The success of implants depends on the biological and mechanical factors of the implant, as well as on the chosen prosthesis, which has different characteristics that can affect the survival of the implant and its marginal bone levels. Therefore, when choosing the type of prosthesis, it is necessary to understand and consider each of these variables and tailor them to the relevant clinical situation. In this narrative review, we will assess the types of prostheses, their different characteristics, timing, and their impact on implant survival and marginal bone levels, as accepted in today’s literature.
Key points
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There is a significant influence of implant-supported restorations and their characteristics on implant survival and success.
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Implant survival and marginal bone loss are influenced by the timing of the restoration placement, the type of restoration, the morphologic characteristics of the restoration, and the chosen occlusal scheme.
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The choice of rehabilitation with regards to the different prosthetic factors should correspond to the clinical parameters in which the implant was placed.
Introduction
The high success rates of dental implants and their rehabilitation are well documented in the literature. The influence of the chosen implant-supported restoration and its characteristics on the success of the implants constitutes a significant factor in their dynamics. There are many variables that influence the choice of rehabilitation type, affecting its survival over time, but each of these variables is an integral part of the implant-restoration relationship. These variables, which are described in the literature, include the implant loading protocol, the transmucosal abutment and its connection to the implant, marginal discrepancy and cervical crown contour, the crown-to-implant ratio and the crown-height space, restoration variables (cemented/screwed; splinted/non-splinted; fixed/removable; prosthesis material), impression and fabrication, provisionalization, and occlusal scheme.
In this narrative review, we will examine the prevailing evidence-based opinion in the literature regarding the impact of decision-making of the choice of rehabilitation on the success of the implant and its survival over time.
Loading protocol
The conventional prosthetic loading protocol (3–6 months prior to loading, depending on bone density and volume) was first established by Brånemark in the 1970s. With the advancement in implant design, surgical technique, and treatment modalities, and in order to speed up the prosthetic recovery of the partially or totally edentulous patient, as well as enhance patient satisfaction and compliance, other protocols have been introduced into practice. These include immediate and early loading times. Immediate loading was defined as an implant being put into function within 1 week post placement, and early loading defined as an implant being put into function between 1 week and 2 months post placement.
An assessment of whether immediate and early loading can achieve clinical outcomes comparable with those of conventional loading must include parameters such as implant success and crestal bone loss around implants. A recent systematic review and meta-analysis attempting to determine these factors in edentulous mandibles found an increased risk of implant loss in immediate loading cases. Similar results were found in a randomized controlled trial (RCT) determining the survival of a single implant in the edentulous mandible after 24 months, which found that immediate loading revealed a lower survival rate than delayed (conventional) loading.
A systematic review and meta-analysis investigating immediate loading of single implants versus early loading found no significant difference between early and immediate loading of a single implant with regards to implant survival rate or marginal bone loss after 3 years. A recent systematic review and meta-analysis comparing the efficacy of immediate loading versus early or conventional loading in fixed prostheses rehabilitations found that when comparing immediate and early loading, there were comparable implant survival rates and marginal bone level changes. However, when comparing immediate and conventional loading, there was a higher incidence of implant failure in immediate loading cases. Based on the current literature, it seems that although there are high success rates for cases of immediate and early loading, with not much of a difference between the 2, there is still a higher success rate for cases of conventional (delayed) loading with regards to implant survival and marginal bone level changes.
Implant-abutment connection
The implant-abutment connection interface is considered to be one of the major factors modulating peri-implant bone level changes. The peri-implant marginal bone level is one of the most important factors determining an implant’s success. Marginal bone loss (MBL) can lead to peri-implant inflammation, soft tissue recession, esthetic problems, and implant failure. Traditionally, peri-implant MBL of less than 1.5 mm during the first year after functional loading and less than 0.2 mm annually is considered normal. ,
Implant connections can be classified into external and internal, with a further subdivision of internal connections into passive joint/flat-to-flat systems, and conical/Morse taper interfaces. The external hexagon connections have been extensively used since the early days of implantology by the Brånemark system with several disadvantages such as abutment micromovement, associated with mechanical and biological complications. Internal connections were designed to reduce mechanical complications such as screw loosening and fracture, and enhance stress dissipation along the implant walls. In addition, internal connections, especially conical connections, were shown to reduce the implant abutment gap and subsequent bacterial penetration. A recent study comparing the long-term survival rate and peri-implant marginal bone loss between external and internal connection abutment found no significant difference in the survival rate between the 2. However, greater MBL was found in implants with an external abutment connection after 1 year, although there was no significant difference in MBL after 5 years. A different study comparing implant connections with a 6-year follow-up found greater MBL in external abutment connections.
