ENDOUROLOGY AND STONE DISEASE
The Impact of Sheath Size in Miniaturized Percutaneous Nephrolithotomy in Adult Patients;
A Matched-pair Analysis
Akif Erbin1*, Burak Ucpinar1, Alkan Cubuk1, Ozgur Yazici1, Harun Uysal2, Metin Savun1,
Seref Basal1, Mehmet Fatih Akbulut1
Purpose: The miniaturized percutaneous nephrolithotomy (mPNL) can be performed by using a very wide range
of different access sheaths (14-22 Fr). It has been well known that tract size is one of the main parameters affecting
the complication rates in PNL. We aimed to compare 21 Fr with 16.5 Fr mPNL tract sizes in adult patients.
Materials and Methods: From May 2013 to April 2018, 604 patients with kidney stone underwent mPNL in our
department. The study was designed as retrospective and match-pair analysis was the preferred method for the for-
mation of groups. The 21 Fr mPNL cases were matched with 16.5 Fr mPNL cases at a 1:1 ratio, according to the
patients’ age, gender, body mass index, American Society of Anesthesiologists (ASA) score, stone characteristics
(stone size, opacity and localization) and hydronephrosis. Patients with solitary kidney, renal anomalies, muscu-
loskeletal abnormalities, and pediatric patients (< 18 years old) were excluded from the study. Both groups (21
Fr and 16.5 Fr) were compared in terms of demographics, stone characteristics, operative data and post-operative
outcomes.
Results: A total of 260 patients were included in the study (130; 21 Fr mPNL group and 130; 16.5 Fr mPNL
group). The operation time was significantly shorter in 21 Fr group (21 Fr; 85.2 ± 37.5, 16.5 Fr; 101.7 ± 37.7 min-
utes, p = 0.001). Complete stone clearance rates were 76.9% and 62.3% in 21 Fr and 16.5 Fr mPNL, respectively
(p = 0.01). There was no significant difference between the groups in terms of overall operative and post-operative
complications. However, in subgroups analysis, post-operative fever was higher in 16,5 Fr mPNL (4 patients in
16.5 Fr, no patients in 21 Fr group, p = 0.044); steinstrasse, renal colic and post-operative JJ stent requirement rates
were higher in 21 Fr mPNL procedure (p: 0.018, p: 0.031 and p: 0.046, respectively). The hospitalization time was
significantly higher in 21 Fr (p = 0.01).
Conclusion: Although 21 Fr mPNL procedure has advantages such as better success rates and shorter operation
time, some post-operative complications (steinstrasse, renal colic, post-operative JJ stent requirement) are against
of 21 Fr mPNL when compared with 16.5 Fr mPNL procedure. Further randomized prospective studies with larger
patient volume are needed to confirm these results.
Keywords: kidney stone; miniaturized; nephrolithiasis; percutaneous nephrolithotomy; sheath sizes
INTRODUCTION
Main treatment modalities for urinary tract stones are extracorporeal shockwave lithotripsy
(ESWL), ureterorenoscopy (URS), percutaneous neph-
rolithotomy (PNL) and open or laparoscopic surgery.
With technological advancements, endourologic pro-
cedures (URS and PNL) have gained more popularity
among other surgical treatments. Since its first descrip-
tion in 1976, percutaneous nephrolithotomy (PNL)
has become the mainstay of treatment for large kidney
stones(1). The European Association of Urology (EAU)
guidelines on urolithiasis recommends PNL as the first
treatment of choice for kidney stones larger than 2 cm(2).
In standard PNL, renal access is obtained through 24-
30 Fr access sheaths. Attempts to minimize the blood
loss during PNL by reducing the sheath size and hence,
decreasing the area of parenchymal and infundibular
1Department of Urology, Haseki Traning and Research Hospital, Istanbul, Turkey.
2Department of Anesthesiology, Bezmialem Vakif University, Istanbul, Turkey.
*Correspondence: Department of Urology, Haseki Traning and Research Hospital, Istanbul, Turkey.
