N. Economides, O. Pantelidou, A. Kokkas & D. Tziafas
Department of Endodontology, School of Dentistry, Aristotle University of Thessaloniki, Greece.

Introduction.
Recent research in endodontics has been focused on the ability of various techniques and filling materials to enhance optimal tissue healing. Root-end filling materials designed to stimulate hard and soft tissue repair in the periradicular tissues are highly recommended (Friedman1991, Guttman & Harrison1994).Mineral trioxide aggregate (MTA) is a relatively new material with numerous clinical applications in endodontics (Torabinejad & Chivian1999). It has been reported to stimulate healing of the periradicular tissues to almost normal preoperative status, when used as a root-end filling material. Experimental studies in infected dog teeth and healthy monkey teeth showed less inflammation with the MTA than with amalgam (Torabinejad et al.1995, Torabinejad et al.1997), whilst formation of a cementum-like matrix was also observed. Nevertheless, the mechanism controlling periradicular tissue healing and hard tissue repair needs to be elucidated further.
The aim of the present study was to evaluate the short-term response of periradicular tissues to MTA, when used as a root-end filling material in optimal tissue conditions, i.e. root-end resection in healthy dog teeth.

Materials and methods.
Two healthy dogs, 15 and18 months of age, were used in the present study. The animals were anaesthetized with pentothal (20 mg kg_1body weight) and intubated with a cuffed endotracheal tube before beginning the experimental procedures. All procedures were conducted in accordance with the protocol outlined by the General Secretariat of Research and Technology in Greece regarding the recommended Standard Practices for Biological Investigations.
Twenty-four root canals of single- and double-rooted premolars were divided equally into four groups that were treated sequentially so that observation periods of 1week (right teeth of the first animal), 2 weeks (right teeth of the second animal), 3 weeks (left teeth of the second animal) and 5 weeks (left teeth of the first animal) were created.
The teeth were opened through occlusal access cavities and the pulps were removed with barbed broaches. The root canals were then cleaned and instrumented to the apical delta with the step-back technique to a size 40 master apical file. The canals were irrigated with 1% sodium hypochlorite before the use of each instrument. After instrumentation, the canals were dried with paper points and obturated with laterally condensed gutta-percha and Roth 801root canal sealer (Roth International Ltd, Chicago, IL, USA). The access cavities were filled with amalgam.
At the same session, a full-thickness triangular muccoperiosteal flap was raised to gain access to the periradicular tissues of the teeth. The cortical bone was removed with a size 6 round bur in a low-speed hand piece using sterile saline spray. After apical resection, root-end cavities were prepared to a depth of 3 mm using a size 2 round bur followed by a size 34 inverted cone bur in a low-speed hand piece with the use of copious sterile saline spray. Eight root-end cavities were filled with IRM (Dentsply Detrey GmbH, Konstanz, Germany) and 14 with ProRoot MTA (Dentsply Simfra, Paris, France). The materials were mixed according to the manufacturer’s instructions and placed in the cavities using an MTA carrier. Light pressure was then applied to the MTA using wet cotton pellets. The surgical flaps were sutured with absorbable gut sutures, and the animals maintained on standard diet.
The animals were killed using an overdose of pentothal and the jaws were immediately dissected and fixed in 10% buffered formalin solution. Radiographs were taken of all operated teeth. Blocks, including one root and the surrounding alveolar bone each, were cut after demineralization in 5%trichloroacetic acid and the specimens were embedded in paraffin. Longitudinal sections, 7 mm thick passing through the root apex, were stained with haematoxylin and eosin. The tissue responses were evaluated by light microscopy at postoperative periods of 1, 2, 3 and 5 weeks.
Two of the canals, filled with MTA for 2 and 3 weeks, were analysed with scanning electron microscopy (JEOL JSM-840A). After fixation, the soft tissue was removed by mechanical means. The specimens were immersed in 1% sodium hypochlorite solution for 4 h and dehydrated in alcohol. The surface structure and composition of the resected roots, MTA and the newly formed calcified deposits were evaluated.