Critical values of mechanical stress and strain during traumatic cervical spinal cord injury: Clinical study with the use of Finite Element Modelling

Abstract

Study design: Prospective case-control study. Objective: Aim of the study was to assess critical values of mechanical stress and strain in the cervical spinal cord based on the Finite Element Method (FEM) simulations of specified clinical cases. Summary of background data: The knowledge about the values and distribution of tension and deformation, which are noted at the moment of traumatic spinal cord injury (tSCI) may enable determination of the range of primary and secondary injury. Methods: Total of 28 patients after cervical spine (C-spine) injury were enrolled, 14 with neurological symptoms of tSCI (study group, SG) and 14 neurologically intact (control group, CG). Both groups were age and sex matched. A three-dimensional (3D) numerical model of the cervical spinal cord containing dura and pia matter together with denticulate ligament was created. The variable boundary conditions were established individually for each case and allowed to reconstruct the moment of the injury in computer environment. Factors differentiating between SG and CG were tested with multiple logistic regression model. The predictors of ASIA scale outcomes were evaluated in the ordinal multinomial probit regression model. Results: There were no correlations between age, sex and the level of injury and the values of stress and strain. The results in longitudinal axis (z), in stress (OR-6.3; 95%CI 3.94-8.78; p < .033) and strain (OR-7.8; 95%CI 3.03-10.19; p < .046) were the risk factors of neurological deficits after tSCI. The cut off value for stress was 8.1 kPa (sensitivity-85.7%; specificity-78.6%; AUC-0.819, p < .001), and for strain 0.0117 (sensitivity-92.9%; specificity-72.5%; AUC-0.645, p < .001). Results in the longitudinal axis (z), in stress and strain correspond with grading in ASIA scale. One grade change in ASIA scale correlates with the decrease in z axis by 4.01 kPa and 0.012 in stress and strain respectively. Conclusions: The severity of damage of osseous and ligament structures of the spine, significantly influences the range of the mechanical stress applied to the spinal cord. Neural tissue of the spinal cord is the most resistant to the mechanical stimulus acting in sagittal direction, distraction appears to be the most destructive component of the injury phenomenon.