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 technique for preparation of the bone cement involves mixing the liquid monomer and powder: two common mixing methods are hand mixing and vacuum mixing. Previous studies have shown that porosity depends on mixing technique. In this study, the fatigue strength of hand-mixed and vacuum-mixed cements is measured and correlated with the pore distribution resulting from each mixing technique. S–N curves show that vacuum mixing improves the fatigue strength by an order of magnitude. However, there is greater variability of fatigue 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-mixed 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 has 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 reliability), the hand-mixed cement is found to have superior fatigue behaviour. The S–N curves can be explained by examination of the fracture surface features. The initiation stage of fatigue cracking is notably different for the two different mixing techniques. The lower fatigue strength of the hand-mixed cement can be explained by the interactions of pores on the fracture surface causing stress concentrations, whereas no such pore interactions occur in the vacuum-mixed cement.