Computation and Collation of Torque Expression in 0.018 Inch and 0.022 Inch Preadjusted Bracket Slots on Passive Insertion of Full-Size Archwire: A Finite Element Study.

Aim: To quantify and compare the prescribed torque expressed in preadjusted edgewise 0.018-inch and 0.022-inch bracket slot on passive insertion of full-size archwire using finite element method. Materials and
Methods: Geometric model of the maxillary arch created with dimensions of the brackets and archwires scanned using microcomputed tomography were converted to finite element model using HYPERMESH V 12.0 software. Material properties were assigned and boundary conditioning was given. 0.017 × 0.025 inch, 0.018 × 0.025 inch, 0.019 × 0.025 inch, 0.021 × 0.025 inch stainless steel wires were inserted and ligated using 0.010 inch stainless steel wire. The amount of torque in each wire was quantified and compared.
Results: 0.018 × 0.025-inch stainless steel archwire in 0.018-inch slot expressed more torque than 0.017 × 0.025-inch wire; in 0.022-inch slot 0.021 × 0.025-inch wire showed more torque. Torque expression in 0.018-inch slot was more than 0.022-inch slot. Torque expressed was less than the prescribed value in both bracket slots. Copyright:

INTRODUCTION

Crown or root inclination of tooth in the buccal or palatal direction produced by twisting the arch wire in the bracket slot is called torque. The natural tooth has its own crown and root inclination for the proper establishment of occlusion and stability.[1] The proper establishment of torque in the anterior and posterior teeth during orthodontic treatment is needed for the establishment of occlusion, smile arc, proper anterior guidance, and cusp-fossa relationship.[2]

Incorporation of torque in orthodontics has evolved from edge-wise brackets to Begg's where the arch wire is twisted to produce torque to Andrew's where torque was incorporated into the bracket's face or base to give desired torque in the tooth. All these systems had the drawback of time-consuming wire bending needed for torque expression.[34]

Evolution of brackets from Andrew's straight wire appliance was done by Roth, where he incorporated more torque in the maxillary incisor brackets and reduced torque in the maxillary canines to counteract the effect. “Super torque” brackets were also introduced for the correction of Class II Division 2 cases.[5] Further improvisation in the preadjusted brackets was done by Mclaughlin, Bennett, and Trevisi to prevent anchorage loss, palatal cusp hanging, and interference with the lateral excursion. They added more lingual crown torque in the lower anteriors and more buccal root torque in the maxillary posteriors.[3]

Various factors which contribute to the torque expression are wire size, slot size, edge bevel, angle of engagement of wire in the slot, wire stiffness, tooth morphology, and ligation method.[6] Studies on slot size have concluded that 0.018-inch slot has better torque control due to the advantage of early filling of the slot with full-size archwire. Studies on the archwire dimension which forms another factor conclude that 0.017 × 0.025 stainless steel archwire in 0.018-inch slot, due to complete engagement of slot has better torque expression than 0.019 × 0.0 25 stainless steel archwire in 0.022 slot where there is a 10° of play between archwire and slot. Play between the archwire and slot is more than given theoretically due to inaccuracies in the slot and wire dimensions during manufacturing.[78]

Various in vitro and in vivo studies have been studied for torque expression with its own disadvantages. Finite element analysis (FEA), is used to study stresses of the model in complex structures such as the stomatognathic system where in vitro or in vivo studies have limitations. Torque expression on insertion of full-size final stage archwires, influence of adjacent tooth on torque expression, and comparison of torque expressed in two slot dimensions in degrees were not studied. Hence, the aim of the study is to compare the amount of torque expressed in two full-size stainless steel archwire during passive insertion in MBT 0.022 and 0.018 slots using the finite element method (FEM) in degrees.

