Digital workflows for 3D-printed customised double-slotted lingual appliances: a case report

Following its introduction in 2003, customised lingual appliances have become widely used and applied for the successful treatment of many complex malocclusions.^1–3 Customised lingual appliances have the advantages of higher bonding strength due to extended customised bracket bases and easier rebonding of failed brackets.^4,5 Customised lingual brackets have also been proven to reduce the incidence of white spot lesions compared with labial brackets.⁶

Customised lingual appliances have made lingual orthodontics easier so that there is confidence in applying the technique in routine practice.⁷ The inaccuracy of the direct manual procedure of lingual bracket placement and wire bending may be minimised due to the advancement of computer-aided design and manufacturing (CAD/CAM). Recent studies have proven the high accuracy in achieving planned treatment outcomes using customised lingual appliances^8,9 which may also be combined with fixed functional appliances for the treatment of complex Class II malocclusions.¹⁰

At present, several brands of customised lingual appliances are available and include Incognito (3M Unitek, Monrovia, CA, USA), Harmony (American Orthodontic, Sheboygan, WI, USA), WIN (Lingual System, Bad Essen, Germany), eBrace (Riton Biomaterial, Guangzhou, China), and Lingual Matrix, (Mumbai, India). These systems share a common feature in that the customised brackets and arch wires are designed and produced by the manufacturers, which leads to disadvantages related to delayed initial bonding due to the delivery time and the need for bracket and arch wire re-orders if the appliances are lost or broken. Additionally, the laboratory fee for customised lingual appliances is usually high due to the complex manufacturing process.

The growing popularity of orthodontic software and three-dimensional (3D) designing freeware has enabled orthodontists to create customised lingual brackets and arch wires. The designs may be sent to a local dental laboratory or the manufacturing process could become totally in-office if the orthodontist acquires a 3D metal printer. If so, the manufacturing time could be shortened, the need for appliance reorders avoided, and the accompanying laboratory costs could also be reduced, which would make lingual orthodontic treatment using customised brackets more cost-effective and versatile.

The present article describes, in detail, the digital workflows for the design and manufacture of customised double-slotted lingual brackets and arch wires and demonstrates appliance efficiency via a non-extraction case report.

A 26-year-old female patient presented with a chief complaint of crowding in both arches. Her medical and dental history were non-contributory. The patient requested comprehensive orthodontic treatment with an aesthetic appliance.

The extraoral examination revealed a slightly convex profile with a normal nasolabial angle (Figure 1). The patient also had a mild facial asymmetry as the chin deviated to the right side. During posed smiling, approximately 70% of the upper incisor crown height was visible. No sign of a temporomandibular joint disorder was noted.

Figure 1.

On intraoral examination, the patient had a mild Class III molar relationship on both sides, a mild Class III canine relationship on the right side, and a Class I canine relationship on the left side (Figure 2). The upper and lower arch forms were ovoid. There was moderate crowding in both arches with arch length discrepancies of 7.8 mm and 7.9 mm in the upper and lower arch, respectively. The maxillary and mandibular right central incisors were in an edge-to-edge relationship. The mandibular molars and second premolars were mesially inclined. The upper dental midline was co-incident with the facial midline, but the lower dental midline deviated 1.5 mm to the left.

Figure 2.

The lateral cephalometric analysis revealed a skeletal Class I relationship with a normally positioned maxilla and mandible (SNA, 81.8°; SNB, 78.2°; ANB, 3.6°). The lower anterior facial height was within the normal range (FMA, 27.4°). The upper incisors were normally inclined (U1-SN,101.7°) while the lower incisors were slightly proclined (L1-MP, 96.8°). The soft tissue findings showed that the upper and lower lips were normally placed (upper lip to E-line, -1.2 mm; lower lip to E-line, -0.7 mm) (Table I). The panoramic radiograph showed the presence of all teeth except the mandibular third molars and confirmed the mesial inclination of the mandibular molars and second premolars (Figure 3).

Figure 3.

