RESUMEN
<p><b>OBJECTIVE</b>To investigate the prevalence and configuration of the isthmuses in the apical 6 mm of the mesial and distal roots of Chinese mandibular first molar by means of micro-computed tomography.</p><p><b>METHODS</b>Thirty-six extracted human mandibular first molars were selected. Specimens were subject to micro-CT and a slice thickness of 30 microm was obtained in the apical 6 mm of the roots examined. The number of sections showing isthmuses at each apical level was recorded. Three-dimensional images of isthmuses of mandibular first molars were reconstructed and observed.</p><p><b>RESULTS</b>The mesial roots of human mandibular first molars had a high incidence of isthmus. The isthmus incidence was greatest 4-6 mm from the apex in human mandibular first molar, with prevalence figures of 49.5%-66.1% and 17.3%-17.8% in mesial and distal roots, respectively. The chi-square test indicated a significant difference in the distribution of isthmuses between the two roots (P < 0.01).</p><p><b>CONCLUSIONS</b>The mesial roots of human mandibular first molars have a high incidence of isthmus, which may have clinical implications especially when surgical endodontics is performed on the mesial roots of mandibular molars.</p>