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Measurement accuracy
of the acetabular cup position using
an inertial portable hip navigation
system with patients in the lateral
decubitus position
Hiromasa Tanino
*, Ryo Mitsutake & Hiroshi Ito
Accurate cup placement is critical to ensure satisfactory outcomes after total hip arthroplasty.
Portable hip navigation systems are novel intraoperative guidance tools that achieve accurate
cup placement in the supine position; however, accuracy in the lateral decubitus position is under
debate. A new inertial portable navigation system has recently become available. The present study
investigated the accuracy of measurements of the cup position in 54 patients in the lateral decubitus
position using this system and compared it with that by a goniometer. After cup placement, cup
abduction and anteversion were measured using the system and by the goniometer, and were then
compared with postoperatively measured angles. Absolute measurement errors with the system
were 2.8° ± 2.6° for cup abduction and 3.9° ± 2.9° for anteversion. The system achieved 98 and 96%
measurement accuracies within 10° for cup abduction and anteversion, respectively. The system was
more accurate than the goniometer for cup anteversion (p < 0.001), but not for abduction (p = 0.537).
The system uses a new registration method of the pelvic reference plane and corrects intraoperative
pelvic motion errors, which may aect measurement accuracy. In the present study, reliable and
reproducible intraoperative measurements of the cup position were obtained using the inertial
portable navigation system.
Total hip arthroplasty (THA) is one of the most eective interventions for patients with degenerative hip dis-
ease. Accurate component positioning is critical to ensure satisfactory postoperative outcomes and minimize
complications. Acetabular cup malposition aer THA increases the risk of dislocation, impingement, leg length
discrepancies, and increased polyethylene wear1,2. Accurate acetabular cup placement is one of the most chal-
lenging aspects of THA, even for experienced surgeons; therefore, surgical guidance tools, such as navigation
systems and robotics, have been developed to increase the accuracy of cup placement. Despite the accumulated
radiographic benets of navigation systems3–11, this technology is currently underutilized for THA12. e follow-
ing disadvantages of navigation systems have been reported: the inaccurate registration of a patient’s anatomy
and position, longer operation times, the need for dedicated preoperative imaging, and high acquisition costs.
Several types of portable hip navigation systems, including accelerometer-based portable hip navigation sys-
tem, mini-optical portable hip navigation system, augmented reality-based portable hip navigation system, and
new accelerometer-based portable hip navigation system combined with an infrared stereo camera, are currently
available13–25. ese systems are novel intraoperative guidance tools that incorporate accurate cup placement
achieved by large console navigation systems as well as the usability and convenience of conventional surgical
procedures21. Accurate cup placement with portable hip navigation systems in the supine position has already
been reported13,14,18,20. In previous study of portable hip navigation system, it was described that the supine
position seemed to be better for anteversion accuracy than the lateral decubitus position13. e registration
methods of the pelvic reference plane were dierent between the spine position and the lateral decubitus posi-
tion. One prospective, randomized, controlled study of portable hip navigation system in the lateral decubitus
position reported accurate cup placement, whereas the other prospective, randomized, controlled study did
not15,21. Accuracy of portable hip navigation system in the lateral decubitus position is still under debate13,15,21.
OPEN
Department of Orthopaedic Surgery, Asahikawa Medical University, Midorigaoka-Higashi 2-1-1-1,
Asahikawa 078-8510, Japan. *email: tanino@asahikawa-med.ac.jp
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A retrospective study from a designer-surgeon series26 recently reported accurate cup measurements in not only
the supine position, but also the lateral decubitus position using a new inertial portable hip navigation system
(INS), which uses a novel registration method of the pelvic reference plane and corrects intraoperative measure-
ment errors caused by intraoperative pelvic motion. Based on our knowledge, apart from this designer-surgeon
series26, there have been no clinical ndings reported on INS. erefore, the measurement accuracy of the cup
position using INS in the lateral decubitus position was investigated herein and then compared with that by a
goniometer and those described in the literature.
