Introduction
The first permanent molar (FPM), the first permanent tooth to erupt without replacing the primary tooth, plays a critical role in the development of dental arches and occlusion in children during the mixed dentition period[1]. The FPM contributes to the establishment of stable posterior occlusal contact and supports functional harmony between the dentition and the temporomandibular joint[2]. During the mixed dentition phase, the FPM serves as the largest and strongest tooth in each arch, occupying a strategically key position for distributing occlusal forces effectively to the surrounding alveolar bone through its broad root base[3]. The vertical position of the FPM also plays a fundamental role in establishing the vertical dimension of occlusion and increasing lower facial height, thereby influencing skeletal and facial development[4,5].
Although the FPM is crucial for both the initiation and completion of the mixed dentition period, the tooth is also among the most vulnerable teeth during childhood[1]. Additionally, FPM is the most caries-prone permanent tooth, with over 50% of children over the age of 11 years presenting with carious lesions on these teeth[6]. In addition, FPMs can be affected by developmental conditions, such as molar-incisor hypomineralization (MIH)[7]. Extraction of the FPM during childhood or adolescence may become necessary due to deep caries, developmental anomalies, ankylosis, and eruption disturbances.
When considering FPM extraction, multiple factors must be considered, including the patient’s symptoms, age, severity of the affected tooth, extent of pulpal involvement, presence of third molars, skeletal pattern, congenital absence of teeth, sagittal molar relationship, and incisor position[8,9]. When FPM extraction is unavoidable during childhood or adolescence, the optimal timing is generally considered to be between 8 and 10 years of age[8,10]. Clinically, this corresponds to the period after the eruption of the lateral incisor but before the eruption of the second premolar and molar. Radiographically, the root bifurcation of the second molar begins its calcification[8,10]. Extraction during this period increases the likelihood of spontaneous space closure via mesial drift of the adjacent second molar. This can reduce the need for extensive orthodontic intervention and shorten the treatment duration and complexity[8,10-12].
Extraction of the FPM during the mixed dentition period is accompanied by various changes in dental arch development as well as occlusion[13]. Following extraction, the adjacent second and third molars on the extraction side exhibit accelerated root development and eruption[14,15]. Although the space gained from tooth movement may contribute to resolving crowding and is often associated with a reduced incidence of proximal caries within the affected arch, an increased prevalence of occlusal surface caries has been observed in second premolars and second molars adjacent to the extraction site[16]. Cephalometric studies have reported that the bilateral extraction of mandibular FPMs often results in lingual tipping and retrusion of the mandibular incisors, downward and forward mandibular growth, and counterclockwise rotation of the occlusal plane[17].
Although numerous studies have described postextraction changes in adjacent teeth, most have been descriptive or focused primarily on the long-term outcomes in the permanent dentition. Only a few studies have quantitatively evaluated vertical and horizontal tooth movement, as well as angular changes, during the mixed dentition period—a dynamic phase of dental development in growing patients. In particular, a relative paucity of structured investigations using panoramic radiographs, which are commonly employed and accessible diagnostic tools in pediatric dentistry, exists to quantify these changes in growing patients.
Therefore, this study aimed to evaluate the vertical, horizontal, and angular changes in the adjacent second molars, as well as the angular changes in the adjacent premolars, following the extraction of the FPM in growing patients, using panoramic radiographs. This study was conducted to provide clinically relevant data to support long-term treatment planning and facilitate occlusal management following FPM extraction during the mixed dentition period.
Materials and Methods
This study was performed in accordance with the principles of the Declaration of Helsinki and approved by the Institutional Review Board of Pusan National University Dental Hospital (IRB approval number: PNUDH-2025-05-007).
1. Study Participants
A retrospective analysis was performed on clinical records collected between January 2013 and December 2022, at the Department of Pediatric Dentistry, Pusan National University Dental Hospital.
The study included patients who underwent FPM extraction during Hellman’s dental ages IIIA - IIIC. A total of 169 cases of FPM extraction were initially identified using the National Health Insurance Service procedure codes for molar (U4413) and complicated tooth extractions (U4414).
