Lisa A. Pruitt

Nanoindentation of biological materials

Nanoindentation has recently emerged as a powerful tool for measuring nano- and microscale mechanical properties in tissues and other biomaterials. This technique has been used to measure the mechanical properties of microstructural features in bone and teeth, investigate variations in mechanical properties with changes in tissue organization or composition in mineralized and soft tissues, and map mechanical properties spatially in complex biomaterials. Continuing advancements in indentation data analysis will increase the methods utility in the characterization of biomaterials.

Plasticity-induced damage layer is a precursor to wear in radiation-cross-linked UHMWPE acetabular components for total hip replacement

The mechanism for the improved wear resistance of cross-linked ultra-high-molecular-weight polyethylene (UHMWPE) remains unclear. This study investigated the effect of cross-linking achieved by gamma irradiation in nitrogen on the tribologic, mechanical, and morphologic properties of UHMWPE. The goal of this study was to relate UHMWPE properties to the wear mechanism in acetabular-bearing inserts. Wear simulation of acetabular liners was followed by detailed characterization of the mechanical behavior and crystalline morphology at the articulating surface. The wear rate was determined to be directly related to the ductility, toughness, and strain-hardening behavior of the UHMWPE. The concept of a plasticity-induced damage layer is introduced to explain the near-surface orientation of the crystalline lamellae observed in the wear-tested acetabular liners. Cross-linking reduces abrasive wear of acetabular components by substantially reducing--but not eliminating--the plasticity-induced damage layer that precedes abrasive wear.

Nanoindentation of polydimethylsiloxane elastomers: Effect of crosslinking, work of adhesion, and fluid environment on elastic modulus

With the potential to map mechanical properties of heterogeneous materials on a micrometer scale, there is growing interest in nanoindentation as a materials characterization technique. However, nanoindentation has been developed primarily for characterization of hard, elasto-plastic materials, and the technique has not been validated for very soft materials with moduli less than 5 MPa. The current study attempted to use nanoindentation to characterize the elastic moduli of soft, elastomeric polydimethylsiloxane (PDMS) samples (with different degrees of crosslinking) and determine the effects of adhesion on these measurements using adhesion contact mechanics models. Results indicate that nanoindentation was able to differentiate between elastic moduli on the order of hundreds of kilo-Pascals. Moreover, calculations using the classical Hertz contact model for dry and aqueous environment gave higher elastic modulus values when compared to those obtained from unconfined compression testing. These data seem to suggest that consideration of the adhesion energy at the tip-sample interface is a significantly important parameter and needs to be taken into account for consistent elastic modulus determination of soft materials by nanoindentation.

Study of fatigue resistance of chemical and radiation crosslinked medical grade ultrahigh molecular weight polyethylene.

The aim of this work is to understand the role of chemical and radiation induced crosslinking on the fatigue crack propagation resistance of medical grade ultrahigh molecular weight polyethylene (UHMWPE). In recent years, the need to improve the tribological performance of UHMWPE used in total joint replacements has resulted in the widespread utilization of crosslinking as a method to improve wear resistance. Although crosslinking has been shown to drastically improve the wear resistance of the polymer, the potential trade-off in fatigue properties has yet to be addressed. Fatigue crack propagation resistance is a concern in tibial inserts where large cyclic stresses are sufficient to drive the growth of subsurface cracks that potentially contribute to delamination wear mechanisms. For clinical relevance, the combined effects of sterilization and aging are examined in two commercially available crosslinked resins. Nonsterile and unaged resins serve as a control. To evaluate the effect of crosslinking, a comparison is made to uncrosslinked resins. Scanning electron microscopy is used to provide an understanding of fatigue fracture mechanisms in the crosslinked polymers. The results of this study show that the current level of crosslinking used in orthopedic resins for enhanced wear resistance is not beneficial for fatigue crack propagation resistance.

