Effect of Different Dental Implant Prosthetic Joints on Marginal Bone Loss: Emerging Findings from a Bayesian Network Meta-Analysis (NMA) and Systematic Review

: Background: A key point in assessing dental implant prosthetic joints is their mechanical strength and biological response under the masticatory loading. The aim of the present systematic review was to evaluate the marginal bone loss and prosthetic behaviour of different internal/external bi-phasic implants. Methods: Randomized Clinical Trials (RCTs) have been considered for analytic purposes. The article screening was conducted on the Pubmed/MEDLINE, EMBASE and Google Scholars databases through an electronic process. Eligibility and risk of bias assessments were conducted for an article to be included in the data process. A series of pairwise meta-regressions for continuous variables was conducted considering the mean differences and 95% CI at two different timepoints: baseline and 1-year follow-up. The meta-analysis was performed comparing the following groups: internal conical prosthetic joint with index (IC), external hexagon bone level position (EI), internal tri-channel connection bone level position (ITC), internal hexagon 1 mm below the bone level (HI), internal hexagon bone level position (HI crest), cone morse 1 mm below the bone level (CM), cone morse bone level position (CM crest) and internal octagon bone level position (IO). The following parameters were considered for descriptive data synthesis: sample size, implant manufacturer, prosthetic joint type, prosthetic complications, marginal bone loss, study outcomes. Results: A total of 247 papers were identified by the electronic screening and 241 were submitted for the full text assessment. The eligibility process excluded 209 articles, and 32 studies with a low risk of bias were considered for the qualitative synthesis and further statistical methods. At the baseline, the CM showed a more effective efficiency and reduced marginal bone loss compared to IC, EI, ITC, internal hexagon, cone morse and internal octagon ( p < 0.05). CM showed the lower rate of prosthetic complications and structural device failure including abutments and joint components under the loading compared to other joint types. Conclusion: Within the limits of the present investigation, the heterogeneity, the weight of the study model considered and the inherent differences between the dental implant properties, the pure CM showed a more consistent control of marginal bone loss at short-and medium-term follow-up. Despite the low rate of cumulative complications for all joints considered, the CM abutment joints were less prone to prosthetic failure at an early and medium-term follow-up.


Introduction
The implant-abutment joint is a well-known factor in two stage implantology due to the related biological and biomechanical implications.A submerged implant is considered to be a supportive protocol to avoid early biofilm colonization and preserve the osteointegration process during the early healing phases from bacteria and local inflammatory stress [1,2].On the contrary, immediate functional loading could emphasize solicitations at the level of the peri-implant marginal components and consequently produce a consistent instability in the peri-implant soft and hard tissues [3].As such, the implant-abutment design, the length and the stability of the prosthetic joint and the platform components' tolerance play a key role in the creation of a hypothetical bacterial reservoir and sustaining a chronic inflammatory status, triggering peri-implant marginal bone loss (MBL) [4].In the literature, the implant's success is considered with a −1.5 mm MBL during the first year after loading and afterwards <0.2 mm/year [5][6][7].Different factors have been recognized as being correlated with peri-implant marginal bone resorption.The most extensively investigated factor is the peri-implant inflammation reaction that is recognized to be a consequence of bacteria colonization at the level of the dental implant interfaces [8].The biofilm adhesion is determined a few seconds after the surface's exposure to the oral cavity environment [8].The prosthetic joint micro-gap could potentially generate a reservoir of bacteria and biofilm subproducts resulting in chronic inflammation at the level of the surrounding tissues [9].In fact, the microleakage is able to produce a pump effect under the functional loading determined by the mismatch generated by the abutment joint prosthetic components [9,10].At present, microleakage prevention seems to be the main challenge for transmucosal dental implants design.In the literature, many different prosthetic joint design have been investigated for this purpose [11,12].The most common implant joint in the market are external connection, internal connection and conical/cone morse joint [7,13].The aim of the present systematic review was to determine the more recent evidence regarding dental implant prosthetic joint design through a network meta-analysis.

