Mechanical and Clinical Evaluation of a Shape Memory Alloy and Conventional Struts in a Flexible Scoliotic Brace
Mechanical and Clinical Evaluation of a Shape Memory Alloy and Conventional Struts in a Flexible Scoliotic Brace
Institute of Textiles 0 1
Clothing 0 1
The Hong Kong Polytechnic University 0 1
Hung Hom 0 1
Kowloon 0 1
Hong Kong SAR 0 1
China 0 1
Hong Kong Community College 0 1
The Hong Kong Polytechnic University 0 1
Kowloon 0 1
Hong Kong SAR 0 1
China 0 1
Department of 0 1
0 Clothing, The Hong Kong Polytechnic University , Hung Hom, Kowloon, Hong Kong SAR , China. Electronic mail:
1 Orthopaedics and Traumatology, Li Ka Shing Faculty of Medicine, The University of Hong Kong , Pokfulam, Hong Kong SAR , China
2 Centre for Orthopaedic Surgery , Central, Hong Kong SAR , China
-Smart materials have attracted considerable attention in the medical field. In particular, shape memory alloys (SMAs) are most commonly utilized for their superelasticity (SE) in orthopaedic treatment. In this study, the resin struts of a flexible brace for adolescent idiopathic scoliosis (AIS) are replaced with different conventional materials and an SMA. The corrective mechanism mainly depends on the compressive force applied by the brace at the desired location. Therefore, the mechanical properties of the materials used and the interface pressure are both critical factors that influence the treatment effectiveness. The results indicate that titanium is the most rigid among the five types of materials, whereas the brace with SMA struts presents the best recovery properties and the most stable interface pressure. A radiographic examination of two patients with AIS is then conducted to validate the results, which shows that the SMA struts can provide better correction of thoracic curvature. These findings suggest that SMAs can be applied in orthoses because their SE allows for continuous and controllable corrective forces.
Nitinol; Bending stiffness; Interface pressure; Flexible brace; Material selection; Scoliosis
Scoliosis is the three-dimensional deformity of the
spine and trunk.14 Adolescent idiopathic scoliosis
(AIS) is the most common type of scoliosis, and
emerges at or near the beginning of puberty and before
skeletal maturity (generally between 10 and 16 years
old).19 Non-surgical treatment, such as immobilization
or dynamically correct posture by application of
corrective forces with a spinal cast or brace, is an
important treatment modality to prevent curve
progression and reduce the spinal deformity of AIS
patients with moderate scoliosis and a spinal curve that
ranges from 20 to 45 .
Conventional rigid scoliotic orthoses, such as the
Milwaukee brace, are fabricated with rigid materials,
such as polyethylene and stainless steel, to provide
support and stabilize the trunk and spine of patients.4
Recently, rigid braces are being constructed with more
lightweight materials, such as carbon1 and
polycarbonate,15 to improve their wear comfort. Most of these
materials are widely used because of their good
mechanical properties, low cost, and acceptable
biocompatibility23,24; however, they may not be able to
satisfy the needs of patients because the rigid materials
may cause pain, restrict daily activities and affect
To overcome the problems of rigid scoliotic
orthoses, new flexible braces composed of textile fabrics
and straps have been developed, such as Spinecor,
TriaC, Spinealite8 and a posture correction girdle.11
The researchers claim that these braces have greater
wear comfort and provide users with a better
self-image and greater mobility in daily life. However, the
braces can easily deform, which means that they do not
offer optimal corrective effects.21 Therefore, smart
materials have attracted considerable attention in the
medical field as an alternative option and are expected
to play a crucial role in the translation of laboratory
findings into clinical devices.18 Shape memory alloys
(SMAs) are an example of a smart material.