Wear and surface cracking in early retrieved highly cross-linked polyethylene acetabular liners

BACKGROUND A higher degree of cross-linking has been shown to improve the tribological properties of ultra-high molecular weight polyethylene in laboratory studies; however, its effect on in vivo behavior has not been well established. We investigated in vivo wear mechanisms in retrieved highly cross-linked polyethylene acetabular liners in order to determine if early in vivo wear behavior is accurately predicted by hip-simulator studies. METHODS A total of twenty-four liners (twenty-one explanted and one unimplanted highly cross-linked liners and two explanted ethylene-oxide-sterilized non-cross-linked liners) were examined for this study. The average age of the patients was 59.9 years, and the average time in vivo was 10.1 months. Articular surface damage on the front and back sides of the liners was assessed with an optical scoring system. Surface quadrants were assigned a grade from 0 to 3 according to the observed wear mechanisms and the percentage of surface affected. The micromechanisms of liner damage were evaluated with use of scanning electron microscopy. RESULTS The average front and back-side explant damage scores were 11 (range, 2 to 26.5) and 6.7 (range, 3.7 to 13.3), respectively. There was consistent evidence of early surface deformation and cracking. All explants exhibited some form of surface change, including surface cracking, abrasion, pitting, or scratching. The original machining marks on the liner surface were observed to be either unaltered, drastically distorted, or absent. CONCLUSIONS Highly cross-linked ultra-high molecular weight polyethylene acetabular liners that were retrieved at an average of ten months after implantation exhibited signs of surface damage that had not been predicted by in vitro hip-simulator studies. These devices had not failed clinically as a result of wear. The discrepancy between in vitro and in vivo wear surfaces may be due to variability in terms of in vivo lubrication and cyclic loading or may represent early surface damage mechanisms that are not well demonstrated by long-term simulator studies.

The yielding, plastic flow, and fracture behavior of ultra-high molecular weight polyethylene used in total joint replacements

The yielding, plastic flow, and fracture behavior of UHMWPE plays an important role in wear and failure mechanisms of total joint replacement components. The primary objective of this study was to compare the yielding, plastic flow, and fracture behavior of two implantable grades of UHMWPE (GUR 1120 vs 4150 HP). The first part of this work explored the hypothesis that up to the polymer yield point, the monotonic loading behavior of UHMWPE displays similar true stress strain behavior in tension and compression. Uniaxial tension and compression tests were conducted to compare the equivalent true stress vs strain response of UHMWPE up to 0.12 true strain. During monotonic loading, the equivalent true stress strain behavior was similar in tension and compression up to the yield point. However, investigation of the unloading behavior and permanent plastic deformations showed that classical deviatoric rate independent plasticity theory may dramatically overpredict the permanent strains in UHMWPE. A secondary goal of this study was to determine the ultimate true stress and strain for UHMWPE and to characterize the fracture surfaces after failure. Using a fracture mechanics approach, the critical flaw sizes were used in combination with the true ultimate stresses to predict the fracture toughness of the two resins. A custom video-based strain measurement system was developed and validated to characterize the true stress-strain behavior up to failure and to verify the accuracy of the incompressibility assumption in calculating the true stress-strains up to failure. In a detailed uncertainty analysis, theoretical expressions were derived for the relative uncertainty in digital video-based estimates of nominal strain, true strain, homogeneous stress, and true stress. Although the yielding behavior of the two UHMWPE resins was similar, the hardening and plastic flow behavior clearly discriminated between the GUR 1120 and 4150 HP. A statistically significant difference between the fracture toughness of the two resins was also evident. The long-term goal of this research is to provide detailed true stress strain data for UHMWPE under uniaxial tension and compression for future numerical simulations and comparison with more complex multiaxial loading conditions.

Clinical fracture of cross-linked UHMWPE acetabular liners.

Highly cross-linked ultrahigh molecular weight polyethylene (UHMWPE) is increasingly used as a bearing material in total hip replacements. Cross-linking of UHMWPE has been shown to increase wear resistance but decrease its fracture resistance. We analyzed the clinical fracture failure of four cross-linked UHMWPE total hip replacement components of four different designs via microscopic observation of the fracture surfaces, and found that in all cases fractures initiated at stress concentrations in an unsupported region of the component (termed the elevated rim). Finite element analyses (FEA) of each individual implant design were then conducted. Results from this analysis demonstrated that the predicted magnitude and orientation of maximum principal stress due to mechanical loading of the elevated rim was sufficient to propagate initiated fatigue cracks in each case. FEA also predicted that cracks may arrest after some amount of growth due to a steep stress gradient near the initiation site. Further, while anatomical positioning of the implant and material properties affect the risk of fracture, we examined whether these failures are strongly related to the notched elevated rim design feature that is common to the four failed cases presented here. We believe that cross-linked UHMWPE remains an excellent bearing material for total hip replacements but that designs employing this material should mitigate stress concentrations or other design features that increase the risk of fracture.