Systematic reviews evaluating MBL of external and internal connections , showed lower MBL for implants with internal connections when compared to external connections. However, there was no difference in implant survival and complication rates. ,
Transmucosal abutment
There are various factors that can affect MBL around implants, including implant surface, design, and the use of platform switching and implant abutment connection. Establishing an adequate peri-implant biological width is a particularly important factor, providing a barrier to bacterial penetration and inflammatory infiltration. , Regarding the importance and effect of the prosthetic abutment height, it’s been shown , , that over a follow-up time of up to 3 years, implants with a higher prosthetic abutment (2-4 mm) show less MBL compared to implants with a shorter abutment. A recent systematic review analyzing the influence of abutment height on marginal bone loss concluded that longer abutments were correlated with less marginal bone loss. Another systematic review concluded that abutment height can influence early bone loss around implants, but found insufficient evidence to determine its impact on late bone loss around implants.
Regarding implant morphology, studies comparing MBL and soft tissue changes between concave and convex abutments, , found that over a short-term follow-up (up to 1 year), there were no significant differences between the 2 with regards to soft and hard tissues. A recent systematic review and meta-analysis comparing concave versus convex abutments, found that over short follow-up periods, concave abutments had a better effect on MBL than convex abutments, with no influence on soft tissues.
Cervical crown contour
The success of implants is directly influenced by plaque control and maintenance of oral hygiene, in order to prevent inflammatory changes to the soft and hard tissue, which can ultimately lead to bone loss and peri-implantitis. The contour of the restoration has a major role in enabling adequate oral hygiene and preventing plaque accumulation and marginal tissue inflammation. The terms used to describe restoration contours are the emergence angle and the emergence profile. Studies have shown a correlation between the emergence angle and MBL. A recent study found an association between emergence angle and profile and plaque accumulation. There is evidence that an emergence angle of >30° combined with a convex emergence profile of the abutment or prosthesis is associate with an increased risk of peri-implantitis , and lower maintenance of the gingival margin. However, a recent retrospective study comparing the emergence angle and MBL at various sites, found no correlation between the 2, and a cross-sectional study comparing difference emergence angles and their influence on MBL and soft tissue health around implants found no correlation between the 2.
Another factor which can influence long-term implant success is the concept of platform-switching versus matched connection. Platform switching involves reducing the abutment diameter relative to the implant diameter, which seems to reduce crestal bone loss and enhance soft tissue preservation. Several studies have found that hard and soft tissues appear to respond more favorably to the use of implants with platform switching restorations. A systematic review and meta-analysis evaluating platform-switching implants found greater bone preservation compared to regular platform implants. Similarly, a recent systematic review and meta-analysis found that platform-switched abutments helped preserve crestal bone levels and improved marginal bone level around implants.
Crown to implant ratio and crown height space
The crown-to-root ratio of teeth is defined as the physical relationship between the portion of the tooth within the alveolar bone compared with the portion not within the alveolar bone, as determined by a radiograph. This serves as a prognostic indicator which aids in the evaluation of a tooth as an abutment for fixed or removable restorations. The minimal crown-to-root ratio recommended is 1:1, although ideally the crown should be smaller than the supporting root. Initial implant dentistry advised the same limit of a 1:1 crown-to-implant (C/I) ratio, fearing that high C/I ratio may lead to complications stemming from unfavorable occlusal forces, such as nonaxial forces, on the bone surrounding the implant. However, when comparing C/I ratios with crown-to-root ratios, it was found that the guidelines associated with natural teeth should not be applied to implant restorations. Recent systematic reviews and meta-analyses , evaluating the influence of C/I ratios on implant survival and MBL found no relationship between C/I ratio and implant survival, and no increase in MBL for increased C/I ratio. Similar data were observed in studies comparing C/I ratios of short implants, showing no significant differences in success compared to longer implants.