Search Strategy
The present study has been registered on International Prospective Register of Systematic Reviews (PROSPERO) prot.n.CRD42024500303.The database screening has been performed following the PRISMA guidelines and checklist (Suppl.S1) (Preferred Reporting Items for Systematic Reviews and Meta-analyses) [14] and was conducted on the Pubmed/MEDLINE database, Google Scholar, Scopus and Web of Science using the following keywords: (dental implant* OR dental prosth* OR implant-supported prosth* OR endosseous implant*) AND (conical OR tapper OR tapered OR fractional OR locking)) and (internal connection OR internal hexagon OR non conical OR non tapered OR internal tri-channel OR butt-joint)) AND (survival OR success OR bone loss OR bone level OR complications).The PICO question has been detailed:

Inclusion Criteria
The articles written in English language were included with no restrictions regarding the date publication.The titles and abstracts list were considered for a first-level initial screening by two independent reviewers (FL, IA).Clinical trials were included for descriptive synthesis and meta-regression.

Study Data Extraction
The following parameters were extracted from the selected studies: publication date, study model design, population size, age, marginal bone loss, prosthetic complications, follow-up.For the scope of this article, a specially designed electronic database form has been used (Excel, Microsoft Office 360, Redmont WA, USA).

Risk of Bias (RoB)
The Risk of Bias Rating Tool for Human and Animal Studies (OHAT) tool has been assessed to measure the risk of bias of the articles assessed.The assessment categories were "low risk", "unclear risk" and "high risk" of bias [15].
The tool categories considered were: random sequence generation, allocation concealment, blinding of patients and personnel, blinding of outcome assessment, attrition bias, reporting bias and other biases [15].

Heterogeneity and Meta-Analysis Assessment
The high heterogeneity is determined by the differences in articles' publication year, study model design, healing duration period, and sample size.The meta-regression has been conducted through the dedicated statistical software package Review Manager (RevMan 5.0, The Nordic Cochrane Centre, The Cochrane Collaboration, Copenhagen, Denmark) and the freely available MetaInsight v5.1.2software (Shinyapps, Leicester, UK) for continuous variables with full R code [16].

Inconsistency Assessment
The node-splitting measurement has been conducted to evaluate the inconsistency of the variables' comparison.No inconsistency was considered at p > 0.05.The level of evidence was assessed by the CINeMA (Confidence in Network Meta-Analysis) system [17,18].

Study Data Analysis
Bayesian NMA was performed using the Bayesian framework with random-effects hierarchical models through the freely available MetaInsight v5.1.2software (Shinyapps, Leicester, UK) for continuous variables with full R code [16].A forest plot of relative effects from Bayesian random effect has been calculated to evaluate the consistency model and the significance of the ranks.The I 2 test considered a low heterogeneity with a value <40%.For I 2 test > 40%, the heterogeneity was further investigated through meta-regressions.The data were presented considering the mean differences and the 95% CI of the means.The surface under the cumulative ranking curve (SUCRA) rank-o-gram has been applied to assess the robustness of the comparison categories.

Prosthetic Complications and Joint Failure
The most common prosthetic joint complications, including provisional/final crown debonding, provisional/final abutment screw loosening, provisional/final framework fracture, chipping/cracking of veneering material, no mechanical failure and prosthetic joint failure, have been detected for all included studies (Tables 2 and 3).The MBL change was similar in the groups.No relevant complicaitons were detected in the soft tissue.

Meta-Regression MBL
A plot of the relative effects from the Bayesian random effect consistency model is shown in Figures 4 and 5  Prosthesis 2024, 6, FOR PEER REVIEW 13

Discussion
This NMA evaluates the effect of different implant-abutment prosthetic joints on marginal bone loss through direct and indirect pairwise comparisons.A total of eight studies have been conducted from 2005 to 2015.The risk of bias represented one of the key assessments within the present investigation.A total of 36.36% of the articles included reported a low risk of bias while only 18 studies reported the adoption of a randomized approach for the population sample allocation.Due to the significance of the marginal bone loss for dental implant survival rate, we compared the early healing period of a submerged implant at the baseline (uncovering) and after 1 year.The CM abutment joint showed a significantly lower amount of marginal bone loss evidence compared to the other connections at the baseline.The same results emerged 1 year after the loading, except for the comparison between the HI and CM groups which revealed a similar resorption rate at the 1-year timepoint.Significantly higher resorption patterns for EH implants have been observed during the 1-year follow up.Several studies in the literature documented the internal flat-to-flat and conical press-fit joint as more favourable compared to an external abutment joint with a lower peri-implant resorption rate [27,30,51].This evidence seems to support the findings observed in the present NMA.The reasons for periimplant bone resorption are multifactorial, and they include surgical, mechanical and biological factors, including several comorbidities [32].The bone level positioning vs. subcrestal implant's effects on marginal bone loss resulted in non-clear evidence from the NMA due to the low effect size documented in the meta-regression.The abutment joint stability and interface micro-gaps could play a role in functional microleakage creating a critical bacterial reservoir at the level of the crestal bone interface [52,53].In the literature, the press-fit joint has been proposed to reduce the micro-gaps and improve the stability of the implant-abutment joint [32,51].A precise adaptation of the prosthetic components is certainly important to produce a higher stability at the implant interfaces avoiding the creation of gaps [54].The prosthetic joint precision fit is also a theoretical critical point for external and internal flat-to-flat systems, where the loading and the bending impairment could produce a marginal decoupling of the components [51].The presence of an interface