Tel: +90 506 543 1062. Fax: +90 212 529 4400. E-mail: akiferbin@hotmail.com.
Received July 2018 & Accepted December 2018
injury, gave rise to the concept of miniaturization. Al-
though a clear definition does not exist in the literature,
the miniaturized PNL (mPNL) is accepted as the use
of 14-22 Fr access sheaths by EAU Urolithiasis Guide-
lines Panel(3). The mPNL technique was introduced by
Jackman et al in 1998(4). Recently, systems with even
smaller diameters, such as ultra mini-PNL (11–13 Fr)
and microperc (4.8–10 Fr), have been introduced as al-
ternative techniques to reduce procedure-related mor-
bidity(5,6). Smaller access sheaths were initially intro-
duced for paediatric use, but are now widely utilised for
the adult patients.
The mPNL can be performed by using a very wide
range of different access sheaths (14-22 Fr). The pri-
mary goal of PNL is to achieve maximal stone clear-
ance with minimal morbidity. It has been well known
that tract size is one of the main parameters affecting
the complication rates in PNL. However, reducing the
Urology Journal/Vol 16 No. 6/ November-December2019/ pp. 536-540. [DOI: 10.22037/uj.v0i0.4676]
tract size may adversely affect some procedure-related
factors such as operation time(3). There is no clear data
on which tract size has more advantages in adult mPNL
procedures. The aim of the study was to compare 21 Fr
with 16.5 Fr mPNL tract sizesin adult patients, using
1:1 match pair analysis.
MATERIAL AND METHODS
Study design
The study protocol was approved by the Institutional Re-
view Board at Haseki Training and Research Hospital.
From May 2013 to April 2018, 604 patients with kidney
stone which have underwent mPNL in our department
were evaluated for inclusion. The study was designed as
retrospective and match-pair analysis was the preferred
method for the formation of groups. We used our stone
database to identify the procedures which were applied
through 21 and 16.5 Fr access sheaths. The 21 Fr mPNL
cases were matched with 16.5 Fr mPNL cases at a 1:1
ratio, according to the patients’ age, gender, body mass
index (BMI), American Society of Anesthesiologists
(ASA) score, stone characteristics (stone size, stone
opacity, stone localization) and hydronephrosis (HN)
(7,8). Patients with solitary kidney, renal anomalies, mus-
culoskeletal abnormalities, and pediatric patients (< 18
years old) were excluded from the study. Patients who
had missing data during follow up period were also ex-
cluded. Both groups (21 Fr and 16.5 Fr) were compared
in terms of demographics, stone characteristics, opera-
tive data and post-operative outcomes. Treatment suc-
cess was defined as ‘complete stone clearance’ with no
residual fragments. Operative and post-operative com-
plications were evaluated according to the Satava and
modified Clavien-Dindo classification system, respec-
tively(9,10). Satava classification system was introduced
in order to define the possible operative complications.
In the following years, it has been widely used for many
endourological procedures.
Preoperative evaluation
Before surgery, all patients signed an informed con-
sent form. Patient assessment included medical history,
physical examination, complete blood count, coagu-
lation tests, serum biochemistry, urinalysis and urine
culture. Anticoagulant drugs were discontinued at least
7-10 days before the operation. All patients were eval-
uated preoperatively by intravenous urography and/or
non-contrast abdominal computed tomography (CT).
Stone size was determined by the measurement of the
stones’ longest diameter. In case of multiple calculi, the
sum of the greatest diameter of each stone was calculat-
ed. All patients had sterile urine culture prior to surgery.
Second generation cephalosporins were administered as
antibiotic prophylaxis. The first dose was administered
intravenously when anesthesia was initiated, and the
second dose was given 12 hours later.
