Accuracy of lingual fixed retainers fabricated using a CAD/CAM bending machine

Abstract

Objective

Lingual fixed retainers, made from 0.0175-inch 3-strand twisted stainless steel wire (TW) and 0.016 × 0.022-inch straight rectangular wire (RW), are generally used in clinical practice. This study aimed to calculate their accuracy by comparing the discrepancy between computer-aided customized retainers made from these two types of wires.

Methods

Eleven orthodontic patients were selected, resulting in 22 maxillary and mandibular three-dimensional printing dental models. Two types of lingual fixed retainers were bonded from canine to canine. To determine the accuracy, five points were chosen for each model, resulting in 110 selected points. The absolute values of the distances on the x-, y-, and z-axes were measured to compare the accuracy of the two types of computer-aided retainers.

Results

The accuracy of the two types of retainers did not differ significantly in the x- and z-axes, but only in the y-axis (P < 0.01), where RW-fixed retainers exhibited a slightly but significantly increased distance compared to the TW.

Conclusions

Both types of retainers showed high accuracy; however, RW had a slight but statistically significant difference along the y-axis compared with TW. This type of computer-aided design/computer-aided manufacturing bending machine is limited to two dimensions, and the dental arch is curved. Therefore, RW may require slight manual adjustment by the practitioner after manufacturing.

INTRODUCTION

Teeth tend to return to their original position after orthodontic treatment.¹ It is inevitable that a rapid-to-slow relapse occurs during the remodeling of periodontal structures.² Orthodontic treatment is crucial, but maintaining teeth positions during retention periods is also a significant aspect of orthodontic treatment. Unfortunately, fewer than half of patients can maintain the position of their teeth for 10 years, and less than 10% can do so for 20 years.³ Therefore, retainers are commonly used to prevent teeth from reverting to their original positions. There are two types of retainers: removable and fixed. Controversy exists over which type to use. Indeed, fixed retainers carry risks, such as calculus accumulation and plaque formation, which can contribute to the development of periodontal disease.⁴ In contrast, removable retainers offer the advantage of easy removal and facilitate easier cleaning. However, the efficacy of removable retainers heavily relies on patient compliance; lack of cooperation can result in relapse.⁵ With an increasing emphasis on enhancing quality of life, invisible fixed retainers are preferred, leading to the widespread adoption of lingual fixed retainers in clinical practice.⁶ Ideally, fixed retainers should be easily placed and possess flexibility to accommodate biological tooth movement. Thus, multistranded wires have been introduced to make fixed retainers.⁷ However, stainless-steel rectangular wires have greater stiffness than multistranded wires. Fracture decreases with thicker retainer wires, and they often function as a splint in patients with less periodontal support.⁸ A study has also shown that rectangular wires have a better retention effect than multistranded wires and can prevent unnecessary tooth movement.⁹ Rahimi et al.¹⁰ showed that both rigid and flexible retainers have no significant clinical effect on tooth retention. Regardless of the type of lingual fixed retainer, its clinical use depends on its purpose.

Nevertheless, the manual fabrication of lingual fixed retainers is difficult because of their time-intensive processes and accuracy issues, potentially leading to unnecessary side effects, such as torque, which can cause unnecessary tooth movement. With technological advancements, a new type of lingual fixed retainer has been developed using computer-aided design/computer-aided manufacturing (CAD/CAM). These retainers can be made using a custom-cut or custom-bent machine.¹¹ The custom-cut machine, such as Memotain (CA-Digital, Hilden, Germany), utilizes a nitinol sheet for cutting. It is made of a Ni-Ti material, which is flexible and fits the tooth surface very well through computer design. This causes minimal stimulation of the tongue and results in high patient satisfaction. However, because it is made of Ni-Ti, its hardness is low, causing it to break easily during later retention. Additionally, it incurs additional costs and time to purchase an additional nitinol sheet.^12-14 In contrast, the custom-bent machine can use wires commonly used in clinical settings, offering a more cost-effective and practical solution. Consequently, custom-bent machines are frequently preferred in clinical practice.¹⁵ The increasing popularity of CAD/CAM-made lingual fixed retainers in clinical practice can be attributed to their numerous advantages over conventional multistranded stainless-steel wires. These advantages include improved accuracy, time efficiency, preservation, and the ability to maintain suitable tooth movement as an active lingual retainer.¹²

A custom-bent lingual fixed retainer made using CAD/CAM has been newly invented, and as such, there is limited research available regarding its accuracy and effectiveness. This study aimed to assess the accuracy of two types of wires fabricated using CAD/CAM technology in three dimensions, followed by a comparative analysis of their precision.

MATERIALS AND METHODS

The study was approved by the Institutional Review Board of the Kyung Hee University Dental Hospital (IRB no. KH-DT-23037). The requirement to obtain informed consent was waived.

