Sound bony ankylosis of the spine almost invariably occurs after pyogenic infection in the intervertebral space. Animal experiments were undertaken to study the mechanism of this process as a possible means of inducing ankylosis of the spine artificially.

A single lumbar interspace in each of seventy-four adult mongrel dogs was treated with an irritant (sodium morrhuate, talc, potassium stearate and oleate, or croton oil), a proteolytic enzyme (sterile human gastric juice, clostridial collagenase, or X-108), or a suspension of bacteria (killed Staphylococcus aureus, killed Neisseria gonorrhoeae, or living Staphylococcus aureus).

Solid intercorporeal fusion was obtained in all the twenty-three animals treated with living Staphylococcus aureus. Only one fusion was obtained in the fifty-one animals treated in other ways, and in this instance there appeared to have been a surgical infection in the interspace.

A single inflammatory stimulus does not appear to be sufficient to induce interbody fusion in the dog's spinal column. A sustained process in which the intervertebral disc and cartilaginous plates are digested by proteolytic enzymes appears to be required to provide the chemical environment in which the proliferation of new bone and fusion can occur.