Ⅰ. Introduction

Dental implants are used to restore completely and partially edentulous arches. Studies have demonstrated the effectiveness of implant treatments, including for partially edentulous cases, with high survival rates for both implants and implant-supported prostheses.^1,2 Recent meta-analyses have reported a 10-year survival rate of approximately 94.6% for such implants.² Despite this high survival rate, dental implants can still fail or develop complications, with marginal bone loss (MBL) being one of the most common causes.³

MBL during the first year of implant function is generally considered a natural part of the bone remodeling response to surgery and establishment of the implant-bone interface. However, MBL occurring after the first year is often indicative of clinical complications, including immunological reactions and surgical or prosthodontic problems.⁴

A substantial proportion of early MBL stabilizes over time without further progression. However, disruptions involving the microbial balance at the implant-mucosa interface can facilitate biofilm accumulation, significantly increasing the risk of peri-implant mucositis.⁵ If inadequately managed, peri-implants can progress to peri-implantitis, leading to further MBL and, in severe cases, implant failure. Thus, MBL serves not only as a marker of clinical alterations but also as a critical factor directly influencing the long-term success of dental implants and as a predictor of peri-implantitis, highlighting its value in identifying and addressing potential implant complications before they progress.^6,7

While biological factors undoubtedly play a significant role in MBL progression, prosthetic aspects such as implant design, connection type, and restoration techniques are equally critical in influencing peri-implant bone stability. A deeper understanding of prosthetic factors is essential, because they can be modified and optimized through proper treatment planning, thereby minimizing MBL and improving long-term outcomes.^8,9,10

Therefore, through this review, we aimed to comprehensively examine the causes and factors contributing to MBL after implant placement through a comprehensive literature analysis, with a particular focus on prosthetic aspects. By emphasizing the prosthetic perspective, this study sought to provide evidence-based insights and strategies to enhance clinical outcomes and support the long-term success of implant treatment.

Ⅱ. Materials and Methods

A comprehensive literature review concerning MBL and its associated factors after implant placement was conducted using major academic search engines and databases. These search tools included PubMed, Google Scholar, Cochrane Database, ScienceDirect, Wiley Online Library, and an integrated electronic journal and database search engine of the Dankook University Library. Keywords, such as implant fixture failure, implant length, implant diameter, implant positioning, MBL, peri-implantitis, platform switching, implant prosthesis design, and emergence profile, were used to conduct a comprehensive search of relevant studies.

Ⅲ. Results

A total of 236 articles were initially identified and their relevance to the research topic served as the primary criterion for the initial selection process. Subsequently, articles addressing comparable topics were subjected to a more detailed evaluation with a focus on the level of evidence and publication date. Priority was given to high-quality evidence, such as systematic reviews and meta-analyses, as well as to the most recent studies published within the last 5 years. Additionally, key factors, including study design, sample size, and analytical methodologies, were rigorously assessed to ensure the inclusion of clinically and academically robust research findings.

Based on this approach, 36 articles were selected for detailed review and descriptive analysis. These articles formed the basis for investigating the causes and contributing factors of MBL involving dental implants. The selected prosthetic factors associated with MBL were connection type; emergence profile; residual cement; fixture length, diameter, depth; crown-to-implant ratio; splinting; and cantilever design. The results are presented descriptively

1. Connection type

In a comprehensive retrospective study analyzing 355 dental implants that had been functional for more than 4 years after loading, MBL was measured using radiographic images over a mean follow-up period of 6.73 years (range: 4–12 years). The results showed that implants with an internal connection (IC) exhibited a mean MBL of 0.15 mm, whereas those with an external connection (EC) demonstrated a significantly higher mean MBL, of 0.47 mm.¹¹ A randomized controlled trial was conducted to evaluate the performance of implants with different prosthetic interfaces and designs in partially edentulous posterior mandibles. Over a 3-year post-loading period, the study reported mean MBL values of 0.67 mm for implants with an IC and 1.24 mm for those with an EC.¹² A systematic review was conducted analyzing 17 studies that directly compared the MBL between implants with IC and EC. This comprehensive analysis concluded that IC implants demonstrate superior outcomes in terms of MBL, consistently exhibiting lower bone loss than in EC cases. These findings underscore the potential advantages of IC in enhancing peri-implant bone preservation and supporting the long-term success of dental implants.¹³

