Evaluation of root resorption of mandibular anterior teeth at the end of stage I with three different alignment archwires

INTRODUCTION

In the initial phase of orthodontic treatment, focusing on tasks such as crowding correction, minor tooth adjustments, alignment, and leveling, clinicians seek wires that apply light and continuous forces efficiently, minimizing treatment duration and potential damage to supporting structures.^[1] Nickel titanium (NiTi) wires, particularly those with a low modulus of elasticity, have revolutionized orthodontics since their introduction. Initially described by Andreasen and Hilleman and further developed by Burstone and Miura.^[2] NiTi archwires offer various advantages owing to their unique crystalline forms: Martensitic (M) and austenitic (A). These wires can be engineered to exhibit superelasticity, a property reliant on the coexistence of these phases, enabling them to absorb deflection stresses and exert consistent forces over a wide range of deflections, particularly beneficial in aligning archwires.^[3] NiTi wires can be classified based on their crystal structure, phase transformation, and stiffness into stabilized martensitic, superelastic active austenitic, thermodynamic-active martensitic, and graded thermodynamic categories, each offering distinct force characteristics. Notably, thermodynamically active martensitic NiTi alloys, such as those incorporating copper, exhibit preset transition temperature ranges, allowing for controlled force application.^[4] Copper NiTi (CuNiTi) archwires, for instance, improved shape memory, reduced hysteresis, and enhanced thermal stability, promoting faster tooth movement with lower activation loads and diminished discomfort for patients. Despite manufacturers’ claims and theoretical advantages, studies reveal discrepancies in aligning efficiency and root resorption among NiTi alloys, prompting the need for comprehensive investigations.^[5,6] External apical root resorption (EARR), a concern in orthodontics, necessitates accurate diagnostic tools like cone-beam computed tomography (CBCT) to assess three-dimensional changes in root surfaces. While continuous forces applied during initial alignment facilitate tooth movement, they may also exacerbate EARR, highlighting the importance of understanding how different NiTi alloys impact this phenomenon. Therefore, this study aims to evaluate and compare EARR levels associated with stabilized nitinol (high force), superelastic active austenitic NiTi (medium force), and thermodynamic-active martensitic NiTi (low force) using CBCT, filling a crucial gap in understanding the interplay between different NiTi alloy characteristics and EARR outcomes in orthodontic therapy.

MATERIAL AND METHODS

Primary outcome methodology

Evaluation of root length

To analyze EARR using CBCT in mandibular anterior teeth, the linear length between the root apex and cementoenamel junction (CEJ) was measured by a single examiner. The maximum linear length between CEJ and root apex was measured using axial multiplanar reconstruction at a 0.25-mm isometric voxel, which is based on an axial-guided navigation technique.^[7] The axial movement of the cursor on sagittal or coronal multiplanar reconstruction defined the reference points [Figures 1-3].

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Figure 1:

Reference points Cemento enamel junction(CEJ) to Root apex (yellow colour arrows) to measure linear length (red line) (a) Reference points to measure linear length (CEJ to Root apex) on central incisor; (b) Reference points to measure linear length (CEJ to Root apex) on lateral incisor; (c) Reference points to measure linear length (CEJ to Root apex) on canine.

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Figure 2:

Axial multiplanar reconstruction with identification of cementoenamel junction (CEJ) as a reference point. (a) Coronal (b) sagittal (c) multiplanar reconstruction identifies CEJ at the intersection of the axial cursor with coronal and sagittal cursors.

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Figure 3:

Axial multiplanar reconstruction with identification of root apex as reference points. (a) Coronal (b) sagittal (c) multiplanar reconstruction identifies the root apex at the intersection of axial cursor with coronal and sagittal cursors.

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The distance between the reference points was marked in the sagittal or coronal multiplanar reconstruction, providing measurements in millimeters [Figure 4]. EARR was measured before and after levelling and alignment, and data were recorded using Microsoft Office ExcelTM 2007.

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Figure 4:

Tracing with CBCT software measuring tool on (a and b) sagittal and (c and d) coronal.

