The tribological difference between biomedical steels and CoCrMo-alloys

In orthopedic surgery, different self-mating metal couples are used for sliding wear applications. Despite the fact that in mechanical engineering, self-mating austenitic alloys often lead to adhesion and seizure in biomedical engineering, the different grades of Co-base alloys show good clinical results, e.g., as hip joints. The reason stems from the fact that they generate a so-called tribomaterial during articulation, which consists of a mixture of nanometer small metallic grains and organic substances from the interfacial medium, which act as a boundary lubricant. Even though stainless steel also generate such a tribomaterial, they were ruled out from the beginning already in the 1950s as "inappropriate". On the basis of materials with a clinical track record, this contribution shows that the cyclic creep characteristics within the shear zone underneath the tribomaterial are another important criterion for a sufficient wear behavior. By means of sliding wear and torsional fatigue tests followed by electron microscopy, it is shown that austenitic materials generate wear particles of either nano- or of microsize. The latter are produced by crack initiation and propagation within the shear fatigue zone which is related to the formation of subsurface dislocation cells and, therefore, by the fact that an Ni-containing CrNiMo solid solution allows for wavy-slip. In contrast to this, an Ni-free CrMnMo solid solution with further additions of C and N only shows planar slip. This leads to the formation of nanosize wear particles and distinctly improves the wear behavior. Still, the latter does not fully achieve that of CoCrMo, which also shows a solely planar-slip behavior. This explains why for metallurgical reasons the Ni-containing 316L-type of steels had to fail in such boundary lubricated sliding wear tribosystems.

Paper 13: Arthroplasty of the Hip Using Foreign Materials: A History

Arthroplasty is a surgical procedure which provides an artificial joint. The operation should give ( a) relief from pain; ( b) a stable joint; and ( c) an improvement in function. The loss of function of a joint may be caused by injury or disease.

Since 1894 there have been many attempts to restore function using various artificial materials either to separate or replace the joint surfaces, and the following is a list of the ‘milestones’ in this field:

(1) The use of foils to cover the femoral head. (2) Floating joint interposition. (3) The introduction of cobalt–chromium alloy. (4) The replacement of the femoral head. (5) The use of acrylic and other plastics for implants. (6) The replacement of the acetabulum. (7) Total hip replacement. (8) The introduction of acrylic dental polymer to anchor components in bone.

This paper will discuss the various materials and devices which have been used, and some of the complications which consequently have occurred.

Wear evaluation of cobalt-chromium alloy for use in a metal-on-metal hip prosthesis

Wear of the polyethylene in total joint prostheses has been a source of morbidity and early device failure, which has been extensively reported in the last 20 years. Although research continues to attempt to reduce the wear of polyethylene joint-bearing surfaces by modifications in polymer processing, there is a renewed interest in the use of metal-on-metal bearing couples for hip prostheses. Wear testing of total hip replacement systems involving the couple of metal or ceramic heads on polymeric acetabular components has been performed and reported, but, until recently, there has been little data published for pin-on-disk or hip-simulator wear studies involving the combination of a metallic femoral head component with an acetabular cup composed of the same or a dissimilar metal. This study investigated the in vitro wear resistance of two cobalt/chromium/molybdenum alloys, which differed primarily in the carbon content, as potential alloys for use in a metal-on-metal hip-bearing couple. The results of pin-on-disk testing showed that the alloy with the higher (0.25%) carbon content was more wear resistant, and this alloy was therefore chosen for testing in a hip-simulator system, which modeled the loads and motions that might be exerted clinically. Comparison of the results of metal-on-polyethylene samples to metal-on-metal samples showed that the volumetric wear of the metal-on-polyethylene bearing couple after 5,000,000 cycles was 110-180 times that for the metal-bearing couple. Polyethylene and metal particles retrieved from either the lubricant for pin-on-disk testing or hip simulator testing were characterized and compared with particles retrieved from periprosthetic tissues by other researchers, and found to be similar. Based upon the results of this study, metal-on-metal hip prostheses manufactured from the high carbon cobalt/chromium alloy that was investigated hold sufficient promise to justify human clinical trials.

Metal on metal bearings in hip arthroplasty

Periprosthetic osteolysis caused by wear debris released from the bearing surface of polyethylene components is the major problem in contemporary hip arthroplasty. Several types of metal on metal prostheses were developed in the 1960s, but by the mid 1970s they were completely displaced by polyethylene bearings. There have been several generations of all metal components with significant variation in design, tolerances, and bearing surface quality. A number of these hips have survived for more than 25 years because of low wear rates and minimal osteolysis. Identification of the characteristics that contributed to long term function is important. The historical development and clinical results of metal on metal hip arthroplasties are presented. Factors that led to the abandonment of the metal on metal bearings are related to: (1) the early success of the Charnley prosthesis; (2) the frictional torque issue; (3) carcinogenesis concerns; (4) metal sensitivity concerns; (5) high infection rates; and (6) increased strain rates in periprosthetic bone and fatigue fractures of the acetabular floor. The accumulated experience to date enables one to evaluate all the factors with a different perspective and makes the use of newer metal on metal bearings a viable option in younger patients.