The term “tooth eruption” often refers to the moment a tooth emerges into the oral cavity. However, it is defined as the process by which a tooth transitions from its developmental position within the alveolar bone to its functional position within the oral cavity and continues to erupt gradually throughout one’s lifetime, even after achieving occlusion [1]. If the process of tooth eruption does not occur properly, it can disrupt the formation of a normal dental arch and hinder facial growth [2]. Therefore, timely treatment is of utmost importance; a delay can complicate the treatment and lead to a poorer prognosis.

Age, tooth morphology, gender, and individual characteristics are some variables that affect the timing of tooth eruption [3]. Accordingly, it is necessary for a dentist to diagnose whether all permanent teeth follow a normal eruption process at the appropriate time [4]. Delayed tooth eruption (DTE), where teeth erupt later than the normal eruption period, can be clinically observed without the need for radiographic imaging. Timely discovery of DTE aids in the formulation of a comprehensive treatment plan [5]. DTE includes chronologic DTE, wherein eruption is delayed beyond the chronological age, and biologic DTE, wherein eruption is delayed despite the complete development of two-thirds of the root [5]. The treatment methods for DTE are categorized as follows: follow-up, space maintenance after primary tooth extraction, orthodontic traction, surgical exposure, replacement after extraction of a tooth with a delayed eruption (via autotransplantation, prosthetics, or implant), and space regaining [5-7].

The eruption process of mandibular premolars occurs vertically [8]. The prevalence of eruption disturbances among Koreans is 4.5%, with mandibular premolars accounting for 7.2% of these cases, and the leading causes are an abnormal eruption pathway and DTE [9]. The prevalence of eruption disturbances among Japanese individuals was 4%, with mandibular premolars accounting for 12.4% of these cases [10]. In addition, the prevalence rate of impacted mandibular premolars is reported to be 0.2% - 0.3% [11]. Disturbances in tooth eruption may lead to issues such as space loss, collapse of occlusion, and resorption of the adjacent tooth roots [6,12].

Therefore, this study aims to explore DTE to intervene at the appropriate time in cases of eruption issues with mandibular premolars. Clinically, the prevalence of DTE in permanent teeth was reported to be 4.3% [4]. Studies on DTE in permanent and retained primary teeth, eruption disturbances in mandibular premolars, and systemic conditions associated with DTE have been conducted previously [6,12-17]. However, the prevalence of DTE in mandibular premolars and the criteria for selecting treatment methods, apart from chronological age and degree of root development, remain unclear. The choice of each treatment method is determined by the clinician [4,5,7,12,17,18]. Therefore, the goal of this research is to investigate the prevalence, causes, average treatment duration, and distribution of dental conditions according to treatment methods in cases of DTE of mandibular premolars.

This study was conducted with approval from the Institutional Review Board (IRB) of Seoul National University Dental Hospital (IRB No: S-D20210029).

This study aimed to determine the prevalence, causes, treatment duration, and tooth condition in cases of delayed eruption of the mandibular premolars. In the current study, a prevalence rate of 5.19% for DTE in mandibular premolars was observed, with no statistically significant differences based on gender or side. Furthermore, consistent with previous studies [4,6,21], a higher prevalence of DTE in mandibular second premolars than in mandibular first premolars was observed. Although the prevalence reported in this study was higher than that (4.3%) reported by Johnsen in permanent teeth [4], a direct comparison between the two studies is challenging due to differences in the types of teeth examined. Additionally, Johnsen observed that delayed eruption predominantly occurred in mandibular second premolars and attributed the most common cause of DTE in permanent dentition to insufficient space for the eruption of succedaneous permanent teeth. The differences in findings between the two studies may be attributed to the fact that Johnsen’s study included all the permanent teeth, whereas this study focused solely on the mandibular premolars.

In the study by Henklein et al. [18], which focused on DTE in permanent teeth with Nolla stage 8 or higher, 70.7% of the DTE was observed in premolars; moreover, 68.7% of the cases were not associated with any specific reason, but featured retained primary teeth. Similar findings were observed in the current study, wherein 78.84% of the mandibular premolar DTE cases lacked a specific cause. In most instances, succedaneous permanent teeth erupt naturally; yet occasionally, primary teeth may impede the eruption of their permanent successors, leading to potential benefits from primary tooth extraction [22]. Mucosal barriers can contribute to DTE in cases where no specific cause is evident on the panoramic radiograph, and surgical removal of these teeth may facilitate the eruption of the permanent tooth [23,24]. A similar case was observed in the current study (Table 2).

