A review on research on development of ligament constitutive relations on macro, meso, and micro levels

Ligaments are densely connective soft tissues capable of maintaining stability and function of knee joint. As an important factor, the constitutive relation of ligament would affect its biomechanics and further play an essential role in the research on ligament injury, healing and treatment. The objective of this paper is to provide an overview of the current research on ligament constitutive relations on the macro, meso, and micro levels as well as the anatomy and histological structure of ligament. Some studies of biomechanical behaviors during ligament injury and healing periods have also been investigated. Based on the research on ligament constitutive relation in the past three decades, a discussion of some research perspectives is also presented, such as a validated accurate measuring method of in situ strain in ligament, a new constitutive relation involving the distribution of ultra-structural properties, and a rational estimation of ligament injury and healing process by the change of its ultra-structural or histological characteristics.