Clinical effectiveness of ramus mini-implants in orthodontic traction of impacted mandibular second molars

Abstract

Objective

The main objective was to analyse the clinical effectiveness of ramus mini-implant-assisted traction of mandibular second molars with eruption disturbances.

Materials and Methods

A prospective study was carried out during a 3-year period. A total of 16 patients with 19 impacted mandibular second molars underwent surgical exposure followed by implant-assisted orthodontic traction. The pre- and post-treatment cone-beam computed tomography, and pre-, in-, and post-treatment panoramic radiographs were collected and measured for the changes in space, angles and alveolar bone at pre-, in-, and post-treatment stages.

Results

Mandibular second molars showed progressive uprighting (the angle between MM2 and the mandibular plane increased to 95.70 ± 11.96°, p < .0001) and improved root parallelism (the angles between MM2 and the mandibular second premolar decreased to 8.45 ± 7.06°, p < .0001) after the treatment. While molar crowns exhibited no significant deviation from the standard arch form, roots predominantly shifted lingually (2.29 ± 1.84 mm lingually at post-treatment, p < .05). Regarding alveolar bone changes, there was a significant increase of bone height at the distal side of the mandibular first molar (p < .05), coupled with a decrease of bone height at the mesial side of the second molar (p < .05). Root lengths of all patients indicated no statistical significance before and after treatment (p = .63).

Conclusion

Mini-implants placed at the mandibular ramus region are clinically effective in the orthodontic traction of impacted mandibular second molars. The orthodontic traction favours periodontal regeneration between first and second molars and bears no or minimal risk of root resorption of mandibular second molars.

Introduction

Although eruption disturbances of mandibular second molars (MM2) are relatively rare [1, 2], early diagnosis and treatment are critical due to the significant role of mandibular molars in facial bone and dental arch development and function [3]. However, the treatment can be very challenging due to the limited visibility and operating space.

Currently, treatment modalities for these eruption disturbances include traditional orthodontic uprighting (like segmental arches, loops, and springs), surgical uprighting, tooth autotransplantation, and replacement by the third molar [4, 5]. Surgical uprighting and tooth autotrassplantation are usually used when the tooth does not respond to conventional orthodontic methods [6]. But possible pulp necrosis, ankylosis, root fracture or resorption may happen [7]. Traditional orthodontic uprighting may cause anchorage loss and discomfort due to additional devices [5, 8].

In recent years, mini-implants have become increasingly popular for bone anchorage in treating impacted teeth [9, 10]. They provide several benefits over traditional orthodontic methods, including improved efficiency, increased comfort, precise control and avoidance of the use of dental anchorage. The mandibular ramus can offer abundant bone quantity and optimal mechanical positioning for mini-implants that can be used for molar traction [11]. It may be effective, but no clinical studies have been reported.

In this study, we aimed to explore the effectiveness of ramus mini-implant-assisted traction of impacted mandibular second molars. Changes in space, angulation, alveolar bone and root length were investigated to assess its efficacy as a potential treatment approach in the future.

Methods

Study population

This was a prospective clinical study examining a cohort of patients diagnosed with impacted mandibular second molars between 2019 and 2022. The inclusion criteria were: (1) Bilateral or unilateral impacted mandibular second molar. (2) Mesioangular or vertical impaction. (3) Impaction depth ≥ 4 mm [12]. The exclusion criteria were: (1) Missing mandibular first molars (MM1), (2) Significant caries, periodontal lesions, periapical lesions.

Treatment progress

After extraction of the mandibular third molar (if any), the maximum circumference of the crown of MM2 was exposed and bone resistance along the traction path was addressed. A single-hole lingual button with an elastic chain was then cemented on the buccal part of the occlusal surface of MM2. A mini-implant (10–12 mm, VectorTAS, Ormco) was placed in the mandibular ramus. Specifically, flapping surgery was performed to expose the anterior ramus bone platform. Then, a pilot hole was made with a motor-driven handpiece. A ramus mini-implant was inserted at an entry site 4–6 mm medial to the external oblique ridge and 4–8 mm above the occlusal plane with an angulation of 30–45° to the sagittal plane [13] Subsequently, the elastic chain was applied from the button to the mini-implant, exerting an upward and distal tipping force on the impacted second molar. Finally, after the impacted second molars were tractioned to the occlusal plane, alignment was further refined using either fixed appliances or clear aligners. (Fig. 1)

Data collection and measurement

All patients received panoramic radiographs and cone-beam computed tomography (CBCT) at initial orthodontic records and the end of orthodontic treatment. To minimize radiation exposure, monitoring of MM2 was accomplished with panoramic radiography only during the treatment.