Mandibular premolars with Nolla stage 7 or lower DTE indicate developmental delay. Several studies have indicated that odontogenesis of the mandibular second premolar exhibits significant variability [2,3]. Park et al. [25] investigated developmental delays in Korean patients aged 6 to 12 years and reported that after maxillary second premolars and maxillary second molars, mandibular second premolars exhibited the highest prevalence of developmental delay, with an overall prevalence of 0.84% for mandibular premolar DTE. Instances of initial enamel calcification at the age of 13 have been reported in the case of the highly variable mandibular second premolar [26,27]. Therefore, the longer treatment duration observed in teeth belonging to the Nolla stage 7 or lower group compared to those in the Nolla stage 8 or higher group may be attributed to the inclusion of teeth with developmental delays in the current study (Table 5).

Physical obstructions, such as benign tumors, odontomas, and supernumerary teeth, should be removed to allow for the eruption of permanent teeth [5,7]. Cysts and benign tumors are commonly found in the canine and molar regions of the jaw [28], with odontomas being the most common odontogenic tumors within the oral cavity [29,30]. In the current study, the benign tumors included ameloblastic fibro-odontomas and odontogenic keratocysts, both of which are prevalent in the mandibular posterior region and can lead to impeded eruption of succedaneous permanent teeth, necessitating surgical removal [28,31]. Surgical treatment was performed for all cases involving benign tumors, odontomas, and supernumerary teeth in the present study. There are two main arguments regarding the treatment approach after the surgical removal of the physical obstacle along the eruption pathway. The first is that the tooth with delayed eruption should be exposed during the surgical removal of the obstacle [32], and the second suggests removing the obstacle and ensuring sufficient space for the succedaneous permanent tooth, thereby promoting spontaneous eruption [33]. Di Biase [34] supported the second approach, stating that if the dental follicle of the succedaneous permanent tooth is undamaged and adequate eruption space is ensured during surgery, 54% - 75% of the teeth would erupt spontaneously after 18 months. This aligns with the findings of the current study, where the duration of surgical treatment was 17.87 months.

Primary teeth with nonvital pulp often undergo cystic transformations, which can potentially cause delayed eruption in succedaneous permanent teeth. Thus, extraction of the nonvital tooth is necessary when symptoms arise [5,7]; this was one of the predominant causes of DTE in the present study (Table 2). The extraction of nonvital teeth can frequently lead to DTE in succedaneous permanent teeth. This phenomenon is often attributed to the premature extraction of primary teeth with nonvital pulp before the resorption of the roots, causing abnormal tissue changes in the upper region of the succedaneous permanent teeth and resulting in the formation of thick and fibrous gingiva [35]. As shown in Table 7, cases involving space maintenance after primary molar extraction exhibited longer treatment durations compared to follow-up alone, suggesting that this could be a contributing factor.

Ankylosis is the most common cause of delayed exfoliation of primary teeth and can lead to delayed eruption of succedaneous permanent teeth [36,37]. In a study by Kurol and Thilander [38], 73.1% of ankylosed primary teeth exhibited a mean or 6-month delayed shedding period without any intervention, and 23.1% showed a 12-month delayed shedding period, followed by an exchange with the succedaneous permanent teeth. Messer and Cline [36] reported that 62% of ankylosed primary teeth were naturally exchanged with succedaneous permanent teeth, whereas 38% required extraction. These findings suggest that space regaining should be performed in cases of insufficient eruption space due to tilting of the adjacent teeth caused by ankylosed primary teeth; additionally, orthodontic traction or surgical treatment might be necessary. In accordance with the treatment methods outlined in previous researches [36-38] for ankylosed primary molars, treatment methods were employed based on criteria such as eruption stage, impaction depth, and tilting of adjacent teeth. This encompassed follow-up, space maintenance after primary molar extraction, space regaining, and surgical treatment; however, only two out of seven cases with naturally erupted succedaneous permanent teeth were observed, which was lower than that seen in previous studies (Table 2). This phenomenon may be attributed to the nature of the university hospital setting, where patients who naturally experience eruptions are less commonly encountered.

Insufficient eruption space often results in an abnormal path of eruption for the tooth; occasionally, it can trigger a delayed eruption of the succedaneous permanent tooth, in which case adequate space for eruption must be created [39]. Most teeth with insufficient eruption space typically result in crowding as they erupt. However, in some cases where the lack of eruption space acts as a physical obstacle, and eruption may not occur naturally, it becomes necessary to create space through methods such as space regaining or extraction [5]. In this study, among the 12 cases with insufficient eruption space, six cases saw spontaneous eruption of the permanent successor teeth without any intervention. For four cases, space regaining was employed, and for two cases, extraction was performed to either secure eruption space or as a treatment method for the affected teeth (Table 2).

