Biomedical Engineering Reference
In-Depth Information
Figure 8.1
Charnley hip stem. (Reproduced with kind per-
mission from DePuy International Ltd, Leeds, UK.
by
DePuy International Ltd.)
As an alternative to the use of metals for the femoral head, ceramics, such
as alumina and zirconia, can be used and they will be attached to a metal stem
[19, 20]. These ceramic heads have lower surface roughness values than metals,
typically 0
.
002 µm. Recent advances in the materials for femoral heads is the use
of coatings, which have a very high wear resistance. Smith and Nephew have
produced Oxinium, which is a metal with a ceramic surface.
The acetabular part of a hip joint replacement can be a one-piece design
(Figure 8.2) that fixes directly into the acetabular bone. This design is manu-
factured from either ultrahigh-molecular-weight polyethylene or cobalt chrome
molybdenum alloy, depending on the bearing coupling the surgeon requires with
the femoral head. The ultrahigh-molecular-weight polyethylene acetabular cup has
a metal wire attached to its outside so that the part can be seen on X-rays. After
manufacture, the ultrahigh-molecular-weight polyethylene bearing surface has an
average surface roughness of around 1
.
29 µm, which is higher than the surface
finishes achieved on the surfaces of metals and ceramics. A two-piece design
of the acetabular part is also available that consists of a metal shell (that fixes
into the acetabular bone) with a liner that inserts into the shell. Liners are made
from ultrahigh-molecular-weight polyethylene or a ceramic. Highly cross-linked
polyethylene is now in use as a bearing surface for the acetabular part and early
clinical results show that the wear rates are far reduced compared with conventional
ultrahigh-molecular-weight polyethylene [16].
The range of bearing material combinations available for hip replacement
implants consists of metal against polymer, metal against metal, ceramic against