PNL technique
After the induction of general anesthesia, a 5 Fr ure-
teral catheter was placed to the ureter and fixed on the
Foley catheter in the lithotomy position. The patient
was then repositioned to the prone position. Percutane-
ous access was achieved under C arm fluoroscopy (Sire
Mobil Compact, Siemens) guidance. The puncture was
performed with an 18 gauge percutaneous access nee-
dle (Boston Scientific Corporation, Natick MA). After
achieving access to the pelvicalyceal system (PCS), a
0.035 inch guidewire (Boston Scientific Corporation,
Natick MA) was advanced through the needle into the
PCS or ureter. The track was dilated sequentially using
fascial dilators and the 16.5 or 21 Fr metallic sheaths
(Karl Storz, Tutlingen, Germany) were advanced over
their metal dilators under fluoroscopic guidance. A rig-
id 12 Fr nephroscope (Karl Storz, Tuttlingen, Germany)
was advanced through the sheath. Stone disintegration
was achieved using a Holmium YAG Laser lithotripter
(Sphinx, Lisa laser, USA) and 550 μm laser fibers at
an energy of 1.0–1.5 J and a rate of 8–10 Hz. Stone
fragments were removed with tipless nitinol stone bas-
kets(Boston Scientific, Natick, MA, USA).At the end of
the procedure, retrograde pyelography was performed
to assess the integrity of the pelvicalyceal system. If
no sign of perforation was detected under fluoroscopy
and if there was no sign of evident bleeding, procedures
were terminated in a tubeless fashion and the incision at
the access tract site was sutured (tubeless mPNL) with
or without placing a JJ stent. Otherwise, a nephrostomy
tube was left in place. All procedures were performed
Miniaturized Percutaneous Nephrolithotomy size-Erbin et al.
Table 1. Demographic data and stone characteristics of patients included in the study.
21Fr mPNL (n:130) 16.5Fr mPNL (n:130) p
Sex (female/male)* 40/90 48/82 .296
Age (years) * 46.6 ± 12.9 45.6 ± 12.4 .550
BMI (kg/m2)* 27.1 ± 4.3 27.6 ± 4.5 .379
ASA score* 2.0 ± 1.0 1.8 ± 1.1 .712
Previous ESWL / surgery
ESWL 33 (25.4%) 26 (20.0%) .302
PNL 31 (23.8%) 27 (20.8%) .553
Open surgery 13 (10.0%) 12 (9.2%) .834
Stone opacity (opaque / non-opaque)* 120/10 116/14 .393
Stone localization * .938
Isolated lower calyx 27 27
Isolated middle calyx 6 6
Isolated upper calyx 7 7
Isolated pelvis 30 30
Multiple calyx 57 59
Partial staghorn 3 1
Stone size (mm) * 26.0 ± 8.6 26.3 ± 8.6 .323
Hydronephrosis (mild/severe) * 91/39 95/35 .584
Operation side (right / left) 71/59 63/67 .323
* 1:1 matching parameters
Abbreviations: BMI; Body Mass Index ASA; American Society of Anesthesiologists ESWL; Extracorporeal Shockwave Lithotripsy
Vol 16 No 06 November-December2019 537
by two experienced urologistsat the tertiary referral
center.
Post-operative evaluation
A complete blood count and renal function test accord-
ing to the glomerular filtration rate measured by the
Cockroft-Gault formulawere obtained from all patients
within 6 hours after the operation. On first post-opera-
tive day, a plain x-ray of the kidneys, ureters and blad-
der was obtained. In cases with a nephrostomy tube, the
tube was removed on first or second post-operative day
after obtaining an antegrade nephrostography which
was performed to prove the lack of obstruction in ip-
silateral ureter. If leakage from the nephrostomy tract
persists longer than 48 hours, this situation was defined
as ‘prolonged urine leakage’ and a JJ stent was placed.
All patients were evaluated with renal function tests and
a non-contrast abdominal CT 1 month after the opera-
tion.
Statistical analysis
Data were analysed by using Statistical Package for the
Social Sciences software package version 20 (SPSS
Inc., Chicago, IL, USA). Quantitative data were ex-
pressed as mean ± standard deviation on tables and
categorical data were expressed with frequency (n)
and percentages (%). The distribution of the variables
was measured by the Kolmogorov Smirnov test. Inde-
pendent t test was used to compare independent groups.
