The Interplay of Design and Materials in Orthopedics: Evaluating the Impact of Notch Geometry on Fatigue Failure of UHMWPE Joint Replacements

Each year, roughly 750,000 patients in the US receive a total joint replacement (TJR), or a synthetic medical device that serves to replace the natural joint to restore function and relieve pain. TJRs have had a long history of use in the hip, knee and shoulder, yet still retain the same standard design of a hard-on-soft bearing coupling. Today, those bearings are primarily composed of hard cobalt chrome (CoCr) surface articulating against ultrahigh molecular weight polyethylene (UHMWPE), a polymer with notable mechanical toughness and biocompatibility that have driven its 50-plus years of use in vivo. TJRs have seen tremendous success for older patient cohorts for whom these devices were designed. However, increasing demand from younger patients has motivated the need for more durable materials that can sustain higher, more variable loading for a longer time.;Explanted devices that have been retrieved from patients following failure have provided significant insight into the vulnerabilities of different UHMWPE formulations currently in the market, especially with respect to component fracture. Three cases examined in this work describe fracture failures of UHMWPE components seen in the knee, hip and shoulder to demonstrate existing tradeoffs in material sterilization and processing.;Incidences of component fracture like the three reviewed in this work have motivated significant study of UHMWPE's mechanical deformation as a function of microstructural changes. In addition, computational studies have sought to establish how changes in design may reduce local stresses in UHMWPE components to mitigate failure. Material and design influences have thus predominantly been studied in isolation. The latter half of this work demonstrates how previous methodologies using linear elastic fracture mechanics (LEFM) and theoretical approaches to notch fatigue can been merged to elucidate the influence that notch geometry (notch-root radius) has on crack behavior in UHMWPE formulations.;The fatigue crack propagation (FCP) behavior was mapped using the Paris law to compare the relative crack speeds at a given applied Delta K for each notch geometry and material formulation. Crack growth ahead of each notch was found to overlap with sharp crack data, further supporting the use of the Dowling approach in characterizing near-notch crack growth. This overlap in data implied that mechanisms of crack growth near the notch were similar to those further away from the notch (outside the "notch-affected zone", as calculated for each notch radius using the Dowling approach). The congruency of all notch-emanating crack data also revealed microstructure-driven trends between each material cohort that have been noted in previous sharp crack studies.;This study demonstrated that fatigue crack growth in UHMWPE primarily defers to microstructural influences, even when considering varying notch geometries within the vicinity of a crack. This work demonstrates that this methodology of investigating notch effects on crack behavior can be leveraged for polymeric materials, despite its primary origin from crystalline metals. Furthermore, by mimicking previous specimen types, sample dimensions and loading conditions, the methodology used in this work is easily translatable to orthopedic manufactures or research groups seeking to evaluate notch effects in novel UHMWPE formulations. Results shown here reveal that blunter notches do serve to mitigate catastrophic failure by reducing local driving forces (lower Delta K) within a larger notch-affected zone than sharp notches. However, this reduction may not offset the tradeoff in fatigue properties exhibited by highly crosslinked UHMWPE and a focus on optimizing the microstructure of this polymer may be more prudent for increasing its durability in vivo. (Abstract shortened by UMI.).