The larger the amount of overextended material, the greater is the intensity of damage to the periradicular tissues. In spite of being cytotoxic, clinical trials have shown that substances used for irrigation or intracanal medication may have no influence on the occurrence of postoperative symptoms. Most irrigants and medications are cytotoxic to the host tissues.

Chemical irritation includes apical extrusion of irrigants or intracanal medications. Mechanical irritation causing periradicular inflammation includes mainly overinstrumentation and overextended filling materials. All these three phenomena are interconnected, as one is directly dependent on the other. The intensity of pain will depend on several aspects, including intensity of the injury, intensity of tissue damage, and intensity of the inflammatory response. Non-microbial causes are represented by chemical or physical factors that can inflict damage to the periradicular tissues, and thereby can be responsible for the development of interappointment pain. The qualitative factor is more difficult to be controlled. Therefore, the quantitative factor is more likely to be under the control of the practitioner. Steps to prevent the apical extrusion of debris, crown-down techniques, irrespective of whether hand- or engine-driven instruments are used, which extrudes less debris and should be selected for the instrumentation of infected root canals. Virtually all instrumentation techniques promote apical extrusion of debris. This will enhance the nutrient supply to the remaining bacteria within the root canal, which can then proliferate and cause exacerbation of a chronic periradicular lesion. Iatrogenic overinstrumentation promotes the enlargement of apical foramen, which may permit an increased influx of exudate and blood into the root canal. The incidence of postoperative pain in re-treatment cases with periradicular lesions has been demonstrated to be significantly high.įorcing microorganisms and their products into the periradicular tissues can generate an acute inflammatory response, the intensity of which will depend on the quantitative (number) and qualitative (virulence) nature of the extruded microorganisms. Consequently, there would be a transient disruption in the balance between aggression and defense in such a way that an acute inflammatory response is mounted to re-establish equilibrium.

During chemomechanical preparation, if microorganisms are extruded into the periradicular tissues, the host would face a situation in which the host will be challenged by a larger number of irritants than it was before. In asymptomatic chronic periradicular lesions associated with infected teeth, there is a balance between microbial aggression from the infecting endodontic microbiota and the host defenses at the periradicular tissues. Extrusion of infected debris to the periradicular tissues during chemomechanical preparation is one of the principal causes of postoperative pain.

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