One Abutment at One Time Concept for Platform-Switched Morse Implants : Systematic Review and Meta-Analysis

vertical bone loss of immediate loading of implant crowns using the one abutment at one time (AOT) protocol and implants with abutment removal (AR). This systematic review with meta-analysis was reported according to the PRISMA statement, with guidance from the Cochrane Collaboration Handbook. A total of 103 publications were identified in the PubMed database and reference lists of examined articles. After the screening of titles and abstracts, the eligibility of eight full-text articles was assessed. Five studies published between 2010 and 2015 were included in the meta-analysis. There was less peri-implant vertical bone loss at implants using an AOT protocol than at implants using AR protocol (WMD -0.19, 95% CI -0.26 to -0.13; p<0.0001; random-effects model). In conclusion, the use of the AOT protocol with platform-switched Morse implants results in less bone loss than do AR procedures, but this effect may not be clinically relevant. The preservation of marginal bone level achieved with the AOT protocol may not enhance the aesthetics. These results should be interpreted with caution. One Abutment at One Time Concept for Platform-Switched Morse Implants: Systematic Review and Meta-Analysis


Introduction
were the first authors to use the term "one abutment -one time" concept to refer to the connection of an immediate non-removal abutment in post-extractive implant (1).This clinical trial showed at 36 months after loading a statistically significant mean difference of 0.2 mm upper peri-implant marginal bone level in favor of the maintenance of the abutment.Other trials (2-4) also showed a less vertical bone loss after 12 months using definitive abutments, although this may be not clinically perceptible.
Nowadays treatment with implant may incorporate divergent option of treatment that does not follow a general rule.Long-term implants switching platform immediately loaded into smokers had the same results to non-smokers if the abutments were screwed after implant placing and no longer removed (5).Two times dis/reconnections of abutment did not show significant differences in periimplant soft and hard tissues when compared with definitive abutment (6).Researchers have been trying to evaluate whether a definitive abutment has advantage over standard guideline (1,2,(5)(6)(7).
Albrektsson et al. proposed criteria for the success of a dental implant, including radiographic evidence of crestal bone around the implant, 1.5 mm bone loss in the first year and <0.2 mm bone resorption annually after 1 year of loading (8).Abrahamsson et al. studied the effects of abutment disconnection and reconnection on the periimplant soft-/hard-tissue complex in dogs (9).They observed that abutment handling resulted in marginal bone resorption due to tissue reactions initiated to establish proper biological width, which moved the mucosal barrier apically relative to the soft tissue.Other factors, such as microgaps between the implant and abutment (10), micromovement at the implant-abutment interface (11), microleakage between the implant and abutment (8), and abutment disconnection and/or reconnection (1-3) also affect bone remodeling.
There is a biologic base for the use of non-removal abutment placed after implant insertion.However, this option treatment needs assessment of the potential clinical benefit and risk associated with the technique.The liability of excess cement remainder in the periodontal area and its consequences has been discussed as an adverse outcome, in case with abutment margin depths and immediate cemented restoration (12), which may be prevented with abutment that allow screw-retained crowns (6).Another disadvantage associated with definitive abutment after surgery is the difficulty of selecting the appropriate definitive standard abutment immediately after implant insertion, concerned to high soft tissue and wall bone variance (12) that can be prevented by using customized abutments (13).

Aim
The aim of this systematic review and meta-analysis was to compare the peri-implant vertical bone loss of immediate loading of implant crowns using the one abutment at one time (AOT) protocol and implants with abutment removal (AR).

Methods
This systematic review with meta-analysis was reported according to the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) statement (14,15), with guidance from the Cochrane Collaboration Handbook (16).A protocol was designed a priori and registered in the PROSPERO database (registration number CRD42015029682).

Search Strategy
A comprehensive search in PubMed database was performed to identify studies published in English that compared peri-implant marginal bone loss with the use of immediate, platform-switched restorations with definitive abutments and those with provisional abutments.The search was performed in September 2016 using the following strategy: ((implant-abutment OR dental implantabutment) and (one abutment-one time OR one abutment one time OR definitive abutment OR one abutment one time concept OR immediate loading)) and (bone loss OR bone level OR bone preservation).Reference lists of original articles and reviews were searched to identify additional studies that could not be located in the electronic database.

Eligibility Criteria and Outcome
Two reviewers (JSS and TSS) independently screened the search results and identified potentially relevant studies based on titles and abstracts.Potentially relevant studies were read in full, and those fulfilling the eligibility criteria were included in the meta-analysis.Disagreements between reviewers (JSS and TSS) were resolved by consensus or by a third reviewer (PRSM-F).
The following elements to define the eligibility criteria were used: (1) population: patients undergoing implantbased prosthetic rehabilitation, (2) comparison groups: implant placement using an AOT protocol versus AR protocol, (3) predefined outcome: peri-implant vertical bone loss, and (4) study type: randomized clinical trials (RCTs).Studies that did not measure bone loss using radiography or computed tomography, and those with mean follow-up time <3 months were excluded.

