Although a calcium hydroxide paste was placed in the prepared canals, the number of positive canals had increased in the period between visits. However, the number of microorganisms had only increased to 0.93% of the original number of CFU (sample 1). It is concluded that a calcium hydroxide and sterile saline slurry limits but does not totally prevent regrowth of endodontic bacteria.

Within the limitations of this study, no significant differences in healing of periapical radiolucency was observed between teeth that were treated in one visit (without) and two visits with inclusion of calcium hydroxide for 4 weeks. The presence of a positive bacterial culture (CFU<10(2)) at the time of filling did not influence the outcome of treatment.

Two sets of teeth with apical periodontitis were collected at different geographic locations to study the identity of bacteria left in the root dentinal tubules. Root dentin of 20 of these teeth was cultured from three locations between pulp and cementum (A, B, and C). In addition dentin from eight teeth was examined histologically. Using the culturing technique bacteria were found in 77% of the dentin samples from set 1 (Amsterdam) and in 87.5% of the dentin samples from set 2 (Glasgow). At greater distance, in layer C, from the pulp bacteria were found in 62% (13 of 21) of the dentin samples. Twenty-three percent (3 of 13) of set 1 and 25% (2 of 8) of set 2 contained >50,000 colony-forming units/mg of dentin in layer C. In layers closer to the pulp higher numbers of anaerobic bacteria and gram-positive rods were found, as well as a larger number of bacterial species. Histological sections showed bacterial penetration in dentinal tubules in 5 of 8 teeth. In the other three teeth where the colony-forming units/mg recovered was <10,000, no histological signs of tubule penetration was seen. It seems clear that, in more than half of the infected roots, bacteria are present in the deep dentin close to the cementum and that anaerobic culturing of dentin is more sensitive than histology to detect these bacteria.

In this article the question of the relevance and consequences of bacteria remaining in the tubules of root dentine after cleaning and shaping of the root canal space is addressed. A second aim is to discuss the necessity of clinical measures taken to eradicate those bacteria that are presumed to survive there. The available clinical and experimental evidence supports the use of antibacterial dressings in cases where the root canal space remains temporarily unobturated after removal of necrotic and infected pulp tissue. There is no evidence, however, that special measures should be taken to kill the bacteria in the dentinal tubules. Should time permit, a sound obturation technique immediately following the cleaning, shaping and disinfection phases allows the remaining bacteria in the tubules to be either inactivated or prevented from repopulating the (former) canal space. In the vast majority of cases, those bacteria appear not to jeopardize the successful outcome of root canal treatment.

These results indicate that endodontic pathogens do not occur at random but are found in specific combinations.