Introduction
The ectopic eruption of the permanent first molar is a localized eruption disturbance in which the tooth erupts mesially, deviating from its normal eruption path. The ectopically erupted first molar becomes locked under the distal contour of the second primary molar, which causes abnormal root resorption of the primary tooth. The ectopic eruption of the first molar occurs in approximately 3 - 4% of the general population [1], and root resorption of the second primary molar caused by ectopic eruption is more pronounced in the maxilla than in the mandible [2].
Ectopic eruption of the permanent first molar is primarily diagnosed through routine radiographic examinations performed between the ages of 5 and 7 years. An impacted first molar overlapping with the distobuccal root of the primary molar or pathological root resorption of the adjacent second primary molar on the radiograph can be considered an ectopic eruption of the first molar [3]. Young classified ectopic eruption into two types [4]. The “jump” type is a reversible form, whereby the ectopic eruption of the maxillary first molar spontaneously resolves and erupts into its normal position. In contrast, the “hold” type represents an irreversible form, wherein the maxillary first molar becomes impacted against the cervical area of the distal surface of the second primary molar, thereby interfering with its eruption. However, clinically distinguishing between these two types can be challenging. If this eruption disturbance is not treated promptly, progressive root resorption of the second primary molar can result in its premature loss. This condition may lead to a reduction in the arch length, mesial tipping of the first molar, and second premolar impaction [5-7]. For this reason, after treating ectopic eruptions, it is crucial to preserve the second primary molar in order to maintain space and facilitate occlusal development.
Confirming the duration for which a second primary molar exhibiting advanced root resorption can be maintained after the treatment of ectopic eruptions is difficult. Previous studies have indicated that once the eruption path of the permanent first molar is normally restored, further root resorption of the second primary molar is expected to cease. In such cases, the resorbed areas may be repaired through the deposition of secondary dentin [5]. Once the ectopic eruption of the first molar is resolved, even second primary molars with significant root resorption can survive for an extended period and function adequately in terms of occlusion and space maintenance [8]. However, studies evaluating the prognosis of second primary molars after treatment of ectopic eruption remain very limited.
There are various appliances available for resolving the ectopic eruption of the permanent first molar. Among these devices, the Halterman appliance has advantages over other devices due to its simple laboratory fabrication process, compact size, and ability to make direct adjustments intraorally and extraorally [9]. However, the potential for excessive loading on the second primary molar, which serves as the anchorage unit, must be considered.
Thus, this study aimed to retrospectively evaluate the 24-month survival rates of the adjacent second primary molars after the treatment of ectopic eruption of the maxillary first molars using the Halterman appliance.
Materials and Methods
This retrospective study was conducted after the approval of the Institutional Review Board (IRB) of the Dental Hospital at Chosun University (IRB No. CUDH-2407-001).
1. Study subjects
This study included patients who visited the Department of Pediatric Dentistry at Chosun University Dental Hospital and three private pediatric dental clinics between January 2017 and December 2021, and who were diagnosed with ectopic eruption of the maxillary first molars through clinical and radiographic examinations and received orthodontic treatment using the Halterman appliance. To ensure consistency and uniformity in comparisons, this study included only cases where a unilateral Halterman appliance was applied, utilizing the second primary molar as the sole anchorage. The exclusion criteria were as follows:
(1) Insufficient clarity of radiographs or inaccurate documentation in electronic medical records.
(2) Severe dental caries in the second molar or evidence of root resorption with pulpitis.
(3) Pathological findings such as cysts or tumors in the maxillary posterior region.
(4) Craniofacial anomalies or syndromes linked to the dental disorder.
Overall, 131 second primary molars from 113 children (66 boys and 47 girls) were analyzed. The children were predominantly between the ages of 6 and 7 years (Table 1).
2. Study methods
Patients’ electronic medical records and radiographs were evaluated to determine whether and for how long the second primary molars survived up to 24 months of follow-up. In the follow-up period, cases in which the second primary molars were either maintained without issues, had exfoliated naturally, or were extracted, yet the deciduous permanent teeth erupted immediately without a space maintainer, were included in the survival analysis. Cases where clinical symptoms such as mobility and spontaneous pain occurred because of severe root resorption, leading to early extraction, were considered failures.
Patients’ age at treatment initiation, degree of root resorption in the affected second primary molar, mesial angulation of the maxillary permanent first molar, and gingival emergence status of the permanent first molar were considered.
The degree of root resorption in the second primary molar was evaluated using panoramic radiographs and assessed based on the criteria proposed by Barberia-Leache et al. [10], which categorizes resorption into four grades: grade 1, mild and limited resorption of the cementum or minimum dentin penetration; grade 2, moderate resorption of the dentin without pulp exposure; grade 3, severe resorption of the distal root, which results in pulp exposure; grade 4, very severe resorption, which involves the mesial root of the second primary molar (Fig. 1).
