mandatory, for increasing the probability of the endodontic-treatment success. however, these temporary filling-materials create coronal seals which only remain hermetic during less than 30 days in average (mainly because of the bacteria the saliva contains). some temporary filling-materials may remain hermetic during 40–70 days. however the estimated standard-deviations of these higher average-durations are important and their computations used observations from dye-based tests, which are less reliable than saliva-based tests. final restoration molars and premolars that have had root canal therapy should be protected with a crown that covers the cusps of the tooth. this is because the access made into the root canal system removes a significant amount of tooth structure. molars and premolars are the primary teeth used in chewing and will almost certainly fracture in the future without cuspal coverage. anterior teeth typically do not require full coverage restorations after a root canal procedure, unless there is extensive tooth loss from decay or for esthetics or unusual occlusion. placement of a crown or cusp-protecting cast gold covering is recommended also because these have the best ability to seal the treated tooth. there is insufficient evidence to assess the effects of crowns compared to conventional fillings for the restoration of root-filled teeth, decision of restoration should rely on the clinical experience of the practitioner and the preference of the patients. if the tooth is not perfectly sealed, the canal may leak, causing eventual failure. a tooth with a root canal treatment still has the ability to decay, and without proper home care and an adequate fluoride source the tooth structure can become severely decayed (often without the patient's knowledge since the nerve has been removed, leaving the tooth without any pain perception). thus, non-restorable carious destruction is the main reason for extraction of teeth after root canal therapy, accounting for up to two-thirds of these extractions. therefore, it is very important to have regular x-rays taken of the root canal to ensure that the tooth is not having any problems that the patient would not be aware of. endodontic retreatment endodontic treatment may fail for many reasons: one common reason for failure is inadequate chemomechanical debridement of the root canal. this may be due to poor endodontic access, missed anatomy or inadequate shaping of the canal, particularly in the apical third of the root canal, also due to the difficulty of reaching the accessory canals which are minute canals that extend in from the pulp to the periodontium in a random direction. they are mostly found in the apical third of the root. exposure of the obturation material to the oral environment may mean the gutta-percha is contaminated with oral bacteria. if complex and expensive restorative dentistry is contemplated then ideally the contaminated gutta percha would be replaced in a retreatment procedure to minimise the risk of failure. the type of bacteria found within a failed canal may differ from the normal infected tooth. enterococcus faecalis and/or other facultative enteric bacteria or pseudomonas sp. are found in this situation. endodontic retreatment is technically demanding; it can be a time-consuming procedure, as meticulous care is required by the dentist. retreatment cases are typically referred to a specialist endodontist. use of an operating microscope or other magnification may improve outcomes. currently, there is no strong evidence favoring surgical or non-surgical retreatment of periapical lesions. however, studies have reported that patients experience more pain and swelling after surgical retreatment compared to non-surgical. when comparing surgical techniques, the use of ultrasonic devices may improve healing after retreatment. application of nanomotor implants have been proposed to achieve thorough disinfection of the dentine. there is no evidence that the use of antibiotics after endodontic retreatment prevents post-operative infection. instruments and equipment used