Scheu Article - Material Properties & Clinical Aspects of a Novel Aligner Foil

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Fabian Karbach, Jens Johannes Bock

Material properties and clinical aspects of a novel aligner foil Originally published at Quintessence Publishing in Kieferorthopädie 2021;35(3):223-235

KEY WORDS Aligner, aligner material, digital orthodontics ABSTRACT The present article describes the material properties of a novel aligner foil and presents the performance of an orthodontic treatment.

Manuscript receipt: 27.06.2021, acceptance: 10.07.2021

Introduction The first of today’s aligner treatments dates back to 1944 1-3. Since the mid-1990s, orthodontic therapy with sequential splints has experienced a significant boom due to the introduction of CAD/CAMsupported processes.

Recent study results especially on Invisalign treatments (Align Technology, Santa Clara, California, USA) can be summarised as follows4-9: Therapy of more complex malocclusions is possible, To a limited extent comparable to the effectiveness of fixed appliances, Individual tooth movements are more difficult (e.g. extrusions, derotations), Use of attachments and approximal enamel reductions are very often necessary, High level of patient compliance is required (20 to 23 h wearing time daily).

Various thermoplastics have been recommended in the past as basic materials for splint production.

The requirements on the material properties of the thermoforming foils proved to be diverse and dependent on the indication10-12. The standard materials used to date have been single- or multilayer polyethylene (PET-G) or polyurethane materials (Tab. 1).

Apart from commercial providers, a concept was developed in 1998 by Tae-Weon Kim to plan and implement chairside fabricated sequential splint treatments in the dental office 10. The basic principles included: Use of foils of different thicknesses (0.5 mm/ 0.65 mm/0.75 mm), Splint material: polyethylene terephthalate glycol copolyester (PET-G), Setup creation on the plaster cast, Vacuum and pressure thermoforming processes, Regular impressions, Activations using specific forceps instruments, if necessary.

Further development and clinical application were continued by Pablo Echarri and supplemented by both analogue and digital planning aids11.

In the course of treatment implementation, various factors proved to be decisive for treatment success11-14. In addition to the precise and clinically oriented planning, the material type, processing technique, material thickness and accuracy of fit, among other things, proved to be particularly important11-14.

Seo et al.12 used simulated tooth movements in the finite element model to show that both 0.5- and 0.75-mm–thick aligner foils apply effective forces for the dentoalveolar transformation processes.

According to their results, about 6% more stress would be expected in the periodontal ligament when using the stronger foil.

Lombardo et al.13 indicated the dependence of the fit according to the manufacturer by means of three-dimensional (3D) micro-computed tomography (micro-CT) evaluations and showed a significant variance between the products. On the other hand, Mantovani et al. 14 was able to demonstrate a comparably accurate fit for Invisalign (Align Technology) and CA Clear Aligner (Scheu Dental, Iserlohn, Germany) splints.

Target In the following paragraphs the mechanical properties of a new splint material are described and its integration into a digital workflow is demonstrated using a patient case study.

Mechanical properties of a novel splint material As a new development, a three-layer composite of abrasion-resistant copolyester material and a thermoplastic elastomer was presented (Tab. 1, Fig. 1). For improved clinical use in comparison with conventional PET-G materials, the following properties were specified: high elasticity, good wearability, continuous force output, low force level.

The processing of the new type of splint material was oriented to the previously known workflow. The basic instructions are summarized in Table 2.

The material tests on the thermoformed foils were carried out using a universal testing machine (ZwickRoell 1445, ZwickRoell, Ulm, Germany), which was equipped with a 100 N force transducer (Xforce, ZwickRoell) and a miniaturised three-point flexural strength test apparatus in a water bath (inhouse special production). In modification of the standard commonly used for composites for the determination of flexural strength properties (DIN EN ISO 178), the testing machine was designed with a contact distance of L = 8 mm. The support and compression fin radius of the structure corresponded to 0.5 mm. The water bath, filled with distilled water, was controlled by an external thermostat (Julabo, Seebach, Germany) via a spiral heat exchanger. Before the test, the water bath was preheated to 37°C (± 0.1 K). During the test, the temperature of the water bath was digitally recorded and maintained

within the specified temperature range.

