Wear tests of orthopaedic biopolymers with the biolubricant augmented by a visco-supplement

Abstract

The lubricant in which both natural and replacement joints must operate is synovial fluid. In vitro wear tests of such prostheses and their biomaterials generally employ a lubricant based on bovine serum, as this contains proteins that have a critical impact in reproducing clinically valid wear rates in the laboratory. However, it is also recognised that bovine serum is not identical to synovial fluid, a key difference being that synovial fluid includes hyaluronic acid but bovine serum does not. Few clinically valid wear tests employing a lubricant that includes hyaluronic acid have been undertaken, and the results have been contradictory. To try and inform these results, a series of wear tests were undertaken using a clinically validated wear screening device to investigate the influence of the addition of hyaluronic acid to the lubricant on the wear rates on two orthopaedic biopolymers. When a clinically used hyaluronic acid solution, Ostenil ® , was added to a dilute bovine serum lubricant, it was found that wear factors were not greatly changed compared with dilute bovine serum alone. Additional wear tests were undertaken with Ostenil ® added to distilled water. Here, wear was much lower but not clinically valid as transfer films were formed on the test plates.

Potential Synthetic Biolubricant as an Alternative to Bovine Serum

“…Previous pin-on-plate studies using bovine serum as the lubricant have also shown this pattern of little change in the S a values and it was suggested that these surface roughness results indicated that there was no transfer film on the plate surface [27,28,31]. As the S a remains similar for the duration of the wear test, this means that there was little change to the average roughness of the plates.…”

“…This can be compared to the wear factors of 1.1 × 10 −6 mm 3 /Nm and 1.6 × 10 −6 mm 3 /Nm when using dilute bovine serum as the lubricant in the same rig [27,28]. For failed and explanted Charnley hips, the wear factor has been found to be 2.1 × 10 −6 mm 3 /Nm [29].…”

“…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]. The plate weight change was actually measured in these previous studies, but as it was not reported it is likely that the plates showed zero wear or wear within the precision of the balance used.…”

“…This sample size (four test pins and four test plates) has, however, been used in previous published works [28,[41][42][43][44][45][46]. These tests were sufficient to indicate that the new lubricant (0.5% gellan gum fluid gel) produced wear factors closer to those measured with bovine serum than 0.75% gellan gum/alginate tested in a previous study [16].…”

“…The 0.5% gellan gum fluid gel lubricant was tested in a four-station, multidirectional, pin-onplate wear test rig (see Figure 6) [16,28] to determine the effect on the wear of UHMWPE pins against stainless steel plates. To provide the multidirectional motion, rotational motion was applied to the test pins and reciprocating sliding was applied to the plates.…”