Robots In Spine Biomechanics
Wafa Tawackoli, Michael A.K. Liebschner
Department of Bioengineering
Rice University
Motivation
In vitro study of human spine for various
complex physiological loading.
Prediction of stress fracture risk
Approximately 700,000 vertebral
fractures occur each year in USA
– Everyday activities
• Trauma (i.e. Car accident, Sports)
• Occupational ($54 billion/year)
– Relatively low impact office duties
– High impact manual labor
– Osteoporosis (~$13 billion/year)
Anatomy
Annulus fibrosus
Posterior Elements
Nucleus pulposus,
Facet Joint
COR
Cortical Shell (rim)
Trabecular bone
Cramer, 1995
Intervertebral Disc
Vertebra
Primary Goals
To understand the biomechanical behavior of spinal
segments under complex physiological loading



3D motion path
Simulation of in vivo complex
loading
Investigate stress fracture risk
base on physiological loading
A 3D coordinate system
+ Z Rotation
 Total of 6 load components may
be applied
+ X Direction
+ Z Direction
Three forces
Three moments
+ X Rotation
+ Y Rotation
 Each load component may
produce 6 displacement
components
Three translations
Three rotations
+ Y Direction
 36 load displacement curves can
be generated
Complications
• Mechanical Properties are difficult to
ascertain.
• Spine movies in a complex 3-Dimensional
pattern.
• However, it is important to apply such
complex motion during in vitro studies.
Biomechanical Methods
1. In vivo experiments (including imaging
studies, i.e. stereoradiography) (Tibrewan, Pearcy)
2. Mechanical Testing (Panjabi, Hansson, Adams)
3. Computational Modeling (finite element
analysis) (Uppala, Williams)
Biomechanical Methods (cont’d)
• Mechanical Testing Devices
– Pulley system (Crawford, Panjabi, Patwardhan)
– Uniaxial system (Adams, Panjabi, Brickmann)
(Servo-Hydraulic or Pneumatic)
• Mechanical Testing Methods
– Uniaxial compression/tension
– Shear
– Bending (Flexion, Extension, Lateral, Torsion)
– Compressive axial preload (Follower Load)
Biomechanical Methods (cont’d)
Spine Testing Machine:
• Pulley system
• Linear servo actuator (Parker-EBT 50)
• 6 DOF Transducer (ATI-Omega 160)
• Bi-axial tilt sensor (range of ~60o)
• Optical tracking system
• Compressive axial preload capability
(up to 2250 N)
Biomechanical Methods (cont’d)
Extension
Flexion
Force
Top View
Force
Sagittal View
Side View
Cable
guide
ATI-160
U-Shape Bracket
Dead Weights
Limitations
Measurement of spinal rigidity in single plane
is very complex
• Unconstrained Motion- 6 Degrees of Freedom (DOF)
• 2 DOF applied force + moment
• Lack of knowledge of disc degeneration (tears or
lesions)
Our Approach
Measurement of spinal rigidity under complex loading
(Fatigue, Creep, Stress Relaxation)
• Decrease DOF of unconstrained motion
• Increase DOF of applied forces and moments
• Apply helical axis of motion (path of minimum
resistance)
• Load and displacement boundary conditions.
Concept of KUKA Robotic Arm
• 6 Degree of Freedom
Wrist
Arm
Link arm
Rotating column
• PC computer
• Windows based program (GUI
Base frame
software)
• Manual and automatic control
• Simple modular system
Coordinate Systems
Coordinate systems (can be defined
by the operator):
• Sensor & Tool coordinate systems
• Base coordinate system
• Virtual coordinate system
Sensing and Control Process (1)
Displacement
EZ
NZ
Load
Hybrid Control = { load control & displacement control }
Sensing and Control Process (2)
Forces and torques measured by the
ATI transducer can be re-calculated
to a virtual coordinate system in
order to sense the real effecting
forces and torques between spinal
segment and the transducer.
The optical tracking system
allows for comparison in
movement between each
vertebra.
Motion Envelope
Ω
Reference (Home)
Position
φ
Foundation Points
Top View of Motion Envelope
(Manually determined)
Boundary condition (i.e. Bending moment of 5 N.m.)
Conclusion
• Human spine is a complex system therefore complex
motion behavior is expected
• Hybrid control for biomechanical testing is
recommended
• 6DOF robotic testing system can be applied to the
delineation of in vitro spine kinetics
Acknowledgment
•
Computational and Experimental Biomechanics Lab
•
KUKA USA Robotics
•
KUKA Development Labs
•
ATI Industrial Automation
•
Joe Gesenhues (Ryon Engineering Lab, Rice University)
Thank You
Robots in
Biomechanics
Research