Another factor to consider is the crown height space (CHS) which is the distance from the crest of the bone to the plane of occlusion. A higher CHS can result in a greater bending moment, thus increasing the stress on the implant-bone interface. An in-vitro study measuring the transfer of axial and nonaxial load in fixed implant restorations with various C/I ratios and CHS found that CHS was more significant than C/I ratio in assessing biomechanical effects. A recent finite element analysis showed that the CHS is more responsible for marginal bone stress than high C/I ratio or an implant’s length.
Screw versus. cemented restoration
Several studies have compared the effect of screw versus cemented restorations on the marginal bone loss and survival rates of implants between the different retention systems. A systematic review assessing survival rates and complications between cemented and screw-retained restorations did not find an overall advantage of one system over the other; however, cement-retained restorations exhibited more biological problems such as higher implant failure and marginal bone loss. Another systematic review found no significant differences between cement and screw-retained restorations with regards to implant survival.
A recent retrospective study assessing the marginal bone loss around implants supporting splinted fixed bridges found statistically higher marginal bone loss for implants supporting cement-retained bridges compared to screw-retained ones. However, a systematic review and meta-analysis comparing the 2 retention systems for fixed implant-supported restorations with a follow-up of between 1 and15 years found less marginal bone loss and less prosthetic complications for cement-retained restorations, although the differences were not clinically significant.
The general consensus advises proper case selection for using either of the systems, taking into account the advantages and disadvantages and making a clinical choice based on the specific case requirements.
Splinted versus non-splinted restorations
When planning the restoration of adjacent implants, the question of splinting the restoration is dependent upon several factors such as implant length, occlusion, oral hygiene, abutment connection, and the possibility of achieving passive fit for the restoration framework. It was suggested that splinted restorations provide better load sharing, thus reducing the stress on the cortical bone, while non-splinted restorations enable better oral hygiene, passive fit for the restoration framework, and enhanced contour and emergence profile. A systematic review and meta-analysis assessing the MBL, implant survival rate, and prosthetic complications of splinted and non-splinted fixed restorations concluded that there was no difference in MBL and complications between the 2. However, splinted restorations with external abutment connections were associated with less implant failure. Another recent systematic review and meta-analysis assessing the same parameters in short implants (≤8.5 mm) found similar results and concluded there was no difference between splinted and non-splinted short implants with regards to survival rate, MBL, and mechanical complications. A systematic review comparing splinted and non-splinted designs for a maxillary overdenture supported by 4 implants reported similar conclusions and found that the survival rates of implants and overdentures were not influenced by the splinting design.
A recent retrospective study analyzing the biological and mechanical complications of splinted and non-splinted restorations concluded that splinted implants had a higher risk of biological complications and lower risk of mechanical complications. Implants splinted between 2 other implants showed the highest risk of biological complications. This finding was also observed in a cross-sectional study aiming to identify the influence of various prosthetic features on peri-implantitis, which found a higher prevalence of peri-implantitis in splinted-middle implants. A recent case study identifying cases in which the middle implant of a 3-implant splinted restoration had to be removed, observed that in these failed cases, the prosthetic platform of the middle implant was coronally positioned compared to the mesial and distal implants. Similar results were reported in a recent retrospective study, concluding that in cases of 3 or more adjacent implant-supported prostheses, the central implants were more predisposed to marginal bone loss compared to the adjacent ones.
Regarding full-arch prostheses, a recent retrospective study investigating the long-term implant survival rate and MBL over an observational period of at least 20 years found lower success rates for implants supporting fixed and removable full-arch rehabilitations compared to implants supporting fixed single-unit and multi-unit rehabilitations.
