Assessment of the Relationship Between Labial Gingival Thickness and Underlying Bone Thickness in Maxillary Anterior Teeth by Two Digital Techniques

doi:10.21203/rs.3.rs-729394/v1

The aim of this study is to noninvasively assess the relationship between the labial gingival thickness (GT) and the underlying bone thickness (BT) of the maxillary anterior teeth by two digital techniques. GT and BT were measured at 2、4 and 6 mm apical to the cemento-enamel junction (CEJ) by two digital techniques: M1- cone-beam computed tomography (CBCT) and M2- digital intraoral scanning (DIS) combined with CBCT (30 healthy participants, 172 maxillary anterior teeth). Pearson's correlation coefficient was calculated to assess the correlation between GT and BT. A significant negative correlation was identified between GT and BT at 2、4 mm apical to the CEJ for central incisors (CI), lateral incisors (LI), and canines (CA) both by M1 and M2, but not at 4 mm apical to the CEJ for CA (M2) or at 6 mm apical to the CEJ for all (M1, M2). The labial BT was demonstrated < 1 mm in most cases (85% of the CI; 97% of the LI; and 90% of the CA). Within the limitation of this study, it is concluded that the GT and BT seems to be negatively correlated in most cases, which should be cautious clinically.

Dentistry

gingival thickness (GT)

bone thickness (BT)

digital intraoral scanning

CBCT

Gingival biotype (GB) is closely associated with the long and short term esthetic outcomes of implant placement, restorative treatments, periodontal therapy in the esthetic region of the anterior maxilla, especially in the cases of immediate implantation, the implant is required to be placed in the ideal three-dimensional position with a thick GB and a thick labial bone plate¹, a complete labial bone plate with a thickness at least 1–2 mm is necessary to prevent bone resorption on the labial side of the implant^2,3, as well as to ensure the implants stability and long-term esthetic outcome of the soft tissue. Moreover, the thickness of gingiva is another factor affecting the long-term outcome of anterior esthetic effect^4,5. Compared with thick GB, patients with a thin GB are more likely to have gingiva recession and alveolar bone retraction after implantation, which is associated with a high risk of esthetics⁶. Gingival thickness (GT) and the underlying alveolar bone thickness (BT) in esthetic zone seem to play a decisive role in the treatment outcomes.

Accurate measurement of GT and the underlying BT before implant placement is beneficial to prevent complications such as soft tissue recession, implant exposure^7,8. However, limited information is available regarding the relationship between the labial BT and GT due to the lack of a standardized measurement technique for measuring the thickness of hard and soft tissue at the same site⁹, there are relatively few studies on this issue and whether GT is specifically correlated with the thickness of the labial bone is still controversial and no consensus has been reached^10,11. There are many methods to measure GB, GT and BT, such as the transgingival probing with periodontal probe^10,12, which observe GB by the outline of the periodontal probe through the soft tissue. However, reports on the accuracy of this method disagree with different studies^10–12. For GT, such as the direct puncture method which requires local anesthesia and is invasive¹³, the ultrasonic detection method which is non-invasive but its accuracy is limited for determining GT in some regions of the posterior teeth¹⁴. The cone beam computed tomography (CBCT) is an objective method for determining the thickness of soft and hard tissue¹⁵, some literature have indicated that CBCT has high precision in measuring BT^16,17, but its low resolution and contrast limit its visualization on soft tissue^11,18−20.

The limitations of the above methods for measuring GT and BT warrant further studies in this area. Digital intraoral scanners (DIS) enabled the acquisition of data directly from the oral cavity and has been widely used in dentistry²¹. DIS images with more accurate outlines and greater resolution compared with CBCT was applied to measure the volume of periodontal tissues²², and has been demonstrated the higher precision and more reliability²³. DIS improves the reproducibility and reduces the variance in measurements between different investigators. Several studies showed the accuracy of DIS on single elements or part of the arch commonly used in restorative or prosthodontic dentistry, such as crowns, inlays, onlays, and short bridges^24–28, while the accuracy of DIS in multiple units or the full arch is still controversial²⁹. The precision was clinically acceptable when scanning less than half of the arch³⁰. Moreover, no consensus on the accuracy of DIS for soft tissue has been reached so far^31,32. The DIS data can be combined with CBCT data. Whether the measurement of GT and BT on the superimposed data generated by CBCT data and DIS data will be more precise is still unknown.