Data Extraction
Two reviewers (JSS and TSS) extracted data independently using a predefined protocol.Disagreements between reviewers (JSS and TSS) were resolved by consensus or by a third reviewer (PRSM-F).The following data were recorded: study design, sample size, characteristics of study groups, implant characteristics, follow-up period, radiographic evaluation and measurements of peri-implant marginal bone levels.

Assessment of Risk of Bias
Two reviewers (JSS and TSS) independently assessed trials quality using the Cochrane risk of bias tool (16).Quality was assessed in six domains: selection bias (random sequence generation, allocation concealment), performance bias (blinding of participants and personnel), detection bias (blinding of outcome assessment), attrition bias (completeness of outcome data), reporting bias (selective reporting), and other bias.All domains were judged as having low, high or unclear risk of bias.

Statistical Analysis
The primary endpoint was the change in peri-implant vertical bone level in millimeters from baseline.It were pooled data with a random-effect meta-analysis with weighted mean differences (WMDs) and 95% CIs reported.Heterogeneity was investigated by the Cochran Q test using a cut-off of 10% for significance and quantified using the I2 index [100% x (Q-df)/Q].It was used subgroup analysis to assess whether the different follow-up times led to different results.A random-effects meta-regression analysis was used to assess the significance of the differences.R2 index was used to quantify the proportion of variance explained by the follow-up time.Two-sided p-values lower than 0.05 were considered statistically significant.The data were analyzed using the statistical software Review Manager 5.3 (Cochrane IMS, Copenhagen, Denmark).Meta-regression was performed by using RStudio (version 0.98.1083).

Results
The search strategy resulted in the identification of 103 records from the PubMed database and reference lists of included articles.After the screening of titles and abstracts, the eligibility of eight full-text articles was assessed.Five studies (1,3,4,6,7) (four multicentre RCTs, and one prospective RCT) published between 2010 and 2015 were included in this review (Fig. 1).

Risk of Bias
In the five studies included in this review, participants were randomly assigned using a random number generator to one of two treatment groups.However, it was observed a high risk of selection bias in most studies (1,3,4,7), since the allocation was not concealed.In the study by Luongo et al. (7), protocol deviations including breaking of the random codes were described.A low risk of detection (1,4,6,7) and attrition (1,3,4,7) bias was observed in four of the five (80%) studies.The risk of performance and The one abutment at one time concept reporting bias was judged as unclear for all studies.In addition, there was insufficient information to permit judgment on other biases (Fig. 2).

General Characteristics and Health Status of Included Patients
The five studies included 174 patients (intervention [AOT protocol], n=89; control [standard protocol], n=85).The mean patient age was 55.2 years, and the sex distribution was similar in the two groups.
Patients' health status was characterized as "good" in two studies (3,4), although one patient had well-controlled diabetes.In the three remaining studies (1,6,7), general health status was defined by inclusion and/or exclusion criteria.In one study (1), periodontal health status was controlled by excluding patients with full-mouth plaque and bleeding scores >25%.Two studies (3,4) excluded patients with plaque indices ≤2, based on periodontal screening and recording performed during the first visit.
In the remaining two studies (6,7), information regarding periodontal health status was unclear.Patients who smoked >10 (1,3,6) or >20 (4) cigarettes per day were excluded from four studies.In one study (7), the authors included non-smokers and smokers.

Implant Characteristics
A total of 258 implants were placed using definitive (n=123) and provisional (n=135) abutments.Four of the five (80%) studies used similar surgical protocols for dental implant placement.Implants were placed with the implant-abutment interface at the bone crest level in these studies (1,3,4,6), whereas Luongo et al. placed implants at least 1 mm beneath this level to the palatal wall (7).In one study (4), implants were placed immediately into extraction sites.In the study by Luongo et al. (7) (12%) post-extractive implants were included in the definitive abutment group and one (1%) in the removal abutment group (7).Custom abutments for single-tooth  implants were used in two studies (3,4), whereas abutments provided by the respective manufacturers were used in the remaining studies (1,6,7).Four studies (3,4,6,7) provided information on provisional abutment disconnection and reconnection, but protocols were not uniform.Grandi et al. (3,4), Canullo et al. (1), and Koutouzis et al. ( 6) used cement-retained implant crowns; whereas the implantretained method used by Luongo et al. (7) was unclear.Table 1 summarizes the major characteristics of the studies.

Radiographic Evaluation and Measurement of Peri-Implant Vertical Bone Levels
In all studies, the peri-implant marginal bone level was measured using periapical radiographs and digital imaging software, taking into account the distal and mesial surfaces of each implant.Linear measurements were made using the most coronal portion of the implant shoulder margin and the most coronal point of bone-implant contact.An increase in the vertical distance between landmarks on consecutive radiographs was considered to be indicative of peri-implant marginal bone loss.All studies used radiographs as the baseline for the following radiographic evaluations.The minimum and maximum follow-up periods were 3 and 36 months following implant placement.
At the 3-month follow up, two studies (1,6) reported no difference in mean peri-implant bone loss between treatments performed with the AOT and standard protocols.Within 3 to 6 months after loading, results varied among studies (3,4,6).At 12 months, differences in peri-implant bone levels were observed between groups in two studies (2)(3)(4).Compared with provisional implants, definitive abutments resulted in less bone loss.Canullo et al. (1) reported similar results during long-term follow up (18 and 36 months; Table 2).

