Finite element biphasic modelling of articular cartilage : an investigation into crystal induced damage.
Some of the most common diseases and disorders that occur in the adult population
are those which affect the joints of the musculo-skeletal system. Most joint diseases
cause damage to articular cartilage, the soft tissue that acts as a bearing surface
within the load-bearing joints. The function of articular cartilage is to provide a
wear-resistant joint surface with a very low coefficient of friction and to reduce the
compressive stresses experienced at the end of the long bones.
Osteoarthritis can be described as the progressive degeneration of articular cartilage.
This disorder causes large areas of cartilage in the load-bearing regions of the joint to
become split and fragmented, resulting finally in the exposure of underlying bone.
Osteoarthritis has been associated with a number of conditions including the
deposition of crystals within the tissue. It has been postulated that crystal deposits
have the potential to cause mechanical damage to articular cartilage. Two possible
damage mechanisms have been identified; localised tissue damage in the vicinity of
the crystal aggregate and surface damage induced by the presence of an aggregate.
Articular cartilage is a biphasic material as it consists of a fluid phase, composed
mainly of water, and a porous-permeable solid phase. The biphasic nature of the
tissue was modelled using the ABAQUS/Standard Finite Element software. Failure
criteria for the tissue were investigated using this technique and radial stress and
radial strain were found to be reliable predictors of damage. Damage threshold
values were determined for radial tensile stress (72 kPa) and radial tensile strain
(0.144). A finite element model was then developed to investigate the propensity for
crystal deposits to cause damage to a cartilage layer under cyclic loading conditions.
It was predicted that aggregates embedded deep within the cartilage layer do not have
the potential to cause either local or surface damage. Aggregates nearer the articular
surface have the potential to cause localised tissue damage, and it was found that this
was dependent upon their stiffness.