Storz Medical Lithotripsy Index Predicts Success of Shock Wave Lithotripsy in Ureteric Stones
Sarp Korcan Keskin1,2*, Mandy Spencer3, Benjamin W Turney1
Purpose: The aim of this study is to evaluate the factors affecting treatment success in patients who underwent
Shock wave lithotripsy (SWL) for ureter stones and to investigate the effect of Storz Medical Lithotripsy Index
(SMLI) on treatment effectiveness in ureteric stones.
Method: Prospective data were collected on patients undergoing SWL treatment for ureter stones between Janu-
ary 2013 and May 2021. Stone location, number, and size were determined with Non contrast CT (NCCT) for all
patients. All patients underwent SWL with a Storz Modulith SLK lithotripsy machine with local anaesthesia. The
total amount of energy applied to the stone was calculated using the Storz Medical Lithotripsy Index (SMLI). All
patients were evaluated for stone-free status by X-ray at least 2 weeks after treatment. The success of the procedure
was defined as the patient being completely stone free (SF) or detection of residual fragments < 4 mm that did not
require further treatment
Results: A total of 1199 patients with ureter stones were included in the study. The mean age of the patients was
43.11 ± 10.65 (18-73), and the mean BMI was 27.87±8.12(19.02-38.65). During SWL, 89.3% of patients demon-
strated excellent pain tolerance (1070/1199). A total of 119 patients could not tolerate pain during SWL (10.7%).
Treatment success was associated with fewer treatment sessions (2.04±1.64 vs. 2.50 ± 1.48; p < 0.001), smaller
stone size (7.35±2.99 vs. 9.02 ± 3.81; p < 0.001) and higher SMLI/stone size (29.70 ± 17.48 vs. 24.98±16.01; p <
0.001). In the univariate and multivariate regression analysis, the factors affecting the success of the treatment were
the number of sessions (OR: 1.147), stone size (OR: 1.112), SMLI/stone size (OR: 1.115) and pain tolerance (OR:
0.740).
Conclusion: In the treatment of ureteral stones with SWL, number of sessions, stone size, SMLI/stone size, and
pain tolerance are the factors affecting success. SMLI per stone size is a statistically significant factor for predict-
ing SWL success.
Keywords: urolithiasis; SWL; treatment; predictive factor; SMLI; ureteric stones
INTRODUCTION
Urolithiasis is one of the most common diseases treated by urologists(1). In recent years, the prev-
alence of urolithiasis has increased to 10.6% in men
and 7.1% in women. High recurrence rates (39% at 15
years) are seen in both genders(2). Urinary stones have
the potential to recur in two-thirds of patients within
20 years, and stone recurrence can be lifelong(3). There-
fore, with the important developments in technology,
minimally invasive treatment options have gained great
importance in order to minimize the cost and harms of
repetitive treatments(4).
Numerous options are available for the treatment of
urolithiasis, including shock wave lithotripsy (SWL),
ureteroscopy, as well as open or laparoscopic proce-
dures(5, 6). The application of these treatment methods
differs for each patient and the response to the treatment
may be different for each patient(7,8).
SWL is a minimally invasive treatment method com-
monly used to treat patients with upper urinary tract
stones. Stone clearance rates were determined between
69-96% for ureteral stones in high-volume centers(9).
1Oxford University Hospitals, NHS Foundation Trust, Urology Department, UK.
2Bahcesehir University Medical School, Department of Urology, UK.
3Oxford University Hospitals, NHS Foundation Trust, Radiology Department, UK.
*Correspondence: Consultant Urological Surgeon, Oxford University Hospitals, UK. Adjunct Professor, Bahcesehir University Medical
School, Dept of Urology, Istanbul, Turkey. Tel: +44 0755 4373556. Email: urologum@gmail.com, sarp.keskin@ouh.nhs.uk.
Received August 2022 & Accepted January 2023
Treatment success depends on appropriate patient se-
lection, improved SWL efficacy, and optimal disease
management. In the current literature, there are various
reported factors that can affect stone clearance rates
(10,11). Clinical parameters such as the patient's body
mass index, stone location, skin-to-stone distance, stone
diameter or stone volume, and Hounsfield unit values
are among the strong predictive parameters for treat-
ment success(8,12,13).