Participants were screened according to predefined inclusion and exclusion criteria. Individuals who had undergone orthodontic treatment before or after extraction were excluded. Additional exclusion criteria included bilateral FPM extractions, congenital absence of primary or permanent teeth, and radiographic records of inadequate diagnostic quality due to artifacts or poor resolution.
Panoramic radiographs were taken by experienced radiologic technologists using a standardized protocol. The inclusion criteria for panoramic radiographs of adequate quality were as follows:
(1) Contrast: Black areas (air spaces), white areas (enamel, dentin, and bone), and gray areas (soft tissue) are distinguishable[18].
(2) Completeness of objects: Superior (orbital floor), inferior (inferior mandibular border), and left and right lateral (cervical spine) boundaries are clearly visible[18].
(3) Distortion of size and shape: The anterior teeth are not narrowed; they widen horizontally, and the ramus, posterior teeth, and condyles are evenly enlarged so that they are symmetrical. There is no overlap of the structure of the object, and the shape of the occlusal plane is good[18].
(4) Sharpness: The outer boundary of an object is clear and not obscured[18].
(5) Resolution: The boundary between the periodontal ligament and the lamina dura and dentinoenamel junction is clearly visible[18].
(6) Right-left marker: There is a label marker right and left[18].
(7) Artifacts/ghost image: No internal or external artifacts or ghost images[18].
(8) Either the pre- or post-extraction radiograph shows a mesiodistal size discrepancy of less than 1 mm between the right and left mandibular second molars[19].
This retrospective study analyzed electronic medical records (EMR) and panoramic radiographs of selected patients. The assessed variables included demographic data (age and sex), specific tooth extractions, and relevant dental history.
2. Panoramic Radiograph
Measurements were obtained by comparing panoramic radiographs taken immediately before extraction with the most recent available images. For each participant, the FPM extraction side was designated as the experimental group (EG) and the contralateral nonextraction side as the control group (CG).
Fig. 1 illustrates the reference lines and landmarks used for this study. Reference lines for measurements were established based on the method described by Chen et al.[20]. The midline was defined as the line connecting the nasal septum and the maxillary suture. The occlusal plane was identified as the representative plane formed by the average contact points of the posterior teeth on radiographic images. The definition of the variables used for the measurements is summarized in Table 1. All the measurements were performed by a single dentist. A built-in calibration bar was used to account for magnification in the measurements.
1) Mesial movement of the second molar
The mesial shift of the second molar was assessed by measuring the horizontal distance from the midline to both the mesial and distal aspects of the tooth on a panoramic radiograph. The shortest distance from the midline to the point of maximum convexity on the mesial crown contour was defined as the center-to-crown mesial (CCM) measurement. Similarly, the shortest distance from the midline to the point of maximum convexity on the distal crown contour was measured and was defined as the center-to-crown distal (CCD).
2) Occlusal movement of the second molar
The eruption level of the second molar was evaluated by measuring the vertical distance from the occlusal plane to the mesial and distal cementoenamel junctions (CEJs) on the panoramic radiograph. The shortest distance from the occlusal plane to the mesial CEJ of the second molar was defined as the occlusal plane to the mesial CEJ distance (OPM). Similarly, the shortest distance from the occlusal plane to the distal CEJ of the second molar was measured and defined as the occlusal plane to the distal CEJ (OPD).
3) Inclination angles of the premolar and second molar
The measurements included the mesial inclination angles between the occlusal plane and the long axes of the second premolar (P2 angle) and second molar (M2 angle), as well as the angle between the long axes of the second premolar and second molar (P2-M2 angle). The long axes of the second premolar and second molar were defined following the methods described by Ismail et al.[21] and Park et al.[22]. For second premolars, the long axis was determined by drawing a line connecting the tip of the most centrally positioned cusp to the root apex. For the second molars, the long axis was defined as the perpendicular line drawn from the midpoint of the crown to the tangent at the most prominent point of the root bifurcation. If the apex appeared wide due to the early root development, the midpoint of the apical width was used.
3. Statistical Analysis
The data are presented as frequency and percentage for categorical variables and as mean ± standard deviation (SD) for numeric variables. One examiner performed all measurements twice, with a 2-week interval between the sessions. The intra-examiner error between the two measurements was determined using the intraclass correlation coefficient (ICC), based on a two-way mixed analysis of variance. ICC values for all variables ranged from 0.99 to 1.00, indicating a high level of intra-observer agreement. The average of the two measurements was used for statistical analysis.
