Requirements for more aesthetic solutions have seen an increased demand by orthodontic patients recently. Fixed lingual appliances provide the perfect aesthetic aspect, although it requires a highly-skilled orthodontist due to the fact that mispositioning of a lingual bracket has a more noticeable impact on the resulting tooth movements compared to traditional straight-wire appliances. The choice of material for the archwire used for retraction is one of the factors that have an impact on the tipping of the anterior segment.
Aims: The aim of the study was to analyze a 3D finite elements model of a maxilla after removing the upper first premolars, simulating en-masse retraction of the anterior segment using a fixed lingual orthodontic appliance, with different wire materials: stainless steel (SS) and titanium-molybdenum alloy (TMA).
Materials and Methods: A 3D model for the maxilla, the upper teeth and the PDL was constructed. The first upper molars were removed and a fixed lingual appliance was designed, consisting of a 16x16 mil mushroom-shaped archwire and lingual brackets. The model was imported into ANSYS software to generate a finite elements mesh. A 100 newton horizontal force was applied at the distal end of the archwire behind the last molar. The maximum values for the Von-mises stress equivalent and horizontal tooth displacement were evaluated. Additionally, the M/F ratio of both the anterior and posterior segments was computed.
Results and Discussion: In both types of material of the archwire, the initial movement of the anterior teeth was backwards and downwards (extrusion). Inversely, the posterior teeth moved backwards and upwards (intrusion). The movement in posterior teeth was higher when SS was used. Posterior teeth moved more initially than the anterior teeth, regardless of the type of material. The anterior teeth moved when a TMA wire was used compared to SS. The TMA wire suffered more deformation than the SS. Regardless of the material, the deformation was most noticeable in the posterior half of the archwire. The M/F ratio of both anterior and posterior segments was higher when a TMA wire was used. Both wires produced M/F ratio within the uncontrolled tipping range.
Conclusions: The initial total movement of the anterior teeth during en-masse retraction using a fixed lingual appliace is higher when TMA archwire compared to an SS archwire. The deformation was most noticable in the posterior half of the archwire. Using TMA for the space closure stage with a fixed lingual appliance produces higher M/F ratios compared to an SS archwire, which indicates that TMA is able to achieve movements closer to pure translation (bodily movements) compared to SS.