Methods
Patients
e present study analyzed 58 consecutive cases of primary THA performed using INS between May 2023 and
July 2023. e Institutional Review Board of Asahikawa Medical University (AMU23081) approved the present
study and waived the need for informed consent due to the retrospective design. is study was performed in
accordance with the ethical standards of the 1964 Declaration of Helsinki. We excluded 1 patient (1 hip) with
pin loosening during surgery and three patients (3 hips) in whom the cup angles measured by a goniometer
were not available. erefore, 54 procedures were ultimately analyzed. Among the cases examined, average age
was 66years (range, 44–85years), average height and body weight were 155cm (range, 136–177cm) and 62kg
(range, 40.6–112.0kg), respectively, 43 were female and 11 were male, and THA was performed on the le side
in 19 cases and the right side in 35. Forty-six patients were preoperatively diagnoses with osteoarthritis, 5 with
osteonecrosis of the femoral head, and 3 with femoral neck fracture.
Surgical procedure
ree surgeons at Asahikawa Medical University Hospital (HT, HI, and RM) performed all surgeries. Patients
were correctly positioned in the lateral decubitus position, namely, the patient’s sagittal plane was coplanar with
the operating table, while the patient’s longitudinal axis was in line with the long axis of the operating table. A
standard posterior approach with repair of the posterior so-tissue in a lateral decubitus position was used for
all patients. A lateral position xation device was employed to ensure that patients were xed on the operating
table. All patients underwent THA using INS (Navbit Sprint; Navbit Pty Ltd., Sydney, Australia) for cup place-
ment. e surgical team had used several other portable hip navigation systems for more than four hundred
THA prior to the present study21–23.
INS contains inertial sensors, including accelerometers and gyroscopes, and consists of a disposable naviga-
tion unit (Navigation Device), Device Mount, bone pins, and Impactor Fitting (Fig.1). With a patient in the
lateral decubitus position, the Device Mount was percutaneously xed to the operated iliac crest using two bone
pins. Prior to the surgical approach, the pelvic reference plane was registered with the patient in the lateral decu-
bitus position. e Navigation Device was attached to the Device Mount (Fig.1a) and obtained the gravity vector,
which represented the transverse axis. e operating table was tilted 10° to the le and then 10° to the right.
e Navigation Device acquired the orientation at each tilted position and the axis of rotation linking these two
orientations was measured, which represented the longitudinal axis of the patient. e third and nal axis was
then calculated as being perpendicular to the rst two, which represented the anteroposterior axis of the patient
(the table tilt method) (Fig.2)26,27, thereby dening the functional pelvic reference plane of the patient, against
which cup abduction and anteversion angles were measured. e Navigation Device was removed from the
Figure1. A new inertial portable hip navigation system. In the lateral decubitus position, the Device Mount
was percutaneously xed to the operated iliac crest using two bone pins. e Navigation Device was attached to
the Device Mount (a). When it was ready to place the nal acetabular component, the Navigation Device was
returned to the Device Mount to update the pelvic coordinate system (b). e Navigation Device was xed to
the cup impactor by the Impactor Fitting and showed radiographic cup abduction and anteversion (c).
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Device Mount until the acetabulum was ready to place the nal acetabular component. Once the nal acetabular
component was ready to be placed, the Navigation Device was returned to the Device Mount to update the pelvic
coordinate system (Fig.1b), which corrected intraoperative measurement errors caused by intraoperative pelvic
motion. e Navigation Device was then xed to the cup impactor by the Impactor Fitting, which enabled the
Navigation Device to display radiographic cup abduction and anteversion (Fig.1c)28. With considering Lewin-
nek safe zone (cup abduction 30°–50°, anteversion 5°–25°)1, 40° abduction and 20° anteversion were the targets
for cup placement in all patients, as reported by Domb etal.3. Following the supplemental xation of screws
for all hips, the cup impactor was reattached to the acetabular cup and the Navigation Device was reattached to
the Impactor Fitting in order to remeasure cup abduction and anteversion angles because screw xation may
change the cup position29. A cross-linked polyethylene liner was then inserted. In the present study, the majority
of femoral stems used were cemented CMK Original Concept stems (Zimmer Biomet, Warsaw, IN, USA), while
cementless stems (POLAR; Smith & Nephew, Watford, United Kingdom: S-ROM; Depuy, Warsaw, IN, USA)
were inserted into nine hips. A cementless, hemispherical acetabular component (R3; Smith & Nephew) and
32-mm ceramic or Oxinium heads were used in all cases. e postoperative rehabilitation program was identical
for all patients. Ambulation was initiated on the rst postoperative day and immediate full-weight bearing was
permitted using a walker and crutches.
