In-vitro assessment of the forces generated by lingual fixed retainers

IntroductionThe aim of this article was to assess the effect of wire adaptation on the lingual surfaces of mandibular anterior teeth with 3 types of lingual retainers on the development of vertical and labiolingual forces.MethodsTen retainers (canine to canine) were constructed from each of the following wires: Wildcat 0.0195-in heat-treated 3-strand twist-flex wire (GAC, Bohemia, NY); Penta-one 0.0215-in 6-strand as received; and Penta-one 0.0215-in 6-strand after heat treatment at 350°C for 4 minutes (both, Masel Orthodontics, Carlsbad, Calif). The retainers were bonded on each tooth of an acrylic resin model, and the model was installed in the Orthodontic Measurement and Simulation System. The vertical and labiolingual forces generated were measured for wire displacements up to 0.2 mm in 0.02-mm increments.ResultsWire displacement of 0.2 mm exerted forces as high as 1 N on the teeth. In the vertical direction, the highest force levels were recorded for the as-received Penta-one 0.0215-in and the lowest from its heat-treated counterpart. In the horizontal plane, the as-received Penta-one 0.0215-in exerted the highest forces.ConclusionsThe forces recorded from the lingual retainer wires during 0.2-mm simulated intrusion-extrusion and buccal-lingual movements might generate high forces that exceed 1 N and be large enough to produce unwanted tooth movement during retention. The only significant determinant of the generated forces was the amount of wire displacement and not the type of wire used in this study.