Factors influencing open gingival embrasures in orthodontic treatment: a retrospective clinical study
Open gingival embrasure space (OGES), also known as the "Black Triangle," refers to the visible triangular gap formed when the interdental gingival papilla cannot completely cover the gingival embrasure space . The presence of OGES not only disrupts the harmonious aesthetic appeal during smiling but may also lead to food impaction , thereby affecting periodontal health . According to research reports, orthodontic treatment can contribute to the occurrence of OGES , with statistics indicating a high incidence rate of 35.4–43.7% among orthodontic patients . With the increasing pursuit of aesthetics and the growing number of orthodontic patients, the open gingival embrasure space in front teeth has garnered increasing attention. Although previous studies have explored the influencing factors of OGES, there is still controversy regarding these factors, possibly due to differences in research sample size, inclusion and exclusion criteria, and measurement items. Most of studies focuses on gender , age , gingival biotype , oral hygiene status , treatment duration , tooth extraction , and crowding , but often ignores the consideration of tooth movement variation during treatment. In particular, there is a lack of reports utilizing cone beam computed tomography (CBCT) technology to conduct in-depth research on the influencing factors related to OGES. The null hypothesis of this study is that there is no correlation between the occurrence of OGES in the central incisor area and factors such as gender, age, duration of treatment, method, tooth extraction, vertical and sagittal skeletal patterns, dentoalveolar height, inclination and movement of central incisors ( P < 0.05, with a study power set at 0.8). Our objective is to test this hypothesis by examining the relationship between these factors and the development of OGES.   Sample collection and grouping This study has been approved by the Medical Ethics Committee of West China Hospital of Stomatology (Approval Number: WCHSIRB-D-2024-153), and followed the contents in the Declaration of Helsinki concerning human subjects. We retrospectively collected data from patients who completed treatment at the Orthodontics Department of West China Hospital of Stomatology, Sichuan University, between January 2016 and December 2023. The presence and location of Open Gingival Embrasures (OGES) were descripted in medical records. This study focuses on the analysis of OGES in the central incisor area, with specific inclusion and exclusion criteria as follows: Inclusion criteria: Patients in the permanent dentition stage; Complete information, including intraoral digital photographs before and after orthodontic treatment (including frontal intraoral digital photographs), lateral cephalometric radiographs, and complete medical records; All patients have received oral hygiene education. Exclusion criteria: The presence of OEGS in the central incisor region before treatment; OGES exists between the lateral incisors and canines, but not between the central incisors; The presence of a gap between the maxillary and mandibular central incisors, or the presence of supernumerary teeth between the central incisors before treatment; Congenital missing incisors or extraction of incisors during treatment; Patients who underwent orthognathic treatment or received periodontal surgery during treatment; Patients who underwent secondary orthodontic treatment; Poor oral hygiene, severe gingival bleeding, acute hypertrophic gingivitis in the anterior tooth region, deep overbite, restorations in the central incisor region, or tooth crown defects involving tooth contact points, which affect the judgment of the gingival embrasure space region; Systemic diseases, such as diabetes. After screening, a total of 330 patients met the inclusion criteria. Patients with OGES in the central incisor area before treatment (31 people), those without OGES between the central incisors but with OGES in other areas (5 people), and other patients who did not meet the criteria were excluded, and finally, patients who met the requirements were included. As shown in Fig. , in this study, dentists graded the shape of the gingival papilla in the central incisor area using the Papillary Fill Index (PFI) proposed by Jemt et al.  Among them grades 0, 1, and 2 were included in the OGES group, while those with grade 3 were included in the Non-OGES Group (those with grade 4 were excluded). Ultimately, the participants were divided into the OGES group (130 individuals who developed OGES in the central incisor region after treatment but had no OGES before treatment) and the non-OGES group (200 individuals who had no OGES in the central incisor region before and after treatment). Intraoral digital photographs and lateral cephalometric radiographs before and after orthodontic treatment were collected from all patients, and the images were saved in JPG format. Information such as gender, age at the first visit, duration of orthodontic treatment (accurate to the month), tooth extraction, and extraction site was recorded by a doctor. Additionally, for the OGES and non-OGES groups, CBCT data before and after orthodontic treatment were saved for 39 and 33 individuals, respectively. The flowchart of this experiment is shown in Fig. . Landmarking and measuring of lateral cephalometric radiographs A dentist finished cephalometric analysis by an platform [accessible at ( https://www.zhibeicloud.com )]. The system has an average error as low as 0.94 ± 0.74 mm and an average accuracy rate of 89.33% . The landmarks used for cephalometric analysis are shown in Fig. . T1 and T2 represent the measurement items before and after orthodontic treatment respectively. The treatment measurements before and after for U1-SN (°), U1-NA (mm), IMPA, and L1-NB (mm), as well as their changes and absolute values of changes before and after treatment, are represented as △U1-SN (°), △U1-NA (mm), △IMPA, △L1-NB(mm), |△IMPA|, |△L1-NB(mm)| (Note: △ = T2–T1). CBCT marking and measurement All CBCT images were formatted into standard DICOM images and reconstructed into continuous slices of 0.3 mm in thickness. A dentist used the software Mimics Research 21.0 (Materialise NV, Leuven, Belgium) to perform axial adjustments on pre- and post-treatment CBCT scans and to conduct measurements. Position the central incisor in all three spatial planes. The method of orientation adjustment and measurement of the CBCT image is shown in Fig. . By manipulating the sagittal and coronal planes, an optimal view is achieved for measuring the desired parameters. Table lists the names of the measurement items and landmarks. The variations in the distance from the contact point to the alveolar ridge crest, as well as the alterations in the angle formed by the incisal edges of the two central incisors, were calculated both before and after treatment. These changes are denoted as △ICP-ABC and △R (Note: △ represents the difference between T2 and T1). Sample size calculation, quality control and statistical analysis The determination of the sample size in this study was calculated by a specific formula and guided by a previous study . With a Z value of 1.96, which corresponds to a significance level of P = 0.05, and the expected prevalence of OGES being 39.6% as the medium value of the reported range between 35.4 and 43.7% , along with a d value of 0.06 , the calculations indicated that a minimum of 255 participants were required for the study. A dentist randomly selected 20 lateral cephalometric radiographs and CBCTs, and continuously marked and measured them over a period of time, then re-measured them after 2 weeks. Intraclass correlation coefficient (ICC) was used to test the consistency of the results of the same measurement project in two occasions. After the repeatability and consistency of the measurements met the standard (ICC > 0.75), formal measurements began. Statistical analysis was performed using SPSS 25.0 software (IBM Corp, Armonk, N.Y, USA), with a significance level set at P < 0.05. When comparing the differences in categorical variables such as gender, orthodontic treatment method, tooth extraction, sagittal and vertical skeletal patterns between two groups, Pearson’s chi-square test, continuity-corrected chi-square test, or Fisher’s exact probability method were used according to the type of contingency table and the minimum expected frequency. Continuous variables were tested for normality (Kolmogorov–Smirnov test or Shapiro–Wilk test) and homogeneity of variance. If they followed a normal distribution and had equal variance, the two-sample t-test was used. If they followed a normal distribution but had unequal variance, the two-sample t’ test was used. If they did not follow a normal distribution, the Mann–Whitney U test (also known as the Wilcoxon rank-sum test) was used. Variables that were statistically significant in the univariate analysis were extracted, and variables with multicollinearity (VIF > 10) were removed, followed by binary Logistic regression analysis.   This study has been approved by the Medical Ethics Committee of West China Hospital of Stomatology (Approval Number: WCHSIRB-D-2024-153), and followed the contents in the Declaration of Helsinki concerning human subjects. We retrospectively collected data from patients who completed treatment at the Orthodontics Department of West China Hospital of Stomatology, Sichuan University, between January 2016 and December 2023. The presence and location of Open Gingival Embrasures (OGES) were descripted in medical records. This study focuses on the analysis of OGES in the central incisor area, with specific inclusion and exclusion criteria as follows: Inclusion criteria: Patients in the permanent dentition stage; Complete information, including intraoral digital photographs before and after orthodontic treatment (including frontal intraoral digital photographs), lateral cephalometric radiographs, and complete medical records; All patients have received oral hygiene education. Exclusion criteria: The presence of OEGS in the central incisor region before treatment; OGES exists between the lateral incisors and canines, but not between the central incisors; The presence of a gap between the maxillary and mandibular central incisors, or the presence of supernumerary teeth between the central incisors before treatment; Congenital missing incisors or extraction of incisors during treatment; Patients who underwent orthognathic treatment or received periodontal surgery during treatment; Patients who underwent secondary orthodontic treatment; Poor oral hygiene, severe gingival bleeding, acute hypertrophic gingivitis in the anterior tooth region, deep overbite, restorations in the central incisor region, or tooth crown defects involving tooth contact points, which affect the judgment of the gingival embrasure space region; Systemic diseases, such as diabetes. After screening, a total of 330 patients met the inclusion criteria. Patients with OGES in the central incisor area before treatment (31 people), those without OGES between the central incisors but with OGES in other areas (5 people), and other patients who did not meet the criteria were excluded, and finally, patients who met the requirements were included. As shown in Fig. , in this study, dentists graded the shape of the gingival papilla in the central incisor area using the Papillary Fill Index (PFI) proposed by Jemt et al.  