Association of prosthetic angles of the Implant Supracrestal Complex with peri‐implant tissue mucositis

Abstract Objectives The aim of this study was to investigate the association of the Mucosal Emergence Angle (MEA) with peri‐implant tissue mucositis. Material and Methods Forty‐seven patients with 103 posterior bone level implants underwent clinical and radiographic examination. Three‐dimensional data from Cone Bean Computer Tomography and Optica Scan were transposed. Three angles were defined: MEA, Deep Angle (DA) and Total Angle (TA) and measured at six sites for each implant. Results There was a significant correlation between MEA and Bleeding on Probing for all sites with an overall odds ratio of odd ratio 1.07 (95% confidence interval [CI] 1.05–1.09, p < 0.001). Sites with MEA ≥ 30°, 40°, 50°, 60°, and 70° had a higher risk for bleeding with an odds ratio of 3.1, 5, 7.5, 11.4 and 33.55, respectively. When all 6 sites of an implant prostheses had MEA ≥ 40°, the risk of having bleeding at all 6 sites was 9.5 times higher (95% CI 1.70–52.97, p = 0.010). Conclusions Maintaining MEA no wider than 30°−40° is advisable, while the aim should be to keep this angle as narrow as clinically feasible. Registered in Thai Clinical Trials Registry: http://www.thaiclinicaltrials.org/show/TCTR20220204002.


| INTRODUCTION
There is an emerging body of evidence pointing towards significant interrelations between the condition of the peri-implant tissue and the design of the prosthetic elements .
Three cross-sectional studies (Katafuchi et al., 2018;Majzoub et al., 2021;Yi et al., 2020) have suggested that prosthesis contour of more than 30°as it appears in periapical radiographs, is correlated with increased risk for peri-implantitis in bone level implants. Two cross sectional studies which followed similar methodology did not confirm the correlation of wide contour with peri-implantitis (Inoue et al., 2020, Lops et al., 2022. Furthermore, convexity of the prosthesis profile has been correlated with increased recession (Siegenthaler et al., 2022), marginal bone loss  and peri-implantitis when combined with overcontouring of the prosthesis (Yi et al., 2020). At the same time, defining the complex three-dimensional (3D) structure of the prosthesis contour solely by means of the 2-dimensional interproximal projection on periapical radiographs might be insufficient. In addition, the Glossary of Prosthodontic Terms (2017) 9th Edition, as well as previous research (Yotnuengnit et al., 2008), defines Emergence Profile (EP) and Angle (EA) only in relation to the "circumscribed soft tissues," which cannot be accounted for in periapical radiographs. Nevertheless, the findings of these studies collectively warrant further investigation with a precise and reproducible protocol.
The aim of the present cross-sectional study was to investigate the association of the prosthesis angle at the point of emergence with peri-implant tissue mucositis, by means of defining and reproducibly measuring on transposed 3D radiographic and optical imaging. In particular, the study defined and

| MATERIAL AND METHODS
This study was approved by the Ethics Committee at the Faculty of Dentistry, Chulalongkorn University (HREC-DCU 2022-024) and was registered at the Thai Clinical Trials Registry (TCTR20210709003).
The study followed the STROBE statements.

| Patient sample
Systemically healthy patients (ASA classification I and II) (Daniel et al., 2023) aged 20 years or older, having received implant therapy at the postgraduate clinics of Prosthodontics or the clinic of Implants and Aesthetics, Faculty of Dentistry, Chulalongkorn University and currently in maintenance for at least 3 months were considered for the study. Patients in regular maintenance visits between July 4 and November 30, 2022 were invited to participate in the study if the following conditions were met: (1) Posterior Implant Supported Single Crown.
(3) Implants restored with the use of an abutment.

