Influence of diameter and the potential for large diameter metal-on-

Bearing diameter in metal-on-conventional polyethylene bearings has been limited due to the wear volume produced (Charnley et al., 1969). Theoretically, following Archand’s wear law as described in equation 4, section 2.1.3, a larger diameter should increase both the contact area and sliding distance thereby increasing the wear rate (Affatato, 2012). This has been widely confirmed , with a 4-10% increase in wear with increasing diameter from 28 mm to 36 mm in metal-on-polyethylene bearings (Shen et al., 2011) and a 5-9% increasing diameter from 22 mm to 28 mm in alumina-on-polyethylene bearings (Clarke et al., 1996).

0

Ratio of wear to fluid gain before testing

Number of cycles x 10⁶

Larger differences have been seen comparing 28 mm diameter metal-on-polyethylene bearings to 46 mm diameter bearings with wear rates of 14 and 51 mm³/mc respectively (Muratoglu et al., 2001).

The introduction of crosslinking to reduce polyethylene wear has made it more challenging to distinguish the influence of diameter on wear. Increasing wear has been reported in alumina-on-90 kGy irradiated polyethylene with wear rates of 10.8 and 16.7 mm³/mc reported in the 36 and 44 mm diameter bearings yet wear in the small, 28 mm diameter bearings was negligible (Zietz et al., 2013).

Muratoglu et al., (2001) reported no measurable wear in polyethylene crosslinked at 95 kGy at diameters of 22, 28, 38 and 46 mm thus making it difficult to establish a relationship between wear and diameter. The wear of sequentially crosslinked polyethylene, however, has reported wear in 32-52 mm diameters but shown poor correlation with head size (Herrera et al., 2007).

Due to the low wear of crosslinked polyethylene, the use of larger diameter vitamin-E highly crosslinked polyethylene bearings have been proposed (Oral et al., 2012) but no studies have previously considered the influence of diameter in vitamin-E blended highly crosslinked bearings. The current study has shown that an increase in diameter from 28 mm to 52 mm significantly increased wear by 31%. The bearing sizes tested represent the extremes of diameter size and showed a difference in wear rate of 2.3 mm³/mc. In a study by Herrera et al., (2007), the increase in wear rate between two extremes of diameter was more pronounced than the current study with 1.4 ± 0.7 mm³/mc in 32 mm diameters and 4.8 ± 3.1 mm³/mc in 52 mm diameters. The lower wear rate than that

observed in other 52 mm diameter bearings may be explained by the highly polished surface finish (Ra 0.004 ± 0.001 µm) achieved on the heads (Herrera et al., 2007). Although the wear rate of the 52 mm diameter bearings in the current study was greater than the reported wear in the Herrera et al., (2007) study; the wear rates reported in the both diameters of the current study were similar or lower than the literature overall, Figure 4:44. The polyethylene wear rate of the 52 mm diameter bearings was comparable to 36 mm diameter bearings crosslinked at 95 kGy (Galvin et al., 2010) and 36 mm diameter hindered phenol anti-oxidant stabilised polyethylene irradiated at 115 kGy (Partidge et al., 2013).

The 28 mm diameter bearings in the current study, produced wear rates comparable to the 5 ± 2 mm³/mc of 28 mm diameter metal-on-100 kGy crosslinked polyethylene (Fisher et al., 2006) and the 6.1 mm³/mc in 28 mm diameter 0.1 wt% vitamin-E blended polyethylene crosslinked at 100 kGy (Lerf et al., 2013). In bearings of the same material as that tested in the current study, wear rates of 1.81 mm³/mc have been reported at a diameter for 40 mm (Traynor et al., 2011) highlighting the potential for differences in wear between laboratory studies even when following ISO standard conditions.

Figure 4:44: Graph showing wear rates reported in the current study comparable to that reported in the literature for standard test conditions

Differences in design can also lead to differences in wear rates. Significant differences between the two bearing diameters tested in the current study have been acknowledged, Table 4:13, which may have confounded results. Liner thickness is believed to influence the stresses within polyethylene with thinner liners increasing the wear rate and leading to an increased risk of fracture (Oonishi et al., 1998). In a hip simulator study considering only liner thickness, however, a decrease of 3 mm in thickness resulted in a decrease of wear by 19%

(Shen et al., 2011), attributed to decreasing contact area despite increased

Conventional Crosslinked Vitamin E incorporated Current study 0

insignificant increases in wear (Herrera et al., 2007). The liner thickness in the current study varied by 8.4 mm in the loaded areas which represents a much larger difference in thickness than previous studies and therefore may have produced a greater difference in wear than if comparable thicknesses were used. Other differences in the design of the liners, for example different locking mechanisms to prevent rotation may have also affected wear rates with different stress distributions in the material. Different locking mechanisms have been shown to alter micromotion and backside wear of the liners (Williams et al., 1997). Wear of the locking rosette of one of the large diameter bearings of the current study was observed and therefore undetected wear of this feature in the other bearings may have also occurred. This would increase the wear rate reported, as would the increased clearances of the larger bearings (Teoh et al., 2002). However, detailed inspection of the back surfaces of the remaining liners did not indicate any damage in these bearings.

Table 4:13: Summary of design differences between 28 and 52 mm diameter metal-on-polyethylene bearings

Design Parameter 28 mm bearings 52 mm bearings

Liner thickness, mm 10.20-11.40 3.00

Locking mechanism Notches on the side of liners

Rosette locking at the bottom of the liner Offset of centre of the inner

diameter from the edge, mm -2.43 0.00

Clearance, µm 100-450 325-681

Despite these design differences and different wear rates, the wear behaviour of the bearings appeared similar under standard simulator conditions. All liners, regardless of size, showed the same highly polished wear area within which light scratching could be observed suggesting abrasive wear mechanisms as described widely previously (McKellop, 2007, Sorimachi et al., 2009). The

generation of polyethylene particles size distributions similar to those reported widely also suggests a similar wear mechanism. Both diameter bearings produced mainly spherical shaped particles, with 38 and 48 % of particles sized between 0.1- 0.2 µm for the 28 and 52 mm diameter bearings, respectively.

Particles generated in crosslinked polyethylene have been reported to include fibrils and small granules (Galvin et al., 2010) as in the current study. The observation of nanometre sized particles (27 and 21% of the particles generated in the 28 and 52 mm diameter bearings) has been reported elsewhere for crosslinked polyethylene with 71% of particles produced in metal-on-polyethylene crosslinked at 95 kGy below 0.1 µm (Galvin et al., 2010). This represents a greater number of nanometre sized polyethylene particles in the crosslinked polyethylene but may be due to the different particle extraction protocols used. In a study by Saikko et al., (2002), particles of spherical shape were reportedly generated in both conventional and crosslinked polyethylene but the introduction of crosslinking produced smaller particles, mean sized 0.23 µm compared to the 0.28 µm particles generated in conventional polyethylene.

This suggests that vitamin-E included in crosslinked polyethylene has little influence on particle size distribution which correlates with a pin-on-plate study comparing virgin and vitamin-E containing polyethylene (Bladen et al., 2013).

Despite the low wear rates reported in crosslinked polyethylene with or without vitamin-E, the revision rate of larger (44 mm) diameter metal-on-polyethylene bearings has been reported to be higher than in smaller diameters according to the England, Wales and Northern Ireland National Joint Registry (2013). This may suggest there may be issues with the adoption of large diameters metal-on-polyethylene bearings which limit the lifespan of the bearing. However, the

numbers of implants and retrievals was markedly smaller than other cohorts and the reason for revision was not stated making it difficult to determine whether this is due to increased wear or other causes.