Does mineral trioxide aggregate reinforce the immature roots? LETTER TO THE EDITOR Dear Sir, I read with great interest the article entitled ‘Compar- ison of fracture resistance in root canals of immature sheep teeth after filling with calcium hydroxide or MTA’ by Andreasen et al. (1) published in your esteemed journal. I want to share few of my thoughts regarding this study. I appreciate the great work of the writers; however, in my opinion, this article has some technical errors: 1 As mentioned in the discussion, the reduction in fracture strength of teeth by long-term use of calcium hydroxide has been shown in the previous study of the authors (2). Therefore, the primary purpose of the present study was to evaluate the effect of MTA on fracture strength of teeth in comparison with calcium hydroxide. According to Table 1, the teeth in MTA group have greater fracture resistance; however, the small sample size of the study makes the differences statistically insignificant. With proper sample size, MTA may show reinforcing effect on the tooth (3, 4). This study may serve as a pilot for another study with sufficient sample size. 2 I have also a question about statistical analysis. The data of the study have been analyzed by the multiple t-tests; however, for comparing four groups, one-way analysis of variance (anova) followed by a proper post hoc test such as Tukey test is recommended assuming the normality of the data and homogeneity of variances. Why did the authors select multiple t-tests instead of anova? Comparing multiple groups by a series of independent sample t-tests is a type of ‘multiple testing’ (5). In this case, for comparing four groups of the study, a series of six independent samples t-tests is needed wherein the alpha (type I error) of 0.05 increases to 0.19 (1)(0.95)4). Therefore, the family error rate is much more than the alpha level defined in the statistical analysis (i.e., 0.05) (5). Contrary to the results of the study, anova shows that even the difference between calcium hydroxide and control group is not significant. I feel that this study can be more refined using sufficient sample size and proper statistical analysis. Amin Salem Milani 1 , Mohammad Asghari Jafarabadi 2 1 Department of Endodontics, Faculty of Dentistry, Tabriz University of Medical Sciences, Tabriz e-mail: Amin.salemmilani@gmail.com; 2 Department of Statistics and Epidemiology, Faculty of Health and Nutrition, Tabriz University of Medical Sciences, Tabriz, Iran References 1. Andreasen JO, Munksgaard EC, Bakland LK. Comparison of fracture resistance in root canals of immature sheep teeth after filling with calcium hydroxide or MTA. Dent Traumatol 2006;22:154–6. 2. Andreasen JO, Farik B, Munksgaard EC. Long-term calcium hydroxide as a root canal dressing may increase risk of root fracture. Dent Traumatol 2002;18:134–7. 3. Bortoluzzi EA, Souza EM, Reis JM, Esberard RM, Tanomaru- Filho M. Fracture strength of bovine incisors after intra- radicular treatment with MTA in an experimental immature tooth model. Int Endod J 2007;40:684–91. 4. Hatibovic-Kofman S, Raimundo L, Zheng L, Chong L, Fried- man M, Andreasen JO. Fracture resistance and histological findings of immature teeth treated with mineral trioxide aggre- gate. Dent Traumatol 2008;24:272–6. 5. Sainani KL. The problem of multiple testing. PM R 2009;1: 1098–103. Response The writer of this letter to the editor raises a question about the statistical methods used. The purpose of the study was to analyze whether the use of 1 month Ca(OH) 2 followed by MTA would weaken the tooth in comparison with a control tooth (saline control). The fracture strength for the saline control tooth was 310.3 ± 63 MPa (n = 9) in comparison with 327 ± 84 MPa (n = 8) for Ca(OH) 2 + MTA. The writer suggests that an anova test should have been used, considering that we also had a Ca(OH) 2 Table 1. Mean fracture strength after 100 days treatment period of teeth with the canals filled with either saline, CH, MTA or with CH for 30 days, then with MTA Ò for the rest of the period Treatment group n Fracture strength, MPa (SD) Saline 9 310.3 (±63.04) CH 6 225.5 (±78.84) MTA 7 330.8 (±99.13) CH + MTA 8 326.7 (±84.03) Dental Traumatology 2011; 27: 497–498; doi: 10.1111/j.1600-9657.2011.01052.x Ó 2011 John Wiley & Sons A/S 497