Correlating material properties to matrix constitution on porcine tibial plateaux

Osteoarthritis (OA) is one of the most common musculoskeletal diseases, mainly affecting people over the age of 60. This disease has multiple symptoms which include wear and loss of articular cartilage and debilitating pain. Its initiation and progression is not well understood and, at the moment, there is no cure. It is important that efforts are made to understand OA because as global life expectancy increases, diseases which affect the ageing population will become more predominant. However, before it is possible to understand what happens to cartilage in the diseased state, the interaction of cartilage structure, mechanical properties and external stimuli in healthy cartilage must be characterised first.
The knee joint is commonly affected by OA. Knee OA is often unicompartmental (UKOA), occurring in either the medial or lateral compartment alone. The location of the lesions in UKOA has been observed to be very repeatable among patients. We believe that this could be explained by variations in cartilage properties over articular surfaces coupled with loading patterns.
The distribution of material properties, glycosaminoglycans(GAG) concentration and cartilage thickness in healthy cartilage on six porcine tibial plateaux were examined. Approximately 1000 measurements were taken of in-situ mechanical properties of articular cartilage-on-bone through indentation using a novel mechanical testing machine on whole specimens. The Whole Articular Surface Indentation Machine (WASIM) possesses 5 degrees of freedom (DOF) and allows the interchange of several probes such as an indenter (hemisphercal with 1.35 mm radius), high resolution laser and ultrasound (US). The three dimensional geometry of articular surfaces were obtained by laser scanning. Routines were developed to analyse this topography, automatically calculate the surface normals and then rotate the specimen relative to the indenter; normal indentation was performed at every test point (displacement controlled, 10 percent per second (pps) strain rate). GAG concentrations in cartilage plugs (4 mm) were obtained using DMMB and an imaging method which combined a cationic contrast agent with fluoroscopy.
Mechanical properties (elasticity modulus) were found to vary between 1 and 40 MPa depending on the spatial location of the test points on the articular surface. The differences in elasticity moduli between regions on the tibial plateaux correspond to the position of the meniscus and load-bearing areas. Central areas on each tibial compartment were found to have the highest stiffness, while peripheral areas, as well as areas uncovered by the meniscus were found to have a lower stiffness. Concentrations of GAG were found to be between 1 and 20 μg/mg depending on the location of test points. Overall, results showed heterogeneous distribution of elastic modulus, GAG concentration and cartilage thickness in terms of their spatial location over the tibial plateaux. Repeatable patterns were observed between all of the six tested specimens. These patterns were correlated to the position of the menisci, which lead to grouping parameters in regions. Region analysis showed inverse correlations ( r ) between GAG concentration and elastic modulus (-0.1<r <-0.6), and direct correlation between GAG concentration and cartilage thickness (0.1<r<0.6) across specimens. Finally, an analysis of the gradient changes in the mechanical properties over the tibial plateaux showed that steep gradients (i.e. relatively large changes in stiffness) occurred in locations that are associated with UKOA lesion locations in human joints.
This work provides insight into the variation, distribution and correlation of mechanical properties, GAG concentration and cartilage thickness in healthy cartilage. Additionally, a testing device which can automatically scan and rotate uneven surfaces to allow normal indentation was introduced. This machine has the potential to measure mechanical properties and biological composition at the same test points in whole specimens, and therefore create distribution maps of different parameters.
Further work needs to be carried out on healthy human articular cartilage to assess whether similar patterns are found. Also, implementation of these methods can be used to assess deterioration at different stages of disease.

Advisor: Gill, H
Hulley, P
Price, A
Ye, H