With information about a patient's bone mechanical properties orthopaedic operations could be optimised to reduce intra- and post-operative complications. However, there is currently no reliable method of measuring a patient's bone mechanical properties in vivo. We have previously investigated microindentation, using a 1.5mm diameter spherical indenter tip, and found no correlation between these measurements and compression testing measurements. We hypothesised that by using a larger diameter indenter tip we would closer match bone millimetre-scale mechanical properties.

20 bone samples were taken from 20 patients undergoing hip replacement surgery. The samples were machined from the femoral neck calcar cortical bone into 6×3×3mm parallelepiped specimens, aligned with the osteons along the long axis. The samples were micro-computed tomography (CT) scanned to calculate porosity. Microindentation was performed using a 6mm diameter, sapphire, spherical indenter tip. 12 indentations were performed in a grid and the reduced moduli were calculated using the Oliver-Pharr method. Compression testing was then performed to failure and the apparent elastic modulus was calculated for each sample.

A moderate correlation was found between the indentation reduced moduli and compression testing elastic moduli (r=0.52, r2=0.275, p=0.018). In addition, a moderate correlation was found between the indentation reduced moduli and CT-measured porosity (r=0.5, r2=0.251, p=0.025) and a strong correlation was found between compression testing moduli and porosity (r=0.75, r2=0.568, p<0.001).

Using large-tip spherical microindentation, indentation reduced moduli correlated significantly with compression testing apparent elastic moduli in these 20 cortical bone specimens. Microindentation using a large, spherical indenter tip may predict the mechanical properties of bone at the millimetre length scale and shows promise as a potential future clinical decision aid in surgery.