A modified serial extraction method in a patient with crowding and a congenitally missing mandibular left second premolar

3.1 Course of treatment

Initially, caries control and oral hygiene instructions were provided. Early extractions were carried out following the standard SE sequence (C → D → 4); however, in the mandibular dentition, the treatment protocol was modified because of the congenitally missing tooth. To shift the mandibular dental midline to the right, the mandibular right primary canine was first extracted, after which a lingual arch was placed in the mandibular arch, and the mandibular anterior teeth were moved rightward using a sectional arch. Simultaneously, a separating module was inserted between the mandibular left second primary molar and the first molar to upright the mandibular left first molar (Fig. 4). By the time the mandibular right and left first primary molars had been extracted, the mandibular canines on both sides had already erupted, resulting in insufficient eruption space for the first premolars. Therefore, hemisection of the mesial roots of the mandibular right and left second primary molars was performed to guide the eruption of the first premolars (Fig. 5). After eruption of the first premolars, the mandibular right first premolar and the distal roots of the mandibular right and left second primary molars were extracted to facilitate the eruption of the mandibular right second premolar (Fig. 5). At that time, an increase in overbite was observed; therefore, a Bionator appliance was introduced to increase the vertical dimension and improve the skeletal Class II relationship (Fig. 4). The Bionator was used for 1 year and 4 months. During this period, the lateral teeth were replaced and the second molars began to erupt. Although improvement in overbite and overjet was achieved subsequently, stable occlusion was not obtained because the lateral teeth were still in the process of erupting. For the second phase of treatment, a multibracket treatment was initially planned, however, due to the father’s job transfer, the family had to move to a distant location, so a tooth positioner “Pre-Finisher®” (TP Orthodontics, Tokyo, Japan) was used instead after confirming the patient’s compliance and occlusal condition (Fig. 5). A tooth positioner was therefore used to promote occlusal settling (Fig. 4). The patient was instructed to use the tooth positioner for at least 13 h per day for 24 months. After occlusal settling was achieved, posterior interdigitation and a favorable interincisal angle were confirmed. The tooth positioner was then discontinued, and wraparound retainers were placed in both the maxillary and mandibular arches for 2 years.

Fig. 4.Treatment progress. (A) Rightward movement of the mandibular midline and uprighting of the mandibular left first molar (7 y 2 m). (B) Completion of sectional arch therapy and placement of the Bionator appliance (9 y 5 m). (C) Completion of Bionator therapy and placement of the tooth positioner (12 y 0 m).

Fig. 5.Treatment progress as observed on panoramic radiographs. (A) Early hemisection of the mesial roots of the mandibular right and left second primary molars to facilitate distal eruption of the mandibular first premolars. (B) Because of the congenitally missing mandibular left second premolar, the mandibular left first premolar was not extracted, and its eruption was guided distally. The mandibular right first premolar had already been extracted. (C) The maxillary left first premolar was extracted, with planned extraction of the maxillary right first premolar.

