Recent advancements toward increasing the longevity of total hip replacements (THR) have made it possible to consider younger patients as candidates for this procedure. These include the development of highly crosslinked ultra high molecular weight polyethylenes (UHMWPE) and the re-introduction (most recently in the US) of metal-on-metal (MOM) articulating couples.

Early MOM designs (e.g. McKee-Farrar, Müller) were made of cast cobalt chrome (CoCr) with no polyethylene liner (a.k.a. “direct”), and to this date continue to show some degree of clinical success (20 to 30 years in-vivo). Since that time, improvements in materials and manufacturing techniques as well as clinical information from retrievals have led to the development of a new design, incorporating an UHMWPE liner between the CoCr inlay and the titanium alloy (Ti6Al4V) ace-tabular shell (a.k.a. “sandwich”). A previous study has reported that couples employing the polyethylene liner show lower wear than their solid metal counterpart [1]. It is hypothesized that the compliant properties of the polyethylene liner may reduce the overall stress which may result in the lower wear reported previously. In order to test this hypothesis, two-dimensional axisymmetric finite element analyses (FEA) were performed on simplified models representing the two MOM design types; both with and without the UHMWPE liner (i. The results indicate that the presence of an UHMWPE liner resulted in nearly a 58% decrease in the peak contact stress and an estimated 60% increase in contact area for this loading regime. Additionally, the underlying compressive stresses are more uniformly spread through the thickness of the implant for the sandwich design, resulting in a better overall stress distribution. Since lower contact stresses typically result in lower wear, it is postulated that the lower wear reported elsewhere for the sandwich MOM design is attributable to the compliant properties provided by the UHMWPE liner.

Recently, highly crosslinked polyethylenes have emerged as an alternative bearing surface with tremendous potential for clinical success. However, the term highly cross-linked polyethylene refers to a great many materials, each manufactured under drastically different processing parameters, such as type of irradiation, dose, and warm versus cold state. It has been widely shown in laboratory hip simulator testing, that the wear resistance of UHMWPE improves significantly with increasing cross-link density, but the measurement of this parameter is somewhat controversial. While both swell testing of the polyethylene (direct) and trans-vinylene content (indirect) both yield information regarding the actual degree to which the material is crosslinked, no study to date has examined the exact relationship between these two tests. In evaluating the clinical performance of highly crosslinked polyethylenes, it is crucial that they be characterized according to the specific parameters by which they were manufactured. onship. Micro-Fourier Transform Infrared Spectroscopy (FTIR) and swelling measurements were performed on samples irradiated by either electron beam or gamma sources at varying doses, in both the cold and warm state. The trans-vinylene content was obtained from the ratio of the peaks at 965 cm-1 and 2022 cm-1, while the crosslink density was computed from Flory network theory.

The information for crosslink density was plotted versus trans-vinylene content to obtain the precise relationship between these two highly sensitive tests. This information can be used to aid in the clinical evaluation of commercially available highly crosslinked polyethylenes, and to improve our understanding of the very complex relationship between wear and the physical and chemical properties of UHMWPE.