Vibration modes of injured spine at resonant frequencies under vertical vibration

Study Design: A detailed three-dimensional finite element model of the spine segment T12-Pelvis was developed to investigate dynamic characteristics of whole lumbar spine with injured cases. Objective: This study investigates the motion mechanism of the human lumbar spine and the effect of component injuries on adjacent spinal components under whole body vibration. Summary of Background Data: Several investigations have analyzed the influence of injured spines on adjacent spinal components under static loadings. However, it is not clear how the spine injury affects dynamic characteristics of whole lumbar spine and adjacent components of the injured segment under vibration. Methods: The T12-Pelvis model was used to obtain the modal vibration modes of the spine at resonant frequencies. Injury conditions of the spine were simulated and tested, including denucleation and/or facetectomy with removal of capsular ligaments. Results: The results indicate the first-order vertical resonant frequency of the intact model is 7.21 Hz. After the denucleation at L4-L5, it decreases by more than 4% compared with the intact condition. All the injured conditions including disc injury and ligament injury decrease the resonant frequency of the spine. Due to the denucleation at L4-L5 the anteroposterior displacements of the vertebrae from L2 to L5 decrease and the vertical displacements of the vertebrae from L1 to L4 increase under vibration. The denucleation also decreases the rotational deformations of the vertebrae from L1 to L5. The material property sensitivity analysis shows intervertebral discs have a dominating effect on variation of vertical resonant frequency of the spine. Conclusion: The denucleation may decrease cushioning effects of adjacent motion segments at the injured level under vibration. The injured condition may increase the vertical displacement amplitudes of the spine above the injured level. All the injured conditions may decrease the resonant frequency of the spine system.