Pearson Correlation test was used to examine the re-
lationship between variables. Pearson Chi-Square and
Fisher Exact tests were used to compare the categorical
data. The data were analysed at 95% confidence level
and the threshold forstatistical significance was accept-
ed as P < 0.05 for all analyses.
RESULTS
A total of 260 patients were included in the study (130;
21 Fr mPNL group and 130; 16.5 Fr mPNL group). Pa-
tient demographics and stone characteristics were sim-
ilar between groups and are demonstrated in Table 1.
Table 2 summarizes the operative data. Nephrostomy
tube and JJ stent placement rates were significantly
higher in 16.5 Fr. There was no significant difference
between the groups in terms of intraoperative compli-
cations, which were classified according to the Satava
classification system. Overall intraoperative compli-
cations occurred in 4 patients (3,1%) in 21 Fr and 2
patients (1.6%) in 16.5 Fr (p: 0.693). Grade 2a com-
plication (pelvicalyceal system perforation proven by
contrast media extravasation) observed in 3 patients
in 21 Fr and 1 patient in 16.5 Fr. These patients were
treated with prolonged ureteral stenting up to 4 weeks.
Grade 2b complication (severely bleeding requiring ter-
mination of the procedure) observed only 1 patients in
16.5 Fr. This patient was treated with blood transfusion
and supportive treatment.
Complications and post-operative outcomes are sum-
marized in Table 3. When post-operative complications
were compared according to the modified Clavien-Din-
do classification, overall and subgroup complication
rates were comparable between groups. Grade 4 or
grade 5 complications were not observed in any pa-
tient. Fever was observed in 4 patients in 16.5 Fr during
post-operative period, whereas, none of the patients in
Table 2. Operative details of patients included in the study.
21 Fr mPNL (n:130) 16.5 Fr mPNL (n:130) p
Operation time (min) 85.2 ± 37.5 101.7 ± 37.7 .001
Fluoroscopy time (min) 5.0 ± 3.5 3.5 ± 3.1 .001
Access .225
Solitary
Lower pole 109 (83.8%) 104 (80.0%)
Middle pole 11 (8.5%) 7 (5.4%)
Upper pole 4 (3.1%) 11 (8.5%)
Multiple access 6 (4.6%) 8 (6.2%)
Intercostal access 7 (5.4%) 14 (10.8%) .112
Nephrostomy placement 89 (68.5%) 114 (87.7%) .001
JJ stent placement 21 (16.2%) 44 (33.8%) .001
Intraoperative complic ation .693
21 fr mPNL 16.5 fr mPNL p
Overall complications, n (%) 28 (21.5%) 23 (17.7%) 0.437
Complications, n (%)
Fever (>38 °C) 0 4 (3.1%) .044
Renal colic 9 (6.9%) 2 (1.5%) .031
Steinstraisse 10 (7.7%) 2 (1.5%) .018
Urine leakage 11 (8.5%) 6 (4.6%) .211
Post-operative DJ placement 19 (14.6%) 9 (6.9%) .046
Pleural effusion 1 (0.8%) 2 (1.5%) .563
Hemoglobin drop (gr/dl) 1.9 ± 3.4 1.1 ± 3.0 .051
Blood transfusion 3 (2.3%) 3 (2.3%) .000
Embolization 0 1 (0.8%) .318
Modified Clavien-Dindo classification .238
Grade 0 103 (79.2%) 107 (82.3%) .531
Grade 1 5 (3.8%) 8 (6.2%) .395
Grade 2 3 (2.3%) 2 (1.5%) .653
Grade 3a 0 2 (1.5%) .157
Grade 3b 19 (14.6%) 11 (8.5%) .120
Hospitalization time (hours) 68.1 ± 32.7 51.3 ± 31.6 .001
Complete stone clearance, n (%) 100 (76.9%) 81 (62.3%) .010
Table 3. Comparison of post-operative outcomes of 21 fr mPNL and 16.5 fr mPNL groups.
