The effects of surgical exposures of dental pulps in germ-free and conventional laboratory rats

The purpose of this study was to observe the pathologic changes resulting from untreated experimental pulp exposures in germ-free rats as compared with conventional rats with a normally complex microflora. The pulp tissues of these rats were exposed by drilling through the occlusal surface of the maxillary right first molar with a carbide round bur mounted in a jeweler's spindle-topped hand mandrel. After varying postoperative time intervals (1 to 42 days), the animals were killed and the appropriate tissues were serially sectioned. By the eighth day, vital pulp tissue remained only in the apical half of the roots in the conventional animals. Complete pulpal necrosis with granulomas and abscess formation occurred in all older specimens. Evidence of repair was uniformly lacking. In contrast, no devitalized pulps, apical granulomas, or abscesses were found in the germ-free animals. Dentinal bridging began at 14 days and by 21 and 28 days was complete, regardless of the angle or severity of the exposure. These results, even in the face of gross food impactions, indicate that the presence or absence of a microbial flora is the major determinant in the healing of exposed rodent pulps.