The effect of storage time in saline solution on the material properties of cortical bone tissue

Background: The use of saline in preserving bone specimens may affect the mechanical properties of specimens. Yet, the reported effects varied and contradicted to each other, with a lack of investigating constitutive material parameters. Therefore, we quantified the effects of preservation time on the constitutive properties of cortical bone. Methods: We collected 120 specimens from the mid-diaphysis of six male bovine femora, which were assigned to five groups, including fresh-frozen for 60 days (-20 degrees C), storage in saline for 3, 10, 36 and 60 days (25 degrees C). All specimens underwent quasi-static three-point bending tests with a loading rate of 0.02 mm/s. Using the optimization method combined with specimen-specific finite element models, the Young's modulus, tangent modulus, yield stress, effective plastic strain, yield strain, ultimate stress, and toughness were calculated. Findings: Saline preservation resulted in a significant decrease of Young's modulus, yield stress, ultimate stress and pre-yield toughness (P < 0.001), and a significant increase of effective plastic strain (P = 0.034). After 10 days of preservation, yield stress and pre-yield toughness decreased 14.9% and 21.4%, respectively, and they continued to decrease with longer preservation time. After 36 days of preservation, Young's modulus and ultimate stress decreased 19.2% and 17.3%, respectively, and continued to decrease with longer preservation time. Our data also showed changes of material properties collected after 3-day saline preservation, while the low statistical power must be considered for this group. Interpretation. Saline preservation impacts on mechanical properties of cortical bone tissue and the effect is already observable after 3 days.