No abstract

The vital first phase of the overall materials study to protract the life of the total joint replacements is the identification of the fracture toughness and fatigue properties of bone cements. Information gained from fatigue testing, performed in a manner which simmulates in vivo conditions, and fracture toughness, which is a measure of the propensity of a crack to propagate, is the first step towards the prediction of the life of the total joint replacement. This study is concerned with the fracture toughness of Zimmer and Simplex-P cold-curing bone cements. Following cement fabrication conditions which closely approximate clinical procedures, fracture toughness testing was conducted on cement specimens which were immersed in bovine serum at 37 degrees C in order to simulate in vivo conditions. In addition, a similar study was completed on specimens, tested in air at ambient temperature for purposes of comparison. Results of this procedure, when analyzed by a Student's t-test at the 95% confidence level with eight degrees of freedom, indicate that both Zimmer and Simplex-P exhibit a higher fracture toughness in the simulated physiological environment. In order to determine whether the addition of barium sulfate to these cements compromises the fracture toughness, the above described testing rationale was repeated, indicating the existence of a complicated relationship between the different testing environments and barium sulfate. The importance of these results lies in the fact that an increased fracture toughness indicates that a cement will inherently exhibit a greater degree of resistance to the propagation of cracks, which could contribute to the ultimate failure of the total joint replacement.

The vital first phase of the overall materials study to protract the life of total joint replacements is the identification of the fracture toughness and fatigue properties of bone cements. Information gained from fatigue testing, performed in a manner which simulates in vivo conditions, and fracture toughness, which is a measure of the propensity of a crack to propagate, is the first step towards the prediction of the life of the total joint replacement. Part I of this study identified the fracture toughness characteristics of two acrylic bone cements, while the present work has been concerned with determining the fatigue behavior by means of tests conducted in a rotating bending fatigue apparatus. Fatigue specimens were fabricated under conditions which approximated clinical procedures and then tested while immersed in bovine serum at 37°C in order to simulate in vivo conditions. In addition, a similar study was completed on specimens tested in air at ambient temperature for purposes of comparison. Testing was conducted in both of these environments on specimens containing zero and 10.0 wt % BaSO4. Cyclic loading frequency was maintained between 1200 and 1400 cycles/min in order to insure that crack propagation was the sole mechanism of failure, i.e., failure via cyclic thermal softening was obviated. Results of this phase of the study, when analyzed by a Student t‐test at the 90% confidence level with four degrees of freedom, indicate that the fatigue life of specimens tested in bovine serum at 37°C is superior to that of specimens tested in air at ambient temperature. The addition of BaSO4 to Simplex‐P cement, while not significant at the 90% confidence level, was significant in increasing the fatigue life in air at the 80% confidence level; however, this effect was not noticeable when testing in bovine serum at 37°C. Examination of the fracture surfaces enabled calculation of the critical stress intensity factor which when compared with values from an earlier work showed good agreement.

Conclusion (2) on p. 622 should be changed to read:(2) When fabricated a t 5 psi, Zimmer acrylic bone cement shows superior fracture toughness for all additive concentrations and testing environments. Simplex-P acrylic bone cement containing 10 w/o Bas04 demonstrates superior fatigue behavior when fabricated a t 5 psi and tested in air a t 22OC while Zimmer cement containing 10 w/o Bas04 exhibits superior fatigue behavior when fabricated a t 5 psi and tested in bovine serum at 37°C in order to simulate in uiuo conditions.

To obtain further details of the unusual behavior of the “hard elastic,” “springy” form of polypropylene (SPP), various loading conditions were studied with an Instron model 1130. Specifically, the variation of the elongation at rupture with change in gauge length and strain rate has been determined. The nature and degree of the specimen hysteresis resulting from various loading cycles have been characterized. The ability of this material to recover from large extensions customarily expected to be permanent deformation has been identified and the time dependency of this recovery has been determined for strain levels of 10, 25, 50, and 100 percent of the original sample length. The effects of submitting the material to conditioning treatments, e.g., immersing in saline solutions of various electrolyte concentrations and autoclaving for 30 min at 250°F and 15 psig have been studied. Characterization of the stress relaxation behavior of the polymer has been made. Fatigue behavior has been identified utilizing a novel testing apparatus which permits load‐cycling of the fiber. Rheovibron work has facilitated calculation of the activation energy of the second order relaxation of SPP from tan δ vs temperature data. Stoll abrasion testing results on SPP tubular knit fabric and multifilament yarn have been compared to normal polypropylene and Dacron®.