Group differences in participant characteristics were analyzed using the chi-square or Fisher’s exact tests for categorical variables and the independent t-tests or Mann-Whitney U tests for continuous variables, as appropriate. Differences between paired data were analyzed using a paired t-test or Wilcoxon signed-rank test. Comparisons among the three groups were performed using analysis of variance (ANOVA) with Scheffé’s post-hoc test or the Kruskal-Wallis test with Dunn’s post-hoc test. The Shapiro-Wilk test was employed to assess normality. All statistical analyses were performed using SPSS Statistics for Windows, version 29.0.2.0. (IBM Corp., Armonk, NY), and p-values < 0.05 were considered statistically significant.
Results
1. Participant Characteristics
Between January 2013 and December 2022, 169 cases involving FPM extraction were identified at the Department of Pediatric Dentistry at Pusan National University Dental Hospital. Of these, 32 patients (38 cases) met the research criteria and were included in the final analysis (Fig. 2). Tables 2 and 3 present the descriptive statistics of the study groups.
The mean duration of follow-up was 29.4 months. At the time of extraction, most patients (26 individuals, 68.4%) were classified as Hellman’s dental stage IIIB. The most common reason for extraction was developmental dental anomalies, which accounted for 25 cases (65.8%), followed by dental caries (10 cases, 26.3%) and eruption disturbances (3 cases, 7.9%) (Table 4).
2. Positional Changes of the Second Molar and Premolar in the Maxilla Following the Extraction of the FPM
Table 5 summarizes the positional changes in the maxillary second premolars and molars following FPM extraction. Regarding the mesial displacement of the second molar, CCM and CCD showed average shifts of 5.9 mm and 4.5 mm, respectively, on the extraction side (Table 5, p < 0.0001 and p= 0.003). In contrast, no significant horizontal changes were observed on the nonextraction side (Table 5, p= 0.784 and p= 0.928).
For vertical displacement toward the occlusal plane, OPM and OPD on the extraction side exhibited mean movements of 7.4 mm and 10.0 mm, respectively (Table 5, both p < 0.0001). On the non-extraction side, OPM and OPD showed corresponding mean values of 5.1 mm and 5.7 mm (Table 5, both p < 0.0001). The magnitude of the vertical displacement was significantly greater on the extraction side than on the non-extraction side (Table 5, mesial: p= 0.004; distal: p < 0.0001).
Following the extraction of the FPM, P2 angle reduced by an average of 5.5°, indicating distal tipping of the crown (Table 5, p= 0.005). No significant angular changes were observed on the non-extraction side (Table 5, p= 0.204). In terms of second molar angulation, M2 angle increased by an average of 23.9° on the extraction side, indicating mesial tipping of the crown (Table 5, p < 0.0001). However, no statistically significant change was detected on the non-extraction side (Table 5, p= 0.088). Lastly, P2-M2 angle increased by an average of 29.5° on the extraction side, whereas no significant change was observed on the non-extraction side (Table 5, p < 0.0001 and p= 0.251).
3. Positional Changes of the Second Molar and Premolar in the Mandible Following FPM Extraction
Table 6 displays the positional changes in the mandibular second premolars and second molars following FPM extraction. On the extraction side, the mesial and distal surfaces of the second molar exhibited substantial mesial displacement, with average shifts of 10.1 mm and 10.4 mm, respectively (Table 6, both p < 0.0001). On the non-extraction side, mesial movement was also observed, with mean shifts of 4.5 mm at the mesial surface and 3.6 mm at the distal surface (Table 6, p < 0.0001 and p= 0.006, respectively). The extent of mesial movement was significantly greater on the extraction side than on the non-extraction side for both surfaces (Table 6, both p < 0.0001).
In terms of vertical eruption toward the occlusal plane, OPM and OPD on the extraction side exhibited mean movements of 4.1 mm and 3.5 mm, respectively (Table 6, both p < 0.0001). On the non-extraction side, vertical displacement was also observed, with mean values of 6.1 mm for OPM and 4.2 mm for OPD (Table 6, both p < 0.0001). Although vertical movement occurred on both sides, the non-extracted side exhibited greater magnitude, with a statistically significant difference observed only at the mesial site (Table 6, mesial: p= 0.024; distal: p= 0.342).
