EFFECT OF A SALINE ENVIRONMENT ON THE CREEP FRACTURE OF POLYMERIC IMPLANT MATERIALS

Abstract

Through the years, many polymers have been introduced into medical applications. Most publicised is silicone, the materials used in breast augmentation device. Failure of a medical device is both detrimental to the patient as well as to the reputation of the manufacturer. As such it is therefore important from the beginning of the design phase that polymeric materials be chosen carefully. Polymeric materials, like many other engineering materials, are affected by environmental elements, no matter how harmless the environment the application is designed for. Therefore in medical applications, particularly in implants, it is important to understand the behavioural aspects of polymeric materials when subjected to an environment of bodily fluids.

Some polymeric implants materials commonly used in medical implants were selected for evaluation : Delrin, Ultra High Molecular Weight Polyethylene (UHMWPE), Polyetheretherketone (PEEK) and Polysulphone. These polymeric materials were subjected to static axial stress in a saline environment to simulate application in bodily fluid. This is equivalent to performing static creep in a saline environment. Each polymeric material is subjected to a variety of stress levels starting from the materials yield stress. The time to fracture for each stress level is recorded. To predict the minimum stress to failure, a mathematical model derived from a three-element spring-dashpot configuration with a spring in series with a Voigt Model and the Reiner-Weissenberg Energy Failure Criterion was applied to the recorded data points. Submitting the data points and the mathematical model to a statistical computational package Statistical Analysis System (SAS) NLIN, the minimum stress level to fracture was estimated.

The minimum stress to failure for the selected polymeric materials in a saline environment at room temperature (27°C) was found to be : • Delrin 10. 7 MPa • UHMWPE 5.8 MPa • Polysulphone 43.0 MPa • PEEK 59.0 MPa Comparing with information collected previously for Delrin in air (15 MPa), this shows a significant drop in strength in this material. At an elevated temperature of 37°C in saline, the previously documented minimum stress level for Delrin is 5 MPa. This indicate that both the saline and elevated temperature will result in a decline in the minimum stress level for Delrin. Similarly, for Polysulphone, previously documented minimum stress level in air is 54 MPa, whereas in saline it falls to 43 MPa. For PEEK, the minimum stress level at 27°C in saline is 59 MPa and an elevated temperature of 37°C is 56MPa; demonstrating only a small change. This indicate high stability of this material and its suitability in biomedical applications. The minimum stress level determined for the UHMWPE was not compared because of lack of previous study. However, this result is important because it explains the failure found in hip implant in mid to long term study. The contact stresses in mating parts in a hip is documented as between 3.45 - 6.90 MPa and when compared to our estimated minimum stress of 5.8 MPa for UHMWPE, failure is quite possible in the mid to long-term duration of application. Fractographs from scanning electron microscope were studied. Sodium chloride crystals were found in two of the materials : Delrin and Polysulphone. None were found in PEEK or UHMWPE. However, it is inconclusive for UHMWPE because no fractographs for long term fracture were taken. The crystals in Delrin and Polysulphone were found in specimens where time to rupture is 10 hours and beyond. The rate of growth of crystal in Delrin (with 0.3% carbon) is 0.l?m per hours whereas that in natural Delrin is 3.4 ?m per hour. No estimation is possible for Polysulphone because only one fractograph shows crystal aggregation. Another mathematical model derived from a three-element model with a spring in parallel to a Maxwell Model and using the Reiner-Weissenberg Energy Failure Criterion was investigated. Using the same initial parameters for initiating the statistical computational model, the estimates were obtained. The estimates from the new model are comparable but further evaluation may be required to evaluate the potential of this new model.

Overall the main objective of this thesis is achieved. With the new information on UHMWPE, Delrin, PEEK and Polysulphone, biomedical engineers are able to design medical devices such that they are not exposed to stresses higher than the minimum stress limit and therefore improving their longevity.