Francisco Medel

Bacterial adherence to separated modular components in joint prosthesis: a clinical study.

Bacterial adherence on total joint replacement implants may lead to biofilm formation and implant‐related osteoarticular infection. It is unclear if different biomaterials in the prosthetic components are more prone to facilitate this bacterial adherence, although ultrahigh molecular weight polyethylene (UHMWPE) component exchange in modular systems has been clinically utilized in the early management of these infections. To clarify if the amount of clinically adhered microorganisms was related to the material or the component, we investigated retrieved implants from infected joint replacements. Thirty‐two patients were revised after confirmed implant‐related infection through positive cultures. Eighty‐seven total joint components (hip and knee) were obtained and separately sonicated following a previously published protocol. Cultures were quantified, and detected colony forming units (CFU) were adjusted according to the component surface and compared based on the component material and location. Variable adherence of bacteria to chrome cobalt alloys, UHMWPE, hydroxyapatite coated components, and titanium alloys. The commonest isolated organisms were Staphylococcus epidermidis (23 of 87 components) and Staphylococcus aureus (10 of 87). Twelve components did not show any microorganism adhered despite location in an infected joint, with positive cultures in other components. A mixed linear model adjusted for random effects (the random effect being the infected patient) obtained convergence for the CFU/mm2 variable, but could not confirm a significantly higher adherence to a particular component or to a particular biomaterial. Therefore, the bacterial adherence primarily depends on the infective microorganism and the response of each individual patient, rather than materials or components.

Fractography evolution in accelerated aging of UHMWPE after gamma irradiation in air.

We studied the fracture surface evolution of ultra high molecular weight polyethylene (UHMWPE) specimens, manufactured from GUR 1050 compression moulded sheets, after gamma sterilisation in air followed by different aging times after thermal treatment at 120 degrees C. Degradation profiles were obtained by FTIR and DSC measurements after 0, 7, 14, 24 and 36h aging. We observed by SEM the morphology patterns at these aging times, in surface fractographies after uniaxial tensile test of standardised samples. The results pointed out clear differences between short and long aging times. At shorter times, 7h, the behaviour was similar to non-degraded UHMWPE, exhibiting ductile behaviour. At longer times, 24-36h, this thermal protocol provided a highly degraded zone in the subsurface, similar to the white band found after gamma irradiation in air followed by natural aging, although closer to the surface, at 150-200mum. The microstructure of this oxidation zone, similarly found in gamma irradiated samples shelf-aged for 6-7 years, although with different distribution of microvoids, was formed by fibrils, associated with embrittlement of the oxidised UHMWPE. In addition, the evolution of the oxidation index, the enthalpy content, the mechanical parameters, and the depth of the oxidation front deduced from the fractographies versus aging time showed that a changing behaviour in the degradation rate appeared at intermediate aging times.

Polyethylene Oxidation in Total Hip Arthroplasty: Evolution and New Advances

Ultra-high molecular weight polyethylene (UHMWPE) remains the gold standard acetabular bearing material for hip arthroplasty. Its successful performance has shown consistent results and survivorship in total hip replacement (THR) above 85% after 15 years, with different patients, surgeons, or designs. As THR results have been challenged by wear, oxidation, and liner fracture, relevant research on the material properties in the past decade has led to the development and clinical introduction of highly crosslinked polyethylenes (HXLPE). More stress on the bearing (more active, overweighted, younger patients), and more variability in the implantation technique in different small and large Hospitals may further compromise the clinical performance for many patients. The long-term in vivo performance of these materials remains to be proven. Clinical and retrieval studies after more than 5 years of in vivo use with HXLPE in THR are reviewed and consistently show a substantial decrease in wear rate. Moreover, a second generation of improved polyethylenes is backed by in vitro data and awaits more clinical experience to confirm the experimental improvements. Also, new antioxidant, free radical scavengers, candidates and the reinforcement of polyethylene through composites are currently under basic research. Oxidation of polyethylene is today significantly reduced by present formulations, and this forgiving, affordable, and wellknown material is still reliable to meet today’s higher requirements in total hip replacement.