In Storz Medical SWL devices, the total energy applied
to the stone is also measured with a proprietary Storz
Medical Lithotripsy Index (SMLI). There is very lim-
ited data about the effectiveness of treatment success
according to the dose of applied energy(4). The aim of
this study is to evaluate the factors affecting treatment
success in a large cohort of patients who underwent
SWL and to investigate the effect of SMLI on treatment
effectiveness.
METHODS
Patients who received SWL treatment for ureteral stones
between January 2013 and May 2021 were included in
Urology Journal/Vol 20 No. 4/ July-August 2023/ pp. 203-207. [DOI:10.22037/uj.v20i.7410]
ENDOUROLOGY AND STONE DISEASES
the study. Written informed consent was obtained from
each patient. The study was designed in accordance
with the Declaration of Helsinki.
Stones were detected with non-contrast computed to-
mography (NCCT). Patients with pregnancy, multiple
kidney stones, active urinary infection, irregular coag-
ulopathy and using anticoagulants were excluded from
the study.
The study design is retrospective. The sample size was
obtained with all patients who met the inclusion criteria
between the specified dates.
Stone localization, number and stone size of the patients
were determined with the help of NCCT. All patients
were treated on a Storz Modulith SLK-F2 lithotripsy
machine without anaesthesia. X-ray and ultrasound
were used to target the stone. SWL was performed by
one of the team of trained radiographers. The study
protocol with a protocol of 4000 shocks at 2Hz.The to-
tal amount of energy applied to the stone was record-
ed using the Storz Medical Lithotripsy Index (SMLI).
No patients had stents in situ and no medical expulsive
therapy was given before or after the treatment.
Recording only the maximum energy level and the
number of shocks in the treatment reports may be mis-
leading and it may not be understood whether sufficient
energy has been applied to the stone. SMLI was created
by STORZ MEDICAL to control the applied energy.
SMLI refers to the energy applied during a shock wave
therapy. SMLI gives a net number representing the total
energy dissipated in an average area of 12 mm.
Typical values for SMLI observed in clinical practice
range from 180-220 for most stones. However, depend-
ing on the individual stone characteristics, lower and
higher values will suffice.
The number of shock waves, energy level, SMLI val-
ues, patient position, stone targeting method (X-ray or
ultrasound), radiation dose received, and patient pain
tolerance evaluations during SWL were recorded.
An experienced operator is required to provide pain
control. While the pain tolerance of patients who can
tolerate appropriate energy during ESWL is evaluated
as excellent, lowering the energy level due to pain or
interrupting the procedure is classified as pain intoler-
ance(14).
All patients were evaluated for stone-free by X-ray
at least 2 weeks after treatment. Ultrasonography or
NCCT was used to assess stone free status. The success
of the procedure was defined as the patient being stone
free (SF) or detection of fragments < 4 mm.
Statistical analysis
Data were evaluated with SPSS 25.0 (IBM, NY, USA)
statistics program. The normality of the distribution of
the data was questioned with the Kolmogorov-Smirnov
test and Q-Q plot. Independent sample t-test and Mann
Whitney U test were used. Factors affecting stone-
free success were evaluated with univariate and mul-
tivariable logistic regression analysis(Model: Tretment
Counts, Stone Size(mm), Number of Shocks, Energy
Level, SMLI, SMLI/Stone Size, Radiation Dose, Side
(Ref:right), Patient Position(Ref:supine), Imaging
(Ref:X-ray), Site (Ref:Lower ureter), Pain Tolerance
(Ref:Excellent); (assumption linearity 69.9%)). ROC
curves were created and areas under the curves (AUC)
were calculated to compare the predictive power of dif-
ferent features. Significant p value was determined as
< 0.05.
RESULTS
A total of 1199 patients with ureter stones were in-
cluded in the study. The mean age of the patients was
43.11 ± 10.65(18-73), and the mean BMI was 27.87
± 8.12(19.02-38.65). During SWL, 89.3% of patients
demonstrated excellent pain tolerance (1070/1199). A
total of 119 patients could not tolerate pain during SWL
(10.7%). Demographic characteristics of the patients
are given in Table 1.
