The Journal of Indian Prosthodontic Society

: 2020  |  Volume : 20  |  Issue : 3  |  Page : 255--268

A systematic review and meta-analysis of the attachments used in implant-supported overdentures

Sachin Haribhau Chaware, Smruti Tushar Thakkar 
 Department of Prosthodontics, MGV's KBH Dental College and Hospital, Nasik, Maharashtra, India

Correspondence Address:
Dr. Sachin Haribhau Chaware
Professor, Department of Prosthodontics, MGV's KBH Dental College and Hospital, Nashik, Maharashtra


Aim: To evaluate the survival rate, tissue response, and patient satisfaction of different attachments used in implant overdenture. Settings and Design: Systematic Review and Meta-analysis. Materials and Methods: Electronic search of peer-review articles published between 2001 and 2019 assessing the attachments used for implant-supported overdentures was done according to PRISMA Guidelies. The review evaluated sixteen articles related to survival of the attachments, the reaction of the soft and hard tissues along with repair and maintenance of the attachments, and overall performance of the overdenture attachments. Statistical Analysis Used: There is statistically significant heterogeneity (Q =374.7403, df = 15, and P < 0.0001). The statistics of fixed-effect model reported an MD of − 0.0880 (95% CI = −0.1536; 0.0225). Result: The review evaluated the 16 articles that met with the inclusion and search criteria. The studies were the combination of bar and ball attachments and their subtypes, magnetic and bar attachments, and locator in combination with other attachments. The meta-analysis of combined 16 studies reported acceptable heterogeneity among 16 studies (I 2 = 96%) and reported to be statistically significant (P < 0.01). Conclusion: The survival rate of attachments was in the range of 95.8%–97.5% for bar, 96.2%–100% for ball, 90%–92% for magnet and locator attachments were in the range of 97% after a mean follow-up period of 3 years. The bar attachments reported moderate tissue reaction in the form of mucosal changes, gingival inflammation, and bone resorption. The locator attachments require higher maintenance and repair. The magnetic attachments produce higher bone resorption and readily displace under functional force. Patient satisfaction and compliance was higher for ball, locator, and bar attachments as well as low for magnetic attachment. Thus, the ball and locator attachments excellently perform in terms of survival rate, tissue response, and patient satisfaction.

How to cite this article:
Chaware SH, Thakkar ST. A systematic review and meta-analysis of the attachments used in implant-supported overdentures.J Indian Prosthodont Soc 2020;20:255-268

How to cite this URL:
Chaware SH, Thakkar ST. A systematic review and meta-analysis of the attachments used in implant-supported overdentures. J Indian Prosthodont Soc [serial online] 2020 [cited 2020 Aug 13 ];20:255-268
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Full Text


Implant-supported attachment overdenture is the most advanced treatment as the attachments secure the retention and improve the masticatory efficiency and overall performance of the complete denture.[1],[2],[3]

Tooth-supported dentures have certain inaccuracies due to the strategic location of the abutment tooth, hard and soft tissues supporting the abutment, angulation of the abutment teeth, and presence of the existing caries and attachment loss and type of maxillomandibular relation. The selection of attachments is totally dependent on the support and angulation of the abutment.[4],[5] However, tooth-supported denture still provide better support in the guarded prognosis case as it has its periodontal proprioception.[6] In case of implant-supported overdenture, the implant provides rigid and stable support to the overdenture attachment. The location, selection, angulation, and placement are not significant problems as there are wide varieties of attachments available in relation to the width, diameter, and length of the implant. The implant-supported overdenture has a scope of specific selection and customization of the attachments in relation to the ridge contour and the type of force (axial and/or nonaxial force).[2],[3]

Retrospectively, the attachment concept was in practice since the 1960s, with the bar and ball attachments being the first ones and were proven both clinically predictable and effective by Mericske-Stern et al. in 1997.[7] The locator attachments were introduced in 2001 (Zest Anchors, Inc., homepage, Escondido, CA, USA).[8] Progressively, attachments were much popular due to their inherent ability to enhance retention and stability of the complete denture.[9] The absolute retentive capacity of overdenture attachments was reported by Wismeijer et al. (1999)[10] and Epstein et al.[11] The resiliency of the attachments in relation to stress dissipation was suggested by Leung and Preiskel.[12] Petropoulos et al.[13] defined the “release period” as the time required for the attachment system to lose retention or disengage from the abutment during forced separation, indicating the clinical significance of retention and stability of the prosthesis under function. According to retentive means, the attachments can be classified into 1) frictional, 2) mechanical, 3)frictional-mechanical and 4) magnetic attachments.[14] They can be rigid or resilient based on the type of movement.

