Surgery Today
https://doi.org/10.1007/s00595-020-02171-8

ORIGINAL ARTICLE

Usefulness of a finger‑mounted tissue oximeter with near‑infrared
spectroscopy for evaluating the intestinal oxygenation and viability
in rats
Yuhi Suzuki1 · Masayoshi Yamamoto1 · Kosuke Sugiyama1 · Toshiya Akai1 · Katsunori Suzuki1 ·Takafumi Kawamura1 · Mayu Sakata1 · Yoshifumi Morita1 · Hirotoshi Kikuchi1 · Yoshihiro Hiramatsu1 ·Kiyotaka Kurachi1 · Naoki Unno1,2 · Hiroya Takeuchi1
Received: 14 July 2020 / Accepted: 26 September 2020
© The Author(s) 2020

Abstract
Purpose To investigate the utility of the device for evaluating intestinal oxygenation and viability using an animal model.
Methods Sprague–Dawley rats underwent laparotomy under general anesthesia, and the blood vessels in the terminal ileum
were clamped to create ischemia. We measured the regional tissue oxygenation saturation ­(rSO2) using an oximeter after 1,
3, and 6 h of vessel clamping. Ischemic tissue damage was assessed using a histological score. The intestine was reperfused
after each clamping period, and intestinal ­rSO2 and survival rate were evaluated.
Results When reperfusion was performed at 1 and 3 h after ischemia, r­ SO2 increased after 10 min, and it improved to thesame level as for normal intestine after 1 h; all rats survived for 1 week. In contrast, after 6 h of ischemia, ­rSO2 did notincrease after reperfusion, and all animals died within 2 days. The histological scores increased after 1 h of reperfusion,
with longer clamping periods.
Conclusion A finger-mounted tissue oximeter could evaluate intestinal ischemia and the viability, which is thus considered
to be a promising result for future clinical application.
Keywords Rat · Intestine · Ischemia reperfusion · Near-infrared spectroscopy

Introduction
Intestinal ischemia caused by a strangulated ileus and nonocclusive mesenteric ischemia is frequently seen in gastrointestinal surgery. Most closed-loop ileus cases are associatedwith intestinal ischemia, and once the ischemia becomesirreversible, the intestine cannot be preserved even if thestrangulation is released [1–3]. Intraoperative decisions must
be prompt and accurate to determine whether the intestine
can be preserved. However, currently, most surgeons relyon personal experience to make these decisions, and new* Yuhi Suzuki
otto.yuhi419@gmail.com
1

Second Department of Surgery, Hamamatsu University
School of Medicine, 1‑20‑1 Handayama, Hamamatsu,
Shizuoka 431‑3192, Japan

2

Division of Vascular Surgery, Hamamatsu Medical Center,
328 Tomitsuka, Hamamatsu, Shizuoka 432‑8580, Japan

monitoring devices that can provide objective evaluations
are needed. Currently, intestinal fluorescence imaging using
indocyanine green (ICG) is widely used clinically to objectively evaluate intestinal ischemia. However, problems withthis technique include its complexity, quantitativity, andrepeatability [4, 5].
A finger-mounted tissue oximeter (Toccare; Astem Co.,
Ltd., Kawasaki, Japan) is a newly developed device formeasuring ­rSO2 using near-infrared spectroscopy (NIRS).
This device enables iterative and quantitative ­rSO2 evaluation of the target tissue/organ using a simple method and has
been reported to be useful in obstetrics and gynecology, vascular surgery, and plastic surgery [6–11]. If this device canbe used to evaluate intestinal ischemia, we can determine
whether the intestine can be preserved, intraoperatively,
using a rapid and quantitative evaluation. In this study, weevaluated the usefulness of a finger-mounted tissue oximeterto assess intestinal ischemia, in an animal model.

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Surgery TodayMaterials and methods
Animals
Male Sprague–Dawley rats (400–550 g, n = 128) from JapanSLC (Hamamatsu, Japan) were maintained under a 12/12-h
light/dark cycle with constant conditions of room temperature and air humidity. Prior to the experiment, all rats wereallowed free access to a standard diet and water.
This study was approved by the Institutional Animal Careand Use Committee of Hamamatsu University School of
Medicine (Permission number: 2017090). All experimental
procedures were conducted in accordance with the Fundamental Guidelines for Proper Conduct of Animal Experiment and Related Activities in Academic Research Institutions (Ministry of Education, Culture, Sports, Science andTechnology of Japan) and the National Institutes of Health
Guide concerning the Care and Use of Laboratory Animals.

