The Deformation, Yielding, and Fracture of Ultra-High Molecular Weight Polyethylene for Use in Total Joint Replacements

Each year close to one millions patients within the United States, receive a total joint replacement (TJR) to alleviate pain from severe debilitating osteoarthritis. TJRs can comprise hip, knee, shoulder, and even elbow and ankle replacements. Though these implants differ in anatomical function they have a consistent theme, a hard-on-soft bearing couple consisting of hard cobalt chrome (CoCr) and soft ultra-high molecular weight polyethylene (UHMWPE). UHMWPE is a semi-crystalline polymer with 2-6 million g/mol where long molecular chains create high entanglements and help give the material high energetic toughness. These molecular characteristics also provide a low coefficient of friction desirable for TJRs. This coupling pair has been the standard of care for nearly sixty years however not without some complications. TJRs primarily fail from wear debris that is liberated from the UHMWPE bearing surface. This wear debris is caused by successive plastic deformation from implant loading leading to crack initiation below the implant surface. Crack initiation leads to fatigue crack growth with the eventual liberation of debris. As a result of this detriment, there have been several changes to the formulations that make up UHMWPE. These changes primarily include radiation cross-linking to improve wear resistance. Increased wear resistance comes with concomitant trade-offs to the mechanical properties of the material.Radiation cross-linking through gamma irradiation, introduces free radicals to the material. Free- radical need to be eliminated otherwise they will react with the body and cause the polymer to oxidize in an in vivo environment. To alleviate these free-radicals post processing is performed. This post processing usually consists of thermal annealing treatments either above or below the melt temperature of UHMWPE. More recently, UHMWPE materials have moved away from post irradiation annealing in favor of antioxidant additions to the material. These antioxidants, such as vitamin E, are added to stabilize the material after irradiation and to prevent any possibility of oxidative embrittlement during in vivo operation. All of these unique additions to UHMWPE pose the important question of how the material’s fundamental mechanical properties are affected. There is a plethora of research data on the mechanical properties of UHMWPE and some of its material formulations, however when one dives into the procedural methods of these studies there are significant inconsistencies.These inconsistencies are rooted in the procedures used to analyze and create material mechanical properties. Unlike metallic materials where methods for analyzing mechanical properties are very well understood, polymeric materials offer a more complex challenge when interpreting their constitutive behavior. This is extremely important when polymeric materials, such as UHMWPE are used in safety critical applications such as TJRs. These challenges increase when UHMWPE is tailored through combinations of resin type, radiation cross-linking, and antioxidant additions. As a result there is a need to answer from a methodological perspective how the mechanical properties of UHMWPE change with different material formulations under different loading scenarios.This dissertation provides a comprehensive and thorough assessment of the mechanical properties of UHMWPE across 12 different material formulations focusing on how the methods used to analyze mechanical behavior can be extremely important. First, a comprehensive microstructural analysis is performed through differential scanning calorimetry (DSC) and small angle x-ray scattering (SAXS) to gather microstructure data for its potential effect on mechanical properties. Then tensile deformation in UHMWPE and its various material formulations are investigated. Engineering versus true tensile stress-strain data is looked at to elucidate the differences between analysis methods for determining elastic properties, yield, post yield, and ultimate behavior. Tensile constitutive properties are then compared to properties determined from compression and nanoindentation in an effort to understand material deformation trends across measurement methods. Then microstructure and tensile analysis are applied in the determination of the elastic-plastic fracture, or J-integral, toughness behavior of UHMWPE. Finally, this study concludes with a mechanistic analysis of the crack growth mechanisms to validate fracture toughness methods