SMAs are most commonly utilized for their
superelasticity (SE) in orthopaedic treatment.12 SE is
the condition in which the SMA reverts back to its
original shape after mechanical loading is applied at
temperatures between the austenite finish (Af)
temperature and the highest temperature at which
martensite transformation can no longer be stress
induced (Md) without the need for thermal activation.6
SE can also be regarded as the ability to provide
continuous and controllable corrective forces that
allow orthoses to function accordingly.20 Nickel
titanium (NiTi) or nitinol is a widely recognized and
accepted SMA in the medical industry17 because of its
Corrective forces are a crucial factor for the
effectiveness of orthoses, and the amount of corrective force
induced is affected by the mechanical properties of the
material used. Rigid orthoses can apply corrective
forces more consistently while flexible orthoses are
more comfortable to wear. However, the relationship
between the mechanical properties of a material and
the interface pressure that is applied by non-surgical
orthopaedic devices is seldom discussed in the current
literature. Previous studies have either focused on one
type of implant material and its mechanical
performance7 or measuring the interface pressure2 of
orthopaedic implants; however, these issues are usually
examined separately. Few studies have examined the
interface pressure of non-invasive orthopaedic devices
constructed with different types of supportive
materials. This study is therefore carried out with the goal to
address the current knowledge gap by developing and
evaluating supportive materials for non-invasive
orthopaedic devices and identifying appropriate
supportive inserts that can exert optimal corrective forces
onto targeted parts of the body.
MATERIALS AND METHODS
Fabrication of Non-invasive Orthopaedic Device
The non-invasive orthopaedic device11 in this study
is a girdle that enables the posture correction of
preteen and teenage girls with early scoliosis and fitted
with new supportive materials. This girdle improves
posture deviation and reduces the progression of the
spinal curvature of scoliosis patients by exerting
optimal amounts of corrective forces onto targeted areas of
the body. Figure 1 is a sketch of the design and
highlights the primary materials used for this girdle, which
is a flexible brace fabricated with knitted fabrics, and
incorporates elastic straps (shoulder and waistband)
and resin bones (RBs) as the supportive material.
Originally, this girdle was fabricated in accordance
with 27 measured places on the body. However, it is
now produced in 3 sizes: small, medium and large.
Each different sized girdle has 3 zippers to
accommodate wearers with different body shapes. The only
difference between them is the length of the girdle. The
small size is suitable for subjects who are between 150
and 154 cm in height, the medium size for subjects
between 155 and 159 cm in height and the large size for
subjects between 160 and 164 cm in height. During the
fitting session, only the paddings need to be inserted
into the pockets in accordance with the radiographic
images. A prosthetist confirms the fit of the flexible
Preparation and Mechanical Characterization
Preparation of Strut Samples
The RBs were used as the supportive struts for the
orthopaedic device and embedded into the fabric
channels that were sewn onto the posture correction
girdle. All of these RBs have the same width (5 mm)
but with 3 different thicknesses of 1.3, 1.5 and 1.8 mm.
In this study, four other types of materials are used as
the struts for comparison purposes: acrylic (ACR),
aluminum (AL), titanium (TI) and NiTi (Ti–55 wt%
Ni). The size of these struts was determined in
accordance with the original RBs used in the girdle. All of
the samples were placed in a conditioned room (23 ±
2 C, 50 ± 10% relative humidity) for 40 hours prior
to the commencement of testing.
Heat Treatment of SMA
A binary NiTi SMA that contains approximately 50
at.% nickel and 50 at.% titanium was purchased from
Jiangyin Fasten-plt Materials Science Co. Ltd., an
SMA supplier in Mainland China. The binary NiTi
SMA was exposed to heat to create SE. The binary
NiTi SMA was treated in an oven at 800 C for 1 h and
then cooled down by using a water-quenching method
based on the method in Yeung et al.22 and Zhou et al.26
Bending and Recovery Test
The strut materials underwent a bending and
recovery test which was conducted through a 3-point
bending method by using an Instron tensile tester
(Instron , Model 4411, U.S.). The span length of the
support frame for the test samples is 165 mm, which
was determined in accordance with the waistband of
the posture correction girdle. After a sample was
placed onto the support frame at a right angle to the
edges of the supporting points, the test started with the
load nose moving downward at a uniform rate of (5 ±
1) mm/min. Three bending depths of 5, 10 and 15 mm
were tested. Five samples of each type of material were
used in the experiment, and the load-deflection curves,
stiffness (N/mm) and recovery (%) of the different
materials were calculated and compared.
