To simulate physiological conditions in vivo, fresh bone—including the surrounding tissue—would be the best testing material. For mechnical testing, high numbers of bones are needed and testing often has to be performed over several days, which requires bone preservation. Changes in mechnical properties after preservation have been discussed in the literature, and they are the subject of some controversy. Sonstegard and Matthews (1997) found a decrease of 10% in stiffness after freezing of trabecular bones, and they suggested that the reason might be the trabecular damage caused by freezing-expansion of interstitial fluids. Pelker et al. (1983)
found a slight increase in stiffness after freezing. Panjabi et al. (1985)
and Linde and Sorensen (1993)
found similar mechanical properties after freezing bones for different periods of time. Before bone preservation, early post mortem changes lead to alteration in bone stiffness—especially in the first 24 hours (Linde and Sorensen 1993
). These changes must be considered when interpreting in vitro results.
Besides fresh frozen bones, embalmed human cadaveric bones are common in mechanical studies. There are also varied and contradictory opinions in the literature as to whether embalmed bone reflects realistic physiological conditions. In experiments with cat bones, Goh et al. (1989)
found that formalin fixation caused a reduction in energy absorption and increased the brittleness of the bone. Currey et al. (1995)
reported a decrease in impact strength of bovine bone after fixation with formaldehyde. Wingerter et al. (2006)
showed similar results for fixation of rat bone with a 10% formalin solution. In contrast, van Haaren et al. (2008)
recently showed that fixation with formaldehyde had no effect on the mechanical properties of goat bones after a fixation period of up to 1 year, and they suggested that embalmed bones can be safely used for mechanical testing purposes.
There have been few studies on human bone tissue. Ohman et al. (2008)
analyzed small specimens extracted from 2 human femoral diaphyses. After a 4 week of fixation using a 4% formalin solution, they found that there were no significant differences in yield stress, ultimate stress, and hardness compared to the fresh bone. In addition, they found an increase in yield and ultimate stress after a fixation period of 8 weeks. Burkhart et al. (2010)
recently published similar results after comparing formalin-fixed and fresh human bones. They analyzed human specimens taken from the subtrochanteric region of the femur and reported an increase in stiffness values as well as axial and torsional loads after 6 weeks of fixation in 4% formalin solution. Another study found no change in stiffness after a storage time of 100 days after ethanol fixation (Linde and Sorensen 1993
Zech et al. (2006) compared embalmed, fresh, and artificial human calcanei. Artificial calcanei had mechanical characteristics that were different from those of embalmed and fresh frozen cadaveric specimens, which does not support the use of artificial calcanei for mechanical implant testing. Fresh frozen and embalmed specimens appeared to be equally adequate for mechanical testing. Furthermore, Zech suggested that the use of paired specimens is not necessary because of the low variation in mechanical strength in unpaired cadaveric specimens. Cristofolini et al. (1996)
found that composite bones differ from cadaveric bones in some of their mechanical characteristics. Zdero et al. (2008)
compared artificial and human bones and they could not find any significant difference regarding screw pullout force and shear stress.
Our testing setup was chosen for comparison of 2 different and frequently used bone types: embalmed and fresh frozen bones. To simulate realistic conditions, we used the whole proximal part of the bone including the shaft for testing of axial loading forces in the physiological axis of the bone. The different unpaired human cadaveric femur bones were harvested from different donors; thus, we could not compare the different fixation techniques on a paired bone model. Zech et al. (2006)
found a low degree of variation in mechanical strength in similar unpaired cadaveric bone specimens. For a better comparison of the specimens, we chose a similar group of donors concerning sex and age distribution. All donors had had osteoporosis documented in their clinical history, but T-scores were not provided. The cortical BMD of embalmed and fresh frozen bones was therefore documented using pQCT measurements prior to mechanical testing.
The cortical and cancellous BMD of the different cadaveric specimens were similar, as were the stiffness and length measurements of the samples.
When comparing embalmed cadaveric bones to fresh frozen cadaveric bones, we found similar stiffness, axial load, and pullout force of cancellous and cortical bone screws. Slightly higher pullout forces for cancellous screws and lower pullout forces for cortical screws in fresh frozen bones can be explained by interindividual differences between the specimens used. There may be an effect of the procedure of embalming on the bones, especially concerning rigidity and brittleness. Van Haaren et al. (2008) and Zech et al. (2006)
did not find any significant differences between embalmed and fresh frozen animal and human bones regarding their mechanical properties. Goh et al. (1989)
found an increase in brittleness and Currey et al. (1995)
found a reduction in impact strength, while Burkhart et al. (2008) and Ohman et al. (2008)
found that cortical bones are more rigid after an embalming procedure. We found similar mechanics in embalmed and fresh frozen human cadaveric bones. They can therefore be recommended for biomechanical testing purposes.
As expected, the reference group of composite bones had higher values for all mechanical parameters tested although they showed the same Pauwels type of fracture (type III). These differences can be explained by the fact that these composite “bone analogs” are designed and optimized to be a model for “average” and non-osteoporotic human femora regarding mass density and stiffness distribution. This was why composite bones were used as a reference group in this study.