It is also necessary to establish a better understanding of the correlation between GT and BT. The purpose of this study is to analyze the correlation between the thickness of gingiva and the thickness of the underlying bone in the aesthetic area of maxillary anterior teeth through two digital techniques: CBCT (M1) and DIS combined with CBCT data (M2), and to provide a theoretical basis for the clinical selection of the aesthetic restoration, implant treatment in clinical practice.

Sample Selection

Thirty medical students (18 males and 12 females) aged 20 to 26 years in Zhejiang Provincial People’s Hospital were enrolled in this study. Informed consent was obtained from the participants, and this study was approved by the Ethics Committee of Zhejiang Provincial People’s Hospital (2018KY015), which was performed according to the principles of the Declaration of Helsinki.

Inclusion criteria: all subjects had a healthy periodontal condition with a periodontal probing depth of no more than 3 mm, a bleeding index of ≤ 2 (according to the bleeding index standard proposed by Mazza in 1981), and no gingiva recession in maxillary anterior teeth.

Exclusion criteria: pregnant or lactating females; fillings or crowns in the maxillary anterior dentition; tooth malocclusion; use of any medication affecting the soft tissue; cigarette smoking; and a history of orthodontic therapy. Each subject was given instructions on maintaining oral hygiene and had their teeth cleaned one week before the test. In result, 172 teeth were included in the study (4 CIs, 4 LIs were excluded because of caries, periapical disease, torsional teeth).

Method 1 (M1): CBCT measurements

GT and BT was assessed using CBCT (Planmeca ProMax 3D and Helsinki, Finland) by one experienced dental radiologist and before scanning the lips and cheeks were retracted by a sterile plastic retractor. All scans were performed at 80 kV and 6.0 mA for 15 seconds (voxel size: 0.15 mm; gray scale: 15 bits; focal spot: 0.5 mm; and field of view: 12 x 9 cm). Image reconstruction for visual analysis was performed using the Romexis software (Romexis Viewer 3.5.1R, Planmeca, Helsinki, Finland). Measurements of the labial BT and GT for each tooth were performed at 2, 4, 6 mm apical to the CEJ on the mid-labial aspect perpendicular to the axis of the tooth in the sagittal plane (Fig. 1). All sites were measured by the same clinician. To assess intra-examiner reliability, duplicate registration was performed by the two examiners at an interval of 24 hours.

Method 2 (M2): DIS+CBCT measurements

GT and BT were assessed after registration between oral soft tissue images obtained by DIS and hard tissue images generated by CBCT. The teeth and gingival surfaces were cleaned and dried before intraoral scanning. The complete maxillary dentition, the labial and palatal gingival tissue were scanned by an experienced technician using the Trios intraoral scanner (3Shape Trios ®, Copenhagen, Denmark). The DIS data were exported into stereolithography (STL) format and matched with the CBCT data. Three highly radiopaque and relatively stable positions in the teeth were chosen as fiducial markers and were used as references to match the STL files with the CBCT images(acquired in M1) according to a best-fit algorithm using TRIOS software (3Shape Trios®, Implant Studio, Copenhagen, Denmark). The outline of the soft tissue on the labial side was visible as a yellow line. GT is defined as the distance from the surface of the gingiva to the surface of the alveolar bone or the surface of the tooth (Fig. 2-5).

Measurements of labial GT and BT were performed at 2 mm, 4 mm, 6 mm apical to the CEJ at the mid-buccal aspect of each single rooted tooth, perpendicular to the axis of the tooth (L). Distance from CEJ to the alveolar bone crest (BC) was also obtained (BC-CEJ) (Fig 1, Fig 5). All sites were measured by the same clinician. To assess intra-examiner reliability, duplicate registration was performed by the two examiners at an interval of 24 hours.

Statistical analysis

The data were analyzed using SPSS 22.0 (SPSS Inc, Chicago, IL, USA). Quantitative data was presented as the means and standard deviations (SDs), and the normality of the data distribution was assessed by one-way variance analysis. The Friedman test was applied to compare the mean BT and GT between the three tooth types CI、LI、CA. If a significant difference was observed, each tooth types were compared using the Nemenyi post-hoc test. Pearson's correlation coefficient was applied to measure the correlation between the mean GT and BT between each teeth type. P < 0.05 was considered statistically significant.

