What is the role of second molars in leveling the curve of Spee? A finite element analysis study.

INTRODUCTION: This study aimed to assess the effect of the mandibular second molars on the stress distribution and initial displacements during leveling the curve of Spee using different archwire thicknesses and materials by means of finite element analysis. METHODS: After construction of all anatomic structures, including the mandibular alveolar bone, periodontal ligament, and dentition, 0.022-in slot brackets and 0.022 x 0.028-in molar tubes were placed on the buccal surfaces of the teeth. Different archwires were modeled, with 2 different thicknesses (0.016-in and 0.017 x 0.025-in) and 3 different materials (nickel-titanium, stainless steel, and titanium molybdenum). Two different models were created: The mandibular dentition (1) with and (2) without second molars. The initial teeth movements and periodontal ligament stress distribution after leveling were analyzed for each of the models and archwires. RESULTS: The initial stress levels with all archwires were higher when the second molars were also included. The highest von Mises stresses were 16.75 N/mm(2) with the 0.017 x 0.025-in stainless steel archwire. Periodontal stresses were mostly concentrated in the apical area of the incisors. The tendency of labial tipping increased with the attachment of second molars. Thicker archwires with all materials were noticed to enhance the anterior tipping of the incisors. In both scenarios, the least movement occurred on the x-axis. CONCLUSIONS: The inclusion of the second molars enhanced the stress level and the initial anterior tipping of the incisors. However, the bonding of the second molars did not have any effect on the initial displacement in the transverse direction for all the archwires used. As the archwire dimension increased, higher stress values were observed on the whole mandibular dentition.