Experimental groups
Three groups of rats were used in this study:• Control group (n = 23): laparotomy was performed with-out clamping. Control groups were used in the experiments for ischemic intestine (n = 4) and ischemia/reperfusion (I/R) injured intestine (n = 19).
• Clamp group (n = 38): the blood flow of intestine wasclamped. This group was sub-dived into two groups.
Each group was subjected to ischemic injury for either
1 h (Clamp 1 h group; n = 28) or 6 h (Clamp 6 h group;n = 10).
• I/R group (n = 67): ischemic manipulation as previously
described was performed for each appropriate period.
After the clamp was released, 1 h of reperfusion wasapplied. This group was sub-divided into three groups.
Each group was subjected to I/R injury for 1 h (1 h I/Rgroup; n = 24), 3 h (3 h I/R group; n = 23) and 6 h (6 h I/Rgroup; n = 20). In the survival experiment, five rats fromeach group were used. ­rSO2 and mortality was compared
between 1 and 3 h I/R group versus 6 h I/R group.

Experimental details of the ischemia/reperfusion
models
The experimental procedures were based on those describedin previous studies [12–14]. Briefly, the rats were intraperitoneally anesthetized with 0.4 mg/kg medetomidine, 2.0 mg/
kg midazolam, and 5.0 mg/kg butorphanol, and injections
were repeated every 2 h to maintain anesthesia. Under anesthesia, laparotomy was performed through a ventral midline

13

abdominal incision. The blood vessels in the terminal ileum
were clamped using bulldog forceps (C-42-S-2; Natsume
Seisakusho Co., Ltd., Tokyo, Japan) for specific lengths oftime. For the reperfusion evaluation, clamping forceps werereleased at specific time points to restore blood flow. In thesurvival experiment, animals were monitored twice a day for
1 week after operation regarding general appearance, viability, morbidity, and behavior of stools. Coma, effort respiration, heavy vomit, hematemesis, diarrhea, and melena were
defined for humane endpoint. If any of the symptoms wereobserved, then the rats were euthanized within 1 h of onset.
We euthanized all rats with 200 mg/kg pentobarbital
sodium after each experiment. All efforts were made toprotect the animals and minimize their suffering during thestudy.

Measuring intestinal ­rSO2
rSO2 was measured by gently placing the finger-mounted
tissue oximeter on the surface of the terminal ileum untilthe value stabilized after a few seconds, and the value wasrecorded manually. During measurement, a 5-mm-thickblack rubber plate was placed on the dorsal intestine to prevent near-infrared scattering. The oximeter and rubber plate
were covered with an echo probe cover (CIV-Flex™ Transductal Cover; Civco Medical Solutions, Orange City, IA) tomaintain an aseptic surgical field.

ICG angiography
After clamping the intestine to stop the blood flow for 1 h,
0.075 mg of ICG (Daiichi Sankyo Co., Ltd., Tokyo, Japan)
was injected through the right femoral vein. After 20 s, thesurface of the intestine was observed using an NIR camera
system (Photodynamic Eye; Hamamatsu Photonics K.K.,
Hamamatsu, Japan), which activates ICG with emitted light(wavelength, 760 nm) and filters out light with a wavelength
below 820 nm. The light source to detect ICG consisted of
760-nm light-emitting diodes, and the detector was a chargecoupled device camera. Fluorescence intensity was quantified using the ImageJ software program (National Institutesof Health, Bethesda, MD) as previously described [15].

Histological analysis
After I/R manipulation, the terminal ileum was excised andfixed with 10% formalin overnight at room temperature,then embedded in paraffin wax. Formalin-fixed and paraffin-embedded slides were counterstained with hematoxylin
and eosin, and histopathological grading was determinedusing the Park/Chiu score as follows: 0 = normal mucosa;
1 = subepithelial space at the tips of the villi; 2 = extensionof subepithelial space with moderate lifting; 3 = massiveSurgery Today

lifting down the sides of the villi, with some denuded tips;
4 = denuded villi, dilated capillaries; 5 = disintegration of
the lamina propria; 6 = crypt layer injury; 7 = transmucosal
infarction; and 8 = transmural infarction [16–19]. The histopathological score was evaluated independently by threeobservers. A median score for all sections was evaluated.

