Dependence of in vitro fatigue properties of PMMA bone cement on the polydispersity index of its powder.

Four variants of poly (methyl methacrylate) (PMMA) bone cement were used, the difference being in the method used to sterilize the powder (three different dosages of gamma irradiation and ethylene oxide gas) and, hence, in the molecular weight of the powder. For each cement powder, the number-average molecular weight and weight-average molecular weight (and, hence, the polydispersity index, PDI) were determined using gel permeation chromatography. For each of the cured cements, the fatigue lives (N(f)) of specimens, at loads corresponding to stresses (S) of +/-10.0 MPa, +/-12.5 MPa, +/-15.0 MPa, and +/-20.0 MPa, were determined using the protocol detailed in ASTM F2118-03. Hence, the values of the three Weibull parameters were determined for each cement set-S combination. From these results, one index of the fatigue life of the cement, namely, the Weibull mean fatigue life (N(WM)), was computed for each combination. For each cement, the Olgive equation was fitted to the S-N(f) results, yielding an estimate of another fatigue property, the cement's fatigue limit. Best-fit empirical relationships (1) betweenlnN(WM), S, and PDI, and (2) between the estimated fatigue limit and PDI were obtained. These relationships may be used in the development of new cement powder sterilization methods.