Soft Active Dynamic Brace for Spinal Deformities

Athar Ali

, Vigilio Fontanari

, Marco Fontana

and Werner Schmölz

Department of Industrial Engineering, University of Trento, Italy

Medizinische Universität Innsbruck, Austria

Keywords: Dynamic Brace, Scoliosis, Corrective Orthosis, Twisted String Actuator, Spinal Exoskeleton.

Abstract: Scoliosis is a 3D deformity of the spine which not only limits the daily activities but in severe cases results in

damaging the musculoskeletal, respiratory and nervous system. A conventional way to treat spine deformity

is to wear braces. Braces are usually static, rigid and passive and they do not allow the mobility to the spine.

This causes the issues of spine stiffness and weakening of the muscles around the spine, which results in other

spine complexities such as the flat back. In this study, we have developed a soft active dynamic brace which

not only applies the 3D corrective forces but also allow the mobility to the spine. The brace applies the

corrective forces using elastic bands, whose tension is being controlled using lightweight twisted string

actuation (TSA) mechanism. TSA generates a higher pulling force using low torque motors, which not only

reduce the weight of the device but also the metabolic cost.

1 INTRODUCTION

Scoliosis is an abnormality of the spinal curve and

every year over 600,000 people are being treated with

this disease in United States (Ogilvie, 2010). Patients

with scoliosis feel comfortable in the deformed pose

and with the growth of the spine in the wrong posture,

the deformity or the cob angle also increases. The

conventional way to treat the spine deformity is to use

braces. A brace is mostly recommended to the

adolescence patients whom spine is still growing and

their cob angle is less than 30-40

. If the cob angle is

greater than 40

than surgery is imminent(Zaina et al.,

2014). Several braces have been developed in the mid

20th century such as Milwaukee(Blount et al., 1958;

Lonstein & Winter, 1994), Boston(Emans et al.,

1986; Périé et al., 2003), Chêneau(Hopf & Heine,

1985; Rigo & Weiss, 2008), Charleston(Lee et al.,

2012) and Lyon brace(De Mauroy et al., 2008). These

braces differ based on their rigidity, symmetry,

openings (posterior/ interior), breathing technique

and principle of correction(Grivas et al., 2018). Some

braces are constructed to apply de-rotation and

tractive force to the spine (Lonstein & Winter, 1994)

https://orcid.org/0000-0002-5936-447X

https://orcid.org/0000-0001-8236-522X

https://orcid.org/0000-0002-5691-8115

https://orcid.org/0000-0003-2962-2594

or pure spine bending (Wiemann et al., 2014), while

others are custom-made to provide three-point

pressure bending along with de-rotation on abnormal

spine curves and apices (Park et al., 2018; Rigo &

Weiss, 2008).

Although braces are quite effective in limiting the

curve progression or even correcting the cobb angle.

However, their passive, rigid, and static designs limit

the motion of the spine column. Results in stiffening

of the spine and weakening of the muscles around it.

Rigid braces affect cardiopulmonary efficiency and

also cause skin breakdown and abnormal bone

deformation. Therefore, designing a brace or spinal

exoskeleton which allows the necessary movements

and bring back the patient to correct posture is

needed. That kind of dynamic brace can also be used

as an assistive device after post-operative

rehabilitation to stabilize the spine and keeping the

spine in a correct posture.

Some soft braces such as Spinecor (Gutman et al.,

2016; Wong et al., 2008), ScoliSMART

(Morningstar, 2013) and Tria-C (Veldhuizen et al.,

2002) have also been developed to enhance mobility

and comfort, but they are passive and have no control

Ali, A., Fontanari, V., Fontana, M. and Schmölz, W.

Soft Active Dynamic Brace for Spinal Deformities.

DOI: 10.5220/0010343301690174

In Proceedings of the 14th International Joint Conference on Biomedical Engineering Systems and Technologies (BIOSTEC 2021) - Volume 1: BIODEVICES, pages 169-174

ISBN: 978-989-758-490-9

169

over the amount of force being applied. The

University of Colombia developed an active dynamic

brace named ROSE(Murray et al., 2020; Park et al.,

2018). It applies corrective forces at the different

cross-sections of the torso while allowing the

mobility. ROSE uses two-layer Stewart platform

which is actuated using eight series actuators. These

actuators increased the weight as well as the power

consumption of the device. Braces are supposed to be

worn over 18 hours a day which limits the use of

ROSE dynamic brace.

This article presents the design of the novel soft

active dynamic brace using twisted string actuation

mechanism. This article focuses on the design

requirements and modelling of twisted string actuator

for the soft active dynamic brace to treat scoliosis.

The objective of the research is to develop a brace

which can apply controlled forces and allow the

movement of the spine to overcome the limitation of

passive rigid braces.

