Biomedical Engineering Reference
In-Depth Information
FIGURE 5.2
A typical total hip joint replacement is made primarily of metal. The ball of the femoral hip stem
fits into a pelvic acetabular cup that is lined with ultra high molecular weight polyethylene (UHMWPE) for fric-
tion-free motion.
Photograph of the PROFEMUR
W
Z minimally invasive hip stem with modular necks courtesy of Wright
Medical Technology, Inc.
preferred selection of alloys of titanium or cobalt-chrome for hip, knee, and dental implants.
Other typical properties of metallic materials include a high density and much greater stiff-
ness than most natural materials they replace, which lead to undesirable stress shielding.
Stress shielding prevents the tissue from being exposed to normal levels of mechanical
loading and has been observed after implantation of metal joint replacements to lead to loss
of adjacent bone, which requires loading to maintain its mass. Certain metals known as
shape memory alloys (e.g., nitinol) can be bent or deformed and still return to their original
shape when the stress is released. These metals have found application in eye glasses and
coronary artery stents that can be inserted through a catheter while collapsed and then
spring into a cylindrical shape once they are pushed beyond the confines of the catheter.
Metallic devices are typically made by investment casting, computer-aided design and
machining (CAD/CAM), grinding, or powder metallurgy techniques. The specific steps
involved in the fabrication of a medical device will depend on factors such as final geome-
try of the implant, the forming and machining properties of the metal, and the costs of alter-
native fabrication methods.
