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Success and survival rates of mandibular overdentures supported by two or four implants: a systematic review.


The use of implant-retained dentures has been an alternative for treating the edentulous mandible, since it allows fixation of the prosthesis to the edentulous ridge. (1) This has encouraged dental professionals to display and perform prostheses, such as overdentures retained by implants, as an option to solve the problem of edentulous patients. Treating patients by implant placement is a popular option for restoring function and esthetics, (2) and for improving oral function, masticatory efficiency and individual satisfaction. (3)

There is still some debate regarding the number of implants required to properly support dentures. Studies have shown good results for oral rehabilitation with implant-retained prostheses; however, overdentures with 2 and 4 implants must be compared in the long term, with respect to clinical characteristics, patient satisfaction and ease of care. (4)

In addition to the success rate of osseointegrated implants supporting dental prostheses, the prosthesis itself should also be taken into consideration when determining the overall success of the rehabilitation treatment. (5) Thus, the goal of this systematic review was to demonstrate the variations in success or survival rates of overdentures when 2 or 4 implants are used to treat the edentulous jaw.


Search strategy

The search was conducted by 2 reviewers, independently, using electronic databases (Pubmed/ Medline, Embase, Cochrane), and was limited to clinical trials reporting the success of mandibular overdentures on 2 or 4 implants. A broad search was undertaken from January 1980 to January 2013 to identify the studies by title and abstract. The terms "overdenture," "implant-retained," "implant-supported," "success," "dental prosthesis," "complete mandibular denture" and "prosthetic outcome" were combined using "and" or "or," and a total of 967 studies were identified for initial screening.

The titles and abstracts of all reports identified through the electronic searches were assessed by 2 reviewers, and both the inclusion criteria (publications in English, follow-up of at least one year, mandibular overdentures, success rates, survival rates, failure rates or prosthesis function rate of overdentures on two or four implants, and ball and/or bar attachments) and the exclusion criteria (laboratory studies, case reports, and cross-sectional data) were applied. The literature search yielded a total of 967 citations, 951 of which were excluded because they did not meet the inclusion criteria. Any disagreement between the reviewers in the search process was resolved either by reaching a consensus or by including the reference for subsequent review. Hand-searching of the selected journals, as well as searching the references of the selected studies was also performed at this point to include any paper not found in the electronic search. Finally, full-text copies of the remaining 16 studies (Tables 1 and 2) were obtained for a more detailed evaluation. The reasons for excluding another 5 studies are listed in Table 3.

The final studies that were included after passing the second phase of the review process were classified according to the quality assessment of pre-established characteristics, in order to document the methodological strength of each paper. (6) The variables investigated in the quality assessment are listed in Table 4. The sample size was considered adequate when the sample size calculation was presented. Measurement methods were considered valid when a measurement error test was presented. Each study was classified based on the following scores:

* low (0-5 points),

* medium (6-8 points) or

* high (9-10 points).


The database search initially yielded 967 titles. Sixteen studies (7-22) were evaluated initially, but only 11 met the inclusion criteria and were ultimately selected for analysis (7-17) (Figure 1). Methodological issues such as sample size, follow-up, attachment system, quality of assessment and results for implants and prostheses were described (Tables 1 and 2). Studies with unclear methodological issues, such as reports with a different number of implants, were excluded (18-22) (Table 3).

This systematic review included randomized controlled trials, (16) non-randomized controlled trials, (9-12) prospective studies (7,8,13,15,17) and retrospective studies. (14) Randomized clinical trials (RCT) present a better evaluation of treatment results when adequate methods are used. However, since RCTs were uncommon for the topic researched, the clinical guidance provided to dental clinicians would be limited if this review were based solely on RCTs. Thus, it was decided that other study designs of overdentures retained by 2 or 4 implants should be included.

Only one study (10) showed a medium degree of quality and methodological consistency (Table 1); most showed a low degree, and none showed a high degree. The sample size, statistical analysis and confounding factors were the most significant flaws.

The main results evaluated in the studies with overdentures (OD) focused on the influence of the number of implants on OD success. Several data were evaluated, including implant success and survival rates, prosthesis failure and functional rates. Five studies described overdenture performance with 2 implants, (7,9-12) and five reported the use of 4 implants. (13-17) One study (8) showed results for both 2 and 4 implants. In regard to the attachment system, the ball attachment was the most commonly used system for 2 implants, and bars, for 4 implants.