Removable versus fixed prosthesis
There are several factors that have been reported to be contributing to peri-implant disease and early implant crestal bone loss. They include general factors like poor oral hygiene and a history of periodontitis as well as local factors like the implant abutment interface, contour and emergence profile, and restoration material. Another factor assessed is the effect of the type of prosthetic restoration on marginal bone loss around implants. A retrospective study analyzing the prevalence and risk of peri-implant disease based on the type of prosthesis found higher bone loss for implants supporting removable prostheses compared to implants supporting single crowns or fixed partial restorations . However, a recent systematic review comparing the MBL between fixed and removable implant supported restorations found similar results between the 2. Another systematic review and meta-analysis analyzing bone loss in the posterior edentulous mandible concluded that 4-implant–supported overdentures showed less bone loss compared to 2-implant–supported overdentures.
Occlusion
Natural teeth are supported by the periodontal ligament which contains mechanoreceptors and acts as a shock absorber for the teeth. Implants, however, are directly connected to bone, and are therefore different with regards to load transmission and distribution. The general recommendations for occlusal schemes of implant-supported single crowns or fixed partial dentures include a mutually protected occlusion with anterior guidance and evenly distributed contacts with wide freedom in centric relation. Although occlusal schemes for implant-supported prostheses share similar concepts to those developed for natural dentition, unlike teeth, the impact of occlusion on peri-implant disease is controversial in the literature.
Compressive forces, exerting inward on the implant, increase the surrounding bone density over time, while shear forces accelerate the resorptive process. Overload is defined as a force that exceeds the capacity of the prosthesis, restorative components, implant fixture, or the supporting bone to withstand without damage. Excessive occlusal load on implant-supported prostheses can lead to high stress on the peri-implant bone tissue, generating stress at the bone-implant contact point, which can increase the incidence of crestal bone loss. A systematic review investigating occlusal overload and marginal bone loss around implants concluded that occlusal overloading can significantly contribute to peri-implant bone loss only under inflammatory conditions. Another systematic review assessing possible effect of traumatic occlusal forces on the peri-implant bone levels reported animal studies in which the presence of excessive overload generated peri-implant bone loss, even in the absence of inflammation. However, the cause-and-effect relationship between traumatic occlusal forces and peri-implant bone loss in humans was poorly reported. A recent systematic review analyzing the relationship between occlusal overload and peri-implant bone loss concluded there is an association between the 2, although further investigation is necessary.
An evidence consensus statement attempting to determine the level of scientific evidence between occlusal overload and implant complications found a lack of scientific evidence regarding which implant occlusal scheme can minimize or eliminate complications. The few number of studies suggesting a potential relationship between implant failure and occlusion-related factors (bending moments, occlusal overload) lacked evidential support.
Marginal discrepancy, impression, and fabrication technique
The literature varies between clinically acceptable marginal discrepancy levels of 60 to 150 μm. A systematic review evaluating the impact of marginal misfit on the clinical outcomes of implant-supported fixed restorations found insufficient evidence regarding the effect of a marginal misfit on clinical outcomes. Marginal gaps did not have negative effects on initial osseointergration or peri-implant bone stability over time.
A systematic review evaluating the in vitro accuracy of digital implant impression taking compared with conventional impressions found that digital impressions are a valid alternative to conventional impressions for cases of single implants or partial dentures, though conventional impressions seem to be more accurate. In cases of complete edentulism, there were inconclusive findings between the 2 impression techniques.
A recent retrospective clinical study analyzing the fit of immediate full-arch prostheses fabricated using conventional and digital impressions, and assessing cumulative survival rate and marginal bone level found high survival rates for both methods used. An RCT comparing the outcome of digital versus analog procedures for the restoration of single implants found no significant differences in peri-implant marginal bone loss between digital and analog procedures.
Accuracy between the intaglio of the restoration and its congruency with the supporting implants, in cases of splinted fixed dental prostheses (FDPs), is defined by the passive fit. Passive fit between the prosthesis and supporting implants is one of the main factors preventing subsequent mechanical and biological complications. Compromised fit between screw-retained restorations and the implant is thought to create uncontrolled strains on the restoration components and on peri-implant tissues, leading to biological and technical complications (eg, bone loss, screw loosening, and loss of implant and/or prosthesis).