2 ACTIVE DYNAMIC BRACE

The active dynamic brace is a soft brace developed to

treat spine deformities and enhance the comfort and

mobility of the spine. The brace uses a compact

lightweight actuation mechanism to apply corrective

forces to the spine. Following sections explain the

brace working principle, design and actuation

mechanism etc.

2.1 Working of Brace

The active dynamic brace uses elastic resistance to

generate corrective movement of the spine with the

movement of the patient. In the long term, this will

reprogram the neuromuscular system and will be able

to slow or stop the curve progression and improve the

overall posture of the patient. Elastic resistance levels

are being controlled using a compact low powered

actuation mechanism which results in controlled

corrective forces being applied to torso.

The objective of the brace is to improve the spinal

alignment and pain relief by offloading the muscles,

nerve roots and joints of the spine. It will stabilize the

spine by reinstructing the movement patterns while

keeping the spine in the correct (de-rotated) posture.

Steady correction in spinal and postural alignment will

help to obviate the typical deterioration of posture and

progression of spinal degeneration disease.

Active dynamic brace treatment will provide in

pain relief and help in the postural correction in

adolescences. It will prevent the progression of cobb

Figure 1: Dynamic Active Soft Brace.

angle in adults with scoliosis and other spinal

deformities like kyphosis. It will provide effective

control over scoliosis while preserving the near-

normal movement. It will reduce the risk of muscle

atrophy observed in rigid braces. The dynamic active

brace will be able to provide specific localized control

of the spine and body posture, combined with curve

specific corrective movements.

2.2 Brace Design

In the proposed design shown in Figure 1, the

corrective bands are attached to a contoured body vest

which covers the end of the rib cage. It consists of

four 50 mm wide elastic bands which provide

thoracic rotation, shoulder rotation and left lateral

flexion. Firstly, the right thoracic flap (orange band)

is attached to the lower right corner of the vest and

wrap around the rib cage to finally attach to the pelvic

back of the body. This band provides thoracic rotation

in a counterclockwise direction and attached using

BIODEVICES 2021 - 14th International Conference on Biomedical Electronics and Devices

170

velcro crocodile strips. The tension in the band can be

adjusted to keep the spine at the correct posture. The

second flap (Tosca green band) is attached to the left

thoracic base. This flap wraps around the abdominal

part of the body and goes all the way to the right half

of pelvic back. Tension in this band is adjusted bit less

as compared to the orange flap to keep the spine

rotated in a counterclockwise direction. Third band

(purple flap) attached to the left shoulder, rotates

around the rib cage and back and finally attached in the

front of the pelvic belt. This band generates clockwise

shoulder rotation and left lateral flexion at T12. The

fourth band, the right shoulder flap generates clock-

wise shoulder rotation and clockwise shoulder tilt.

The tensions in the elastic bands are being

controlled using twisted string actuators. A Twisted

String Actuator (TSA) is a simple, cheap, portable

and compact mechanism and is an alternative to

conventional gear systems. In the TSA, a string that

is co-axially attached to the motor shaft acts as a high-

ratio gear, which yields the potential to generate high

output force with low input torque.

2.3 Actuation Mechanism

The dynamic active soft brace uses twisted string

actuators as an actuation mechanism. Actuation

module consists of Pololu Brushed DC gear motor

2386 with gear ratio 150:1, equipped with optical

encoders. DC gear motor shaft is attached to the 3mm

four-hole mounting hub, which twists the four

Dyneema fishing strings as shown in Figure 2.

Actuation module for a single TSA weighs

approximately 150 grams. Resulting in total device

weight nearly 1Kg including power. Which is

significantly less than the other braces. When a string

is attached to the motor and twisted it behaves like a

gear with non-linear transmission ratio. Table 1

describes the parameters of the twisted string

actuator.

Figure 2: Twisted String Actuator.

Table 1: TSA parameters.

Parameter Value

String Length (

)

200mm

String Radius (r) 0.725mm

Spring Constant of the band (K) 133 N/m

String Material Dyneema

Motor Output Power 1.2 Watt

Output power at max efficiency 0.68 W

No load Current 70 mA

Current at max efficiency 0.31 A

2.4 Device Actuation Modelling

To effectively control the twisted string actuation

mechanism, it is important to estimate the contraction

of the string length based on the rotation of the

actuator shaft. The shaft rotation θ can be measured

through optical encoder attached to the motor. A

conventional twisted string model can be derived

from the string’s helix geometry as shown in Figure 3

(Igor et al., 2021). The contraction of length X as a

function of twist angle θ as shown in Figure 3 can be

written as:

𝑟



𝜃



𝐿



𝑋



𝐿



0 (1)

𝑋𝐿



𝐿





𝑟



𝜃



(2)

Where L

is the original length of the string

bundle before twisting and r is the radius of string

bundle after 5 turns. According to the conventional

mathematical model of the TSA, a string of length L

twisted by a motor for an angle θ contracts by X

amount.