Specimen preparation and conditions The thermoforming foils (foil thickness 0.75 mm) were heated according to the manufacturer’s instructions (BIOSTAR, Scheu Dental) and placed over a smooth strip of polypropylene (thickness 2 mm, 90 × 60 mm). From the central area of the resulting blank, sample strips (40 × 105 mm) were cut out with the aid of a lever cutter (IDEAL 1058, Krug & Priester, Balingen, Germany). The samples prepared in this way were stored in water at 37°C for 24 h prior to testing and then placed in the previously described test machine. The specimens rested for another hour in the machine before the test was started.

The bending test was performed with a traverse speed of v = 1 mm/min. The starting point (deflection = 0 mm) of the test results was from the initial contact between the fin and the specimen, which was manifested by the increase in the measured force and registered by the testing software (testXpert II, ZwickRoell).

After reaching the initial contact, the fin was shifted to the stress point of the specimen (deflection 0.3 mm) and the initial bending force F0 was determined. Over the following 24 h, the course of the bending force Ft was recorded. The stress relaxation S is a result of the ratio of the bending force to the initial bending force (S = Ft/F0).

The cyclic loading and unloading test was performed with the same parameters as the fatigue loading test. In contrast to the fatigue loading test, however, the stress point (deflection 0.3 mm) was only maintained for 16 h.

After this loading phase, the fin was brought back to the starting point of the test and the specimen was unloaded for 8 h. This was followed by the next loading phase. In total, each specimen was subjected to seven cycles of loading and unloading and the bending force curve during the loading phases was recorded.

The analysis of the force-displacement data from the loading cycles additionally allowed the determination of the permanent deformation of the foil caused by the deflection.

Discussion of the test results The results of the static fatigue loading are presented in Figure 2. The conventional aligner material exhibits a very high initial bending force, which drops rapidly in the course of the loading test. After 24 h, only 38% of the initial measured bending force remains. In contrast, the three-layer CA Pro sheet has a lower initial bending force, which decreases less rapidly over the testing period. At the end of the testing period, the CA Pro sheet retains 51% of the initial bending force. According to these results, CA Pro aligners should have a more constant force application over the treatment period than conventional aligner foils. A lower initial force also increases the patient’s comfort.

Comparable to the results of the static permanent load, CA Pro is characterized by a less noticeable force decay compared with conventional aligner material (Fig. 3). Throughout all the loading and unloading phases, the force exerted by CA Pro drops less dramatically. In addition to the drop in force over time, the cyclic test also provides information about the plastic deformation of the foils under physical load. Figure 4 shows that after the first loading phase, the conventional aligner foil already exhibits 50% plastic deformation after the first loading phase. In contrast, CA Pro retains only 30% of the deflection. This trend continues over all testing cycles (Fig. 4). After six loading and unloading cycles, the conventional material has retained 70% of the deflection, while CA Pro has only undergone 50% plastic deformation.

Applied to aligner therapy, this means that CA Pro exerts a force on the teeth over a longer distance and time and can thus contribute to more efficient tooth movement in aligner therapy.

Figure 5 shows the clinically driven behaviour of the new material when placed in colouring fluids.

For CA Pro it was found that discoloration only occurs at the margins, if at all.

Clinical use of the novel splint material As a clinical example, the treatment procedure of a 36-year-old female patient is described (Fig. 6). The intraoral diagnostics can be summarized as follows: permanent dentition, Angle Class I, generalized attachment loss with exposed cervical areas and wedge-shaped defects, anterior narrow spaces in the maxilla and mandible, various rotating and inclining positions.

The patient was fully informed that orthodontic treatment with aligners was possible on condition that the splints were worn for at least 22 h per day and that the resolution of the narrow spaces was only possible within the respective dental arches by approximal enamel reduction to avoid further attachment loss.

After the intraoral scans had been taken and the occlusal references recorded, the 3D analysis to prepare the therapeutic steps was performed with OnyxCeph planning software (Image Instruments, Chemnitz, Germany). Due to the high-detail reproduction of modern intraoral scanners, it is possible to investigate difficult situations. In addition to an occlusogram, Figure 8 shows the extent of palatal destruction of the enamel of the maxillary incisors due to the artificial removal of the mandibular anterior teeth. A significant improvement of the incisor positions was defined as a major treatment goal to minimise the habitual influences on the dentoalveolar structures in the long term.

