## 1 Introduction  
The aim of endodontic treatment is to remove bacteria from the root canal system and prevent the migration of microorganisms or their products into the periapical tissues, thus creating an effective barrier. Retreatment or periapical surgery can be required in case of failure of endodontic treatment. Periapical surgery consists of curetage of the infected or inflammatory tissue, excision of the root apex of the infected or damaged root (apicoectomy), root-end cavity preparation (retropreparation), and retrograde placement of a filling material to avoid contact of the root canal system with the periapical tissues [1]. It has been demonstrated that it is sufficient to reset 3 mm of the root tip as 93% of the lateral canals and 98% of the apical branches are removed [2]. The minimal level angle resection technique has resulted in less exposure of the dentinal tubules, thus preventing bacterial leakage and contamination [3]. The optimal root apex cavity has parallel walls and is 3-4 mm deep in accordance with the anatomical contours of the root canal space [2]. The preparation of the root-end is traditionally carried out with the use of tungsten carbide burs or, nowadays, with the use of ultrasonic and laser tips [4]. However, the conventional method may result in some complications including non-parallel cavity walls, access difficulties to the root-end and perforation of the lingual dentin [5]. Ultrasonic tips have significantly reduced the risk of perforation by enabling the preparation of smaller, more centralized and more parallel-walled root end cavities [6]. The placement of retrograde filling material into the apical cavity is to provide apical sealing that prevents residual irritants' movement from the root canal into the periadicular tissues [7]. Because of its high sealing and biocompatibility properties, mineral trioxide aggregate (MTA) is commonly used as a retrograde filling material. Nevertheless, MTA has some negative features like discoloration, application difficulties and long curing time [8]. On the other hand, Biodentine is a tricalcium silicate (Ca3 SiO5) that has been presented as a biomaterial that can be used as a substitute for dentin. Biodentine offers advantages such as a shorter hardening time than MTA, no discoloration, easy manipulation, and similar compressive strength to dentin [9]. Additionally, glass ionomer cement is a type of retrograde filling material that has been used for many years. Their most important advantage is that they can chemically bind to tooth structures. The conventional glass ionomer cements possess sufficient biological compatibility [10]. Due to leaking retrograde fillings, inadequate coronal restorations and incompletely prepared isthmuses, surgical attempts to keep the remaining micro-organisms in the root canal system and just to prevent communication with the periapical tissues may fail [11]. With the aid of an operating microscope, complete preparation of the entire root canal system, use of effective disinfection and irrigation protocols, and hermetic obturation of all canals, accordingly high success rates can also be expected in orthograde retreatment after failed or unsuccessful apical surgery [12, 13]. Orthograde retreatment should always be the first option after failed root canal treatment instead of apical surgery. If a patient experiences post-treatment disease in a tooth that underwent apical surgery instead of orthograde retreatment, the endodontist should still formulate a treatment plan [14]. However, available data on the results of non-surgical retreatment after failed apical resection are limited. There are few case reports describing a non-surgical retreatment approach following failed apical resection [15, 16]. The purpose of this study is to examine retrograde filling materials' bonding ability following retreatment in cases of the recurrence of apical periodontitis after apical resection using different techniques and methods.