There are several attachments used for implant-supported overdenture, which includes studs, bar, bar with clip, bar with coping, recently “O” ring attachment, and locator attachment (Zest Anchors, Inc., homepage, Escondido, CA, USA). All attachments produce excellent retention, stability, and support; however, the survival is the most valuable criterion for the long-term prognosis, hence the follow-up is crucial to study the behavior of attachments under axial and nonaxial forces. Tissue response, peri-implant mucosal changes, bone resorption, and loss of attachment indirectly assess the survival of attachment. Eventual patient response in terms of compliance in placement and removal of the prosthesis, oral hygiene, and overall satisfaction are the most significant criteria for evaluation of the success of the attachments.

Eventually, the success of the overdenture attachment depends on the selection of the attachment-related factors that simulate the clinical situation. The factors that govern the selection of attachment are based on proper diagnosis of intraoral structures and various factors such as bone type, inter-arch space, cost-effectiveness, amount of retention needed, expected level of oral hygiene, amount of available bone, patient's social status, patient's expectation, maxilla–mandibular relationship, inter-implant distance, and status of the antagonistic jaw.[15] The primary objective of the systematic review and meta-analysis is to study the survival rate, tissue response, and patient satisfaction of different attachments used in implant overdenture.


This systematic review was designed according to the guidelines of the Preferred Reporting Items for Systematic Review and Meta-Analysis (PRISMA) statement.[16],[17]

Study protocol

Before the start of the systematic review, a protocol was developed and registered (Prospero ID: 163013) aiming to answer the P.I.C.O. question that rendered the following P.I.C.O. definitions:

Population: Patients, older than 18 years and in good general health, with complete edentulism requiring placement of implant-supported overdentures in either one or both the archesInterventions: Any attachment designed for implant-supported overdentures. The attachments used were ball, bar, ball-bar, magnetic, locator, ERA, Dalla Bona, conus, and combinations of various attachmentsComparison: Attachments were compared with the objective of evaluating various attachments and their combinations on the basis of survival rate, tissue response, and patient satisfactionOutcomes: Primary outcomes was the survival rate of the attachments with various follow-up periods, maintenance visits by the patients, tissue response evaluated both clinically as well as radiographically, patient satisfaction using Visual Analog Scales (VAS) whereas Retention, stability, support were the additional outcomes.

Study design

A combination ofin vivo studies involving either or both the arches, comparative prospective and retrospective case–control studies, and randomized clinical trials were included.

Exclusion criteria

Review articles on the topicStudies with partially edentulous subjects with implant supported prosthesisPreclinical studies in animal modelsIn vitro studiesArticles published in a different language than English.

Search strategy

Three electronic databases were used as sources in the search for studies satisfying the inclusion criteria: (a) The National Library of Medicine (MEDLINE via Pubmed); (b) Google Scholar; and (c) Cochrane library. These databases were searched for studies published until June 2019. Keywords used were: Overdenture, implant supported denture, precision attachment denture, and dental implant. Two independent investigators performed the study selection. Data extraction was performed independently by two investigators, and duplicates were removed using Mendeley and thereafter discussed to find an agreement. All reference lists of the selected studies were checked for cross-references. In addition, the following journals were hand-searched from the years 2001 to 2019: Journal of Prosthetic Dentistry, Journal of British Dental Association, Journal of Oral Implantology, The International Journal of Oral and Maxillofacial Implants, The International Journal of Prosthodontics, and Journal of Advanced Prosthodontics. Related articles were identified from the existing reviews and study design (networking meta-analysis) PICOS framework [Table 1].{Table 1}


The initial phase is the primary screening of the identified articles. Due to the broad nature of the initial search, references were further filtered according to title, abstract, and keyword. Following the initial search, a reference list of the retrieved articles was manually searched for additional articles that met the inclusion criteria. Additionally, authors and keywords were searched again in Google Scholar to ensure that all relevant articles were uncovered.