Data collection
One week aer surgery, pelvic anteroposterior radiographs were performed on patients in the supine position
and were used to assess cup abduction. Cup abduction was measured as the angle between a line drawn through
both acetabular teardrops and the line through the face of the acetabular component. All patients included in
the present study underwent a pelvic computed tomography (CT) scan in the supine position one week aer
surgery. On axial CT images obtained through the central position of the cup, cup anteversion was measured
perpendicular to a line drawn across the ischial spines. Digital measurements of postoperative cup angles (ViewR;
YOKOGAWA, Japan) were performed by one observer, as previously reported21–23. Data obtained for cup antever-
sion were converted to radiographic denitions and then analyzed28. Main outcome measures were as follows:
the absolute dierence between cup angles displayed on INS during surgery and those measured aer surgery,
dened as the measurement error, postoperatively measured cup abduction and anteversion angles, the time
required for pin insertion and registration, and the operation time (including the time required for pin insertion
and registration). e cup abduction and anteversion angles were also measured by a goniometer during surgery
aer measurements with INS, the method is same as previous study19, and accuracy measured by the goniometer
was compared with the measurement error.
Statistical analysis
e Shapiro–Wilk test was rst performed to assess the normality of data distribution for continuous variables.
Data were analyzed using a two-tailed independent t-test for normally distributed data or a Mann–Whitney U
test for non-normally distributed data. A two-tailed paired t-test was also used to compare measurement errors
with the absolute dierence between cup angles measured by the goniometer during surgery and postoperatively
measured angles. Pearson’s correlation was used to identify continuous factors aecting measurement errors.
p values < 0.05 indicated a signicant dierence. SPSS Version 24 (SPSS Inc., Chicago, IL, USA) was employed
for statistical analyses.
Figure2. e table tilt method. A gravity vector was acquired by the Navigation Device, which represented
the transverse axis (b). e operating table was tilted 10° to the le and then 10° to the right (a and c). e
Navigation Device acquired the orientation at each tilted position and the axis of rotation linking these two
orientations was measured, which represented the longitudinal axis of the patient. e third and nal axis was
then calculated as being perpendicular to the rst two, which represented the anteroposterior axis of the patient
(the table tilt method) (b)26,27, thereby dening the functional pelvic reference plane of the patient, against which
cup abduction and anteversion angles were measured.
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Results
Mean postoperatively measured cup abduction and anteversion of the acetabular cup were 39.0° ± 5.6° (range,
24°–50°) and 18.6° ± 5.9° (range, 2.9°–31.6°), respectively (Table1). Cup angles displayed on INS during surgery
and those measured by the goniometer during surgery are also shown in Table1.
Mean absolute measurement errors were 2.8° ± 2.6° (range, 0°–12°) for cup abduction and 3.9° ± 2.9° (range,
0.1°–14.5°) for cup anteversion. Regarding cup abduction, measurement errors > 5° and > 10° were observed in
eight (14.8%) and one hip (1.9%), respectively. Concerning cup anteversion, a measurement error > 5° was noted
in een hips (27.8%) and > 10° in two hips (3.7%). No hip had a measurement error > 10° for both cup abduction
and anteversion. Scatterplots of the measurement errors of INS revealed a few outliers (Fig.3). Measurement
errors for cup abduction and anteversion did not correlate with age (p = 0.698, 0.748), height (p = 0.771, 0.189),
weight (p = 0.294, 0.330), diagnosis (p = 0.307, 0.284), sex (p = 0.614, 0.237), or the operated side (p = 0.517, 0.355).
Absolute dierences between cup angles measured by the goniometer during surgery and postoperatively
measured angles are shown in Table2. e absolute measurement error was not signicantly dierent from the
absolute dierence for cup abduction (p = 0.537), whereas the absolute measurement error was signicantly
smaller than the absolute dierence for cup anteversion (p < 0.001) (Table2).
Table 1. Mean cup abduction and anteversion angles measured postoperatively, displayed on INS, and
measured by the goniometer.
