Dynamic tensile testing of ligaments from the human cervical spine

Abstract

This study focuses on establishing appropriate experimental methodology to facilitate investigation of the dynamic stress-strain characteristics of soft bio-tissues. Dynamic mechanical tests were conducted on ligaments from the human cervical spine (neck), using a tensile split Hopkinson bar device. The strain rates imposed were of the order of 102-103/s. As ligaments are extremely soft and extensible, specific test protocols relating to Hopkinson bar testing were developed to enable the acquisition of reliable and accurate data. These encompass aspects such as detection of transmitted waves of small magnitudes, a means to perform tests with absolutely zero preloading, and generation of a sufficiently long loading pulse. Comparison with static test results reveals that the initial physiologic (toe) region for dynamic loading is noticeably shorter, while the accompanying stress levels and strength are elevated and able to attain values more than two to three times the static values. However, the elongation capacity of ligaments under dynamic loading is reduced. Simple empirical equations were found to provide suitable fits to the response prior to attainment of the ultimate stress.