100% Positive Material Identification

Understanding Implants in Knee and Hip Replacement (continued)
Figure 3: Example of a rotating platform knee replacement implant: front view, side view, and flexed
In rotating platform implants, a
similar metal implant is inserted into
the tibia, but the polyethylene tray is
placed on a circular stem that allows
slight rotation of the tray on the metal
tibial platform during knee motion. This
mobile-bearing design may allow for
slightly greater range of motion in the
knee. [Figure 3]
Rotating platform implants were
originally designed with the younger,
more active patient in mind. Some data
suggests that there is less wear of the
polyethylene component with this type
of implant, but a clinical benefit has not
been established. (This choice of fixedbearing
versus rotating platform implants
is also relevant to people who are
candidates for partial knee replacement,
in which only one compartment of the
knee is replaced.) [Figures 4 and 5]
During knee replacement surgery
these implant components are usually
connected to the bone with cement, a
technique that generally yields excellent
results. However, the orthopedic
community is studying “cementless”
fixation of implants - a technique already
in widespread use in hip replacement - in
which the components have a trabecular
metal surface (one that resembles a
honeycomb). This porous design permits
the growth of new bone tissue on the
surface of the fixed metal components.
Knee Implant Materials, Shapes and
Sizes
As described, knee implants generally
include components made of metal and
plastic. In order to ensure a smooth,
gliding motion, and to avoid friction,
metal surfaces never come into contact
with one another. Metal components are
usually made of cobalt chrome (which
contains nickel) or titanium (which is
known for its strength and lightness).
Titanium-only implants are available for
patients who are allergic to nickel, but
it is more difficult to obtain the highly
polished surface that facilitates smooth
motion with titanium than it is with
cobalt chrome.
Based on the successful use of ceramic
components in hip replacement, one
manufacturer has developed a ceramic
implant for the knee. “However,” Dr.
Windsor notes, “we won’t know for quite
a while if this will offer any advantage in
the knee, which is a very different kind of
joint.”
The patient’s size, weight, and gender
also have a bearing on implant selection.
While once there were only a few sizes
available, surgeons now have a wider
selection to choose from. This includes a
gender-specific “woman’s knee”, which
is designed to accommodate a woman’s
narrower bone structure, especially on
the femur. “This development reflects
Figure 4: Example of a fixed bearing unicondylar knee replacement
Figure 5: Example of a mobile bearing unicondylar knee replacement
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