Ankylosed teeth, characterized by abnormal fusion of the root cementum and surrounding bone, typically maintain their existing occlusal levels despite the continual eruption of the adjacent teeth. As a result, the deposition of alveolar bone can lead to ankylosed teeth being infraoccluded or submerged[1,2]. The etiology of ankylosis remains unknown. Various hypotheses have been proposed including familial patterns, traumatic injury to Hertwig’ s epithelial root sheath, bone growth deficiencies, localized metabolic disruption, inflammation, infection, and chemical or thermal irritations[3]. The incidence rate of ankylosed teeth ranges between 1.3% and 38.5%, with primary teeth being affected more commonly than permanent teeth[1]. Ankylosis in the primary dentition most frequently affects the mandibular first molars, followed by the mandibular second molars, and the maxillary molars.

Extensive bony ankylosis of the primary molars can interfere with the eruption of the permanent successors and, consequently, affect occlusal relationships[2]. Previous studies also reported on complications, such as prolonged retention of the affected teeth, extrusion of the opposing teeth, periodontal destruction due to occlusal forces and food impaction, increased susceptibility to dental caries, and eruption disturbance of the permanent successors, especially in association with untreated infraocclusion[3,4]. Numerous treatment methods for ankylosed primary molars are available, including periodic observation, restorative treatment, space management, and surgical extraction of ankylosed teeth. The treatment choice is typically selected based on the degree of infraocclusion, the presence and stage of development of the successor tooth, and the onset time[5]. Decreased arch length is a common complication, occurring in 28 - 43% of ankylosed primary molars, especially primary mandibular second molars. When the adjacent first permanent molar tilts toward the ankylosed tooth, it can reduce the arch length as well as impede vertical growth[6]. Therefore, space management is indicated for patients with moderate to severe infraocclusion of the primary molar. Space management can ensure easier access for surgical extraction, normal eruption, and occlusion[7].

While both removable and fixed appliances have been used to regain space, removable appliances can pose challenges in terms of patient compliance and cooperation. The therapeutic effectiveness relies heavily on patient compliance to wear the appliance[8]. Ensuring stable anchorage is paramount during the mixed dentition stage due to physiological root resorption and increased mobility of the primary teeth during the exfoliation stage. Thus, more effective and stable appliances are needed.

In this case report, we described two patients with mixed dentitions who underwent orthodontic treatment with a fixed space regainer appliance that utilized ankylosed teeth for skeletal anchorage. Both patients exhibited favorable treatment outcomes, including adequate space regaining and appropriate uprighting of the adjacent permanent molar within a short treatment time, without experiencing anchorage loss.

This study was conducted with the approval of the Institutional Review Board (IRB) of Kyungpook National University Dental Hospital (IRB No: KNUDH-2023-04-03-00). The patients’ legal guardians provided informed consent for the publication of this case report, as well as related clinical photographs and radiographic images of the patients.

Ankylosed primary molars pose significant challenges due to their potential to disrupt the eruption of permanent successors and affect occlusal relationships. Traditional treatment, including periodic observation, restorative treatment, space management, and surgical extraction, has varying degrees of success and patient compliance issues[9,10]. This study described orthodontic space regaining treatment using a novel fixed appliance, leveraging the ankylosed tooth for skeletal anchorage. The results of our two cases demonstrated the effectiveness of this novel method in addressing space management and facilitating the eruption of the permanent successor.

The three-step treatment process described in this case report began with the use of a fixed appliance for space regaining, which also distalizes and uprights the adjacent first permanent molars. In both cases, this crucial step effectively created the necessary space for the surgical access to the ankylosed tooth. Following successful space regaining, the ankylosed primary molar in both cases was surgically extracted, and the permanent successor erupted uneventfully. After the surgical extraction, a space maintainer was inserted to preserve the newly created space, thereby facilitating the spontaneous eruption of the permanent successor. This sequential treatment approach not only addressed the immediate occlusal and spatial concerns but also set the stage for optimal long-term dental development[7]. In cases where there are more complex occlusal issues or significant malalignment of the teeth, combining our novel fixed appliance for space regaining with comprehensive orthodontic treatment using brackets and wires can be beneficial. The degree of space regaining achieved with the appliance and the treatment duration are carefully adjusted according to the timing and necessity of comprehensive treatment.