Measured postoperatively Displayed on INS during surgery Measured by the goniometer during surgery
Mean cup abduction 39.0° ± 5.6° 40.2° ± 4.5° 38.2° ± 6.0°
(range, 24°–50°) (range, 29°–50°) (range, 24°–55°)
Mean cup anteversion 18.6° ± 5.9° 17.8° ± 3.3° 22.4° ± 4.5°
(range, 2.9°–31.6°) (range, 9°–32°) (range, 13°–33°)
Figure3. Measurement errors of INS (non-absolute values). A positive value for the measurement error was
recorded when the cup angles displayed on INS during surgery were larger than postoperatively measured
angles, while negative values indicated that the cup angles displayed on INS during surgery were smaller than
postoperatively measured angles.
Table 2. Measurement accuracy of the cup position with INS and a goniometer.
Measurement error (INS) Absolute dierence between cup angles measured by a goniometer and postoperatively
measured angles p value
Cup abduction 2.8° ± 2.6° 3.1° ± 3.0° 0.537
Cup anteversion 3.9° ± 2.9° 6.1° ± 4.6° < 0.001
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e mean operative time was 70.4 ± 20.0min (range, 46–155), and the mean time required for pin insertion
and registration was 3.6 ± 2.1min (range, 1–11).
Discussion
e present study examined the accuracy of measurements of the cup position in patients in the lateral decubitus
position by INS, which uses a new registration method of the pelvic reference plane (the table tilt method) and
corrects intraoperative measurement errors caused by intraoperative pelvic motion. INS achieved 98 and 96%
measurement accuracies within 10° for cup abduction and anteversion, respectively. Intraoperative measure-
ments of cup anteversion were more accurate using INS than the goniometer, whereas those of cup abduction
were not. erefore, we obtained reliable and reproducible intraoperative measurements of the acetabular cup
position using INS.
Current imageless hip navigation systems calculate the implant position in relation to a reference plane, and
various registration methods have been used, including the methods to use the anterior pelvic plane, the oper-
ated anterior superior iliac spine and the spinous process of L5 vertebra30, the functional pelvic plane, and the
longitudinal axis of the patient and the gravity vector. Since incorrect registration results in inaccurate navigation,
concerns have been raised over the accuracy of registration methods. Many large console imageless navigation
systems employ the anterior pelvic plane, which is dened by the right and le anterior superior iliac spines
and pubic symphysis, as the pelvic reference plane for cup abduction and anteversion. However, diculties
are associated with identifying these anatomical landmarks, particularly in obese patients or in patients in the
lateral decubitus position. First-generation portable hip navigation systems, including the accelerometer-based
portable hip navigation system (HipAlign; OrthAlign Inc., Aliso Viejo, CA, USA) and mini-optical portable hip
navigation system (Intellijoint HIP; Intellijoint Surgical Inc., Kitchener, Canada), use an alternate method to
register the pelvic reference plane of a patient in the lateral decubitus position. e registration probe is manually
positioned parallel to the longitudinal axis of the patient in the lateral decubitus position to obtain the coronal
pelvic reference plane. Although this coronal registration does not need to identify the anatomical landmarks,
such as anterior superior iliac spine and pubic symphysis, and eliminates concerns regarding palpation errors
in obese patients, it assumes that the pelvis is held in a strict lateral decubitus position at the beginning of sur-
gery and is dependent on the operating surgeon’s judgement. Conicting ndings have been reported on the
accuracy of rst-generation portable hip navigation systems in the lateral decubitus position. Two prospective,
randomized, controlled studies compared the accuracy of cup placement with the accelerometer-based portable
hip navigation system and the conventional technique in the lateral decubitus position. One study indicated that
cup placement was more accurate in the navigation group21, whereas the other did not15. e table tilt method
is used for coronal registration by INS, which assumes that the pelvis is held in a strict lateral decubitus position
at the beginning of surgery; however, this method does not require the identication of anatomical landmarks
and is not dependent on the operating surgeon’s judgement.
e pelvis is oriented by surgeons into a strict lateral decubitus position at the beginning of surgery and it is
generally assumed to remain in this position throughout the procedure. However, due to manipulations of the leg
as well as the levering eect of retractors for adequate exposure, intraoperative pelvic motion inevitably occurs.
