Biomedical Engineering Reference
In-Depth Information
Figure 5.4
Plowing friction resulting in fatigue wear of bearing material.
Figure 5.5
Various surface profiles of the pin used for
pin-on-disc configuration tests. Adapted from Besong
et al
.[12].
There are clear differences in the contact stress between the pin and plate con-
figuration, and misalignment between the pin and the plate may raise the stress
within the tribological system. The plowing friction shown in Figure 5.4 is also an
important consideration, because the increase in friction and wear on the lower
bearing material is enhanced by plowing.
For the past 20 years, laboratory studies on polyethylene wear have been con-
ducted using unidirectional or reciprocating linear wear-type testing machines
consisting of a polymer pin on a metallic or ceramic flat. In recent years, mul-
tidirectional motions have been introduced to more realistically address
in vivo
kinematics (Figure 5.6) [13-15]. Unidirectional reciprocating motion causes the
polyethylene surface to appear to become oriented and strain hardened. It appears
that the multidirectional motion due to physiological gait patterns causes the sur-
face layer to be constantly redrawn and reoriented at acute angles. This leads to
shearing of polyethylene particles from the surface, producing wear.