MATERIALS AND METHODS

Pre adjusted edgewise stainless steel brackets of 0.018 inch and 0.022-inch slot dimensions (AO mini master series) with prescribed torque [Table 1] from right maxillary second premolar to left second premolar and four dimension stainless steel archwires - 0.017 × 0.025 inch, 0.018 × 0.025 inch, 0.019 × 0.025 inch, and 0.021 × 0.025 inch were selected. Dimensions were obtained using microcomputed tomography scanning (Zeiss xradia, versa XRM 500). Brackets were mounted on a holder at an angle to the radiation firmly and scanning was done after all parameters set [Figures 1 and 2].

Table 1

Prescribed torque values of Mini Master series MBT brackets

	Central incisor	Lateral Incisor	Canine	1st premolar	2nd premolar	1st molar	2nd molar
Torque	17°	10°	-7°	-7°	-7°	-14°	-14°

Figure 1

Interior view of Zeiss Xradia, Versa XRM 500, and mounting of sample

Figure 2

Scanning of brackets using microcomputed tomography

Two-dimensional slices obtained were saved in DICOM file (Digital imaging and communications in medicine created by National electrical manufacturers association) and converted to stereolitheography using 3D slicer (version 4). The finite element model was obtained through three steps-preprocessing, processing, and postprocessing.

Processing

The geometric model was constructed by dividing the model into simpler elements which were connected at specific points called nodal points and meshing was done. Geometric models of the teeth were constructed using standard dimensions given in Wheeler's textbooks on anatomy, physiology and occlusion.[9] Each tooth was assigned with PDL thickness of 0.025 mm around which bone was modeled.

Images of brackets and archwires were also converted into elements [Figures 3-6]. The finite element model was constructed from the geometric model using HYPERMESH 12.0 software (by Altair engineering Inc., Michigan, USA). Models were assigned with material properties – Young's modulus of elasticity and Poisson's ratio [Table 2]. Models were given boundary conditions to constraint movements of structure that were not intended to be displaced during load deflect. 0.017 × 0.025 inch, 0.018 × 0.025 inch, 0.019 × 0.025 inch, and 0.021 × 0.025-inch stainless steel wires were inserted in the FEA model and ligation was done for each wire insertion. The amount of torque expression was calculated in each wire [Figure 7].

Figure 3

Creation of geometric model of brackets from DICOM images

Figure 6

Finite element model of the maxillary arch with boundary conditioning and load application

Table 2

mechanical properties of bone, tooth, pdl, cementum, brackets, arch wires

Materials	Young’s modulus (Mpa)	Poisson’s ratio
Tooth	19600	0.3
Compact bone	13700	0.3
Cancellous bone	1370	0.3
PDL	0.667	0.45
Cementum	14000	0.3
Brackets	20000	0.3
Archwires	20000	0.3

Figure 7

Torque expression of central incisor in 0.018 × 0.025-inch slot (a) 0.017 × 0.025 inch archwire (b) 0.018*0.025 inch archwire. Torque expression of central incisor in 0.022 × 0.025-inch slot (c) 0.019 × 0.025 inch archwire (d) 0.021*0.025 inch archwire

Creation of geometric model of buccal tubes from DICOM images

Creation of geometric model of maxillary arch-occlusal view

Solution processing

Stress distribution and displacement of each tooth were analyzed by Abaqus 6.14 software(developed by 3ds Dassault systems Simulia corp.). The amount of torque expressed on insertion of each wire was quantified in degrees by applying moment and calculated using the vector formula.[10] cosθ = a.b/|a| |b|. The obtained values were sent to statistical analysis.

RESULTS

The statistical results of the data obtained from the finite element study are represented in Table 3. Comparison of two wires in 0.018 × 0.025-inch slot showed torque expression was better in 0.018 × 0.025-inch archwire than 0.017 × 0.025-inch archwire [Graph 1]. Comparison between two slots showed torque expression was better at 0.018 × 0.025-inch slot than 0.022 × 0.025-inch slot.

