On the magnitude and variability of the fatigue strength of acrylic bone cement

When used for the fixation of orthopaedic implants poly(methyl methacrylate ) bone cement is prepared during surgery, and polymerises in situ. The tech nique for preparation of the bone cement involves mixing the liquid monomer and powder: two common mixing methods are hand mixing and Vacuum mixing. P revious studies have shown that porosity depends on mixing technique. In th is study, the fatigue strength of hand-mixed and vacuum-mixed cements is me asured and correlated with the pore distribution resulting from each mixing technique. S-N curves show that vacuum mixing improves the fatigue strengt h by an order of magnitude. However, there is greater variability of fatigu e strength associated with vacuum-mixed cement. This is correlated with the appearance of an occasional large pore in the vacuum-mixed cement. If the cross-sectional area is corrected to take account of porosity in vacuum-mix ed cement, an 8% increase in the association of the data is found. Using a two-parameter Weibull model, it can be shown that the vacuum-mixed cement h as a greater Weibull life at the 50% probability-of-survival level. However , if a probability of-survival close to 100% is required (i.e. high reliabi lity), the hand-mixed cement is found to have superior fatigue behaviour. T he S-N curves can be explained by examination of the fracture surface featu res. The initiation stage of fatigue cracking is notably different for the two different mixing techniques. The lower fatigue strength of the hand-mix ed cement can be explained by the interactions of pores on the fracture sur face causing stress concentrations, whereas no such pore interactions occur in the vacuum-mixed cement. (C) 2000 Elsevier Science Ltd. All rights rese rved.