Among them grades 0, 1, and 2 were included in the OGES group, while those with grade 3 were included in the Non-OGES Group (those with grade 4 were excluded). Ultimately, the participants were divided into the OGES group (130 individuals who developed OGES in the central incisor region after treatment but had no OGES before treatment) and the non-OGES group (200 individuals who had no OGES in the central incisor region before and after treatment). Intraoral digital photographs and lateral cephalometric radiographs before and after orthodontic treatment were collected from all patients, and the images were saved in JPG format. Information such as gender, age at the first visit, duration of orthodontic treatment (accurate to the month), tooth extraction, and extraction site was recorded by a doctor. Additionally, for the OGES and non-OGES groups, CBCT data before and after orthodontic treatment were saved for 39 and 33 individuals, respectively. The flowchart of this experiment is shown in Fig. .   A dentist finished cephalometric analysis by an platform [accessible at ( https://www.zhibeicloud.com )]. The system has an average error as low as 0.94 ± 0.74 mm and an average accuracy rate of 89.33% . The landmarks used for cephalometric analysis are shown in Fig. . T1 and T2 represent the measurement items before and after orthodontic treatment respectively. The treatment measurements before and after for U1-SN (°), U1-NA (mm), IMPA, and L1-NB (mm), as well as their changes and absolute values of changes before and after treatment, are represented as △U1-SN (°), △U1-NA (mm), △IMPA, △L1-NB(mm), |△IMPA|, |△L1-NB(mm)| (Note: △ = T2–T1).   All CBCT images were formatted into standard DICOM images and reconstructed into continuous slices of 0.3 mm in thickness. A dentist used the software Mimics Research 21.0 (Materialise NV, Leuven, Belgium) to perform axial adjustments on pre- and post-treatment CBCT scans and to conduct measurements. Position the central incisor in all three spatial planes. The method of orientation adjustment and measurement of the CBCT image is shown in Fig. . By manipulating the sagittal and coronal planes, an optimal view is achieved for measuring the desired parameters. Table lists the names of the measurement items and landmarks. The variations in the distance from the contact point to the alveolar ridge crest, as well as the alterations in the angle formed by the incisal edges of the two central incisors, were calculated both before and after treatment. These changes are denoted as △ICP-ABC and △R (Note: △ represents the difference between T2 and T1).   The determination of the sample size in this study was calculated by a specific formula and guided by a previous study . With a Z value of 1.96, which corresponds to a significance level of P = 0.05, and the expected prevalence of OGES being 39.6% as the medium value of the reported range between 35.4 and 43.7% , along with a d value of 0.06 , the calculations indicated that a minimum of 255 participants were required for the study. A dentist randomly selected 20 lateral cephalometric radiographs and CBCTs, and continuously marked and measured them over a period of time, then re-measured them after 2 weeks. Intraclass correlation coefficient (ICC) was used to test the consistency of the results of the same measurement project in two occasions. After the repeatability and consistency of the measurements met the standard (ICC > 0.75), formal measurements began. Statistical analysis was performed using SPSS 25.0 software (IBM Corp, Armonk, N.Y, USA), with a significance level set at P < 0.05. When comparing the differences in categorical variables such as gender, orthodontic treatment method, tooth extraction, sagittal and vertical skeletal patterns between two groups, Pearson’s chi-square test, continuity-corrected chi-square test, or Fisher’s exact probability method were used according to the type of contingency table and the minimum expected frequency. Continuous variables were tested for normality (Kolmogorov–Smirnov test or Shapiro–Wilk test) and homogeneity of variance. If they followed a normal distribution and had equal variance, the two-sample t-test was used. If they followed a normal distribution but had unequal variance, the two-sample t’ test was used. If they did not follow a normal distribution, the Mann–Whitney U test (also known as the Wilcoxon rank-sum test) was used. Variables that were statistically significant in the univariate analysis were extracted, and variables with multicollinearity (VIF > 10) were removed, followed by binary Logistic regression analysis.   