DYNAMIC LOADING AFFECTS THE MECHANICAL-PROPERTIES AND FAILURE SITE OFPORCINE 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 porc ine model which ensured a homogeneous population for age, weight, gene tic background and physical activity. Background. Spinal motion segmen ts comprise of viscoelastic materials, and as a result the rate of loa ding will modulate mechanical characteristics and fracture patterns of the segments. Methods. Twenty-six cervical porcine spines were excise d immediately post-mortem with ail soft tissue intact. Spines were the n separated into two specimens each consisting of three vertebral bodi es and the two intervening intervertebral discs (C2-C4 and C5-C7) and loaded to failure under five loading rates (100, 1000, 3000, 10000 and 16000 N s(-1)). After the specimens failed, they were dissected to de termine the mode of failure. Results. Dynamic loading increases the ul timate load compared with quasi-static loading (100 N s(-1)), whereas the magnitude of dynamic loading (1000-16000 N s(-1)) appears not to h ave a significant affect. Stiffness behaved in a similar manner. The d isplacement to failure of specimens decreased as load rate increased, although there was a diminishing effect at high load rates. Furthermor e, failure at low load rates occurred exclusively in the endplate, whe reas failure of the vertebral body appeared with greater frequency at higher load rates. Conclusions. The mechanical characteristics and res ulting injuries of porcine spinal motion segments were affected as the loading rates changed from quasi-static to dynamic. The modulating fa ctors of the mechanical characteristics of the spine need to be unders tood if valid models are to be designed which will increase the unders tanding of spinal function, and are important for choosing better inju ry prevention and rehabilitation programmes. Relevance Documenting cha nges in failure patterns, as a function of load rate assists in recons truction of the injury event. While understanding changes in mechanica l properties assists in the formulation of justifiable injury avoidanc e strategies. (C) 1997 Elsevier Science Ltd.