The existence of butterfly effect and its impact on the dentinal microhardness and crack formation after root canal instrumentation.

To compare the microhardness and crack formation in root dentine presented with butterfly effect in lower premolars. Sixty mature lower premolars were selected and divided into the control and experimental groups. Teeth in the experimental group were instrumented up to size 30/.04. The roots were cut horizontally into twelve parts of 1-mm-thick cross-section and were numbered accordingly. They were divided into coronal, middle, and apical root sections. Sections were then viewed under a microscope to determine the presence of butterfly effect and subsequently scored. 8 teeth from both control and experimental groups with the highest and lowest score were selected. Crack formation was inspected and classified into four different types of cracks. Microhardness test was performed using a Vickers hardness test. Higher frequency of butterfly effect was found in the apical root section and root dentine with butterfly effect were harder mesiodistally. The middle and apical root sections with butterfly effect were harder than the coronal section. No significant difference of dentine hardness between the control and experimental groups. Cracks only occurred in the experimental group and presented in buccolingual direction with a higher rate of Type 1 and Type 2 cracks. Prevalence of butterfly effect in lower premolars increased from coronal to apical with increased hardness mesiodistally. More buccolingual cracks were found in radicular dentine with butterfly effect and most of them exhibited Type 1 and Type 2 cracks. Roots of lower premolar with butterfly effect may be susceptible to a higher rate of vertical root fracture in buccolingual direction, especially after root canal treatment. Thus, special attention should be given not to overload instruments during root canal preparation.