The fate of mechanically induced cartilage in an unloaded environment

According to mechanobiologic theories, persistent intermittent mechanical stimulation is required to maintain differentiated cartilage. In a rat model for bone repair, we studied the fate of mechanically induced cartilage after unloading. In three groups of rats, regenerating mesenchymal tissue was submitted to different loading conditions in bone chambers. Two groups were immediately killed after loading periods of 3 or 6 weeks (the 3-group and the 6-group). The third group was loaded for 3 weeks and then kept unloaded for another 3 weeks (the (3 + 3)-group). Cartilage was found in all loaded groups. Without loading, cartilage does not appear in this model. In the 3-group there was no clear ongoing endochondral ossification, the 6-group showed ossification in 2 out of 5 cartilage containing specimens, and in the (3 + 3)-group all cartilage was undergoing ossification. These results suggest a tendency of the cartilage to be maintained also under unloaded conditions until it is reached by bone that can replace it through endochondral ossification.Additional measurements showed less amount of new bone in the loaded specimens. In most of the loaded specimens in the 3-group, necrotic bone fragments were seen embedded in the fibrous tissue layer close to the loading piston, indicating that bone tissue had been resorbed due to the hydrostatic compressive load. In some specimens, a continuous cartilage layer covered the end of the specimen and seemed to protect the underlying bone from pressure-induced resorption. We suggest that one of the functions of the cartilage forming in the compressive loaded parts of a bone callus is to protect the surrounding bone callus from pressure-induced fluid flow leading to resorption.