Figure 3: Schematic depiction of a section of a twisted

string.

2.5 Experimental Evaluation

To verify the actuation model of the brace, a setup

was designed consisting of Pololu DC gear motor

Encode

Moto

Gea

Mounting

Hub

String

Ban

Soft Active Dynamic Brace for Spinal Deformities

171

2386 with optical encoder and a gear ratio of 150:1 to

twist the four 0.4mm Dyneema fishing strings of 20

cm (200mm) attached to the motor shaft and elastic

band. A laser displacement sensor (keyence lk-g152)

and a load cell were used to measure the actual

position and pulling force of the string. The setup

configuration can be seen in Figure 4. The string

length contraction and the elastic resistance force

graphs can be seen in Figure 5 and Figure 6.

Figure 4: Actuation model verification setup.

Figure 5: String length contraction with shaft revolutions.

Figure 6: Force to control elastic resistance with respect to

the shaft rotation.

The model tracks the position effectively with the

RMSE of 0.17386 cm (1.7386mm). While the force

model shows the RMSE of 0.7242 N.

2.6 Life Cycle

Evaluating the life cycle of the string is one of the

important aspects of twisted string actuator. This

study evaluates the life cycle of the twisted string

actuator in different twisting regions. In the

experiment, the effect of the number of motor shaft

rotations per cycle on string behaviour is studied. The

cycle represents the twisting of the string for a

specific number of turns (20,30,35,45) and then it's

untwisting back to the original position.

The experimental setup consists of the DC gear

motor with an optical encoder attached to it, a current

sensor that measures the system current and motor

torque to report the system failure. A controller is

programmed to conduct the cycles. A short delay of a

few seconds is introduced after each cycle to reduce

the motor temperature. The data of the number of

turns and motor current is logged in the file with the

duration equals the experiment time. Thus, having a

record of the number of cycles of twisting that motor

endured by analysing the system failure point through

motor current.

Figure 7: Life cycle test for twisted string actuator.

Region up to 20turns represents the low

contraction region and have a higher life cycle of

2712. While the 35 turns cycle corresponds to the

starting of the overtwisting phase and represents the

maximum contraction limit without over twisting the

string with a cycle life of 1080. The 45 turns are the

limit where overtwisting is possible without

untwisting issues. It can be seen from Figure 7 that

the life cycles of the TSA reduce while going for the

high contraction of the string. Therefore, the active

dynamic brace operates below the overtwisting zone.

Motor Module

Displacement Sensor

Elastic Band

Load Cell

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3 CONCLUSIONS

The dynamic active brace will preserve mobility and

effectively correct or stabilize the curve progression.

It will improve posture and body aesthetics. The

dynamic active brace will not cause muscle atrophy

as it will strengthen the muscles around the spine,

providing lasting results after the treatment.

The dynamic active brace will have certain

advantages over the rigid conventional braces. The

elastic nature of the bands will allow the greater

mobility and it will increase the physiotherapy

performance. It will be able to solve the issues of

stiffening of the spine and flat back problems. It has

a compact, light design and can be wearable under the

clothes, hence solving the socio implications.

Modular design and cheap motor mechanism of the

device solves the economic implications as well.

Rigid braces are not effective for obese patients.

In contrast to rigid braces, the dynamic active brace

can be a practical solution for obese patients as the

extra weight will not restrain the dynamic action of

the elastic flaps.

4 FUTURE WORK

In future, the focus of the research will be on the

validation of the device. The interface pressure

between the body and the elastic bands needs to be

measured using a body measurement system by

Tekscan® or similar. This will help to evaluate the

relation between the tension in bands and the amount

of force it is applying on the torso. The muscle

activation also needs to be measured using

electromyography sensor to validate the mobility of the

spinal muscles in comparison to the rigid brace. There

is a lot of potential in the improvement of twisted string

actuation technology such as working on self-sensing

supercoiled polymer twisted string actuation strings

(Zhang et al., 2020). The advance stretch(Vu et al.,

2019) to evaluate the motion of the spine and the

tactile(Sferrazza et al., 2019) sensors can be embedded

with elastic band to measure the stretch in the bands

which will be help full to measure the six-dimensional

stiffness of the torso. After the validation of device,

the goal is to go for the clinical trials of the device.

ACKNOWLEDGEMENTS

This project has received funding from the Italian

Ministry for Education, University, and Research

(MIUR) through the "Departments of Excellence"

program.

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