Wear Tests of a Potential Biolubricant for Orthopedic Biopolymers

Abstract

Most wear testing of orthopedic implant materials is undertaken with dilute bovine serum used as the lubricant. However, dilute bovine serum is different to the synovial fluid in which natural and artificial joints must operate. As part of a search for a lubricant which more closely resembles synovial fluid, a lubricant based on a mixture of sodium alginate and gellan gum, and which aimed to match the rheology of synovial fluid, was produced. It was employed in a wear test of ultra high molecular weight polyethylene pins rubbing against a metallic counterface. The test rig applied multidirectional motion to the test pins and had previously been shown to reproduce clinically relevant wear factors for ultra high molecular weight polyethylene. After 2.4 million cycles (125 km) of sliding in the presence of the new lubricant, a mean wear factor of 0.099 × 10 −6 mm 3 /Nm was measured for the ultra high molecular weight polyethylene pins. This was over an order of magnitude less than when bovine serum was used as a lubricant. In addition, there was evidence of a transfer film on the test plates. Such transfer films are not seen clinically.The search for a lubricant more closely matching synovial fluid continues.

Potential Synthetic Biolubricant as an Alternative to Bovine Serum

“…Previous work reporting on the same material combination (UHMWPE against stainless steel) lubricated with a 0.75% gellan gum/2% alginate mixture gave wear factor values for UHMWPE of 0.099 × 10 −6 mm 3 /Nm [16]. The UHMWPE wear factor measured for 0.5% gellan gum fluid gel is approximately 2.5 times greater than 0.75% gellan gum and is closer to the values reported for bovine serum.…”

“…A more important finding was that, as shown by surface roughness analysis, the plate surfaces had no evidence of a transfer film. In the previous pin-on-plate studies using 0.75% gellan gum/alginate as the lubricant, the wear of the plates was not reported [16] but there was evidence of a transfer film. Other work investigating the wear properties of UHMWPE pins against stainless steel plates using bovine serum as the lubricant also have not reported the wear of the plates but they do report that no transfer film was present [27,28].…”

“…Analysis of the wear debris when using 0.75% gellan gum/2% alginate mixture as the lubricant in the Thompson et al (2015) study [16] showed particle sizes ranging from 50 to 400 nm. No larger particles were found however, it must be noted that the particle analysis performed was done solely by using the nanoparticle tracking analysis tool leading to a maximum particle size of 2000 nm in diameter, therefore any larger particles will not have been measured in the Thompson et al paper.…”

“…There are many published papers that discuss possible alternatives to bovine serum [11][12][13][14][15][16][17][18] but the search for an alternative lubricant still continues. It is well understood that any proposed alternative lubricant will need to be safe; produce wear rates and particles that match those produced in the body; and be cost effective with no ethical limitations.…”

“…This lubricant was based on the lubricant tested by Thompson et al (2015) [16]. The lubricant tested in the study by Thompson et al (2015) [16] (0.75% gellan gum/2% alginate mixture) has been shown to have similar non-Newtonian properties to synovial fluid, but it gave lower wear rates than that expected for bovine serum [16,19].…”