Miniaturized Percutaneous Nephrolithotomy size-Erbin et al.
Endourology and Stone Diseases 538
Vol 16 No 06 November-December2019 539
21 Fr experienced fever (p = 0.044). Steinstrasse was
encountered in 10 (7.7%) patients in 21 Fr and 2 (1.5%)
patients in 16.5 Fr (p = 0.018). Renal colic during
post-operative period was more common among 21 Fr,
as well (p:0.031). Post-operative JJ stent placement was
required in 19 patients (14.6%) in 21 Fr and 9 patients
(6.9%) in 16.5 fr (p = 0.046). Both groups were similar
in terms of hemoglobin drop, blood transfusion rates
and bleeding which requires angioembolization.
The hospitalization time was significantly higher in
21 Fr (68.1 ± 32.7 vs 51.3 ± 31.6 hours, p = 0.01).
Complete stone clearance was achieved in 100 patients
(76.9%) in 21 Fr and 81 patients (62.3%) in 16.5 and the
difference was statistically significant (p = 0.01).
DISCUSSION
Different types and sizes of instruments are available
for PNL procedure and selection between these differ-
ent instruments are dependent on surgeons’ preference.
We compared two different sheath sizes, 21 Fr and 16.5
Fr, which were both classified under the name of mPNL
and found out that21 Fr had higher operation success
rates with decreased operation duration, whereas, 16.5
Fr had shorter fluoroscopy duration. In terms of compli-
cations; steinstrasse, renal colic and need for post-oper-
ative JJ stent placement was more common among 21
Fr, whereas, fever was more common among 16.5 Fr.
The duration of an operation is an important parameter
especially in high-risk patients. Application of gener-
al anesthetic agents for a prolonged duration may have
negative impacts on patients’ overall health(11).Percuta-
neous nephrolithotomy procedure is performed under
continuous irrigation. So, prolonged operation time
may increase the intrapelvic pressure, especially in
mPNL, and thereby increase the risk of pelvicalyceal
rupture, septic and metabolic complications. Many
studies have demonstrated the limitation of mPNL pro-
cedures as longer operation times when compared with
conventional PNL procedure(3). Laser lithotripters are
the commonly preferred method for stone fragmenta-
tion in mPNL cases and laser fragmentation of stones
during mPNL is quite time consuming. In addition to
laser lithotripters, miniaturized ultrasonic and pneu-
matic lithotripters, with or without aspiration mecha-
nisms, can also be preferred for stone fragmentation.
However, stone fragmentation and aspiration with these
instruments during mPNL cases is not as fast as frag-
mentation with large-bore instruments during conven-
tional PNL cases. Additionally, fragmentation of stones
into very small pieces (dusting) is required, since bigger
fragments can not be expelled out via smaller access
sheaths. We emphasized that using a 21 Fr sheath in-
stead of a 16.5 Fr sheath, allows bigger fragments to
be expelled out and thereby, may shorten the operation
time by decreasing the duration of laser fragmentation.
The fluoroscopy time was longer in 21 Fr mPNL. We
use serial fascial and metallic dilators for tract dilata-
tion. Fluoroscopy is most needed during tract dilata-
tion. Traction dilatation occurs at more steps in 21 Fr
operations and this causes prolonged fluoroscopy time.
The majority of urologists utilize fluoroscopy to obtain
renal tract access. But radiation exposure is the major
drawback both for surgeon and patient. Ultrasound
guidance access can be preferred to minimize radiation
exposure(12).
Many studies have compared the effectiveness of mPNL
and conventional PNL. A meta-analysis including 18
studies (2 randomized controlled trials, 6 non-rand-
omized comparative studies, and 10 case series) have
demonstrated the equal effectivity of mPNL and con-
ventional PNL(3). In a recently published randomized
prospective study, equal effectivity of mPNL and
conventional PNL have been shown for the treatment
of large kidney stones(9). Even though there are some
studies which have compared mPNL with conventional
PNL in the literature, there are no studies which have
compared different sheath sizes of mPNL in adult pa-
tients. In our study, 21 Fr was significantly superior to
16.5 Fr in terms of complete stone clearance. We have
emphasized that, effective retrieval of stone fragments
from a larger access sheath was the reason of higher
stone free rates.
