Phospholipids as boundary lubricants in wear tests of prosthetic joint materials

Abstract

The wear of ultra-high molecular weight polyethylene (UHMWPE) bearing surfaces in prosthetic joints is a significant clinical problem. Wear studies on prosthetic joints are therefore necessary in the improvement of their long-term performance. However, there is no practical and reliable lubricant available for the wear tests. The present study was the beginning of the development of “artificial joint fluid” for this purpose in the authors' laboratory. In prosthetic joints, effective boundary lubrication is known to take place. At the first stage, phospholipid–water dispersions were investigated. The phospholipids studied were dipalmitoylphosphatidylcholine (DPPC), which according to some studies functions as a boundary lubricant in the natural joints, and soybean lecithin, which contains some of the same phospholipids as joint fluid. The material combination used in the tests was UHMWPE against Co–Cr–Mo alloy. The tests were run with a pin-on-flat reciprocator, a uniaxial hip joint simulator, and a three-axis hip joint simulator. Phospholipids had a remarkable effect on the wear of UHMWPE. Surprising features in the wear of UHMWPE in regard to boundary lubrication and polyethylene transfer were observed. Some macroscopic polyethylene debris was produced, but the actual wear of the polyethylene component was still negligible, quite irrespective of whether the polyethylene transfer to the Co–Cr–Mo counterface was absent, mild or heavy. However, the fact that polyethylene transfer occurred in some tests and that the wear was negligible differs from typical clinical observations. More tests are needed and will be done to clarify the role of phospholipids as boundary lubricants in prosthetic joints.