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Evaluation of root volume

Digital Imaging and Communications in Medicine datasets were imported into ITK-Snap software (version 3.2, http://www.itksnap.org) to generate stereolithographic (STL) data [Figure 5]. The radiographer assigned a number to each STL image; the primary investigator (P.P.) was blinded to the codes. The corresponding STL images for T0 and T1 were imported into Geomagic (Geomagic, Cary, N.C.).

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Figure 5:

Three-dimensional segmentation of the lower anterior teeth.

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The reference plane was constructed between the highest point of the labial and palatal CEJ. T0 to T1 STL images were segmented immediately below the reference plane, and the root portion was analyzed.^[8] Root volumes were computed using Geomagic software [Figure 6].

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Figure 6:

Three-dimensional segmentation of individual root at the reference plane and root volume was measured.

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RESULTS

Participant flow

A total of 66 patients were invited to participate in the study, with 22 participants each in the CuNiTi group, superelastic NiTi group, and nitinol group. The participant flow is depicted in the consolidated standards of reporting trials (CONSORT) flowchart [Figure 7].

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Figure 7:

Consort flowchart of participants through each stage of the trial. CuNiTi: Copper nickel-titanium, NiTi: Nickel-titanium.

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DISCUSSION

This study was designed to determine whether there are any clinical differences between nitinol, superelastic NiTi, and CuNiTi archwires in terms of root resorption and alignment efficiency. The results showed a statistically significant difference between the three types of NiTi arch wires.

Superelastic NiTi archwires need to be bent by 50–70° to effectively harness their super-elastic properties. This level of deformation is achievable in clinical settings due to the degree of crowding and reduced interbracket distances found in lower anterior crowding cases. The main difference between superelastic and thermal wires is in the force levels. Thermally active wires produce significantly lower working forces than superelastic wires of the same size.^[9,10] In vitro studies^[11] prefer thermal wires to superelastic wires during the alignment phase because of their lower working forces and ability to express the particular characteristic of superelasticity at lower deflection levels. Moreover, these wires permit so-called “full-bracket engagement” at the start of treatment and in subjects with severe dental crowding, decreasing the risk of generating excessive forces.

In the present study, the end of the leveling phase was chosen as the time point for the evaluation of the effect of the three different arch wire materials, owing to the possibility of using only one type of arch wire material before the continuation of treatment that would require both types of arch wires. In addition, the amount of root shortening 6–12 months after bracket placement is of high predictive value for the severity of root resorption after the completion of treatment.^[12] Mandibular incisors were selected for the study because they are more prone to tooth resorption, while a small reduction in their length is easy to detect using radiographic techniques.

The results of the present study reflected an overall reduction in root length and root volume in the three groups after levelling and alignment. CuNiTi showed the least root resorption, followed by superelastic NiTi and Nitinol.

The results of the current study were consistent with those of Nabbat and Yassir^[13] and Alzahawi et al.^[14] who found that lower incisors appeared to be significantly affected by root resorption in the group treated with super-elastic NiTi wires. Jain et al.^[5] observed higher root resorption in super-elastic NiTi wires (0.59 mm) than in heat-activated NiTi (0.50 mm); however, the difference was not statistically significant. This may be related to the continuous force exerted by the super-elastic NiTi wires. Whereas, intermittent forces could produce less root resorption because the period of inactive tooth movement provides the chance for the resorbed cementum to heal. Weiland^[15] stated that the perimeter, area, and volume of resorption lacunae were significantly larger (140%) when the teeth were moved with superelasticity than when they were moved with steel wires. The continuous force of 0.8-1 N used for tipping tooth movement is more detrimental than an initially higher but rapidly dissipating force. This finding underscores the importance of continuous radiographic follow-up during orthodontic treatment to monitor root resorption evaluating root resorption, especially when using superelastic NiTi and nitinol wires. The results also highlight the efficiency of thermally active CuNiTi wires in producing significantly lower working forces, which could be optimal in preserving the integrity of periodontal structures, thus minimizing root resorption during the levelling phase of mechanotherapy.

An orthodontic force can be defined as heavy or light according to the ratio between the magnitude of the force applied and the affected root–bone surface. Because of the surface area of the periodontal structure involved, an ideal archwire should be able to deliver differential forces to the arch segments.

