A new bone surrogate model for testing interbody device subsidence

Area of Science:

• Spinal surgery
• Biomechanical engineering
• Orthopedic research

Background:

• Human cadaveric vertebrae are crucial for biomechanical testing of interbody devices.
• Bone surrogates like synthetic vertebrae and polyurethane foam blocks are used when cadavers are unavailable.
• Synthetic vertebrae aim to mimic vertebral components, offering an alternative to foam blocks for subsidence testing.

Purpose of the Study:

• To compare the accuracy of synthetic vertebrae and polyurethane foam blocks in measuring interbody device subsidence against human cadaveric vertebrae.
• To evaluate how variations in interbody device size and placement affect subsidence measurements in different bone models.

Main Methods:

• An in vitro biomechanical study was conducted using synthetic vertebrae, polyurethane foam blocks, and human cadaveric vertebrae.
• Indentors were subjected to uniaxial compression on the endplates of these models, with central and peripheral placements.
• Subsidence, failure force, and indentation stiffness were measured using custom extensometers and force-displacement curves.

Main Results:

• Human vertebrae showed varied subsidence based on indentor placement, with peripheral placement reducing subsidence.
• Polyurethane foam blocks exhibited insensitivity to indentor placement, mimicking central placement in human vertebrae but not peripheral.
• Synthetic vertebrae were sensitive to placement but overestimated stiffness and underestimated subsidence compared to human vertebrae.

Conclusions:

• Synthetic vertebrae accurately model endplate geometry but overestimate structural properties, leading to inaccurate subsidence modeling.
• Polyurethane foam blocks offer better central subsidence accuracy than synthetics but lack robustness for varied placements.
• Current bone surrogates lack the material property distribution necessary for precise modeling of interbody device subsidence in human cadaveric vertebrae.