172 anterior teeth of 30 participants with a mean age of 24 years was included. Two sets of recordings for M1 and M2 measurement of GT and BT were performed by two examiners at an interval of 24 hours to achieve intra-examiner reliability. Duplicate recordings of GT (0.875, 0.91 respectively; P < 0.001) and BT (0.812, 0.846 respectively; P < 0.05) demonstrated perfect agreement between different examiners. Therefore, the values measured by one of the examiners were used for further statistical analysis.

1、The correlation between BT and GT measured by M1

The results of correlation between BT and GT measured by M1 were shown in Table 1. In 172 anterior teeth, the mean thickness of underlying labial bone at the CI, LI, and CA was 0.74mm, 0.54mm, and 0.70 mm, respectively, and the mean corresponding GT was 1.14 mm, 1.00 mm, 0.94 mm respectively. There was a strong negative correlation between BT and GT at 2 mm, 4 mm apical to the CEJ in all maxillary anterior teeth, no correlation was found between BT and GT at 6mm apical to the CEJ in this three teeth types.

2、The correlation between BT and GT measured by M2

The results of correlation between BT and GT measured by M2 were shown in Table 2. The mean GT of the CI, LI, and CA was 0.98 mm, 0.88 mm, 0.83 mm and the underlying BT was 0.65 mm, 0.50 mm, 0.60 mm respectively. There was a strong negative correlation between BT and GT at 2 mm apical to the CEJ in this three teeth types, a weak negative correlation between BT and GT at 4 mm apical to the CEJ in CI, LI. No correlation between BT and GT at 4 mm apical to the CEJ in CA and 6 mm apical to the CEJ in this three teeth types.

3、Comparison of buccal bone and gingival thickness with respect to tooth type

There was a significant difference in the mean thickness of buccal bone in the studied teeth in M1(P=0.009) and M2 (P=0.030). The BT of the CI was greater than that of LI and CA. Concerning GT, a difference was also found among tooth types (P=0.008, 0.016 respectively) and the thinnest was observed in the CA. (Table 3).

4、Distribution frequency of the labial BT and GT at different dental positions and different sites in M2

Analysis of the distribution frequency of labial BT measurements at 2, 4, 6 mm apical to the CEJ of the three types tooth was shown in Fig. 6-11. For the CI, 20 (11.63%) sites had a thickness less than 0.5 mm, 142 (82.56%) sites had a thickness of 0.5-1 mm, 6 (3.43%) sites had a thickness of 1.0-1.5 mm, and no sites had a thickness more than 1.5 mm. At the LI, 91 (52.91%) sites were 0.5-1.0 mm, and 4 (2.33%) sites were 1.0-1.5 mm. A total of 73 (42.44%) sites had a thickness less than 0.5 mm, no sites exhibited a thickness of 1.5 mm or more. For the CA, 163 (90.56%) sites were less than 1.0 mm thick, 14 (7.78%) sites were 1.0-1.5 mm thick, and only 3 (1.67%) sites more than 1.5 mm thick were detected.

As for the GT，at the CI，90 (52.33%) sites had a thickness less than 1.0 mm, 66 (38.37%) sites had a thickness of 1.0-1.5 mm, 12 (6.98%) sites had a thickness greater than 1.5 mm. At the LI, 120 (69.77%) sites had a thickness less than 1.0 mm, and 40 (23.26%) sites were 1.0-1.5 mm thick. A total of 8 (4.65%) sites had a thickness greater than 1.5 mm. For the CA, 132 (73.33%) sites were less than 1.0 mm, 44 (24.44%) sites were 1.0-1.5 mm, and 4 (2.22%) sites more than 1.5 mm were detected.

