changes of the mandible after orthodontic treatment with and without extraction of four premolars

objectives: this study was designed to assess the changes of the mandible of patients who underwent orthodontic treatment with or without extraction of four premolars. materials and methods: eighteen class i bimaxillary protrusion patients treated with extraction of four first premolars and retraction of anterior teeth and 18 class i non-extraction patients with a mean age of 16.38±0.4 years were selected. cephalometric analysis was performed before and after treatment. twenty-four variables for analyzing the hard and soft tissues of the mandible were compared between the two groups. repeated measures anova was used for the comparison of the two groups fallowed by paired t-test. the relationship between the soft and hard tissue variables was studied using the pearson’s correlation coefficient. results: in both groups, the mean value of angular measurements related to b point and pogonion (pog) decreased with treatment (p<0.05). similarly, the symphysis depth of soft tissue decreased (p=0.008). the mean angular value of y-axis increased in both groups after treatment (p=0.007). the mean changes in hard tissue symphysis depth after treatment were different in the two groups (p=0.021). vertical, horizontal and rotational changes in soft tissue b point (b’) and pogonion (pog’) followed their underlying hard tissue changes (p<0.05).  conclusions: points b and pog showed backward movement after orthodontic treatments in both extraction and non-extraction patients. changes in b’ and pog’ were directly influenced by the changes in the corresponding points of the underlying hard tissue. orthodontic treatments with and without extraction of premolars produced insignificant changes in vertical facial dimension.