Does Cyclic Stress Play a Role in Highly Crosslinked Polyethylene Oxidation

BackgroundMinimizing the impact of oxidation on ultrahigh-molecular-weight polyethylene components is important for preserving their mechanical integrity while in vivo. Among the strategies to reduce oxidation in modern first-generation highly crosslinked polyethylenes (HXLPEs), postirradiation remelting was considered to afford the greatest stability. However, recent studies have documented measurable oxidation in remelted HXLPE retrievals. Biologic prooxidants and physiologic loading have been proposed as potential mechanisms.Questions/purposesIn our pilot study, we asked: (1) Does cyclic stress induced by wear or (2) by cyclic compression loading increase oxidation and crystallinity of remelted HXLPE? (3) Does oxidative aging reduce the wear resistance of remelted HXLPE?MethodsRemelted and annealed HXLPE prisms (nxa0=xa01 per test condition) were tested in a wear simulator for 500,000 cycles. After wear testing, some samples were subjected to accelerated aging and then wear-tested again. Wear track volumes were characterized by confocal microscopy. Thin films (200-μm thick) were microtomed from wear prisms and then used for Fourier transform infrared spectroscopy oxidation and crystallinity assessments. Remelted HXLPE compression cylinders (nxa0=xa01 per test condition) were subjected to fatigue experiments and similar oxidation characterization.ResultsRemelted HXLPE qualitatively showed low oxidation indices (≤xa01) when subjected either to cyclic loading or aging alone. However, oxidation levels almost doubled in near-surface regions when remelted HXLPE samples underwent consecutive cyclic loading, artificial aging, and cyclic loading steps. The type of loading (wear versus compression fatigue) appeared to not affect the oxidation behavior in the studied conditions. Annealed HXLPE showed higher oxidation (oxidation indexxa0>xa03) than remelted HXLPE and delamination wear. No delamination wear was observed in remelted HXLPE in agreement with its comparatively low oxidation levels (oxidation indexxa0<xa03).ConclusionsWith the numbers available in our pilot study, the findings suggest that cyclic stress arising from a wear process or from cyclic compression may trigger the loss of oxidative stability of remelted HXLPE and contribute to synergistically accelerate its progression. Further studies of the effect of cyclic stress on oxidation of remelted HXLPE are needed.Clinical RelevanceRetrieval studies are warranted to determine the natural history of the in vivo oxidation and wear behavior of first-generation, remelted HXLPE.

Fatigue and Fracture of UHMWPE

Publisher Summary Ultra-high molecular weight polyethylenes (UHMWPEs) have motivated numerous studies on fatigue and fracture properties. UHMWPE formulations should represent a balance between design considerations, wear resistance, oxidation stability, and fatigue and fracture properties as well. The clinical significance of the fatigue and fracture properties of UHMWPE materials depends on the prosthetic device. The fatigue resistance of UHMWPE is now widely accepted as critical to the structural performance of orthopedic implant bearings, and such data are now commonplace in regulatory approval submissions. With regard to fatigue properties, both the total life (stress analysis based) and defect tolerant (fracture mechanics based) approaches have been employed to characterize conventional and highly crosslinked UHMWPE materials, although the limitations associated with total life experiments have rendered fatigue crack propagation experiments prevalent. The aim of this chapter is to provide a theoretical background on fatigue and fracture concepts, as well as the different philosophical approaches and experimental techniques available with special regard to UHMWPE. It also outlines the main findings on fatigue and fracture properties of contemporary medical UHMWPEs.

Chapter 30 – Fatigue and Fracture of UHMWPE

Publisher Summary nUltra-high molecular weight polyethylenes (UHMWPEs) have motivated numerous studies on fatigue and fracture properties. UHMWPE formulations should represent a balance between design considerations, wear resistance, oxidation stability, and fatigue and fracture properties as well. The clinical significance of the fatigue and fracture properties of UHMWPE materials depends on the prosthetic device. The fatigue resistance of UHMWPE is now widely accepted as critical to the structural performance of orthopedic implant bearings, and such data are now commonplace in regulatory approval submissions. With regard to fatigue properties, both the total life (stress analysis based) and defect tolerant (fracture mechanics based) approaches have been employed to characterize conventional and highly crosslinked UHMWPE materials, although the limitations associated with total life experiments have rendered fatigue crack propagation experiments prevalent. The aim of this chapter is to provide a theoretical background on fatigue and fracture concepts, as well as the different philosophical approaches and experimental techniques available with special regard to UHMWPE. It also outlines the main findings on fatigue and fracture properties of contemporary medical UHMWPEs.