The mesial angulation of the maxillary first molar was determined using the method described by Bjerklin and Kurol [1]. The reference points were the orbital horizontal line, which connected the lowest points of the bilateral orbits on the panoramic radiograph, and the vertical line along the mesial surface of the maxillary first molar. The angle between the reference lines was defined as the mesial angulation of the maxillary first molar (Fig. 2).
The gingival emergence status of the permanent first molar was classified as “emerged” or “unemerged” based on panoramic radiographs and information recorded in the electronic medical records immediately prior to the initiation of treatment. Cases in which the crown of the first molar was exposed above the gingiva at the time of appliance placement were classified as “emerged,” whereas cases in which the crown was covered by gingiva and an operculectomy was performed to expose the crown prior to appliance placement were classified as “unemerged.”
3. Statistical analysis
Data were analyzed using the IBM SPSS version 29.0 (SPSS Inc., Chicago, IL, USA). The survival rates of the second primary molars were analyzed according to each factor using the Kaplan-Meier method, and differences in the survival rates were compared using the log-rank test. Furthermore, the Cox proportional hazards regression model was employed to assess the potential risk factors for survival, and the hazard ratio between the groups for each factor was analyzed. All statistical significance levels were set at α = 0.05.
Results
1. Cumulative survival rates
The survival rates of 131 second primary molars were analyzed by examining whether and when premature loss occurred during the follow-up period. Of the 131 teeth, 40 were prematurely lost, while 91 survived. The 24-month cumulative survival rate was 69.5%, with a mean survival time of 20.248 ± 1.024 months (Fig. 3).
2. Risk factor analysis
1) Patients’ age at treatment initiation
The cumulative survival rates by age groups were 50.0%, 62.3%, 70.0%, 86.4%, and 87.5% in groups aged 5, 6, 7, 8, and 9 years, respectively. This trend exhibited an increasing survival with age, although the difference was not statistically significant (Table 2, Fig. 4, p= 0.163). The Cox proportional hazards regression model also revealed that there was no statistically significant correlation between age and risk of premature loss of the second primary molar (Table 3, p= 0.075).
2) Degree of root resorption in the second primary molars
The 24-month cumulative survival rates were 90.9% for grade 1, 88.6% for grade 2, 68.9% for grade 3, and 33.3% for grade 4. A significant decrease in survival was observed with the severity of root resorption in the second primary molar (Table 4, Fig. 5, p < 0.001). Furthermore, the risk of premature loss of the second primary molar was approximately 8.7 times higher in grade 4 than in grade 1 (Table 3, HR = 8.718, p < 0.001).
3) Mesial angulation of the permanent first molar
In the Cox proportional hazards regression model, the mesial angulation of the permanent first molar was not a significant factor in the survival of the second molar (Table 3, p= 0.418).
4) Gingival emergence status of the permanent first molar
Survival analysis, which categorized the patients into “emerged” and “unemerged” groups based on the gingival emergence status immediately prior to appliance placement, revealed no statistically significant difference in survival rates between the groups (Table 5, Fig. 6, p= 0.723). In the Cox proportional hazards regression model, there was no significant difference in the risk of premature loss of the second primary molar between the two groups (Table 3, p= 0.729).
Discussion
Ectopic eruption of the permanent first molars can be classified as either reversible or irreversible. In irreversible cases, orthodontic treatment can restore the eruption pathway of the maxillary first molars, enabling normal eruptions, provided that the second primary molar has undergone resorption. However, with delayed treatment, root resorption of the second primary molar may continue, potentially leading to premature loss and complications, including space loss, crowding of the dentition in the affected area, and second premolar impaction [7].
Previous studies have reported that two-thirds of ectopic eruptions are reversible [4], and in most cases, the eruption pathway spontaneously resolves by the age of 7 years when the ectopic eruption is reversible [5]. Silva et al. [11] stated that spontaneous eruption rarely occurs after the age of 7 years, generally requiring orthodontic intervention. Furthermore, Bjerklin and Kurol [1] suggested that the diagnosis of irreversible ectopic eruption should be made between the ages of 7 and 8 years. Moreover, Im et al. [12] proposed that early treatment may inadvertently promote infection and accelerate the premature loss of the second primary molars. However, this study found no significant correlation between patient age at treatment initiation and the survival rates of second primary molars. Determining the appropriate timing for intervention based solely on patient age is considered challenging. Delaying treatment in anticipation of spontaneous eruption may accelerate the root resorption of the second primary molar, resulting in its premature loss. Therefore, various factors, including age, should be considered when deciding on the timing of treatment.