A total of 22 maxillary and mandibular scan datasets from 11 patients who underwent orthodontic treatment at the Department of Orthodontics, Kyung Hee University Dental Hospital, were selected. The inclusion criteria were: (1) no missing teeth, (2) clear shape of all maxillary and mandibular anterior teeth, (3) crowding of the teeth by less than 2 mm, (4) bonding of the retainer from the canine to the contralateral canine, and (5) completion of orthodontic treatment by December 2023. This study utilized a bending machine, Bender II (FixR, YOAT Corp., Lynnwood, WA, USA), and used two different types of wires: (1) a 0.0175-inch 3-strand twisted stainless steel wire (TW; Dentaflex, Dentaurum GmbH & Co., Ispringen, Germany) and (2) a 0.016 × 0.022-inch straight rectangular wire (RW; Dentaurum GmbH & Co.).

Existing three-dimensional (3D) oral scan files (Medit i700, Medit Co., Seoul, Korea) of selected patients were used and transferred to a 3D printing software (RayWare 2.9.2, SprintRay Inc., Los Angeles, CA, USA). The models were directly printed using Graphy S-100M gray resin (GR-RE-MOD-001, Graphy Inc., Seoul, Korea), resulting in 22 3D-printed dental models, including both maxillary and mandibular models. Two types of retainers, designed using CAD/CAM, were fabricated using a single bending machine and glued to a 3D-printed model. A flowchart is shown in Figure 1.

Figure 1.

Study flowchart.

STL, stereolithography; 3D, three-dimensional; Mx, maxilla; Mn, mandible.

Computer-aided retainer design

Three dimples were created using a round bar on both sides of the canine, the right incisor of the maxillary teeth, and the left incisor of the mandibular teeth. Subsequently, a plane conducive to the position of the retainer was created. The models were scanned using an intraoral scanner (Mediti700, Medit Co.), and the resulting stereolithography (STL) files were transferred to the CAD software (FixR, YOAT Corp.). By selecting premade dimples, the software generated a plane and designed the retainer along the lingual surface of the teeth. The same 3D-printed dental model was employed to print both types of retainers (Figure 2).

Figure 2.

Procedure for design and fabrication of lingual fixed retainers on the three-dimensional (3D)-printed models. A, Make the dimples on the 3D-printed dental model using a round bar; B, final three dimples on the model; C, make a reference plane through the dimples; D, click three points and design the retainer; E, 0.0175-inch 3-strand twisted stainless steel wire bonded on the model; F, 0.016 × 0.022-inch straight rectangular wire bonded on the model.

Measurements

After bending, CAD software FixR (YOAT Corp.) was used to generate a computer-designed retainer and model the STL file. The retainer was fabricated on the model through three dimples using instant adhesive and then scanned again. Rapid Form 2006 software (3D systems Inc., Rock Hill, SC, USA) was used to overlay the computer-designed STL file with the scanned STL file obtained after actual bending. A reference plane was established through the mesial contact points of the two central incisors and mesiobuccal cusps of the first two molars. Five interproximal surfaces were set up from the canine to the contralateral canine. The interproximal plane was perpendicular to the reference plane and passed through the contact point between the two teeth and the center point of the reference plane. The distance between the outermost point where the computer-designed lingual fixed retainer and the actual product intersected the interproximal plane was measured. A total of 110 points were calculated for each group (Figure 3).

Figure 3.

Measurement of absolute distance. A, Overlay two scan data stereolithography files and set a reference plane; B, click the contact points between the two teeth; C, perpendicular to the reference plane and passing through the contact points between the two teeth forms an interproximal plane; D, measure the distance between two outermost points.

Statistical analysis

All statistical analyses were performed using SPSS Statistics version 27 (IBM Corp., Armonk, NY, USA). All measurements were taken by a single examiner. The data were measured twice within a 1-week interval, and the intraclass correlation coefficient (ICC) was tested. Results with ICC values exceeding 0.85 indicated high consistency and reliability. We divided the 3D space into three vectors, and the absolute value of the distance between two points on each vector was measured. All continuous variables were tested using the Shapiro−Wilk test for normal distribution. The results show that the data for the x- and z-axes were not normally distributed. The means, standard deviations, medians, and interquartile ranges were calculated. A total of 220 points were calculated, with 110 points per group. The Mann−Whitney U test was used to compare retainer accuracy on the x-axis (horizontal) and z-axis (sagittal), whereas a paired t test was used to assess accuracy on the y-axis (vertical). The threshold for statistical significance for all tests was set at P < 0.05. Additionally, if the absolute distance was less than 0.5 mm, it indicated the clinically applicable range.¹⁶

RESULTS

The 3D overlay software revealed that the mean values of the absolute distance in both groups were < 0.5 mm in all three axes, indicating high accuracy regardless of the wire type used. However, a difference in accuracy was observed between the two wire types, with RW (x-axis: 0.07 mm, y-axis: 0.32 mm, z-axis: 0.16 mm) demonstrating lower accuracy compared to the TW (x-axis: 0.07 mm, y-axis: 0.24 mm, z-axis: 0.18 mm). Furthermore, a significant difference was detected on the y-axis between the two groups (P = 0.001). Notably, the x- and z-axes exhibited higher precision than the y-axis for both groups (Table 1).

