Abstract
Introduction
The increase in revision joint replacement surgery and fractures of bone around orthopaedic implants may be partly addressed by keeping bone healthy around orthopaedic implants by inserting implants with mechanical properties closer to the patient's bone properties. We do not currently have an accurate way of calculating a patient's bone mechanical properties. We therefore posed a simple question: can data derived from a micro-indenter be used to calculate bone stiffness?
Methods
We received ethical approval to retrieve femoral heads and necks from patients undergoing hip replacement surgery for research. Cortical bone from the medial calcar region of the femoral neck was cut into 3×3×6mm cuboid specimens using a diamond wafering blade. Micro-indentation testing was performed in the direction of loading of the bone using a MicroMaterials (MicroMaterials, UK) indenter, using the high load micro-indentation stage (see Figure 1). To simulate in vivo testing, the samples were kept hydrated and were not fixed or polished. From the unloading curve after indentation, the elastic modulus was calculated, using the Oliver-Pharr method using the indentation machine software. To assess which microindentation machine settings most precisely calculate the elastic modulus we varied the loading and unloading rates, load and indenter tip shape (diamond Berkovich tip, 1mm diameter Zirconia spherical tip and 1.5mm diameter ruby spherical tip).
Following this, for 11 patients' bone, we performed compression testing of the same samples after they were indented with the 1.5mm diameter ruby spherical tip to assess if there was a correlation between indentation values of apparent elastic modulus and apparent modulus values calculated by compression testing (see Figure 2). Platens compression testing was performed using an Instron 5565 (Instron, USA) materials testing machine. Bluehill compliance correction software (Instron, USA) was used to correct for machine compliance. The strain rate was set at 0.03mm/s. The apparent elastic modulus was calculated from the slope of the elastic region of the stress-strain graph. The correlation between values of apparent modulus from compression testing and indentation were analyzed using IBM SPSS Statistics 22.
Results
The most precise results were obtained using a spherical indenter tip (1.5 mm diameter ruby ball), rather than a sharp Berkovich tip, high load (10N), a loading rate of 100 mN/s and unloading rate of 300 mN/s with a pause of 60 seconds at maximum load. We also used multiple load cycles with a peak load of 10N (see Figure 3). Using these optimal settings we calculated the mean elastic modulus over 10 cycles of testing with six indents on one sample to be 11.8 GPa (+/− 1.01). There was a moderate correlation between indentation and compression values of apparent modulus (r=0.62, n=11, p=0.04).
Discussion
By using a spherical indenter tip and fast unloading it was possible to get precise apparent modulus values. The apparent modulus derived from micro-indentation, correlated moderately with that derived from direct compression testing. This early data suggests that microindentation may become a clinically relevant test of bone quality in real time.
