Biomechanical properties of CAD/CAM-individualized nickel-titanium lingual retainers: an in vitro study

This study aimed to determine and compare the mechanical properties of novel CAD/CAM(computer-aided design/manufacturing)-individualized nickel-titanium (NiTi) and conventional twisted wires.

Seven NiTi wires (angular, 0.20 × 0.20 mm up to 0.50 × 0.50 mm) and two twisted steel wires (round, 0.40 mm and 0.44 mm) were investigated in three-point (3PBT) and four-point bending tests (4PBT) using the Orthodontic Measurement and Simulation System. All wires were loaded up to a distance of 3.3 mm, and resulting forces were measured. Additionally, in the biomechanical test (BMT), CAD/CAM-individualized NiTi retainers (0.35 × 0.35 mm) were subjected to intrusive and extrusive deformation of 0.1, 0.2, and 0.4 mm.

For twisted wires, typical plastic deformations were found in 3PBT (0.40 mm: 0.3 mm; 0.44 mm: 0.4 mm), and maximum resulting forces for these reached 1.3 N and 3.2 N for the 0.40 mm and 0.44 mm wires, respectively. Corresponding values in the 4PBT totaled 0.8 N for the 0.40 mm and 1.9 N for the 0.44 mm twisted wire. The NiTi samples showed plastic deformations in the 3PBT for the 0.25 × 0.25 mm dimension and in 4PBT for the 0.35 mm × 0.35 mm version. At a deflection of 1.5 mm, both tests revealed significantly different forces for the NiTi wires of different dimensions (p ≤ 0.001). In the BMT, no remarkable loading plateau or plastic deformation was observed. The resulting forces were significantly higher for extrusive than for intrusive deformation (p ≤ 0.001).

Individualized NiTi retainers feature specific elasticity and dimensional stability, and more deflection is necessary to achieve permanent deformations. The CAD/CAM production process did not significantly influence the biomechanical properties of the NiTi wires.