Transpalatal arch hinge mechanic using dental and skeletal anchorage for molar intrusion: a biomechanical approach

1. Introduction

The lack of antagonistic teeth and the absence of occlusal contact can lead to the extrusion of the opposing maxillary molar into the edentulous space [1]. The elongation of the dentoalveolar process may cause occlusal interferences and functional disturbances, resulting in difficulties during prosthetic reconstruction. In this context, orthodontic intrusion of overerupted molars is considered the first-line conservative option, which allows restoration of the occlusal plane and avoids invasive procedures (e.g., endodontic treatment, periodontal surgery, fixed prosthesis) [1].

Several conventional appliances have been proposed in recent decades to achieve molar intrusion and reduce the risk of side effects. On the one hand, to prevent molar crown tipping during intrusive movement, either buccal and lingual forces were applied, or a transpalatal arch (TPA) was placed [2]. On the other hand, to avoid extrusion of adjacent teeth during molar intrusion, conventional fixed mechanics involved a multi-tooth anchorage unit, resulting in patient discomfort and difficulties with oral hygiene [3–6]. In recent years, skeletal anchorage has been suggested to provide an ideal force system for molar intrusion, and several methods have been proposed, including miniplates in the zygomatic buttress area and extra-radicular bone screws in the infrazygomatic region, whose position should be evaluated using CBCT [7–9]. However, miniplate insertion requires a complex and invasive surgical procedure (e.g., surgical flap, bone exposure), and infrazygomatic mini-implants may be associated with screw failure and soft-tissue irritation, as the insertion site may be covered by the movable mucosa [10–11]. So, the use of orthodontic screws in the alveolar process has spread widely in recent years because of their ease of placement, minimal patient compliance required, and the ability to be immediately loaded after initial wound healing [1,12–18].

In the present article, we aimed to highlight the effects of a biomechanical approach in two orthodontic cases of molar intrusion through a TPA hinge mechanic, by using dental anchorage and by using palatal miniscrews in order to achieve an intrusive movement.

2. Case 1: TPA hinge mechanic for molar intrusion

In 1997, when TADs were not yet used in the orthodontic clinical practice, a 37-year-old female presented with an extruded maxillary right first molar due to a missing mandibular right first molar. The molar had overerupted beyond the marginal ridge of the adjacent second molar by about 3 mm on the disto-buccal cusp and 2 mm on the mesio-buccal cusp (see Figures 1 and 2). The treatment plan called for maxillary molar intrusion to allow the prosthetic rehabilitation of the missing lower first molar. Two permanent implants were placed in the mandibular right first and second molar area to achieve complete osseointegration during the intrusion of the maxillary overerupted molar.

The appliance designed to obtain molar intrusion is shown in Figure 3.

The anchorage unit was made of rigid .036” stainless steel wire (Remanium, DENTAURUM GmbH & Co. KG, Ispringen, Germany) soldered on orthodontic mini pads and bonded on the palatal surface of right first and second premolars, left canine, first and second left molars, and second right molar, as shown in Figure 4.

The active unit was a rigid .036” stainless steel modified TPA (Remanium, DENTAURUM GmbH & Co. KG, Ispringen, Germany) positioned between the overerupted first molar and the first left molar. On the right side, it was inserted into a rectangular tube .036” × .072” on the palatal surface of the overerupted molar; on the left side, it was inserted into a horizontal round tube soldered on the .036” stainless steel wire (anchorage unit) at the first upper left molar area, to obtain a hinge system (see Figure 5). The rigid TPA could allow avoidance of palatal tipping, providing the tooth with an obligatory direction of intrusion.

Approximately 100 g of intrusive force was applied by a NiTi open coil positioned between the anchorage and active units to achieve an intrusion of 0.4–0.6 mm per month, as shown in Figure 6. The insertion of the TPA into the round tube was from the distal side to avoid disinsertion under the force delivered by the NiTi coil.

The TPA hinge system works as a third-class lever. Generally, a lever consists of a beam or rigid rod pivoted at a fixed hinge, or fulcrum. Based on the locations of the fulcrum, load (output force), and effort (input force), the lever is divided into three types. In relation to the described appliance, the fulcrum was the hinge; the elastic power chain applied the effort; the load/resistance was the tooth to be moved. Since the force was applied between the resistance and the fulcrum, the appliance worked as a third-class lever.

