The results of these few experiments may be summarized as follows: In none of the cases did injury to the knee joint, in the form of removal of the articular cartilages and a thin layer of the underlying bone, cause any apparent loss of efficiency of the joint.
In all the cases there was regeneration, to greater or less extent, of bone and cartilage, except in the case where infection was present.
The bone apparently regenerated from the osteogenetic tissue in the marrow spaces.
The regeneration of cartilage apparently took place in the following manner: (1) degeneration of a few cells at the border of the injury: (2) proliferation and hypertrophy of the next layer of cells; (3) out-growth of cartilage from the remaining cartilage, periosteum, and tissue of the marrow spaces, (4) transformation of the fibrous tissue into fibro-cartilage, the process being apparent first in the vicinity of the old cartilage and at the junction of the fibrous tissue and bone, (5) and finally transformation of the fibro-cartilage into hyaline cartilage.
In some cases there was formation of exostoses, and this was very marked in one case. In these cases there were large areas of apparently isolated cartilage, and bone formation seemed to be progressing from them. It may be that these cartilage foci were, originally, fragments that were displaced during the operation. The bone formation in the exostoses was all of the cancellous variety.
Finally, it may be stated that bone and cartilage of the condyle of the femur of rabbits do regenerate after serious injury, to such an extent that an adequate bearing surface is again formed.
In conclusion, I wish to express my sincere thanks to Dr. Leonard W. Ely, Associate Professor of Surgery and Chief of the Department of Orthopedics, for choosing so interesting and important a subject and supplying the necessary material; for his advice, instruction, and supervision during operations, autopsies, and subsequent work; and for the many necessary stimuli given during the course of the work.