Fatigue properties of acrylic bone cement: S-N, P-N, and P-S-N data

The determination of the fatigue properties of a material is a fundamental criteria in the engineering design process. The fatigue properties are governed by a number of factors, one of which is the inherent scatter in the data. In order to take into account this scatter in the results, the concept of the probability of failure (P) is introduced and interconnected with the well known stress (S) versus number of cycles to failure (N) data. This report determines the S-N curve, P-N curve, and P-S-N contour for the three leading acrylic bone cements: Surgical Simplex P, Zimmer LVC, and Zimmer Regular. Tensile specimens were fabricated according to ASTM D638 specifications and tested in uniaxial, zero-tension fatigue. The resulting stress versus number of cycles to failure data was subjected to a nonlinear least-squares analysis to determine the mathematical expression of the fatigue curve. Statistical analysis showed excellent fit of the data to the predicted curve for estimation of the endurance limit of each cement. The results indicated that Zimmer LVC had the highest endurance limit while the limit of Simplex-P and Zimmer Regular were significantly lower. No significant difference was noted in the endurance limit between Simplex-P and Zimmer Regular. The probability of failure at each stress level was determined with respect to fatigue distribution functions. Using the normal distribution function and previous S-N data, the P-S-N contour was generated for each cement. The P-S-N contour fully describes the fatigue characteristics of the material.