A study on the macro- and micro-dynamic characteristics of lumbar spine under different frequencies based on finite element analysis

Whole-body vibration is a critical occupational hazard linked to spinal degeneration, yet the multi-scale biomechanical mechanisms underlying vibration-induced damage remain poorly understood. This study aimed to investigate the effects of different frequency loadings on macro- and micro-scale dynamic mechanical properties of lumbar spine. A macro- and micro-scale model of the lumbar spine was established in this study, and a specific boundary displacement method, based on Saint-Venant's principle, was used to apply sinusoidal excitation forces of different frequencies to the lumbar spine osteon. Modal and transient dynamic analyses were then conducted. he first axial fundamental frequency of the entire lumbar spine was 10.27 Hz. The macro-bone units of the L1 vertebral body exhibited the largest response amplitude to varying frequency dynamic loads, while those of the L3 vertebral body showed the smallest. Under 5 Hz sinusoidal excitation, the Von-Mises stress amplitudes in the L1 and L3 vertebral bodies were 0.344 MPa and 0.187 MPa, respectively. At 9 Hz, the Von-Mises stress amplitudes increased to 0.890 MPa in L1 and 0.490 MPa in L3, respectively. The stress in the micro-bone units was approximately 2.38 to 17.33 times higher than that in the macro-bone units. The stress ratio varied depending on working conditions and the spatial positioning of the bone unit, with less influence from the excitation force frequency. Under varying frequency excitations, both the macro- and micro-bone units demonstrated cyclic stress responses. The amplitude of the stress response curve increased as the excitation frequency approached the first axial resonance frequency. Meanwhile, the ratio of stress amplitude responses between the micro- and macro-bone units remained consistent under different frequency excitations. In the micro-bone unit structure, the adhesive line and the outermost bone plate bore the main stress.