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

Macrostress and Macrostrain Finite Element Predictions in Articular Cartilage for
Correlation with Gene Expression Studies
1. ABSTRACT
J.Y.S. Li 1 , A. Metcalfe 2 , S. Evans 3 , E.J. Blain 4 and C.A. Holt 5
Articular cartilage has a complex inhomogeneous microstructure consisting of
interstitial fluid, proteoglycans and collagen fibres. Simple uni-axial compression can
induce a variety of local stresses and strains inside the tissue different to those measured
at bulk tissue level. In order to better understand the effects of mechanical loading on
the regulatory response of chondrocyte cells found in articular cartilage, use of finite
element modelling coupled with mechanical loading and gene expression studies is
proposed. Current articular cartilage constitutive models represent the explants as flat
surface axis-symmetric cylinders. However, cutting of the explants to achieve a flat
surface to reduce discrepancies between FEA and experimental results can cause trauma
to the chondrocytes and affect biological results. The effect of modelling uncut explants
as flat surface axis-symmetric cylinder was investigated. Results show that modelling of
explant specific surface curvature is essential for accurate macrostress and macrostrain
finite element predictions in articular cartilage during mechanical loading.
2. INTRODUCTION
Like other tissues in the human body, articular cartilage contains regulatory cells called
chondrocytes. Chondrocytes regulate the catabolic and anabolic activities of the tissue
depending on external environment stimuli, such as mechanical loading. This
mechanism allows the composition and organisation of the extracellular matrix to be
constantly altered to cope with functional demands. In cases where this mechanism fails
to maintain metabolic balance, articular cartilage thinning occur leading to
Osteoarthritis.
An ‘appropriate’ amount of mechanical loading is required for healthy articular
cartilage; however, at the same time, both under loading and excessive loading can
cause degeneration of the tissue. Gene expression studies are typically used to analyse
the effect of mechanical loading on the biological responses of articular cartilage 1,2,3 ,
where cylindrical explants are extracted using a biopsy punch and subjected to different
loading conditions. Although the compressive loading can be generally quantified for
the bulk tissue by the use of load cells and platen displacement measurements, little is
1 PhD student, Institute of Medical Engineering and Medical Physics, School of Engineering, Cardiff
University, Cardiff CF24 3AA, Wales, UK
2 Orthopaedics Registrar, Cardiff and Vale NHS Trust, University Hospital of Wales, Wales, UK
3 Professor, Institute of Medical Engineering and Medical Physics, School of Engineering, Cardiff
University, Cardiff CF24 3AA, Wales, UK
4 Cardiff Academic Fellow, Division of Pathophysiology and Repair, School of Biosciences, Cardiff
University, Cardiff CF10 3AX, Wales, UK
5 Reader, Institute of Medical Engineering and Medical Physics, School of Engineering, Cardiff
University, Cardiff CF24 3AA, Wales, UK