Objective: To determine the effects of extraction and the number of teeth extracted on changes in occlusal function by measuring occlusal contact area and force before and after orthodontic treatment with a fixed appliance. Materials and Methods: Female patients treated with a fixed appliance were divided into nonextraction (n=36), two maxillary premolar extraction (n=31), and four premolar extraction (n= 18) groups. Bite pressure-sensitive films were used to analyze the occlusal contact area and force. Measurements were performed before treatment (Pre-Tx), immediately afterward (After-Tx), and 2 years later (2Y After-Tx). The data were analyzed using a linear mixed model and the post hoc Bonferroni test. Results: The occlusal contact area and force after treatment decreased significantly compared with Pre-Tx values but were increased 2 years later in all groups. There were no significant differences in occlusal contact area or force during the entire observation period among the three groups (P >.05). The occlusal contact area and force in the nonextraction and two maxillary premolar extraction groups recovered to Pre-Tx levels 2 years later (P <.05). The occlusal contact area in the four premolar extraction group was significantly lower than the Pre-Tx level after 2 years of retention (P <.05). Conclusions: The occlusal contact area and force showed a tendency to decrease immediately after treatment and then gradually increase to pretreatment levels during the observation period. However, the occlusal contact area did not recover fully after 2 years in the four premolar extraction group.
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