In conditions of equilibrium, R x a = F x b, where R is the resistance, a is the distance between the resistance and the fulcrum, F is the applied force, and b is the distance from the point of application of the force and the fulcrum, R = F x b/a.

Considering the appliance (see Figure 7), the intrusive force was not applied directly on the overerupted molar. Still, the point of application of the force (P) was shifted to the point of insertion of the NiTi coil spring on the modified TPA, about 1/5 of the distance between the fulcrum and the resistance. Thus, a was equal to the intermolar width, and b was equal to 4/5 of the intermolar width. So, R = F x b/a, which equals, say, R = F x 4/5.

Another concept to consider was that the direction of the applied force was not perpendicular to the occlusal plane (90°), but it was about 45° to it. Thus, according to trigonometric rules, the intrusive force applied to the molar was F = sin (45°) x F. Thus, the force applied to the molar was about 70% of the force delivered by the coil spring. Consequently, it was possible to calculate the final force that should be applied to the overerupted molar, which was R = [sin (45°) x F] x [b/a], equal to R = [70% x F] x [4/5]. So, the 100 g force delivered by the NiTi coil spring corresponded to about 55 g of intrusive force on the molar.

The force that would tend to contract the molar (F = cos (45°) x F) was counteracted by the stiffness of the TPA.

After 5 months of intrusion, the overerupted molar was in an optimal position, and an adequate vertical space was obtained. Since the anchorage was provided by dental units, a small amount of extrusion of the right bicuspids may have occurred.

To prevent relapse of maxillary molar intrusion, a stainless steel ligature was immediately placed to replace the NiTi coil spring. After one week, the hinge TPA was removed, and a passive bonded splint (.017” x .025” Stainless Steel rectangular wire) was positioned from the maxillary right second molar to the maxillary right first bicuspid on both palatal and buccal sides, until the prosthetic rehabilitation of the missing lower molar was completed (Figure 8). The intraoral photographs and an OPG detail at the end of treatment are shown in Figures 9 and 10.

3. Case 2: TPA hinge mechanic and TADs for molar intrusion

In 2019, a 46-year-old female presented with an extruded left maxillary first molar secondary to a missing left mandibular first molar. The molar had overerupted beyond the marginal ridge of the adjacent second molar by about 3 mm on the palatal cusp and 2 mm on the buccal cusp (see Figures 11 and 12). Maxillary molar intrusion was required to allow prosthetic rehabilitation of the lower arch. During the intrusion of the maxillary overerupted molar, a permanent implant was placed in the mandibular left first molar area to achieve complete osseointegration.

The approach selected for this treatment was similar to the previous one, but anchorage was achieved with TADs (see Figures 13 and 14). More in detail, two self-drilling titanium-alloy miniscrews (OSSTEM, Seoul, Korea, 1.6 mm × 8 mm) were placed into the attached gingiva on palatal side: the first one was positioned in the left paramedian region along the axis of the overerupted first molar at 45° angle relative to a line perpendicular to the occlusal plane [19–20]; the second one was positioned between the maxillary right second premolar and the maxillary right first molar and it was screwed with an inclination that allowed the hole in the head to be parallel to the occlusal plane (hole size 0.8 mm).

The hinge TPA was made of a rigid .036” stainless steel wire (Remanium, DENTAURUM GmbH & Co. KG, Ispringen, Germany). On one side, it was bonded to the occlusal surface of the overerupted first molar; on the other side, the modified TPA was inserted into the hole in the miniscrew head. Before insertion, the 0.9 mm stainless steel wire was thinned to fit through the 0.8 mm hole. In this way, the miniscrew could act as a hinge, and rotation of the system could occur around the head of the miniscrew.

The intrusive force was delivered by the elastomeric power chain (Morita ClosedClear Rocky Mountain Orthodontics LX 2849) pulled from the head of the left miniscrew to the occlusal surface of the extruded molar.

