Introduction - Materials and methods.
T. Lambrianidis, A. Kapalas & M. Mazinis Department of Endodontology, School of Dentistry, Aristotle University of Thessaloniki, Thessaloniki, Greece.Introduction.
External cervical root resorption has been associated with intracoronal bleaching of discolored endodontically treated teeth (Harrington & Natkin 1979, Lado et al. 1983, Goon et al. 1986, Friedman et al. 1988), with the reported incidence of root resorption ranging from 0 to 6.9% (Friedman et al. 1988, Holmstrup et al. 1988). Preliminary reports that related root resorption to trauma or to the use of heat (Harrington & Natkin 1979) have been dismissed by studies reporting root resorption in teeth without history of trauma (Lado et al. 1983, Friedman et al. 1988, Goon et al. 1986), and cases in which heat was not utilized (Friedman et al. 1988, Goon et al.1986).
The exact mechanisms by which bleaching agents leak to the periapical tissues and initiate the resorptive process are not fully understood. It has been shown that dentine is permeable to hydrogen peroxide (H2O2) (Fuss et al.1989, Rotstein1991a) and the amount of its diffusion may reach up to 82% of the total amount applied, depending on morphologic and chemical variations as well as differences in the remaining dentine thickness (Rotstein 1991a). The seepage of H2O2 is significantly enhanced by the presence of cementum defects at the cemento-enamel junction (CEJ), frequently found in all groups of teeth and in different areas of the same tooth (Rotsteinet al.1991b). Severalauthors (Harrington&Natkin 1979, Rotstein et al. 1991c) have claimed that H2O2 penetrates through the dentinal tubules into the surrounding medium causing cellular and tissue destruction, thus initiating an inflammatory process that may be followed by root resorption. Bacterial intervention through the gingival sulcus or the pulp chamber may be a contributing factor in the development of root resorption (Harrington & Natkin 1979, Rotstein et al. 1991c). It has also been suggested that bleaching agents denature dentine exposed at the CEJ, which then acts as an alien tissue that may trigger an immunological response (Lado et al.1983). This hypothesis is supported by the fact that H2O2 alters the chemical structure of dentine and cementum making them susceptible to resorption (Rotstein et al. 1992a). A more recent study (Dahlstrom et al. 1997) indicated that H2O2 induces pH changes and generates hydroxyl radical, an extremely reactive and toxic chemical species that can degrade components of connective tissue, particularly collagen and hyaluronic acid. This may be one of the mechanisms underlying periodontal tissue destruction and root resorption, following intracoronal bleaching. Bleaching agents have been reported to cause a pH decrease in the microenvironment of the cervical periodontal ligament, thus enhancing root resorption (Kehoe 1987). However, reports of the pH changes taking place during intracoronal bleaching are contradictory (Kehoe 1987, Fuss et al.1989).
In all these cases, no barrier had been placed between the endodontic filling material and the pulp chamber (Harrington & Natkin 1979, Lado et al. 1983, Friedman et al.1988, Goon et al. 1986, Holmstrup et al.1988). Since dentinal tubules are oriented incisally, placing a protective base material at the lower level of the CEJ could reduce the leakage of H2O2 to the periodontal tissues (Rotstein et al.1992b, Brighton et al.1994). Different protective base materials have been compared. No significant differences were found between them, although Cavit (ESPE, Premier Sales Corp., Norristown, PA, USA), zinc oxide-eugenol cements and the intermediate restorative material (IRM), type 3 (Caulk Dentsply, Milford, DE, USA) were shown to be slightly superior (Rotstein et al. 1992b, Brighton et al. 1994). It has been demonstrated that a 2-mm barrier should be placed at the CEJ level, in order to reduce the radicular penetration of H2O2 (Rotstein et al.1992b). Tronstad et al. (1981) suggested that calcium hydroxide (Ca(OH)2) should be placed intracoronally after the removal of the bleaching agent to establish an alkaline pH at the root surface, which may inhibit root resorption.
It has also been suggested that a layer of Ca(OH)2 be placed as a biological seal in direct contact with the root-canal obturation material before placing the barrier (Baratieri et al. 1995). This layer may maintain an alkaline pH in the CEJ area that could neutralize H2O2 during bleaching procedures (Baratieri et al. 1995). However, there are no available data in support of this statement.
