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

Clinical studies have identified threshold HU values above which the risk of lumbar implant instability increases significantly; however, these threshold values have a very wide range. Furthermore, there is a lack of rationale for HU value thresholds based on biomechanical models to predict fusion failure. The purpose of the study is to provide biomechanical evidence for using an HU threshold to predict lumbar fusion failure.

A fused model of the L4-L5 lumbar segment with screws and an interbody cage was developed. The model was then numerically investigated under standard static loading conditions (flexion, extension, lateral bending and axial rotation). The von Mises stresses were investigated in different elements of the fused segment, including the vertebral trabecular bone, with HU values varied from 256 to 90.

The highest stresses in the trabecular bone of the vertebral bodies were observed in the areas of contact with the screws during extension, lateral bending, and axial rotation in the lumbar segment. High stress concentrations were also found in the contact zones between the cage and the vertebral body during compression and extension. At HU values of 165 and above, the maximum stress in the vertebral trabecular bone was far below its ultimate strength. At HU = 140, the stress exceeds the ultimate strength during flexion. At HU = 125, the stress exceeds the ultimate strength under four loading conditions: flexion, extension, left bending, and clockwise axial rotation. The highest stresses in the cortical bone (100 MPa) and in the screws (180 MPa) did not reach critical values for cortical bone damage even at the lowest HU value of 90.

This study suggests that, within the assumptions of this single-level finite element model, an HU value of approximately 125 represents a transition point at which trabecular bone stresses approach or exceed predicted strength under multiple loading modes in lumbar fusion with screw fixation. Cortical bone remained intact even at the lowest trabecular bone HU value of 90.