Biomedical Engineering Reference
In-Depth Information
Y
Disc
(fixed)
X
Y
X
Pin
Wear track on disc
Y
X
Y
X
Figure 5.6
Example of multidirectional motion of pin on
disc. Adapted from Nakanishi
et al
.[15].
It has been recognized for a number of years that water does not produce
adequate boundary lubrication in joint simulations of
in vivo
wear. It has also been
confirmed that natural synovial fluid used as a lubricant in joint simulations affects
the tribological characteristic of bearing materials in joint prostheses [14].
The synovial fluid found in the joint cavity can be regarded as a dialysate of
blood. The large molecules in blood cannot penetrate the synovial membrane and
are excluded from the joint cavity. The polysaccharide hyaluronic acid (HA) is the
main component of the joint fluid. It is produced by the synovial membrane and
forms a high molecular mass complex [16, 17]. Both aging and arthritis can reduce
both the concentration and the relative molecular weight of HA, and thus reduce
the viscosity of the synovial fluid.
Bovine serum is the most commonly used lubricant in
in vitro
wear tests.
Boundary lubricants that occur in bovine serum include proteins of various
molecular weights and phospholipids [18, 19]. In some cases, a water-based liquid
that contains the principal constituents of synovial fluid is used for wear tests in
order to reduce considerable individual variability of bovine serum, which may
affect the wear results (Table 5.1). The wear of the polyethylene in the pin-on-plate
tests with water as a lubricant are highly variable, however, and depend on the
formation of a polymer transfer film on the counterface. The wear factors in the
bovine serum lubricated tests are more consistent and transfer onto the counterface
does not occur. Bovine serum is, therefore, considered to be more representative
of
in vivo
conditions [20, 21].