Statistical analysis
All statistical analyses were performed using the EZR version 1.41 software program (Saitama Medical Center, Jichi
Medical University, Saitama, Japan), which is a graphicaluser interface for R (The R Foundation for Statistical Computing, Vienna, Austria). More precisely, EZR is a modified
version of R commander designed to add statistical functions
frequently used in biostatistics [20]. Before analyzing either­rSO2 or fluorescence intensity, we used the Shapiro–Wilk
test to confirm normality of the distribution. The paired t-test
was used for compatible conditions. For multiple comparisons, two-way analysis of variance (ANOVA) or one-way
repeated ANOVA and Holm’s test for post hoc were used tocompare intestinal ­rSO2 or fluorescence intensity. Evaluating Spearman’s rank correlation analysis was performed toanalyze the interrelation between the logarithms of ­rSO2 andperiod of intestinal ischemia. The Kruskal–Wallis test andSteel–Dwass test were used to compare Park/Chiu scores.
All results except for histopathological scoring and survival
analysis were presented as the mean ± standard error of themean. Park/Chiu score was presented with a dot plot. For thesurvival analysis, the Kaplan–Meier method was used anddifferences were compared by the log rank test. For each test,
p-values < 0.05 were considered to be significant.

Results
Evaluation of ischemia by intestinal vessel clamping
We first created a model of intestinal ischemia by clamping
the vessels of the terminal ileum and evaluated the ischemia
by ICG fluorescence and a histological analysis. ICG fluorescence of the terminal ileum showed no ICG fluorescencein the area after 1 h of vessel clamping (Fig. 1a, b). The
fluorescence intensity of the clamped area was significantly
lower than that for normal intestine (Fig. 1c). Histologically,
intestinal tissue showed ischemic damage limited to the villi
after 1 h of vessel clamping, and these findings extended
into the seromuscular layer after 6 h of clamping (Fig. 2a).
A quantitative analysis by Park/Chiu score showed a significant increase in scores over time (Fig. 2b, c). These results
indicated that the clamped area was indeed ischemic, in thismodel.

Evaluation of ­rSO2 in the ischemic intestine
To verify the usefulness of the finger-mounted tissue oximeter to assess intestinal ischemia, we evaluated intestinal­rSO2 after 1 h of vessel clamping (Fig. 3a). As shown in
Figs. 3b, c, the r­ SO2 of the ischemic area was lower thanthat for the normal intestine. In order to verify the sequential ­rSO2 change, r­ SO2 values were tracked hourly for 6 h

Fig. 1  Evaluation of intestinal
ischemia in an animal model byusing ICG fluorescence imaging. a Experimental design of
the ICG fluorescence. b Evaluation of ischemia by ICG fluorescence. A macroscopic viewof the normal/clamped intestine.
Upper image: under white light.
Lower image: fluorescent image
before (left) and 20 s after ICG
injection (right). c Quantification of fluorescence intensity
using the ImageJ software
program (n = 10). s second; ICG
indocyanine green

13Surgery Today
Fig. 2  Evaluation of intestinal
ischemia in an animal model by
Park/Chiu histological scoring.
a Experimental design of Park/
Chiu scoring. b A representative
microscopic view of intestinal tissue after the indicatedduration of vessel clamping.
Upper image: Normal intestinal
mucosa (left) and denuded tipof villus (right). Lower image:
Necrosis of mucosa and muscularis propria. c Histologicalquantification of tissue ischemia
according to the Park/Chiu scoring system (n = 4 per group).
Scale bar, 100 µm. h hourFig. 3  rSO2 measurement of theintestine. a Experimental designof ­rSO2 measurement of 1-hclamped intestine. b Representative images of 1-h clamped
intestine and intestinal r­ SO2
measurement. Upper image:Intestine of clamped region
was more reddish brown thannormal intestine. Lower image:Intestinal ­rSO2 was measured
under “finger-mounted” state.
The value in the dotted circle
indicates ­rSO2. c ­rSO2 fornormal and ischemic intestine
after 1 h of vessel clamping(n = 14). h hour; ­rSO2 regional
tissue oxygenation saturation; Mmeasurement of ­rSO2(Fig. 4a). During the clamping of the blood flow for 6 h,
the macroscopical appearance of the intestine and mesentery showed ischemic and congestive change characterized
by reddish color change. The changes that derived fromischemia and congestion changed from mild to severe overtime. Eventually the intestine became dark red and mesentery became blood red with hematoma (Fig. 4b). Along with13

the changes in the macroscopical appearance, intestinal ­rSO2
was decreased significantly over time (Fig. 4c).

