The authors concluded that maxillary overdenture supported by six dental implants connected with a bar was the most successful treatment for survival of both implants and overdenture. The authors' conclusion reflected the evidence presented, but several limitations in the review process make the reliability of the conclusions uncertain.

To assess the survival of implants and maxillary overdentures, and the condition of surrounding hard and soft tissues in patients treated with implant-supported maxillary overdentures.

MEDLINE, EMBASE and Cochrane Central Register of Controlled Trials (CENTRAL) were searched for studies published in English. Search dates spanned 1800 to August 2009. Search terms were reported. Reference lists of identified relevant reviews and studies were handsearched.

Randomised controlled trials (RCTs), cohort studies and case series that reported detailed information on maxillary overdentures supported by root-form endosseous implants were eligible for inclusion. In cases where data for implant-supported fixed full dentures were combined, only studies where it was possible to extract data for the overdenture were considered. Treatment of patients had to be initially planned for an overdenture. Studies had to enrol at least five patients with a follow-up period of at least one year. Eligible outcomes measured included survival of implants, survival of maxillary overdentures and condition of hard and soft tissues surrounding the implants.

The number of implants placed to support the overdenture varied (range was one to eight implants). Bone graft procedures were varied (for example, onlay block graft procedures, elevation of the floor of the maxillary sinus). Different implant systems were used; most were Branemark implants. Both splinted (bar) and unsplinted (ball) designs were used. Bar anchorage designs were used in most studies. The kind of opposing dentition was not described in most studies. Conditions of hard/soft tissues were assessed with different kinds of radiographs. Details of the included patients were not reported.

Two reviewers independently assessed the studies for inclusion. Disagreements were resolved by discussion with a third reviewer.

Two reviewers independently assessed study quality using specific study design related modified forms designed by the Dutch Cochrane Collaboration. No further details were reported. Disagreements were resolved by discussion with a third reviewer.

Two reviewers independently extracted data to enable calculation of event rates (ERs) and 95% confidence intervals (CIs). Disagreements were resolved by discussion with a third reviewer.

Where possible, weighted event rates and 95% CIs of outcomes were pooled using random-effects meta-analysis. The meta-analysis was stratified according to the number of implants (six or more implants and four or fewer implants) and anchorage design. Implant survival rates (SRs) were used to complement the event rates. Statistical heterogeneity was assessed using Cochran's Q test and I² statistic.

Thirty-one studies were included (n=796 patients, range five to 100 patients, n=3,116 implants). There were two RCTs and 29 case series. Follow-up periods ranged from 12 to 180 months. The results of the quality assessment were not reported.

Implant survival: Implant survival rate was 98.2% per year in case of six or more implants and a bar anchorage (ER 0.018, 95% CI 0.014 to 0.023, I²=38.6%; n=98 patients, seven studies). In cases of four or fewer implants and a bar anchorage, implant survival rate was 96.3% per person (ER 0.037, 95% CI 0.030 to 0.046, I²=51.2%; n=75 patients, four studies). In cases of four or fewer implants and a ball anchorage, implant survival rate was 95.2% per year (ER 0.048, 95% CI 0.032 to 0.070, I²=53.2%; n=55 patients, three studies).

Survival of maxillary overdentures: In cases of four or fewer implants and a bar anchorage, survival rate for overdenture loss was 96.5% per year (ER 0.035, 95% CI 0.022 to 0.069, I²=51.2%; n=75 patients, four studies). In cases of four or fewer implants and a ball anchorage, survival rate for overdenture loss was 88% (n=8 patients, one study).

Condition of surrounding hard and soft tissues: Loss of marginal bone and probing depth both varied. Loss of marginal bone was 0.23mm in 27 months to 2.45mm in 19 months (n=14 studies). Probing depth was 0.21mm to 3.6mm (n=six studies). Bleeding on probing was reported in four studies.

Maxillary overdenture supported by six dental implants connected with a bar was the most successful treatment for survival of both implants and overdenture. Treatment with four implants and a bar was the second-line option. Treatment with four or fewer implants and a ball attachment system was the least successful.

The review question was clear with regard to eligible interventions, study designs and outcome measures. Eligible patients were not clearly defined. Three databases were searched for articles published in English, which raised the possibility of language and publication biases. Review processes were conducted in duplicate, which minimised the risk of error and bias. A quality assessment tool was used (no further details or results were reported). The decision to combine study results using a meta-analysis may have not been justified given the presence of significant heterogeneity. The authors acknowledged the limitations of the few controlled trials, small sample sizes and likelihood of heterogeneity.

The authors' conclusion reflected the evidence presented, but several limitations in review processes (such as high risk of language bias, unclear quality of included studies and unjustified statistical analysis) make the reliability of the conclusions uncertain.

Practice: The authors stated that it was not wise to give a practical implication at the moment given the limited number of patients included in the meta-analysis and differences in the implant systems, surgical procedures and kinds of opposing dentition.

Research: The authors did not state any implications for research.

None stated.

This is a critical abstract of a systematic review that meets the criteria for inclusion on DARE. Each critical abstract contains a brief summary of the review methods, results and conclusions followed by a detailed critical assessment on the reliability of the review and the conclusions drawn.