Pure moment testing for spinal biomechanics applications: Fixed versus sliding ring cable-driven test designs

Area of Science:

• Biomechanics
• Spinal Surgery
• Biomedical Engineering

Background:

• In vitro multi-axial bending tests are standard for assessing spinal kinematics after surgical interventions.
• Cable-driven setups are common due to minimal infrastructure requirements.
• Traditional
• fixed ring
• designs raise concerns regarding the accuracy of pure moment loading.

Purpose of the Study:

• To directly compare the loading accuracy of a traditional
• fixed ring
• apparatus with a novel
• sliding ring
• approach.
• To evaluate the impact of these designs on spinal kinematics under pure moment loading.

Main Methods:

• Flexion-extension bending was performed on artificial and cadaveric spine models.
• Loading conditions were measured using an in-line multiaxial load cell.
• Direct comparison of loading states between the
• fixed ring
• and
• sliding ring
• systems.

Main Results:

• The
• fixed ring
• system applied 50-60% lower flexion-extension moments than intended.
• Non-trivial anterior-posterior shear forces were imposed by the
• fixed ring
• design.
• Non-uniform loading occurred along the specimen length with the
• fixed ring
• system.

Conclusions:

• Traditional
• fixed ring
• systems can deviate from pure moment loading conditions.
• The novel
• sliding ring
• modification corrects errors in the original test design.
• The proposed
• sliding ring
• design is recommended for future in vitro spine biomechanics studies using cable-driven pure moment apparatus.