Meta-Analysis
The five studies were included in the meta-analysis for the evaluation of peri-implant vertical bone loss.There was less peri-implant vertical bone loss at implants using an AOT protocol than at implants using AR protocol (Weighted mean difference [WMD] -0.19, 95% CI -0.26 to -0.13; p<0.0001; random-effects model).The subgroup analysis showed no differences in the bone loss at the first 6 months of follow-up.However, WMD of peri-implant vertical bone loss between AOT and AR protocols was found statistically significant at 6 months <t≤12 months (WMD -0.40, 95% CI -0.53 to -0.27; p<0.0001; random effects model) and 1 year <t≤3 years (WMD -0.16, 95% CI -0.27 to -0.05; p=0.004; random effects model).The test of heterogeneity among all studies showed heterogeneity (p<0.0001,I2=96%), as well as the test for subgroup differences (inconsistency across the subgroups) (p<0.0001,I2=87.4%) (Fig. 3).

Unclear
The one abutment at one time concept Figure 3. Forest plot of mean difference of effects of AOT and AR protocols on peri-implant vertical bone loss.

Meta-Regression
The meta-regression analysis showed an increase of the WMD in the bone loss with the increase in the follow-up time, although not statistically significant (p=0.132;y=-0.0670-0.0145x).According to the prediction equation, it is expected an increase of approximately 0.2 mm in WMD for each year in follow-up time.Heterogeneity was not explained by meta-regression for follow-up time (adjusted R2=0%) (Fig. 4).

Discussion
In this systematic review, the PRISMA recommendations and Cochrane methods were used to evaluate the best evidence for the use of the AOT protocol as an option to limit bone loss after implant placement.Marginal bone loss associated with the immediate loading of platformswitched implant crowns using the AOT protocol was compared with that resulting from the use of the AR protocol.In the included studies, peri-implant bone level was evaluated for 258 implants (123 with definitive and 135 with provisional abutments).Follow-up periods ranged from 3 (1,6) to 36 (1) months.
Many factors can affect the peri-implant bone loss level, which is considered to be a criterion for the success of implant therapy (8).Hard-and soft-tissue remodeling may be related to implant preparation, soft tissue-inflammation and biomechanical factors (17).Following the first reports of the effects of abutment disconnection and reconnection on hard and soft tissues (9,18), RCTs confirmed that the AOT concept limits marginal bone loss in comparison with AR.
The studies included in this review showed variable degrees of peri-implant bone level change, partially due to the differences in the duration of follow up.Two studies reported no difference in peri-implant bone loss at 3 months (1,6), whereas two studies showed significant differences favoring the use of definitive abutments at 12 months (3,4).The measurement of horizontal and vertical marginal bone changes is considered important because inflammatory cell infiltrate at the implant-abutment junction affect bone remodeling in both directions (2,19,20).This meta-analysis showed that higher follow-up period was associated with less bone loss for one abutment at one time.The first 6 months did not show differences for both treatments.Thus, future studies should be done with longer (>1-year) follow-up.
It was found variation among the included studies in implant system used, clinician, surgical protocol, implant shoulder position, implant placement site, implant diameter, abutment type, and patients' periodontal biotype.Although gingival biotype may affect peri-implant remodeling, and thus the outcome of dental implant procedures (21,22), three studies did not describe patients' periodontal status before implant placement.Patients' tissue biotypes were assessed in two studies (1,6), but the correlation of this parameter with the outcome was not examined in one of these studies (1).Koutouzis et al. found no correlation between bone wall thickness and peri-implant mucosal height 6 months after implant placement involving two ARs, concluding that this treatment yielded results similar to those obtained with the use of definitive abutments (6).
This systematic review with meta-analysis has some limitations and the results should be interpreted with caution.The quality of evidence of the included studies was not optimal; no study had an overall low risk of bias.Standardization of peri-implant marginal bone level measurement was lacking, and only two-dimensional examination was performed in the included studies.Short follow-up periods and the use of different surgical The one abutment at one time concept protocols likely affected the results.Finally, this review was based on a small sample with considerable heterogeneity.Further randomized clinical studies involving the vertical and horizontal measurement of peri-implant bone levels, evaluation by three-dimensional cone-beam computed tomography, and longer follow-up periods are needed.
In conclusion, this review demonstrated that the use of the AOT protocol with platform-switched Morse implants results in less bone loss than do AR procedures, but this difference may not be clinically relevant.Thus, the preservation of marginal bone level achieved with the AOT protocol may not enhance aesthetics.

Figure 1 .
Figure 1.Flow diagram showing study selection for meta-analysis.

Figure 2 .
Figure 2. Risk of bias summary showing review authors' judgments about each risk of bias domain for each included study.

Figure 4 .
Figure 4. Scatter plot for the meta-regression of the association between the mean difference of the peri-implant vertical bone loss comparing the two protocols and the follow-up time in months.

Table 1 .
General characteristics and implant data of studies included in the systematic review Studies

Table 2 .
Peri-implant vertical bone loss according to follow-up time across studies