Table 3

Comparison of torque values between 0.018×0.025 inch and 0.022×0.025-inch slot

	n	Mean	SD	95% CI for mean		F	P
				Lower bound	Upper bound
Central incisor
0.017×0.025	10	14.01	0.00	14.01	14.01	22,406.25	0.001**
0.018×0.025	10	15.88	0.06	15.83	15.93
0.019×0.025	10	12.24	0.00	12.24	12.24
0.021×0.025	10	14.40	0.00	14.40	14.40
Lateral incisor
0.017×0.025	10	7.80	0.05	7.77	7.83	1757.63	0.001**
0.018×0.025	10	8.39	0.03	8.37	8.41
0.019×0.025	10	7.30	0.00	7.30	7.30
0.021×0.025	10	8.12	0.04	8.09	8.15
Canine
0.017×0.025	10	–5.84	0.23	–6.00	–5.67	74.06	0.001**
0.018×0.025	10	–6.50	0.05	–6.53	–6.47
0.019×0.025	10	–5.20	0.16	–5.31	–5.09
0.021×0.025	10	–6.10	0.28	–6.30	–5.90
1st premolar
0.017×0.025	10	–5.25	0.00	–5.25	–5.25	3082.05	0.001**
0.018×0.025	10	–6.20	0.00	–6.20	–6.20
0.019×0.025	10	–5.42	0.00	–5.42	–5.42
0.021×0.025	10	–5.60	0.05	–5.63	–5.57
2nd premolar
0.017×0.025	10	–5.41	0.09	–5.48	–5.35	231.24	0.001**
0.018×0.025	10	–6.01	0.00	–6.01	–6.01
0.019×0.025	10	–5.56	0.05	–5.59	–5.53
0.021×0.025	10	–5.78	0.05	–5.81	–5.75

CI: Confidence interval, SD: Standard deviation, **p < 0.001 statistically significant

Graph 1

Comparison between groups based on various wires

DISCUSSION

Root positioning of the teeth is vital for stability, function, and esthetics; In straight wire appliances, the rectangular archwires are used for torque control and correction. Factors such as bracket placement, slot dimension, wire dimensions, tooth morphology, wire radius, wire properties, and method of ligation affect the torque expression.[6]

Burstone stated that a clinically efficacious moment is between 5 and 20 Nmm.[11] J Odegaard et al. concluded that the play between the wire and bracket was 5° for 0.018 × 0.025-inch wire in 0.018 slot and 20° for 0.016 × 0.022-inch wire in 0.018 slot. Elimination of the play resulted in better torque expression.[12]

Gioka and Eliades in their study suggested engagement of full-size archwire in high torque brackets were to be used.[2] In vitro study conducted by Sifakakis et al., an orthodontic measurement and stimulation system was used to stimulate tooth movement close to clinical situations. The shortcoming of the study was that it failed to take into account the intra-oral factors influencing the tooth movement such as intra-oral aging and saliva.[6] The FEA is a highly precise technique used to analyze structural stress and initial displacement.

The result of the present study showed that torque expression on insertion of 0.017 × 0.025-inch stainless steel wire in 0.018-inch slot and insertion of 0.018 × 0.025-inch stainless steel archwire and Torque expression in the insertion of 0.019 × 0.025-inch stainless steel wire in 0.022-inch slot were compared.

The torque expressed in each tooth on insertion of various full-size archwires in each dimension were less than the torque values prescribed by the company due to various factors such as bracket dimension wire size, wire radiusing, tooth morphology and bracket positioning.[5]

In the FEM study, they are treated as isotropic and linear-elastic materials. The shape of the tooth used in the study is based on the common morphology of the maxillary teeth in general. There are variations in the morphology of the tooth for each individual which will affect the amount of torque expressed.[13]

SUMMARY AND CONCLUSION

The present study quantified and compared the torque expression in 0.018 inch and 0.022-inch slot on passive insertion of full-size archwire by finite element method. The results of the present study within its limitations concluded that:

Torque expression in 0.018 × 0.025-inch stainless steel archwire in 0.018-inch slot is more than 0.017 × 0.025 inch wire

Torque expression in 0.021 × 0.025-inch stainless steel archwire in 0.022-inch slot is more than 0.019 × 0.025 inch wire

Torque expression in 0.018-inch slot is more than 0.022-inch slot.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.