The influence of water removal on the strength and toughness of cortical bone

Abstract

Although the effects of dehydration on the mechanical behavior of cortical bone are known, the underlying mechanisms for such effects are not clear. We hypothesize that the interactions of water with the collagen and mineral phases each have a unique influence on mechanical behavior. To study this, strength, toughness, and stiffness were measured with three-point bend specimens made from the mid-diaphysis of human cadaveric femurs and divided into six test groups: control (hydrated), drying in a vacuum oven at room temperature (21 degrees C) for 30 min and at 21, 50, 70, or 110 degrees C for 4 h. The experimental data indicated that water loss significantly increased with each increase in drying condition. Bone strength increased with a 5% loss of water by weight, which was caused by drying at 21 degrees C for 4 h. With water loss exceeding 9%, caused by higher drying temperatures (> or =70 degrees C), strength actually decreased. Drying at 21 degrees C (irrespective of time in vacuum) significantly decreased bone toughness through a loss of plasticity. However, drying at 70 degrees C and above caused toughness to decrease through decreases in strength and fracture strain. Stiffness linearly increased with an increase in water loss. From an energy perspective, the water-mineral interaction is removed at higher temperatures than the water-collagen interaction. Therefore, we speculate that loss of water in the collagen phase decreases the toughness of bone, whereas loss of water associated with the mineral phase decreases both bone strength and toughness.

Fig. 1

Each force–displacement plot from the three-point bending tests of cortical bone provided the mechanical properties. As an example, a plot from a control specimen is compared to specimens dried at room temperature and at 70 °C showing the loss of ductility caused by low level of drying and the loss of strength at a high level of drying.

Fig. 2

Cross-sectional images of vascular pores within cortical bone were converted to binary form to calculate porosity (black area per total area).

Fig. 3

There was a significant increase in water loss with each successive increase in temperature and drying time (p≤0.0013). Drying at 50 °C and above in a vacuum oven for 4 h removed more water than what would be expected to exist in the pores.

Fig. 4

Non-linear relationships existed between some mechanical properties of bone and water loss: (A) strength increased following vacuum drying at room temperature but decreased following vacuum drying at elevated temperatures, (B) a decrease in toughness occurred with increases in water loss, with the greatest change caused by drying at room temperature and (C) there was an increase in stiffness with an increase in water loss.