Compliant layer bearings in artificial joints. Part 2: Simulator and fatigue testing to assess the durability of the interface between an elastomeric layer and a rigid substrate

Abstract

Artificial joints have been much improved since their introduction but they still have a limited lifetime. In an attempt to increase their life by improving the lubrication acting within these prostheses, compliant layered polyurethane (PU) joints have been devised. These joints mimic the natural synovial joint more closely by promoting fluid film lubrication. In this study, tests were performed on compliant layer joints to determine their ability to function under a range of conditions. Both static and dynamic compression tests were undertaken on compliant artificial hip joints of two different radial clearances. Friction tests were also performed before and after static loading. In addition to this, knee wear tests were conducted to determine the suitability of a compliant layer in these applications. In the knee tests, variations in experimental testing conditions were investigated using both active and passive rotation and severe malalignment of the tibial inserts. The static compression tests together with the friction studies suggest that a small radial clearance is likely to result in 'grabbing' contact between the head and cup. The larger radial clearance (0.33 mm) did not exhibit these problems. The importance of the design of the compliant layer joints was highlighted with delamination occurring on the lateral bearings during the knee wear studies. The bearings with a layer 2 mm thick performed better than the bearings with a layer 3 mm thick. Tests conducted on flat PU bearings resulted in no delamination; therefore, it was concluded that the layer separation was caused by design issues rather than by material issues. It was found that, with careful material choice, consideration of design, and effective manufacturing techniques, the compliant layer joint functioned well and demonstrated durability of the union between the hard and soft layers. These results give encouragement for the suitability of these joints for clinical use.

Friction in metal-on-metal total disc arthroplasty: Effect of ball radius

“…When the friction factor decreases, with increasing Sommerfeld number, a mixed lubrication regime is indicated, whereas an increasing friction factor, with increasing Sommerfeld number, indicates a fluid film regime; a constant friction factor indicates a boundary lubrication regime (Jones et al, 2009). The viscosity of the lubricant was measured using an AR-G2 cone-on-plate rheometer (TA Instruments, West Sussex, UK) under 0.5% constant strain, at 37º C. The viscosity of diluted new born calf serum was found to be 1.44 ± 0.4 mPa.s.…”

Preservation of Native Aortic Valve Flow and Full Hemodynamic Support with the TORVAD Using a Computational Model of the Cardiovascular System

“…The typical clinical operating range of the device is between 8,600 and 9,800 rpm, 43 represented by the shaded region of the plot.…”

Future directions in knee replacement

“…Regarding friction and wear, the fundamental question is the nature of the lubrication mechanism between the surfaces [81]. Fluid film lubrication will result in low friction, which could improve the kinematics and eliminate the 'stick periods' characteristic of metal on UHMWPE bearings [82], which have a coefficient of friction in the range 0.05-0.2.…”

“…Further studies, focusing mainly on debonding, were carried out in another laboratory [81]. Flat polyurethane layers with a thickness of 2 mm and 3 mm were tested, as well as contoured surfaces -as is usual in TKR.…”