The preparation of a virtual setup (module VTO) and the calculation of the individual therapy steps were also carried out using the OnyxCeph software. In addition to determining the space required, the occlusal relation was checked in the occlusogram (Fig. 8) The calculated dental arches were generated as an STL file and fabricated in a 3D-printing process in order to thermoform a total of seven foils of the new CA Pro+ material. The processing of the splint material is based on known working steps and proved to be uncomplicated. To improve clinical retention, the foils were not cut out in a garland shape.

Before inserting the initial aligners, the proximal enamel was reduced in both anterior segments, according to the protocol (Fig. 9).

Compared with conventional splint materials, the aligners were noticeably more elastic (Fig. 10).

Both the insertion and the patient’s wearing comfort were improved. The treatment course was monitored clinically. After 10 weeks, the following results were recorded (Figs. 11 and 12): significant improvement of the dental arch, significant improvement of the anterior occlusal relation, resolution of the narrow spaces, slight residual gap in the area of teeth 11/21, Incomplete derotations in the maxilla/mandible.

The treatment continued with a new intraoral scan and the planning of two more splints for each jaw (Figs. 13 and 14). After a total of 16 weeks, a very good treatment result was achieved (Fig. 15). The success of the treatment was secured with fixed retainers in the maxilla and mandible (Figs. 16 and 17).

Conclusion The development of new materials in aligner therapy requires intensive material testing and should be confirmed by clinical studies. Initial clinical experience shows that the new type of splint material appears to be useful and has some advantages compared with conventional materials. Further clinical studies are necessary to confirm these initial clinical results.

References 1. Kesling HD. The philosophy of tooth positioning appliance. Am J Orthod 1945;31:297–304.

2. Weir T. Clear aligners in orthodontic treatment. Aust Dent J 2017;62 Suppl 1:58–62.

3. Hennessy J, Al-Awadhi EA. Clear aligners generations and orthodontic tooth movement. J Orthod 2016;43:68–76.

4. Rossini G, Parrini S, Castroflorio T, Deregibus A, Debernardi CL. Efficacy of clear aligners in controlling orthodontic tooth movement: a systematic review. Angle Orthod 2015;85:881–889.

5. Iliadi A, Koletsi D, Eliades T. Forces and moments generated by aligner-type appliances for orthodontic tooth movement: A systematic review and meta-analysis. Orthod Craniofac Res 2019;22:248–258.

6. Galan-Lopez L, Barcia-Gonzalez J, Plasencia E. A systematic review of the accuracy and efficiency of dental movements with Invisalign (R). Korean J Orthod 2019;49: 140–149.

7. Papageorgiou SN, Koletsi D, Iliadi A, Peltomaki T, Eliades T. Treatment outcome with orthodontic aligners and fixed appliances: a systematic review with meta-analyses. Eur J Orthod 2019;42:331–343.

8. Charalampakis O, Iliadi A, Ueno H, Oliver DR, Kim KB. Accuracy of clear aligners: A retrospective study of patients who needed refinement. Am J Orthod Dentofacial Orthop 2018;154:47–54.

9. Buschang PH, Ross M, Shaw SG, Crosby D, Campbell PM. Predicted and actual end-of-treatment occlusion produced with aligner therapy. Angle Orthod 2014;85:723–727.

10. Kim TW, Stückrad P. Das Clear-Aligner Konzept. ZWP online, 2011.

11. Echarri P. Clear Aligner. Madrid: Ripano-Verlag 2011.

12. Seo JH, Eghan-Acquah E, Kim MS, et al. Comparative analysis of stress in the periodontal ligament and center of rotation in the tooth after orthodontic treatment depending on Clear Aligner Thickness-Finite Element analysis study. Materials 2021;14:324.

13. Lombardo L, Palone M, Longo M, et al. MicroCT X-ray comparison of aligner gap and thickness of six brands of aligners: an invitro study. Prog Orthod 2020;21:12.

