Choosing an endodontic sealer for clinical use is a decision that contributes to the long-term success of non-surgical root canal treatment (NSRCT) 1) . Sealers are used as a thin tacky paste which functions as a lubricant and luting agent during obturation, allowing the core obturation material, such as gutta-percha points or other rigid materials, to slide in and become fixed in the canal 2,3) . Sealers can fill voids 4) , lateral canals 5) , and accessory canals where core obturation materials cannot infiltrate 6,7) . If the sealer does not perform its function, microleakage may cause NSRCT failure via clinically undetectable passage of bacteria, fluids, molecules or ions between the tooth and restorative material 8,9) . Knowing the qualities and characteristics of an endodontic sealer is critical to determining the best selection and application for each clinical case.Endodontic sealers are categorized by composition based on setting reaction and composition: zinc oxideeugenol, salicylate, fatty acid, glass ionomer, silicone, epoxy resin, tricalcium silicate, and methacrylate resin sealer systems (Table 1). Some novel sealers contain fillers or ceramic powders including calcium hydroxide, mineral trioxide aggregate (MTA), and calcium phosphate; however, they are fundamentally composed of the above sealer matrices. Until recently, many review articles were published within sealer types [10][11][12][13][14][15][16] . However, few reviews have been published that cover all sealer types 17,18) . Therefore, in this comprehensive review, a historical perspective of each sealer type will be discussed first, followed by a description of the properties of all sealer types, such as setting time and solubility, sealing ability, antimicrobial activity, and biocompatibility and cytotoxicity. Sealer attributes such as the rheology 19) , radiopacity 20) , and tooth discoloration [21][22][23] have been shown to be satisfactory and will not be discussed in detail.
CURRENT ROOT CANAL SEALERS AND HISTORY
Chelate formationMany dental luting agents set by way of a chelation reaction, the formation of metal complexes with polydentate (usually organic) ligands 24) . Two of the most common chelates used in dentistry are eugenolates and salicylates. For eugenolates, the setting reaction starts with water that hydrolyzes the zinc oxide to form zinc hydroxide. The zinc hydroxide and eugenol chelate and solidify 25) . For salicylates, the ion is calcium, usually formulated using calcium oxide. Although uncommon, fatty acids have also been used as ligands for chelate sealers, in conjunction with zinc oxide.