1. Correlation between GT and BT

The correlation between labial GT and BT in the maxillary anterior region were evaluated in the present study. No correlation was found between BT and GT at either 6 mm or 4 mm apical to the CEJ using M2 method, while there was a significantly negative correlation between BT and GT at other sites. This finding was consistent with the result of some other researchers such as Cao et al. who used a new method of CBCT combined with indirect gingival imaging to analyze the relationship between labial soft and hard tissues of the maxillary anterior teeth. Their study reported that GT at the site 2 mm below the labial central alveolar ridge in the upper anterior region was negatively correlated with BT²⁰. Stein et al. also found the negative correlation between BT and GT in the proximal labial side of the maxillary anterior teeth using the transparency method with a gingival marginal periodontal probe¹¹. However, Fu et al. measured the thickness of soft and hard tissue among the maxillary anterior teeth (2 mm apical to the alveolar crest) using both a caliper and an imaging method, in contrast, they found a positive correlation between the thickness of soft tissue and the thickness of the underlying bone⁶. This disagreement among different researchers highlights the controversy correlation between the thicknesses of soft and hard tissues commonly exist and used in preoperative planning. Some reports speculate this inconsistency among different research may be related with different measurement methods, different phenotype in the ethnicity of patients, and differences in measuring position^6,11,20. However, the results of this study further suggest that the clinical evaluation of esthetic areas is complex for implantation timing (either immediate, early, or delayed) and for indications of aesthetic risk (such as gingiva recession or detachment). Furthermore, individual methods of judging soft tissue biotype for aesthetic risk using only the naked eye are unreliable.

Many studies have found that GB is a comprehensive result of several factors not only the thickness of the gingiva, but also such as the thickness of the underlying bone, the dimensions of the dentogingival complex, the morphology of crown, the bone crest of cemento-enamel junction and so on^9,10,33,34. GT in conjunction with the underlying BT showed the highest correlation with GB. The standard method for determining GT is transparency probing by a periodontal probe. However, the assessment of GB must also take into account other factors, such as the shape of the underlying bone and the size of the teeth. Furthermore, the GT can decrease due to age and other factors, and is the main cause of periodontal detachment and gingiva retraction³⁵. Previous studies have shown that the risk of gingiva retraction in patients with a thin GB was significantly higher than in patients with a thick GB in periodontal and implantation treatments³⁶, and moreover, GB can be changed through surgical techniques to augment soft tissue such as gingiva grafting or sub-epithelial connective tissue grafting^37,38.

Due to the mentioned inconsistencies on the determination of GB, clinical evaluations for indications of immediate implantation cannot solely rely on transparency probing technique. Eghbali et al. discussed the unreliability of visual examination for determining GB, the study revealed that roughly half of the participating patients with thin gingiva had been mistakenly classified as other biotype³⁹. Kan et al. used calipers to directly measure GT at the point 2 mm under the middle margin of the extraction sockets, the result showed that gingiva < 1 mm should be designated as the thin biotype, while >1 mm was defined as thick biotype¹⁰. Claffey et al. measured GT at the point 2 mm submarginal by puncture and subsequently proposed that 1.5mm would suffice as a maximal value for the thin biotype while gingiva >2 mm should be designated as a thick GB⁴⁰. In this study, we found significant differences between GT measured by CBCT imaging alone and CBCT imaging matched with DIS, strongly suggesting that assessments of GB for clinical procedures should be more cautious.

2. Frequency distribution of buccal bone and gingival thickness

Our results showed that the average thickness of the labial gingiva of the CI was 0.98 mm, and the average BT was 0.75 mm. The average thickness of labial gingiva of the LI was 0.88 mm, and the thickness of bone was 0.54 mm. The mean thickness of CA labial gingiva was 0.84 mm, and the thickness of the bone was 0.69 mm. In general, the proportion of GT > 1.5 mm was 7.1% for CI, 4.8% for LI, and 2.2% for CA. The proportion of BT for maxillary anterior teeth >1 mm was also significantly less, with proportions of the thick biotype for the CI, LI, CA of 3.3%, 2%, and 9%, respectively. The proportion of people who actually meet the criteria indicating for implantation of in the esthetic area is very rare, according to Buser et al.¹. Clinical recommendations of immediate implantation of anterior teeth should be made with great caution. Nevertheless, this study found significant correlations between GT and BT at different sites in the anterior tooth region. Most notably, there was a significant negative correlation between the thickness of gingiva and the thickness of bone wall at 2 mm and 4 mm below the boundary between enamel and dentin bone. Therefore, when evaluating the esthetic risks prior to implantation or in anterior regions, it will be ill-advised to justify that a thick GB is necessarily corresponds to a thick gingiva and a thick bone plate, since this often leads to incorrect clinical decisions.

