Effects of Computer Assisted Design-Computer Assisted Manufacturing Nickel-Titanium Fixed Lingual Retainers Versus...

Little information is available on tooth displacement and periodontal ligament (PDL) stress distribution caused by computer assisted design-computer assisted manufacturing (CAD/CAM) retainers. Therefore, the purpose of this study was to compare CAD/CAM and multistrand retainers in terms of PDL stresses, risk of root resorption, and displacements of mandibular anterior teeth with 4 different inclinations using finite element analysis.

In this computer-based finite element simulation, eight models consisting of the six mandibular anterior teeth, their PDLs, and surrounding alveolar bones were created. Four models simulated conventional fixed retainers with multistrand wires; four others simulated CAD/CAM nickel-titanium fixed retainers. Within each retainer group, four different tooth inclinations relative to the horizontal plane (inclination angles of 110°, 100°, 90°, and 80°) were modeled. All models were subjected to a vertical pressure of 42 psi (0.289 MPa, exerted on the incisal surfaces) and a lingual force of 10 N. Calculated were PDL stresses, the risk of root resorption (defined as areas where PDL stress exceeded 0.0047 MPa), and dental displacements along the buccolingual and intrusive/extrusive axes.

With fixed multi-strand retainers, the buccolingual displacements of incisor and canine crowns are 3.10, 6.10, and between 2.82 and 5.32 µm in the lingual side. As the inclination increases, displacement and strain are often increased. In the CAD/CAM models, the buccolingual displacements of the crown of incisors and canines were between 2.94 and 4.76 µm and 3.34 and 4.01 µm in the lingual side, respectively. By increasing the inclination, the displacement and strain often decreased.

Although this FEA study showed some differences between models, no clinical recommendation is possible. Clinical decisions must also consider breakage modes, undetected debonding risk, and patient follow-up.