Finite element study of controlling factors of anterior intrusion and torque during Temporary Skeletal Anchorage Device (TSAD) dependent en masse retraction without posterior appliances: Biocreative hybrid retractor (CH-retractor)

Abstract

Objectives: To evaluate, using the finite element method (FEM), the factors that allow control of the anterior teeth during en masse retraction with the Biocreative hybrid retractor (CH-retractor) using different sizes of nickel-titanium (NiTi) archwires and various gable bends on the stainless-steel (SS) archwires.

Materials and methods: Using FEM, the anterior archwire section, engaged on the anterior dentition, was modeled in NiTi, and another assembly, the posterior guiding archwire, was modeled in SS. Two dimensions (0.016 × 0.022- and 0.017 × 0.025-inch NiTi) of the anterior archwires and different degrees (0°, 15°, 30°, 45°, and 60°) of the gable bends on the guiding wire were applied to the CH-retractor on the anterior segment to evaluate torque and intrusion with 100-g retraction force to TSADs. Finite element analysis permitted sophisticated analysis of anterior tooth displacement.

Results: With a 0° gable bend all anterior teeth experienced extrusion. The canines showed a larger amount of extrusion than did the central and lateral incisors. With a gable bend of >15°, all anterior teeth exhibited intrusion. Bodily movement of the central incisor required a 30°∼45° gable bend when using anterior segments of 0.016 × 0.022-inch NiTi and 15°∼30° gable bend with the 0.017 × 0.025-inch NiTi.

Conclusions: With the CH-retractor, varying the size of the NiTi archwire and/or varying the amount of gable bend on the SS archwire affects control of the anterior teeth during en masse retraction without a posterior appliance.

Keywords: Biocreative orthodontic strategy; En masse retraction; FEA; Gable bend; NiTi; TSAD; Torque.

Figure 1.

(A) Composition of CH-retractor: (1) Anterior archwire (0.016 × 0.022- or 0.017 × 0.025-inch NiTi); (2) Posterior archwire (0.017 × 0.025-inch SS); (3) Osseointegrated mini-implant (C-implant). (B) Pre and post–CH-retractor application with gable bends and superimposition (13-year-old male patient).

Figure 2.

Illustration of CH-retractor. Anterior NiTi wire (0.016 × 0.022 or 0.017 × 0.025), posterior guiding wire (0.017 × 0.025 SS). (A) Without gable bend model; (B) With gable bend model: An intrusive force on the incisor bracket by gable bends applied to the main archwire combined with a retraction force will produce a torquing moment, and it is called “Biocreative Type I torque control mechanics.”

Figure 3.

The finite element analysis model. (A) Teeth; (B) lateral views of teeth, PDL, alveolar bone of the maxillary dentition; Schematic representation of the coordinate system; (C) Y-Z plane; and (D) X-Z plane.

Figure 4.

Schematic illustration of CH-retractor with gable bend. (A) C-implant was assumed to be placed between second premolar and first molar, anteroposteriorly; (B,C) CH-retractor was engaged on six anterior teeth, and guiding wire was inserted into the head of C-implant.; (D) Gable bend was applied on posterior extended wire.

Figure 5.

Comparison of main archwire dimension and gable bends (retraction force = 100 g, hook length = 7 mm). (A) Z-axis displacement (Z-axis: [+] = intrusion, [−] = extrusion). (B) Y-axis displacement (Y-axis: [+] = proclination, [−] = retraction).

Figure 6.

Biomechanical illustration of CH-retractor system without gable bend (A) and with gable bend (B). Black dot: center of resistance. F_V indicates vertical force generated by gable bend; F_H, horizontal force generated by orthodontic elastics; F_N, normal force generated as a reactive force of F_V; F_F, friction force between C-implant head and SS archwire; M_FV, moment generated by vertical force; abd M_FH, moment generated by retraction force.