3.2 Treatment results

Facial photographs taken after the completion of treatment (14 y 1 m) showed good bilateral symmetry. In the profile view, the mandibular retrusion and mild protrusion of both the upper and lower lips observed at the initial presentation were improved, resulting in a favorable facial appearance. Intraoral examination after the completion of treatment showed correction of the dental midline deviation, with appropriate overbite and overjet achieved. In addition, Angle Class I relationships were obtained in the premolar and molar regions, resulting in stable occlusion (Figs. 2,6). Root parallelism was satisfactory, with no significant root resorption or periodontal problems. The maxillary right and left third molars remained impacted in the bone. Cephalometric analysis revealed no change in the angle between the sella (S), nasion (N), and subspinale point A (A) (SNA), while forward and downward growth of the mandible was observed due to the use of the Bionator appliance, resulting in an increase in the angle between the SN line and supramentale point B (B) (SNB) and an improvement in ANB from 5.0° at the initial presentation to 2.5° at the end of treatment. The FMA showed no substantial change, increasing from 35.0° to 36.0°. Interestingly, the U1 (Upper first incisor) to NA (line from the nasion to point A) angle changed from 30.0° to 18.0° during Bionator therapy, indicating substantial lingual tipping of the maxillary anterior teeth. Subsequently, with the use of the tooth positioner, the angle increased to 22.0°, reflecting labial tipping. The L1 to NB distance ultimately reached 6.5 mm, which was close to the predetermined treatment goal. The FMIA remained unchanged before and after Bionator therapy at 63.0°; however, following the use of the tooth positioner, the angle decreased to 57.0° at the end of treatment, indicating labial tipping of the mandibular anterior teeth and approaching the Japanese normative value. Consequently, the interincisal angle increased from 127.0° before Bionator therapy to 141.0° after Bionator therapy, and subsequently decreased to 130.0° at the end of treatment (Figs. 3,7, Table 1). A favorable occlusal relationship was maintained for 2 years and 7 months after the completion of treatment (Fig. 8) and for 8 years and 7 months post-treatment, demonstrating long-term stability (Fig. 9).

Fig. 6.Findings at the end of dynamic treatment (14 y 1 m). (A) Facial photographs. (B) Intraoral photographs. (C) Panoramic radiograph.

Fig. 7.Superimpositions of pretreatment (black; 6 y 11 m), before Bionator therapy (orange; 9 y 6 m), after Bionator therapy (blue; 10 y 10 m), and posttreatment (red; 14 y 1 m). BN, Bionator therapy.

Fig. 8.Findings at 2 years and 7 months of follow-up posttreatment (16 y 8 m). (A) Facial photographs. (B) Intraoral photographs. (C) Panoramic radiograph.

Fig. 9.Findings at 8 years and 7 months of follow-up posttreatment (22 y 8 m). (A) Facial photographs. (B) Intraoral photographs. (C) Panoramic radiograph.

4.1 Treatment planning in pediatric orthodontics

In pediatric orthodontic treatment, determining the most appropriate treatment strategy can often be challenging. Because children are still growing, prioritizing skeletal correction during the growth period may reduce the need for a second treatment phase and shorten the overall treatment duration [16]. In our clinical practice, particularly in cases involving maxillary protrusion, deep overbite, and crowding, overbite control is given the highest priority. In other words, space-related problems, including crowding, are addressed while simultaneously controlling the overbite. This treatment sequence is considered effective in advancing pediatric orthodontic treatment. We believe that overbite control represents a critical step in terms of saving both time and labor and should be prioritized over transverse expansion. Based on this concept, treatment in the present case prioritized correction of the deep overbite, while skeletal improvement was supported through mandibular growth and an increase in occlusal vertical dimension. At the same time, because the mandibular left second premolar was congenitally missing and there was substantial space deficiency, early extraction of the contralateral first premolar was necessary to guide the eruption of the posterior teeth and facilitate alignment. In cases such as this one, where a modified treatment strategy was required because of a congenitally missing tooth, early improvement of the overbite allows for effective space control through occlusal guidance strategies, including active interproximal reduction of primary teeth and hemisection [17, 18]. This approach can contribute to the effective resolution of mandibular crowding. Furthermore, previous reports have indicated that slice cutting and hemisection are most appropriately performed at approximately 8 to 9 years of age [18]. The timing of treatment in the present case was therefore also considered appropriate.