Table 1.

Comparison of the absolute distance between stainless steel rectangular wire and 3-strand twisted stainless steel wire

Axis	0.016 × 0.022-inch stainless steel rectangular wire	0.0175-inch 3-strand twisted stainless steel wire	P value
x†	0.07 (0.03, 0.17)	0.07 (0.03, 0.12)	0.177
y‡	0.32 ± 0.19	0.24 ± 0.18	0.001**
z†	0.16 (0.07, 0.31)	0.18 (0.08, 0.30)	0.750

^†Mann−Whitney U test for the absolute distance (mm). Values are presented as median (interquartile range).

^‡Paired t test for absolute distance (mm). Values are presented as mean ± standard deviation.

**P < 0.01.

DISCUSSION

Lingual fixed retainers have become the gold standard for maintaining the stability of the anterior teeth following orthodontic treatment.¹⁷ CAD/CAM technology is widely utilized in the dental field, leading to a surge in studies focusing on lingual retainers produced by CAD/CAM. Following some short-term studies, lingual retainers produced by CAD/CAM achieved satisfactory results in terms of accuracy, stability, and periodontal performance.¹⁸ Currently, two types of machines utilize CAD/CAM technology to manufacture lingual fixed retainers: custom-bent and custom-cut machines. The primary advantages of a custom-bent machine are its time-saving and cost-effective nature. Patients can receive their retainers 10 minutes after the intraoral scanning procedure, offering convenience and efficiency. Although extensive research exists on custom-cut machines, there are inadequate studies focusing on custom-bent machines.

Two types of wires are available for custom-bent machines: multistranded steel wires and rectangular straight wires. Through experimental analysis, we evaluated the accuracy of the fabricated retainers on the three axes. The results indicate a slight deviation between the computer-designed models and the actual product across all axes, with an average discrepancy of < 0.5 mm. In this study, we found no significant differences in accuracy along the x- and z-axes, but RW showed slightly less accuracy along the y-axis than TW. The observed difference in accuracy along the y-axis can be attributed to the mechanism of the custom-bent machine. Specifically, a pin inside the machine hammers the inserted wire in a two-dimensional direction, guided by the imported 3D oral scan STL file. However, the shape of the dental arch is not just a horizontal plane; it is an arc with curvature at both the front and back. After orthodontic treatment, teeth tend to relapse, and their positions change in three dimensions.¹⁹ The custom-bent machine in this study could not bend the retainer in three dimensions. This limitation is less significant with a multistranded steel wire, as its round shape allows for highly fitted adaptation to the tooth surface. In contrast, the RW can only form an angle with the lingual plane of the teeth when placed, leading to a slight but significant error in the y-axis direction compared to the multistranded steel wire. Manual adjustment is sometimes necessary to ensure the optimal fit and function of RW retainers in clinical settings. This need for adjustment is more commonly observed in the maxilla because of its greater curvature and wider fan-out compared with the mandible.

The unnecessary force exerted on the teeth by the wire is typically attributed to two factors: the force generated by the wire itself and the force resulting from the wire's deformation due to external factors, such as biting.^19,20 Previous experiments have indicated that the multistranded stainless steel wire retainer exhibits a smaller torsional load transfer force and greater flexibility compared to a rectangular straight wire retainer. Consequently, multistranded retainers tend to store a larger force, which increases the risk of breakage. Additionally, owing to the rotational nature of this retainer, which is achieved through several stainless-steel wires, it can unwind.⁹

There are unavoidable factors in this experiment that may influence accuracy. Dental models may contain machine errors owing to the 3D printing process. Additionally, errors may have occurred during the bonding of the retainer to the model. Further studies considering the refinement of RW retainers fabricated with a custom bending machine and the long-term stability of TW and straight RW retainers in terms of unwinding are needed.

CONCLUSIONS

From this study, both the 0.0175-inch three-strand twisted and 0.016 × 0.022-inch straight stainless steel wires made by CAD/CAM custom-bent machines exhibited high accuracy with < 0.5 mm; however, TW retainers showed higher accuracy overall. Slight but statistically significant differences were observed on the y-axis between the two groups. 0.016 × 0.022-inch RW retainers may require manual adjustment by the practitioner after manufacturing. Stainless steel wire retainers, made by CAD/CAM custom-bent machines, are recommended for clinical use due to their accuracy, cost-effectiveness, and time-saving benefits, even though some minor improvements are needed for better adaptation.