A review focusing on MBL associated with bone-level (BL) and tissue-level (TL) implants examined the findings of 19 studies.¹⁴ The analysis concluded that there was no clear evidence indicating a significant advantage of one implant type over another in terms of MBL. These findings suggest that the differences in MBL between BL and TL implants are not sufficiently significant to establish a clear preference, highlighting the need for further research to identify the factors affecting MBL in each implant design. However, platform switching in BL implants has a favorable effect in terms of MBL.¹⁴

Platform switching, which offsets the implant fixture and abutment, redistributes mechanical stress at the interface and reduces inflammation by distancing the microgap from the crestal bone. Consequently, this approach has been widely shown to lower MBL, improve tissue stability, and preserve marginal bone integrity.^15,16 Additionally, one study confirmed the long-term benefits of platform switching, demonstrating its sustained effect in minimizing MBL over an extended observation period.¹⁷ These findings may suggest the clinical importance of incorporating platform switching in implant design to enhance peri-implant bone stability and improve long-term outcomes.

2. Emergence profile

MBL arises from a complex interplay of biological and mechanical factors and is influenced by various elements, such as external aspects of the prosthetic and tissue responses. The emergence profile (EP), as a biological factor, is crucial for reducing plaque accumulation by achieving harmonious integration with soft tissues; however, the EP and one of its components, the emergence angle (EA), represent an ongoing discussion regarding their impact on MBL. Yi et al.¹⁸ demonstrated a direct relationship between overcontoured EP and EA, reporting significant MBL when EA exceeded 30° and even when it remained below 30°. One study explained the external contour of prostheses using two key angles: the divergence angle (DA) at the point where the abutment ascends from the bone and the mucosal emergence angle (MEA) at the mucosal margin. They emphasized that excessive contouring of the DA can cause MBL (remodeling), whereas over-contouring of the MEA may lead to inflammation of the peri-implant tissue.¹⁰ Therefore, prosthesis contour not only affects MBL but also significantly influences the health of peri-implant tissues. This emphasizes the critical importance of maintaining appropriate prosthetic angles to mitigate inflammation and ensure long-term stability.¹⁹

Conversely, a number of studies have argued that contour does not have a significant impact on MBL. An EA > 30° leads to significant MBL under prolonged functional loading, whereas an EA < 30° does not pose a substantial risk, suggesting that EA alone may not be the primary determinant of MBL.^20,21 Additionally, studies on MBL across different implant sites found that EA was significantly greater in posterior regions than in anterior or premolar regions; however, no significant differences in MBL were observed based on implant location.²²

3. Margin cement

Another critical factor that influences MBL is the presence of residual cement, which can compromise peri-implant health and contribute to biological complications. MBL around dental implants can be significantly influenced by residual cement as it may compromise the protective barrier against microbial colonization and promote peri-implant inflammation. A comparison of the extent of residual cement between prosthetic retention types revealed that cement-retained restorations were associated with a higher frequency of biological complications, particularly MBL, with residual cement identified as a potential contributing factor.²³

Furthermore, residual cement was detected in 72.48% of dental implants, with a markedly higher prevalence at the equigingival or subgingival margins than at the supragingival margins. Implant restoration surfaces with subgingival abutment margins demonstrated a 3.66-fold greater risk of residual cement occurrence, compared with those with supragingival or equigingival margins.²⁴ This increased prevalence of residual cement at subgingival margins has been strongly associated with a significantly greater degree of bone loss. Additionally, residual subgingival cement is strongly associated with peri-implant mucositis, a condition that serves as a risk factor for increased probing depth, crestal bone loss, and progression to peri-implantitis.²⁵

4. Fixture length, diameter, depth

The diameter and length of dental implants have not been shown to exert a significant impact on MBL.²⁶However, it has been noted that, although the absolute MBL may remain the same, its relative impact can differ depending on the length of the implant fixture.²⁷ For shorter implants, maintaining adequate surrounding bone is crucial to ensuring long-term success. Short implants with IC result in lower MBL, resulting in improved survival rates.²⁷ The depth of implant insertion exhibited a significant correlation with MBL, with deeper insertion being associated with increased initial MBL. Implants placed at depths > 1.0 mm had a significantly higher initial MBL than did those placed at depths < 1.0 mm. Deeper placement of smooth surface implants below the alveolar crest also resulted in increased MBL, compared with those at the crestal level.²⁸ When the rough/smooth surface border was positioned 2 mm beneath the crest, MBL occurred, along with the formation of 1.88 mm of junctional epithelium and 1.05 mm of connective tissue contact.²⁹

However, a greater insertion depth reduces the likelihood of implant thread exposure. Initial MBL was primarily observed within 6 months after loading in subcrestally placed implants, with the coronal bone on the implant shoulder acting as a biological shield.⁸ Subcrestal implants showed superior bone levels and less thread exposure, especially in patients with thin biotypes, as they supported the reestablishment of biologic width.³⁰ These findings emphasize the importance of carefully determining the optimal placement depth to minimize MBL and thread exposure.