P2 angle decreased by an average of 8.7° on the extraction side (Table 6, p= 0.021), indicating distal tipping of the crown following FPM loss. No significant change in angulation was observed on the non-extraction side (Table 6, p= 0.495). For the second molars, no significant angular changes were identified on the extraction side (Table 6, p= 0.997). However, on the non-extraction side, M2 angle decreased by a mean of 11.1° (Table 6, p < 0.0001), indicating distal inclination of the crown. P2-M2 angle did not reveal a statistically significant difference (Table 6, p= 0.062).
4. Comparison of Tooth Movement Between Extraction and Non-extraction Sides in the Maxilla and Mandible
After FPM extraction, the horizontal mesial movement of the second molar was significantly greater in the mandible than in the maxilla. Specifically, the mandibular CCM and CCD exhibited mean mesial shifts of 10.1 mm and 10.4 mm, respectively, compared with 5.9 mm and 4.5 mm in the maxilla (Table 7, p= 0.001 and p < 0.0001, respectively).
Concerning vertical displacement in the direction of the occlusal plane, the second molar in the maxilla demonstrated a significantly greater eruption than that observed in the mandible. The mean OPM and OPD in the maxilla were 7.4 mm and 10.0 mm, respectively, while the corresponding values in the mandible were 4.1 mm and 3.5 mm (Table 7, p= 0.003 and p < 0.0001, respectively).
Regarding the angular changes in the second premolar after extraction of the FPM, both arches demonstrated a tendency for distal tipping of the crown. However, the difference between the maxilla and mandible was not significant (Table 7, p= 0.376). Conversely, the second molar exhibited a pronounced angular change in the maxilla, with the crown tipping mesially by an average of 23.9°, whereas no angular change was observed in the mandible. The interarch difference was statistically significant (Table 7, p < 0.0001). P2-M2 angle increased by an average of 29.5° in the maxilla, which was significantly greater than the 8.7° change observed in the mandible (Table 7, p < 0.0001).
5. Positional Changes of the Second Premolar and Molar According to Hellman’s Dental Age at the Time of First Molar Extraction
Table 8 presents the extent of positional changes in the second premolar and molar according to the Hellman’s dental age at the time of FPM extraction. Most measured variables demonstrated no statistically significant association with dental age at the time of extraction.
Although both the mesial and vertical movements of the maxillary and mandibular second molars tended to be significant in the IIIA group and decreased progressively from IIIB to IIIC, these differences were not statistically significant (Table 8, p > 0.05). The only variable that exhibited a statistically significant difference across the dental age groups was the mandibular CCM. For this parameter, the IIIA group exhibited the greatest displacement with a mean value of 7.3 mm, followed by 3.1 mm in the IIIB group and 2.2 mm in the IIIC group, indicating a decreasing trend in mesial movement with increasing dental age (Table 8, p= 0.040).
Discussion
This retrospective study used panoramic radiographs to quantitatively evaluate the positional and angular changes in the adjacent second premolars and molars following early extraction of the FPM in growing patients.
In this study, the most common reason for early FPM extraction was developmental dental anomalies (65.8%), followed by dental caries (26.3%), as shown in Table 4. Developmental anomalies included MIH and molarincisor malformations (MIM). The condition could be diagnosed as MIH according to the guidelines of the European Academy of Pediatric Dentistry[23] or as MIM based on the diagnostic criteria described by Lee et al.[24]. Given that some cases initially classified as dental caries lacked clinical photographs or sufficient EMR documentation to confirm the presence of developmental anomalies, the actual proportion of early FPM extractions attributable to developmental dental anomalies may have been underestimated. Eruption disturbances were the least common reason for extraction (3 cases, 7.9%), with one case each of impaction, ankylosis, and primary failure of eruption.