Skeletal correction in the present case was achieved using a functional orthodontic appliance. Functional orthodontic appliances have been reported to improve overjet and overbite and to promote mandibular growth to some extent; however, their effectiveness is strongly influenced by the patient cooperation and varies among individuals [19]. To date, various functional appliances have been developed. Representative appliances include the Herbst appliance [20], Bionator [21, 22], Activator [23], and Frankel appliance [24]. In addition, functional appliances consisting of separate maxillary and mandibular components include the Twin Block appliance [25] and the Bite Jumping Appliance [26, 27, 28, 29]. Among these options, the Bionator–a removable monoblock appliance originally introduced by Balters et al. [30]—was selected for the present case. One of its advantages is that it allows skeletal correction while enabling dental adjustments and guiding posterior tooth eruption through selective trimming of the resin. In our clinical practice, the Bionator is frequently used for this purpose. Regarding the effect of the Bionator on mandibular advancement and growth, Almeida et al. [31] and Illing et al. [32] reported significant effects on mandibular growth. Accordingly, in the present case, the Bionator was used to improve the skeletal Class II relationship, and favorable outcomes were achieved without the need for a second treatment phase with a multibracket (fixed appliance) system. Furthermore, no recurrence of crowding was observed 8 years and 7 months after treatment completion, indicating favorable long-term stability.

This dual orthodontic technique, which simultaneously addresses skeletal and dental correction, is considered advantageous. We refer to this approach as the Minimum Multibracket System (M.M.B.S.) and apply it as a treatment strategy during the mixed dentition period. A key element of this approach is bite control. Although we usually use the Bionator, the approach may also be applied using other functional orthodontic appliances, provided that forward positioning of the mandible and adequate bite control can be achieved. Furthermore, the subsequent use of a tooth positioner allows for efficient and effective treatment in terms of both time and appliance use. This approach may eliminate the need for finishing treatment with a multibracket system or substantially shorten its duration, thereby offering potential advantages for both patients and dentists.

4.2 Early extraction and the use of functional orthodontic appliances

The ideal timing for initiating orthodontic treatment has long been a subject of debate. When a patient in the early mixed dentition presents with severe crowding due to substantial arch-length discrepancy and has been diagnosed as requiring tooth extraction as part of the treatment plan, a decision must be made as to whether premolar extraction should be performed early during the mixed dentition or later after premolar eruption in the permanent dentition. Regardless of the timing, the purpose of tooth extraction is to create sufficient space to properly align the teeth within the basal bone [7, 33]. Early extraction is commonly associated with SE. When SE is performed, teeth are extracted sequentially in the order of C, D, and 4, while monitoring tooth eruption. The advantages of SE include: (1) utilization of physiologic tooth movement; (2) reduction in overall treatment duration, particularly the duration of multibracket therapy; (3) improved stability of treatment outcomes; (4) shortening of the retention period; and (5) improvement in oral hygiene [5, 7, 8, 11, 34]. On the other hand, its disadvantages include: (1) an increased risk of deep overbite; (2) lingual inclination of the mandibular anterior teeth; (3) persistence of residual spaces; and (4) alterations in tongue function [6, 10, 13]. These limitations restrict the indications for SE.

SE is primarily indicated for severe crowding associated with Class I malocclusion. However, Dewel [35] stated that “in cases with severe skeletal Class II discrepancies, treatment should primarily focus on Class II orthopedic correction, with SE serving as an adjunctive procedure”. Thus, in cases with abnormal maxillomandibular relationships (e.g., Class II and Class III malocclusions), SE programs must be implemented with great caution, with the understanding that skeletal malocclusions should be corrected using appliances before completion of the permanent dentition [6]. Because the present case was diagnosed as skeletal Class II, it would not ordinarily be considered an appropriate indication for SE. However, to compensate for the inherent disadvantages of SE, the treatment strategy combined extraction with a functional orthodontic appliance capable of advancing the mandible and increasing the occlusal vertical dimension. For this reason, the present approach was intentionally defined as early extraction combining SE with a functional orthodontic appliances, rather than SE alone.