Interface Pressure on Soft Mannequin
In this study, the interface pressure of the posture
correction girdle is measured by using a soft
mannequin25 to reliably compare the different supporting
materials under controlled conditions because
physiological variations found in a human being, such as
body sway, can be minimized accordingly. Since the
mannequin and girdle are both symmetrical, the
pressure sensors were only placed onto 3 positions of one
side of the body or the right side of the underbust,
waist and pelvis. The underbust level was determined
as the end of the sternum, and the waist level was
determined as the middle point between the 10th rib
and the pelvis. The Pliance -xf-16 system with a 3 9 3
socket sensor, which provides a sensing area of 30 9 30
mm2 and pressure range from 0 to 200 kPa, and a
Pliance -xf analyser are used in all of the pressure
tests because the non-invasive orthopaedic device is
similar to a garment and the Pliance X System shows
high test-retest and inter-rater reliability with good
linearity when measuring the interface pressure
produced by pressure garments.9 Each sensor measured
the pressure for 5 s. Figure 2 shows the 3 positions of
the pressure sensors during the measurement process.
The sensor position, stretch length of the elastic band
and brace location on the mannequin were marked
before the pressure measurement started. The sensor
can be temporarily attached onto the mannequin until
the pressure measurement is finished. Therefore, the
pressure differences due to the usage of the device are
eliminated. The girdle-wearing process was repeated
three times on the mannequin, and pressure
measurements were recorded at the first and third donning.
After obtaining the measurements, the mean of the
measurements recorded at the 3 locations was
calculated for comparison purposes. The pressure changes
(in percentage) between the first time and third time
that the flexible brace was placed onto the mannequin
The relationship between flexural stiffness and the
interface pressure at the level of the underbust, waist or
pelvis was analysed by using Microsoft Excel charts.
The flexural stiffness of the 4 newly sourced types of
struts with 3 different thicknesses was determined from
the bending and recovery tests, while the interface
pressure of the girdle with the different struts was
measured through pressure tests including those for the
first donning of the girdle. Scatterplots and polynomial
trendlines are used to show the trends in the statistical
data by using the R2 value.
In Vivo Testing
Material Selection Criteria
In the original design of the posture correction
girdle, RBs were used as the supportive insertions.
Although RBs accommodate body shape well due to
their high elasticity, the materials are often too soft to
establish supportive forces. Also, after donning the
brace, the supportive material should maintain its
shape, so that supportive forces can be maintained.
Therefore, the most suitable supportive insertion
material for the flexible brace should have high
strength yet good recovery at the same time. High
strength means that the struts can help to provide
support. Good recovery means that the struts can fixed
onto the body over its entire length when the posture
correction girdle is worn, and the girdle should be able
to recover back to its original shape without
deformation. Therefore, both stiffness and recovery
properties of the newly selected struts should excel those of
the original design.
Recruitment of Scoliotic Subjects
After the tests were carried out in the laboratory,
the most suitable material was selected for use as the
new supportive struts of the flexible brace. The effects
of the new and original struts were then compared by
conducting a wear trial. In this study, the subject
selection criteria for both the pressure test and the
radiographic examination are as follows: females
between the ages of 10 and 14; diagnosis of progressive
scoliotic deformity; and a primary Cobb’s angle
between 25 and 40 . The study was approved by The
Hong Kong Polytechnic University and the
Institutional Review Board of the University of Hong Kong/
Hospital Authority Hong Kong West Cluster (HKU/
HA HKW IRB). Informed consent was obtained from
the volunteer subjects and their parents.
Two 14-year-old females participated in the wear
trial. They were diagnosed with a Risser sign 4-5 and
Scurve spinal deformity, and a Risser sign 1 and C-curve
spinal deformity respectively. They had a Cobb’s angle
of 34 (T5–T10, apex at T7)/24 (T12–L4, apex at L2)
and 28.9 (T10–L3, apex at L1) respectively. They
weigh 48 and 45.3 kg, are 155 and 162 cm in height,
and have a body mass index (BMI) of 19.9 and 17.3,
respectively. Based on the material test results, the
subjects were asked to wear the posture correction
girdle with either the SMA or RB struts for 2 h10 and
then undergo a radiographic examination to evaluate
the effectiveness of the girdle when it is donned. Two
independent orthopaedic doctors were invited to
measure the Cobb’s angles of each radiographic image.