The results of this study reflected a statistically significant reduction in the average root length of the six mandibular anterior teeth at the post-alignment time point with nitinol, superelastic, and CuNiTi. A similar observation was reflected in the average root volume between the 2 time points in the three groups. The amount of resorption observed in our study with superelastic NiTi wires (0.53 mm) is close to the values reported by Jain et al. (0.59 mm).^[5] However, the root resorption observed in our study with CuNiTi (0.25 mm) was significantly lower than that reported by Jain et al. (0.50 mm).^[5] This difference could be due to variations in the manufacturer and batch differences. It has been established that wires of similar types from different manufacturers do not necessarily possess similar properties because these manufacturing conditions are not consistent. Furthermore, it is established that notable differences exist from production lot to production lot of wires from the same manufacturer.^[16]

Alzahawi et al.^[14] reported significantly higher values than the root resorption values in the current study, which could be due to overestimation usually observed with periapical radiographs. The greater reduction in root length and volume observed in canines compared to central and lateral incisors may be attributed to the increased load at the apical area during distal tipping and uprighting movements. These findings highlight the importance of considering differential forces across the dental arch to optimize outcomes and minimize adverse effects during orthodontic treatment.

While the current study provided valuable insights, prior studies evaluating the efficacy of initial aligning archwires often faced limitations such as variations in bracket schemes, ligation techniques, extraction methods, and observation periods. To address these gaps, this study standardized these variables, ensuring a robust evaluation of the three types of NiTi wires.

In terms of alignment efficiency, CuNiTi wires outperformed superelastic NiTi and nitinol wires, requiring the shortest time to achieve alignment. This finding is consistent with the study by de Castro Serafim et al.^[17] who also reported superior alignment with CuNiTi wires. The ability of CuNiTi wires to express their superelastic properties fully, even with smaller deflections, likely contributed to their efficiency in achieving alignment. This contrasts with findings by Abdelrahman et al.^[2] and Pandis et al.,^[18] where no significant differences in alignment efficiency were noted among different NiTi wires. These differences may stem from variations in study designs, including wire dimensions, irregularity indices, and observation periods. The authors in their study were not conclusive about the amount of irregularity, and the observation period was expanded to 6 weeks until the initial alignment was completed with a 0.014-inch arch wire. The authors reasoned that large deflections in the wire were required to express the super-elastic property, which was not fully expressed in their study.

Pandis et al.^[18] reached a similar conclusion by comparing the efficiencies of CuNiTi and NiTi archwires. He reported that severe crowding (>5 mm on the irregularity index) showed a significantly higher probability of duration of crowding alleviation relative to dental arches with a score of <5 (138.5 vs. 113.1 days). The values reported in the current study were CuNiTi versus superelastic NiTi versus nitinol (105, 112, and 118 days). Pandis et al.^[18] confined a 0.16 dimension wire during the entire period of 6 months observation, while the current study employed full dimensions rectangular wires in the three groups, achieving complete slot engagement, greater deflection, and higher superelasticity, resulting in increased alignment efficiency. The results of the present study are consistent with a clinical trial by phermsang-ngarm^[19] who reported that heat-activated NiTi wires required less alignment time than super-elastic NiTi wires. However, studies by Nabbat and Yassir^[13] Azizi et al.^[20] Atik et al.^[21] Aydin et al.^[12] found that both superelastic NiTi and CuNiTi were equally effective in the aligning stage of orthodontic treatment. The abovementioned studies had limitations that could hinder the complete expression of the properties of the respective NiTi wires, such as an irregularity index that requires non-extraction orthodontic treatment, smaller-dimension round wires, and a limited span of observation spanning around 6–12 weeks.

CONCLUSION

The following are the salient conclusions from the present study.

1. Root resorption of mandibular incisors after levelling was statistically different between the three groups of NiTi wires

2. Root resorption was minimal with CuNiTi and maximum with conventional nitinol wires

3. CuNiTi exhibited superior alignment efficiency over superelastic NiTi and conventional nitinol wires.