When premolars are extracted at an early stage, changes in intercanine width can also be a matter of debate. In non-extraction orthodontic treatment, intercanine width may be increased, which has led to the assumption that intercanine width naturally increases during growth. However, Moorrees et al. [36] concluded that the intercanine width of the mandibular deciduous teeth increases by about 3 mm during the replacement of the mandibular anterior teeth, but then gradually decreases and eventually returns to a width comparable to that observed at approximately 7–8 years of age. Similar findings have been reported in Japanese populations. These studies have shown that the mandibular primary intercanine width continues to increase until approximately 9 to 10 years of age, coinciding with the replacement of primary canines by the permanent canines. Subsequently, the intercanine width gradually decreases and stabilizes at a level comparable to that observed at around 7 to 8 years of age [37]. Thus, Graber et al. [6] stated that expecting an increase in mandibular intercanine arch width through the use of orthodontic appliances after 10 to 11 years of age is inconsistent with current understanding of growth and development. Therefore, in the present case, early extraction-oriented treatment was initiated at 7 years of age and was associated with maintenance of the mandibular intercanine width. This approach helped prevent relapse, including recurrence of crowding, without the need for excessive transverse expansion, and allowed for efficient treatment.

4.3 Use of a tooth positioner during the second treatment phase

At the finishing stage of orthodontic treatment, various approaches are available for establishing an optimal functional occlusion.

In the present case, after completion of Bionator therapy (at 10 years and 10 months of age), the patient’s father was transferred for work, making it difficult to continue regular monthly visits. Nevertheless, further occlusal settling was required, which would normally necessitate tooth movement using a multibracket appliance, along with careful oral hygiene management. Therefore, it was decided to continue treatment using a removable appliance. Recently, removable appliances have been reported to be more aesthetically acceptable and better tolerated by patients [38]. Among removable options, aligners and tooth positioners are commonly considered. It has been reported that aligners may allow better maintenance of periodontal health compared with fixed appliances [39]. However, because aligners are fabricated separately for the maxillary and mandibular arches, they have disadvantages, including difficulty in establishing proper interarch occlusion and the need to apply relatively large forces during insertion and removal, which may reduce ease of use.

In contrast, the tooth positioner, introduced in 1945, is fabricated from an elastic material, allowing easy insertion and use [40, 41]. There have been reports that, during the finishing stage of orthodontic treatment, tooth positioners can correct minor discrepancies associated with bracket positioning, increase the number of interarch occlusal contacts, and facilitate correction of the axial inclination of the anterior teeth [41, 42]. In addition, as a monoblock appliance, the positioner is designed to allow the maxillary and mandibular molars to occlude in an Angle Class I relationship, and has therefore been used to guide the mandible into a Class I relationship [43]. However, treatment outcomes with a positioner are known to be influenced by patient compliance [44], which represents an inherent limitation of removable appliances. In the present case, the patient demonstrated good compliance during Bionator therapy, and the elastic material of the positioner was considered advantageous for maintaining adequate appliance fit during tooth movement. Therefore, a tooth positioner was selected for the present case. The absence of fixed appliances during adolescence may have provided practical advantages for both the patient and the dentist.

In the present case, a prefabricated tooth positioner was used. When a plaster was poured into the tooth positioner, the interincisal angle measured on the resulting cast was 127° (Fig. 10). At the initiation of tooth positioner use, the patient’s interincisal angle was 141.0°. Because the target value was set at 131°, the tooth positioner was applied with the intention of achieving slight overcorrection. As a result, tooth movement proceeded in accordance with the tooth positioner setup, leading to labial inclination of the maxillary and mandibular anterior teeth. At the completion of treatment, the interincisal angle had decreased to 130°, indicating a favorable interincisal angle.

Fig. 10.Use of a tooth positioner. (A) Photograph of the tooth positioner. (B) Plaster cast of the tooth positioner, showing an interincisal angle of 127°.

4.4 Limitations

This report has some limitations that should be considered. First, because this is a single case report, the results cannot be generalized to a broader patient population, and careful patient selection is therefore essential. Second, the treatment strategy used in this case relies on patient compliance, and long-term follow-up is necessary to assess treatment outcomes and durability.