Effects of upper-molar distalization using clear aligners in combination with Class II elastics: a three-dimensional finite element analysis

Abstract

Introduction: The effects of upper-molar distalization using clear aligners in combination with Class II elastics for anchorage reinforcement have not been fully investigated yet. The objective of this study is to analyze the movement and stress of the whole dentition and further explore guidelines for the selection of traction methods.

Methods: Three-dimensional (3D) finite element models are established to simulate the sequential molar distalization process, including the initial distalization of the 2nd molar (Set I) and the initial distalization of the 1st molar (Set II). Each group set features three models: a control model without Class II elastics (model A), Class II elastics attached to the tooth by buttons (model B), and Class II elastics attached to the aligner by precision cutting (model C). The 3D displacements, proclination angles, periodontal ligament (PDL) hydrostatic stress and alveolar bone von Mises stress in the anterior area are recorded.

Results: In all of the models, the maxillary anterior teeth are labial and mesial proclined, whereas the distal moving molars exhibit distal buccal inclination with an extrusion tendency. With the combination of Class II elastics, the anchorage was effectively reinforced; model C demonstrates superior anchorage reinforcement with lower stress distribution in comparison with model B. The upper canines in model B present an extrusion tendency. Meanwhile, the mandibular dentition in models B and C experience undesired movement tendencies with little discrepancy from each other.

Conclusions: Class II elastics are generally effective for anchorage reinforcement as the upper-molar distalization is performed with clear aligners. Class II elastics attached to an aligner by precision cutting is a superior alternative for maxillary anchorage control in cases that the proclination of upper incisors and extrusion of upper canines are unwanted.

Keywords: Class II elastics; Clear aligners; Finite element analysis; Molar distalization.

Conflict of interest statement

The authors declare that they have no competing interests.

© 2022. The Author(s).

Figures

Fig. 1

Finite element models. Two group sets including six sub-models were presented. Set I (A1, B1, and C1) represents the initial distalization of the 2nd molar, Set II (A2, B2, and C2) simulated the initial distalization of the 1st molar after 2 mm distalization of the 2nd molar. Models A (A1, and A2) indicated maxillary models to simulate the upper-molar distalization using clear aligners without Class II elastics. Models B (B1, B2) were designed to simulate the upper-molar distalization using clear aligners in combination with Class II elastics by buttons. Models C (C1, and C2) were designed to simulate the upper-molar distalization using clear aligners in combination with Class II elastics by precision cutting. The red circles represent the temporal-mandibular joint

Fig. 2

The boundary conditions, mesh figure, and coordinate systems. a The boundary conditions. Label A, the movement of temporal and maxilla bones were restricted for all degrees of freedom of the nodes at its superior region; Label B, the mandibular was also fixed at the lower margin of mandibular body. b A figure showing the mesh. c. Image showing the global and local coordinate systems

Fig. 3

The movement tendency of the maxillary dentition. The coordinate system was based on the entire dentition (global coordinate system). Set I, initial distalization of the second molar; Set II, initial distalization of the first molar. Model A, control model without anchorage reinforcement; Model B, Class II elastics attached to the tooth by buttons; Model C, Class II elastics attached to the aligner by precision cutting. As shown by the arrows, with the distalization of the upper molars, the anterior and posterior teeth move in opposite directions due to the exerted reciprocal force. The x-axis represented the coronal plane (+ left, −right), the y-axis represented the sagittal plane (+ posterior, −anterior), and the z-axis represented the vertical plane (+ superior, −inferior)

Fig. 4

The movement tendency of the mandibular dentition in models with Class II elastics. The coordinate system was specified based on the entire dentition (global coordinate system). As shown by the arrows, with the combination of Class II elastic, the mandibular dentition of models B and C moved forward with a little extrusion tendency of molars and intrusion of anterior teeth

Fig. 5

Bar charts of three-dimensional displacement for the maxillary and mandibular dentition (in mm). The coordinate system was considered based on the entire dentition (global coordinate system)

Fig. 6

Three-dimensional displacement of the maxillary and mandibular anterior anchorage units. The vector diagrams and color maps showed initial patterns of movement. The anterior anchorage units exhibited labial and mesial inclination with a rotation center at the intersection of the apical and middle thirds of the roots for the maxillary incisors and the apical third of the roots for the mandibular incisors. The histograms show total displacement of the maxillary and mandibular anterior teeth displayed as crown and root displacement respectively (in mm). AI, model A1; AII, model A2; BI, model B1; BII, model B2; CI, model C1; CII, model C2. The coordinate system was centered on each tooth (local coordinate system); the positive value for the x-axis represents the mesial surface of the teeth, the positive value for y-axis represents the palatal surface of the teeth, and the z-axis represents a positive direction towards the apex of the maxillary teeth and the incisor/occlusal of lower teeth

Fig. 7

The movement tendency of the upper molars. The coordinate system was centered on each tooth (local coordinate system). In group set I, the 2nd molar demonstrated distal and buccal inclination with extrusion tendency, while the 1st molar presented mesial and buccal inclination with intrusion tendency. In group set II, the 1st molar showed distal and buccal inclination with extrusion tendency, while the 2nd molar demonstrated a tendency for mesial and palatal inclination with little intrusion

Fig. 8

PDL hydrostatic stress and the von Mises stress of alveolar bone in maxillary anterior area (MPa). The front side represents buccal. For each sub-image, from middle to lateral are central incisors, lateral incisors, and canines. Positive values indicate tensile stresses for PDL hydrostatic stresses, while negative values represent compressive pressures. In all of the six models, the highest compressive stress of the PDL was concentrated on the labial cervical region and apex for upper incisors, and on the mesio-buccal cervical and apex area for upper canines. The stresses of alveolar bone were mostly distributed on the labial surface and concentrated on the cervical and apex region

Fig. 9

PDL hydrostatic stress and the von Mises stress of alveolar bone in mandibular anterior area (MPa). The front side represents buccal. For each sub-image, from middle to lateral are central incisors, lateral incisors, and canines. Positive values indicate tensile stresses for PDL hydrostatic stresses, while negative values represent compressive pressures. The stress of PDL and alveolar bone was concentrated on the labial cervical region for incisors, and on mesio-buccal cervical region for canine

Fig. 10

Bar charts of PDL hydrostatic stress and von Mises stress of alveolar bone in maxillary and mandibular anterior area. Positive values indicate tensile stresses for PDL hydrostatic stresses, while negative values represent compressive pressures