ABSTRACT
<p><b>OBJECTIVE</b>To study the change of the cracks and the influence on overall displacement of the cracked first mandibular molar under different loadings.</p><p><b>METHODS</b>Three-dimensional finite element models of first mandibular molar with cracks of different depth and length and a control model with no crack were created firstly. Then six loading conditions were applied to the models simulating the real mastication. The changes of the cracks and displacement of the teeth under the six loadings conditions were obtained by finite element method.</p><p><b>RESULTS</b>The length and depth of the cracks increased, the cracks of the occlusal surface become wider, and the crack was the widest under the fourth loading condition. Moreover, the edge of the cracks was irregular. The adjacent nodes were not in the same plane. The crack of the distal surface was the widest under the sixth loading condition. Compared to the teeth without cracks, the overall displacement of the teeth with cracks increased, but the increment was limited.</p><p><b>CONCLUSION</b>The change of the cracks is closely related to the initial crack forms and loading conditions.</p>