Fracture resistance of calcium hydroxide-treated teeth filled with water- and PBS-mixed MTA
Background: Traumatic dental injuries and dental caries are the main reasons why permanent teeth with immature, thin-walled roots lose their vitality and require endodontic treatment. Currently there are three endodontic treatment options to treat teeth with irreversibly damaged dental pulps and incomplete root formation. These are: pulp preservation, pulp regeneration and apical barrier induction with calcium hydroxide (CH) or production by placement of calcium silicate materials e.g. Mineral Trioxide Aggregate (MTA) in the apical part of the root canal.MTA apexification can be undertaken as a one-visit treatment or two-visit treatment with a short-term CH pre-medication. The majority of in vitro studies on influence of CH on fracture strength showed reduction in the mechanical properties of radicular dentine after 5 weeks exposure (Yassen & Platt 2013). MTA may increase the resistance to vertical root fracture of endodntically treated teeth treated in one-visit (EL-Ma'aita et al. 2014). Aim: To test, in vitro, the fracture strength of extracted human teeth restored with 5 mm apical plugs of MTA mixed with either water or Ca- and Mg-free Phosphate Buffered Saline (PBS), to establish whether these materials strengthen the root, and if an initial CH dressing has any effect on the fracture strength. Materials and methods: One hundred and eighty freshly extracted human permanent single-rooted teeth were used. The crowns of these teeth were removed and 10 mm long roots were prepared until a Largo Peeso drill of a size 150 ISO could be inserted 1 mm beyond the apical root-end, to resemble thin-walled incompletely formed roots. The roots were randomly allocated to nine groups (n=20). All samples were irrigated with equal volume of 5.25% sodium hypochlorite and 10% citric acid using a standardised protocol. Group Irrigation only - control group, received irrigation only; Group 2/52 CH - control group, dressed with calcium CH for 2 weeks; Group 12/52 CH - control group, dressed with CH for 12 weeks; Group MTA(W) - filled with water-mixed MTA (MTA-W); Group 2/52 CH + MTA(W) - dressed with CH for 2 weeks and filled with MTA-W; Group 12/52 CH + MTA(W) - dressed with CH for 12 weeks and filled with MTA-W; Group MTA(PBS) - filled with PBS-mixed MTA (MTA-PBS); Group 2/52 CH + MTA(PBS) - dressed with CH for 2 weeks and filled with MTA-PBS; Group 12/52 CH + MTA(PBS) - dressed with CH for 12 weeks and filled with MTA-PBS. All samples were stored for 4 weeks at 37⁰C and 100% humidity. Following the storage period, 18 samples from each group were mounted in acrylic resin, with simulated periodontal ligament using a polyether impression material. The roots were subjected to a compressive force using an Instron® Universal testing machine until fracture, and the maximum force at fracture (F-max) were recorded in Newtons (N). A metal jig was designed and fabricated for this purpose which permitted the prepared tooth specimens to be loaded by the tip of a chisel at 130⁰ to the long axis of the tooth in a lingual-labial direction. The fracture type and depth were recorded. The interface between the material and the tooth structure of the remaining 2 samples from each group underwent element mapping using a Scanning Electron Microscope (SEM) with an Energy-dispersive X-ray detector. Several images of the dentine and the dentine-MTA interface were taken using the SEM. Results: Fracture resistance data were analyzed statistically by one-way ANOVA and the Tukey’s comparison of means tests. The mean F-max (± SD) were: 462.34 ± 205.42 for Group Irrigation only; 598.9 ± 194.67 for Group 2/52 CH; 706.56 ± 240.66 for Group 12/52 CH; 920.41 ± 403.05 for Group MTA(W); 446.68 ± 201.07 for Group 2/52 CH + MTA(W); 409.17 ± 211.52 for Group 12/52 CH + MTA(W); 852.61 ± 375.7 for Group MTA(PBS); 832.36 ± 328.73 for Group 2/52 CH + MTA(PBS); 513.87 ± 272.44 for Group 12/52 CH + MTA(PBS).Significant differences (P < 0.05) in fracture resistance were found between groups: 12/52 CH +MTA(W) vs 12/52 CH, 2/52 CH + MTA(PBS), MTA(PBS), MTA(W); 2/52 CH + MTA(W) vs 2/52 CH + MTA(PBS), MTA(PBS), MTA(W); Irrigation only vs 2/52 CH + MTA(PBS), MTA(PBS), MTA(W); 12/52 CH + MTA(PBS) vs 2/52 CH + MTA(PBS), MTA(PBS), MTA(W); and 2/52 CH vs MTA(W). The Weibull moduli analysis flagged up, missed by the Tukey analysis of fracture strength data, issues concerning higher dependability of groups 12/52 CH vs 2/52 CH + MTA(W) and 12/52 CH + MTA(PBS); and 2/52 CH + MTA(PBS) vs 2/52 CH. The analysis showed also poorer dependability of samples in group MTA(W) vs 2/52 CH + MTA(PBS) and MTA(PBS).The results of Chi-square calculation for two types of fracture: split and comminuted (Chi-square = 19.06, P = 0.015) and three depths of fracture: above the cylinder, into the cylinder and vertical root fracture (Chi-square = 50.46, P = 0.00002) rejected the null hypotheses that the experimental groups and fracture types and depths are independent. In the SEM examination of groups filled with MTA mixed with PBS, it was possible to distinguish the interfacial layer between the cement and dentine, and a 50-200 μm layer of altered dentine adjacent to MTA. No such findings could be noted in groups filled with MTA mixed with water. Conclusions: This study, with its limitations, has confirmed that: 1) CH dressing for up to 12 weeks had no negative effect on fracture resistance of human roots; 2) There is no significant change in fracture resistance of human teeth that received a 2- and 12-week treatment with CH and were restored with MTA-W, or treated with CH for 12 weeks, and restored with MTA-PBS, in comparison with teeth that received no treatment; 3) One-visit apexification with MTA mixed with either water or Ca- and Mg-free PBS can significantly improve fracture resistance of human teeth. One-visit apexification with MTA mixed with Ca- and Mg-free PBS is the more dependable treatment option to strengthen thin-walled teeth; 4) If CH dressing is required for disinfection of a thin-walled tooth before apexification with MTA, it is better to use it short-term (2 weeks) and obturate the root canal with MTA mixed with Ca- and Mg-free PBS. This will have a root strengthening effect on the human teeth; 5) Obturation with MTA apical plug has negative effect on fracture resistance of human roots treated with CH for extended periods (12 weeks); 6) The fracture force has no influence on the type or depth of root fracture in teeth treated with either CH or MTA apexification. The type of apexification treatment may have an influence on the depth and the type of root fracture; 7) MTA mixed with PBS can produce an interfacial layer between the cement and the dentine even in teeth that received pre-treatment with CH; 8) The element diffusion from MTA into dentine is possible when MTA is mixed with PBS, even if pre-treatment with CH was used. When MTA is mixed with water, CH pre-medication seemed to prevent the element migration to dentine.
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