To assess saggittal changes of MM2 and root parallelism, angles between the long axis of MM2 and the mandibular plane, occlusal plane, first molar, and second premolar were measured using panoramic radiographs. (Fig. 2) Using CBCT, we evaluated the detailed three-dimensional changes for specific tooth landmarks. Vertical changes were quantified by measuring the distances from the mesial buccal cusp, distal buccal cusp, central fossa, mesial root apex, and distal root apex of MM2 to the occlusal plane at the pre and post treatment stages. Saggittal changes were mearsure from the six landmarks to the tangent line of MM1 distal end. (Fig. 3A and B) Similarly, horizontal changes were measured relatively to the standard arch.

We also measured the changes in alveolar bone around MM2 and the root length using panoramic radiograph. Alveolar bone changes were determined by measuring the distance from the crown’s most convex point (mesial or distal end) to the top of the alveolar ridge pre, in, and post treatment. (Fig. 3C and D) Root length changes were assessed by measuring mesial root lengths pre-, in-, and post treatment. Also, pre-treatment Nolla stage of each tooth was measured.

Statistical analyses

Normality was assessed using the Shapiro-Wilk and Kolmogorov-Smirnov tests. Differences among pre-, in-, and post- treatment phases (vertical dimension change, root parallelism, and alveolar bone alteration) were analysed using one-way ANOVA and the Kruskal-Wallis test. Multiple comparisons across different treatment phases were conducted by employing Tukey’s and Dunn’s tests. Correlations between pre-treatment and post-treatment data (three-dimensional changes and root length) were assessed using paired t-tests. Correlations between the changes of root length and pre-treatment Nolla stage were evaluated by using Spearman correlation test.

SPSS (IBM SPSS Statistics) and GraphPad Prism (GraphPad Software) were used for the statistical analyses. A significance level of p < .05 was considered statistically significant.

Results

This study included 16 patients (8 males and 8 females). Thirteen patients had unilateral MM2 eruption disturbances and 3 had bilateral MM2 eruption disturbances. In total, 19 mandibular second molars were included in the study. The mean age of the patients was 17.38 ± 4.84 years (17.00 ± 4.14 years for males and 17.75 ± 5.73 years for females). The average impaction depth was 6.99 ± 2.61 mm. The average treatment duration for orthodontic traction was 7.47 ± 3.61 months, and overall orthodontic treatment duration was 18.47 ± 5.25 months. All the ramus mini-implants were stable during orthodontic traction of impacted mandibular second molars, rendering the success rate to be 100%.

The inclination angle of MM2 and root parallelism significantly differed across pre-, in-, and post-treatment. By the end of treatment, the angle between MM2 and the mandibular plane increased to 95.70 ± 11.96° (p < .0001), and the angle between MM2 and the occlusal plane rose to 80.72 ± 10.28° (p < .0001). Conversely, the angles between MM2 and the mandibular first molar (8.96 ± 8.27°, p < .0001) and mandibular second premolar (8.45 ± 7.06°, p < .0001) decreased gradually. (Fig. 4)

The vertical and sagittal changes of MM2 in the three-dimensional view showed a significant uprighting after treatment. For vertical change, three measurement points of crown exhibited closer to the occlusal plane after treatment. The mesial buccal cusp, the central fossa and the distal buccal cusp of MM2 were located -0.1 ± 1.1 mm (p<.0001), -0.97 ± 2.30 mm (p<.001) and 0.58 ± 1.58 mm (p<.0001) to the occlusal plane at post treatment, respectively. Regarding root position changes, the mesial and distal root showed a movement away from the occlusal plane after treatment (-15.68 ± 2.31 mm, p < .05; -14.79 ± 2.57 mm, p < .05), while the root bifurcation did not move significantly after treatment (-9.47 ± 2.09 mm, p = .21). (Fig. 5A and C) For sagittal changes, crown position showed an increase in distance to the MM1 at all three measurement points after treatment. The mesial buccal cusp, the central fossa and the distal buccal cusp of MM2 was located 3.16 ± 0.86 mm, 6.10 ± 0.78 mm and 8.27 ± 1.28 mm The mesial buccal cusp, the central fossa and the distal buccal cusp of MM2 was located 3.16 ± 0.86 mm, 6.10 ± 0.78 mm and 8.27 ± 1.28 mm to the vertical line of the most convex point of the mandibular first molar (all p < .001) to the vertical line of the most convex point of the mandibular first molar (all p < .001). Changes in root position showed a reduction in sagittal distance from the mandibular first molar at all three measurement points after treatment. The mesial and distal root shifted mesially, located 9.56 ± 3.20 mm and 11.94 ± 2.36 mm to the vertical line of the most convex point of the mandibular first molar (all p < .0001). The root furcation was also moved mesially and located 7.90 ± 1.38 mm (p < .001) to the vertical line of the most convex point of the mandibular first molar. (Fig. 5B and D)