The prosthesis survival rates for 2 implants ranged from 92%10 to 100%. (12) However, when the protocol described by Payne et al. (23) was used to define success rate based on different criteria (success, survival, unknown, loss, retreatment by relining or replacement), the rate ranged from 23%9 to 100%. (12) For other authors, (8,13,14,16,17) individual criteria were used to define the outcomes. As can be observed, these individual criteria are reflected in the results. Authors who used the criteria successfully were stricter and sought to follow predetermined evaluation criteria. In contrast, those who used survival rates, considered only the prostheses that remained in use, despite the fact that complications may have occurred.

High survival rates were observed in the studies with four implants. The results were based on personal evaluation and criteria established by the authors regarding prosthesis function (8,13,15-17) or failures. (14)


The parameters for prosthetic success showed great variability among the studies as a result of different methodologies and criteria. This becomes a challenge for significant synthesis and comparison of results. The authors used the terms success and survival as two independent characteristics. Success was defined as rehabilitation that remained unchanged, not requiring any intervention during the entire observation period, and that proceeded under ideal conditions. Survival was defined as the rehabilitation remaining in situ with or without modification and clinical management over the observation period, (24) even when repairs were made. Thus special attention had to be given when comparing the results of each parameter.

The different results were a consequence of different sample sizes, follow-up periods, attachment systems and complications. An adequate evaluation should take into consideration all of these aspects to show the prosthetic success in general, and any conclusions should be addressed carefully before applying them to dental practice.

Considering that randomized, controlled clinical trials present excellent quality, further evaluation should be conducted based on this type of study, to compare appropriate sample sizes with 2 and 4 implants and different attachment systems. It is important to highlight that any unexpected complications requiring clinical intervention should be classified as treatment complications and described in the clinical trials (prospective and retrospective), seeing that a trend was observed to report only positive results.

There were wide-ranging results for overdentures retained by 2 implants based on pre-established criteria. (23) The only study with a 100% success rate was a short-term follow-up, (12) and, most probably, the evaluation period was not long enough to show any complications. Although some authors suggested that failures usually occur during the first year and decrease over time, (9-11,25) medium- and long-term follow-ups are recommended to assess prosthesis performance, (8) in order to determine the real trend.

A minimum of 2 and up to 4 implants are required for rehabilitation with overdentures. (26) Some authors concluded that 2 implants are enough and presented this option as a low-cost alternative. (4,12) However, this alternative has some limitations and a number of complications related to the prosthetic components, (9-11) and also to non-axial load transfer, posterior bone resorption and prosthesis rotation (27) The protocol with 4 implants showed better results and optimum survival rates in four studies. (13,15-17) In two other studies, (8,14) the long-term follow-up results may justify these rates. Four implants provide better stability, and avoid both movement and excessive loading, which may compromise osseointegration. (14)

The attachment system plays an active role in prosthesis success rates, reflected in prosthetic maintenance and complications. Ball attachments revealed complications, such as being time-consuming and incurring costs related to wearing and loosening of matrix and patrix components that had to be changed. They also incur movement in different directions, resulting in damage and ring wear. (9-11) This kind of attachment is used basically with two implants. In contrast, the bar attachment showed better results for rehabilitation with two and four implants. Some studies suggested that implants splinted with bars are a favorable alternative to minimize rotational movement and provide vertical load transfer, (14,16) and to avoid implant micromovement and prosthetic repairs. (28) Bearing this in mind, it is important that these complications be recorded when analyzing prosthesis maintenance during follow-ups.

Walton (10) evaluated the success rate based on pre-established criteria (21) to assess any difference in prosthetic maintenance between ball and bar attachments. After a 3-year follow-up, the bar-clip system exhibited a statistically significant difference in its success rate. The ball attachment required 5 times more repair than the bar system, thus lowering its success rate to 23%.

Other factors are also important in analyzing the results, such as maintenance period, types of complications and failure, time and type of repair, complexity of procedures, and prosthesis design. Although it is difficult to predict the complications, these factors should be considered during treatment planning and be informed to the patient, since they will influence treatment success.

Not only is the success of implants and prosthesis at issue, but the occurrence of complications that must be considered in evaluating the treatment. Biological failures, costs and patient satisfaction regarding function and esthetics should also be included in the final results.

Few studies showed that mandibular overdentures retained by 4 implants are an adequate alternative for an edentulous mandible, based on better success rates and prostheses survival, especially with the bar attachment system.


Given the limitations of this review, mandibular overdentures with 4 implants showed better results with respect to survival and success rates, especially those with a bar connection. Further studies comparing these two treatment types are necessary to improve the scientific evidence in this area. With this in mind, long-term, randomized and appropriately designed clinical studies are recommended.


The authors would like to thank Danilo Gonzaga for his critical comments and suggestions.


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Isabelle de Sousa Dantas

Mariana Barbosa Camara de Souza

Maria Helena de Siqueira Torres Morais

Adriana da Fonte Porto Carreiro

Gustavo Augusto Seabra Barbosa

Department of Dentistry, Universidade Federal do Rio Grande do Norte--UFRN, Natal, RN, Brazil.