During SWL, 89.3% of patients demonstrated excel-
lent pain tolerance (1070/1199). A total of 119 patients
could not tolerate pain during SWL (10.7%). The pro-
cedure was mostly performed under X-ray guidance
Endourology & Stone Diseases 204
Storz Medical Lithotripsy Index Predicts Success of SWL-Keskin et al.
n=1199
Treatment Counts 2.18 ± 1.60(1-10)
Stone Size(mm) 7.85 ± 3.34(2-23)
Number of Shocks (median(IQR) 2790 (200-3000)
Energy Level 6.27 ± 1.32(1-8)
SMLI 193.98 ± 91.25
Radiation Dose (median(IQR) 480 (100-750)
Side Right 498 (41.5%)
Left 701 (58.5%)
Patient Position Supine 1087 (90.7%)
Prone 112 (9.3%)
Imaging X-Ray 1179 (98.3%)
Ultrasonud 20 (1.7%)
Site Upper Ureter 671(55.9%)
Lower Ureter 528(44.1%)
Table 1. Patient demographics
` Succesfull(n=838) Unsuccesfull(n=361) p
Treatment Counts 2.04 ± 1.64 2.50 ± 1.48 < 0.001
Stone Size(mm) 7.35 ± 2.99 9.02 ± 3.81 < 0.001
Number of Shocks 2822.40 ± 1938.60 2696.99 ± 906.63 0.240
Energy Level 6.29 ± 1.33 6.24 ± 1.32 0.574
SMLI 194.19 ± 92.04 193.48±89.53 0.920
SMLI/Stone Size 29.70 ± 17.48 24.98 ± 16.01 < 0.001
Radiation Dose 479.47 ± 393.23 466.48 ± 370.46 0.594
Side Right 355 (42.4%) 143 (39.6%) 0.375
Left 483 (57.6%) 218 (60.4%)
Patient Position Supine 765 (91.3%) 322 (89.2%) 0.279
Prone 73 (8.7%) 39 (10.8%)
Imaging X-Ray 823 (98.2%) 356 (98.6%) 0.585
Ultrasonud 6(1.8%) 4(1.4%)
Site Upper Ureter 470(56.1%) 224(62%) 0.032
Lower Ureter 368 (43.9%) 137 (38%)
Table 2. Comparison of the factors affecting the success of ESWL
(98.3%) and in the supine position (90.7%).
Complete stone clearance was achieved in 50.4%
(604/1199) of the patients. The number of patients
with clinically insignificant residual fraction were 228
(19.5%). Overall success rate was therefore 69.9%. A
total of 361 patients (30.1%) did not respond to treat-
ment.
Treatment success was associated with fewer treat-
ment sessions (2.04 ± 1.64 vs. 2.50 ± 1.48; p < 0.001),
smaller stone size (7.35 ± 2.99 vs. 9.02 ± 3.81; p <
0.001) and higher SMLI/stone size (29.70 ± 17.48
vs. 24.98 ± 16.01; p < 0.001) (Table 2). Better success
and fewer treatment sessions were seen in the patient
group with better pain tolerance (p = 0.001). A signifi-
cant positive correlation was found between SMLI and
the number of shocks and energy levels (r = 0.567 for
number of shocks, r = 0.409 for energy levels).
In the univariate and multivariate regression analysis,
the factors affecting the success of the treatment were
the number of sessions (OR: 1.147), stone size (OR:
1.112), SMLI/stone size (OR: 1.115), and pain toler-
ance (OR: 0.740) (Table 3).
In the ROC Curve analysis for SMLI/stone size, a cut-
off value of 18.92 has a sensitivity of 74% and a spec-
ificity of 66.9% (AUC: 0.699, CI 95%: 0.588-0.712) .
DISCUSSION
This study, in line with other research demonstrates that
the success of SWL depends on stone size, number of
treatment sessions. Patients who can tolerate pain have
fewer sessions and their success in treatment increases.
This study is the first to evaluate the value of SMLI and
SMLI/stone size.
SWL is an effective, minimally invasive treatment
method with similar effects and complication rates
as retrograde urethrography in the treatment of many
stones(12,15). Among the factors affecting the success
of SWL, there are many factors related to the techni-
cal features of the device, stone characteristics and the
structure of the patient(10). According to the literature,
while the success of SWL is 80-85% in stones less than
20 mm, the success falls between 30-65% in stones over
20 mm(16,17). The decrease in the chance of success, es-
pecially in lower pole stones, was seen as a challenge
for SWL. However, several studies have demonstrat-
ed good outcomes with lower pole stones(4,18-20). In our
study, the success of SWL in lower calyceal stones was
found to be 65.5%.
