Investigation of Dentin Hardness in Roots Exhibiting the Butterfly Effect Assil A. Russell, BDS, Li Hong (Chris) He, BDS, DDS, PhD, and Nicholas P. Chandler, BDS, MSc, PhD Abstract Introduction: Most vertical root fractures occur in root canal treated teeth, and they usually run in a buccolingual direction. The butterfly effect is an optical phenomenon seen in some sections of tooth roots. The aim was to investigate the microhardness of dentin in mesiodistal and buccolingual cross sections of roots exhibiting the effect. Methods: Thirty extracted single-rooted teeth were allocated according to patient age: group 1, 15–24 years; group 2, 25–44 years; and group 3, 45 years and older. Roots were embedded in acrylic and cut into ten 1-mm-thick cross sections. Sections were viewed under a light microscope and coded (1 or 2) according to presence or absence of the butterfly effect. A root scored 20 when all levels featured the butterfly appearance. The 2 teeth with the highest score from each group and 2 control teeth with the minimum score (10) were selected. Two adjacent, consecutive cross sections were chosen from the middle of the roots. Vickers microhardness testing was carried out on the dentin walls. Results: Mean hardness scores were highest mesiodistally (83.7 kgf/ mm 2 ) and lowest buccolingually (56.4 kgf/mm 2 ), a significant difference (P = .028). This trend was found across all age groups. Conclusions: Root sections with the butterfly effect are harder mesiodistally. This might explain the high prevalence of vertical root fractures that run buccolingually. (J Endod 2014;40:842–844) Key Words Dentin, endodontics, vertical root fracture T he majority of vertical root fractures (VRFs) occur in root canal treated teeth, and they usually run in a buccolingual direction (1, 2). The butterfly effect is seen in some cross sections of tooth roots (3). Vasiliadis et al (4) reported that dentinal tubular sclerosis differed in mesiodistal and buccolingual directions, noting a characteristic butterfly shape. Sclerosed dentin is more translucent than normal dentin (5, 6). Russell et al (7) reported that teeth with the butterfly effect had a higher density of dentinal tubules buccolingually than mesiodistally, suggesting that this may affect hardness of dentin. A search of the literature revealed no previous studies examining hardness of dentin and the butterfly effect. The aim of this study was to investigate the hardness of dentin in mesiodistal and buccolingual cross sections of roots exhibiting the butterfly effect. Materials and Methods Ethical approval was granted from the University of Otago, Dunedin, New Zealand to collect 30 single-rooted human teeth of known age. They were divided into groups of 10: group 1, 15–24 years; group 2, 25–44 years; and group 3, 45 years and older. Roots were embedded in acrylic (Vertex Self Curing; Vertex-Dental BV, Zeist, The Netherlands) and cut into 1-mm-thick cross sections (Accutom 50 saw; Struers A/S, Ballerup, Denmark). Each root yielded 10 sections, which were marked to indicate orientation. These were viewed with a light microscope (EHT; Olympus, Tokyo, Japan) at Â10 magnification by 2 calibrated examiners and given a score. A score of 1 represented no butterfly effect where the dentin had uniform color, and 2 represented the butterfly effect with alternating shades of dentin (Fig. 1). Examiners reached a consensus for each section, and scores were summed. Twenty represented a tooth with the effect present in all sections, and 10 represented a tooth with the effect totally absent. From each age group, the 2 teeth with the highest overall scores were selected for further examination. As controls, 2 teeth with a score of 10 (no effect) were selected. For each of the 8 teeth, 2 adjacent sections were chosen from the middle of the root to give 16 specimens. To remove surface defects, sections were polished with silicon carbide paper of increasing grit (P500 to P4000; 3M Europe, Diegem, Belgium) and reexamined with the microscope to identify any remaining scratch lines and need for further polishing. Each section was then indented with a square-based pyramid diamond indenter to determine Vickers hardness (Shimadzu Ltd, Tokyo, Japan). The indenter was set to 1 kg (10 N) load for 30 seconds. Four indents were made per specimen on the mid-mesial, mid-distal, mid-buccal, and mid-lingual aspects (Fig. 1). Indents were made a consistent distance from the lumen, with the tip of the diamond facing the luminal space. Sections were then placed in 1% aqueous methylene blue dye and rinsed with water to increase visibility. From the Sir John Walsh Research Institute, Faculty of Dentistry, University of Otago, Dunedin, New Zealand. Address requests for reprints to Dr Nicholas P. Chandler, School of Dentistry, University of Otago, PO Box 647, Dunedin 9054, New Zealand. E-mail address: nick. chandler@otago.ac.nz 0099-2399/$ - see front matter Copyright ª 2014 American Association of Endodontists. http://dx.doi.org/10.1016/j.joen.2013.11.005 Basic Research—Biology 842 Russell et al. JOE — Volume 40, Number 6, June 2014