**1. Introduction**

External root resorption (ERR) happens when the periodontal ligament of the cementum is either destructed or removed [1]. Damage to the cementum uncovers the root surface to osteoclasts that can resorb dentin. With additional stimulation provoked by sulcular bacteria in the neighboring area, root resorption constantly progresses [2]. ERR of a permanent tooth is generally unfavorable because it may cause irreversible damage and ultimately loss of the tooth. However, in its early stages, ERR can be stabilized by repairing the cementum with calcium silicate-based cement [3].

Calcium silicate-based cements are hydraulic materials consisting of tricalcium silicate, dicalcium silicate, and tricalcium aluminate [4,5]. The first tricalcium silicate-based cement was mineral trioxide aggregate (MTA), which is a derivative of Portland cement. The physical, chemical, and biological properties of MTA have been studied for decades, and it produces favorable results when applied to direct pulp capping, regenerative endodontic procedure, apical retrograde filling, and repair of ERR or perforations [6]. Tricalcium silicate enhances proliferation and differentiation of dental pulp cells [7–9]. However, ProRoot MTA (Dentsply Tulsa Dental Specialties, Tulsa, OK, USA) contains heavy metal components such as bismuth oxide [5]. It also has a long setting time and handling difficulty, and can discolor the tooth and gingiva [10,11]. Novel calcium silicate-based cements have been produced overcome these shortcomings.

Biodentine (Septodont, Saint-Maur-des-Fossés, France) is composed mostly of tricalcium silicate, zirconium oxide, and calcium carbonate powder, which are mixed with a supplied solution that includes calcium chloride [12,13]. The reduced setting time compared to MTA is achieved by diminishing the particle size and adding calcium chloride to expedite the reactions [13–15]. The substitution of bismuth oxide with zirconium oxide may also play a role in reduced setting time, because this component has been reported to expedite the primary hydration reaction [13]. Previous studies of this material's effects on dental pulp stem cells demonstrated its biocompatible ability, odontoblast differentiation ability, and mineralization potential [12,16]. Endocem Zr (Maruchi, Wonju, Korea) and RetroMTA (BioMTA, Seoul, Korea) were developed to cause less tooth discoloration compared to ProRoot MTA [17]. These materials have a reduced setting time compared to ProRoot MTA and are easy to handle [18,19]. Bismuth oxide is replaced by zirconium oxide as a substitute radiopacifier [17]. RetroMTA is composed of fine hydrophilic particles that do not originate from Portland cement [19].

To the best of our knowledge, no study has evaluated the biocompatibility and calcium nodule formation ability of various calcium silicate-based cements on human gingiva-derived stem cells (GDSCs). Therefore, the aim of the present study was to evaluate the cytotoxic effects of four calcium silicate-based cements on GDSC compared to that of intermediate restorative material (IRM; Caulk Dentsply, Midford, DE, USA). IRM is a commonly used temporary filling material that is highly toxic to human stem cells [18,20]; therefore, we used IRM as a negative control. We also evaluated the mineralization potential of the four calcium silicate-based cements on GDSCs.