Mesial movement of the adjacent second molar was evaluated after the extraction of the FPM. In the maxilla, CCM and CCD exhibited mean mesial shifts of 5.9 mm and 4.5 mm, respectively (Table 5, p < 0.0001 and p= 0.003). In the mandible, both CCM and CCD showed pronounced mesial displacement, with mean values of 10.1 mm and 10.4 mm, respectively (Table 6, both p < 0.0001). A comparison between the maxillary and mandibular arches revealed a significantly greater mesial movement in the mandible than in the maxilla (Table 7). This difference may be attributed to the wider mesiodistal crown dimension of the mandibular FPMs than their maxillary counterparts, as well as the greater leeway space typically present in the mandible. Additionally, differences attributable to the root morphology of the maxillary and mandibular second molars should be considered. Maxillary second molars typically have three roots, whereas mandibular second molars generally have two[25-28]. Teeth with a greater number of roots and a broader, more triangular root surface area can provide stronger anchorage[29]. However, a previous meta-analysis by Saber et al.[13] reported that spontaneous space closure following FPM extraction, driven by mesial migration of the second molars, is more commonly observed in the maxilla than in the mandible, although this difference was not statistically significant. Nevertheless, a direct comparison with the present study is limited due to differences in extraction timing and the absence of standardized follow-up intervals. Furthermore, molar occlusal relationships and facial patterns may also influence the closure of extraction spaces. According to the study by Alexander et al.[30], the amount of space loss following premature loss of the primary first molar was significantly greater in patients with a leptoprosopic facial pattern and end-on molar occlusion. These findings suggest that the extent of space closure by the second molars after extraction of the FPM may likewise be affected by molar occlusal relationships and skeletal facial patterns. Therefore, future studies should take these factors into consideration.
Following the extraction of the FPM, the vertical movement of the adjacent second molar toward the occlusal plane was investigated. In the maxilla, OPM and OPD on the extraction side showed mean vertical displacements of 7.4 mm and 10.0 mm, respectively (Table 5, both p < 0.0001). In the mandible, the corresponding OPM and OPD exhibited mean vertical movements of 4.1 mm and 3.5 mm, respectively (Table 6, both p < 0.0001). This reflects a greater degree of eruption in the maxilla than in the mandible (Table 7). This difference may be attributed to anatomical variations between the maxilla and mandible. The maxilla typically has a lower bone density and a higher proportion of cancellous bone than the mandible, potentially resulting in reduced resistance to bone remodeling and resorption[12,31]. Consequently, tooth eruption and vertical movement are more likely to occur in the maxilla than in the mandible.
After extraction of the FPM, the angulation of the adjacent second molar relative to the occlusal plane was assessed. In the maxilla, the mesial tipping of the second molar was 23.9° (Table 5, p < 0.0001), whereas in the mandible, no statistically significant change in axial angulation was observed (Table 6, p= 0.997). This difference may be explained by the developmental angulation patterns of second molars within the alveolar bone. Maxillary second molars typically develop with a distally inclined crown, and during eruption, follow the distal surface of the first molar, gradually tipping mesially[32]. In contrast, mandibular second molars tend to develop with a mesially inclined crown, and during eruption, they are usually upright against the distal surface of the FPM[32]. When the FPM is extracted prematurely, the maxillary second molar loses the distal support provided by the tooth and may erupt with a greater mesial inclination than that on the non-extraction side. Conversely, the mandibular second molar, lacking distal guidance, is more likely to erupt while maintaining its mesial inclination during the developmental stage. According to previous studies on the eruption pattern of third molars following early extraction of the second molars, the positional changes of the third molars were comparable to the angulation changes of the second molars observed in our study after early extraction of the FPM[33]. On panoramic radiographs, maxillary third molar germs that exhibited distal angulation prior to second molar extraction tended to erupt through the extraction space with mesial angulation, ultimately becoming more parallel to the adjacent maxillary FPM[33]. Similarly, mandibular third molar germs that initially showed mesial angulation generally erupted either with uprighting or, in some cases, with further mesial tipping, thereby aligning more parallel to the mandibular FPM[33]. Additionally, P2 angle showed distal tipping in both arches, consistent with the findings of previous studies[34]. The degree of distal tipping was 5.5° in the maxilla (Table 5, p= 0.005) and 8.7° in the mandible (Table 6, p= 0.021), with the greatest changes observed in the mandible (Table 7). This study evaluated the two-dimensional tooth movement using panoramic radiographs. Further investigations using cone-beam computed tomography (CBCT) are warranted to assess three-dimensional changes in tooth position following early extraction of the FPM.
