ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

pressures higher than normal contact forces.
Figure 3 – Mean friction coefficient during the stance phase for healthy and OA groups.
5. DISCUSSION
In this study, frictional properties of knee OA articular cartilage have been assessed
during gait. Although first peak of maximum normal contact force could be an indicator
of OA, the measurement of frictional properties highlights more significantly the
presence of this disease. Actually, the results of this study showed that surface friction
force for OA articular cartilage could reach up to four times the magnitude of the
surface friction force for healthy articular cartilage. These differences are mainly
attributed to the friction coefficient which showed also significant differences in
magnitude for both cohorts during the swing phase. Limitations in this study are mainly
attributed to the modeling process. First, confined compression configuration may not
be appropriated to represent the real mechanical behaviour of articular cartilage in
physiological conditions since it yields 1D homogeneous deformation which is not
likely to occur in real situations. Also, it has to be mentioned that confined compression
configuration is known to yield higher fluid pressurisation compared to other
configurations. Therefore, the effective friction coefficient will be relatively low for this
configuration. In physiological conditions, it is actually not known if fluid
pressurisation reaches very high levels as those reached in confined compression
simulations. Thus, it is difficult to estimate the in vivo effective friction coefficient
based on the weeping mechanism. Also, the effective friction coefficient in this study
depends only on one mechanism of lubrication. To date, no mechanism of lubrication
can describe perfectly the in vivo lubrication process of articular cartilage during daily
activities. It has been suggested that a combination of existing theories may be a good
representation of the lubrication process in vivo [13]. Values used in this study to
represent the structural and mechanical properties of OA articular cartilage were the
same for all subjects. However, it is known that some of these properties are varying
depending on the grade of the disease. Future works might be accomplished to measure
these parameters on human articular cartilage and for different grades of OA. If such
parameters are measured, and if significant differences in frictional properties are found
between the cohorts regrouped per grade of OA, then measurable parameters in vivo
having an impact on the frictional properties have to be identified. If this is
accomplished, these parameters could therefore be used to screen the disease in its early
phases and progression. However, to date, there exists no way to measure mechanical
properties such as the equilibrium friction coefficient in vivo using non-invasive
techniques.