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Enabling Longitudinal Assessment of Ankle-foot Orthosis Efficacy
for Children with Cerebral Palsy
Shanshan Chen, Christopher L. Cunningham,
John Lach
Charles L. Brown Dept. of Electrical &
Computer Engineering
Bradford C. Bennett
Motion Analysis and Motor
Performance Lab
Department of Orthopedic Surgery
UVA Center for Wireless Health
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Cerebral Palsy
• Neuromuscular Disorder
• Wide Spectrum
• Pathological Gait Pattern
Children with equinus pattern gait (A)
and crouch pattern gait (B) deformity
• How to Treat?
• Surgeries:
• Muscle lengthening/transferring
• Ankle-foot orthosis (AFO)
• Very popular
Severe Crouch Gait
by OpenSim
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Uncertain AFO Efficacy
• However..
• Does AFO really help?
• How much does it help?
• What happens in real life?
Quantitative Measure
Continuous, Longitudinal Study
• Patient Self-Report?
• Unreliable
• Can’t provide a continuous document
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Current Clinical Approach
• In-lab Optical Motion Capture
System
• Vicon® instrumented gait lab
• High precision, industrial standard
• Limitations
• Expensive
• In-clinic, unnatural environment
• Inconvenient to use due to many markers
• Discontinuous data if marker drops
• Line of sight
• Discontinuous data if sight is blocked
• Short-term data
• A few cycles of gait data
• No idea what’s going on outside the lab
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Inertial BSNs in Gait Analysis
• Inertial Body Sensor Networks
(BSNs)
• Promising Platforms for Portable Gait
Analysis
• Fall risks assessment
• Knee joint angle tracking
• Gait speed estimation
• Less Invasive and More Wearable
• Potential for continuous longitudinal
analysis
• Apply for assessing AFO efficacy
• Mold the sensors in the AFOs!!
TEMPO 3.1 System
6 DOF motion sensing
a wrist watch form factor
Developed by the INERTIA
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Project Description
• Lay the groundwork for a continuous longitudinal study for
children with CP
• For the first time, evaluate efficacy of AFO in the long-term, in the real
world
• Tailor AFOs for each individual
• Employ Inertial BSNs for Assessing AFO Efficacy
• Design a TRUE continuous, longitudinal monitoring system
• Enhancing the ease of use
• Molding the inertial BSNs in the AFO
• Designing user friendly interface
• Elongating battery life
• Track gait parameters accurately for valid analysis
• Enabling Study
• Addressing technical challenges posed by specific applications
• Validation of methodology against Vicon®
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Enabling Study Objectives
• Overcome Technical Challenges
• Spatial Parameters Extraction
• Integration drift
• Mounting error
• Pathological Gait
• Multi-plane movement
• Irregular gait pattern
• Validate on Real Subjects
• Coordination Challenges
• Difficult for CP subjects to walk
• Synchronization between Vicon® system and TEMPO system
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Outline
• Medical Background in Quantitative Assessment
• Methodology to Overcome Technical Challenges
• Initial Experiments on Healthy Subject
• Validation Experiments on CP Subjects
• Future Work
• Conclusion
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Quantitative Assessment
• Medical Hypothesis for AFO
• Help to correct pathological gait by
limiting out-of-plane motion and
increase the stability
• Limit the excessive knee flexion and
resisting dorsiflexion for the crouch gait
• Limit the excessive plantar flexion for
the equinus gait and promote heelstrike
• Ankle Joint Angle as Primary Gait
Parameter
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With and Without AFO Comparison
-- Ankle Joint Angle
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With and Without AFO Comparison
-- Shank Angular Velocity
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Outline
• Medical Background in Quantitative Assessment
• Methodology to Overcome Technical Challenges
• Initial Experiments on Healthy Subject
• Validation Experiments on CP Subjects
• Future Work
• Conclusion
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Angle Extraction from Inertial BSNs
• Accelerometer
• Provide inclination information of the sensor node by utilizing
gravitational factor
• 𝜃 = 𝑎𝑡𝑎𝑛2(𝑥, 𝑠𝑞𝑟𝑡(𝑦 2 + 𝑧2))
• Gyroscope
• Indicates the angular velocity of the inertial frame
• 𝜃 = 𝑤𝑑𝑡
• Differential Inclination of Shank and Foot
• Ankle_Angle = Shank_Angle – Foot_Angle
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Minimizing Integration Drift
• Complimentary Filter
• Accelerometer is good at long term displacement estimation
• – low pass, keep low frequency component
• Gyroscope is good at short period displacement estimation
