Does the trimline extension and attachment size affect maxillary arch expansion in clear aligner therapy ? A finite element study

The impact of numerous conventional orthodontic treatment methods on arch width has been effectively examined.^1–6 Nevertheless, the ongoing search for progress in orthodontics has heralded the advancement of clear aligners (CA), with the objective of providing patients with enhanced comfort, reduced treatment duration, improved oral hygiene, and increased compliance throughout treatment.⁷

CA therapy has proven to be successful in achieving maxillary dental arch expansion.^8–14 This method involves precise digital planning of tooth movements and allows the posterior teeth to be moved buccally through a combination of tipping and bodily movement.¹² Additional studies asserted that adding attachments enhances the efficacy of tooth movement, and increasing the number of attachments applied to the posterior teeth provides greater control.^15,16

Aligner providers have developed an array of attachment shapes, sizes and material features to enhance the accuracy of tooth movement.^15–19 According to prominent researchers, a high trimline extension of an aligner results in a greater surface area over which force may be applied to the teeth, leading to improved efficiency and control.¹⁷ As a result, ClearCorrect’s aligner material has been specifically designed with a high and flat trimline that extends approximately 1.5 mm ± 0.5 mm above the gingival margin. The objective of this change is to simultaneously enhance the accuracy of tooth movement and improve root control.¹⁷

The configuration of the attachment has a significant mechanical influence as it is capable of altering the force value and torque generated by the aligner.^20,21 The effectiveness of aligner treatment has been improved by attachments which can be related to and modify the force system required to move the teeth. The force arises from the geometric disparity between the teeth and the aligner material which is subsequently transferred to a larger tooth contact region that is often less well delineated.²² Attachments therefore, facilitate the application of complex and diverse forces to achieve tooth movement.^23,24 Gaining knowledge of the suitable attachment configuration for a certain clinical situations can aid orthodontists in achieving the most effective force and moment to avoid any adverse effects. To make an informed decision and digital plan, the orthodontist must have knowledge of aligner materials and attachment characteristics.

A study conducted by Ahmad et al.¹⁹ examined the impact of attachment size on force delivery. It was reported that both the force and moment increased as the attachment size became larger. Additionally, it was observed that a larger attachment size resulted in better alignment of the teeth in the intended direction of movement.¹⁹ Therefore, was concluded that a suitable force magnitude can be achieved by selecting the appropriate attachment size.

Finite element analysis (FEA) is a computational simulation method employed for intricate biomechanical study. The benefits of this technique encompass the capacity to produce several simulations with precise control over variables and exceptional consistency. This enables the creation of a comprehensive database for future clinical trials by eliminating redundant repetition.^25–28

There are few studies which focus on enhancing the control of maxillary posterior teeth during arch extension using CAs.²⁶ The literature has not directed sufficient attention to the impact of connection geometry on the bucco-lingual/expansion movement of the posterior teeth which therefore justifies experimental investigation. Hence, the aim of the present study was to evaluate the effects of different attachment sizes and aligner trimline extensions on the expansion of maxillary molars using FEA. A further objective was to analyse the displacement and stress distributions of the teeth, aligner, and periodontal ligament (PDL), and to guide clinical practice by establishing an optimal attachment protocol.

Aligners are frequently utilised in orthodontic therapy to treat malocclusions. The control and predictability of tooth movement during CA therapy is affected by factors related to the trimline extension of the aligner material, the position, geometric design, and dimension of applied attachments.^15–17 Recently, computer technological advancements have led to the utilisation of FEA as a valuable tool in researching orthodontic biomechanics.^{6,25,26,28–32} The present study aimed to thoroughly assess maxillary molar expansion and the subsequent movement pattern of the molars using CAs by applying FEA. Furthermore, the FEA approach was utilised to research the impact of the trimline extension of the aligners and the size of attachments on tooth movement.

Aligners may successfully treat minor to moderate discrepancies in dental alignment by buccal crown tipping of the posterior teeth with a reasonably predictable outcome.^13,33 The various attachment configurations used with CA are crucial in determining the efficiency of different movements produced by the applied forces.^19,30,34 Horizontal rectangular attachments are generally preferred for controlling torque in the posterior teeth during maxillary expansion.²⁶ Ahmad et al.¹⁹ contended that the size of the attachment utilised in aligner therapy impacted the force and moment distribution, consequently affecting the control of tooth movement. Therefore, in the present investigation, the use of horizontal rectangular attachments on the maxillary first and second molars was favoured as a benefical method of regulating root movement during buccal tooth movement. Furthermore, Elshazly et al.¹⁷ conducted a study on trimline design and the edge extension of aligners, to determine the impact of the trimline modification on tooth movement. In support, different aligner trimline models were used to ascertain the impact of the aligner extension on the molars during maxillary arch expansion in the present study.

