Stress in the proximal femur (Compression not tension!)

Mechanics of cartilage on bone

The stiffness of articular cartilage increases dramatically with increasing rate of loading, and it has been hypothesised that increasing the stiffness of the subchondral bone may result in damaging stresses being generated in the articular cartilage. The juxtaposition of these tissues in a joint inevitably means that changes in one will affect the other. Surprisingly, however, little is understood of the nature of these effects or which are the most important parameters governing this interplay. To begin to investigate this, a parametric finite element model of an idealised joint has been developed (Fig 1). The model incorporates layers representing articular cartilage, calcified cartilage, the subchondral bone plate and cancellous bone. Taguchi factorial design techniques (1), employing a two-level full-factorial and a four-level fractional factorial design, were used to vary the material properties and thicknesses of the layers over the wide range of values found in the literature. So far we have examined the effects on the maximum values of von Mises stress in each of the tissues. Not surprisingly, the stiffness of the cartilage turned out to be the main factor that determined the stress in the articular cartilage. This, and the thickness of the cartilage, also had the largest effect on the stresses in all the other tissues with the exception of the subchondral bone plate, in which stresses were dominated by its own stiffness. Perhaps a bit surprisingly, the stiffness of the underlying subchondral bone had no effect on the stresses generated in the cartilage.

Two consequences arise from these results. The first is that stiffening of the subchondral bone, either the bone plate or the cancellous bone, has very little effect on the stresses generated within the cartilage. Whilst not precluding the involvement of subchondral bone in osteoarthritis, this does not support the hypothesis that cartilage damage may arise by this mechanism. Secondly, the softening and increased thickness seen in the early stages of OA should reduce the stresses generated for a given load in all the tissues and would appear, therefore, to be protective. However, a concomitant reduction in strength and / or fatigue resistance could be important, but these were beyond the scope of this study.

References

1) Dar F, Meakin JR, Aspden RM. Statistical methods in finite element analysis. J Biomech 2002; 35:1155-1161.

2) F. H. Dar and R. M. Aspden. A finite element model of an idealised diarthrodial joint to investigate the effects of variation in the mechanical properties of the tissues. Proc.Instn.Mech.Engrs.[H], J.Eng.Med. 217 (5):341-348, 2003.