Lanthanide-Doped UHMWPE for In Vivo Wear Monitoring and Increased Oxidative Stability

The capability to detect the onset of unusual wear of the polyethylene tibial insert in total knee replacement may allow the surgeon to intervene earlier and more effectively, leading to a less invasive revision surgery in which only the insert is replaced or in which other corrective action is taken, such as realignment, before serious bone loss occurs. We report on a lanthanide-doped UHMWPE that may permit in vivo wear monitoring. The lanthanide acts as a tracer. Excellent correlation between the gravimetric and tracer-determined wear rates of UHMWPE has been obtained in laboratory tests and previously reported. However, the biocompatibility of the lanthanides must first be carefully evaluated if prosthetic components containing them are to be implanted in humans. Four lanthanide elements that together span almost the full range of lanthanide atomic numbers were evaluated: cerium (58), europium (63), gadolinium (64), and lutetium (71). The effect of their trivalent chlorides on cell proliferation and apoptotic cell death of human osteoblasts and mouse murine macrophages was examined. The induced cell response by the four lanthanides was similar for osteoblasts but element specific for macrophages. A similar cytocompatibility studies using the cobalt(II) cation (Co2+) suggest this ion is markedly less biocompatible with osteoblasts than the lanthanide ions. Equally significant, initial results suggest that another beneficial effect of lanthanide doping is to increase the resistance of UHMWPE to oxidative degradation. Europium stearate concentrations in the range of 465 to 4650 ppm largely inhibited the mechanical degradation of nitrogen-packaged, gamma-sterilized UHMWPE subjected to oxidative accelerated aging. The lanthanide-doped UHMWPE may thus offer the possibility of in vivo wear monitoring combined with increased material oxidative stability. Further work is required for proof of concept.