In clinical practice, different PBs may react differently to inflammation and various types of dental treatment. Accurate pre-treatment diagnosis of the PB of patients is necessary to obtain the ideal effect. In this study, we conducted a quantitative analysis of the CGM morphology in terms of the clinical parameters of upper anterior teeth using 3D digital models with the aim of providing an accurate reference for the esthetic analysis and computer-aided design of anterior teeth, determining the cutoff value of gingiva and crown clinical parameters, and establishing clinical guidelines to offer quantitative guidance for periodontal biotyping.
Among the 56 participants included in this study, the thick biotype accounted for the largest proportion (69.6%), while thin biotype accounted for only 30.4%. Furthermore, there was statistically significant difference in PBs between males and females, which is consistent with the results reported by De Rouck et al . However, Lee et al.  found that sex had no significant influence on PBs, with the thin biotype accounting for 21.8% of the individuals evaluated. In contrast, Frost et al.  reported that the thin biotype accounted for only 7% of their study participants. It can be speculated that sample size and ethnic differences may be the major factors contributing to the inconsistency in these results.
The contour of the gingival margin is determined by parameters such as the gingival angle, papilla width, and papilla height. The GA averages of the maxillary central incisor, lateral incisor and canines of all study participants were 98.19±7.69°, 96.24±10.03° and 89.45±6.63°, respectively. However, Olsson et al.  reported GAs for the maxillary central incisor, lateral incisor and canines of 86.60°, 82.80° and 80.29°, respectively. Differences in measurement methods may account for these inconsistencies. Olsson et al.  determined GA with the cosine function using intra-oral images to generated 3D digital models. This is a simple, convenient and accurate method of reflecting the spatial positional relation of the teeth and gingiva. The logistic regression model used in this study showed that GA (P = 0.016, OR = 1.206) is an independent influencing factors of PBs. Our study also showed that the central incisor GAs of the thin and thick biotypes were 92.73± 6.21° and 101.68±8.03°, respectively. These results are consistent with those reported by Olsson  and Zhou Zhixuan et al. , suggesting that the GA of the thin biotype is smaller and the gingival margin more curved than that of the thick biotype.
The morphology of the gingival papilla is a major evaluation index used in various current anterior teeth esthetic evaluation systems. The present study showed that the PWs of the maxillary central incisor, lateral incisor and canines in all the study participants were 10.05±0.79 mm, 7.83±0.60 mm and 7.97±0.65 mm, respectively, which is consistent with the findings of Zhou Zhixuan et al. . The logistic regression model of the right maxillary central incisor indicates that PW has a significant influence on PBs (P = 0.002, OR = 5.048), making the gingival papilla of the maxillary central incisor of the thin biotype narrower, although there are few studies on the correlation between PB and PW.
Olsson et al.  proposed that the PHs for the maxillary central incisor, lateral incisor and canines were 4.16 mm, 4.02 mm and 4.21 mm, respectively, although the results of our study revealed values of 3.65±0.59 mm, 3.37±0.54 mm, 3.28±0.57 mm, respectively. This disparity may be attributed to the differences in study participants and measurement methods. In addition, ANOVA showed a significant difference (P = 0.027) between PH and PB, while logistic multi-factor regression analysis suggested that PH is not an independent influencing factor of PB. De Rouck et al.  also reported a significant difference in PH between PBs, while Olsson  and Stein  et al. claimed that there was no obvious correlation between gingival thickness and PH. Lee et al.  found that the sum of five gingival papilla heights of the MAT larger than 24 mm was the identification standard for the thin biotype, and PB had no obvious correlation with the papillary height between two central incisors. The disparity in measurement methods and periodontal biotyping methods may be the major reasons for these differences.
Using image analysis software to calculate CW/CL, Stein et al.  found that CW/CL and PB were closely related, and therefore could be used to represent the predictive index for gingival thickness. In this study, ANOVA showed no significance difference between CW/CL and PB, although in logistic regression model, CW/CL was eliminated from the regression equation, indicating that it is not an independent influencing factor of PB. This finding is consistent with those of Olsson  and Eger  and may be related to the difficulty in determining the most appropriate reference points, because CL is subject to the influence of attachment loss, gingival inflammation and incisal attrition, while CW is subject to the influence of the gingival papilla . Moreover, differences in ethnicity and region may lead to different crown morphologies.
By measuring casts, Olsson  reported BLWs of the maxillary central incisor, lateral incisor and canines of 7.33±0.56 mm, 6.51±0.57 mm and 8.29±0.65 mm, respectively, indicating a significant correlation between gingival thickness and BLW. According to our study, the BLWs of the maxillary central incisor, lateral incisor and canines are 7.22±0.53 mm, 6.56±0.52 mm, and 8.38±0.48 mm, respectively, with no correlation between BLW and PB found. This discrepancy may be attributed to differences in the study participants and the PB diagnosis method.
Tarnow et al.  reported that the esthetic effect of the gingival papilla was associated with the position of the contact area. In this study, the CSWs of the maxillary central incisor, lateral incisor and canines were 4.39±0.72 mm, 3.56±0.56 mm, and 2.62±0.57 mm, respectively, and the CS/CL values were 59.7%, 62.3% and 63.5%, respectively. Moreover, compared with thin biotype, the contact surface of the thick biotype is wider, and the most apical portion of the contact area is closer to the gingival margin, although no significant differences were found between PBs. Gobbato et al.  categorized the maxillary central incisors, finding that the most apical portion of the contact area in the triangular group was closer to the incisal edge, while that in the square group was closer to the gingival margin.
Most previous studies have focused on the correlation of PB with the morphology of the soft and hard tissues [5, 6, 7, 8, 10, 11, 12, 18, 20, 23, 24, 25], with little investigation of the influence of independent factors on PB diagnosis efficiency. The logistic regression results of our study show that the right central incisor GA and PW are important predictive factors of PB, with the probability of a thick biotype diagnosis increasing 1.206 times for every 1° increase in GA, while the probability of a thick biotype diagnosis increases 5.048 times for every I mm increase in PW. This supports the hypothesis that “compared with thick biotype, the free gingival margin at the labial side of the thin biotype is more curved and the gingival papilla narrower” [5, 9, 17, 26]. We found that the GA, PW and combined AUC were 0.807, 0.881, and 0.935, respectively, indicating that the combination of GA and PW improve the diagnostic efficiency of PB. In this study, the GA and PW of the right maxillary central incisors of 95.95° and 10.01 mm, respectively, were identified as the optimal cutoff values to categorize individuals as thick biotype. This implies an increased probability of categorizing individuals as thick biotype when the GA and PW of the right maxillary central incisors are ≥95.95° and ≥10.01 mm, respectively. In an analysis of the relationship between gingival thickness and PB based on ROC curves, Frost et al.  failed to identify a suitable gingival thickness threshold for diagnosis a thick biotype.
This exploration of CGM parameters and their correlation with PB using 3D digital models is limited by the small sample size, uneven sex ratio, and single focus on the correlation of the right maxillary central incisor PB with CGM clinical parameters. Therefore, in future studies, it is necessary to expand the sample size, balance the sex ratio, and take into consideration the correlation of the periodontal biotypes at different teeth positions with CGM. In addition, the limited number of influencing factors included in this research may have ignored the influence of other factors on PB; therefore, future investigations should evaluate the influence of factors such as alveolar bone morphology, keratinized gingival width and gingival thickness to provide more powerful evidence for the diagnosis of PBs in the clinic. In addition, randomized controlled trials are required to verify the potential of GA and PW for improving the accuracy of PB diagnosis.