14. Mantovani E, Castroflorio E, Rossini G, Garino F, Cugliari G, Deregibus A, Castroflorio T. Scanning electron microscopy evaluation of aligner fit on teeth. Angle Orthod 2018;88:596–601.

Fabian Karbach Dr. (Doctor of chemistry) SCHEU-DENTAL GmbH Am Burgberg 20 58642 Iserlohn

Jens Johannes Bock Dr. med. dent.

Am Schlossgarten 1 36037 Fulda

Karbach / Bock

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Tab. 1 Table showing the different materials used.

Aligner

Manufacturer Material

CA Clear Aligner

SCHEU-DENTAL Polyethylene terephthalate glycol copolyester (PET-G)

Invisalign

Align Technology

CA Pro

SCHEU-DENTAL Three-layer copolyester elastomer

Multilayer thermoplastic polyurethane

Fig 1. The new CA Pro film: Three-ply structure. Outer layers: abrasion resistant copolyester material; core: thermoplastic elastomer (TPE).

Fig. 2 Recovery force and stress relaxation in N, depending on time. Bending test with 0.3-mm deflection.

Fig. 3 Recovery force and stress relaxation in N, depending on time during cyclic loading and unloading. Bending test with 0.3mm deflection.

Fig. 4 Deformation of the foil material as a percentage of the initial value, depending on the number of cycles. Bending test with 0.3-mm deflection.

Fig. 5 Discolouration of the CA Pro foil after immersion in various solutions. TPE = thermoplastic elastomers.

Fig. 6 Initial intraoral situation. Angle Class I on both sides, anterior narrow spaces and rotations in the maxilla and mandible.

Fig. 7a to c Initial intraoral situation in the digital model indicating the traumatic bite relation of the incisors. Virtual removal of teeth 41 and 42 and red colouring in the occlusogram (OnyxCeph).

Fig. 8a to c Virtual treatment planning: balancing the narrow spaces through approximal enamel reduction and compensation of the tooth misalignments through virtual tooth movements. Calculation of the intermediate steps = staging (Aligner Module, OnyxCeph).

Fig. 9 Intraoral situation after slicing in the maxilla and mandible before insertion of the first splints.

Fig. 10a and b After creating the virtual models, the output of the dental arches 3D printed, followed by the production of the aligners (CA Pro, SCHEU-DENTAL). Maxilla: three splints (a); Mandible: four splints (b).

Fig. 11 Interim intraoral results after 10 weeks. Improvement of the tooth positions has been achieved. Slight residual gap in the maxilla and incomplete derotations.

Fig. 12a to c Intermediate intraoral results in the digital model. Significant improvement of the incisor relations in the detailed image and occlusogram (OnyxCeph).

Fig. 13a and b Virtual treatment planning in the VTO (OnyxCeph) module. Shaded grey-blue area: planned fine adjustment with further aligners.

Fig. 14a and b Fine adjustment with additional aligners. Maxilla and mandible: two more splints.

Fig. 15 Intermediate intraoral results after a total of 16 weeks. Significant improvements of the anterior tooth positions.

Fig. 16 Securing of the treatment result with fixed retainers in the maxilla and mandible.

Fig. 17a to j Comparison between initial and final situation.

Tab. 2 Brief description of the novel splint material.

Notes Indications

Sequential splint therapy to correct malocclusion through aligner therapy

Contraindicatio Known incompatibilities with the used materials, ns no adhesive buildup possible with acrylates Procedure

Vacuum and pressure moulding at 150 to 175°C Insulating foil or CA Pro+ for 3D-printed models If necessary, block out undercut areas Must be processed within 15 min after opening the barrier bag Trimming and finishing with specific instruments

Cleaning/care

No oxidising agents or solvents Optimal: specific cleaning powders for daily use

Available material thicknesses

0.5 mm 0.65 mm 0.75 mm

a b c b a c a b a b c a b a b c h e d i f a j g b

Fabian Karbach

Adress Dr. Fabian Karbach, Email: fabian.karbach@scheu-group.com

Material properties and clinical aspects of a novel aligner foil KEY WORDS Aligner, aligner material, digital orthodontics ABSTRACT The present article describes the material properties of a novel aligner foil and presents the performance of an orthodontic treatment.