In vitro spine testing using a robot-based testing system: comparison of displacement control and "hybrid control"

Area of Science:

• Biomechanics
• Robotics
• Spine Engineering

Background:

• Current in-vitro spine testing uses load or displacement control, which lack adaptability to specimen changes.
• Existing methods struggle to realistically govern load/motion application to spinal specimens.

Purpose of the Study:

• To develop and evaluate a novel hybrid control algorithm for in-vitro spine biomechanical testing.
• To compare the performance of the hybrid control algorithm against traditional displacement control.

Main Methods:

• A robotics-based spine testing system was programmed with a hybrid control algorithm, combining load and displacement control.
• Preliminary testing used a rigid-body-spring model; subsequent testing involved cadaveric spine specimens.
• The hybrid control algorithm actively managed secondary degrees-of-freedom to apply pure moments to functional spinal units (FSUs).

Main Results:

• The hybrid control algorithm successfully applied pure moments to FSUs (flexion/extension, lateral bending, axial rotation) while minimizing coupled forces/moments.
• Nonlinear S-shaped moment-rotation curves were observed, consistent with FSU neutral and elastic zones.
• Hybrid control actively minimized off-axis forces, leading to a larger neutral zone and range of motion compared to displacement control.

Conclusions:

• The hybrid control algorithm offers a more adaptive and realistic approach to in-vitro spine biomechanical testing.
• This advanced control strategy enhances the ability to study FSU biomechanics, particularly the neutral zone and range of motion.
• Robotics-based systems with hybrid control can improve the accuracy and applicability of spine testing.