Discussion
This NMA evaluates the effect of different implant-abutment prosthetic joints on marginal bone loss through direct and indirect pairwise comparisons.A total of eight studies have been conducted from 2005 to 2015.The risk of bias represented one of the key assessments within the present investigation.A total of 36.36% of the articles included reported a low risk of bias while only 18 studies reported the adoption of a randomized approach for the population sample allocation.Due to the significance of the marginal bone loss for dental implant survival rate, we compared the early healing period of a submerged implant at the baseline (uncovering) and after 1 year.The CM abutment joint showed a significantly lower amount of marginal bone loss evidence compared to the other connections at the baseline.The same results emerged 1 year after the loading, except for the comparison between the HI and CM groups which revealed a similar resorption rate at the 1year timepoint.Significantly higher resorption patterns for EH implants have been observed during the 1-year follow up.Several studies in the literature documented the internal flatto-flat and conical press-fit joint as more favourable compared to an external abutment joint with a lower peri-implant resorption rate [27,30,51].This evidence seems to support the findings observed in the present NMA.The reasons for peri-implant bone resorption are multifactorial, and they include surgical, mechanical and biological factors, including several comorbidities [32].The bone level positioning vs. subcrestal implant's effects on marginal bone loss resulted in non-clear evidence from the NMA due to the low effect size documented in the meta-regression.The abutment joint stability and interface micro-gaps could play a role in functional microleakage creating a critical bacterial reservoir at the level of the crestal bone interface [52,53].In the literature, the press-fit joint has been proposed to reduce the micro-gaps and improve the stability of the implant-abutment joint [32,51].A precise adaptation of the prosthetic components is certainly important to produce a higher stability at the implant interfaces avoiding the creation of gaps [54].The prosthetic joint precision fit is also a theoretical critical point for external and internal flat-to-flat systems, where the loading and the bending impairment could produce a marginal decoupling of the components [51].The presence of an interface is able to produce a physiological reaction determined by bacterial contamination and marginal micromovements [55].This evidence is histologically accompanied by a chronic inflammatory infiltrate at the level of the peri-implant tissues [55].The mechanical complications seem to be a heterogeneous occurrence that transversely affects all systems considered with a cumulative rate ranging from 0 to 8.53% including major and minor events and a cumulative prosthetic success rate >95%.The screw loosening/fracture is a common joint complication for both internal flat-to-flat and conical connections with index [12].Despite the limited follow-up of the present investigation, no abutment joint decoupling has been documented for CM joints in any of the studies included.Although the implant prosthetic joint could be considered as a relevant risk factor for late implant failure, no significant evidence in this NMA has been detected.A consistent critical point in the present NMA is that implant success could be determined by the type of rehabilitation.In fact, the present investigation considered no limitations for either provisional or final restoration.In addition, the surrounding bone loss is mainly influenced by many factors other than prosthetic joint type, including gingival biotype and thickness, bone width, bone density, biologic width around the implant and other factors related to prosthetic provision.These aspects could be considered a relevant limitation of the present study and the methodology.On the other hand, the lack of homogeneity of the study data represented a critical factor for the network meta-analysis where bone level position data were not available after 1 year for the CM and HI groups, but only at the baseline.These data could be useful for improving the relevance of the findings in relation to the biomechanical behaviour of the different prosthetic joints and the bone level depth.
The biomechanical behaviour could significantly affect the MBL, creating a confounding factor for the NMA calculations and masking the effects of the variables.On the other hand, a rigid inclusion criteria approach could reduce the sample size leading to the investigation making more assumptions.Another limitation of the present study is determined by the follow-up, where very few studies carried out more than 24 months of follow-up.Further long-term randomized clinical trials including implants with similar macro-/and micro-topography are necessary for an equal and comparative evaluation.

