Fatigue Loading Causes Earlier Modulus Reduction Than Creep Loading in Healing Rabbit Medial Collateral Ligaments

During normal daily activities, ligaments experience a range of stresses. These stresses can be repetitive, sustained or a complex combination of the two [1]. The stress-strain curve of ligaments is comprised of two parts: the toe region with increasing modulus, and the linear region which has constant modulus. Normal physiologic loading occurs between 5% and 10% of the ultimate tensile strength (UTS), and the transition from the toe region to the linear region occurs at approximately 15% of UTS [2]. Ligaments healing from an injury have decreased failure properties compared to normal ligaments [3], which may correspondingly affect the ability of healing ligaments to function under physiologic loads. Modulus reduction was found to be a suitable marker of damage in normal ligaments [4]. The purpose of this study was to investigate the effect of long-term fatigue and creep loading on modulus changes in healing medial collateral ligaments (MCLs).