## 4 Discussion  
This clinical study was conducted in teeth with a healthy uninflamed pulp using two different capping materials ProRoot MTA and RetroMTA, which are hydraulic biocompatible cements [18] with similar biological effects [19]. The null hypothesis was rejected for the parameters of hard tissue formation, inflammatory pulp response, and pulpal findings, except for intensity and giant cells number after 30 days, and for continuity after 60. As for the hard tissue bridge, the RetroMTA results of this study showed better results regarding the parameters continuity, morphology, and localisation, while ProRoot MTA performed better regarding the parameter thickness. However, after 60 days, the only significant parameter was continuity, in favour of ProRoot MTA. The complete bridge formation is expected to take place 60-90 days after pulp capping [20; 21]. The findings of this study are in accordance with one histological study, which evaluated direct pulp capping performance in dog's teeth, where no significance could be found between the two test pulp capping materials RetroMTA and ProRoot MTA, where a hard tissue bridge was formed in direct contact to the vital pulp [18]. [PERSON] et al. also sustained high success rate up to 1 year, with no significant differences between outcomes treated with ProRoot MTA (96.0%) and RetroMTA(96.0%) [22]. The dentine bridge, in the form of a calcified barrier, is usually considered a favourable response to capping therapy. In conjunction to vital signs, it supports that the pulp condition is healthy, leading to physiologic dentin deposition. [PERSON], [PERSON] et al. reported that a calcified barrier was found in almost 70% of teeth [23], compared with 55%-64% in previous studies [24; 25]. The thickness of the dentine bridge in the 60 days sample of RetroMTA is less than 30 days. It is proof that the thickness of the dentinal bridge may vary depending on the location and angle of sectioning; therefore, it is more critical to evaluate the condition of pulp beside the bridge thickness, continuity, and quality, while assessing the success of the direct pulp capping material. The initiation is induced throughout initiation of reparative dentinogenesis in the case of MTA [26; 27]. Pulp cells are aligned in proximity to crystals, built in a homogenous zone along the pulp-MTA interface. After MTA hardening, calcium oxide is formed, which can react with tissue fluids to produce calcium hydroxide [28]. After contact with pulp tissue, MTA presents some structures that are similar to calcite crystals, which attract fibronectin, which is generally responsible for cellular adhesion and differentiation [29]. Some studies found necrotic tissue next to the hard tissue bridge, which is assumed to be caused by MTA throughout coagulation in contact with pulp connective tissue [26]. The randomized controlled trial, conducted by [PERSON] et al. [23], using three different MTA materials (ProRoot, Ortho, and RetroMTA), stated that the partial pulpotomy performed in permanent teeth showed favourable results, while clinical and radiographic results showed no difference after one year. In contradiction, one study found significantly worse results after 8 weeks using RetroMTA concerning pulp morphology and hard tissue bridge thickness compared to ProRoot. The hard tissue was described as a tubular reparative dentine. However, one study reported a lack of mineralized tissue and rather regular dentin with parallel tubules and rather amorphous and irregularly calcified tissue with lacunae of necrotic debris [23]. This finding may explain the changes after 60 days, and the less favourable performance of RetroMTA after 60 days. The quality of the hard tissue bridge relates to the pulpal health state after application of MTA and the pulp capping procedure [30; 31]. For tooth survival, the pulp status and inflammatory response after pulp capping material application is more important compared to the thickness of the dentine bridge.