Purpose

The purpose of the current study is to evaluate, using a finite element model, the impact that osteochondral allograft cartilage thickness has on contact pressures, and to simulate whether a mismatch of the subchondral bony interface relative to the host-recipient site results in altered contact pressures or biomechanics.
Study Design: Properties of articular cartilage and subchondral cortical bone were incorporated into a finite element model to create a simulation of an osteochondral lesion (diameter: 10mm, height: 10mm, cartilage thickness: 2mm, subchondral bone thickness: 8mm). Five osteochondral plugs were constructed to fill the defect, with cartilage-to-bone ratios ranging from 1:9 to 1:1. The plugs were individually inserted into the lesion and loaded with a static downward force of 5000N. Resultant stresses and displacements were measured.

Results

The 1:1 matched cartilage-to-bone ratio with the recipient site was deemed to be
optimal based on its resultant stress and displacement. A deficiency of 1mm cartilage displaced to a lesser extent than the 1:1 match and consequently endured greater stress per unit of cartilage volume, whereas an excess of 1mm also displayed similar displacement to the 1mm deficiency plug, but had greater cartilage volume and was therefore able to distribute less stress per unit of cartilage volume. An excess of 2mm displaced to a similar extent as an excess of 1mm despite having a greater cartilage volume, but displays a unique pattern of strain. An excess of 3mm underwent similar stresses to the other plugs, although the majority of displacement was seen in the cartilage of the surrounding recipient site as opposed to in the plug itself.

Conclusion

The relationship between the ratio of cartilage-to-bone in an osteochondral allograft and that of its surroundings significantly impacts the distribution of stresses and predilection for micro-motion at the repair site.