PU foams, of three different densities, were used in this study. Closed cell PU foam of density 0.16 g.cm-3 and 0.32 g.cm-3 (American Society for Testing and Materials, ASTM, Grade 10 and Grade 20) [1 ] was used to model low and medium density cancellous bone respectively. Open cell rigid foam of density 0.09 g.cm-3 was used to model very low density cancellous bone. All PU foams were purchased in block form, with dimensions 130 × 180 × 40 mm, from Sawbones® Europe AB, Malmö, Sweden. The foam densities were supplied by Sawbones® Europe AB.
Using a sharpened tube, six cylindrical cores of 9 mm diameter were drilled from each of the three different density PU foam blocks. The cores were taken using the method described by Li and Aspden [14 (link)], in which the cylindrical axis of the core was roughly perpendicular to the surface of the PU block (this is the preferred orientation of the "trabeculae"). The exact diameter of the PU cylinders was determined as an average of four measurements; this was necessary to account for the inhomogeneity of the 0.09 g.cm-3 open cell PU foam in particular.
For this study, two different cylinder lengths were chosen to test for any buckling or shape effects. A cylinder, of length of 7.7 ± 0.2 mm, was chosen so that results could be compared with those from a published study of human OP cancellous bone [14 (link)]. In order to investigate the effect of specimen dimensions, a cylinder, of length 3.9 ± 0.1 mm, was also investigated. This length was obtained from a standard for testing rubbers [16 ]. The reason for choosing this standard was to ensure that the specimens did not bulge during compression; rubbers have a Poisson's value of about 0.5 and so maintain an almost constant volume during compression; as a result, they bulge more than most other materials [17 (link),18 (link)]. Dimensions were measured with digital vernier callipers (Fisher Scientific UK Ltd., Leicestershire).
Six cylinders were prepared for each cylinder length and each density of PU foam block. The required cylinder length was achieved by either using a small pair of scissors, for the 0.09 g.cm-3 PU foam, or by rubbing the PU foam cylinder on a sheet of sandpaper (medium grade M2, SupaDec, RS Components Ltd., Northamptonshire, UK), for the 0.16 g.cm-3 and 0.32 g.cm-3 PU foams.
Using a sharpened tube, six cylindrical cores of 9 mm diameter were drilled from each of the three different density PU foam blocks. The cores were taken using the method described by Li and Aspden [14 (link)], in which the cylindrical axis of the core was roughly perpendicular to the surface of the PU block (this is the preferred orientation of the "trabeculae"). The exact diameter of the PU cylinders was determined as an average of four measurements; this was necessary to account for the inhomogeneity of the 0.09 g.cm-3 open cell PU foam in particular.
For this study, two different cylinder lengths were chosen to test for any buckling or shape effects. A cylinder, of length of 7.7 ± 0.2 mm, was chosen so that results could be compared with those from a published study of human OP cancellous bone [14 (link)]. In order to investigate the effect of specimen dimensions, a cylinder, of length 3.9 ± 0.1 mm, was also investigated. This length was obtained from a standard for testing rubbers [16 ]. The reason for choosing this standard was to ensure that the specimens did not bulge during compression; rubbers have a Poisson's value of about 0.5 and so maintain an almost constant volume during compression; as a result, they bulge more than most other materials [17 (link),18 (link)]. Dimensions were measured with digital vernier callipers (Fisher Scientific UK Ltd., Leicestershire).
Six cylinders were prepared for each cylinder length and each density of PU foam block. The required cylinder length was achieved by either using a small pair of scissors, for the 0.09 g.cm-3 PU foam, or by rubbing the PU foam cylinder on a sheet of sandpaper (medium grade M2, SupaDec, RS Components Ltd., Northamptonshire, UK), for the 0.16 g.cm-3 and 0.32 g.cm-3 PU foams.
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