Changes in Smile Parameters Following Surgical-orthodontic Treatment for Skeletal Class III Malocclusion

Abstract PurposeTo analyze differences in smile parameters following surgical-orthodontics in skeletal Class III patients with mandibular prognathism and to identify factors affecting smile parameters after orthognathic surgery.Materials and Methods:These retrospective study comprised of thirty-four patients, diagnosed as having skeletal Class III malocclusion with pure mandible prognathism, were treated with surgical-orthodontics in Taipei Medical University Hospital. Hard and soft tissue variables were measured through lateral cephalometry, and smile esthetics were measured using extraoral photographs. Postoperative changes in smile parameters and differences between sexes were analyzed using the t test. The relationship among smile parameters were analyzed using the Pearson correlation. Random forest methods and decision tree deep learning models were used to identify the variables associated with asymmetry after surgery.Results:Postoperatively, the buccal corridor (p = 0.024), upper lip height (C1) (p = 0.003), and smile index (p = 0.031) were significantly different. The upper midline and right chelion were significantly different between sexes. A very strong positive correlation was observed between the upper lip height and upper lip ratio and between the lower lip height and lower lip ratio; a strong positive correlation was observed between lower teeth exposure and the interlabial gap and between the right chelion and left chelion. By contrast, a strong negative correlation was observed between lower teeth exposure and the smile index and between the smile index and interlabial gap. The following smile parameters associated with asymmetry exhibited the most changes after surgery: lower lip area, interlabial gap, smile index, buccal corridor, and arc ratio.Conclusions:Following orthognathic surgery, marked changes were noted in skeletal, dental, and soft tissue parameters, but smile esthetics changed less from the frontal view. The most changes regarding asymmetry after surgery were noted in the lower lip area, interlabial gap, smile index, buccal corridor, and arc ratio in this order.