Preservation of the interdental papilla is an essential part of the functional and esthetic rehabilitation of dental treatment. It has been described that the morphology of the interdental papilla is strongly related to bone volume in the interproximal space [4,5,6,7,8,9]. In addition to recession of the interdental papilla related to periodontal disease, recession can also occur in a healthy periodontium due to anatomical and physiological predispositions . Therefore, we examined only periodontally healthy patients without any tooth position discrepancies. To have a homogeneous sample, we only studied the papilla between maxillary central incisors, because contact points may vary in different regions, which may influence the shape of the interdental papilla.
Periodontal phenotype is determined by gingival phenotype defined as three-dimensional volume of the gingiva and by bone morphotype (thickness of the buccal bone plate) . There is evidence reporting a correlation between gingival thickness and buccal bone plate ; therefore, it was recommended to assess periodontal phenotype in a standardized and reproducible way by an assessment of gingival thickness . In this study, two different non-invasive methods were employed: gingival probe transparency, a method most commonly used in similar studies [16, 19,20,21, 28, 29], and less standard method of ultrasonic measurement [30,31,32] of accurate thickness of gingival tissues. A comparison of both methods showed a significant correlation and there seems to be a threshold value of 1-mm gingival thickness between the thick and thin phenotypes assessed by probe transparency, which supports previously published findings by Kan et al. . Therefore, these results support the fact that such a straightforward method of phenotype assessment using gingival probe transparency is as reliable as other methods which are often more time-consuming or require some additional costs for appliances. However, assessment of gingival probe transparency in patients with gingival pigmentation should be made with caution.
Despite the fact that papilla recession was present in 34.3% of cases with the thick phenotype and in 60% of cases with the thin phenotype, there was no statistically significant correlation between the interdental papilla of maxillary central incisors and gingival thickness. This result supports previously published studies by Kim et al.  and Singh et al. . Some authors assume that a thick phenotype is more resistant to physical trauma and has a lower risk of papilla recession due to a better blood supply and adequate amount of dense fibrous tissue . The thick phenotype is also associated more strongly with square-shaped tooth crowns with the contact point located more apically, and requires less tissue to fill the interproximal space [22, 23]. This assumption was confirmed by Chow et al.  who observed that gingival tissues were significantly thicker when the papilla was competent. Opposite results were published by De Lemos et al.  who noted a significantly higher presence of the papilla in the thin phenotype group. In that study, however, the phenotype was evaluated visually only, which may have biased the assessment due to subjectivity. Most of the other authors studied the correlation between phenotype and papilla height as the only descriptive parameter of the papilla. The results suggested that increased papillary height was associated with a thin phenotype [15, 16, 28, 29], which may have been influenced by different tooth shapes [15, 21, 23, 29, 35]. As a tooth becomes triangular, which is more typical for thin phenotype subjects, the contact point can be seen more coronally, and a longer papilla is present. Yin et al.  have recently published that papilla width has a significant effect on phenotype, making the papilla of maxillary central incisors of the thin biotype narrower. Our study failed to find an effect on papilla width with gingival thickness. In the study by Yin et al.  papilla width was assessed as the distance between the gingival zeniths of two adjacent teeth. The incongruity in measurement methods of papilla width may be a major reason for different results. However, there have been few studies on the correlation between the gingival thickness and papilla width, and more research needs to be done.
We also compared papilla characteristics—papilla fill, height and width between each other. Papilla height was measured as the distance from the tip of the papilla to the connecting line of the gingival zeniths of maxillary central incisors; therefore, it is important to point out, that the values of papilla height do not include complete supracrestal connective tissue attachment (i.e. they do not represent the true height of the papilla). The results showed that a papilla assessed as normal, i.e. one which fills the whole interdental space, seemed to be shorter than that classified as a Class 1, where a slight reduction of papilla fill is present. Chang et al. found in their study that papilla height was significantly greater in the group where a complete papilla was present . However, this result was not confirmed in a study by Kim et al. . Both authors measured papilla height on radiographs using radiopaque material and defined it as the distance from the crest of the bone to the tip of the papilla. It can be speculated that different methods of papilla height measurements may be a major factor contributing to inconsistencies in these results. Another possible factor may be the different ages of the participants, as reported by Chow et al. , who noted that papilla height decreased by 0.012 mm with each increasing year of age. In our study, the age of the participants ranged from 20 to 30 years only. We found no significant relationship between papilla fill and papilla width, but a significant correlation was observed between papilla height and width. It seems that there may be an effect of papilla base width on its vertical dimension and therefore may pose as one of the potential risk indicators influencing the presence of interdental papilla.
The mean papilla height was greater in the male group than in the female group. However, the groups were unequal without further statistical correction, the interpretation of the sex comparison should be made with caution. Chow et al. have reported the same results , which is in contrast with the study by Joshi et al. . Many other authors have observed a thin phenotype more frequently in females [16, 21, 38]; in this study, however, no correlation was found. We assume that the greater height of the interproximal papilla found in the male group was due to different tooth forms and position of the contact point, which could be the reason for the only difference between sexes in this study.
The small size of our sample lacking different age groups limits the assessment of gingival phenotype and its correlation with papilla morphology. Therefore, in future studies, it is recommended to expand the sample size. To describe gingival phenotype properly, measurement of keratinized tissue width should be considered, although the importance of this parameter is still a matter of discussion. Also, it is advisable to evaluate other potential risk indicators, such as tooth form (shape/dimension, the width of the approximal tooth surfaces, course of the CEJ) or tooth angulation, which appear to be other potential factors influencing the interdental papilla because of a different shape and position of the contact point. Finally, another potential factor, buccolingual tooth position, which may affect the gingival phenotype and thickness of the alveolar bone, should be added in future studies to provide more convincing evidence.