The following treatment objectives were established: (1) to align and level the dentition; (2) to correct the Class III molar and canine relationships to Class I; (3) to correct the edge-to-edge occlusion and achieve a normal overjet and overbite; (4) to correct the lower dental midline deviation; (5) to preserve the upper incisor inclination and retract the lower incisors; (6) to maintain the lower anterior facial height; and (7) to maintain the upper and lower lip position.

Due to the moderate crowding, the extraction of the four second premolars was the first treatment consideration. However, this option would retract the upper and lower incisors, which may flatten the facial profile. The second option was a non-extraction treatment plan in which space creation would be achieved by the combination of interproximal stripping, molar uprighting, and total arch distalisation. Because the patient did not want tooth extractions and the teeth had a triangular morphology which facilitated interproximal reduction, the second option was selected.

Initially, the patient chose customised lingual appliances for the upper arch and labial stock ceramic brackets for the lower arch. However, the day following bonding, the patient requested the replacement of the ceramic brackets with customised lingual appliances in the lower arch to achieve maximum aesthetics and remove the prominence of her lower lip due to the bulk of the lower labial brackets.

Initially, a digital impression was obtained and the data imported into orthodontic software (Autolign, Diorco, Gyeonggi-do, Korea) for tooth and gingiva segmentation. Subsequently, an ideal orthodontic setup was constructed incorporating corrected tip and torque values based on the established treatment objectives (Figure 4). Upon patient approval, a virtual double-slotted lingual bracket set was selected from the software library and placed on the virtual setup models according to the arch form. The arch form could be modified so that the brackets were positioned as close as possible to the lingual tooth surfaces. After confirmation of the final bracket positions, the teeth and accompanying brackets were virtually moved back to the presenting malocclusion situation. The initial models with attached brackets were exported as standard triangular language (STL) files and imported into freeware (Meshmixer, Autodesk, California, USA) to design the customised lingual appliances.

Figure 4.

The customised lingual bracket base was created by selecting an area of the lingual tooth surface and using ‘extrude’ or ‘offset’ commands. The base of the virtual stock bracket was removed and the bracket body was digitally joined to the bracket base to form the customised lingual bracket. The bracket bases could be engraved with tooth numbers for identification purposes. After designing all customised brackets, the data were exported as STL files and 3D-printed using a cobalt-chromium alloy, selective laser melting technology, and a metal 3D printer (NCL-M1100, Chamlion, Nanjing, China). Alternatively, wax patterns of the customised brackets could be formed using a resin 3D printer and castable resin following which, the brackets could be cast via the ‘lost wax’ technique. The fabricated brackets were finally sandblasted with 50 μm aluminum oxide particles at 3 bar pressure and polished using a metal polishing kit (PN 0309B, Shofu, Tokyo, Japan). Additionally, the brackets were put on the patient’s 3D-printed study models to perform required adjustments of the bracket bases to achieve a precise fit against the tooth lingual surfaces.

The virtual initial models incorporating the designed customised lingual brackets were also exported and printed using a 3D printer (Sonic Mighty 4K, Phrozen, Hsinchu, Taiwan) to make indirect bonding trays by a double-layer thermoforming technique.¹¹ The inner soft layer was formed from BIOPLAST foil (Scheu, Iserlohn, Germany) while the outer hard layer was constructed from BIOCRYL foil (Scheu, Iserlohn, Germany). Alternatively, transparent silicone or 3D-printed indirect bonding trays could also be used.

Treatment commenced by bonding all teeth with 0.018” × 0.025” double-slotted customised lingual brackets except the mandibular right central incisor due to inadequate space. A dual-cured resin cement (RelyX U200, 3M Unitek, Monrovia, CA, USA) or a low-viscosity orthodontic adhesive (Enlight, Ormco, Glendora, CA, USA) combined with a universal primer (Single Bond Universal, 3M Unitek, Monrovia, CA, USA) may be used. The initial alignment and levelling were achieved using 0.012”, 0.014”, and 0.016” nickel-titanium round arch wires combined with open coil springs to open space for the mandibular right central incisor and left second premolar. Because of the flexibility of nickel-titanium arch wires, they were difficult to shape and so stock arch wires (Lingual straight wire, Ormco, Glendora, Calif) were selected from three standard arch forms at this initial period.^12,13 The arch wire customisation is of minimal importance at this early stage because the nickel-titanium arch wires would not immediately express their arch form due to their flexibility. After gaining enough space, the mandibular right central incisor was bonded directly guided by the customised bracket base. The arch wire sequence was restarted from a 0.012” nickel-titanium round arch wire and progressed through stiffer arch wires.

