Biomedical Engineering Reference
In-Depth Information
Although some cementless stem designs have good long-term outcomes,
cementless stems can still loosen if a strong bond between bone and the stem is
not achieved. Additionally, the acetabular component survival is relatively poor
(Capello et al., 2003; Kearns et al., 2006). This is mainly because cementless hip
replacements still suffer from polyethylene wear debris and osteolysis. There-
fore, a second-generation of metal-on-metal bearing systems for total hip
replacements have received a lot of attention since the 1980s (Dumbleton and
Manley, 2005). However, this design raises questions on the release of metal
ions and puts higher criteria for the wear resistance of metal components (mainly
cobalt±chromium alloys).
In summary, cemented hip replacements are usually recommended for older
patients while cementless hip replacements are more suitable for younger
patients who can achieve bone ingrowth onto the prosthesis more easily.
2.1.3
Knee implant design and construction
The knee can be simply considered as a hinge joint. In reality, it includes more
complicated motions like rolling and gliding. There are over 150 knee implant
designs in the market today but, generally, all of them are composed of three
parts: a curved femoral component to replace the lower end of the thighbone, a
flat tibial component to replace the top surface of the shinbone, and a dome-
shaped patella component to replace the kneecap.
To provide smooth motion and minimize wear, all articulating interfaces are
between metals and plastics (Table 2.1). Specifically, the femoral component is
usually made of cobalt±chromium alloys. The tibial component has a metal
platform and an ultrahigh density polyethylene cushion. The patella component
is also made of ultrahigh density polyethylene. Titanium and titanium alloys
tend to have a subordinate role in knee replacements due to their less superior
wear properties than cobalt±chromium alloys. To date, wear is still the major
reason for a knee revision.
2.2
Challenges for current metal hip and knee
implants
Over a million patients undergo hip and knee replacement surgeries every year
in the US (Table 2.2), and a significant percentage of these surgeries are revision
surgeries. Recent statistics pointed out that the chance of a hip replacement
lasting 20 years is approximately 80% (AAOS, 2007a) and the chance of a knee
replacement lasting 20 to 25 years is about 95% (AAOS, 2007b). Although this
is generally acceptable for older patients who may not live longer than this, it is
not acceptable for younger and more active patients receiving hip and knee
replacement. Thus, novel hip and knee prosthesis with improved efficacy are in
high demand and are now under investigation by many researchers.
