Abstract
INTRODUCTION
Osteoarthritis (OA) can be artificially simulated ex vivo on healthy articular cartilage (AC) samples by use of proteolytic enzymes. In this article we will present preliminary analyses of the physical degradation of AC when subjected to alternating mechanical stresses. Since AC damage due to OA is believed to be mechanically induced, the first step towards the realisation of an improved understanding of degenerative behaviour of AC under physiological loading conditions is to perform ex vivo tests which mimic such conditions at best.
METHODS
Porcine AC was subjected to biochemical stimulation or left as native AC. Biochemical degradation was performed using combinations of trypsin and Matrix Metalloproteinases (MMPs) to induce the loss of proteoglycan and collagen. A comparison of the biochemical and mechanical properties, topography and difference in response to mechanical damage between the digested AC and healthy AC was made using White Light Interferometry (WLI), Atomic Force Microscopy (AFM) and mechanical testing. The mechanical damage was induced by subjecting AC to shear under physiological and non physiological conditions. The AC was mechanically tested in a Phosphate Buffered Saline (PBS) bath. After mechanical testing, biochemical analysis of the collagen and aggrecan content of the tissue and PBS present in the bath during the mechanical test was performed. Collagen content was determined by measurement of the amount of hydroxyproline (HPRO), and aggrecan content by the amount of glycosaminoglycans (GAG). The mechanical test was either performed on healthy (native) AC or on AC which had first been digested.
RESULTS AND DISCUSSION
After mechanical testing, very small collagen damage and a very high ECM damage in the native AC following the mechanical test was observed. This seems to be in line with the development of AC damage during OA; the first part of the AC structure to be affected and damaged is the ECM. The collagen is believed to be more stable and degrades both mechanically and chemically only after the ECM has started degrading. Another possible explanation for this could be the fact that the collagen is able to resist shear stresses very well due to the fibrils being aligned parallel to the surface hence limiting the onset of damage. In the digested AC, both ECM and collagen damage are considerably higher since both parts of the structure have already been partially degraded. Experiments have also shown a dramatic decrease of interstitial fluid pressurisation in the digested samples hence exposing the solid constituent of AC to further degradation during mechanical testing. This could shed light on the nature of the progression of OA.
CONCLUSIONS
This study allows us to better understand damage in AC and its effect on biomechanical, structural, biochemical properties and on the mechanical response of the tissue at physiological conditions. Future work will also include the use of Atomic Force Microscopy in order to characterise the surface and evaluate local mechanical properties using force – indentation curves. Mechanical stimulation of living AC in order to induce biochemical changes in the tissue due to abnormal stresses and strains on the chondrocytes will also be investigated.
