A procedure and criterion for bone cement fracture toughness tests

Nowadays, two procedures, based on the recommendation of two American standards (ASTM E399 and ASTM D5045), are used to determine the fracture toughness, K-Ic, of bone cement. However, there is a lack of knowledge about the equivalence of the two testing methods applied to bone cement. Additionally, in spite of the recommendation of several authors to introduce a rejection criterion for specimens based on the size of defects found in the fracture surface, no data are available about the effect of porosity within the material on the K-Ic of bone cement. The aims of this study were to verify whether the K-Ic values calculated for bone cement using the two procedures are comparable and whether macroporosity within the tested samples affects the K-Ic value of bone cement, and, if so, to establish a rejection criterion for specimen selection. Samples of pure polymethyl methacrylate (PMMA) were tested by both procedures. Additionally, samples showing defects (macroporosity) of different sizes and located in different positions within the specimen were tested. The K-Ic value determined following the ASTM E399 procedure was 13 per cent lower than that calculated following the ASTM D5045 procedure. In the first series a lower data scatter was observed. Also, the presence of macroporosity on the fracture surface of the specimen affected the K-Ic value of bone cement. Therefore, the mechanical behaviour of samples was affected by defects within the material. Since it is possible to mould specimens without macroporosity, it seems recommendable to reject specimens with macroporosity on the fracture surface before calculating the K-Ic, value of bone cement.