There were no statistically significant horizontal changes pre and post treatment at all three crown measurement points. However, horizontal changes in roots showed a lingual movement. Mesial root, distal root and root furcation of MM2 exhibited lingual movement, with respective displacements of 1.88 ± 2.71 mm, 0.242 ± 2.03 mm, and 2.29 ± 1.84 mm (all p < .05) to the standard arch. (Fig. 6)

Patients were categorized into juvenile (age < 18, n = 13) and adult (age ≥ 18, n = 7) groups based on treatment initiation age. Only mesial and single roots were measured for root length. There was no statistical significance in root lengths among all patients between pre- and post-treatment (p = .63). However, the adult group showed a decrease in root length before and after treatment (0.45 ± 0.34 mm, p < .05), while no significant change was observed in the juvenile group (p = .93). (Fig. 7) We also analysed whether root length changes were associated with different Nolla stages. However, we found that the root length changes were not associated with different Nolla stages (p = .95 > .05), possibly due to the fact that the majority of patients included in this study had a Nolla stage of 9 or 10.

Alveolar bone changes were significant at all three measurement points. The distance form distal of MM2 to the top of the alveolar ridge showed a decrease from pre-treatment. Statistical significance was observed between pre-treatment and in-treatment, and between pre-treatment and post-treatment (p < .0001, p < .05), but not between in-treatment and post-treatment (p = .67). The distance from the mesial of MM2 to the top of the alveolar ridge increased from pre- to in-treatment (p < .01). But no statistical significance was found between pre-treatment and post-treatment or between in-treatment and post-treatment (p = .31, p = .31). The distance from the distal of the mandibular first molar crown to the top of the alveolar ridge decreased between pre- and in-, and pre- and post-treatment (p < .0001, p < .05). However, no significant difference was found between in-treatment and post-treatment (p = .15). (Fig. 8)

Discussion

This study investigated the efficacy of mandibular ramus mini-implants for managing impacted mandibular second molars. The results showed that the mandibular second molars of all patients erupted successfully post-treatment and reached desirable positions. At the same time, the alveolar bone height increased in distal of the first molar and mesial of the second molar. Minimal root resorption was noted among adults, while no root resorption found among adolescents.

The upward movement of the crown and mesial displacement of the roots were indicative of molar uprighting. Notably, the vertical movement of the root furcation of MM2 from pre-treatment to post-treatment was insignificant, suggesting a rotational movement around the centre of resistance. (Figs 4 and 5) For sagittal changes, the roots shifted more mesially than the distal movement of the crown. There was no statistically significant difference in the horizontal movement of the crowns of MM2, but roots exhibited lingual movement.(Fig. 6) Ye et al. [14] suggested that deeply-impacted teeth were likely to experience greater resistance on the crown compared to the root, which can explain our findings that the roots rather than the crowns exhibited ligual movements. In our treatment, we successfully controlled tooth movement to achieve precise lingual root positioning.

There was no statistically significant difference in root lengths in juvenile patients between pre- and post-treatment. This may be attributed to root development of MM2, which usually complete between 13 and 17 years old [15], so adolescent patients in our study continued root development during treatment. Adults had completed second molar root development at pre-treatment and no root development occured during treatment, resulting in minimal post-treatment root resorption (0.45 ± 0.34 mm).(Fig. 7) Root resorption is influenced by force and duration [15, 16]. Hence, prolonged treatment should be minimized, and adequate alveolar bone support ensured during surgical exposure.

Favorable alveolar bone remodelling occurred during treatment. The distance from the distal end of the mandibular first molar to the top of the alveolar ridge decreased after treatment, indicating bone regeneration along the MM2 traction. (Fig. 8) The tooth and alveolar ridge height relationship (The distance of enamel cementum boundary to the top of alveolar ridge < 2 mm) of MM2 pre-treatment was within normal limits. MM2 was rapidly tractioned distally and occlusally during the treatment, in which the periodontium was stretched and the alveolar bone remodelling had not completed, manifested as a decrease in alveolar ridge height. Subsequent to traction, new bone formation resulted in the increase of alveolar bone height, reflected by a reduced distance from the distal MM1 to the alveolar ridge top. (Fig. 9)

For the treatment of impacted mandibular second molars, various treatment techniques have been proposed using fixed orthodontic appliances such as brass wire, springs and loops of different shapes and sizes [6, 12, 17]. However, techniques like brass wire suit better for mild or moderate degree of mesial inclination of impacted second molars [18]. Also, possible loss of dental anchorage, discomfort for the patient may happen during traction stage. Traditional techniques need frequent follow-ups due to the need for regular reactivation of the brass wire or loops to ensure continuous force applicaction [19]. In this study, the mandibular ramus implant-assisted surgical-orthodontic treatment allows absolute anchorage with no dental side effects and has optimal oral hygiene control. More importantly, it is superior to other methods in traction of deeply-impacted molars. Also, it can reduce the number of appointments and improve patient comfort.