rSO2 after I/R
Next, we assessed intestinal ­rSO2 after I/R (Fig. 5a). Whenassessing tissue damage by Park/Chiu scores, the score

Surgery Today
Fig. 4  rSO2 measurement of theintestine hourly. a Experimental
design of intestinal r­ SO2 hourly
measurement during 6-h clamping. b Representable hourly
images of intestine during 6 hclamping the blood flow. All
sequential images were takenfrom the same rat. (c) Changesin ­rSO2 every hour up to 6 hof clamping and secondary
ischemia (n = 6). h hour; rSO2
regional tissue oxygenation
saturation; M measurement of­rSO2

Fig. 5  Intestine after I/R injury.
a Experimental design of I/R
injury among four groups. b
Representative microscopic
images of intestinal tissue
after the indicated durationof I/R. Intestinal tissue which
was not imposed any injury(left). Villi were denuded by
I/R injury (second from theleft). Villus tissue was lost but
lamina propria did not necrotize(second from the right). Mucosa
and muscularis propria became
necrosis (right). c Histologicalquantification of tissue damage according to the Park/
Chiu scoring system after theindicated duration of I/R (n = 6
per group). d ­rSO2 before and
1 h after reperfusion (n = 4 pergroup). Scale bar, 100 µm. I/R
ischemia/reperfusion; rSO2
regional tissue oxygenation
saturation; M measurement of­rSO2

increased as the duration of ischemia increased, reachinga maximum at 6 h (Fig. 5b, c). Next, we assessed intestinal­rSO2 of 1, 3, and 6-h ischemia followed by reperfusion (1 h
I/R, 3 h I/R, 6 h I/R, respectively). After 1 h and 3 h I/R,
­rSO2 recovered to the control level. However, ­rSO2 did notrecover after 6 h I/R (Fig. 5d).

Next, we assessed the short-term ­rSO2 transitions afterI/R injury. We anticipated that if the intestinal ischemia
was irreversible, the rats would not survive for 1 week postprocedure. In the 1 h I/R group (Fig. 6a), ­rSO2 decreased
gradually during the 1-h vessel clamping. After reperfusion, ­rSO2 increased every 10 min and reached an equivalent13

Surgery Today
Fig. 6  rSO2 transition after I/R
injury of 1 h I/R. a Experimental design of ­rSO2 measurement
every 10 min. b ­rSO2 changes
during 1 h of I/R (n = 9 pergroup). h hour; I/R ischemia/
reperfusion; min minute; ­rSO2
regional tissue oxygenation
saturation; M measurement of­rSO2

Fig. 7  rSO2 transition after I/R
injury of 3 and 6 h I/R group.
a Experimental design of ­rSO2
measurement of 3 h I/R group.
b ­rSO2 changes during 3 h ofI/R (n = 8). c Experimental
design of ­rSO2 measurement of
6 h I/R group. d ­rSO2 changes
during 6 h of I/R (n = 5). h hour;
I/R ischemia/reperfusion; minminute; rSO2 regional tissue
oxygenation saturation; Mmeasurement of r­ SO2; n.s. notsignificant

level to the control/normal intestine (Fig. 6b), suggestingthat the intestine might be salvageable after 1 h of ischemia.
After 3 h I/R (Fig. 7a, b), similar to 1 h I/R, ­rSO2 started
to increase after 10 min of reperfusion. In contrast, r­ SO2
did not recover after 6 h of ischemia (Fig. 7c, d). Next, we
focused on the relationship between variance of r­ SO2 andlong-term survival (Fig. 8a). All rats of 1 and 3 h I/R groups
in which r­ SO2 was increased after reperfusion survived for13

1 week after laparotomy, whereas all five rats of 6 h I/Rgroup in which r­ SO2 was not increased after reperfusion diedwithin 2 days (Fig. 8b, c). Necropsy was performed in all
rats which belonged to 6 h I/R group as soon as their death
was confirmed. The necropsy findings did not indicate any
cause of death other than intestinal necrosis. These results
suggest that intestinal ischemia might become irreversible
between 3 and 6 h of vessel clamping. In addition, the ­rSO2

