of surgery, which precluded some assessment of postoperative hematologic changes that are commonly reported, such as drop in hemoglobin. Future prospective randomized controlled trials directly comparing surgical techniques would eliminate these limitations and we are currently conducting a multi-institutional randomized control trial.

Although this investigation is the largest MIP study in North America, future studies using strict SFR criteria will help determine the true role of MIP. These findings will add to the body of critically appraised literature with this new surgical technique, which is widely adopted in other parts of the world. We suggest that MIP is not for every renal stone case but may serve to treat cases that otherwise would be compromised with RIRS or unduly exposed to the morbidity of standard PCNL.

Conclusion

We present North America's first single institution analysis of MIP cases with acceptable outcomes comparable with both RIRS and standard PCNL. The exact role of MIP in renal stone disease needs to be determined by future studies that critically assess their outcomes.

Footnotes

Author Disclosure Statement

Roger L. Sur is a consultant for Karl Storz, Boston Scientific, and Cook Medical Seth Bechis is a consultant for Karl Storz, Boston Scientific.

Funding Information

No funding was received for this article.

Abbreviations Used

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