Comparison of Stress Distribution After Initial Leveling of Mandibular Teeth

Objectives: Leveling and aligning of teeth are in the first phase for an orthodontic treatment with full fixed appliance. A small size of round cross-sectioned wire is recommended as the first archwire for this stage. A curve of Spee in lower teeth causes an intrusive force at lower incisor brackets after archwire placement. This study aimed to describe the pattern of tooth movement and to compare levels of stress between 0.014-inch nickel-titanium and stainless steel wires.
Methods: A mandible with all lower teeth and periodontal tissues was created as a finite element model. All mandibular teeth were well-aligned and presented deep curve of Spee of 4-mm. Twelve teeth from central incisors to first molars were attached with 0.018-inchx0.025-inch slot-size brackets. The 0.014-inch nickel-titanium and stainless steel archwires were placed and allowed for a movement within periodontal space of 0.2-mm. Pattern of right central incisor movement and Von-Mises stress were evaluated and compared between two types of archwires.
Results: After applied intrusive force at four lower incisors for 0.2-mm. Both types of archwires found similar pattern of distributed stress from labial/lingual stress ratio. Maximum stress were shown at both labial and lingual cervical areas with less stress at the lingual one. The location of maximum stress on labial surface was slightly more cervical than lingual one which represented intrusion with labial tipped of tooth. Stainless steel wire generated high stress at labial and lingual surfaces of 5.2905 and 3.7643-MPa respectively which higher than stress from nickel-titanium wire which were only 3.2132-MPa for labial side and 2.2590-MPa for lingual side. The amount of incisal edge labially tipped by stainless steel wire was more than nickel-titanium wire.
Conclusions: Initial archwire leveling with nickel-titanium wires produced less stress distribution than stainless steel. However, pattern of tooth movement after leveling with round wires found flaring of lower incisors.