Previous studies reported that intraoperative pelvic motion with patients in the lateral decubitus position may
vary by up to 32° of roll, 25° of tilt, and 10° of pitch31,32. is motion may result in inaccurate cup placement.
One study demonstrated that for every 1° change in pelvic roll, tilt, and pitch, radiographic cup abduction was
changed by 0.22, 0.19, and 1.00°, respectively, while radiographic cup anteversion was changed by 0.61, 0.75, and
0.00°, respectively32. During non-navigated procedures, the precise orientation of the pelvis at cup placement
is unknown, and traditional navigation and rst-generation portable hip navigation systems do not adequately
compensate for this type of pelvic motion13. INS updates the pelvic coordinate system by returning the Naviga-
tion Device to the Device Mount and corrects intraoperative measurement errors caused by intraoperative pelvic
motion. is may improve measurement accuracy with INS.
Several second-generation portable hip navigation systems, including an augmented reality-based portable
hip navigation system (AR-HIP; Zimmer Biomet Japan, Tokyo, Japan) and a new accelerometer-based portable
hip navigation system combined with an infrared stereo camera (Naviswiss Hip miniature imageless navigation
platform; Naviswiss AG, Brugg, Switzerland), are currently available. ese systems use the functional pelvic
plane, which is dened by the right and le anterior superior iliac spines and the gravity vector, as the pelvic refer-
ence plane, and it is registered in the supine position. When THA is performed in the lateral decubitus position,
the patient is moved into this position aer supine registration (the ip technique). e position tracking system
of a large console navigation system is generally based on infrared light, whereas the augmented reality-based
portable hip navigation system uses a standard red-green-blue camera to track markers within the sterile eld
and quick response codes as augmented reality ducial markers. Regarding an augmented reality-based port-
able hip navigation system in the lateral decubitus position, three prospective, randomized, controlled studies
reported more accurate cup placement than the conventional technique in the lateral decubitus position19,23,
or smaller measurement errors than a rst-generation portable hip navigation system in the lateral decubitus
position16. One cadaver study from the designer-surgeon series reported a mean absolute measurement error
of 4.1°±3.3° for both cup abduction and anteversion in the lateral decubitus position using INS27. One retro-
spective study from the designer-surgeon series reported mean absolute measurement errors of 2.3°±2.8° and
2.7°±2.5° for cup abduction and anteversion, respectively, in the lateral decubitus position26. However, apart
from this designer-surgeon series26, there have been no clinical ndings reported on INS. e time required for
pin insertion and registration by INS currently remain unknown. In the present study, measurement accuracy
for cup abduction appeared to be equivalent to the ndings of the designer-surgeon series, whereas that for cup
anteversion was slightly inferior (Table3). Based on previous ndings on portable hip navigation systems, the
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measurement errors of INS may be lower than those by rst-generation portable hip navigation systems for
cup abduction and anteversion. New registration method of the pelvic reference plane (the table tilt method)
and correction of intraoperative pelvic motion errors by INS might improve the accuracy. In comparisons with
second-generation portable hip navigation systems, the measurement error was similar for cup abduction, but
slightly larger for cup anteversion (Table3). In addition to dierences in the registration methods of the pelvic
reference plane, position tracking systems are dierent among portable hip navigation systems. Currently, little
is known for the eects of dierent position tracking systems on the accuracy of portable hip navigation systems.
Although several portable hip navigation systems require equipment outside of the sterile eld, INS is used
entirely in the sterile eld, removing the need for the surgical team to interact with a system outside of the sterile
eld. INS is compact, the pelvic reference plane is registered aer draping in the lateral decubitus position, and
the time required for pin insertion and registration is short (mean of 3.6min). Furthermore, the ip technique,
which is used with second-generation portable hip navigation systems, is not needed. is technique requires
additional time, particularly during the pre-operation phase, specically to re-drape and re-position the patient
aer the insertion of pins and registration23. We obtained reliable and reproducible intraoperative measurements
of the acetabular cup position using INS; however, the following limitations need to be addressed.
