THE VARIATION OF GROSS FORCE RESPONSE OF SPINAL MOTION SEGMENT DURING CYCLIC LOADING ― A PORCINE BIOMECHANICAL MODEL

摘要

The cyclic loading encountered due to exposure to whole body vibration has been implicated as a risk factor for occupational low back disorders (OLBD, Wilder, 1996). The frequent bending and lifting has been identified as a major risk for acute disc prolapse. Wilder (1985) has shown that cyclic loading causes a decline in the stiffness properties of the motion segments and predisposes them to more risk of buckling injury. Numerous studies have also demonstrated that soft tissues subjected to repetitive loading show creep and stress relaxation behavior because of their viscoelastic properties. The cumulative trauma and residual strain in the soft tissues due to repetitive loading may predispose the motion segment to altered load distribution - hence accelerating the process of disc degeneration. Although the association of load and risk of OLBD has been established, the nature of dose-response is less than clear. The current study developed a unique apparatus using an in vitro porcine spine model to quantify the alteration in the load response under cyclic compression loading at different loading conditions. The purpose of current project is to understand the mechanical gross response of the spinal motion segment during repetitive loading. The porcine spine motion segments were used in the study. Two group of loading condition were applied; one is the compression force evenly distributed on the top of vertebrae, and one is compression force at the anterior cortex of vertebrae. Both loading conditions were loaded for 90,000 cycles at 5 Hz. The total loading period is 5 hours. The loading was set at 200 N compression and 50 N tension from peak to peak. The results showed the spine is not stabilized even after 90,000 cycles of loading, and the evenly distributed loading condition obtained higher deformation than the anterior flexed loading condition.