The Effect Of Formalin Fixation On The Biomechanical Properties Of Bovine Femoral Cortical Bone

It is the gold standard to use fresh-frozen bone to conduct biomechanical tests because freezing has no significant influence on the mechanical properties of bone tissues. But, fresh-frozen bone has a risk of carrying infec tious pathogens. Furthermore, freezing treatments are expensive and need large storage space. Fixed bones by formalin or other chemical solutions do not have these disadvantages, so they are also used in many biomechanical tests. However, chemical solution s have physical or chemical reaction with protein of bone, which will change the mechanical properties of bone. So far, there is no general consent about the effect of fixing on bone-material properties, but many studies still use the fixed bones. Thus, it is necessary to study the effects of fixing on bone-material properties.This paper studied the effect of formalin fixation on the mechanical properties of bone and investigated the differences of the effect among different fixing temperature, fixing duration and anatomic locations of bone specimens. The longitudinal and transverse specimens of bovine femoral cortical bone were chosen as the subject in this study and the 4% formalin solution was the fixing agent. The quasi-static three-point bending test combined with beam theory were used to obtain the material parameters of bone specimens: Young`s modulus, yield stress and strain, ultimate stress and strain, elastic absorption and plastic absorption. The Analysis of Variance and Non-parametric Tests were applied to estimate the differen ces between the material parameters of bone specimens which were fixed at 4 ℃ or 25 ℃ for 4 weeks or 8 weeks respectively and those of Fresh-frozen specimens.The results showed that for longitudinal specimens, formalin fixation at 4℃ for 8 weeks has significant effect on the Young`s modulus and all calculated material parameters were significantly altered when specimens were fixed by formalin at 25 ℃. For transverse specimens, the ultimate strain and plastic absorption were significantly changed when bone specimens were fixed at 4℃ for 8 weeks. Specimens which were fixed at 25℃ for 4 weeks had significantly different yield strain, ultimate strain and plastic absorption from fresh-frozen specimens. Except for ultimate stress and elastic absorption, all calculated material parameters were significant differences between fresh-frozen group and 25℃ 8 weeks fixing group. Considering the four anatomic quadrants, the effect of formalin fixation on the material properties of these quadrants was different for longitudinal specimens and there were no differences for transverse specimens.In conclusion, formalin fixation changed the mechanical properties of bone, which was related with the temperature, duration, osteon direction and the anatomic locations. The formalin fixed bone should not been largely used in biomechanical test and should been restricted to pilot test when it is hard to obtain the fresh or fresh-frozen bone.