Declaration of Interests: The authors certify that they have no commercial or associative interest that represents a conflict of interest in connection with the manuscript.

Corresponding Author:

Isabelle de Sousa Dantas


Submitted: May 13, 2013

Accepted for publication: Sep 28, 2013

Last revision: Oct 14, 2013

Table 1--Results of studies with two implants.

Authors             Sample      Number of   Follow-up
(year)                          implants     (years)

Naert et              43            2          2.5
al. [7] (1988)

Ferrigno et           84            2          10
al. [8] (2001)

Watson et             72            2           1
al. [9] (2002)

Walton [10]       Group 1: 50       2          2-4
(2003)            Group 2: 50

Mackie et             106           2           5
al. [11] (2011)

De Kok et             10            2           1
al. [12] (2011)

                                  Rate for implants

Authors           Attachment     Survival     Success

Naert et             Bar            -          97.7%
al. [7] (1988)

Ferrigno et          Ball      97.1% (2 and      -
al. [8] (2001)                 4 implants)

Watson et            Ball           -
al. [9] (2002)

Walton [10]          Ball           -
(2003)               Bar

Mackie et            Ball           -
al. [11] (2011)

De Kok et            Ball          100%          -
al. [12] (2011)

                  Rate for prostheses

Authors           Survival   Success    Quality
(year)                                 evaluation

Naert et             -        97.7%       Low
al. [7] (1988)

Ferrigno et        98.8%        -         Low
al. [8] (2001)

Watson et          98.6%     83.3% *      Low
al. [9] (2002)

Walton [10]         92%       23% *      Medium
?(2003)             96%       63% *

Mackie et          96.84%    58.5% *      Low
al. [11] (2011)

De Kok et           100%     100% *       Low
al. [12] (2011)

* Six-field protocol by Payne et al. [23] (2001).

Table 2--Results of studies with four implants.

Authors (year)    Sample   Number of   Follow-up
                           implants     (years)

Donatsky [13]       19         4           1

Chiapasco et       194         4         2-13
al. [14] (1997)

Makkonnen et        15         4           5
al. [15] (1997)

Ferrigno et         44         4          10
al. [8] (2002)

Romeo et            20         4           2
al. [16] (2002)

Heschl et           32         4           5
al. [17] (2011)

                                     Rate for implants

Authors (year)    Attachment     Survival            Success

Donatsky [13]        Ball           -                  98%

Chiapasco et         Bar            -                 96.9%
al. [14] (1997)

Makkonnen et         Bar          97.4%                 -
al. [15] (1997)

Ferrigno et          Bar       97.1% (2 and             -
al. [8] (2002)                 4 implants)

Romeo et             Bar            -           100% (Immediate
al. [16] (2002)                                  loading) 97.5%
                                              (Conventional loading)

Heschl et            Bar            -                 98.6%
al. [17] (2011)

                  Rate for prostheses

Authors (year)    Survival   Success    Quality

Donatsky [13]       100%        -         Low

Chiapasco et       98.5%        -         Low
al. [14] (1997)

Makkonnen et        100%        -         Low
al. [15] (1997)

Ferrigno et        97.7%        -         Low
al. [8] (2002)

Romeo et             -        100%        Low
al. [16] (2002)

Heschl et           100%        -         Low
al. [17] (2011)

Table 3--Studies excluded and reason why.

Authors (year)               Exclusion criteria

Adell et al. [18] (1990)     A mean of 6 implants was used

Johns et al. [19] (1992)     A maximum of 4 implants were
Hutton et al. [10] (1995)    placed but only 2 were connected to
Jemt et al. [21] (1996)      the abutments. However, one of the
                             sleepers was activated at some point
                             of the treatment. Thus, 3 implants
                             were evaluated

Fartash et al. [22] (1996)   Individual assessment of success rate
                             for 2 or 4 implants was not reported

Table 4--Characteristics and scores used for quality analysis
of selected papers.

Characteristic                                      Score

Adequate study design: randomized clinical trial,
prospective study, controlled clinical trial,         3
longitudinal study

Clinical trial                                        1

Adequate sample size                                  1

Adequate description of selection process             1

Valid measurement methods                             1

Use of method of error analysis                       1

Blinded measurement evaluation                        1

Valid statistical methods                             1

Confounding factors included in analysis              1
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Author:Dantas, Isabelle de Sousa; de Souza, Mariana Barbosa Camara; Morais, Maria Helena de Siqueira Torres
Publication:Brazilian Oral Research
Article Type:Author abstract
Date:Jan 1, 2014
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