Histological examination of paraformaldehyde-exposed gutta-percha implanted in rats.

Adsorption of some paraformaldehyde was noted in a previous study evaluating its sterilizing effect on gutta-percha (GP). This study examined histologically the effect of this adsorption when formaldehyde-exposed GP was implanted into the subcutaneous connective tissue of rats. GP implants were prepared in cylinder shape using a template designed to standardize size. Fifty GP cylinders were exposed to paraformaldehyde for 7 days before being implanted, while 50 others were implanted without exposure. Fifty rats had two implant sites prepared, at dorsal-interscapular and dorsal-caudal regions. Sham operations were performed on 10 rats to examine the effect of the surgery itself. The animals were killed at 4, 7, 14, 28, and 56 days. There was a significant difference between the two categories of implants only at 7 days, with the GP specimens without paraformaldehyde exposure showing more inflammation than GP with paraformaldehyde specimens (p = 0.043). Although the GP-alone specimens showed greater initial inflammation, both groups recovered in the same time period. One of the GP specimens with paraformaldehyde still showed a moderate/severe response at 56 days, whereas all of the GP-alone specimens showed only none/mild responses. The GP examined appeared to cause more inflammation than was expected.