Hounsfield unit threshold to predict lumbar fusion failure: a finite element study

Area of Science:

• Biomechanical Engineering
• Orthopedic Surgery
• Medical Imaging

Background:

• Clinical studies suggest a link between Hounsfield unit (HU) values and lumbar implant instability.
• Existing HU thresholds lack biomechanical validation for predicting fusion failure.
• A biomechanical rationale is needed to establish reliable HU thresholds.

Purpose of the Study:

• To provide biomechanical evidence for using a Hounsfield unit (HU) threshold to predict lumbar fusion failure.
• To investigate the relationship between vertebral bone density (HU values) and mechanical stress under physiological loading.
• To identify a critical HU value indicative of potential fusion complications.

Main Methods:

• Development of a finite element model of a fused L4-L5 lumbar segment with screws and an interbody cage.
• Numerical simulation of the fused model under standard static loading conditions (flexion, extension, lateral bending, axial rotation).
• Analysis of von Mises stresses in vertebral trabecular bone with varying HU values (90-256).

Main Results:

• Maximum stresses in trabecular bone occurred near screw and cage contact points during extension, lateral bending, axial rotation, and compression.
• At HU 165+, maximum trabecular bone stress remained below ultimate strength.
• At HU 140, stress exceeded ultimate strength during flexion; at HU 125, it exceeded ultimate strength under all four loading conditions.
• Cortical bone and screws did not reach critical damage levels even at HU 90.

Conclusions:

• An HU value of approximately 125 appears to be a transition point for lumbar fusion.
• At HU 125, trabecular bone stresses approach or exceed predicted strength under multiple loading modes.
• Cortical bone integrity was maintained across the tested HU range, suggesting trabecular bone density is the limiting factor.