The inter-observer variability in the Cobb’s angle
measurements is ± 5 . The Cobb’s angle measured
through X-ray images was compared to that measured
without the girdle.
Bending and Recovery Test
In this study, the load-deflection curves and recovery
(%) values of all the supportive struts are recorded and
shown in Fig. 3 and Table 1. The stiffness (N/mm)
values of the samples are listed in Table 2. The results
indicate that the TI is the most rigid among the five types
of materials. The compressive load that the TI struts can
withstand at maximum compressive extension is
significantly increased from 11.6 to 31.4 N with increases in
the strut thickness from 1.3 to 1.8 mm. The SMA struts
can withstand the second highest compressive load
which ranges from 11.5 to 25.1 N, whereas the AL bones
can withstand a compressive load that ranges from 8 to
11.8 N. Since the ACR struts and RBs are more flexible
than the metal struts, they have the lowest bending force.
With regard to recovery, the SMA struts can achieve a
nearly 100% recovery, whereas the TI struts showed a
reduction in recovery from approximately 97–70% for
bending depths between 5 and 15 mm. The AL struts
have poor recovery at only 40%, whereas the plastic
materials ACR and RBs have a recovery of
approximately 90 and 75%, respectively.
Interface Pressure on Soft Mannequin
The pressure (kPa) distribution of the posture
correction girdle with different supportive materials on the
underbust, waist and pelvis areas was recorded on a soft
mannequin and is shown in Fig. 4. With regard to the
pressure induced onto the underbust, the TI struts with
a thickness of 1.8 mm exert the greatest amount of
pressure of 7.6 kPa. The AL struts with the same
thickness exert the second greatest amount of pressure,
followed by the TI struts with a thickness of 1.5 mm and
SMA struts with a thickness of 1.8 mm. In terms of the
plastic materials, the ACR struts and RBs with a
thickness of 1.3 and 1.8 mm apply the least amount of
pressure of 3.1 and 3.3 kPa, respectively. As for the
measured induced pressure onto the pelvis, the AL
struts with a thickness of 1.8 mm induce the greatest
amount of pressure of 5.5 kPa, whereas the TI and SMA
struts both with a thickness of 1.8 mm apply a similar
amount of pressure of 4.4 kPa. The ACR struts with a
thickness of 1.3 and 1.5 mm and RBs with a thickness of
1.8 mm apply the least amount of pressure of
approximately 1.4, 2.1 and 2.7 kPa, respectively. In terms of the
pressure induced onto the waist area, the results are
entirely different. The ACR struts with a thickness of
1.3 and 1.5 mm and RBs with a thickness of 1.8 mm
exert the greatest amount of pressure which ranges from
1.8 to 2.1 kPa. The TI bones with a thickness of 1.8 mm
exert the least amount of pressure of 0.2 kPa.
The changes in the amount of pressure exerted
between the first and third donning of the girdle is
shown in Fig. 5. The girdle with the AL and TI struts
show the greatest changes in pressure. At the level of
the underbust, the greatest change of 24.2% is found
with the girdle that has AL struts with a thickness of
1.5 mm, followed by those with a thickness of 1.3 mm
and then a thickness of 1.8 mm. At the level of the
waist, the girdles that have AL struts with a thickness
of 1.5 mm and TI struts with a thickness of 1.8 mm
show the greatest change in pressure of approximately
24.2 and 18.8%, respectively. At the level of the pelvis,
the girdles with AL and TI struts that have a thickness
of 1.8 and 1.3 mm show a 30.7 and 27.6% difference in
pressure, respectively. Due to small fluctuations in
pressure, only minimal differences are found among
the ACR, SMA and RB struts
The data on the pressure in the areas of the
underbust, waist or pelvis based on the flexural
stiffness of the material are plotted as scatterplots and
polynomial trendlines as shown in Fig. 6. Regarding
the pressure on the underbust, the order 2 polynomial
trendline shows a positive relationship between
stiffness and interface pressure. As the stiffness increases,
the interface pressure also tends to increase. The
figure also shows that the stiffness and interface pressure
are highly correlated (R2 = 0.934). As for the pressure
at the level of the waist, there is a negative relationship
between stiffness and interface pressure with an
obvious non-linear pattern. As the stiffness increases, the
interface pressure tends to decrease. The R2 values
show that the stiffness and interface pressure are highly
correlated (R2 = 0.962). With regard to the pressure
on the pelvis, there is a positive relationship between
stiffness and interface pressure. The R2 values show
that the stiffness and interface pressure are also
correlated (R2 = 0.695).