The aim of this study was to determine whether a reduction in the radicular penetration of H2O2 during thermocatalytic bleaching took place when a layer of Ca(OH)2 was placed as a supplementary barrier to the protective base material. Materials and methods.
Twenty-eight intact single-rooted human premolars freshly extracted for orthodontic reasons from young adults were placed in formosaline. The soft tissues covering the root surface were gently removed with a sterile gauze soaked in 5.25% sodium hypochlorite.
Standard access cavities were prepared and the root canals located. Coronal flaring was performed with Gates-Gliddendrills sizes 1, 2 and 3 (Premier Dental Products Co., Norristown, PA, USA). The root canals were instrumented using K-Flex files (Kerr Manufacturing, Romulus, MI, USA) in a rotational motion to a master apical file size 30. A 5.25% sodium hypochlorite solution was used as an irrigant and was delivered with a 27- gauge needle. The canals were obturated with guttapercha (Kerr Manufacturing, Romulus, MI, USA) and Rothâ€™s 801 sealer (Roth International Co., Chicago, IL, USA) by means of cold lateral condensation. The coronal third of the gutta-percha was removed 2.5 mm short of the CEJ with hot pluggers. Remnants of gutta-percha and sealer were removed from the access cavity with a cotton pellet soaked in 90%alcohol and a round carbide bur rotated at a low speed. The pulp chamber was rinsed thoroughly with distilled water.
The external root surface including the apical foramen and the apical third of the root were covered with wax. In each tooth, both wax and cementum were removed at the CEJ level, with a round carbide bur at a low speed, in four sites, i.e. mesially, distally, buccally and lingually to simulate standardized defects. The smear layer of the artificial defects (depth 0.5 mm, diameter 1mm) was removed with 17% EDTA and the exposed dentine surface was rinsed with distilled water. All the remaining surfaces were covered with two layers of nail varnish.
Experimental teeth were divided into four groups of seven teeth each: A, B, C and D(Table 1). The intracoronal barriers were placed according to the Steiner-West technique (Steiner & West 1994). In group A, a Ketac-Cem glass-ionomer cement (ESPE-Premier Sales Corp., Norristown, PA, USA) barrier was placed over the guttapercha at the CEJ level. In group B, a layer of Ca(OH)2 was placed in direct contact with the gutta-percha and a glass-ionomer barrier was placed at the CEJ level. In group C, the glass-ionomer cement barrier was placed 1mm apical to the CEJ level and in group D, the barrier was placed1mmapical to the CEJ following application of a 0.5-mm layer of Ca(OH)2. The cement was allowed to set for 48 h.
Table 1. Groups of experimental teeth as divided according to the placement of intracoronal barriers.
Figure 1. Experimental tooth model.
Each tooth was placed in a plastic vial containing 2 mL of distilled water, and was stabilized with Paralm (American National Can, Greenwich, CN, USA). The root including the CEJ was submerged in the solution, leaving only one-third of the crown outside the vial (Fig.1). The teeth were incubated for 24 h at 37 8C. The pH values of the water in the vials were determined (day1) with a digital pH-meter with automatic temperature control (PBS-737; El-Hama Industries, El-Hama, Israel). Following this, 2 mL of saline was placed in the access cavity of two teeth in each group that served as controls. In experimental teeth, a cotton pellet soaked in 2 mL of 30% H2O2 (Superoxol, Union Broach, Long Island City, NY, USA) was placed in the pulp chamber. The teeth were subjected to heat treatment with a 1000-Wphotoflood lamp for 2 min. The procedure was repeated twice with the placement of a freshly soaked cotton pellet. Finally, the pulp chamber was rinsed thoroughly with distilled water and dried with warm air. A dry cotton pellet was placed and the cavity was sealed with approximately 2 mm of Cavit (Premier Dental Products Co., Norristown, PA, USA).The treatment was performed in a manner similar to the clinical procedures of bleaching. The pH measurements were recorded at1, 2, 4 and 10 and 15 days, following renewal of the surrounding medium.
The measured pH data were subjected to the two-way anova to assess whether the mean of the pH measurements differed across groups and whether the mean differences were statistically significant from zero.