Several studies have investigated the diagnostic criteria for ectopic eruption of the maxillary permanent first molar. Chintakanon and Boonpinon [2] reported the difficulty in determining the type of ectopic eruption based solely on the degree of root resorption of the maxillary second primary molar and, the mesial angulation of the maxillary first molar as the most critical factor. Im et al. [12] also argued that the mesial angulation of the permanent first molar is a more reliable diagnostic criterion than the degree of root resorption in the maxillary second primary molar. They particularly noted that when the mesial angulation of the permanent first molar is less than 90°, irreversible ectopic eruption is highly probable. In this study, the average mesial angulation of the maxillary permanent first molar was approximately 86°, which is consistent with the results of a previous study. However, the mesial angulation of the permanent first molar was not a significant factor that affected the survival of the second primary molar. Instead, the degree of root resorption in the second primary molar was found to be a more significant factor. In conclusion, although the mesial angulation of the maxillary permanent first molar has been proposed as an important factor in determining the type of ectopic eruption, the degree of root resorption in the second primary molar has a greater effect on its survival rate.
Ectopic eruption of the maxillary permanent first molar often presents without noticeable symptoms, even in severe cases, making it difficult for the child to recognize the problem. Consequently, it is frequently discovered incidentally during dental visits for unrelated reasons. Diagnosis is usually made through routine radiographic examinations between the ages of 5 and 7 years, before the eruption of the maxillary permanent first molar [3]. Therefore, this study utilized panoramic radiographs, which are commonly used in dental check-ups, for evaluation. Although panoramic radiographs are prone to errors because of overlapping and distortion, Samawi and Burke [13] reported that maxillary and mandibular molars exhibit the least distortion compared with other teeth. Thus, panoramic radiographs can be regarded as the most efficient tool for clinicians for routine use in diagnosing ectopic eruption of the maxillary permanent first molars.
If the eruption pathway of the permanent first molar is restored to its normal position, further root resorption of the second primary molar ceases [5]. Even second primary molars exhibiting significant root resorption can persist for an extended period after the resolution of the first molar’s ectopic eruption, effectively maintaining occlusion and space [8]. However, fixed appliances such as the Halterman device, which induce distal movement of the permanent first molar, may exert excessive stress on the second primary molar used as an anchor, potentially accelerating root resorption [14,15]. Thus, as the degree of root resorption in the second primary molar increases, the survival rate and mean survival time decrease significantly. In particular, grade 4 cases exhibit a risk of premature loss that is approximately 8.7 times higher than that in grade 1. This indicates that the load imposed by the use of a resorbed second primary molar as an anchor for the Halterman device might accelerate root resorption.
When designing an appliance to manage ectopic eruption of permanent first molars, anchorage reinforcement may be necessary if there is mobility or root resorption in the adjacent second primary molars. In such cases, a bilateral Halterman appliance, including the contralateral teeth as part of the anchorage, is commonly used [14]. Cases have also been reported where anchorage was reinforced by placing orthodontic bands on two anterior teeth to fabricate the appliance [16]. Therefore, when the second primary molar exhibited significant root resorption, leading to mobility and anticipated space loss, increasing the anchorage or excluding the affected tooth from the anchorage would be beneficial to prevent early loss of the second primary molar. In this study, for consistent comparison under the same conditions, the subjects were limited to cases where unilateral Halterman appliances were used with the affected second primary molar as the anchorage. Consequently, the survival rates could not be compared according to different anchorage designs. Therefore, further investigations are needed to better understand this topic.
In this study, the Halterman appliance was used for 1 to 8 months, with a mean duration of approximately 3 months. Although effective in resolving ectopic eruptions, prolonged use of the Halterman appliance may impose sustained mechanical loading on the second primary molar, potentially accelerating root resorption. Although the present study did not specifically investigate the association between treatment duration and the prognosis of the second primary molar, further research is necessary to clarify the relationship between appliance duration and the progression of root resorption in the anchorage tooth. Such investigations could provide valuable insights into minimizing the adverse effects of prolonged appliance use and developing evidence-based treatment protocols to preserve the long-term function and stability of the anchorage tooth.
In cases where the second primary molars prematurely exfoliate after orthodontic treatment of the maxillary permanent first molars, space maintainers are commonly used to preserve the eruption space for the second premolars. However, it is often difficult to completely prevent space loss. Once space loss occurs, even if it is regained through orthodontic treatment, the second premolars may erupt ectopically toward the palatal side, requiring additional orthodontic interventions. In this study, due to the short follow-up period, the prognosis of the second premolar eruption was not evaluated. More studies with a longer-term follow-up period are required to address this issue.
Conclusion
This retrospective study evaluated the 24-month survival rates of second primary molars after treatment of ectopic eruption of the maxillary permanent first molar using the Halterman appliance. The degree of root resorption in the second primary molar was identified as the most significant factor affecting survival, with more severe resorption leading to a decrease in survival rate and duration. Patient age at treatment initiation, mesial angulation of the permanent first molar, and gingival emergence of the permanent first molar did not significantly affect survival.
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