In clinical practice, different PBs may have different reactions to inflammation and various dental treatment. To obtain an ideal treatment effect, it is necessary to judge the PBs of patients in advance. This paper quantitatively analyzes the CGM morphology clinical parameters of upper anterior teeth using 3D digital models, in the hope of providing more accurate references for the esthetic analysis and design of computer-aided anterior teeth, determining the cutoff value of gingiva and crown clinical parameters, and establishing clinical guidelines to offer quantitative guidance for periodontal biotyping.
This research shows that among the 56 experimental subjects, thick biotype accounted for the most, around 69.6%, while thin biotype accounted for 30.4%; and there was statistical difference in PB between males and females,which is consistent with the results by De Rouck et al . However, Lee et al.  held that gender had no significant influence upon PBs, and thin biotype held about 21.8%. According to Frost NA et al. , thin biotype held about 7%. Sample size and ethnic differences therein may be the major reasons for the inconsistency of results.
The contour of gingival margin is determined by parameters like gingival angle, papilla width, and papilla height. The GA averages of the maxillary central incisor, lateral incisor and canine of all experimental subjects were 98.19±7.69°, 96.24±10.03° and 89.45±6.63°, respectively. However, Olsson et al.  argued that the GAs for the maxillary central incisor, lateral incisor and canine were 86.60°, 82.80° and 80.29°, respectively. The measurement methods may be the major reason for the inconsistency of the results. Olsson et al. Worked out GA through cosine function using intra-oral photos. This experiment directly measured GA through 3D digital models, which is more simple, convenient and accurate in reflecting the spatial positional relation of teeth and gingiva. The logistic regression model in this experiment showed that GA (P=0.016, OR=1.206) was one of the independent influence factors of PBs. Research showed that the central incisor GAs of thin biotype and thick biotype were 92.73± 6.21° and 101.68±8.03°, respectively. The conclusion is consistent with the viewpoint of Olsson  and Zhou et al. , suggesting that the gingival angle of thin biotype is smaller and the gingival margin more curved.
The morphology of gingival papilla is a major evaluation index for current various anterior teeth esthetic evaluation systems. This research showed that: the PWs of the maxillary central incisor, lateral incisor and canine of all the experimental subjects were 10.05±0.79mm, 7.83±0.60mm and 7.97±0.65mm, respectively, which is consistent with the findings by Zhou et al. . The logistic regression model of the right maxillary central incisor indicates that PW has a significant influence upon PBs (P=0.002, OR=5.048), i.e., making the gingival papilla of the maxillary central incisor of thin biotype narrower, but there are few studies invoving the correlation between PB and PW.
Olsson et al.  held that the PH for the maxillary central incisor, lateral incisor and canine was respectively 4.16mm, 4.02mm and 4.21mm, but the results in this experiment are 3.65±0.59mm, 3.37±0.54mm, 3.28±0.57mm. This disparity may be attributed to the differences in experiment subjects and measurement methods. Besides, ANOVA test shows that there is statistical difference (P=0.027) between PH and PB, while logistic multi-factor regression analysis suggests that PH is not an independent influence factor of PB. De Rouck et al.  also held that there was statistical difference in PH between PB, while Olsson  and Stein  et al. claimed that there was no obvious correlation between gingival thickness and PH. Lee SP et al.  found out that the sum of five gingival papilla heights of the MAT larger than 24 mm was the identification standard for thin biotype, and PB had no obvious correlation with the papillary height between two central incisors. The imparity in measurement method and periodontal biotyping method may be the major reason for the above differences.
Stein et al.  measured and calculate CW/CL using image measurement software, finding that CW/CL and PB were closely related, and therefore could be taken as the predictive index for gingival thickness. The ANOVA test in this research shows that there is no obvious statistical significance between CW/CL and PB, but in logistic regression model, CW/CL is eliminated out of the regression equation, indicating that it is not the independent influence factor of PB. This is consistent with the research results by Olsson  and Eger . This is possibly related to the difficulty in determining the proper 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 gingival papilla . Moreover, differences in ethnicity and region may lead to different crown morphologies.
Olsson  obtained the BLWs of the maxillary central incisor, lateral incisor and canine through measuring casts, which were 7.33±0.56mm, 6.51±0.57mm and 8.29±0.65mm, respectively, aruging that gingival thickness and BLW were significantly correlated. This research believes that the BLWs of the maxillary central incisor, lateral incisor and canine are 7.22±0.53mm, 6.56±0.52mm, 8.38±0.48mm, respectively, with no correlation between BLW and PB found. Such an imparity may be attributed to the differences in experimental subjects and PB judgement method.
Tarnow et al.  held that the esthetic effect of gingival papilla was associated with the position of the contact area. This research shows that the CSWs of the maxillary central incisor, lateral incisor and canine are 4.39±0.72mm, 3.56±0.56mm, 2.62±0.57mm, respectively, and the CS/CL is respectively 59.7%, 62.3% and 63.5%. Moreover, compared with thin biotype, the contact surface of thick biotype has a large width, and the most apical portion of the contact area is closer to gingival margin, but no statistical difference has been found between PBs. Meanwhile, Gobbato et al.  categorized the maxillary central incisor, 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 gingival margin.
Most of the research focuses on the correlation of PB with the morphology of soft and hard tissues [5-8, 10, 11, 17, 19, 22-24], with little involving the influence of independent factors on PB diagnosis efficiency. The logistic regression results in this research show that the right central incisor GA and PW are important predictive factors of PB; and when GA increases every 1°, the probability of experimental subjects being diagnosed as thick biotype increases 1.206 times, and when PW increases every 1 mm, the probability of being diagnosed into thick biotype increases 5.048 times. This supports the hypothesis that “compared with thick biotype, the free gingival margin at the labial side of thin biotype is more curved and the gingival papilla narrower” [5, 9, 16, 25]. It is discovered that the GA, PW and combined AUC are 0.807, 0.881, 0.935, respectively, indicating GA and PW combined diagnosis can help improve the diagnostic efficiency of PB. In this experiment, when the GA and PW of the right maxillary central incisor are 95.95° and 10.01mm, it turns ou to be the optimal cutoff value to categorize experimental subjects into thick biotype. It indicates that when the GA and PW of the right maxillary central incisor are GA≥95.95° and ≥10.01mm, there is a higher possibility of categorizing experimental subjects into thick biotype. Frost NA et al.  analyzed the relation between gingival thickness and PB using ROC curve, failing to find a suitable gingival thickness threshold to judge thick biotype.
This research explores the CGM parameters and their correlation with PB using 3D digital models, but it is beset by the problems of small sample size, uneven gender 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 gender ratio, and take into consideration the correlation of the periodontal biotyps at different teeth positions with CGM. In addition, the influence factors included into this research are limited, which may have ignored the influence of other factors upon PB, so future research is to include the influence factors like alveolar bone morphology, keratinized gingival width and gingival thickness, in the hope of providing powerful evidence to clinicians’ diagnosis of PBs. To clarify whether GA and PW can benefit the PB diagnosis efficiency, it is also necessary to carry out randomized controlled trial for verification.