Starting from R=F x b/a, we considered that b=a and that the direction of the force delivered by the elastic chain was about 55° to the occlusal plane. Thus, the intrusive force was R = sin (55°) x F, meaning that the force applied on the molar was about 80% of the force delivered by the power chain. As described by the previous case, the force that would tend to contract the molar (F = cos (55°) x F) was counteracted by the stiffness of the TPA.

So, a force of about 70 g was delivered by the elastomeric power chain to obtain an intrusive force of about 55 g or less on the molar. The elastic chain was replaced every four weeks to obtain an intrusion of about 0.65–1 mm per month.

After three months of intrusion, the overerupted molar was in an optimal position, and an adequate vertical space was obtained. To prevent any relapse of the maxillary molar intrusion, the elastomeric power chain was immediately replaced by a stainless steel ligature, which extended from the left miniscrew to the occlusal surface of the molar, on which it was bonded. After one week, the hinge TPA was removed, and two passive bonded splints (.017” x .025” stainless steel rectangular wire) were positioned from the maxillary left second premolar to the maxillary left second molar on the buccal side, until the prosthetic rehabilitation of the missing lower molar was completed. The intraoral radiograph and photographs at the end of the treatment are shown in Figures 15 and 16.

4. Appendix to Case 2

The intrusive force can also be delivered by the elastomeric power chain pulled from the head of the miniscrew to the modified TPA, as shown in Figure 17.

Starting again from R=F x b/a, and considering that the force delivered by the power chain was about 70° to the occlusal plane, the amount of intrusive force was R= [sin (70°) x F] x [b/a].

The intrusive force was not applied directly on the overerupted molar (as shown in Case 2). Still, the point of application of the force (P) was shifted to the point of insertion of the power chain on the modified TPA of about 1/5 of the distance between the fulcrum and the resistance, so R = [94% x F] x [4/5].

In this case, two over-erupted molars were to be intruded; the power chain delivered about 150 g of force to obtain an intrusive force of about 110 g (about 55 g for each molar).

5. Discussion

This article reports two cases of molar intrusion using a TPA hinge mechanic with and without TADs for achieving the intrusive movement through a biomechanical approach.

In 1982, Fontenelle suggested the “hinge mechanics” as an efficient and simple approach to obtain a tooth’s bodily translation around an arc of a circle, keeping both tipping and rotation under control [21–22]. The innovation of this system concerned the possibility of displacing the point of application of the force along the rigid TPA. Indeed, the TPA extended across the palatal vault, and consequently, it was possible to deliver the force through the tooth’s center of resistance.

Since then, the TPA hinge system has been a valuable aid in orthodontic clinical practice, especially for bodily translation through edentulous sites. In this paper, we reported how to take advantage of the hinge mechanism to achieve bodily molar intrusion and explained which biomechanical roles should be considered.

During tooth intrusion, the magnitude of the force should be considered as one of the most critical factors, especially in relation to the risk of root resorption [23–24], because all applied forces were concentrated at the root apex, disturbing the capillary blood flow and initiating the resorptive process [25].

Orthodontics has evolved from a purely mechanical discipline to a biologically integrated science, in which biomechanics is a fundamental pillar of diagnosis and treatment planning. Contemporary literature consistently emphasizes that orthodontic forces must be interpreted within a complex system involving muscles, skeletal structures, and functional patterns.

In this context, growing evidence suggests that systemic factors — particularly vitamin D status — play a significant modulatory role in bone remodeling processes underlying orthodontic tooth movement. Vitamin D is essential for calcium-phosphate homeostasis and osteoblastic activity; therefore, hypovitaminosis D may impair bone turnover, reduce mineral density, and alter the balance between bone resorption and apposition [26–27]. These alterations can adversely affect the response to intrusive forces applied via transpalatal arch mechanics, potentially leading to slower tooth movement, an increased risk of root resorption, and reduced anchorage control. From a biomechanical perspective, the effectiveness of molar intrusion with combined dental and skeletal anchorage depends on a predictable bone response to controlled forces; however, in patients with vitamin D deficiency, this response may be compromised, necessitating adjustments to force magnitude and duration, as well as monitoring protocols [26–27].

