Final Report Summary - CARTILAGE LOADING (Dynamic mechanical properties of cartilage: influence of loading conditions on properties and structure)
Dr Espino held an Intra-European Fellowship (IEF) at the Biomedical Engineering Research Group, School of Mechanical Engineering, University of Birmingham. This group holds expertise on high rates of testing of natural and synthetic biomaterials. Dr Espino was mentored by Prof. David Hukins (Professor in Biomedical Engineering) and Dr Duncan Shepherd (Reader in Biomedical Engineering). Dr Espino can be contacted at: d.m.espino@bham.ac.uk The group's website is as follows: http://www.birmingham.ac.uk/research/activity/mechanical-engineering/bio-micro/bio-medical/index.aspx
Fellowship research
Articular cartilage is a load bearing material found at articulating joints in the body. Articular cartilage also enables smooth joint articulation. Osteoarthritis is associated with degeneration of joints including its articular cartilage. Degeneration of the articular cartilage increases friction during movement making activities such as walking difficult and painful. Degeneration includes damage to the surface of articular cartilage (e.g. collagen) and changes to its mechanical properties.
One limitation in experimental testing of cartilage has been testing at rates associated with physiological loading. This has led to limitations in understanding of the mechanical behaviour of articular cartilage. However, the Biomedical Engineering Research Group (University of Birmingham) has recently developed an experimental approach to test cartilage using dynamic mechanical analysis (DMA) at high loading frequencies. This method has enabled characterisation of viscoelastic properties of cartilage at high loading rates. For example, cartilage properties can be determined at frequencies relevant to gait patterns during walking and impulsive heel strike rates associated with the early onset of lower-limb osteoarthritis.
DMA enables a structure's storage (k') and loss (k'') stiffness to be determined. k defines the energy stored which is available for subsequent elastic recoil, while k'' defines the viscous dissipation of energy. This can be combined with a frequency sweep to determine frequency dependent viscoelastic properties of cartilage. The frequency sweep is vital because it enables changes in the k':k'' ratio with frequency to be determined. Theoretically, this ratio provides a measure of the proportion of energy stored to energy dissipated. In principle this provides a measure of predisposition to fracture, as fracture is induced by excess energy storage.
The aim of the fellowship was to determine how loading conditions affect the maintenance of healthy cartilage. This is a necessary step to subsequently understand the mechanics by which loading conditions may be implicated in osteoarthritis. Three clear objectives were stated for the fellowship:
1. to determine viscoelastic properties of cartilage from a range of locations of knee and hip joints;
2. to determine differences in collagen orientation;
3. to culture cell lines of chondrocytes (cells that produce cartilage), on simple alginate scaffolds, and apply a variation of loading conditions.
Frequency dependent viscoelastic properties were found for all cartilage samples tested. The k':k'' ratio increasing with frequency for all samples. Therefore, impulsive loading may induce damage to the underlying cartilage by storing more energy in the tissue. From fatigue testing it was found that failure increased significantly with increases in k':k'' ratio (with increased loading frequencies). This provided experimental evidence that excess energy stored in cartilage predisposes it to failure. Rate alone, independent of load, can be responsible for inducing failure. This finding is important as some clinicians believe that only increases in load can be responsible for damaging cartilage.
Gross morphology showed that cartilage from different joints was maintained at different levels of 'health'. For example, cartilage from certain regions of the knee (e.g. tibial plateau) often showed gross signs of damage. However, cartilage from humeral joints rarely showed any signs of damage. Cartilage displaying limited / no signs of damage was typically thin cartilage with values for k' and k'' that were up to an order of magnitude greater than for cartilage from regions such as the tibial plateau. Although changes in the underlying collagen could be used to assess changes in structure of the cartilage, gross morphological changes correlated to measurements of mechanical properties. For example, cartilage thickness correlated significantly to measurements of k' and k''. This finding is important clinically because scanning methods used (e.g. magnetic resonance imaging (MRI)) enable non-invasive assessment of cartilage thickness, whereas collagen assessment would require invasive biopsies. However, three-dimensional (3D) reconstruction of collagen was applied and found to be particularly useful in assessing surface layer roughness. This has potential applications in texturing replacement cartilage biomaterials.
Cells that produce collagen were cultured on alginate scaffolds. It was feasible to apply a variation of loading conditions to these and determine how loading altered cell proliferation (e.g. via live-dead assays). This demonstrates the feasibility of developing a methodology that enables isolation of individual experimental parameters. This provides a novel approach for further studies into response of collagen producing cells found in a range of connective tissues and the impact this has on the mechanical behaviour of such tissues.