Conclusions
Within the limits of the present systematic review and NMA, the CM implant joint showed significantly lower MBL after 1 year compared with external hexagonal and internal flat-to-flat and conical connections.On the other hands, the CM joint reported a lower rate of prosthetic complications and implant-abutment decoupling events.

Figure 2 .
Figure 2. Risk of bias graph: review authors' judgements about each risk of bias item presented as percentages across all included studies [wlr: weighted low risk; ur: unclear risk; weighted high risk: whr].

Figure 2 .
Figure 2. Risk of bias graph: review authors' judgements about each risk of bias item presented as percentages across all included studies [wlr: weighted low risk; ur: unclear risk; weighted high risk: whr].

Figure 3 .
Figure 3. Risk of bias summary: review authors' judgements about each risk of bias item for each included study.
. The higher SUCRA (Surface Under the Cumulative Ranking Curve) values and cumulative ranking curves nearer the top left indicate better performance.The plot represents each data points' contribution to the residual deviance in the NMA in terms of consistency (horizontal axis) and the unrelated mean effect (ume) inconsistency models (vertical axis) along with the line of equality, while the radial SUCRA plot showed that higher SUCRA values indicate better treatments; the size of nodes represents number of participants, and thickness of lines indicates the number of trials conducted.At the baseline, the CM positioned 1 mm under the bone level resulted in the most effective reduction in marginal bone resorption at the baseline.The forest plot for the baseline indicated that CM abutment joint showed a significant advantage in marginal bone loss reduction over HI crest group (MD: 0.74; 95% CI: −1.02, −0.56), HI group (MD: 1.23; 95% CI: 0.96, 1.59), CM crest (MD: −1.09; 95% CI: −1.50, −0.80) and EH (MD: −1.52; 95% CI: −1.81, −1.21) (Figures 4 and 5 ) . The higher SUCRA (Surface Under the Cumulative Ranking Curve) values and cumulative ranking curves nearer the top left indicate better performance.The plot represents each data points' contribution to the residual deviance in the NMA in terms of consistency (horizontal axis) and the unrelated mean effect (ume) inconsistency models (vertical axis) along with the line of equality, while the radial SUCRA plot showed that higher SUCRA values indicate better treatments; the size of nodes represents number of participants, and thickness of lines indicates the number of trials conducted.At the baseline, the CM positioned 1 mm under the bone level resulted in the most effective reduction in marginal bone resorption at the baseline.The forest plot for the baseline indicated that CM abutment joint showed a significant advantage in marginal bone loss reduction over HI crest group (MD: 0.74; 95% CI: −1.02, −0.56), HI group (MD: 1.23; 95% CI: 0.96, 1.59), CM crest (MD: −1.09; 95% CI: −1.50, −0.80) and EH (MD: −1.52; 95% CI: −1.81, −1.21) (Figures 4 and 5 )

Figure 5 .
Figure 5. BASELINE MBL: Radial SUCRA plot (left); The stem plot represents the posterior residual deviance per study arm.The total number of stems equals the total number of data points in the network meta-analysis (NMA).The square root plot showed the average leverage across the arms for each study (right).

Figure 5 .
Figure 5. BASELINE MBL: Radial SUCRA plot (left); The stem plot represents the posterior residual deviance per study arm.The total number of stems equals the total number of data points in the network meta-analysis (NMA).The square root plot showed the average leverage across the arms for each study (right).

Figure 5 .
Figure 5. BASELINE MBL: Radial SUCRA plot (left); The stem plot represents the posterior residual deviance per study arm.The total number of stems equals the total number of data points in the network meta-analysis (NMA).The square root plot showed the average leverage across the arms for each study (right).

Figure 7 .
Figure 7. 1 YEAR MBL: Radial SUCRA plot (left); The stem plot represents the posterior residual deviance per study arm.The total number of stems equals the total number of data points in the NMA.The square root plot showed the average leverage across the arms for each study (right).

Figure 7 .
Figure 7. 1 YEAR MBL: Radial SUCRA plot (left); The stem plot represents the posterior residual deviance per study arm.The total number of stems equals the total number of data points in the NMA.The square root plot showed the average leverage across the arms for each study (right).

Table 1 .
Search strategy for the electronic database screening.

Table 1 .
Search strategy for the electronic database screening.

Table 2 .
Descriptive synthesis of the studies included.

Table 3 .
Descriptive synthesis of the included study.