The novel fixed appliance has several key components: a minitube for retention, a double-helical spring as the active component, and a free arm with a resin ball for the action part (Fig. 7, 8). To upright the mesially tilted permanent molars caused by the ankylosed primary molar, applying a moment proves to be an effective method (Fig. 9). The center of resistance of the first molar is distant from the point where the appliance’s free arm is attached. As the helical spring is activated, it generates a moment that causes the first molar to move distally. This moment generates a controlled tipping force, uprighting the molar’s mesial inclination while simultaneously regaining the lost space[11]. The minitube, serving as the retentive part, is attached to the occlusal surface of the ankylosed tooth using Transbond XT, providing skeletal anchorage. The active component consists of two continuous helical springs, made from 0.5 mm stainless steel wire, which are shaped into loops ranging from 1.3 mm to 3.0 mm in diameter. The 0.5 mm thickness was chosen to ensure the wire fits securely within the appliance’ s tube while providing adequate flexibility to generate controlled and effective forces during treatment. However, one limitation of this study is the lack of precise force measurements using a gauge during the activation of the helical springs. Future research should aim to incorporate force gauges to better refine force application, allowing for more controlled and predictable treatment outcomes.

During each visit, the existing loop is activated and its diameter is gradually increased. Once there is enough space for a larger diameter, a new loop is fabricated to accommodate the next size. The free arm, shaped to match the contour of the permanent first molar, ensures effective force application as the spring straightens, moving the molar distally. The resin ball at the end of the free arm prevents mucosal irritation, enhancing patient comfort. It is recommended to initiate treatment with this device during the mixed dentition phase when the ankylosed primary molar starts causing infraocclusion that impacts adjacent permanent teeth, such as space loss, mesial tipping, or delays in the eruption of the permanent successor. Early intervention helps regain space and prevent complications, especially when there is moderate to severe ankylosis with enough exposed occlusal surface for secure attachment[11].

Rubber dam isolation can be employed to attach a minitube to a submerged tooth at the level of the contact point or interproximal gingiva of the adjacent teeth using a resin curing system. The rubber dam deflects the tongue and buccal mucosa, minimizing bonding failure due to tongue movement while simultaneously protecting against aspiration of the appliance. To ensure a completely dry field, the fit of the rubber dam can be tightened using dental floss knots or wedges[9,10].

In this case report, we presented two patients with mixed dentitions who had ankylosed and infraoccluded mandibular second primary molars and space loss due to the mesial inclination of the first permanent molar. The fixed appliance for space regaining, designed with a minitube on the ankylosed tooth for skeletal anchorage and a double-helical spring mechanism, provided continuous uprighting forces on the adjacent permanent first molar. This novel appliance effectively increased the space needed for the surgical extraction of the ankylosed tooth and facilitated the normal eruption of the permanent successor. In future studies, the use of CBCT scans could be considered to obtain more precise diagnostic information for surgical planning.

In both Case 1 and Case 2, the fixed appliance effectively uprighted the mandibular first permanent molar within a short treatment period (eight weeks in Case 1 and three months in Case 2). This also facilitated the surgical extraction of the ankylosed primary molars. Following the extraction, a lingual arch space maintainer was inserted while waiting for the spontaneous and timely eruption of the mandibular second premolar. Radiographic and clinical assessments over 24 months of follow-up showed normal root development and healthy periodontal tissues around the erupted second premolars, demonstrating the successful outcomes of the treatment approach.

Fixed appliances are generally preferred over removable appliances due to several inherent benefits, such as improved patient cooperation, better chewing habits, easier oral hygiene maintenance, higher treatment efficacy, and shorter treatment duration[12,13]. The novel appliance described in this case report is advantageous because it is simple, provides effective treatment within a short duration, and yields acceptable outcomes without serious side effects, such as undesirable tooth movements due to insufficient anchorage. This treatment can be performed chairside, eliminating the need for band fabrication and impressions. The simple design makes it less bulky compared to removable space regaining appliances. Moreover, without an extended resin plate, patient comfort is enhanced. Additionally, the resin ball on the free arm minimizes soft tissue irritation, further improving patient comfort and reducing the risk of iatrogenic injury.

Throughout the treatment, no wire detachment was observed. The patients exhibited good oral hygiene and did not report any discomfort. Unlike many other orthodontic appliances, which are bulky and commonly result in soft tissue discomfort, this novel fixed appliance only involves the affected area of the mouth, the ankylosed tooth, and adjacent permanent molar, providing superior patient comfort. The ease of application without laboratory procedures is also a substantial advantage[8].

