Biomedical Engineering Reference
In-Depth Information
tional gradation in these implants will allow for a single unitized component to be
manufactured without any chemically or structurally abrupt interfaces leading to
enhanced properties and performance.
9.3.2 Proposed Design of Functionally-Graded Hip Implant
Since there are distinct property requirements at different locations of the femoral
(hip) implant, satisfying all these requirements in a monolithic implant is rather
challenging. However, using the LENS™ process with multiple powder feeders, it
might be possible to produce a functionally-graded implant in its near-net shape
form with site-specifi c properties. Firstly, the idea is to fabricate the basic core
structure of the femoral stem and head assembly using LENS™. This core-
structure can also be hollow, similar to the prototype shown in Figure 9.3.
However, instead of Ti-6Al-4V, the material of choice for the core of the femoral
stem and head assembly will be based on the newer generation beta-Ti alloys,
such as those based on the Ti-Nb-Zr-Ta system. This alloy system exhibits a low
modulus and consists of completely biocompatible alloying elements. Both solid
geometries as well as stems with internal cavities can be processed in order to
achieve an optimum balance of mechanical properties. Since the surface of the
femoral stem is required to exhibit excellent osseointegration properties, addi-
tional roughness can be introduced on the surface of the stem by laser depositing
lines of the same alloy in the form of a grid. The pattern of these lines/grid can
be optimized for achieving the best potential of osseointegration based on trial
in vitro studies.
As discussed previously, one of the primary requirements of the femoral head
(ball part of the ball-socket joint) is excellent wear resistance, especially against
the internal surface of the acetabular cup. Titanium and its alloys typically exhibit
rather poor wear resistance. Therefore, the femoral head is often made of ceramic
or a Co-Cr-Mo based alloy which exhibit very good wear resistance. The acetabu-
lar cup is often made of titanium with the internal surface coated with a special
polyethylene (UHMWPE). While the use of a ceramic femoral head improves the
wear resistance, it typically exhibits poor fracture toughness and the joint between
the head and the stem (made of Ti alloy) creates a weak interface between two
dissimilar materials. A more appropriate approach might be to manufacture the
core of the femoral stem and head assembly in the form of a single monolithic
component and use surface engineering to improve the wear resistance of the
base Ti alloy locally in the femoral head section. Thus, as discussed before, the
LENS™ process will be used to initially deposit an integrated femoral stem-head
assembly using the Ti-Nb-Zr-Ta based beta-Ti alloy and subsequently, a boride
(or carbide) reinforced Ti-Nb-Zr-Ta alloy will be deposited in a graded fashion on
the femoral head section of the implant. In this manner, the core of the femoral
head will be made of the same tough
-Ti alloy as the structural core of the
femoral stem while the surface of the femoral head will comprise of a hard and
wear-resistant metal-matrix composite. An ideal candidate for the outer layers of
the wear - resistant femoral head is the Ti - Nb - Zr - Ta - B system, which forms hard
β
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