Surgery Today
Fig. 8  rSO2 10 min after reperfusion and subsequent survival
outcomes. a Experimental
design of ­rSO2 measurement
and evaluation of long-term
survival. b ­rSO2 changes after
10 min of reperfusion during 1,
3 and 6 h I/R (n = 5 per group).
c Representation of the survival
for each I/R group (n = 5 pergroup). h hour; min minute;rSO2 regional tissue oxygenation saturation; M measurementof ­rSO2

of viable and salvageable intestine recovered rapidly after
reperfusion.

Discussion
Currently, intestinal ischemia is evaluated by ICG and Doppler ultrasound, but these methods are associated with problems such as complexity, quantitativity, and repeatability,and no standard method has been established [4, 21–23]. The
finger-mounted tissue oximeter used in this study is a simple
and quantitative tissue oxygen saturation monitoring device
that uses NIRS technology. In this study, we demonstrated
the usefulness of this finger-mounted tissue oximeter toevaluate intestinal ischemia for the first time. In addition to
analyze intestinal ischemia, we evaluated intestinal viability
by comparing the survival period.
NIRS was first reported in the 1980s and it has been usedprimarily to assess cerebral circulation [23]. Previous NIRS
measuring devices were used to evaluate adults’ cerebral
blood flow to a depth of 1–3 cm [9, 24, 25], but these devices
are unsuitable for assessing intestinal tissue. The thicknessof the small intestinal wall is less than 3 or 4 mm, therefore
the depth of measurement with previous NIRS devices wasconsidered to be too deep [26, 27]. Recently, the cerebral
blood flow of infants or the placental oxygen saturation of
fetuses were analyzed using advanced technology of NIRS
[28–30]. More recently, a finger-mounted tissue oximeter
was developed to measure oxygen saturation of the fetal
head, transvaginally [6]. This finger-mounted tissue oximeterhas also been used to assess lower limb ischemia in patientswith peripheral arterial disease [9, 31].
Measuring ­rSO2 with a finger-mounted tissue oximeteris very simple because the device can be quantified by simply contacting the probe with the target organ. Unlike ICG,
these devices provide repeated measurements. The optimize
measurement depth is reported to be 4–5 mm from the probe[6, 9]. However, it can be also applied to 3-mm-thick tissue,
such as facial skin and subcutaneous tissue of mouse [32].
The rat intestine was approximately 3 mm in thickness, so
we used rubber plates behind the intestine in order to avoid
penetration and the scattering of light.
We showed that ­rSO2 decreased over time after blood flowinterruption. To examine the correlation between the degreeof tissue damage and r­ SO2, hourly r­ SO2 and the Park/Chiu
score was analyzed up to 6 h. The r­ SO2 decreased over time,
and the Park/Chiu score increased over time, suggestingthat finger-mounted oximeter evaluates the degree of intestinal ischemia. ­rSO2 was calculated by the concentrationsof oxyhemoglobin [­ O2Hb] and deoxyhemoglobin [HHb]in the target tissue as ­rSO2 = [O2Hb]/([O2Hb] + [HHb]) [6,11]. We considered the reasons for reducing r­ SO2 by intestinal ischemia to be as follows: the first is the decreasing of
oxyhemoglobin by consuming oxygen in the tissue whenblood flow is blocked. Interruption of arterial blood flowcauses the decrease of oxyhemoglobin supply that results indecrease of oxygen diffusion [16, 17, 33–35]. In our study,
injured tissue by I/R indeed showed ischemic change suchas microvascular collapse, denuded villi and necrosis. The
second is the increase of deoxyhemoglobin by impairment