A critical factor in determining the timing of the FPM extraction during childhood and adolescence is the developmental stage of the adjacent second molar at the time of extraction. The optimal timing for FPM extraction is when the root bifurcation of the adjacent second molar begins calcification, corresponding to Demirjian stage E[12,13,35]. According to Teo et al.[36], three radiographic factors should be considered to achieve successful spontaneous space closure after FPM extraction during the mixed dentition period. The three evaluated factors were (1) engagement of the second premolar within the bifurcation area of the second primary molar, (2) mesial angulation of the second permanent molar relative to the FPM, and (3) presence of the third molar. Space closure was significantly more favorable when all three factors were present (p < 0.001), with a combination of factors (2) and (3) yielding the most optimal results (p < 0.001)[36]. In summary, the decision to perform early extraction of the FPM should consider not only the timing of extraction but also a comprehensive evaluation of the position and angulation of the adjacent second premolar and molar, along with the presence of the third molar.
This study had certain limitations. First, the study was conducted at a single institution and had a relatively small sample size. Second, a skeletal pattern-based classification of the participants was not performed. Hence, further studies are required to investigate the potential association between related variables and tooth movement after FPM extraction. Third, this study was limited to a two-dimensional analysis of tooth movement based on panoramic radiographs. Consequently, distortion from variations in head positioning, superimposition of anatomical structures, and ghost images may have impacted the accuracy of spatial assessments[37,38]. To minimize these concerns, cases exhibiting radiographic distortions, such as asymmetrical magnification between the left and right sides or evident image tilting, were excluded during screening. As measurements crossing the mandibular midline are less reliable due to the limitations of panoramic radiography, such measurements were not performed in this study[39]. Additionally, vertical measurements obtained from the left or right side on panoramic radiographs have been reported to closely correspond with those taken from dry skulls and are therefore regarded as relatively reliable[40,41]. Even though CBCT provides accurate, distortion-free, three-dimensional images of the tooth position, its routine use before active orthodontic intervention is ethically questionable due to the high radiation dose, particularly when no clear clinical indication is available. Accordingly, while the panoramic-radiography-based measurements in this study may not reflect absolute values, they remain meaningful indicators of the overall trends in tooth movement and offer valuable guidance for subsequent treatment planning. These findings provide clinically insightful data on positional changes in adjacent teeth after early FPM extraction, which may guide long-term treatment planning and orthodontic decision-making.
Notably, the results highlight distinct differences in the horizontal and vertical movements, as well as angulation changes between the maxillary and mandibular arches, suggesting that arch-specific considerations should be integrated into future orthodontic planning. The pronounced mesial migration of the mandibular second molars observed in this study indicates that, when extraction is appropriately timed according to the patient’s dental developmental stage, spontaneous space closure can be facilitated, potentially reducing the need for extensive orthodontic space management. Conversely, the greater vertical eruption and mesial tipping of maxillary second molars highlight the importance of monitoring occlusal plane changes and axial inclination to prevent unfavorable occlusal relationships. Incorporating these positional and angular changes into the treatment plan may help preserve arch integrity, guide proper eruption trajectories, and optimize overall orthodontic outcomes. To strengthen these findings, future research should incorporate three-dimensional analyses using CBCT and adopt a prospective design with appropriate control for confounding variables that may affect post-extraction tooth movements.
Conclusion
This study demonstrated that the patterns of positional change in the adjacent teeth following early extraction of the FPM differed between the maxillary and mandibular arches. Specifically, significant mesial displacement of the second molar was observed in the mandible, whereas the maxilla exhibited a relatively pronounced vertical eruption toward the occlusal plane. In terms of angulation, the maxillary second molars demonstrated significant mesial tipping, whereas the mandibular second molars maintained their initial angulation. Additionally, both maxillary and mandibular second premolars tended to tip distally.
These findings underscore the importance for clinicians to recognize arch-specific tooth movement patterns when planning early FPM extraction. Individualized treatment strategies that consider these differences may contribute to improved long-term outcomes in patients with mixed dentition.
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