Treatment continued using 0.016” × 0.016” and 0.016” × 0.022” stainless steel customised arch wires which were engaged to fully express bracket prescription values and arch form. The pre-determined arch forms were exported from the software and printed at a 1:1 ratio to serve as templates. The customised lingual arch wires were formed from straight lengths of wire by using a lingual turret to form the anterior arch curve. Subsequently, a hollow-chop or three-prong plier was used to shape the lateral angled arch wire segments until they matched the templates (Figure 6).¹⁴

Figure 5.

Figure 6.

After 5 months of levelling and alignment, the overjet increased and a Class II molar and canine relationship developed on the left side (Figure 7). Using a diamond disc (SuperFlex, Edenta, Kaltbrunn, Switzerland), interproximal reduction was performed in both arches from the left to the right first molars which reduced the proximal surface of each tooth by approximately 0.3 mm. Two miniscrews (diameter, 1.6 mm; length 8 mm; Medico, Gyeonggi-do, Korea) were placed in the palatal alveolar bone between the maxillary first and second molars. Entire upper arch distalisation forces of approximately 200 g on each side were applied by power chain from the miniscrews to four anterior lingual brackets. Power chains were also applied to the lower arch to close interproximal reduction spaces. The patient was also instructed to wear Class II elastics on the left side and anterior cross elastics (3/16-inch, 3.5 oz) for lower midline correction.

Figure 7.

After 5 months of distalisation, a normal overjet and co-incident midlines were obtained. Two labial ceramic brackets were bonded to the maxillary first and second premolars and first-order bends were placed in 0.016” × 0.022” stainless steel customised arch wires to improve tooth alignment (Figure 8). Additionally, labial composite buttons were bonded to apply vertical elastics for occlusal settling. The finishing stage, which took 2 months, resulted in a total treatment time of 12 months. After appliance removal, fixed retainers were bonded to both arches for long-term stability.

Figure 8.

An evaluation of the treatment outcome showed that all treatment objectives were achieved producing a well-aligned dentition and improved smile aesthetics (Figure 9). The anterior crowding was addressed with solid Class I canine and molar relationships on both sides and a normal overbite and overjet. The upper and lower dental midlines were co-incident with the facial midline and the interdigitation was excellent (Figure 10). The occlusal space for prosthetic restoration of the mandibular left second molars was adequately created. The upper incisal display improved but several black triangles were evident despite the triangular tooth shape inviting interproximal reduction.

Figure 9.

Figure 10.

A lateral cephalometric analysis showed a reduction in the proclination of the upper and lower incisors (U1-SN, 96.7°; L1-MP, 94.1°) despite the non-extraction treatment strategy (Figure 11). The lower anterior facial height slightly increased (FMA, 27.8°). The upper and lower lip projections were maintained (upper lip to E-line, -1.4 mm; lower lip to E-line, -0.2 mm). The panoramic radiograph showed good root parallelism without signs of root resorption. The uprighting of the mandibular molars and second premolars was apparent. The cephalometric superimpositions further confirmed the retraction of the upper and lower incisors, the slight extrusion of the upper incisors, the intrusion of the lower incisors, and the uprighting of the mandibular molars (Figure 12).

Figure 11.

Figure 12.

The one-year post-retention records showed that the treatment results were stable without signs of relapse (Figure 13). The patient was satisfied with the treatment outcome and the aesthetics of the lingual appliances during treatment.

Figure 13.

With the advancement of 3D-printing technology, orthodontists are able to design and manufacture customised lingual appliances, which may increase treatment versatility and reduce treatment costs. The presented case report showed that 3D-printed customised lingual appliances combined with miniscrews could successfully manage moderate crowding by a non-extraction approach over a short treatment time. Additional studies with large sample sizes should be conducted to confirm the effectiveness of the technique.