Does premolar extraction influence the biomechanics when leveling the curve of Spee? A finite element analysis

The aim of this study was to evaluate the effects of first premolar extraction on stress distribution and initial tooth displacement when leveling the curve of Spee with different archwire dimensions and materials by means of finite element analysis.

Overall, 2 finite element models were created to simulate the following therapeutic situations: (1) a complete mandibular dentition with the corresponding anatomic structures and (2) a mandibular dentition with first premolar extractions. In addition, 2 archwire dimensions (0.016-in round and 0.017 × 0.025-in rectangular) and 3 materials (Nickel-Titanium, stainless steel [SS] and titanium molybdenum) were engaged in bracket slots to simulate leveling the curve of Spee. Linear static analysis was performed to assess the von Mises stress distribution (in Newton per square millimeter) and initial displacements (in micrometers) within the teeth and periodontal tissues.

In both models, the highest von Mises stresses were observed with the 0.017 × 0.025-in SS archwire, predominantly in the apical third of the second molars (nonextraction model: 16.75 N/mm²; extraction model: 16.98 N/mm²). The lowest peak stress on mandibular incisors was recorded with the 0.016-in nickel-titanium wire in the extraction model (3.75 N/mm²). Premolar extraction reduced mandibular incisor proclination and decreased posterior uprighting, whereas greater incisor proclination was observed with rectangular archwires compared with round wires. Vertically, increased apical displacement was noted in the roots and the distal cusps of the first and second molars.

This study demonstrated that initial stress distribution and displacement patterns vary depending on archwire selection and extraction protocols. Rectangular SS archwires produced higher stress concentrations, particularly on second molars, whereas premolar extraction was associated with reduced anterior stress and mandibular incisor proclination.