Abstract
Introduction
Hemiarthroplasty is an attractive technique for young and active patients as it preserves more bone stock. Polycarbonate urethane (PCU) has recently been introduced as an alternative bearing material. DSM Biomedical BV (Geleen, The Netherlands) has modified Bionate® PCU 80A (80AI) with C18 groups and produce Bionate® II PCU 80A (80AII) to create a different biointerface and enhance its tribological properties. The aim of this study was to compare friction performance of the articulation of the cartilage against 80AI and 80AII in various lubricants.
Materials and Methodology
A customised multidirectional pin-on-plate reciprocating rig (Fig. 1) was used to perform friction tests of ovine femoral condyles as they articulated against PCU 80A discs (diameter 38 mm, thickness 3.2 mm). The average surface roughness of the cartilage and the PCU discs was approx. 450 nm and 10 nm respectively. 30% (v/v) bovine calf serum (BCS) and bovine synovial fluid (BSF) were used as lubricants. Prior to testing, each disc was fully hydrated in its test lubricant for 6 days. During testing, a static compressive load of 20 N was applied (an average stress of approx. 0.95 MPa). The sliding distance was 25 mm with ±15° rotation over the length of the stroke to produce cross shear. Each test lasted 15 h at a frequency of 1 Hz. Lubricant was kept at 37±1 °C throughout testing. The friction force was measured using full-bridge circuit strain gauges (Fig. 1).
Results
The COF steadily increased during the first hour of testing due to the biphasic nature of the cartilage and then stabilized. Fig. 2 shows the average COF (mean ± 95% confidence limits, n=6) over 15 h. When BSF was used, the average COF decreased from 0.379±0.015 to 0.286±0.039 for 80AI and from 0.316±0.014 to 0.190±0.029 for 80AII, which found to be statistically significant (p = .004 and p < .0005 respectively). In both lubricants, the average COF for 80AII was statistically lower than that for 80AI (p < .0005 for BCS and p = .004 for BSF).
Conclusion
Under the clinically-relevant conditions, the average COF varied with tested lubricants and polymers. The average COF reduced for both polymers when BSF was used. This might be because proteoglycan 4 (PRG4) adsorbs on cartilage surface [1], and thus possibly lowered the COF outcomes. In addition to PRG4, other molecules present in BSF (i.e. hyaluronan and lipids) might also have roles in reduced COF. Moreover, 80AII had a lower friction than 80AI as either lubricant was used. This can be attributed to the higher protein adsorption nature of 80AII, though further investigation is required. Furthermore, when BCS was used the mean COF for 316L stainless steel (0.358±0.032) was lower than that for 80AI (0.379±0.015), which was not statistically different (p = 0.277), and it was greater than that for 80AII (0.316±0.014), which was statistically significant (p = 0.039). In conclusion, 80AII showed statistically superior friction performance over 80AI and SS316L, which would be a potential hemiarthroplasty material among these three.
