Dynamic mechanical behaviour of PMMA based bone cements in wet environment

The mechanical properties of three wet commercial bone cements, namely Braxel (from Bioland), Simplex-P (from Howmedica) and CMW1-G (from DePuy) are investigated by means of stress relaxation and dynamic mechanical analysis (DMA). The geometry of loading that was used is the three point bending method (ASTM D790); all the tests were performed in a water chamber by means of temperature sweeps between 17°C and 57°C and spanning four frequency decades. The results show that viscoelastic properties are strongly dependent on specimen conditioning (i.e. water uptake and heat treatment). The results also show that all the cements that were analysed show mechanical properties which are intermediate between the ones of the cancellous bone and of the metals of which prostheses are normally made. As a consequence, the cement is able to reduce the stress concentrations due to the interfacing of materials which have very different stiffnesses. Moreover, the results of the DMA, particularly the ones concerning the damping factor (tan delta), indicate that at body temperature the bone cements tested show an increased capacity of dissipation, the higher is the loading frequency, thus displaying shock absorbing properties.