Moment measurements in dynamic and quasi-static spine segment testing using eccentric compression are susceptible to...

Area of Science:

• Biomechanics
• Spinal Injury Biomechanics
• Orthopaedic Biomechanics

Background:

• Spine tolerance to bending moments is crucial for evaluating injury prevention devices.
• Eccentric axial compression of cadaver spine segments is a common testing method.
• Preliminary studies indicated unexpected artifact moments during these experiments.

Purpose of the Study:

• To evaluate the static and dynamic effects of test configuration on bending moments during eccentric axial compression.
• To create and experimentally verify dynamic equilibrium equations for loads measured below the specimen.
• To compare moment responses across various test configurations using synthetic and cadaver specimens.

Main Methods:

• Developed dynamic equilibrium equations for loads measured inferior to the specimen.
• Verified equations using quasi-static and dynamic tests on a rubber specimen with a six-axis load cell.
• Compared moment responses from hinge, linear slider/hinge, and roller joint configurations under quasi-static and dynamic conditions.

Main Results:

• Calculated shear force and bending moment curves closely matched measured data.
• Translation constraints and superior mounting structure inertia generated unexpected moments in both static and dynamic tests.
• Dynamic tests showed initial flexion moments in unconstrained configurations, attributed to inertia.

Conclusions:

• Eccentric axial compression can produce artifactual bending moments due to test configuration and dynamic inertia.
• The assumption that bending moments equal compression force multiplied by eccentricity is questionable in constrained or dynamic setups.
• Minimizing mass, moment of inertia, and distance of loading jig structures is recommended to reduce inertial artifacts.