The influence of design, materials and kinematics on the in vitro wear of total knee replacements
Introduction
Improvements in total knee replacement (TKR) designs, materials and sterilisation techniques during the past decade have led to improved clinical performance of these prostheses by reducing the prevalence of delamination and structural fatigue of the ultra high molecular weight polyethylene (UHMWPE) bearings (Stewart et al., 1995; Won et al., 2000; Reeves et al., 2000; Bell et al., 1998). However, in the longer term, concern remains regarding the surface wear of total knee components as the generation and accumulation of micrometre and submicrometre size wear particles has been observed in tissues surrounding knee replacements which were revised for infection in the early years of implantation (Howling et al., 2001). This may lead to osteolysis and long-term failure mechanisms similar to those found in total hip replacements (Ingham and Fisher, 2000). The generation of UHMWPE wear debris from articulating surfaces in total knee replacements is affected by a number of factors. These include resultant knee motion, prosthesis design and bearing materials. Such factors have not previously been systematically analysed as a matrix of variables using physiological in vitro knee simulator testing.
Current TKR devices can be subdivided into two groups based on different fundamental design principles: fixed bearing knees, where the UHMWPE insert snap or press fits into the tibial tray, and mobile bearing designs which facilitate movement of the insert relative to the tray. In mobile bearing knees, motion of the knee is designed to occur at two articulating surfaces. Such designs differ according to the kinematics at the tray–insert interface and the resulting axis of rotation of the knee. Some mobile bearing designs allow both anterior–posterior translation and internal–external rotation at the tray–insert interface whereas in rotating platform mobile bearing knees rotation only is facilitated at the tray–insert counterface, hence reducing the degree of rotation at the femoral–insert articulation. The resulting kinematics at the articulating surfaces of the UHMWPE bearings from different prostheses differ greatly and UHMWPE wear is dependent on the kinematics to which the material is subjected (Wang et al., 1996).
This paper describes a series of studies which compared the wear of fixed bearing and rotating platform mobile bearing total knee prostheses with different bearing materials and under varied kinematic conditions using a physiological knee simulator. The effects of prosthesis design, bearing material, backside motion and kinematics on UHMWPE wear in TKR are discussed.
Section snippets
Materials
The wear of fixed bearing and rotating platform mobile bearing TKRs was investigated using commercially available designs (Fig. 1). The fixed bearing TKRs tested in these studies comprised PFC and PFC Sigma components. The PFC system, which was introduced clinically in the early 1980s, employed an essentially flat-on-flat femoral–insert configuration and used polyethylene, which was gamma irradiated in air. The PFC Sigma implant system was introduced with the femoral component being rounded in
Methods
The wear of fixed bearing and rotating platform mobile bearing TKRs was compared using the Leeds ProSim six-station force/displacement controlled knee simulators (Barnett et al., 2002). Six (n=6) TKR of a single design were tested on the simulator for most test conditions. Femoral axial loading (maximum 2600 N) and extension–flexion (0°–58°) input profiles were adopted from the ISO 14243 (2002) standard for all simulator studies (Fig. 2). The compressive load applied to each knee was offset 5 mm
Study 1: effect of fixed bearing design and materials
The PFC fixed bearing knees, which had inserts manufactured from 1020 UHMWPE and were sterilised by gamma irradiation in air and shelf aged for 29–33 months, exhibited a mean wear rate with 95% confidence limits of 41±14 cubic mm per million cycles (mm3/MC) when subjected to standard kinematics (Fig. 4). In contrast, a mean wear rate of 23±5.9 mm3/MC was observed for the PFC Sigma components with inserts manufactured from 1020 GVF polyethylene. This two-fold reduction in wear with the introduction
Effect of fixed bearing design and materials
The PFC total knee replacement is a non-conforming posterior-cruciate retaining prosthesis with a polyethylene insert that is essentially flat in the sagittal plane. Flat-on-flat articulations, while conforming when in perfect alignment, are susceptible to high edge loading during walking (Bartel et al., 1986). Schai et al. (1998) reported that survivorship of the PFC cruciate retaining knee after 10 years was 90%. However, this increased to 97% when metal-backed patellae and less conforming
Conclusions
The potential for long-term osteolysis in total knee replacement necessitates minimisation of the number of particles generated due to surface wear, particularly for implantation in younger, more active patients. These simulator studies have identified factors which affect the in vitro wear of polyethylene and may also affect in vivo behaviour of total knee replacements. More conforming coronal geometry and use of stabilised polyethylene bearings significantly decreased the volumetric wear rate
Acknowledgements
DePuy International, a Johnson and Johnson Company, provided a studentship for H.M.J. McEwen. Funding for these studies was received from the Engineering and Physical Sciences Research council UK and the Arthritis Research Campaign UK.
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