The current study demonstrated that there was a negative correlation between labial GT and BT in the anterior region. A thin labial BT and GT(< 1 mm) may be expected in over half of the maxillary anterior teeth. It should be aware that the thick gingiva is not related with the thick underlying bone plate in clinical practice.

Acknowledgements

This study was supported by Zhejiang Provincial Natural Science Foundation of China under Grant No. LGF20H140010 and Zhejiang Provincial Traditional Chinese Medicine Science and Technology Foundation of China under Grant No.2019ZA014.

Author contributions

L.W: Study design, clinical procedures, data analysis, interpretation of results, drafting and writing of the final article J.C: Clinical procedures, data acquisition, data analysis, prepared figures, approving final article Y.R: Statistical analysis, approving final article Y.L: Statistical analysis, approving final article F.Y: Study design, clinical procedures, interpretation of results, drafting and approving of the final article. All authors reviewed the manuscript.

Competing interests

The authors declare no competing interests.

Additional information

Correspondence and requests for materials should be addressed to F.Y.

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Table 1. Gingival thickness (GT) and bone thickness (BT) at 2, 4, and 6 mm apical to the CEJ by M1

Level		Central incisors(CI)	Lateral incisors(LI)	Canines(CA)
At 2 mm below the CEJ	GT (mm)	1.26±0.37	1.02±0.37	1.13±0.42
	BT (mm)	0.54±0.39	0.44±0.38	0.42±0.45
	Pearson correlation	-0.728^**	-0.767^**	-0.698^**
At 4 mm below the CEJ	GT (mm)	1.03±0.27	0.90±0.36	0.92±0.37
	BT (mm)	0.93±0.19	0.78±0.31	0.82±0.36
	Pearson correlation	-0.286^*	-0.298^*	-0.282^*
At 6 mm below the CEJ	GT (mm)	1.15±0.45	1.10±0.34	0.88±0.37
	BT (mm)	0.77±0.25	0.41±0.38	0.73±0.37
	Pearson correlation	0.036	-0.079	-0.066

**Confidence levels (bilateral) of < 0.01 indicate a significant correlation;

*Confidence levels (bilateral) <0.05 indicate a significant correlation.

Table 2. GT and BT at 2, 4, and 6 mm apical to the CEJ measured by M2

Level		Central incisors(CI)	Lateral incisors(LI)	Canines(CA)
At 2 mm below the CEJ	GT (mm)	1.10±0.29	0.92±0.32	0.94±0.31
At 2 mm below the CEJ	BT (mm)	0.52±0.27	0.41±0.30	0.42±0.40
	Pearson correlation	-0.707^**	-0.768^**	-0.585^**
	GT (mm)	0.86±0.22	0.76±0.26	0.78±0.22
At 4 mm below the CEJ	BT (mm)	0.76±0.14	0.66±0.22	0.73±0.32
	Pearson correlation	-0.319^*	-0.292^*	-0.197
	GT (mm)	0.98±0.37	0.97±0.31	0.82±0.23
At 6 mm below the CEJ	BT (mm)	0.66±0.13	0.37±0.25	0.59±0.32
	Pearson correlation	0.000	0.163	0.202

**Confidence levels (bilateral) of < 0.01 indicate a significant correlation; *Confidence levels (bilateral) <0.05 indicate a significant correlation.

Table 3. The mean±standard deviation of GT and BT (mm) in maxillary anterior teeth

Tooth types	M1		M2
	GT	BT	GT	BT
CI (mm)	1.14±0.36	0.74±0.28	0.98±0.29	0.65±0.18
LI (mm)	1.00±0.36	0.54±0.36	0.88±0.29	0.50±0.25
CA (mm)	0.94±0.38	0.70±0.40	0.83±0.24	0.60±0.34
P-value	0.009	0.008	0.030	0.016

No competing interests reported.

Assessment of the Relationship Between Labial Gingival Thickness and Underlying Bone Thickness in Maxillary Anterior Teeth by Two Digital Techniques

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Abstract

Figures

Introduction

Materials And Methods

Results

Discussion

Conclusions

Declarations

References

Tables

Additional Declarations

Status:

Journal Publication

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