Abstract
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.
The abstracts were prepared by Nico Verdonschot. Correspondence should be addressed to him at Orthopaedic Research Laboratory, University Medical Centre, PO Box 9101, 6500 HB Nijmegen, The Netherlands.
1] Clausen, J. et al., Trans. 46th Ann. ORS, 2000: 571. Google Scholar
