THE EFFECTS OF CENTRIFUGATION AND TITANIUM FIBER REINFORCEMENT ON FATIGUE FAILURE MECHANISMS IN POLY(METHYL METHACRYLATE) BONE-CEMENT

The contributions to fatigue resistance in poly(methyl methacrylate) bone cement (PMMA) by centrifugation and titanium fiber addition were studied. Three modified bone cements were tested: porosity reduced cement (C-PMMA); titanium (Ti) fiber-reinforced cement (R-PMMA), using a novel method to incorporate fibers; and porosity reduced and Ti fiber-reinforced cement (C-R-PMMA). Specimens of untreated bone cement (PMMA) were included as controls. Non-notched and notched specimens were cyclically loaded in fully reversed bending. For the non-notched specimens, at all stress levels, C-R-PMMA had significantly greater fatigue life than either C-PMMA or R-PMMA (except C-PMMA at 30 MPa). R-PMMA had lower fatigue life than the control at 30 MPa (nominal maximum stress), but higher fatigue life at 20 and 15 MPa. At both 20 and 15 MPa, there was no statistical difference between the fatigue lives of C-PMMA and R-PMMA. Fractography revealed that, at 30 MPa, characteristics of rapid fracture predominated. At 20 and 15 MPa, in contrast, almost the entire fracture surface showed the characteristic fatigue morphology, indicating that subcritical crack growth was predominant. For the notched specimens, C-PMMA showed greater life than PMMA at the highest stress intensity, but the lifetimes converged with decreasing stress intensity. In contrast, the lifetimes for PMMA and R-PMMA diverged with decreasing stress intensity. The effect of centrifugation appeared to be strongest at higher stress intensities, and diminished with decreasing stress intensity. The reinforcing effect of Ti fiber addition increased the notched fatigue life at high initial stress intensities, and the reinforcing effect increased with decreased stress intensity. In both the non-notched and notched fatigue experiments, the combination of porosity reduction and Ti fiber reinforcement (C-R-PMMA) led to substantial improvements in fatigue life at each stress and stress intensity level, suggesting that Ti fiber reinforcement and porosity reduction effects are additive. Fiber reinforcement affects the fatigue crack propagation phase of failure in bone cement, enhancing the fatigue crack propagation resistance. Pore reduction may affect fatigue crack initiation as well as fatigue crack propagation resistance. (C) 1995 John Wiley & Sons, Inc.