Repetitive loading damages healing ligaments more than sustained loading demonstrated by reduction in modulus and residual strength

Healing ligaments have decreased strength compared to normal ligaments, leaving healing ligaments vulnerable to damage accumulation during normal daily activities at functional stresses. Rabbit medial collateral ligament gap scars after 14 weeks of healing were exposed to long-term creep and fatigue loading over a range of functional stresses. In addition to the 58 healing ligaments that underwent in vitro creep and fatigue testing, seven healing ligaments underwent only monotonic failure tests for comparison with residual strength tests that followed creep and fatigue testing. When exposed to repetitive loading during fatigue testing, healing ligaments exhibited modulus reduction earlier than when exposed to sustained loading during creep testing that was occasionally interrupted with unloading/reloading cycles to measure modulus. In other words, after the same loading duration, repetitive loading was more damaging than sustained loading. At modulus reduction, the increase in strain during fatigue was greater than or similar to that during creep. Healing ligaments that were damaged during long-term loading exhibited decreased strength and increased toe-region strain during subsequent residual strength tests. Normal daily activities that result in repetitive loading of a ligament healing from an injury will likely cause damage to accumulate faster than activities that result in sustained loading.