It was concluded from these cases that when a spine fusion is unquestionably solid and fairly massive, there is little increase in length of the fused area. The small increase that most of the cases we studied showed could be accounted for by magnification and other technical factors, but it is impossible to rule out a small amount of growth. The slight decrease in the kyphos in two of our cases suggests some bending of the fusion mass. No definite pseudarthroses could be seen on the roentgenograms but the presence of one or more pseudarthroses could not be ruled out. Microscopic and transient pseudarthroses are considered by us to be the most likely mechanism by which any real increase in length or any true change in angulation occurs. In our experimental studies⁹, microscopic losses of bone continuity in transepiphyseal bone grafts in the distal femora of young rabbits were demonstrated. These defects were of such a nature that they could not be demonstrated by standard clinical or roentgenographic methods.

In our opinion, the end-result study of Hallock and his associates is valuable in that it indicates, from the practical, clinical point of view, what will happen to the average patient after spine fusion in early childhood. However, their data and the observations of the other authors previously mentioned convey an inaccurate and perhaps unintentional impression that considerable growth occurs in a solidly fused spine segment. It would be unfortunate, we believe, to allow this impression to persist since surgeons not familiar with all that is known about growth of the spine after spine fusion might be falsely encouraged on the basis of published data to perform longer and more massive spine fusions in young children. Our study suggests that a long, massive, and completely solid fusion in early childhood will impair spine growth to a significant degree.

We believe that growth of a fused segment of the spine can occur only at the ends of the segment or at the site of gross or microscopic defects in the fusion plate. Pseudarthroses in spine fusions in children are much more frequent, in our opinion, than is generally suspected because of the tension forces exerted by the growing epiphyseal cartilages, as well as the usual stresses caused by motion. These pseudarthroses or stress fractures may be microscopic or grossly visible; they may occur spontaneously at any time and heal spontaneously. The more massive the fusion plate, the less chance there will be that it will break down under the stress of growth and motion. Finally, we believe that the laws that govern bone growth in general apply to the bone of spine fusions. There is in our opinion no such thing as interstitial growth of bone.