Biomedical Engineering Reference
In-Depth Information
2.1 Radiograph of 26-year follow-up of first-generation Charnley low friction
arthroplasty composed of a stainless steel femoral head and high density
polyethylene cup.
socket (acetabulum) of the pelvis. Thus, to mimic this structure, a total hip
replacement usually has three parts: (i) the stem that fits into the femur; (ii) the
ball that replaces the spherical head of the femur; and (iii) the cup that fits into
the worn out acetabulum. As shown in Fig. 2.1, the stem and ball can be one
piece but they can also be modular for additional customization.
Today's hip implants are composed of high performance metal alloys,
ceramics, and polymers (Table 2.1). Specifically, the stem is usually made of
titanium or cobalt±chromium alloys. These metal alloys are designed to possess
good mechanical strength and corrosion resistance. Certain kinds of surface
treatments (such as bioactive coatings, porous structures) are often used to
enhance bone ingrowth onto the hip implant. The ball is commonly made of
cobalt±chromium alloys or ceramic materials (e.g., aluminum oxide and zir-
conium oxide) due to their good wear resistance properties. The ball surface is
smooth in order to minimize rotation resistance against the socket. The
acetabular socket can be composed of metal or ultrahigh molecular weight
polyethylene (UHMWPE).
Although new materials, such as polyetheretherketone (PEEK), are being
investigated for hip stems (Kurtz and Devine, 2007), titanium and cobalt±
chromium alloys are still dominant. This chapter is thus focusing on reviewing
the fundamentals of these alloys summarizing some of the new advances in
developing more efficient metal hip and knee implants.