Achieving absolute passive fit is almost impossible, due to the many steps involved in the prosthesis fabrication process, especially for fixed partial dentures and full-arch restorations supported by several implants. It is therefore accepted to define a level of passive fit which will not lead to long-term clinical complications.
Provisionalization
The role of provisional restorations during immediate implant placement is to restore esthetics and to provide patient comfort. However, it is important to assess the effect of provisional restorations on the hard and soft tissues.
Cases that require bone grafting and reconstructive procedures prior to implant placement, with a long healing period, require an interim prosthesis that will not exert undesirable pressure on the grafting site. Since removable partial dentures (RPDs) can cause unfavorable pressure on the underlying tissues, it is therefore advised, when possible, to use an interim prosthesis that is fixed to the adjacent teeth, like resin-bonded fixed partial dentures, fiber-reinforced composites, and polyethylene ribbons.
After implant placement in cases that are not immediately loaded, provisionalization with an RPD may be needed for cases where there are no adjacent teeth available for a fixed interim prosthesis. Care must be taken to avoid occlusal contacts that can exert undesirable loading on graft sites around the implants, and to minimize the contact between the interim prosthesis and the grafted area. In advanced augmentation cases, when possible, it is advised to use an Essix appliance for the first 2 to 4 weeks, followed by a second interim prosthesis, preferably fixed, or removable in cases of smaller graft sites.
In cases of immediate loading with a provisional prosthesis that provides proper occlusal schemes, osseointegration is not impeded, provided that the loading forces are well-oriented, and that the implant has enough primary stability upon insertion. Similar results were found in a recent systematic review and meta-analysis, concluding that over a 12-month follow-up, immediate interim restorations had no significant impact on peri-implant soft and hard tissues, around immediately placed single dental implants.
Prosthesis material and polishing
The improvement and progression of computer-aided design/computer-aided manufacturing systems as well as the development of new materials and treatment protocols have aided in facilitating advanced options and possibilities for fixed and removable restorations. Examples include high-strength ceramics and monolithic zirconia.
An ideal restoration material should promote esthetics, biocompatibility, and stability over time. Several studies have aimed to investigate the influence of various restorative materials on the survival of dental implants. A systematic review and meta-analysis examining the correlation between the type of restorative material and the clinical outcome of implants concluded that the choice of prosthetic material seems to have no influence on implant survival rates in fixed restorations. Another systematic review and meta-analysis evaluating the impact of prosthetic material on implant survival for fixed dentures found similar results and concluded that prosthetic material selection seems to have no clinically relevant influence on implant survival rates.
In addition to the type of material used for the prosthesis, another important factor to consider is the polishing status of the material’s surface, which is directly correlated with plaque accumulation and subsequent inflammatory response of the soft tissue. A recent retrospective analysis investigating the impact of prosthetic material on peri-implant bone resorption found that zirconia restorations exhibited higher resorption compared to metal-ceramic restorations. A study evaluating the surface roughness values of zirconium oxide found that full polishing protocols substantially reduced surface roughness.
Summary
Implant-supported restorations have become increasingly favored by both dental practitioners and patients over the past decades, providing functional and esthetic solutions following tooth loss. The success and longevity of implants vary among individuals due to numerous biological and technical factors. Beyond the surgical component, factors such as the choice of rehabilitation, its design, and timing also influence the dynamics between implants and their restorations. While this narrative review presents the prevailing consensus in the literature, further research is needed to solidify these findings.
Clinics care points
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The clinical factors with significant literary evidence influencing implant survival or MBL include loading protocol, abutment height, crown-height space, platform switching, splinted middle implant, splinted full-arch restorations, occlusal overload, and fixed provisional restorations.
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The clinical factors with no significant literary evidence on implant survival or MBL include crown-to-implant ratio, screw versus cemented restorations, digital versus analog impressions, prosthesis material, splinted versus non-splinted short FDPs, and immediate fixed restorations on immediate implants.
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The clinical factors with controversial literary evidence on implant survival or MBL include cervical crown contour and removable versus fixed restorations.
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