Aligner deformation greatly impacts the outcomes of clear aligner therapy,³⁵ and significant deformation of the aligner in the occlusal area of the second molar is due to its correlation with the material’s shape and mechanical characteristics. This could explain the decrease in sensitivity as aligners transition from the anterior to the posterior region, as reported in prior research.^9,34 The FEA results of Fan et al. showed that the aligner deformation decreased as the number of attachments increased.³⁶ In the current investigation and compared to the no attachment model, it was found that deformation decreased as the attachment size increased. However, the difference in deformation within these values was insignificant. Therefore, it may be concluded that the presence or absence of attachments on the models does not result in any noticeable variation in aligner deformation. In addition, the maximum aligner deformation in HTLA models was lower than in LTLA models, indicating that aligners with a high trimline extension may have better retention performance. As is well known, FEA studies are performed on ideal model formations to demonstrate potential result trends. In clinical studies, it is common for factors such as aligner fit to deviate from a perfect design.³⁷ It is expected that FEA studies will produce result trends that are as similar as possible to clinical results.

Several studies have demonstrated the constraints of aligner treatment, particularly in arch expansion through bodily tooth movement.^13,14,33 Many authors have indicated the significance of attachments and aligner trimline extension to reduce constraints, enhance the efficiency of tooth movement, and improve predictability by improving aligner retention.^{16,17,19,26,30,33} In the assessment of tooth movement in the transverse direction following expansion, buccal crown tipping of maxillary molars was observed in all models. The low trimline and no attachment model exhibited the highest level of buccal tipping, consistent with prior studies.^17,30 A slight buccal tipping of the molars was observed due to the increased extension of the aligner trimline and the larger size of the attachment. This outcome corresponds with the findings of Ahmad et al.²⁰, who demonstrated that a larger attachment size led to increased force, moment, and improved movement control. Furthermore, the results of the present research corroborated the conclusions obtained by Elhazy et al.¹⁷ that using higher trimline aligners decreased tooth tipping by enhancing the effective force transferred to the cervical region.

The results of the present research supported the conclusions obtained by Mao et al.³⁸ regarding the displacement of molar crowns in a mesiodistal direction. This could be attributed to the occlusal flattening tendency of CAs, affecting the curve of Spee, and the mesial displacement of the crowns of the molar teeth caused by the shortening and expanding of the aligners.³⁷ Aligners with a high trimline extension showed reduced mesial tipping compared to aligners with a low trimline extension, regardless of the attachment. This could be due to improved control of tooth movements facilitated by the increased aligner surface area, as noted by Elhazy et al.¹⁷

In the occlusogingival direction, gingival displacement was found at the buccal cusps and occlusal displacement at the palatal cusps of the molars due to buccal tipping noted in all models after expansion. The higher aligner trimline extension and greater attachment size helped reduce the tipping movements in the vertical direction. The rationale behind this phenomenon is that the high trimline extension of the aligner and larger attachment size (4 mm) enhanced the surface area across which force was applied to the tooth, thereby efficiently controlling tooth movement.^17,19 Furthermore, consistent with numerous studies, the increase in movement differences from the first molar to the second molar, particularly in the vertical direction, could be caused by a decrease in aligner retention from anterior to posterior and the increasing width and flexibility of the aligner-tooth gap.^37–39

It is widely acknowledged that the width of the aligner edge plays a crucial role in the transmission of force. Forces produced by the aligners are transferred to the teeth, which are then conveyed to the surrounding periodontium.²⁰ Considering both the surface area and buccal tipping, the present study identified that the greatest stress on the PDL was focused at the coronal area of the palatal root of the first and second molars. Additionally, the aligner model without attachments and with a low trimline extension exhibited the largest distribution of PDL stress, further supporting the tooth movement findings.³⁰

The present study solely investigated the movement of the maxillary first and second molars with appliance extension using FEA, which limited the ability to accurately represent actual orthodontic treatment conditions. The main limitation of the study was the exclusion of associated factors related to the aligner’s presence in the mouth, aligner wear protocol, and chewing function. The study solely focused on analysing the impact of force transmission from the aligner on the teeth through biomechanical analysis. Hence, there is a requirement for clinical trials to assess the efficacy of moving multiple teeth simultaneously under in vivo orthodontic therapy situations. An additional restriction is that the aligner force applied to the tooth is anticipated to decrease in clinical situations as the aligner deformation occurs more from anterior to posterior.^39,40 However, this could not be replicated precisely in the study due to the exclusive use of the FEA method.

The present FEM study demonstrated that arch expansion using aligners caused buccal tipping of the maxillary molars, whereas buccal tipping movement decreased when the attachment sizes were increased and aligners with a high trimline extension, were utilised.