Biomedical Engineering Reference
In-Depth Information
in the polymer. This material has been shown to have lower wear rates in joint
replacement implants compared to conventional ultrahigh-molecular-weight
polyethylene, although there has been some concern about reduced mechanical
properties [16].
The replacement of joints of the hand and wrist has typically involved the use of
implants manufactured from silicone [17]. Silicone is a rubber that has a Young's
modulus of 30MPa and an ultimate tensile strength of 7MPa [9]. Silicone has
excellent fatigue properties over a wide range of strains. However, silicone has
been shown to have poor crack growth resistance once a crack has been initiated in
the material.
8.3
Joint Replacement Implants for Weight-Bearing Joints
8.3.1
Introduction
Joint replacement implants are commonly used for replacement of diseased
synovial joints that are weight-bearing, such as the hip, knee, and ankle. These
weight-bearing joints are subjected to large forces (typically two to three times body
weight during walking) over 1 million times a year [1]. This section of the chapter
discusses the materials that are used to manufacture the various designs of joint
replacement implants for the hip, knee, and ankle.
8.3.2
Hip Joint Replacement
The natural human hip consists of the articulation between the femoral head and
the acetabulum. Conventional hip replacement surgical procedures involve cutting
off the femoral head, broaching the femoral bone to create the shape of the implant
stem, and reaming the acetabulum. Surgical navigation and robotic systems are
increasing in popularity and have been found to be less invasive and lead to better
positioning of implant parts [18].
There is a variety of designs of hip joint replacement implants available, but
all consist of femoral and acetabular parts that form a ball and socket joint. The
femoral part comprises a stem and a head, as shown in Figure 8.1. The femoral
part can be either one-piece or a modular design where different femoral heads
can be fitted to the stem. The stems are generally made from stainless steel, cobalt
chrome molybdenum alloy, or titanium alloy. Conventionally, metals are also used
for the femoral head with stainless steel or cobalt chrome molybdenum alloy used.
Depending on the design, the diameter of the femoral head is typically between
22mm (Charnley hip, DePuy International Ltd, Leeds, UK) and 32mm (Muller hip,
Biomet, Dordrecht, The Netherlands). During manufacture, the femoral head is
highly polished, with the surface having an average roughness of around 0
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