Orthopedic Joint Implants
Orthopedic joint implants are widely used in orthopedic surgery, particularly for
the hip joint. Each year, more than 250,000 of orthopedic hip joints are implanted
in the United States alone to treat severe hip joint disease, and this number is
increasing every year. Although much research work has been devoted to various
aspects of this topic, there are still several important problems. In the past, most
of the research was conducted by bioengineering and medical scientists, and
participation by the tribology community was limited. In fact, in the past decade
there has been a significant improvement in bearings in machinery, but the design
of the hip replacement joint remains basically the same. This is an example where
engineering design and the science of tribology can be very helpful in actual
The common design of a hip replacement joint is shown in Fig. 20-1. The
acetabular cup (socket) is made of ultrahigh-molecular-weight polyethylene
(UHMWPE), while the femur head replacement is commonly made of titanium
or cobalt alloys. The early designs used metal-on-metal joints in which both the
femoral head and socket were made of stainless steel. In 1961, Dr. Sir John
Charnley in England introduced the UHMWPE socket design. A short review of
the history of artificial joints is included in Sec. 20.3.
Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.
The combinations with UHMWPE have relatively low friction and wear in
comparison to earlier designs with metal sockets. Later, the stainless steel femur
was replaced with titanium or cobalt alloys for better compatibility with the body.
It proved to be a good design and material combination, with a life expectancy of
10–15 years. This basic hip joint design is still commonly used today.
For comparison with the implant joint, an example of a natural joint
(synovial joint) is shown in Fig. 20-2. The cartilage is a soft, compliant material,
and together with the synovial fluid as a lubricant,