After the laboratory tests were completed, the SMA
struts with a thickness of 1.8 mm were selected for the
preliminary wear trial due to the high pressure
performance and maintenance of a proper shape after
examining the stiffness and recovery properties of the 5 types
of struts. This preliminary wear trial was conducted to
determine the effectiveness of the posture correction
girdle with SMA and RB supportive struts. Figure 7a
shows the X-ray images of the scoliosis patient before
and after wearing the flexible brace with SMA struts.
Before wearing the girdle, the Cobb’s angle of the
patient was 34 in the thoracic region and 24 in the
lumbar region, whereas after wearing the girdle, the
Cobb’s angles were 22.7 and 23.8 , respectively.
Therefore, the angle of the spinal curve in the thoracic
region is reduced by approximately 11.3 (reduction of
33% in the Cobb’s angle), which is comparable to that
through conventional bracing treatment.16
Unfortunately, very little improvement was observed for the
lumbar curve. With regard to the effectiveness of the
flexible brace with RB struts, Fig. 7b shows that before
the patient wore the girdle, her Cobb’s angle was 28.9
in the thoracolumbar region, whereas after wearing the
girdle, her Cobb’s angle is 21.4 . Therefore, the angle of
the spinal curve is reduced by approximately 7.5
(reduction of 26% in the Cobb’s angle). According to Fok
et al.,5 the overall percentage of correction of the
posture correction girdle for different Lenke types of curves
is around 10–20%.
A higher stiffness value indicates that a stronger
force is required to bend the material; thus, the
material is more difficult to deform. At the same time, good
recovery of the supportive materials for orthoses is
important because it indicates that the product is
durable and allows for the continuous application of
pressure/force. Material with more stiffness and good
recovery is preferred for the supportive struts of a
flexible orthosis because a flexible brace may not be
robust enough to apply corrective forces. In addition,
the material should be able to maintain its shape after
wearing or subjected to daily movement. The bending
and recovery results obtained in this study explain why
TI has been used in many of the orthoses available
today. Although thin TI struts were used, they do not
easily deform. Therefore, TI struts are more supportive
and lightweight. However, compared to the SMA used
in this study, TI is lacking in terms of recovery, and the
largest difference in the percentage of recovery between
the materials is with a strut thickness of 1.8 mm.
Accordingly, the SMA is selected for the posture
correction girdle because of its relative high stiffness and
excellent recovery properties. Although the TI struts
have the highest compressive load, they cannot
maintain a good shape after several rounds of bending.
Therefore, if TI struts are used on the girdle, they will
easily deform, which means that corrective forces
cannot be applied continuously. Additionally, overly
rigid materials may not be suitable for the flexible
brace because compressive forces cannot be applied
evenly to the 3D shape of the human body as
demonstrated by the pressure tests.
The pressure tests indicate that more rigid materials
apply greater pressure onto the bony parts of the body,
such as the underbust and pelvis areas, while flexible
materials apply greater pressure onto less bony areas,
such as the waist. Figure 8 shows a schematic diagram
to illustrate the findings. Stiffness mostly affects the
pressure on the underbust and positively affects the
pressure on the underbust and pelvis. However,
pressure on the waist is negatively influenced by the
material stiffness. In other words, the interface
pressure is significantly affected by the contour shape if the
material is very stiff. If rigid materials are used in the
fabrication of functional apparel, then moulding is
recommended to accommodate the body curves before
application because of the difficulty in bending rigid
materials, which prevents them from accommodating
the curves of the body. Rigid materials can only exert
high pressure onto convex but not concave body parts.