e rst limitation is related to the cost-to-benet ratio and clinical eects. Although INS is not an expensive
system ($800), the present study demonstrated its radiographic benets, and time required for pin insertion and
registration was 3.6min, it remains unclear whether the magnitude of radiographic benets demonstrated in
the present study are clinically signicant. Future studies are needed to investigate the cost-to-benet ratio and
clinical ndings. However, in our opinion, the reproducibility of contemporary THA appears to be dependent on
the combination of a number of factors, such as the component position, implant design, bearing, and surgical
technique. Although no individual factor in itself may prevent complications and improve clinical outcomes, we
think surgeons should use these variables in combination for excellent THA outcomes.
e second limitation is related to the position of the patient at the beginning of surgery and the correction of
intraoperative measurement errors caused by intraoperative pelvic motion. Coronal registration by INS assumes
that the pelvis is held in a strict lateral decubitus position at the beginning of surgery; however, the actual pelvic
position is not a strict lateral decubitus position, which causes measurement errors. e pelvic reference plane
of INS is xed to the Device Mount, and by extension, to the pelvis. erefore, the pelvic reference plane moves
with the pelvis. is minimizes intraoperative pelvic motion errors. However, the eects of the pelvic position
at the beginning of surgery and whether INS adequately compensates for intraoperative pelvic motion errors
currently remain unclear. Future studies are required to assess the eects of the pelvic position and intraopera-
tive pelvic motion errors.
e third limitation is the location of the hip center. e anatomic center was recommended in order to
decrease the hip joint reaction force33 and the location may be related with the incidence of dislocation aer
THA34. It is dicult to measure the location of the hip center using imageless portable hip navigation systems
at this time. With recent advances in computer technology35–37, future modications of portable hip navigation
system are desirable.
Cup placement has historically been guided according to the safe zone reported by Lewinnek1. It remains con-
troversial whether cup placement inside the Lewinnek safe zone decreases the dislocation rate. Smaller or indi-
vidualized safe zones, including spinopelvic alignment with the pelvic tilt and spinal deformities, have recently
been suggested38. Regardless of the target values set for each individual case, more accurate cup placement is
Table 3. Comparison of ndings of portable hip navigation systems in the lateral decubitus position.
Placement error (absolute dierence between the intended target and the angle achieved) Measurement error
Type of navigation Abduction (°) Anteversion (°) Abduction (°) Anteversion (°)
First-generation portable hip navigation
Tanino21 Accelerometer 3.7° ± 3.0° 6.0° ± 4.5°
Tetsunaga24 Accelerometer 4.1° ± 3.7° 6.8° ± 4.8°
Tanino22 Accelerometer 3.7° ± 3.3° 5.9° ± 3.6°
Kiyohara15 Accelerometer 4.3° ± 3.2° 4.4° ± 2.9°
Kurosaka16 Accelerometer 3° ± 2° 5° ± 4°
Vigdorchik25 Optical 4.1° ± 2.7° 5.3° ± 4.4°
Mei17 Optical 5.2° ± 4.0° 4.8° ± 5.4°
Second-generation portable hip navigation
Ogawa19 AR 1.9° ± 1.3° 2.8° ± 2.2° 2.0° ± 1.5° 2.9° ± 2.0°
Kurosaka16 AR 3° ± 2° 2° ± 2°
Tanino23 AR Median 1° (IQR 0–4.0) Median 2° (IQR 1.9–3.7)
Ohyama18 Accelerometer with an infrared
camera Median 2.1° (IQR 1.0–3.7) Median 2.1° (IQR
0.9–3.1)
INS
Xu26 INS 2.3° ± 2.8° 2.7° ± 2.5°
is study INS 2.8° ± 2.6° 3.9° ± 2.9°
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becoming increasingly important. e results of this non-designer-surgeon series showed reliable intraoperative
measurements of the acetabular cup position using INS.
Data availability
e datasets used and/or analyzed during the present study are available from the corresponding author upon
reasonable request.
Received: 9 September 2023; Accepted: 9 January 2024
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Acknowledgements
We thank Navbit Pty Ltd. for providing pictures of the INS and table tilt method. e author has all rights to the
pictures used in this study.
Author contributions
H.T. participated in the study design, draing of the manuscript, and data collection. R.M. participated in data
collection and draing of the manuscript. H.I. participated in data collection and draing of the manuscript.
All authors read and approved the manuscript.
Competing interests
e authors declare no competing interests.
Additional information
Correspondence and requests for materials should be addressed to H.T.
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