Reduction of mandibular incisors proclination during leveling and aligning phase in non-extraction orthodontic treatment

Abstract

Tooth leveling in mandibular teeth with deep curve of Spee by nickel titanium archwires found intrusive forces at mandibular incisors. These forces which applied labial to center of resistance of teeth caused labial tipping of mandibular incisors occurred. Objectives To compare the results of intrusive forces from different shapes nickel titanium archwires cross- sections by finite element analysis and to determine whether separating the alignment and leveling phases can improve the outcome of the early stages of treatment and lead to less proclination of the mandibular incisors compares to conventional technique. Materials and Methods Construction of finite element model of mandible with all teeth, periodontal tissues and alveolar bone from cone beam computed tomography of selected patient, and construction of brackets, buccal tubes and orthodontic archwires. Finite element analysis was done by applying intrusive forces limited for 0.2 mm from 3 different shapes nickel titanium archwires of 0.016-inch, 0.016 x 0.016-inch and 0.016 x 0.022-inch archwires. Comparison of stress at root surfaces, periodontal tissues and tooth displacement. And comparison between separating of alignment by round nickel titanium archwires with accentuated curve of Spee and leveling by rectangular archwire in experimental group to the conventional alignment and leveling progressively to rectangular archwires in control group. This study was done in 30 skeletal Class I patients with 3-5 mm of anterior crowding and 3-4 mm of curve of Spee depth. Determined changes of mandibular incisor position and inclination with lateral cephalometric analysis and compared curve of Spee depth changes by mandibular teeth model analysis. Results After applied intrusive forces from nickel titanium archwires found the highest stress at the cervical areas in both labial and lingual surfaces but much higher stress and higher vertical level were found on labial surfaces in all models. Comparison among 3 models found the highest stress in round archwire model than square and rectangular archwire models. The highest labial displacement also found in round archwire model. The clinical study found no significant difference of antero-posterior mandibular incisor position (P>0.05). But control group found more proclination of mandibular incisors than experimental group (P<0.0001). Crowding was resolved in both group but curve of Spee reduction in experimental group was 2.88 mm which higher than control group of 1.69 mm (P<0.0001). Experimental group found longer treatment time than control group of 8.50 weeks (P<0.0001). Conclusion Intrusive forces from round archwire produced higher stress on the root surfaces and periodontal tissues at labial side, including more labial displacement. Tooth leveling with rectangular archwire in experimental group found less proclination of mandibular incisors.