A major advantage of mPNL over conventional PNL
is its’ less hemoglobin drop and less transfusion re-
quirement(12). Operating through smaller access sheaths
decrease the injury on renal parenchyma and thereby,
decrease the amount of bleeding during surgery. Even
though our procedures were classified under mPNL, our
sheath sizes were different between groups and bleed-
ing was an important parameter in our study. However,
no significant difference was detected between groups,
in terms of hemoglobin drop, transfusion requirement
and necessity of angioembolization. Besides bleeding,
additional complications may be encountered during
and after PNL surgeries. Operative complications can
be listed as major bleeding and pelvicalyceal system
perforation during surgery. Performing the percutane-
ous access and dilation through an appropriate calyx
at an appropriate angle lowers these complications, so,
tract size can be considered as a determinant factor for
these complications. But, in our study, we have detected
no difference between 21 Fr and 16.5 Fr and conclud-
ed that they are both equally safe in terms of operative
complications.
A wide range of post-operative complications can be
encountered after PNL surgery. According to recent
EAU guidelines, complications like fever can be en-
countered as frequent as 10.8% of all patients and oth-
er complications can be listed as bleeding, pelvi-cal-
yceal perforations, prolonged urinary leakage, thoracic
complications, sepsis, organ injury and death(2). In our
study, overall post-operative complications were sim-
ilar between groups. However, there were significant
differences when complications were evaluated indi-
vidually. Fever was encountered in 3.1% of patients
in 16.5 Fr. In contrast, none of the patients in 21 Fr
group experienced fever. Nephrostomy and/or JJ stent
placement rates were higher in 16.5 Fr group due to
higher rates of residual calculi detected at the end of
each operation.We have emphasized that, higher rates
of instrumentation (nephrostomy or JJ stent) resulted
in higher rates of fever during post-operative period.
Additionally, decreased drainage of irrigation fluid
and increased intrapelvic pressure in 16.5 Fr group
might have resulted in this significant difference. In
our study, we have detected significantly higher rates
of steinstrasse, renal colic and necessity of post-oper-
ative JJ stent placement in 21 Fr group. Steinstrasse is
one of the bothersome complications, which may result
in renal colic episodes, prolonged urinary leakage and
prolonged hospitalization times. We have emphasized
that,higher rates of steinstrasse was due to the creation
Miniaturized Percutaneous Nephrolithotomy size-Erbin et al.
of bigger stone fragments in 21 Fr group and migration
of these stone fragments into the ureter before effective
clearance. If spontaneous passage of these fragments
could not achieved, post-operative JJ stent placement
was inevitable. This was the main reason of increased
hospital stay in 21 Fr group.
Contrary to the fact that the present study is a matched
pair study, it has some limitations, mainly related to its
retrospective nature and non-randomization. The other
important limitation of our study was the lack of our
usage of flexible ureteroscope during surgery. Flexible
instruments might haveincreased the final stone free
status rates and decrease the need for a second-look
procedure. Although the total number of patients was
sufficient, the number of data was small in effective-
ly comparing some parameters (eg. complications) and
this can be listed as another limitation of our study.
CONCLUSIONS
The 21 Fr mPNL procedure has significantly higher
success rates and shorter operation time when com-
pared with 16.5 Fr mPNL. Although overall operative
and post-operative complications were similar between
groups, operative nephrostomy and JJ stent placement
and post-operative fever rates were higher in 16.5 Fr,
whereas; steinstrasse, renal colic, post-operative JJ
stent requirement rates and hospitalization time were
higher in 21 Fr mPNL procedure. Although this is the
first study which evaluates different sheath sizes of
mPNL in adult patients, future prospective randomized
studies are required to clarify which sheath size is more
advantageous in mPNL.
CONFLICT OF INTEREST
The authors report no conflict of interest.
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