RESUMEN
Liners used in orthopedic devices are often made from ultrahigh molecular weight polyethylene (UHMWPE). A general predictive capability for transport coefficients of small molecules in UHMWPE does not exist, making it difficult to assess properties associated with leaching or uptake of small molecules. To address this gap, we describe here how a form of the Vrentas-Duda free volume model can be used to predict upper-bound diffusion coefficients (D) of arbitrary molecules within UHMWPE on the basis of their size and shape. Within this framework, the free-volume microstructure of UHMWPE is defined by analysis of a curated set of model diffusants. We determined an upper limit on D for vitamin E, a common antioxidant added to UHMWPE, to be 7.1 × 10-12 cm2 s-1 . This means that a liner that contains 0.1 wt % or less Vitamin E and has <120 cm2 patient contacting surface area would elute <100 µg/day of vitamin E. Additionally, the model predicts that squalene and cholesterol-two pro-oxidizing biological compounds-do not penetrate over 820 µm into UHMWPE liners over the course of 5 years because their D is ≤7.1 × 10-12 cm2 s-1 . © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 2393-2402, 2018.
Asunto(s)
Materiales Biocompatibles Revestidos/química , Prótesis de Cadera , Ensayo de Materiales , Polietilenos/química , HumanosRESUMEN
This study examined retrieved UHMWPE tibial bearings made from a remelted highly crosslinked (HXL) UHMWPE to determine whether the material is chemically stable in vivo. Retrieved tibial components were measured for changes in ketone oxidation and crosslink density. Oxidation increased with in vivo duration, and a significant decrease in crosslink density with increased mean ketone oxidation index was observed. These results suggest that in vivo oxidation is causing material degradation. Furthermore, a subsurface whitened damage region was found below the articular surface of one bearing, indicating the possibility of a clinically relevant decrease in mechanical properties of this component. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 39-45, 2017.
Asunto(s)
Polietilenos , Tibia , Animales , Oxidación-Reducción , Polietilenos/química , Polietilenos/farmacocinética , Polietilenos/farmacología , Tibia/lesiones , Tibia/metabolismo , Tibia/patología , Soporte de PesoRESUMEN
Ultra-high molecular weight polyethylene (UHMWPE), a common bearing surface in total joint arthroplasty, is subject to material property tradeoffs associated with conventional processing techniques. For orthopaedic applications, radiation-induced cross-linking is used to enhance the wear resistance of the material, but cross-linking also restricts relative chain movement in the amorphous regions and hence decreases toughness. Equal Channel Angular Extrusion (ECAE) is proposed as a novel mechanism by which entanglements can be introduced to the polymer bulk during consolidation, with the aim of imparting the same tribological benefits of conventional processing without complete inhibition of chain motion. ECAE processing at temperatures near the crystalline melt for UHMWPE produces (1) increased entanglements compared to control materials; (2) increasing entanglements with increasing temperature; and (3) mechanical properties between values for untreated polyethylene and for cross-linked polyethylene. These results support additional research in ECAE-processed UHMWPE for joint arthroplasty applications.
Asunto(s)
Polietilenos/química , Artroplastia , HumanosRESUMEN
Two groups of retrieved tibial inserts from one manufacturer's knee system were analyzed to evaluate the effect of a highly cross-linked bearing surface on wear and in vivo oxidation. The two groups ((1) conventional gamma-inert sterilized and (2) highly cross-linked, coupled with the same rough (Ra=0.25) Ti-6Al-4V tray) were matched with statistically similar in vivo duration and patient variables. The retrieved inserts were analyzed for ketone oxidation and wear in the form of dimensional change. The difference in oxidation rate between highly cross-linked and conventional gamma-inert sterilized inserts did not reach statistical significance. Observations suggest that the majority of wear can be accounted for by the backside interface with the rough Ti-6Al-4V tray; however, wear measured by thickness-change rate was statistically indistinguishable between the two bearing materials.
Asunto(s)
Materiales Biocompatibles , Articulación de la Rodilla/cirugía , Prótesis de la Rodilla , Polietilenos , Falla de Prótesis , Tibia/cirugía , Anciano , Artroplastia de Reemplazo de Rodilla , Remoción de Dispositivos , Femenino , Humanos , Masculino , Persona de Mediana Edad , Oxidación-ReducciónRESUMEN
This investigation analyzed retrieved sequentially crosslinked and annealed (SXL) ultra-high molecular weight polyethylene bearings to determine whether the material is chemically stable in vivo. A series of retrieved tibial and acetabular components were analyzed for changes in ketone oxidation, crosslink density, and free radical concentration. Oxidation was observed to increase with in vivo duration, and the rate of oxidation in tibial inserts was significantly greater than in acetabular liners. SXL acetabular bearings oxidized at a rate comparable to gamma-sterilized liners, while SXL tibial inserts oxidized at a significantly faster rate than their gamma-sterilized counterparts. A significant decrease in crosslink density with increased mean ketone oxidation index was observed, suggesting that in vivo oxidation may be causing material degradation. Furthermore, a subsurface whitened damage region was also found in a subset of the bearings, indicating the possibility of a clinically relevant decrease in mechanical properties of these components.
Asunto(s)
Acetábulo/cirugía , Artroplastia de Reemplazo de Cadera/instrumentación , Prótesis de Cadera , Prótesis de la Rodilla , Polietilenos/química , Tibia/cirugía , Remoción de Dispositivos , Análisis de Falla de Equipo , Radicales Libres/análisis , Rayos gamma , Prótesis de Cadera/efectos adversos , Humanos , Cetonas/análisis , Prótesis de la Rodilla/efectos adversos , Ensayo de Materiales , Oxidación-Reducción , Polietilenos/efectos de la radiación , Infecciones Relacionadas con Prótesis , Estudios Retrospectivos , Espectroscopía Infrarroja por Transformada de Fourier , Esterilización , Propiedades de Superficie , Factores de TiempoRESUMEN
Irradiated, thermally stabilized, highly cross-linked UHMWPE bearings have demonstrated superior wear performance and improved in vitro oxidation resistance compared with terminally gamma-sterilized bearings, yet retrieval analysis reveals unanticipated in vivo oxidation in these materials despite fewer or no measurable free radicals. There has been little evidence to date that the oxidation mechanism in thermally stabilized materials is the same as that in conventional materials, and so it is unknown whether oxidation in these materials is leading to chain scission and a degradation of mechanical properties, molecular weight, and crosslink density. The aim of this study was to determine whether measured in vivo oxidation in retrieved, highly cross-linked tibial bearings corresponds with a decreasing crosslink density. Analysis of three tibial bearing materials revealed that crosslink density decreased following in vivo duration, and that the change in crosslink density was strongly correlated with oxidation. The results suggest that oxidation in highly cross-linked materials is causing chain scissions that may, in time, impact the material properties. If in vivo oxidation continues over longer durations, there is potential for a clinically significant degradation of mechanical properties.