Abstract
In knee replacements, vitamin E-doped ultra-high molecular weight polyethylene (UHMWPE) shows a better wear behavior than standard UHMWPE. Therefore, different sets of polyethylene (PE) acetabular cups, i.e. standard UHMWPE and cross-linked polyethylene irradiated with 50 kGy and 75 kGy, were compared, at a molecular level, with vitamin E-doped UHMWPE to evaluate their wear performance after being tested on a hip joint simulator for five million cycles. Unworn control and worn acetabular cups were analyzed by micro-Raman spectroscopy to gain insight into the effects of wear on the microstructure and phase composition of PE. Macroscopic wear was evaluated through mass loss measurements. The data showed that the samples could be divided into two groups: 1) standard and vitamin E-doped cups (mass loss of about 100 mg) and 2) the cross-linked cups (mass loss of about 30-40 mg). Micro-Raman spectroscopy disclosed different wear mechanisms in the four sets of acetabular cups, which were related to surface topography data. The vitamin E-doped samples did not show a better wear behavior than the cross-linked ones in terms of either mass loss or morphology changes. However, they showed lower variation at the morphological level (lower changes in phase composition) than the UHMWPE cups, thus confirming a certain protecting role of vitamin E against microstructural changes induced by wear testing.
Highlights
Ultra-high molecular weight polyethylene (UHMWPE) is the biomaterial used for more than twenty years as acetabular cups in the orthopedic field
As can be seen, cups can be divided into two homogenous groups, statistically different between each other: the first characterized by a higher mass loss, about 100 mg (STD and VE), and the second by a significantly lower mass loss, comprised between 30 and 40 mg (XL-50 and XL-75)
Micro-Raman spectroscopy: unworn samples The data reported in Figure 2 allowed to examine quantitatively the differences among the four sets of unworn acetabular cups, whose average micro-Raman spectra are reported in Figure 3 in the most representative spectral ranges
Summary
Ultra-high molecular weight polyethylene (UHMWPE) is the biomaterial used for more than twenty years as acetabular cups in the orthopedic field This polymer has exceptional properties (chemical inertness, lubricity, impact and abrasion resistance), sadly oxidative degradation may decrease its mechanical properties leading to debris production and possible osteolysis and implant loosening [1,2]. The structure adopted by UHMWPE is based on long molecular chains (with an average molecular weight of 106 Daltons) that fold into crystalline lamellae (10–50-nm thick and 10–50-mm long) possessing orthorhombic structure Those domains are connected by a matrix composed of anisotropic amorphous phase and an intermediate anisotropic disordered phase (‘‘third phase’’), which is characterized by a prevailing trans-conformation and poor lateral order [5]. This structural organization ensures the mechanical integrity and toughness of UHMWPE but might be undermined by the decrease in molecular weight and the increase in crystallinity, which have been reported to deteriorate UHMWPE mechanical properties [6]
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