Surface articulating joint replacement

Ultrahigh molecular weight polyethylene (UHMW PE) is most commonly used for articulation surfaces in joints. It is also used for prosthesis components in total hip replacement. In the latter application, it may be reinforced with carbon fibers to increase wear properties. 

Wear is defined as the loss of material from a surface as the result of relative motion. In this chapter, the wear processes in polymer implants are discussed. Polymers are used in a wide variety of implants in the human body such as joint replacement implants, pacemakers, catheters and heart valves. Wear of polymer implants is almost exclusive to joint replacement implants, such as those used to replace the hip or knee. These implants involve the articulation of a metal or ceramic against a polymer. Typically these implants operate with a mixed or boundary lubrication regime and, therefore, there is contact between the bearing surfaces that can lead to the generation of wear debris. The chapter is divided into sections that cover implants, wear processes, polymers used in implants, the effect of wear debris on the body and, finally, likely future trends. 

Joint replacements, 45-10-45-13 Joint structure modehng, 15-6 Joint-articulating surface motion, 49-1-49-34 ankle joint, 49-2—49-4 elbow joint, 49-20-49-23 hand joint, 49-27-49-32 hip joint, 49-10-49-15 knee joint, 49-4 9-10 shoulder joint, 49-15—49-20 wrist joint, 49-23—49-27 Jones, D., 74-7... 

PE remains the workhorse of joint replacement liners as it functions as a low-friction implant articulating surface. There have been many adaptations and... 

UHMWPE is used as an articulating surface in both hip and knee joint replacements. As an acetabular component in THR (total hip replacement), its annual wear rate was reported to be approximately 0.1 mm/ year or 80 mmVyear against a 32 mm diameter femoral head. Hall et al. [70], and Oonishi et al. [71]. It is approximately a 30-fold reduction in wear rate in comparison with a PTFE socket that was first introduced by... 

In 1971 a metal-backed polyethylene acetabular cup was introduced. This cup provided an eccentric socket which was replaceable, leaving the metal and replacing only the polyethylene. Because of the success of this component, metal-backed high density polyethylene (HDPE) liner is standard for prosthetic acetabular components. Research confirms that metal-backing reduces the peak stresses in the bone cement, and that HDPE forms a successfiil articulating surface for the prosthetic joint. 

Polymers have found two specific applications in orthopedics. First, they are used for one of the articulating surface components in joint prostheses. Thus, they must have a low coefficient of friction and low wear rate when they are in contact with the opposing surface, which is usually made of metal or ceramic. Initially, poly(tetrafluoroethylene), PTFE, was used in this type of application, but its accelerated creep rate and poor stress corrosion caused it to fail in vivo. Ultimately, this material was replaced with... 

Knee-joint prostheses consist of two metal components that replace the destroyed joint surfaces of the thigh bone (femur) and the shinbone (tibia) and that are usually cast from cobalt- or titanium-based alloys. The tibia part is typically extended by a short stem, anchoring the system in the marrow cavity. A UHMWPE insert is used as the articulating counterpart to optimize sliding behavior and to minimize wear see Fig. 8.6. 

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