13

Surgery Todayof venous blood flow. The interruption of venous blood flowcauses reduction of venous drainage to portal vein. Venous
stasis increases tissue pressure and deoxyhemoglobin accumulation that results in hypoxia [16, 17, 35]. Indeed, we
found that tissue of I/R injured intestine showed collapse ofcapillary vessel and venous enlargement.
Furthermore, the rats with recovered ­rSO2 after reperfusion survived for 1 week, and none of the rats with unrecovered ­rSO2 survived. These results suggest that ­rSO2 reflects
the viability of the intestinal tract in real time, and that
finger-mounted tissue oximeters may be used to determine
whether intestine can be preserved intraoperatively. In comparison with 6 h I/R, the tissue damage of 1 h and 3 h I/R
were mild. In addition to the results of histological scoring
after I/R injury, the survival outcome also clearly reflected
the intestinal viability. To determine the actual cause ofdeath of 6 h I/R group as intestinal necrosis is difficult. However, the autopsy of five deceased rats did not suggest otherpossible causes of death such as intraabdominal hemorrhage
or injury of other organs. Therefore, we determined theircause of death as intestinal necrosis of the terminal ileum.
For clinical application, it was necessary to evaluate theviability of the intestine after intraoperative reperfusion anddetermine whether the intestinal tract can be preserved ina short time. Therefore, we analyzed the intestinal r­ SO2 a
short time after reperfusion. Some previous studies of NIRS
showed fluctuation of ­rSO2 after reflow of lower limbs by
vascular occlusion test or endovascular treatment, and r­ SO2
began to improve from early phase after reflow [31, 34, 36].
No studies have been reported to evaluate intestinal viability after reperfusion by using the finger-mounted oximeter.
Thus, we hypothesized that intestinal ­rSO2 reflects tissue
oxygenation after reperfusion and can be used for the rapid
assessment of intestinal viability.
The viable intestine showed rapid recovery of ­rSO2 in
10 min after declamping, and these animals survived for
1 week. In the viable intestine with intact microvessels, reperfusion causes rapid influx of oxyhemoglobin into intestinal
tissue, leading to r­ SO2 increase. Conversely, r­ SO2 of theintestine with impaired microvessels did not increase after
declamping because oxyhemoglobin could not flow into tissue. These findings implied the clinical usefulness of fingermounted oximeter for the rapid intraoperative assessment ofintestinal viability by measuring ­rSO2 shortly after releasing
strangulation. The optimal time point for the human intestine
should therefore be determined.
In addition, this device has potential applications for use
in association with other ischemic diseases. The minimum
horizontal measurement range of this oximeter is about
8 mm; the range form light-emitting diodes to photodiodes[6], suggesting the possibility that it could be applicable tothe verification of narrow-range ischemia such as non-occlusive mesenteric ischemia or incarceration of Richter hernia.

13

The main limitation associated with this study is that thisdevice has not been used to evaluate human intestine. The
measurement depth of 4–5 mm was optimal for the rat intestine, but it is unclear whether this depth is suitable for thehuman intestine as well. New devices with variable depth
have been developed for various types of tissue. For future
clinical application, it is necessary to verify the optimal
depth for the human intestine. In addition, the application
for laparoscopic surgery is also expected. Unfortunately, it is
not possible in the present shape, however, device upgrades
such as improvement of probe structure or wireless datatransmission might make it possible to use this device during laparoscopic surgery in the near future.
Another limitation is the brevity of follow up period afterI/R injury. In this study, the postoperative period was only
seven days and we did not evaluate the non-necrotizing irreversible variation such as post-ischemic bowel stricture [37,38]. Further studies are needed to determine whether theassessment of ­rSO2 can also predicts long-term intestinal
dysfunction.

Conclusions
Real-time ­rSO2 monitoring using a finger-mounted tissue
oximeter was useful in our animal model of I/R injury toassess intestinal ischemia and viability. If this device canbe used clinically, then intraoperative decisions regarding
intestinal preservation can be made rapidly and accurately,
and unnecessary resection or postoperative intestinal necrosis may thus be avoided.
Acknowledgments We highly appreciate the help of Toshiyuki Ojima,
MD, PhD, from Department of Community Health and Preventive
Medicine, Hamamatsu University School of Medicine for giving anadvice of analysis method. In addition, this work was supported by the
project “Development of Medical Devices and Systems for Advanced
Medical Services” from the Division of Medical Device Research in
the Japan Agency for Medical Research and Development, AMED
(awarded to Naoki Unno).

Compliance with ethical standards
Conflicts of interest No benefits in any form have been or will be received from any commercial parties related directly or indirectly to thesubject of this article.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as longas you give appropriate credit to the original author(s) and the source,
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Surgery Todayneed to obtain permission directly from the copyright holder. To view acopy of this licence, visit http://creat​iveco​mmons​.org/licen​ses/by/4.0/.

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