There are three optimal sites for mini-implant-supported molar uprighting: between the roots of the premolar teeth, in the mandibular retromolar pad and in the mandibular ramus. Elastic chains are suitable for molar traction using retromolar pad and ramus mini-implants [20], whereas mini-implants between premolar roots require loops or springs [5]. Although the mini-implant in the mandibular retromolar pad can provide a distal force for distal rotation, upward force is insufficient, which is most needed for impacted molar eruption. Mini-implants positioned between premolar roots can generate upward force, but mesial force is not conducive to uprighting. Therefore, a mandibular ramus mini-implant can provide more suitable traction force, generating both upward and uprighting forces, thereby creating a counterclockwise moment on the molar. However, since the mandibular ramus is located buccally to the molar, traction may induce buccal movement, so alignment of the mandibular second molar is needed after traction [3]. (Fig. 10) Previous research suggests that concurrent removal of the adjacent third molar during second molar uprighting increases the chances of spontaneous eruption of the second molar [21, 22]. Extraction of the third molar before treatment facilitates ease of surgical exposure and access, thereby reducing resistance to MM2 uprighting. Hence, extraction of the third molar prior to treating impacted mandibular second molars is recommended.

One limitation about this study is that three-dimensional coordinate system could be used to analyse the angular and linear changes of tooth using CBCT, as it would more accurate. In this study, panoramic radiograph was used to evaluated the distance and angle changes since this methodology is the most straight-forward and intuitive one and is most widely used to evaluate impacted teeth. These measurements offered direct and easily interpretable data regarding tooth position relative to other anatomical landmarks and were reproduceable. Thus, we used panoramic radiography in our study for angular and linear measurements so that our study can be well reproduced by other researchers. However, due to the two-dimensional limitation of the panoramic radiography, we implemented CBCT to measure the horizontal changes of crown and root, which can’t be measured by panoramic radiograph. We look forward to exploring to measure all tooth movements three-dimensionally in a coordinate system in future research.

The study demonstrated that with mandibular ramus mini-implants, the impacted mandibular second molar can be effectively uprighted, ensuring desirable management of space, angulation, root length, and alveolar bone changes. In addition, the mini-implant assisted treatment mitigates potential side effects associated with dental anchorage, thereby reducing unpredictability. This approach offers a more comfortable and potentially more widely adopted treatment alternative.

Conclusion

• Mandibular ramus mini-implants are effective in orthodontic traction of impacted mandibular second molars.

• Mandibular ramus mini-implant-assisted traction enables periodontal regeneration between first and second molars.

Acknowledgements

This work was supported by National Natural Science Foundation of China (NSFC, No. 82071147 & 82171000), International Orthodontics Foundation 2022 Elite Grants Award (No. IOF-E1) and China Oral Health Foundation (No. 24H0016)

Author contributions

Xuechun Yuan (Conceptualization [equal], Data curation [equal], Investigation [equal], Methodology [equal], Validation [equal], Writing—original draft [lead], Writing—review & editing [equal]), Qianyun Kuang (Conceptualization [equal], Data curation [equal], Investigation [equal], Methodology [equal], Validation [equal], Writing—original draft [lead], Writing—review & editing [equal]), Xiaoyue Han (Data curation [supporting], Validation [supporting], Writing—review & editing [supporting]), Hu Long (Conceptualization [equal], Funding acquisition [equal], Project administration [equal], Supervision [lead], Writing—review & editing [equal]), Xian He (Data curation [supporting], Formal Analysis [equal], Investigation [supporting], Methodology [supporting], Writing—review & editing [equal]), and Wenli Lai (Conceptualization [equal], Funding acquisition [equal], Project administration [equal], Supervision [equal], Writing—review & editing [supporting])

Conflict of interest

The authors declare that there is no conflict of interest.

Data availability

The data in the current study are available from the corresponding author on reasonable request.

Ethics approval and consent to participate

This study was approved by Ethics Committee of West China Hospital of Stomatology, Sichuan University (WCHSIRB-D-2022-468). Informed consent was obtained from all individual participants included in the study.

References

Author notes