The effect of different preconditioning protocols on repeatability of bovine ACL stress-relaxation response in tension.

Mechanical characterization of soft tissues such as ligaments remains challenging. There is variability in the measured material parameters of ligaments, most of which is related to natural tissue variability, but some of it can be a result of using different testing protocols. Generally preconditioning (cyclic loading-unloading) is performed prior to actual tests to reduce the experimental variability. Commonly, preconditioning protocols for ligaments with a small strain level and 10 sinusoidal loading-unloading cycles are used. The effect of preconditioning and its parameters including strain level, number of cycles and number of preconditioning repetitions on the repeatability of tensile stress-relaxation tests are poorly known for knee ligaments. In the present study, forty-eight dumbbell-shaped bovine anterior cruciate ligament (ACL) samples were used to evaluate the repeatability of stress-relaxation response. Different preconditioning protocols with 2% and 6% strain levels and 1, 5 or 10 preconditioning repetitions were applied. After preconditioning, one-step stress-relaxation test was carried out twice with an hour resting period in between the tests. The equilibrium stress showed no systematic bias when only one preconditioning repetition was applied (2.0 ± 3.1% difference and p > 0.05 between repeated tests). Systematic bias in the peak-to-equilibrium stress ratio was not observed when higher strain level and number of repetitions were used (0.5 ± 1.6% difference and p > 0.05 between repeated tests). In conclusion, the commonly used preconditioning protocol is capable of producing repeatable equilibrium stress levels of bovine ACLs from stress-relaxation tests in tension. However, if repeatable peak-to-equilibrium stress ratio is desirable, higher strain and number of preconditioning repetitions are recommended.