The biochemical characteristics of wear testing lubricants affect polyethylene wear in orthopaedic pin-on-disc testing

Abstract

Lubricant protein concentration is known to affect crosslinked polyethylene wear in in vitro testing; however, the biochemical nature of these lubricants may also have a significant effect on wear and dictate its clinical relevance. A modified approach to pin-on-disc testing was implemented to explore the effect of four biochemically different lubricants on the wear of two types of crosslinked polyethylene materials (XLK™ and Marathon™; DePuy Synthes, Warsaw, IN, USA). XLK was associated with higher wear rates than Marathon. In comparison to lubricants containing deionized water, lubricants containing phosphate buffered saline solution and hyaluronic acid increased osmolality by up to 1.2 times and thermal stability by up to 1.4 times. This biochemical change reduced wear by up to 12.5 times. Wear rates for XLK and Marathon differed by a factor of 3.2 using lubricants with phosphate buffered saline solution as the dilutive media, but only 2.0 for lubricants with deionized water. Interestingly, varying the concentration of hyaluronic acid did not have a significant effect on wear, and differences between XLK and Marathon wear rates were not found to be statistically significant when hyaluronic acid was added to the lubricant. The findings of this study showed that increasing the osmolality and thermal stability of lubricants to more clinical levels decreased wear; however, the effect of hyaluronic acid on wear may not be apparent in simplistic pin-on-disc testing. It was suggested that phosphate buffered saline solution be used as the dilutive media of choice in order to better differentiate the ranking of materials while maintaining some clinical relevance.

Development of a Synthetic Synovial Fluid for Tribological Testing

“…In agreement, Guenther et al observed a decrease in the wear of cross-linked polyethylene pins in articulation with CoCr discs, when HA was added to the test liquid [70]. They explain this effect with the higher osmolality and thus an increase in thermal stability of the proteins [70]. However, they performed their tests at a higher sliding speed of 64 mm/s, which led to mixed lubrication rather than boundary lubrication with fewer asperities being in contact, and thus the viscosity was presumably a less important factor for their tests.…”

“…The presence of HA led to lower friction and wear, which was likely because of the viscosity increase. In agreement, Guenther et al observed a decrease in the wear of cross-linked polyethylene pins in articulation with CoCr discs, when HA was added to the test liquid [70]. They explain this effect with the higher osmolality and thus an increase in thermal stability of the proteins [70].…”

“…Adding salts to the test liquid led to an increase of wear (HA NCS RS vs. HA NCS), probably because of the reduction of the viscosity by the additional salt, leading to more asperities being in contact. In contrast, Guenther et al got a reduction in wear of the cross-linked polyethylene pins with the addition of salts (phosphate buffered saline) to a NCS-based test liquid, which they explain as well with higher protein stability because of the increased osmolality [70]. However, this was a test liquid without HA and at higher sliding speeds too.…”

“…A disadvantage of the synthetic synovial fluid is that the viscosity strongly depends on the molecular weight of the HA, thus-if changing to another batch of HA-the concentration of HA has to be determined performing by viscosity measurements. On the other hand, Guenther et al showed that the concentration of HA did not affect wear [70]. However, adjusting the concentration of HA is far less work compared to a hip simulator study necessary to evaluate a different batch of NCS for the HiSi-mix.…”

Silicon Nitride Bearings for Total Joint Arthroplasty

“…Protein adsorption onto bearing surfaces can be thermally or mechanically activated (i.e., frictional heating, pressure, or high shear) [114,115]. Their presence is often difficult to detect (<250 nm), but their effect in reducing wear has been extensively examined [45,62,[116][117][118][119][120][121][122][123][124][125]. The temperature differential between CoCr and Si 3 N 4 cells within the SWM study suggests higher precipitation occurred in the CoCr cells.…”