• – high pass, keep motion change
• Fusing/Combining information of accelerometer and gyroscope
sensors
• 1st Order Butterworth filter, 𝑓𝑐 = 0.3𝐻𝑧
• Time Domain:
𝑆 = 0.98 × 𝐴𝑛𝑔𝑙𝑒𝜔 + 0.02 × 𝐴𝑛𝑔𝑙𝑒𝐴
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Minimizing Mounting Error
• Mounting Calibration
• Sensor Alignment
• Sensors are not affixed to the body in
the global coordinate
• Coordinates re-mapping
• Rotation Matrix
• Euler rotation sequence
• Obtain compensating angles from the
rotation matrix
• Sensor coordinates map back to the
desired coordinates
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Compensating for Multi-plane Motion
• Multi-plane Movement
• Severe motion on planes apart from
sagittal plane
• Multi-axis rotation obtained by looking at
the 3-axis rotation
• Method
• Rotations do not commute with each other
• Derive Euler rotation rate from gyroscope
signal
• Provide less error but when the sampling
rate is sufficiently high, the gyroscope rate
is near Euler rotation rate
Severe Crouch Gait
by OpenSim
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Outline
• Medical Background in Quantitative Assessment
• Methodology to Overcome Technical Challenges
• Initial Experiments on Healthy Subject
• Validation Experiments on CP Subjects
• Future Work
• Conclusion
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Initial Experiments on Healthy Subject
• Prep for Experiments on CP Subjects
• Validating methodology for further development
• Solving the practical issues in the experiments
• Controlling gait variables
• Experiment Setup
• One healthy subject
• Synchronization procedure
• Walked on the treadmill for 1 minute
• 3 Gait Patterns, with/without AFO comparison
• Normal
• Simulated Crouch Gait
• Simulated Equinus Gait
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Healthy Subject -- Normal Gait
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Healthy Subject-- Simulated Crouch Gait
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Healthy Subject – Simulated Equinus Gait
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Outline
• Medical Background in Quantitative Assessment
• Methodology to Overcome Technical Challenges
• Initial Experiments on Healthy Subject
• Validation Experiments on CP Subjects
• Future Work
• Conclusion
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Validation Experiments on Children with CP
• Experiment Setup
• 4 CP Subjects Wearing AFOs
• Vicon® markers and TEMPO instrumented at the
same time
• On Ground Walking for Several Trials
• Within the range of the Vicon® cameras
• About 5 meters each trial
• Unexpected Challenges
• Validation Challenge
• Walking aid devices block the line of sight
• A few cycles – not easy for CP subjects to get on
treadmill
• Mounting Calibration Challenge
• CP subject with crouch gait has difficulty to stand
straight and still
• Need of assistance from the research staff for
holding the subject’s shanks
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Ankle Joint Angle Validation
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Ankle Joint Angle Validation (contd.)
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Ankle Joint Angle Range Validation
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Ankle Angle Range (°)
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Ankle Angle Range by
Vicon
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Ankle Angle Range by
TEMPO
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0
Sub1
Sub2
Sub3
Sub4
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Shank Angular Velocity Validation
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Enabling Study Outcomes
• Sufficient Accuracy for Key Gait Parameters Extraction
• Validated against Vicon® for Children with CP
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Future Work
• Future Plan for CP Subject Study
• Fabricate AFOs and non-AFOs
• with compartments sized for molding TEMPO
• Instruct Children with CP
• wear AFO/non-AFO as they typically do
• charge the device and upload data to the remote site
• Future Plan for Technology Updates
• Molding Inertial Sensors into the AFOs
• Data Streaming Unnecessary
• Data caching for opportunistic offloading
• Ensuring battery life
• More Analysis Enabled to Assess AFO Efficacy
• Gait Speed
• Phase Portrait -- Gait Stability and Complexity
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Phase Portrait Comparison – Healthy Subject
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Phase Portrait Comparison – CP subject
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Conclusion
• Validation of Methodology
• Enabled Continuous Longitudinal Study
• Improve Patient Outcomes
• Tailor AFO for each individual
AFO Manufacturers
Physicians
Caregivers
Network
EMR
Server
Medical
Researchers
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THANKS!