Characterizing the Mechanical and Viscoelastic Response of the Porcine Facet Joint Capsule Ligament in Response to a Simulated Impact

The facet capsule ligament (FCL) is a structure in the lumbar spine that constrains motions of the vertebrae. Subfailure loads can produce microdamage resulting in increased laxity, decreased stiffness, and altered viscoelastic responses. Therefore, the purpose of this investigation was to determine the mechanical and viscoelastic properties of the FCL under various magnitudes of strain from control samples and samples that had been through an impact protocol. Two hundred FCL tissue samples were tested (20 control and 180 impacted). Impacted FCL tissue samples were obtained from functional spinal units that had been exposed to one of nine subfailure impact conditions. All specimens underwent the following loading protocol: preconditioning with five cycles of 5% strain, followed by a 30 s rest period, five cycles of 10% strain, and 1 cycle of 10% strain with a hold duration at 10% strain for 240 s (4 min). The same protocol was followed for 30% and 50% strain. Measures of stiffness, hysteresis, and force-relaxation were computed. No significant differences in stiffness were observed for impacted specimens in comparison to control. Impacted specimens from the 8 g flexed and 11 g flexed and neutral conditions exhibited greater hysteresis during the cyclic-30% and cyclic-50% portion of the protocol in comparison to controls. In addition, specimens from the 8 g and 11 g flexed conditions resulted in greater stress decay for the 50%-hold conditions. Results from this study demonstrate viscoelastic changes in FCL samples exposed to moderate and highspeed single impacts in a flexed posture.