Biomedical Engineering Reference
In-Depth Information
available for attachment to conventional femoral stems where a patient might not
be suitable for the peg.
Future developments in hip joint replacement implants are generally investigat-
ing the use of different bearing surfaces. This includes the combination of ceramic
against metal and the use of diamond-like coatings [20, 21]. An alternative approach
that has been investigated in hip simulator studies in the laboratory is to use a soft
layer of rubber as one of the bearing surfaces (Figure 8.4). The design rationale
behind this is the fact that in natural human synovial joints the articular cartilage
is a low modulus material and the use of a rubber layer may benefit from the
excellent tribological characteristics seen in natural synovial joints [21]. However,
this has yet to reach a clinical investigation.
8.3.3
Knee Joint Replacement
The natural human knee joint is formed by the distal end of the femur, the
proximal ends of the tibia and fibula together with the patella. The meniscus covers
a large proportion of the surface of the proximal tibia. Four ligaments connect the
knee together and provide stability during locomotion: the anterior and posterior
cruciate ligaments and the two collateral ligaments. Knee replacement surgery
involves using a series of jigs to guide an oscillating saw to cut the bone on the
femur and tibia to the appropriate shape for the parts of the implant. In a similar
manner to hip replacement surgery, surgical navigation and robotic systems are
increasing in popularity [18].
There are a huge variety of designs of knee replacement implants including fixed
bearing, mobile bearing, and rotating hinge designs. Various designs also allow
for the preservation of both cruciates, preservation of the posterior cruciate, or
resection of both cruciates. Designs of total knee replacement implants (Figure 8.5)
consist of femoral and tibial parts; some designs may also include a patellar part.
The femoral part is generally made from cobalt chrome molybdenum alloy. The
tibial part is generally a two-piece design consisting of a tibial tray and an insert.
The insert is manufactured from ultrahigh-molecular-weight polyethylene, with
the tray made from a metal. With fixed bearing designs (where the tray and insert
are rigidly fixed together) the tibial tray is manufactured from titanium alloy. With
mobile bearing designs (where the insert is free to move relative to the tray) the
tray is manufactured from cobalt chrome molybdenum alloy, since titanium alloy
has poor wear characteristics. The design rationale with a mobile bearing is that it
will reduce wear and allow for more natural motion [20, 26, 27].
Metal against polymer knee joint replacement implants operate with a bound-
ary/mixed lubrication regime and therefore wear will occur [28]. Knee replacement
implants typically have a 95% survival rate at 10 years, with osteolysis being a major
cause of revision surgery. With fixed bearing designs, wear has occurred on the
top side and bottom side of the polymer insert. Mobile bearing designs have been
shown to have much reduced wear compared to fixed bearing designs [29].
