Biomedical Engineering Reference
In-Depth Information
10
−
9
10
−
8
Pin specimen
Disc specimen
4.0
1.5
3.0
1.0
2.0
0.5
1.0
0
0
20.9 10.0 5.0 0.1
Concentration of oxygen (%)
20.9
10.0
5.0
0.1
Concentration of oxygen (%)
Figure 5.8
Influence of dissolved oxygen on wear of
Co-Cr-Mo alloy. Typically, pin-on-disc tests were performed.
Pin and disc, Co-Cr-Mo alloys; lubrication, simulated syn-
ovial fluid. Adapted from Nakanishi
et al
. [26].
sliding. This suggests that partial dry contact at asperity levels occurred in the
metal-polyethylene ball-in-socket joint that used serum bovine lubrication [24].
As with other experiments, oxygen concentration in our experimental system
is an important factor to be considered. A metallic surface is composed of three
layers including a superficial layer of oxide film, the work-affected layer, and the
base metal. The oxide film generally improves corrosion resistance and decreases
wear, but sufficient oxygen is needed to reform the oxide film after damage from
friction. However, the damaged oxide film in the human body may be difficult to
restore, because the oxygen concentration
in vivo
is low (Figure 5.8) [25, 26].
The oxygen concentration in human joints will also affect the material and wear
properties of polyethylene. Oxidative degradation of polyethylene is known to cause
increased wear rate of the polymer in total joint replacement, leading to failure of
these devices [27, 28].
5.3
Tribological Testing of Tissue from a Living Body
When performing tribological testing on living tissue, it is important to safeguard
against tissue desiccation prior to testing. Living tissues generally contain a
significant amount of fluid, and as a result, material and tribological properties will
vary in accordance with the degree of desiccation. Furthermore, the dampness or
softness of the surface and the individual variations in the geometry of the tissues
also complicate testing as well as introduce challenges in the design of appropriate
methods of fastening the tissue to the testing device. The geometric configuration