| Measurements
Digital Imaging and Communications in Medicine (DICOM) file of CBCT was imported in the treatment planning software (coDiagnostiX version 9.7, Dental Wings Inc). Thereafter, and the intraoral optical scan (STL file) was imported through the standard software option of "alignment to another object" (DICOM). Three pairs of corresponding anatomic regions on natural teeth were defined on the intraoral scan and registration object (DICOM). Transposition of the STL file on the CBCT data was then conducted automatically by the software.
The implant axis and the perpendicular axis of the implant platform were defined. Thereafter, three Vertical Planes (parallel to Implant Axis) were identified: (Figure 1) Each plane included two side views of the Implant Supracrestal Complex, so a total of six sites were identified for each implant ( Figure 1). The following horizonal levels (perpendicular to the Implant Axis) were defined for each site: (Figure 2).

| Clinical diagnoses
Diagnosis at implant level was based on the condition of the periimplant tissue upon clinical and radiographical examination.

| Mucositis
Peri-implant tissue inflammation was recorded by means of bleeding or suppuration on gentle probing with light pressure (Berglundh et al., 2018). Number of BoP sites out of six was reported for each implant. Mucositis at implant level was described by means of the extent of BoP (number of BoP sites out of six) as in Vianna et al. (2018) and Philip et al. (2022).

| Peri-implantitis
Three case definitions were used for Peri-implantitis: (c) the same as above with the bone level threshold set at ≥2 mm (Romandini et al., 2021). Periapical radiographs were inserted in Image J software (National Institute of Health) and marginal bone level at mesial and distal of the implant was calculated in relation to the implant platform after calibration using the implant length and width.

| SAMPLE SIZE CALCULATION
The association of MEA and peri-implant tissue inflammation has not been previously investigated, thus limiting the ability to use specific data for a sample size calculation. Previous studies (Yi et al., 2020), have established the threshold 30°for studying the impact of the CA and found difference in the prevalence of peri-implantitis between 16% and 33%. Albeit not directly related measurements, assuming a minimum effect size of at least 25% to detect the difference in the prevalence of BoP (primary outcome of this study) between MEA ≥ 30 and MEA < 30, a sample size of 90 cases is required using a significant level of 0.05 (two-sided) and study power of 80%.
Adjusting the sample size by 10% for missing data, would result in a minimum sample size of 100 implants. The sample size calculation was based on the hypothesis tests for two independent proportions.

| STATISTICAL ANALYSIS
Descriptive statistics were presented using mean, standard deviation (SD), median, interquartile range (IQR), and percentages as appropriate. Associations between angles and BoP were assessed using threelevel mixed models for logistic regression to account for multiple sites within implant and within patient by using implant level and patient level as random intercept. Logistic regression analysis was done for the implant level, to explore the association between BOP and the MEA ≥ 40°for all 6 sites. The generalized estimating equations (GEE) were performed to account for multiple implants within patients for the analysis at implant level. The association between the number of sites (0−6) with different cut points of the angles and number of bleeding sites at implant level, (0−6) was investigated using GEE with poisson family and log link, and the association between angles and peri-implantitis (case definition) was investigated using GEE with binomial family and logit link. All the models were adjusted for plaque index, abutment type, prosthesis type, and time with prosthesis in function (months). The site was added into the model for the site level analysis. Receiver operating characteristics (ROC) curve analysis was done to assess the cut-point of MEA for predicting the BoP. The nature of this study was exploratory and no sample size calculation was conducted. Statistical testing was done within an exploratory framework at a two-sided significance level of α = .05. All statistical tests were performed using Stata IC15 (StataCorp, 2017).

