Strain rate dependent properties of human craniovertebral ligaments

Craniovertebral ligaments were tested to failure under tensile loading. Ligaments tested included: transverse ligament, anterior atlanto occipital membrane, posterior atlanto occipital membrane, capsular ligaments between Skull-C1 and C1-C2, anterior atlantoaxial membrane, posterior atlantoaxial membrane and the tectorial membrane/vertical cruciate/apical/alar ligament complex. The objective of this study was to obtain mechanical properties of craniovertebral ligaments of a younger population, at varying strain rates representative of automotive crash scenarios, and investigate rate and gender effects for use in numerical models of the cervical spine. There have been few studies conducted on the mechanical properties of human craniovertebral ligaments. Only one study has tested all of the ligaments, and previous studies use older age specimens (mean age 67, from most complete study). Further, tests were often not performed at elongation rates representative of car crash scenarios. Previous studies did not perform tests in an environment resembling in vivo conditions, which has been shown to have a significant effect on ligament tensile behaviour. Fifty-four craniovertebral ligaments were isolated from twenty-one spines, and tested to failure in tension under simulated in vivo temperature and hydration levels, at quasi-static (0.5 s(-1)) and high strain rates (150 s(-1)). Values for failure force, failure elongation, stiffness, and toe region elongation were obtained from force-displacement curves. Values were analyzed for strain rate and gender effects. Increased strain rate produced several significant effects including: higher failure forces for the transverse ligament and capsular ligament (Skull-C1), lower failure elongation for the tectorial membrane complex, higher stiffness for the tectorial membrane complex and capsular ligament (Skull-C1), and lower toe region elongation for capsular ligament (Skull-C1). Gender effects were limited. Ligament tests demonstrated expected rate effects. Younger specimens had a higher failure force and stiffness and failed at lower elongations than older specimens from previous studies. Gender effects suggest there may be a difference between male and female properties, but require further testing to establish greater significance.