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Limited evidence to guide implant prostheses retention system selection

Mohamed-Nur Abdallah BDS, MSc .


Systematic Review Conclusion

There is no evidence of differences in peri-implant marginal bone loss (PMBL) between cement- (CRP) and screw-retained prostheses (SRP) when used with external hexagonal connectors.

Critical Summary Assessment

A limited-quality systematic review suggests that selection between CRP and SRP for cylindrically threaded implants with external hexagonal connectors should not depend solely on possible PMBL.

Evidence Quality Rating

Limited Evidence

Structured Abstract

Clinical Questions:

Does PMBL differ between CRP and SRP when used with external hexagonal connectors?

Review Methods:

The authors searched four electronic databases and hand-searched four related journals up to June 2012. The authors included studies published in English that radiographically assessed PMBL after a period of at least 1 year between CRP and SRP with external hexagonal connections. Only randomized clinical trials (RCTs) or observational studies were included. Two independent reviewers selected the studies and extracted the data. In cases of disagreement, a third reviewer was consulted. For two of the selected studies, the corresponding authors were contacted to obtain missing information or unpublished data. A meta-analysis for each retention system was conducted. Random effects models were used to estimate the PMBL. Homogeneity of the studies was evaluated and publication bias was assessed. Implant success rate was defined as absence of individual implant mobility; absence of persistent and/or irreversible sign and symptoms of infection, pain or ongoing pathologic process suppuration; absence of a radiolucent area around the implant; and the vertical bone loss should be less than 0.2 mm following the implant’s first year of service. Implant survival was defined as the proportion of implants still in place at a certain time, even if they are of no clinical value or even cause side effects.

Main Results:

Only 9 studies were finally selected. From them, 3 evaluated only SRP, 4 assessed only CRP and 2 studies directly compared both retention systems. The study designs for CRP were 4 prospective clinical studies and 2 split-mouth RCTs with a total of 171 patients and 266 placed implants. SRP studies included 3 prospective studies and 2 split-mouth RCTs with a total of 113 patients and 352 placed implants. One of the studies that directly compared between both systems was a RCT, while the other was prospective clinical study. The follow up period varied among studies (12-49.2 months for CRP and 12–48 months for SRP). 5 studies of CRP used single-unit prostheses, whereas only one used both single and multiple-units. The surgical region varied between studies (mandible or maxilla). All CRP studies except one used a two-step surgical approach. Two SRP studies utilized single-unit prostheses and three used multiple units. Three SRP studies used a two-step surgical approach, while the other two used a one-step procedure. The success rate was 100% in two of the CRP studies and 100% survival rate was reported in the rest. In the SRP studies, two mentioned success rate (96.6 - 100%) and three reported the survival rate (92.7 – 100%). Pooled mean PMBL was 0.53 mm (CI 95%, 0.31-0.76 mm) and 0.89 (CI 95% 0.45-1.33 mm) for CRP and SRP, respectively. Indirect comparison between the two systems is not significant.


The evidence is inconclusive to suggest that one retention system causes more PMBL than the other; therefore, the selection of a prosthesis retention system should not be based only on possible PMBL.

Source of Funding:

There was no external funding and all authors were supported by their institutions.


Importance and Context:

Appropriate prosthetic restoration of implants is important for their long-term success. The choice between CRP versus SRP is a complex decision involving many factors. The review topic is important because currently there does not appear to be evidence that one method is clinically or biologically superior to the other. Currently the decision is based on the dentist's preference and patient's choice.(1)

Strengths and Weaknesses of the Systematic Review:

Although multiple databases were searched, only English-language studies were included. The authors did not attempt to search for grey literature, but they contacted the corresponding authors of two selected studies asking for missing information or unpublished data. In this review, the authors clearly stated the inclusion and exclusion criteria. Details of reasons to exclude studies were clear. The authors included tables clearly describing the characteristics of the nine selected studies. A third reviewer was consulted in cases of disagreement. The quality assessment of the selected studies was very limited and not well described. The authors did not discuss the findings of the two studies that directly compared the two retention systems. Two distinct meta-analyses (one for each retention system) were completed. The proposed meta-analysis approach could be questioned as different study types were combined.

Strengths and Weaknesses of the Evidence:

The evidence identified was limited. All studies except one clearly described the inclusion criteria. All selected studies evaluated the PMBL through periapical radiographs, but none reported sample size calculations. Statistical analysis was not performed in one study. Only two studies directly compared the two retention systems, and only one of those was an RCT. The authors performed two separate meta-analyses that indirectly compared the two systems. Tests for heterogeneity and publication bias were not statistically significant, but their power was limited because of the small number of studies. The authors included only one implant design with one type of connector (cylindrical threaded design and an external hexagonal connection), which decreased the heterogeneity and provided a more sound basis for the indirect comparison. However, this comparison might have been affected by the heterogeneity in the surgical procedure, surgical site, type of dental prosthesis and number of prosthetic units. Furthermore, the selected studies did not control for possible confounders that might affect PMBL. The implant success rate was well defined in three studies, whereas the rest stated the survival rate. Follow-up periods varied (12 - 49.2 months), but it was after at least one year of function in all studies. This helped in reducing the publication bias, since most of the PMBL occurs during the first year of function.(2) Three studies accounted for dropouts and the main reason was that patients left the area.

Implications for Dental Practice:

At this time the choice of oral implant design is still up to the dentist's and patient's preference.(1) Evaluation of PMBL is one of the criteria used to assess long-term success of oral implants.(3) There is little evidence to support the use of any of the evaluated implant retention system to diminish PMBL. However, PMBL can be affected by many other factors such as bone quality, surgical technique, implant loading, implant design and position, type of abutment system, and establishment of biological width.(4-7)

Critical Summary Publication Date:


These summaries are not intended to, and do not, express, imply, or summarize standards of care, but rather provide a concise reference for dentists to aid in understanding and applying evidence from the referenced systematic review in making clinically sound decisions as guided by their clinical judgment and by patient needs. American Dental Association © 2019