Consistency test results and characteristics of the study groups Intraclass correlation coefficients (ICCs) from 0.897 to 0.990 ( P < 0.01) confirmed high repeatability and consistency in the two measurements. Of the 330 patients, 130 were in the OGES group and 200 in the Non-OGES group. OGES was present in both maxillary and mandibular incisor areas in 32 (9.67%), only in the maxillary incisor area in 14 (4.23%), and only in the mandibular incisor area in 84 (25.38%). Demographic details and OGES distribution are in Table . Overview and clinical characteristics of the study population Based on the results of the chi-square test, we found a correlation between gender and the occurrence of orthodontic gingival recession (OGES) in the maxillary and mandibular central incisor regions ( P < 0.05). Specifically, female patients exhibited a higher proportion of OGES. However, there was no significant correlation between the treatment methods, as well as tooth extraction, with the occurrence of OGES ( P > 0.05) (Table ). Meanwhile, we discovered a significant association between the initial age at diagnosis, treatment duration, and the occurrence of OGES in the maxillary and mandibular central incisors. The older the initial consultation age and the longer the treatment duration, the more likely OGES is to occur. Comparison of cephalometric measurements in the OGES and Non-OGES groups Participants were classified by ANB and MP-SN (Table ). Chi-square test analysis revealed no statistically significant association between sagittal/vertical skeletal pattern and the occurrence of OGES ( P > 0.05). U1-SN(T1), U1-SN(T2), U1-NA distance(T2), ΔU1-SN, ΔU1-NA correlated significantly with OGES in maxillary central incisors ( P < 0.05), with smaller values indicating higher OGES likelihood. Additionally, a smaller ΔL1-NB distance indicated a greater possibility of OGES. No statistically significant differences were found for the remaining measurement indicators (as shown in Table ). Comparison of CBCT between the OGES and Non-OGES groups After measuring and analyzing the CBCT data of the OGES group and the Non-OGES Group, we found that in the maxillary central incisor region, the distances between ABC and ICP, △ICP-ABC, and A-P overlap before and after treatment in the OGES group were significantly greater than those in the Non-OGES Group ( P < 0.05). The remaining measurement indicators, including TD, △R, root angulation, and crown width-to-height ratio, showed no significant correlation (Table ). In the mandibular central incisor region, the distances between ABC and ICP, and △ICP-ABC before and after treatment in the OGES group were also significantly greater than those in the Non-OGES Group ( P < 0.05). Multivariate analysis of correlated indicators Due to differences and sample size limitations in CBCT measurements compared to the study population and lateral skull X-ray film measurements, this experiment focuses on multivariate analysis of demographic, clinical characteristics, and cephalometric indicators. Binary logistic regression identified initial consultation age, treatment duration, and △U1-SN as significant factors affecting OGES occurrence in the maxillary central incisor area, with older age, longer treatment, and smaller △U1-SN correlating with greater retraction and higher OGES likelihood (Table ). Similarly, initial consultation age and treatment duration were significantly associated with OGES in the mandibular central incisor area.   Intraclass correlation coefficients (ICCs) from 0.897 to 0.990 ( P < 0.01) confirmed high repeatability and consistency in the two measurements. Of the 330 patients, 130 were in the OGES group and 200 in the Non-OGES group. OGES was present in both maxillary and mandibular incisor areas in 32 (9.67%), only in the maxillary incisor area in 14 (4.23%), and only in the mandibular incisor area in 84 (25.38%). Demographic details and OGES distribution are in Table .   Based on the results of the chi-square test, we found a correlation between gender and the occurrence of orthodontic gingival recession (OGES) in the maxillary and mandibular central incisor regions ( P < 0.05). Specifically, female patients exhibited a higher proportion of OGES. However, there was no significant correlation between the treatment methods, as well as tooth extraction, with the occurrence of OGES ( P > 0.05) (Table ). Meanwhile, we discovered a significant association between the initial age at diagnosis, treatment duration, and the occurrence of OGES in the maxillary and mandibular central incisors. The older the initial consultation age and the longer the treatment duration, the more likely OGES is to occur.   Participants were classified by ANB and MP-SN (Table ). Chi-square test analysis revealed no statistically significant association between sagittal/vertical skeletal pattern and the occurrence of OGES ( P > 0.05). U1-SN(T1), U1-SN(T2), U1-NA distance(T2), ΔU1-SN, ΔU1-NA correlated significantly with OGES in maxillary central incisors ( P < 0.05), with smaller values indicating higher OGES likelihood. Additionally, a smaller ΔL1-NB distance indicated a greater possibility of OGES. No statistically significant differences were found for the remaining measurement indicators (as shown in Table ).   After measuring and analyzing the CBCT data of the OGES group and the Non-OGES Group, we found that in the maxillary central incisor region, the distances between ABC and ICP, △ICP-ABC, and A-P overlap before and after treatment in the OGES group were significantly greater than those in the Non-OGES Group ( P < 0.05). The remaining measurement indicators, including TD, △R, root angulation, and crown width-to-height ratio, showed no significant correlation (Table ). In the mandibular central incisor region, the distances between ABC and ICP, and △ICP-ABC before and after treatment in the OGES group were also significantly greater than those in the Non-OGES Group ( P < 0.05).   