Micromechanics of shelf‐aged and retrieved UHMWPE tibial inserts: Indentation testing, oxidative profiling, and thickness effects

Abstract

Understanding the surface micromechanical properties of ultra-high-molecular-weight polyethylene (UHMWPE) may allow for improvement in its wear characteristics. Microtomed sections of two UHMWPE tibial bearings, one that had been irradiation sterilized and shelf-aged, and the other irradiation sterilized and used in a patient, were subjected to depth sensing indentation testing. The microtomed sections exhibited a white band in the subsurface region that is characteristic of oxidation, and the indentation testing, followed by FTIR analysis at the same testing locations, was performed across this region and into the bulk of the material. Indentation testing yielded data leading to hardness, modulus, and energy dissipation factor (EDF). FTIR profiling generated information about oxidation; oxidation indices were calculated by taking a ratio of peak heights at 1,716 cm(-1) (ketone) to 2,022 cm(-1) (methylene). The mechanical properties showed a strong linear correlation with oxidation index above a minimum thickness. Modulus, hardness, and EDF all increased with increasing oxidation. The appropriate thickness-to-indentation-depth ratio was determined by two methods and was found to be approximately 20:1. The mechanical properties through the oxidized region were seen to vary with depth into the sample in a profile similar to the oxidation profile. The differing aging environments of the tibial bearings are hypothesized to have had an effect on both the mechanical and oxidation profiles. The retrieved bearing exhibited a narrower oxidation profile with peaks closer to the surface, and oxidation indices of lower magnitude. The mechanical properties proved similar, with less intense and narrower readings for the retrieved sample. This research is consistent with much of the literature.

PE‐UHMW in Hip Implants: Properties of Conventional and Crosslinked Prosthetic Components

“…Hence, the higher hardness values can be correlated to a higher degree of crystallinity in the region of the white band. A similar relation in non-crosslinked PE-UHMW implants was shown by Wernlé et al [14] The depth profiles of two highly-crosslinked implants, the shelf aged sample, and the retrieval after 2 years in vivo, revealed no maximum (Fig. 4(b)).…”

“…Therefore, higher hardness values in an area about 2 mm below the implant surface are detectable by instrumented nanoindentation. Such a behavior was observed [14] in conventional noncrosslinked materials but has not been reported for highly crosslinked PE-UHMW. The correlations between OI, degree of crystallinity, and micro-hardness at different positions within the implants are demonstrated for non-crosslinked and crosslinked material.…”

“…These results are also in accordance with ref. [14 The non-crosslinked retrieval after 10 years in vivo shows a typical curve of highly oxidative degraded PE-UHMW in the white bands region (Fig. 6(a), dotted line, analyzed position B).…”

Retrieval analyses of highly cross-linked polyethylene acetabular liners four and five years after implantation

“…10 The oxidation depth profile was measured by samples taken close to the pole of the liner. 11 The ability of UHMWPE to absorb particles of body fluids in vivo can affect the oxidation measurements. Thus, in order to evaluate their actual oxidation, the body-fluid components were extracted using cyclohexane at 80˚C for 48 hours for the first case and for 96 hours for the second.…”

Nano- and Microindentation Testing of UHMWPE

“…Gilbert et al [3,6,7] showed that the hysteresis energy (HE, see Figure 33.1) associated with microindentation of UHMWPE varied with the maximum indent depth squared for a spherical indenter, as can be seen in Figure 33.4 for three different UHMWPE materials [3]. Thus, it was determined that a parameter, called the energy dissipation factor (EDF), defined as:…”