Wear characteristics of materials for artificial hip joints
Total hip prostheses have succeeded in significantly improving the quality of life of millions of patients worldwide. Ultra high molecular weight polyethylene (UHMWPE) has been widely and successfully used as a bearing material in orthopaedic prostheses since the 1960s. Despite the recognized success and worldwide acceptance of total hip replacement, the numbers of failures increase after about 15-20 years of use. The negative biological effects of wear particles are considered to be the most important limiting factors for a long implant life. The liberation of sub micron size wear debris into human tissue leads to chronic inflammation. Continuous inflammation caused by UHMWPE particles activates inflammatory cells (macrophages) that stimulate bone resorptive cells (osteoclasts) and eventually a loosening of the implant.
In-vitro experimental wear simulations are helpful in evaluating the importance of the many variables which can influence the generation of the wear debris. However, the difficulties involved in accurately duplicating in-vivo wear conditions, the long time that it takes to carry out the wear experiments and difficulties in accurately measuring the low degrees of wear, have limited the applicability of in-vitro wear testing. Screening methods using simplified experimental conditions, mainly unidirectional sliding (pin-on-disc devices), have shown to yield wear rates markedly different from those obtained in advanced wear simulator tests and from that observed in vivo. There is thus a need for simplified screening methods which mimic the wear process in a more realistic way, still do not require long testing times.
A new test method has been developed based on a modification of an existing pin-on-disc machine. The strain-hardening effect of the polymer pin was avoided by a bi-directional motion. The results indicate that significant wear occurs due to the bi-axial motion and the testing time is drastically reduced compared to that required in a joint wear simulator. The modified pin-on-disc machine was used for tribological investigation of UHMWPE sliding on stainless steel, and on stainless steel coated with diamond-like carbon, titanium nitride or Micronite. Micronite is a new type of coating combined with a very low friction coating material giving enhanced tribological properties. It was also shown that the wear properties of UHMWPE can be affected by a treatment of the polymer (addition of vitamin E (VE) as antioxidant, -irradiation and annealing). The possibility of controlling the wear properties of high-density polyethylene at an early processing stage by using a cold roll-drawing technique was also investigated. The results indicate that molecular orientation induced by the polymer processing is a promising route to improve the wear properties of polyethylene.
Artificial hip joint