Mechanical behaviour of a prototype orthodontic retraction spring: a numerical-experimental study.

The purpose of this study was to examine the mechanical behaviour of orthodontic delta retraction springs. Twelve titanium-molybdenum (0.016 × 0.022 inch) delta loops were studied. The springs were analysed by means of the finite element (FE) method and experimental tests using a platform transducer. Each spring was activated from 0 to 6 mm. Statistical analysis of the data was carried out by one-way analysis of variance and Games-Howell parametric multiple comparison test for heterogeneous variances. FE analysis revealed that the stress level varied from 277 to 1273 MPa. At 6.1 mm (773 MPa), the springs were still in the elastic range. Force levels varied from 0.1 N (10 g) to 2.2 N (224 g) at 1.4-8.1 mm of activation for the numerical study and from 0.44 N (45 g) at 1 mm to 2.02 N (206 g) at 6 mm of activation in the experimental study. The spring rate was within the levels that are appropriate for clinical use (34 g/mm). Vertical forces (Fy) showed constancy and were of low magnitude. The anterior moment/force ratio from the experimental tests was 14 at 3 mm of activation decreasing to 10.7, 8.7, and 7.2, for 4, 5, and 6 mm of activation, respectively. The springs could be activated up to 7 mm without exceeding the elastic limit.