Tribology of Mechanical Systems: A Guide to Present and Future Technologies
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- Ris (Zotero)
- Reference Manager
The everyday activity of walking, not to mention the various weekend recreational sport activities we choose to challenge ourselves with, poses considerable tribological challenges to the human body. For each of the million steps a person typically takes in a year, the accelerations associated with heel-strike and toe-off actions within the stance phase impose loads that are several multiples of body weight through the legs. In the hip joint formed between the acetabulum and femoral head, the near-spherical bearing formed by these conformal surfaces of diameter of several centimeters will experience peak contact pressures of 5 MPa or more. In a typical walking gate with flexion/extension occurring at a frequency around 1 Hz and through an angle of 45° or so, sliding speeds in such a joint will be several centimeters per second. The knee joint formed between the femoral condyles and the tibial plateau has a lesser degree of conformity, thus contact pressures are higher with relative motion, including some extent of rolling in addition to the sliding.
Articular cartilage on the surfaces of these joints, coupled with synovial fluid within, provides most individuals with a natural lubrication system that performs successfully under such contact conditions for a lifetime. However, in diseased arthritic joints, failure of articular cartilage results in painful sliding contact directly against the underlying bone. Revolutionized by the introduction of polymeric bearing surfaces by Charnley in the 1960s, joint replacement has become an increasingly popular and effective treatment for arthritis, with well over a million such devices now being annually implanted worldwide. Though the typical lifetimes of such implants is now roughly up to 20 years, it is limited by wear and remains insufficient to safeguard younger patients against the eventual necessity for revision surgery. This chapter will review the state of understanding of wear as the lifetime-limiting failure process for joint replacements, as well as recent attempts to improve materials, and the efforts to evaluate these joint replacements by appropriately simulative in vitro testing such that observed improvements may translate to realized in vivo performance.