Dynamic loading affects the mechanical properties and failure site of porcine spines

OBJECTIVE: The purpose of this study was to investigate the effect of load rate on the mechanical characteristics of spinal motion segments under compressive loading. DESIGN: An in vitro experiment using a porcine model which ensured a homogeneous population for age, weight, genetic background and physical activity. BACKGROUND: Spinal motion segments comprise of viscoelastic materials, and as a result the rate of loading will modulate mechanical characteristics and fracture patterns of the segments. METHODS: Twenty-six cervical porcine spines were excised immediately post-mortem with all soft tissue intact. Spines were then separated into two specimens each consisting of three vertebral bodies and the two intervening intervertebral discs (C2-C4 and C5-C7) and loaded to failure under five loading rates (100, 1000, 3000, 10 000 and 16 000 N s(-1)). After the specimens failed, they were dissected to determine the mode of failure. RESULTS: Dynamic loading increases the ultimate load compared with quasi-static loading (100 N s(-1)), whereas the magnitude of dynamic loading (1000-16 000 N s(-1)) appears not to have a significant affect. Stiffness behaved in a similar manner. The displacement to failure of specimens decreased as load rate increased, although there was a diminishing effect at high load rates. Furthermore, failure at low load rates occurred exclusively in the endplate, whereas failure of the vertebral body appeared with greater frequency at higher load rates. CONCLUSIONS: The mechanical characteristics and resulting injuries of porcine spinal motion segments were affected as the loading rates changed from quasi-static to dynamic. The modulating factors of the mechanical characteristics of the spine need to be understood if valid models are to be designed which will increase the understanding of spinal function, and are important for choosing better injury prevention and rehabilitation programmes.