Patient compliance is crucial for successful orthodontic treatment outcomes. Non-compliance can increase treatment duration and compromise results[14]. These limitations can be mitigated by using fixed appliances, especially fixed appliances that exert continuous forces using minitubes, regardless of patient compliance. In the current study, our patients and their parents reported high treatment satisfaction, particularly due to the appliance’s compact size, which resulted in minimal soft tissue trauma and discomfort while also requiring less compliance and parental supervision. Furthermore, the overall treatment duration was short, with approximately 4 mm of space regained within 2 months in case 1 and 2 mm within 3 months in case 2, without any complications. In Case 1, based on model analysis, an arch length discrepancy of 3 mm was identified. Considering previous research that reports 1 to 1.2 mm of mesial movement with the use of a lower lingual arch, an overcorrection of 4 mm was planned to account for potential space loss and ensure sufficient space for alignment. In other words, the target of 4 mm space regaining was considered to account for the possibility of overcorrection, which would provide adequate space for the eruption of the permanent successor and establish a proper occlusal relationship. On the other hand, in Case 2, where there was an initial midline deviation due to the early loss of primary left mandibular canine, the treatment focused on achieving space regaining, correcting occlusal relationships, and facilitating the eruption of the successor teeth within the shortest possible timeframe. Although the space gained was 2 mm, which is slightly less than initially desired, it was sufficient to accomplish these primary treatment goals.

Anchorage is an important consideration in designing orthodontic appliances that aim to upright molars. Adequate anchorage is particularly important in children with mixed dentitions, since the prevalence of infraocclusion peaks at 8 - 9 years of age[15,16]. Importantly, when primary teeth are used for orthodontic treatment anchorage, local inflammatory mediators increase, which can lead to faster root resorption, and early exfoliation of the anchoring tooth, resulting in undesirable movements and the need to reconstruct the appliance[17]. Absolute anchorage can be achieved using skeletal anchorage, such as miniscrew implants or bone plates, which reduces the risk of side effects like undesirable movements that are often associated with intraoral anchorage appliances[18]. In the current study, the second primary molar was used as skeletal anchorage as it was ankylosed. No additional anchorage was required, and favorable treatment outcomes were achieved with maximum compatibility and without the occurrence of complications, such as root resorption and tooth mobility. Even in ankylosed molars, there is a risk of anchorage loss when excessive force is applied, depending on the degree of ankylosis. While a simple appliance design has its advantages, the potential for anchorage loss beyond the intended tooth movement should be carefully considered.

Ankylosed primary molars with permanent successor teeth could undergo physiological resorption and exfoliation. However, early extraction is considered when infraocclusion leads to the displacement of the successor molar, significant occlusal issues, tipping of adjacent teeth, or space loss[18]. In both Case 1 and Case 2, infraocclusion of the primary molars was accompanied by displacement of the successor permanent molar, indicating a potential risk of delayed or impacted eruption of the permanent tooth. Consequently, early extraction of the affected primary teeth was deemed necessary. Additionally, the root development of the permanent successor was determined to be between half and two-thirds complete, making this the appropriate time to facilitate proper eruption and alignment while minimizing the risk of complications. By extracting the ankylosed tooth early, the position of the displaced successor tooth was corrected, and spontaneous eruption was observed. Furthermore, space was regained more efficiently, facilitating the timely eruption of the permanent successor and promoting the restoration of normal bone height and vertical alignment, both of which are crucial for optimal occlusal development[2,19]. However, in cases where infraocclusion has progressed to the point that space expansion procedures are necessary before extraction, reevaluation from an orthodontic perspective may be required after the eruption of the second premolar. This consideration should be taken into account when planning treatment.

However, our novel fixed appliance for space regaining has several limitations. Since the appliance is attached to the occlusal surface of the ankylosed tooth, it can only be used on teeth with sufficient exposed occlusal surface for attachment. Moreover, the teeth adjacent to the minitube and helical spring are at higher risk of dental caries due to increased food impaction and plaque accumulation. Any oral hygiene issues identified during orthodontic treatment should be addressed immediately. Furthermore, patients must be motivated to maintain their oral health. Lastly, minitube cementation utilizes a light-cured resin system; thus, the risk of bonding failure increases in children with poor cooperation due to contamination.

In the current study, we described two patients who exhibited favorable space regaining and occlusal relationships following treatment with a fixed double-helical spring appliance for space regaining that uses the ankylosed tooth for skeletal anchorage and, at the same time, effectively resolved the space problems caused by the ankylosed teeth. Therefore, this method may be considered a suitable alternative in patients with mixed dentitions, at risk of unstable anchorage from primary teeth and poor compliance. The novel appliance described can achieve specific treatment goals within a short time without any complications.

This method offers multiple advantages. It is simple, provides effective treatment within a short duration, and yields acceptable outcomes without serious side effects, such as undesirable tooth movements caused by insufficient anchorage.