Although flexible materials cannot exert the same
amount of pressure as rigid materials, they can apply
force onto both the convex and concave parts of the
body. Therefore, when selecting materials for flexible
braces, the rigidity of the materials and an even
pressure distribution should be taken into consideration.
Moreover, the interface pressure on certain parts of
the soft mannequin also changed after repeated
wearing of the brace. As shown in Fig. 6, the AL and TI
struts show the greatest changes, especially at the
underbust and pelvis areas, and these changes are
likely caused by the deformation of the struts. The
tight elastic waist band of the brace applies force to
push the struts against the body, which causes them to
bend. After repeated wear, the struts start to curve (as
demonstrated in Fig. 8). As a result, the pressure on
the underbust and pelvis is reduced while that on the
waist depends on how the material deforms. If the
struts are bent and lean into the most concave point,
the pressure on the waist will increase. If they are bent
but do not lean into the most concave point, the
induced pressure will change slightly. In general, these
results agree with the recovery test results. The SMA,
ACR and RB materials show good recovery and
relatively little change in interface pressure in different
positions. Since the SMA material demonstrates SE,
the SMA was selected as the new supportive material
for the flexible brace after considering the stiffness and
recovery properties of all of the different materials for
the different struts.
The preliminary wear trial demonstrated a positive
relationship between pressure and corrective effects. A
comparison of the material test results and pressure
data with the radiographies showed that straight SMA
struts can induce higher pressure on bony parts
(underbust and pelvis) which are probably able to apply a
greater amount of force to correct thoracic curvatures.
However, these struts cannot easily conform to the
body shape; therefore, the compressive forces induced
through the girdle onto the waist region may be
inadequate, and it is difficult to correct the lumbar
spinal deformity. Flexible RB struts can conform to
the body so that the deformity in the thoracolumbar
region can probably be corrected, although the forces
induced onto the bony parts of body are minimal. To
improve the effectiveness of thetreatment with the
flexible brace, moulded SMA struts are a better option
which could conform to the body curves and improve
the distribution of pressure over the body itself.
The relationship between material stiffness and
corrective forces applied via a flexible brace is
determined in this study. Flexible materials can conform to
the body shape and induce more even pressure onto
different parts of the body. Therefore, these materials
may be suitable for the construction of a flexible brace
for mild scoliosis cases. With regard to more serious
scoliosis cases, materials with greater rigidity, such as
AL, TI and SMAs, are more appropriate choices
because they can restrict body movement to a greater
extent and apply greater amounts of pressure to certain
body parts. However, these materials cannot exert
pressure evenly because they do not easily bend to
conform to the curves of the body. For orthoses that
require an even pressure distribution, moulding in
accordance with the body shape is recommended.
Otherwise, the desired effects of treatment cannot be
achieved. Additionally, because of their SE, SMAs
have excellent recovery properties and can exert
continuous corrective forces. Therefore, the results
indicate that SMAs should be used in non-invasive
This study is a pilot and therefore the sample size
in the in vivo test is small. The SMA struts also
require further controlling of their shape. Pre-shaped
SMA struts are recommended for a larger clinical
trial in the future. On the other hand, each subject
only wore one type of brace. It is recommended that
they try on two types of girdles for a better
comparison. Also, the radiographic images are provided
by the subject herself. Although the Cobb’s angle
measurement is done by the same doctor, different
equipment used may also affect the results. Therefore,
using different X-ray machines should be avoided
when comparing the effect of bracing treatment. In
the radiographic examination, it was assumed that the
correction obtained through bracing is greater than
the potential projection bias.
The work is supported by funding from the RGC
General Research Fund [PolyU 152101/16E] entitled
‘‘Anisotropic Textile Braces for Adolescent Idiopathic
Scoliosis’’ and a research studentship granted to Ms.
CHAN Wing-yu (RUV9) from The Hong Kong
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