Assistant Professor Medical College of Wisconsin Milwaukee, Wisconsin, United States
Introduction: Spinal cord stress and strain during flexion-extension of the neck contributes to spinal cord damage in degenerative cervical myelopathy (DCM), yet direct measurement of these forces is not feasible in humans. The effect of lateral bending or axial rotation of neck on spinal cord stress and strain is also unknown. Quantification of spinal cord stress and strain during neck motion is expected to reveal mechanisms of adverse spinal cord tension, and assist with surgical planning to mitigate these forces.
Methods: Three-dimensional patient-specific finite element models (FEMs) were created for 6 DCM patients (mild [n=2], moderate [n=2] and severe [n=2]). FEMs were developed using MRI-derived geometries of the spine and spinal cord and incorporated material properties from human spinal cord tissue. Flexion-extension, lateral bending and axial rotation of the cervical spine were simulated with a pure moment load of 2 Nm. Peak von Mises stress (VMS) and peak maximum principal strain (MPS) within the spinal cord were calculated at each segment. Segmental range of motion (ROM), spinal cord area, spinal cord compression ratio was measured and regression analysis was performed to determine predictors of spinal cord stress and strain.
Results: A total of 216 segmental measurements were made. Segmental VMS (p < 0.001) and MPS (p=0.027) was significantly higher in extension compared to flexion. The mean segmental ROM was 2.42±1.51 degrees in the sagittal plane, 7.9±3.5 degrees in the coronal plane, and 2.2±1.9 degrees in the axial plane. Segmental ROM in flexion-extension (b=0.81, p< 0.001; (b=0.6, p< 0.001) , and axial rotation (b=4.9, p< 0.001; b= 4.0, p< 0.001) was associated with spinal cord stress and strain, independent of the degree of spinal cord compression. However, segmental range of motion in lateral bending was not associated with spinal cord stress (b=-0.6, p=0.57) or strain (b=-0.09, p=0.93).
Conclusion : These findings reveal a distinct, independent effect of physiological loading modes on spinal cord stress and strain in DCM. Further study is necessary to determine the effect of motion-sparing surgical approaches on spinal cord stress and strain.
How to Improve Patient Care: Our FEMs can assist with surgical decision-making for DCM in cases with clinical equipoise- motion preservation versus fusion, anterior versus posterior approach.