In cases of abnormal eruption pathways of maxillary canines, research suggests that not only the impaction depth but also the angle of inclination of the impacted crown can impact the prognosis and treatment duration [40]. A similar report exists for mandibular second premolars, providing treatment recommendations based on the depth and inclination of impaction [17]. As shown in Table 6, a lower eruption stage was associated with longer treatment periods. Furthermore, in Table 8, an increasing trend in the impaction depth and angle of inclination was observed with more invasive treatment methods, similar to the findings by Ismail et al. [17]. However, the previous study [17] classified based on the maximum value of the impaction depth and angle of inclination for each treatment method, whereas in this study, the 95% CI was calculated for each treatment method, and the minimum and maximum values were recorded in cases of insufficient sample size; additionally, the mandibular first and second premolars were included. Due to the insufficient sample size, statistical errors may occur as we were unable to calculate the 95% CI for the impaction depth and inclination in space regaining. Premolars often erupt and sometimes even erupt in a distal direction [41], with cases of restoration of normal eruption direction, highlighting the need for evaluating the eruption patterns at intervals of 3 - 6 months [42]; this may be due to the presence of the gubernacular cord [43]. The gubernacular cord is a structure of the dental lamina. It undergoes apoptosis, and the remains of the cord are organized into strands that align together. These strands are located in proximity to the reduced epithelium of the enamel organ and extend towards the oral mucosa. This, in conjunction with the coronal osteoclast action and apical osteogenesis of the dental follicle, guides tooth eruption [43]. Therefore, even in cases of ectopic eruption of mandibular premolars, the guidance provided by the gubernacular cord might help restore the normal eruption pathway.

In several studies [5,42], when a deviation of 90° or more from the normal eruption pathway was observed, autotransplantation was suggested as a viable option. In addition, it was suggested that a more invasive treatment approach should be considered if spontaneous positional improvement is not observed during regular radiographic observations. As seen in Table 4, the success rates of all treatment methods, excluding space regaining, ranged from 80.00% to 98.71%. Furthermore, when the initial treatment method proved ineffective, more invasive treatment options are available. Therefore, clinicians should always consider revising their treatment plans with a more invasive approach if there is no improvement in the chosen initial treatment method. However, the choice of treatment was left to the dentist’s discretion, and apart from studies like Ismail et al. [17], very few have proposed treatment methods based on the depth of impaction or angle of inclination of the tooth [5,7,17,44]. The state of root development in the affected tooth, as per the definition of biological DTE, plays a crucial role in determining the treatment approach. However, since there is currently no globally agreed-upon guideline for the treatment of mandibular premolar DTE, clinicians rely on their experiential judgment in clinical practice. Factors such as the tooth’s eruption stage, impaction depth, inclination angle, developmental stage, and root development status also influence the treatment decision. The lack of statistically significant differences in the treatment duration based on the treatment methods in Table 7 may be due to differences in the selection criteria for the treatment methods. Therefore, based on the 95% CI for impaction depth and inclination associated with each treatment method, we have proposed a guideline for the selection of treatment methods for mandibular premolars’ DTE in Fig. 3.

This study has several limitations. The study population consisted of patients from university hospitals, which may limit the generalizability of the findings to the broader population of children and adolescents in Korea. Additionally, being a retrospective study, investigations of the eruption timing and treatment duration were based on panoramic radiographs taken at predetermined intervals, which could potentially introduce discrepancies compared to the actual eruption timing and treatment duration. Furthermore, panoramic radiographs have limitations in providing a three-dimensional view of the tooth position and angle of inclination, suggesting the need for follow-up studies utilizing computed tomography scans to obtain more comprehensive information. Moreover, the relatively small sample size for each treatment method underscores the necessity for future follow-up studies involving larger and more diverse patient groups.

According to this study, the prevalence of DTE in mandibular premolars among Korean children was 5.19%. Establishing an appropriate timing for diagnosis and treatment planning is crucial for addressing DTE in mandibular premolars. Dentists can determine treatment methods based on the cause of DTE, developmental stage of the tooth, depth of impaction, and angle of inclination. This study reveals that the treatment duration for delayed eruption of mandibular premolars tends to increase with the increase in the impaction depth and angle of inclination of the tooth; moreover, the lower root developmental and eruption stages, the higher the treatment duration. Thus, the range of impaction depth and inclination angle according to the different treatment methods may aid in determining the appropriate treatment for DTE. Furthermore, there is a need to discuss objective and agreed-upon criteria for the treatment of delayed eruptions of mandibular premolars.