This study aimed to identify the tooth movement patterns relative to various force angulations (FAs) when distalizing the total maxillary dentition.
Long-term orthodontic movement of the maxillary dentition was simulated by accumulating the initial displacement of teeth produced by elastic deflection of the periodontal ligament using a finite element analysis. Distalization forces of 3 N were applied to the archwire between the maxillary canine and first premolar at 5 different FAs (−30°, −15°, 0°, 15°, and 30°) to the occlusal plane.
Maxillary incisors and molars showed lingual and distal tipping at all FAs, respectively. At a force angulation of 30°, almost bodily distalization of the total maxillary dentition occurred, but incisors showed considerable lingual tipping because of the effect of clearance gap (0.003-in, 0.022 × 0.025-in bracket slot, 0.019 × 0.025-in archwire) and elastic deflection of the archwire. Medial displacement of the maxillary anterior teeth occurred because of lingual tipping during distalization. The occlusal plane rotated clockwise at all FAs because of extrusion of the maxillary incisors and intrusion of the maxillary second molars, and the amounts decreased as FA increased.
Tooth movement patterns during distalization of the total maxillary dentition were recognized. With an understanding of the mechanics, a proper treatment plan can be established.
We studied maxillary tooth movement at various force angulations (FAs) during distalization.
Maxillary incisors and molars showed lingual and distal tipping at all FAs, respectively.
The occlusal plane rotated clockwise at all FAs.
Medial displacement of the maxillary anterior teeth occurred because of lingual tipping.
Class II malocclusion correction can be accomplished with extraction or nonextraction treatment. The decision should be made by a prudent differential diagnosis considering the face, skeletal pattern, and dentition. ,
For nonextraction treatment, headgear and Class II elastics , have been traditionally used for distalization of total maxillary dentition. However, they included undesirable side effects and were highly dependent on patient compliance, therefore alternative treatment modalities such as pendulum and distal jet were introduced because they required only minimal patient cooperation, but unfortunately, some reciprocal anchorage loss occurred during distalization with these methods.
A nonextraction approach has become more popular for Class II treatment by total distalization of the maxillary dentition with the advent of temporary skeletal anchorage devices. For distalization of the maxillary dentition, an indirect skeletal anchorage-supported pendulum and distal jet were introduced to prevent reciprocal anchorage loss.
Miniscrews and miniplates have been placed in various sites and are used widely as direct skeletal anchorage devices for total distalization of the maxillary dentition with various force angulations (FAs) from the posterosuperior to the posteroinferior direction to the occlusal plane in pursuit of desired tooth movement.
Many studies have been conducted to correct Class II malocclusions with various treatment modalities that use conventional and skeletal anchorage devices, but few studies have attempted to predict the tooth movement patterns from a biomechanical point of view. Therefore, this study aimed to recognize tooth movement patterns and identify the biomechanics relative to the FAs during total distalization of the maxillary dentition.
Material and methods
Figure 1 shows the finite element model. The same model was used as in a previous study for total mesialization of the maxillary dentition. Only the right side of the dentition was modeled because of its bilateral symmetry. Finite element models of teeth were constructed on the basis of a dental study model (i21D-400C; Nissin Dental Products, Kyoto, Japan). Surfaces of the teeth were divided into shell elements and were considered to be rigid. The periodontal ligament (PDL) was assumed to be a linear elastic material of 0.2 mm thickness. Young’s modulus and Poisson’s ratio were 0.2 MPa and 0.4, respectively. The alveolar bone was assumed to be a rigid body and was thereby excluded from the finite element model.
The archwire was made of 0.019 × 0.025-in stainless steel (SS) wire, whose Young’s modulus and Poisson ratio were 200 GPa and 0.3, respectively. The size of the bracket slot was 0.022 × 0.025-in, the width was 4 mm for the molars and 3 mm for the other teeth. The bracket was assumed to be rigid. A force of 3 N was applied to the archwire between the canine and first premolar brackets with various FAs in the sagittal plane. The archwire slid along the bracket slots with a frictional coefficient of 0.15. Surfaces of the archwire and the bracket slot were overlaid with contact elements ( Fig 1 ).
There was a clearance gap or play (0.003-in) between the archwire and the bracket slot in the initial position ( Fig 2 , A ), and the bracket rotated 7.2° in the contact position ( Fig 2 , B ). Figure 3 shows the location of the center of resistance, which was determined in a previous study.
Long-term movements of the dentition were simulated by the finite element method (FEM) in the same way as in the previous study. The simulation method consisted of the following 2 steps. First, the initial displacement of each tooth produced by elastic deflection of the PDL was calculated. Second, the alveolar socket was moved in the same direction and the same amount as the initial displacement. Repeating these steps, the teeth moved with their alveolar sockets. The number of repeated calculations was equivalent to the elapsed time after the force application. FEM software, ANSYS (version 19.1; ANSYS, Inc, Canonsburg, Pa), was used for the simulation.
Three-dimensional displacements were measured at the central incisal edge (CIE), the mesiobuccal cusp of the first molar (MBC6), and the distobuccal cusp of the second molar (DBC7). The change of the occlusal plane angle was calculated using vertical displacements of these points.
Figure 4 shows the movement patterns of the maxillary dentition at the number of repeated calculations (N = 400) for each force angulation. The initial tooth positions are illustrated with a pale color. Color contours on the root indicate distributions of mean stress in the PDL. Tipping angles of the central incisor and the second molar are described in Figure 4 . Table I shows the 3-dimensional displacement of CIE, MBC6, and DBC7, whereas Table II shows their tipping angles and occlusal plane angles relative to each force angulation.
|Displacement||0.019 × 0.025-in stainless steel archwire|