The key original concept explored during this fellowship was that the mechanical loading of articular cartilage is a key factor in its maintenance. The key conclusion is that the type of loading (e.g. high rate impact loading) can predispose the cartilage to degeneration.
Fellowship research
Articular cartilage is a load bearing material found at articulating joints in the body. Articular cartilage also enables smooth joint articulation. Osteoarthritis is associated with degeneration of joints including its articular cartilage. Degeneration of the articular cartilage increases friction during movement making activities such as walking difficult and painful. Degeneration includes damage to the surface of articular cartilage (e.g. collagen) and changes to its mechanical properties.
One limitation in experimental testing of cartilage has been testing at rates associated with physiological loading. This has led to limitations in understanding of the mechanical behaviour of articular cartilage. However, the Biomedical Engineering Research Group (University of Birmingham) has recently developed an experimental approach to test cartilage using dynamic mechanical analysis (DMA) at high loading frequencies. This method has enabled characterisation of viscoelastic properties of cartilage at high loading rates. For example, cartilage properties can be determined at frequencies relevant to gait patterns during walking and impulsive heel strike rates associated with the early onset of lower-limb osteoarthritis.
DMA enables a structure's storage (k') and loss (k'') stiffness to be determined. k defines the energy stored which is available for subsequent elastic recoil, while k'' defines the viscous dissipation of energy. This can be combined with a frequency sweep to determine frequency dependent viscoelastic properties of cartilage. The frequency sweep is vital because it enables changes in the k':k'' ratio with frequency to be determined. Theoretically, this ratio provides a measure of the proportion of energy stored to energy dissipated. In principle this provides a measure of predisposition to fracture, as fracture is induced by excess energy storage.
The aim of the fellowship was to determine how loading conditions affect the maintenance of healthy cartilage. This is a necessary step to subsequently understand the mechanics by which loading conditions may be implicated in osteoarthritis. Three clear objectives were stated for the fellowship:
1. to determine viscoelastic properties of cartilage from a range of locations of knee and hip joints;
2. to determine differences in collagen orientation;
3. to culture cell lines of chondrocytes (cells that produce cartilage), on simple alginate scaffolds, and apply a variation of loading conditions.
Frequency dependent viscoelastic properties were found for all cartilage samples tested. The k':k'' ratio increasing with frequency for all samples. Therefore, impulsive loading may induce damage to the underlying cartilage by storing more energy in the tissue. From fatigue testing it was found that failure increased significantly with increases in k':k'' ratio (with increased loading frequencies). This provided experimental evidence that excess energy stored in cartilage predisposes it to failure. Rate alone, independent of load, can be responsible for inducing failure. This finding is important as some clinicians believe that only increases in load can be responsible for damaging cartilage.
Gross morphology showed that cartilage from different joints was maintained at different levels of 'health'. For example, cartilage from certain regions of the knee (e.g. tibial plateau) often showed gross signs of damage. However, cartilage from humeral joints rarely showed any signs of damage. Cartilage displaying limited / no signs of damage was typically thin cartilage with values for k' and k'' that were up to an order of magnitude greater than for cartilage from regions such as the tibial plateau. Although changes in the underlying collagen could be used to assess changes in structure of the cartilage, gross morphological changes correlated to measurements of mechanical properties. For example, cartilage thickness correlated significantly to measurements of k' and k''. This finding is important clinically because scanning methods used (e.g. magnetic resonance imaging (MRI)) enable non-invasive assessment of cartilage thickness, whereas collagen assessment would require invasive biopsies. However, three-dimensional (3D) reconstruction of collagen was applied and found to be particularly useful in assessing surface layer roughness. This has potential applications in texturing replacement cartilage biomaterials.
Cells that produce collagen were cultured on alginate scaffolds. It was feasible to apply a variation of loading conditions to these and determine how loading altered cell proliferation (e.g. via live-dead assays). This demonstrates the feasibility of developing a methodology that enables isolation of individual experimental parameters. This provides a novel approach for further studies into response of collagen producing cells found in a range of connective tissues and the impact this has on the mechanical behaviour of such tissues.
The key original concept explored during this fellowship was that the mechanical loading of articular cartilage is a key factor in its maintenance. The key conclusion is that the type of loading (e.g. high rate impact loading) can predispose the cartilage to degeneration.