The patients' ability to tolerate pain during SWL both
reduces the number of sessions and affects the success
of the treatment. Patients that tolerate the treatment well
allow better stone targeting and reduced stone excur-
sion due to respiration and patient movement. An ex-
perienced operator is required to ensure that adequate
coupling is achieved for pain control. It is also extreme-
ly important for the operator to increase the voltage in
gradual increments to aid the development of pain tol-
erance. Our study is based on the fact that a patient’s
pain tolerance affects SWL treatment and its clinical
outcomes. Energy levels are reduced for patients with
lower pain threshold making total stone disintegration
less likely. It shows that under simple non-opioid an-
algesia, there are a number of independent predictors
for increased pain tolerance during SWL. This results
in better stone-free status in a reduced number of SWL
sessions(21). In our study, it was determined that the
treatment success was better in the group that tolerated
the pain and indirectly the number of sessions was less.
With the widespread use of NCCT, the burden of uri-
nary tract stones has been evaluated more easily. The
size of the stone is typically measured as its maximal
diameter. Due to the irregular structure of the stones,
3D stone volume measurements have also been used
(22). In a study in which the effect of stone burden on
the success of SWL was evaluated, a significant differ-
ence was found between the mean stone volumes in the
successful and fragmented groups. For stone volumes
over 500mm3, the success rate dropped to 27% (23). In
a similar study, stone size was determined as the most
important parameter in predicting the success of SWL
(24). In our study, stone size was found to be larger in
the group with unsuccessful SWL treatment (p < 0.001,
OR: 1.112).
SMLI is a measure of total power delivered by the
machine in a treatment session
This reflects ramping up or down of the energy set-
tings throughout the treatment and any adjustments in
frequency (Hz). It does not reflect how much power
hits the stone. i.e. even if the targeting is not accurate it
could still record a high SMLI. Assuming that a consist-
ent number of shockwaves accurately target the stone, it
would be expected that higher SMLIs would be associ-
ated with greated treatment success. In a small study of
109 patients the SMLI/stone size ratio was significant(4).
However, the sample size of the study is small. In this
study, a cut-off value for SMLI was not specified and
it was argued that it could be done in further studies.
In our study conducted with a total of 2429 patients,
it was found that the SMLI value had an independent
effect on the success of SWL. Successful SMLI/stone
Univariable Analysis Multivariable Analysis
p OR CI %95 p OR CI %95
Tretment Counts 0.008 1.120 1.077-1.195 < 0.001 1.147 1.090-1.207
Stone Size(mm) < 0.001 1.163 1.085-1.146 <0.001 1.112 1.084-1.141
Number of Shocks 0.162 1.000 1.000-1.002
Energy Level 0.593 0.961 1.046-1.316
SMLI 0.681 0.998 0.997-1.000
SMLI/Stone Size 0.001 1.001 0.998-1.003 < 0.001 1.115 0.997-1.122
Radiation Dose 0.976 1.002 0.978-1.012
Side (Ref:right) 0.889 0.937 0.783-1.121
Patient Position(Ref:supine) 0.425 0.835 0.737-1.112
Imaging (Ref:X-ray) 0.321 0.749 0.410-1.368
Site (Ref:Lower ureter) 0.227 1.331 0.979-1.809
Pain Tolerance (Ref:Excellent) < 0.001 0.716 0.625-0.821 < 0.001 0.740 0.650-0.843
Table 3. Univariate and multivariable analysis
Storz Medical Lithotripsy Index Predicts Success of SWL-Keskin et al.
Vol 20 No 4 July-August 2023 205
size threshold value was determined as 20.72. With
these findings, increasing the power proportionally to
the stone size and determining the effective power in
patients who underwent SWL will significantly affect
the success of stone-free.
The study is the first in which SMLI was evaluated and
a threshold score was found to be a factor affecting the
success of SWL. The study has some limitations. The
first of these is that the study was conducted retrospec-
tively. Another limitation is the short follow-up period
after SWL. Due to the heterogeneity of the patients, the
sensitivity of the SMLI cut-off score was low. Further
studies evaluating the effect of SMLI in more homoge-
neous patient groups are needed.
CONCLUSIONS
In the treatment of urinary tract stones with SWL, stone
site, stone size, SMLI, and pain tolerance are the factors
affecting treatment success. SMLI is a surrogate for the
power delivered by the Storz Modulith lithotripter and
may have some predictive value in treatment success.
CONFLICT OF INTEREST
None as declared by the authors.
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