Due to differences and sample size limitations in CBCT measurements compared to the study population and lateral skull X-ray film measurements, this experiment focuses on multivariate analysis of demographic, clinical characteristics, and cephalometric indicators. Binary logistic regression identified initial consultation age, treatment duration, and △U1-SN as significant factors affecting OGES occurrence in the maxillary central incisor area, with older age, longer treatment, and smaller △U1-SN correlating with greater retraction and higher OGES likelihood (Table ). Similarly, initial consultation age and treatment duration were significantly associated with OGES in the mandibular central incisor area.   Previous studies have reported risk factors including gender , age , gingival biotype , oral hygiene status , treatment duration , tooth extraction , and crowding . However, there is still controversy surrounding the perspectives. In our analysis, females are more prone to OGES, which may be associated with gender differences in gingival biology and hormonal levels . However, is not consistent across all research  and may be influenced by the sample size and the interplay of multiple factors. Age also emerged as a significant factor, with an increase in the initial diagnosis age of patients correlating to a higher risk of OGES, aligning with the majority of existing research . Furthermore, the duration of treatment is significantly correlated with the occurrence of OGES, consistent with previous research findings . The sagittal movement of anterior teeth, especially the lingual movement of the maxillary central incisor, was found to significantly correlate with OGES occurrence and supported by multivariate analysis. In our study, we found no evidence to suggest that labial movement of anterior teeth leads to the occurrence of OGES, which is consistent with the findings of Vasconcelos et al.  and An et al. . The inclination of the anterior teeth was found to have a correlation with OGES, with a more lingually inclined position of both the initial and final angles of the maxillary central incisors correlating to a higher likelihood of OGES. In our study, the mean value of U1-SN after treatment for the non-OGES group of maxillary central incisors was 105.31 degrees, which was significantly greater than that of the OGES group at 99.07 degrees. According to Andrews’ Six Elements, teeth that are upright in the center of the alveolar bone are crucial for achieving long-term stable treatment outcomes . In contrast to the maxillary central incisors, we observed no statistically significant correlation between the lingual inclination angle of the mandibular central incisors and the occurrence of OGES. This finding might be attributed to the thinner gingival tissue surrounding the mandibular central incisors, which could predispose them to a higher incidence of OGES. Currently, there is a scarcity of research examining the link between alveolar bone height and OGES using CBCT. In our study, we discovered that the distance from the contact point of the maxillary central incisor to the alveolar ridge crest was greater in patients who experienced OGES. This suggests a potential association between alveolar bone height and the likelihood of OGES, a finding that echoes previous research conclusions  and confirmed the research by Tarnow et al. . The relationship between tooth crowding and the development of OGES is a matter of debate . We assessed the degree of crowding by measuring specific distances and angles of the central incisors, finding a significant association only with the A-P distance of the upper central incisors and OGES occurrence in univariate analysis. Additionally, other factors such as treatment method, extraction of teeth, sagittal and vertical skeletal patterns, root angulation, and crown morphology were compared, but no significant statistical differences were found between the groups. This is consistent with some studies  but not with others . We speculate that differences in the number and characteristics of the patients included may account for the divergence in conclusions between our study and previous ones.   It is acknowledged that there are inherent limitations within the scope of this study. Initially, a number of factors were not taken into account, such as gingival biotype, oral hygiene status, and the practice of interproximal reduction (IPR). Furthermore, the sample size of CBCT scans was constrained, as these were not deemed a clinical necessity for every patient evaluated. In the pursuit of further research, it is recommended to incorporate sophisticated methodologies, such as machine learning, on a larger sample size. Additionally, the conduct of prospective studies may be warranted to enhance our comprehension of the risk factors associated with OGES.   In conclusion, our study’s univariate analysis indicated that the occurrence of OGES in the upper central incisors is significantly associated with initial consultation age, treatment duration, initial and final angular positions and changes of the anterior teeth, and alveolar bone height. For the lower central incisors, similar factors were identified. Binary Logistic regression analysis confirmed that initial consultation age and treatment duration are independent influencing factors for the occurrence of OGES. Therefore, in orthodontic treatment, it is crucial to consider these various factors comprehensively to prevent or reduce the occurrence of OGES, ensuring a balance between facial aesthetics and smile aesthetics.   