| RESULTS
Forty-seven patients (24 male and 23 female) with 103 bone level implant crowns in molar and premolar position were enrolled in the study (Table 1). Patients' average age was 61 years (range: 32-81) and average time with prosthesis in function was 39.17 months (SD 28.05, range 3-120 months). One patient was diagnosed with Diabetes Mellitus Type II under good metabolic control and 3 with hypertension also falling within the ASA type II Classification. Eightyeight crowns were screw retained and 15 cement retained. Crowns were either Porcelain-fused-to-metal (PFM), monolithic Zirconia (all submucosal areas in glazed Zirconia), while 1 crown was nonveneered made of Type III precious alloy (Table 1a).  After adjustment for site, plaque index, abutment type, prosthesis type and time with prosthesis in function (months), there was a significant correlation between MEA and BoP for all sites with an overall odds ratio of 1.07 (95% CI 1.05-1.09, p < 0.001) (Figure 3a).

| Site level analysis
There was no association between specific site location and BoP

| Implant level analysis
Peri-implant tissue inflammation at implant level (Vianna et al., 2018;Philip et al., 2022), is presented in Table 3 and Figure 4. Increase in the number of sites per implant harbouring MEA above all of the cut-off points tested (≥30°, 40°, 50°, 60°) would significantly increase the number of bleeding sites as well (Table 4). When all 6 sites of an implant prostheses had MEA ≥ 40°, the risk of having bleeding at all 6 sites was 9.5 times higher (95% CI 1.70-52.97, p = 0.010).
There was no association between the diagnosis of peri-implantitis (case definitions 1, 2, and 3) and all angles (MEA, DA, and TA), although the results suggest that the more sites with angles 40°, 50°, and 60°, the greater the risk of peri-implantitis (Table 4).

| DISCUSSION
The results of this study suggest that the angle of the prosthesis at The DA on the other hand describes the relation of the abutment as it ascends directly from the bone level. This angle has been shown to influence early marginal bone loss (Souza et al., 2018). The TA was defined in a similar manner as in previous studies on periapical radiographs and represented a rough approximation of the total contour, which also served as a "control," to relate these results to previous work (Katafuchi et al., 2018;Yi et al., 2020).   irrespective of the peri-implant tissue margin. The total angle (TA) in this study was significantly correlated with the DA, which implies that the total contour can be possibly used as a surrogate of the DA. This design feature might be an important parametre in the comprehension of previous findings.
The plaque-induced inflammatory diseases are first manifested as a reversible, marginal tissue inflammation at the coronal margin of the peri-implant soft tissue (Salvi et al., 2022). Experimental studies in humans have documented the causal role of plaque accumulation for the development of peri-implant mucositis (Pontoriero et al., 1994, Zitzmann et al., 2001, while the 2017 World Workshop on the Classification of Periodontal and Peri-Implant Diseases and Conditions (Berglundh et al., 2018) has accepted Peri-implant Mucositis being a precursor to Peri-implantitis based on evidence from observational studies. It is at present well established that patients with poor plaque control are at higher risk of developing periimplantitis. As plaque accumulates on the prosthesis, the impact of plaque will be first manifested on the peri-implant mucosal margin.
Thus, the original concept of EA as described by  The implants included in this study were all bone level placed at posterior sites, aiming to reduce the heterogeneity of the sample.
Statistical models were utilised to account for most common but not all possible confounding factors. Thus factors such as plaque, time with prosthesis, prosthesis and abutment type were accounted for.
Other factors such as patient age, patient periodontal disease status, smoking status, implant diameter, implant type were not accounted for in the analysis. Larger studies in the past have not found any association of the latter group of factors with the studied parametres (Katafuchi et al., 2018). None of the patients in this study was diagnosed with Diabetes, but certain confounding factors such as implant depth and concavity/convexity might have been interesting to account for in future studies. Convexity of the prosthesis contour has been in the past being associated with marginal bone loss , recession (Siegenthaler et al., 2022) and peri implantitis (Yi et al., 2020). Nevertheless, the 3D analysis of the EA has shown that a large number of cases cannot fit within a dichotomous assessment of convexity/concavity, as areas of convexity are followed by areas on concavity in the same contour. The study was funded by the Faculty of Dentistry and the 2nd century Fund, Chulalongkorn University, Bangkok, Thailand.