Fracture evaluation of acrylic bone cements modified with hydroxyapatite: influence of the storage conditions

Fracture and mechanical characterization of bone composite composed of polymethylmethacrylate and hydroxyapatite (HA) at different contents was carried out. Hydroxyapatite is added in order to improve cement biocompatibility, but it is expected that it also affects mechanical properties. Specimens were either stored in air at 37 degrees C for 120 h or in physiological solution (PhS-37), in order to establish the influence of storage conditions upon mechanical behavior. One set of specimens was also postcured at 120 degrees C for 4 h to take into account the influence of free monomer. Fracture experiments revealed some non-linearity in load-displacement records and differences in trends between initiation and propagation values of the fracture surface energies. The trends in the data shows that HA acts as a rigid filler enhancing fracture resistance, flexural modules and yield stress, up to a certain content. Beyond the latter limit, properties suffer a deterioration because the addition of HA also affects the cement porosity. Absorbed water acts as plasticizer leading to a decrease in mechanical properties. The highest propagation strain energies were exhibited by materials aged in PhS-37.