5. Crown-to-implant (C/I) ratio

In implant prosthetic design, the C/I ratio is inherently related to implant length, as it represents the proportional relationship between the length of the crown and implant, and serves as a structural element that can influence load distribution and stability of the marginal bone. According to Pellizzer et al.,³¹ an increasing C/I ratio has been suggested to have a potentially deleterious effect on peri-implant tissues and MBL. Their study, which tracked 369 implants, revealed a trend in which an elevated C/I ratio correlated with a higher average MBL. Specifically, when the C/I ratio exceeded 2, MBL increased, which could compromise long-term implant stability. However, they concluded that the C/I ratio did not have a significant effect on the implant survival rates.

Similarly, Padhye et al.³² reported that, although the C/I ratio does not significantly influence implant survival, advancements in modern implant surface treatments and stress distribution designs have mitigated its effects on MBL. These findings suggest that contemporary technological advancements in implant design may counteract the negative effects of high C/I ratios, particularly in terms of marginal bone preservation.

6. Splinting

In implant prosthetics, splinting, which connects two or more implants into a unified prosthetic structure, is also closely related to prosthetic length because it can influence load distribution, enhance mechanical stability, and reduce excessive strain on individual implants.³³

However, the effect of splinting on MBL remains a topic of discussion. Although splinting is widely acknowledged for its ability to enhance load distribution and improve the mechanical stability of implant-supported restorations, its influence on marginal bone preservation and implant survival rates remains unclear.

In a study by Kim et al.¹¹ comparing MBL between IC and EC implant types in the posterior regions of patients without peri-implantitis, implant splinting did not show a significant difference in MBL outcomes, regardless of implant type. Additionally, a significant difference was observed in the implant survival rates. According to a systematic review and meta-analysis, among 2,768 splinted implants, only 24 failed, resulting in a 99.1% survival rate, whereas among 2,185 non-splinted implants, 51 failed, resulting in a survival rate of 96.5%. These results suggest that, although the differences in MBL were not significant, splinted implants demonstrated a significantly higher survival rate.³⁴

In contrast, Yi et al.³⁵ reported that splinted implants, particularly those involving three or more units in the central position, exhibited a 4.66-fold higher risk of peri-implantitis than single implants did. Although splinting facilitates a more even distribution of occlusal loads, the inherent limitations of the design, including reduced accessibility around centrally positioned implants, may contribute to challenges in maintenance and hygiene management, thereby increasing susceptibility to peri-implantitis.

7. Cantilever design

Studies examining the relationship between cantilever length, implant diameter, and bone loss were reviewed. Khorshid et al.³⁶ reported that smaller-diameter implants combined with longer cantilever extensions were particularly vulnerable to excessive occlusal forces. This increased stress on the implant may increase the risk of peri-implant bone loss, which could compromise both the implant’s stability and surrounding bone health. To prevent excessive stress and maintain long-term bone preservation, it is important to limit the use of extended cantilever designs, particularly when small-diameter implants are used. These findings emphasize the need for careful selection of implant dimensions and cantilever length to optimize mechanical performance and reduce the likelihood of bone loss.

Ⅳ. Conclusion

Platform switching, fixture type, and fixture depth significantly influenced MBL, whereas variables such as fixture diameter and length did not display consistent or significant effects. The emergence profile, C/I ratio, residual cement, and smaller-diameter implants with extended cantilevers are critical factors contributing to MBL. Careful prosthetic planning and advancements in modern implant design are essential to mitigate these effects. The advantages of splinted prosthetic designs in load distribution must be carefully weighed against potential maintenance challenges and the increased risk of peri-implantitis. Given the inherent complexity of these factors and the limitations of retrospective studies, there is a pressing need for controlled prospective studies to enhance our understanding and improve clinical outcomes in implant therapy.