Robots In Spine Biomechanics

Wafa Tawackoli, Michael A.K. Liebschner

Department of Bioengineering

Rice University

Motivation

– 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)

Approximately 700,000 vertebral fractures occur each year in USA

In vitro study of human spine for various complex physiological loading.

Prediction of stress fracture risk

Anatomy

Cortical Shell (rim)

Trabecular bone

Vertebra

Posterior Elements

Cramer, 1995 Intervertebral Disc

Annulus fibrosus

Nucleus pulposus,Facet Joint

COR

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

Total of 6 load components may be applied

Three forces

Three moments

Each load component may produce 6 displacement components

Three translations

Three rotations

36 load displacement curves can be generated

+ Z Rotation

+ X Rotation

+ Y Rotation

+ X Direction+ Z Direction

+ Y Direction

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)

Sagittal View

ATI-160

Dead Weights

Extension Flexion Force

Force

U-Shape Bracket

Cable guide

Side View

Top View

Biomechanical Methods (cont’d)

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

• PC computer

• Windows based program (GUI

software)

• Manual and automatic control

• Simple modular system

Base frame

Rotating column

Link arm

ArmWrist

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)

Hybrid Control = { load control & displacement control }

NZ

EZ

Load

Displacement

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

Top View of Motion Envelope

Ω

φ

Boundary condition (i.e. Bending moment of 5 N.m.)

Foundation Points

(Manually determined)

Reference (Home) Position

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