The influence of swing phase load

Metal-on-metal bearings have been well documented to produce low steady state wear rates under standard hip simulator conditions at values below 1.5 mm³/mc (Fisher et al., 2006). Under these conditions, wear in the current study was also found to be low with a steady state wear rate of 0.40 ± 0.49 mm³/mc.

Although this mean wear rate was slightly higher than similar studies (0.26 ± 0.09 mm³/mc) with the same sized 48 mm diameter bearings and the same double

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heat-treated cast metallurgy (Royle, 2012), this could be due to the small number of bearings tested in both studies. In the current study, 2 out of 4 of the bearings tested produced relatively high wear rates after 1 million cycles yet this was within the range reported in the literature, comparable to the 0.37 mm³/mc reported in 50 mm diameter metal-on-metal bearings (Li et al., 2011), Figure 5:44.

Figure 5:44: Steady state wear rates of metal-on-metal bearings under standard and removed swing phase load conditions in the current study compared to the literature

Although metal-on-metal bearings have been proposed as a low wearing alternative to metal-on-polyethylene bearings, retrieval wear rates as high as 87.73 mm³/year have been reported using a co-ordinate measuring machine (Lord et al., 2011) compared to the wear rate of 0.40 ± 0.49 mm³/mc recorded in the laboratory as in this work. There also appears to be an inherent variation in the clinical wear of metal-on-metal bearings with wear measured using roundness machines ranging from 1.63 to 7.48 µm/year in bearings without pseudotumours (Kwon et al., 2010), equating to approximately 0.08 to 0.38 mm³/year (Medley et al., 1996). The variability has been reported to be greater

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in failed bearings (Kwon et al., 2010, Matthies et al., 2011b). Based on average wear rates, retrievals due to adverse reactions to metal debris show higher wear rates than revisions for other reasons (Lord et al., 2011) although the adverse reactions may be due to the increased wear of these bearings, Table 5:5.

Table 5:5: Mean wear rates reported in retrieved metal-on-metal bearings Reason for

The variable wear behaviour of metal-on-metal bearings has also been observed in highly reproducible simulator studies under standard conditions with some bearings reaching steady state wear and others showing runaway wear behaviour (Bowsher et al., 2009). In many instances, these higher wearing bearings typically show a higher initial run-in period than other bearings (Bowsher et al., 2009, Vassiliou et al., 2006, Annissian et al., 2001). Various design changes influence this run-in wear rate such as clearance and diameter (Dowson et al., 2004a). Several explanations have been proposed for the runaway wear phenomenon including differences in contact pressure (Lee et al., 2010) or differences in the functional arc between bearings (Griffin et al., 2010).

However, these explanations fail to account for differences in wear between bearings of the same design and metallurgy under identical conditions.

Six wear trends of metal-on-metal bearings have been described by Bowsher et al. (2009), Figure 5:45; that of a classical high run-in and low steady state, three forms of “breakaway” wear in which a high run-in is combined with a period of high steady state wear which recovers to a lower wear rate and two “runaway”

wear trends where wear remains high. In the current study, only (a), (d) and (f) wear patterns were identified for the components, although the increased wear rate of MoM S3 between 1 and 2 mc may have resulted in (b) or (c) wear patterns had testing continued.

Figure 5:45: Graph showing the different wear trends documented in MoM bearings; a) classical run in and steady state behaviour, b-d) breakaway wear with recovery and e-f)

runaway wear with no recovery. Image taken from Bowsher et al., (2009)

Higher in vitro wear in metal-on-metal bearings has been reported with the increasing proportion of wear in the cup (Bowsher et al., 2009). However, the current study observed wear to normally predominantly originate from the cup with this contributing to 80% of the total wear reported. Increasing proportions

of head wear resulted in increased total wear suggesting that “well behaving”

bearings can produce a significant proportion of wear from the cup. In the current study, three out of four bearings produced similar levels of cup wear throughout testing suggesting that the variation in wear reported between the three bearings were due to the heads. The remaining bearing (MoM S4) produced higher levels of both head and cup wear leading to greater wear overall. It is still unknown, however, what produced this increased wear.

Bowsher et al., (2009) observed that under standard conditions, the smoother bearings with greater conformity exhibited higher wear rates, contrary to predictions derived from tribological theory. Therefore, it may be small changes within the design or material which produce significant effects on wear such as the presence and location of carbides.

The reduction of swing phase load from 280 N to 100 N has previously been shown to decrease steady state wear of metal-on-metal bearings fourfold (Williams et al., 2004b). A more pronounced run-in wear reduction was observed from 2.03 mm³/mc to 0.13 mm³/mc (Williams et al., 2006), although this study considered run-in wear up to 1 million cycles. The removal of swing phase load, in the current study, showed a 2.5 times reduction in run-in wear rate (0-0.33 mc) and 1.3 times reduction in transition wear rate but a 1.7 fold increase during steady state wear. This increased steady state wear in the current study could be due to the separation of the head and cup during the swing phase which may have produced uncontrolled and intermittent rim loading as a result of the hip wear testing machine design. This was particularly observed between 2-5 mc with increasing head and rim damage, preventing wear from reaching the steady state rate reported under standard conditions. The steady state wear

rates observed with the removal of swing phase load may represent a more clinically relevant condition as rim contact and microseparation probably does not occur with every step. Although this difference in steady state wear was not as noticeable over 2 million cycles, over extended testing periods this increased wear rate would produce significantly more wear in the removed swing phase load condition bearings. Extrapolating the wear rates reported in the current study, after 4 million cycles the standard condition bearings would produce lower wear (2.55 mm³) compared to the removed swing phase load bearings (3.07 mm³), or a difference of 2.57 mm³ at 10 million cycles emphasising the increasing difference between wear conditions and the need for appropriate test duration and conditions.

Cobalt released throughout testing showed a strong linear relationship with wear; levels produced under standard test conditions were similar to those previously reported (Royle, 2012). Despite the linear relationship between wear and cobalt release under both test conditions, these correlations were slightly different between conditions. Under standard conditions the linear relationship observed suggested greater levels of cobalt were released than when the swing phase load was removed if wear was equivalent. It is possible that the loading altered the cobalt release producing greater ionic loss under standard conditions. It has been recognised that increased loading can increase corrosion of the CoCrMo alloy (Yan et al., 2009), possibly suggesting mechanical induced corrosion. Changing the loading of the hip simulator, therefore, may have altered the amount of corrosion, resulting in greater corrosion during the standard loading. The removal of the swing phase load and subsequent separation of components may have also allowed the bearing surfaces to reform