Study of the hysteresis phenomenon in canine anterior cruciate ligaments

Preconditioning is now in common use in biomechanical testing of soft connective tissues. During the first few cycles the tissue behaviour is characterized by a decreasing hysteresis area. However, little is known about the changes occurring during the preconditioning process. The purpose of this study was to investigate the hysteresis phenomenon of ligaments as it is influenced by preconditioning and in vitro enzymatic treatment. Canine anterior cruciate ligament (ACL) was chosen because its mechanical properties and microstructure are relatively well known. A series of experiments were conducted to clarify some of the hysteresis features by combining mechanical testing, enzymatic digestions and pH variations. The area within the hysteresis loops (energy absorption, EA) was measured before and after each treatment. The results showed that the EA before preconditioning is not modified by elastase treatment, however, it was significantly reduced after preconditioning. The fractional EA lost during the preconditioning process increased significantly as compared to the controls. This may be explained by the destruction of elastin, which plays a significant role in the elasticity of ligaments. It was found that hyaluronidase treatment did not significantly affect the hysteresis of preconditioned ligaments. However, the EA during the first cycle decreased significantly as a result of uronic acid digestion leading probably to an exhaustion of the viscous component of the tissue. Hyaluronidase treatment seems to have the same effect as preconditioning on the hysteresis area.