Histologic studies on the fate of soluble collagen injected into canine vocal folds

Collagen is an essential component of the functionally important layered structure of the vocal cord. Soluble bovine collagen appears to be suitable for injection into vocal folds to correct glottic insufficiency. Clinical trials and preliminary laboratory studies with the canine larynx indicate that injectable collagen is safe, effective, easily injected, and well-tolerated. Bovine collagen softens scar tissue, attracts the ingrowth of host fibroblasts, and allows the eventual replacement of the implant by new host collagen. This study examines the fate of collagen injected into canine vocal folds. Two forms of soluble bovine collagen, differing only in the amount of chemically induced cross-linkage, were injected into vocal folds of dogs; the animals were then killed at 12-week intervals for 1 year. Gross and histologic examination showed that both substances tended to be resorbed when injected deeply in the vocalis muscle, but persisted when injected in the plane of the vocal ligament - a site normally composed of dense collagen. The invasion of the implant by active host fibroblasts and the secondary deposition of new host collagen were evident in histologic sections. Examination with polarized light confirmed the presence of birefringence characterizing host collagen. The invasion of fibroblasts and deposition of new collagen in the implant were seen with electron microscopy. Cellular invasion seemed to progress more rapidly in the cross-linked preparation. Injected collagen did not evoke a foreign body reaction in any specimen studied, and was well-tolerated during replacement by host tissue.