M. Aeinehchi, B.Eslami, M. Ghanbariha & A. S. Saffar.
Department of Endodontics, Azad University School of Dentistry, Tehran, Iran.
Department of Oral and Maxillofacial Pathology, Shahid Beheshti School of Dentistry, Tehran, Iran.
Department of Oral and Maxillofacial Pathology, Isfahan School of Dentistry, Isfahan, Iran.
Shahid Beheshti School of Dentistry, Tehran, Iran.

Introduction.
The procedure of pulp capping relies primarily on the ability of pulpal tissue to heal. Various factors affect this process including age, periodontal condition and stage of root formation (Camp et al. 2002). Procedural factors such as the size of exposure, its nature (traumatic, mechanical or carious) and microbial contamination of the site have also been described as determinants of the success of pulp capping (Camp et al.2002).However, the importance of these factors has been challenged (Stanley1989).
A wide array of materials have been used for pulp capping, but calcium hydroxide remains the standard (Camp et al. 2002). Subsequent to pulp capping with this conventional alkaline agent, the adjacent pulp tissue is usually completely deranged and distorted, forming a zone of obliteration. A weaker chemical effect on the subjacent, more apical tissue results in a zone of coagulation necrosis. This layer causes sufficient stimulation to the vital pulp tissue to respond (Stanley 1989). Some question the superiority of calcium hydroxide, because of its degradation over time, tunnel defects through dentinal bridges under it and poor sealing properties (Schuurs et al. 2000).
Mineral trioxide aggregate (MTA ) has been used in pulp-cap procedures in animals, demonstrating remarkable success compared with calcium hydroxide (Abedi et al.1996, Pitt Ford et al.1996, Junn et al.1998, Faraco & Holland 2001).However, no studies have so far evaluated MTA as a pulp-capping agent in humans. The purpose of this study was to compare the properties of MTA with calcium hydroxide in human tooth pulp-capping treatment.

Materials and methods.
Twenty-two intact maxillary third molars free of restorations, from subjects between 20 and 25 years of age which required extraction were selected. Percussion and pulp-sensitivity tests were performed and radiographs were examined to assess pulpal health. Only those teeth that could be extracted without surgery and with minor trauma were included. All subjects were informed of the possible complications of the procedure. Ethical approval was sought and granted. Informed consent was obtained from all subjects.
Lidocaine was used to anaesthetize the teeth. They were subsequently isolated with rubber dam and a conventional Class I cavity of approximately 1mm width was prepared on the occlusal surface. A standard exposure of 0.5 mm diameter was created in the pulp with a high-speed handpiece and 005 round bur by an operator who did not have prior knowledge of the pulp-cap agent to be used. No salivary contamination was allowed. Homeostasis was gained by irrigating the cavity with sterile saline and application of small pieces of cotton before capping with calcium hydroxide (Dycal1, L.D. Caulk, Milford, DE, USA) or MTA (ProRoot1, Dentsply Tulsa, Tulsa, OK, USA) on the contralateral third molars of the same subject.
Using a stiff metal spatula, MTA powder was mixed with saline in a 3:1ratio and then placed over the exposure site with a plastic instrument. Calcium hydroxide paste was mixed and applied to the exposure site with a plastic instrument. Zinc oxide-eugenol (ZOE) cement at a thickness of 2 mm was used over both materials. Amalgam served as the filling material.
A total of 14 teeth were extracted after periods of 1week (two molars),2 months (three molars), 3 months (five molars), 4 months (two molars) and 6 months (two molars). The apex of each tooth was immediately sectioned to allow penetration of 10% formalin for tissue fixation purposes. The teeth were kept in formalin for 1week and subsequently decalcified in10% formic acid for 2-3 weeks. Samples were exposed to ascending concentrations of alcohol (70, 90 and 100%), cleared in methyl salicilate and submerged in paraffin for 12 h. Six-micrometre sections were cut in a buccolingual direction every100 mm. Haematoxylin and Eosin (H&E) staining was used.
An oral and maxillofacial pathologist studied the images taken from slides under a microscope without knowledge of the source of specimens. Hyperaemia and inflammation were recorded under a magnification of x312.5 and categorized as shown in Table 1.

Table 1. Categorization of inflammation and hyperaemia.

Dentine bridge thickness was measured to 10 mm accuracy using computer software (Adobe Photoshop 6). The true thickness of the dentine bridge formed was registered in millimetres. Every sample was also evaluated for severity, type and site of inflammation, presence of necrosis, hyperaemia, calcification other than in the area of the bridge and odontoblastic layer.