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

model has shown good agreement with experimental results for static and dynamic
loadings in vitro [6]. The estimation of fluid pressurisation requires the use of a biphasic
cartilage model for which material parameters are needed. These parameters are
representative of the mechanical properties of articular cartilage. In the case of OA
articular cartilage, it has been shown that it has a larger permeability and water content
than healthy articular cartilage [7]. Though previous studies in the literature have
characterized the frictional properties of OA articular cartilage during in vitro
experiments, no studies have attempted to estimate surface friction for in vivo
conditions such as gait.
The aim of this study is to investigate the differences in frictional properties in terms of
surface friction between OA and healthy knee articular cartilage during gait. The impact
of normal contact force applied at the articulation on the progression of OA will also be
assessed. Since an increase of tissue permeability increases compressive tissue strain
and decreases fluid pressurization [8] and that a decrease in fluid pressurization is more
likely to decrease the quality of the lubrication process at the articular surfaces. We
hypothesize that surface friction will be higher for OA articular cartilage than for
normal articular cartilage.
3. METHOD
3.1 Experimental Protocol
Twenty-five subjects with grade I to IV OA (8 grade I, 5 grade II, 6 grade III, and 6
grade IV) as well as fifteen healthy subjects performed a gait analysis on treadmill.
Each subject was asked to walk at selected self-pace. Treadmill was equipped with two
force-platforms (ADAL 3D, Méd. Développement, France). Four markers fixed on rigid
body were placed on the thighs, the shanks, and the feet by mean of exoskeletal system.
A system of six optical cameras sampling at 120 Hz was used for data acquisition of
kinematics Vicon M460 system (Vicon M460, Vicon Peak Inc., CA). Force-platforms
data were synchronised with kinematics data. Between 16 and 24 gait cycles were
recorded for each subject. Kinematics, kinetics and anthropometric data were introduced
in a generic 3D inverse dynamic model developed to estimate 3D joint forces and
moments at the hip, the knee, and the ankle [9]. A 2D reductionist contact model was
used to calculate tibio-femoral contact forces in the sagittal plane. These contact forces
were then introduced in a uniaxial biphasic-poro-viscoelastic (BPVE) cartilage model to
estimate cartilage displacement in compression. Afterwards, dissipative mechanisms are
used to estimate articular dissipation in compression and shear.
2.2 Numerical Formulation
Articular cartilage was modeled as a BPVE soft tissue [10]. A uniaxial confined
compression configuration was adopted since only the axial displacement (u(z,t)) of the
solid matrix is required in this study to calculate the dissipation of the cartilage.
Articular cartilage of the tibial plateau is confined by its undeformed part and by the
subchondral bone of the proximal tibia. Also, it is compressed by the articular cartilage
of the femur which acts like a piston compressing tibial articular cartilage. The reduced
momentum equation corresponding to this configuration is
zz zt , 1 u(
z, t)
, 0 z h, 0 t
(1)
zkt