Corrosion-Induced Fracture of a Double-Modular Hip Prosthesis: A Case Report

Modularity is an important aspect of total hip replacement design. A modular Morse taper is commonly used to attach the femoral head to the femoral stem. Newer double-modular designs incorporate a second interface at the neck-stem junction. Increased modularity purportedly allows the surgeon to more closely restore patient anatomy, such as limb length, lateral offset, and femoral anteversion, and to better balance the soft tissue to achieve optimal biomechanics. However, modularity also increases the number of mechanical junctions that may lead to fretting (micromotion)1, corrosion2,3, and ultimately fracture. We have evaluated a double-modular PROFEMUR Z stem (Wright Medical Technology, Arlington, Tennessee) after catastrophic failure due to a fractured neck component at the neck-stem junction. This retrieval allowed us to analyze the mechanical failure mechanisms associated with a double-modular design. We hypothesized that modularity at the neck-stem junction resulted in fretting and crevice corrosion, which led to crack initiation and, on the application of an overload event, to fracture. The patient was informed that data concerning the case would be submitted for publication, and he consented. In June 2006, a thirty-year-old man with rheumatoid arthritis who was 6 ft and 6 in (2 m) tall, weighed 242 lb (109.8 kg), and had a body mass index of 29 kg/m2 underwent an uncomplicated right total hip arthroplasty with a cementless PROFEMUR Z hip stem with a long, straight neck (no varus/valgus offset) and a ceramic-on-ceramic articulation. The ceramic-on-ceramic modular implant was considered to be indicated in this particular patient because of his young age, high body mass index, active lifestyle, and profession. A modular stem was used in an effort to reproduce the anatomic femoral anteversion, offset, and lower-limb length to optimize hip abductor and biomechanical function. The hip functioned well until April 2008, when …

Comparison of the effects of gamma radiation and low temperature hydrogen peroxide gas plasma sterilization on the molecular structure, fatigue resistance, and wear behavior of UHMWPE

The effects of gamma radiation and low temperature hydrogen peroxide gas plasma (HPGP) sterilization on structure and cyclic mechanical properties were examined for orthopedic grade ultra-high-molecular-weight polyethylene (UHMWPE) and compared to each other as well as to no sterilization (control). Density was monitored with a density gradient column and was found to be directly influenced by the sterilization method employed: Gamma radiation led to an increase, while plasma did not. Oxidation of the polymer was studied by observing changes in the carbonyl peak with Fourier transform infrared spectrometry and was found to be strongly affected by both gamma radiation and subsequent aging, while plasma sterilization had little effect. Gamma radiation resulted in embrittlement of the polymer and a decreased resistance to fatigue crack propagation. This mechanical degradation was a direct consequence of postradiation oxidation and molecular evolution of the polymer and was not observed in the plasma-sterilized polymer. Both gamma radiation and plasma sterilization led to improved wear performance of the UHMWPE compared to the nonsterile control material.

The effects of gamma radiation sterilization and ageing on the structure and morphology of medical grade ultra high molecular weight polyethylene

Ultra high molecular weight polyethylene (UHMWPE) that had been sterilized with 25 kGy of gamma radiation and non-sterile material processed from the same batch were studied following five years of ageing in air. Differential scanning calorimetry, small angle X-ray scattering, transmission electron microscopy, Fourier transform infra-red spectroscopy and density gradient column measurements were used to characterize the changes in structure and morphology of the UHMWPE as a result of sterilization and shelf ageing. Significant changes were observed. The UHMWPE oxidized and increased in crystallinity as well as density, with chain scission the dominant response to irradiation. The applicability of the results of this study to the evolution of mechanical properties and structural integrity of medical grade UHMWPE for total joint replacement is addressed.