To reduce this risk and obtain better results with intrusive forces, stable anchorage associated with intermittent and light forces should be planned [28–29]. Moreover, the amount of intrusive force delivered should be exactly calculated, especially in periodontal patients. Indeed, the condition of the periodontium and the amount of alveolar bone surrounding the supraerupted tooth should be taken into consideration. In this context, using finite element analysis, Ugarte et al. In 2022, the amount of force reduction needed to avoid root resorption and maintain the efficiency of orthodontic mechanics in periodontally compromised patients was determined [28]. Authors suggested that the intrusive forces should be reduced by about 8–9.3% for each millimeter of bone height loss, highlighting that the buccal skeletal anchorage must be associated with a supplemental strategy to avoid buccal crown tipping. Since about 20 g of intrusive force per root is recommended for a safe intrusive movement [30], in the second case report, we reduced the intrusive force on the overerupted molar to prevent any periodontal side effects.

On this topic, a recent randomized clinical trial by Akl et al. compared the root resorption resulting from miniscrew-supported maxillary posterior dentoalveolar intrusion using two different force magnitudes [24]. In the control group, they applied 20 g of intrusive force per root, whereas in the intervention group, they applied 40 g per root. They reported resorption amounts of 0.84±0.96 mm and 0.93±1.00 mm in the control and intervention groups after 6 months, respectively, with no statistically significant difference between the groups. However, the authors concluded that lighter forces could generally be recommended for their claimed benefits in promoting a favorable tissue reaction.

Among the other most critical factors in molar intrusion, the direction and the point of application of the force should be considered. The line of action of the force should pass through the center of resistance to avoid unwanted rotation or transverse displacement. To avoid rotation and crown-molar tipping, a rigid stainless steel TPA was positioned in both cases. In the case of palatal skeletal anchorage, the TPA passed through the head of the contralateral miniscrew, so that the reaction forces were not discharged on the contralateral molar. In our case report, the miniscrews were inserted entirely into the resistant attached gingiva, and no miniscrew failures occurred, in line with the literature. In this context, studies have shown that the overall success rate of palatal miniscrews was greater than 90% [31–32], and a recent meta-analysis showed that palatal sites had failure rates of 1.3%, 4.8%, and 5.5% for the midpalatal, paramedian, and parapalatal insertion sites, respectively [33]. Indeed, the palatal surface is covered with keratinized gingiva, and the risk of dental root invasion is lower than in the buccal alveolar bone [32], where the contact between the miniscrew and the root may lead to damage to the periodontal structures or to mini-implant failure, especially during intrusion movement [33,35].

Lastly, assessment of skeletal maturation is a key component in planning orthodontic therapy for growing individuals, as it helps determine the most appropriate timing for orthopedic and dentofacial interventions. Evidence from the literature supports the reliability of commonly adopted methods, including cervical vertebral maturation and hand-wrist radiographic analysis, despite some differences in their application and interpretation [36].

Additionally, dental-based indicators, such as those derived from tooth calcification stages and the Demirjian method, have been shown to correlate with skeletal development, although their variability prevents them from being used as stand-alone diagnostic tools [36–37]. The stage of skeletal maturation has a direct impact on treatment effectiveness; for example, both the maturation of the midpalatal suture and vertical skeletal patterns can influence the success of maxillary expansion procedures and changes in overbite over time [37–38].

Conclusion

In conclusion, the findings of these two paradigmatic cases showed that a bodily intrusion of the overerupted molar could be obtained by TPA hinge mechanic with both dental and skeletal anchorage. However, it should be emphasized that TADs showed to make the appliance simpler, cheaper, and more comfortable. Therefore, the miniscrews could be considered as an indispensable orthodontic device in these complex patients, mainly because it could allow avoiding side effects on the other teeth.

Consent for publication

Written consent to publication was obtained from each participant, who was informed that their sensitive data would not be disclosed.

Availability of data and materials

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Competing interests

The authors declare no conflict of interest and that they did not receive support from any organization for the submitted work. All authors certify that they have no affiliations with or involvement in any organization or entity with any financial interest or non-financial interest in the subject matter or materials discussed in this manuscript.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Acknowledgements

None.