Tribological Behaviour of UHMWPE

UHMWPE is used as an articulating surface in both hip and knee joint replacements. As an acetabular component in THR (total hip replacement), its annual wear rate was reported to be approximately 0.1 mm/ year or 80 mmVyear against a 32 mm diameter femoral head. Hall et al. [70], and Oonishi et al. [71]. It is approximately a 30-fold reduction in wear rate in comparison with a PTFE socket that was first introduced by 

Has the above discussion, it is understood that wear of UHMWPE primarily occurred due to adhesive, abrasive, subsurface cracks and delamination. These wear phenomena were affected by several factors, which are not inherent material properties of the articulating surface. A reduction of wear volume is always desirable to increase the longevity of the bearing surface. It was attempted by using various fillers as reinforcing materials in UHMWPE. Although a wide variety of fillers was used, MWCNTs were chosen for discussion in the present chapter in relevance to their superior properties. A detailed discussion on the 

MWCNTs are the rolled graphene sheets, and thus they detain certain properties similar to that of graphene, which acts as a good solid lubricant. MWCNTs also exhibit lubricant characteristics, in addition to their superior mechanical properties. However, many studies have not reported the influence of MWCNTs on the wear behavior of UHMWPE. It is observed from the literature that the presence of MWCNTs could significantly reduce the wear of UHMWPE and thereby increase its longevity as a bearing surface. 

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Y. Xue, W. Wu, O. Jacobs, and B. Schadel, Tribological behaviour of UHMWPE/HDPE blends reinforced with multi-wall carbon nanotubes. Polymer Testing, 25 (2), 221-229,2006. 

A knee joint simulator was designed and built in the bioengineering laboratory at Leeds in the late 1970 s to enable the mechanical and tribological characteristics of current and projected knee joint replacements tp.he assessed. The simulator has been described by Dowson et al and it is shown in Figure 1. Realistic load and motion cycles are applied to the knee joints to simulate any desired activity, but usually walking, and the penetration of the metallic femoral components into the polymeric tibial components after a large number of cycles can then be used to assess the life of the joints. Furthermore, the penetrations recorded, which include both wear and creep, can be related to results of laboratory wear studies of the behaviour of UHMWPE and to the in-vivo performance of total replacement knee joints. The simulator is therefore an Important machine in both the pragmatic and fundamental aspects of total replacement knee join development. 

In this study, the tribological behaviour of the HTOZ-on-UHMWPE sliding pair for cementless knee joint prostheses was evaluated using unidirectional pin-on-disk tests and multidirectional pin-on-plate tests. Results were compared to the friction and wear properties of the TZP-on-UHMWPE sliding pair and the effects of the altered surface characteristics were examined. 

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