Inclusion criteria

Peer-reviewed articles published between January 2001 and June 2019Studies that discuss the use of attachments in implant-supported overdentures in relation to survival rates of various attachments for varied follow-up periodsStudies that discuss the comparison between tissue response considering the clinical and radiographic findingsStudies that discuss the patient compliance and satisfaction with various attachment retained implant-supported overdentures.

Quality assessment

The risk of bias was assessed using the Cochrane Collaboration tool. The selected articles were assessed by the first author, and any variant view of selected articles was further assessed by the second author. The randomized controlled trial studies were evaluated using the following domains: random sequence generation, allocation concealment, blinding of the participant and personal blinding of the outcome assessment, incomplete outcome data, selective outcome reporting, and other bias. The studies were rated further as a risk of bias (low, medium, and high) by the reviewers.

Data management

Data extraction was independently done by two reviewers using the specific format. The specific information was as follows: year of publication, baseline data, study population, diagnostic tool, various available attachments, and follow-up period.

Tools for measuring outcomes: 1)clinical performance was evaluated by scanning electron-microscope, hardness and elastic modulus of attachment systems; 2) fatigue resistance of attachments was measured by strain gauge; 3) Clinical and radiographic evaluation of peri-implant parameters include plaque and bleeding on probing index and marginal bone loss; 4) cumulative survival rates on overall performance and 5)Questionnaires to assess patient satisfaction (OHIP (Oral Health Impact Profile) and VAS (Visual Analog Scale)).

The outcomes were presented for relevant studies in a graphical format where possible. The studies were graphed according to the mean difference (MD) with the level of significance at P < 0.01. In the meta-analysis, heterogeneity was measured as a final calculation of effect size and the confidence interval (CI) around that effect size was calculated by using random-effects and fixed-effects models in the forest plot.

Data analysis

The statistical heterogeneity among studies was assessed using the Q test based on Chi-square statistics (Cochran, 1954) as well as the I2 index [18] in order to know the percentage of variation in the global estimate that was attributable to heterogeneity (I2 = 25%: low; I2 = 50%: moderate; and I2 = 75%: high heterogeneity).

To summarize and compare studies, the mean values of primary (the survival rate of the attachments with various follow-up periods and the requirement of maintenance visits by the patients, tissue response evaluated both clinically and radiographically, and patient satisfaction using VAS) and secondary quantitative outcomes (retention, stability, and support) were directly pooled and analyzed with weighted MDs and 95% CIs. In the case of dichotomous outcome (exposure events), the estimates of the effect were expressed in risk ratios and 95% CIs. Study-specific estimates were pooled with both the fixed- and random-effect models. The publication bias was evaluated using the Begg's and Egger's tests for small-study effects for the main outcome variable.


Study characteristics

The review identified 1360 articles. A review of 89 full-text articles identified 45 articles for qualitative synthesis and 28 articles that met the inclusion criteria, search criteria, and confidence in implant-supported overdenture cases [Figure 1]. A total of 44 articles were excluded from the study with subsequent reason: the studies that were not peer reviewed and the studies that show very less information about the attachment in relation to retentive force, survival under complex axial or nonaxial force, simplicity in use, and outcome of the attachment (inter-reader agreement = 93.02%; kappa = 0.85, P < 0.001; 95%CI: 0.77; 0.98). The articles were distributed according to (1) The clinical and radiographic evaluation of the attachments used in implant-supported overdentures and randomized clinical trials. Finally, a total of 16 studies were evaluated for meta-analysis, out of that six are prospective case–control or comparative analysis, one is retrospective comparative analysis, and nine are randomized clinical trials.{Figure 1}

Quality assessment of the included studies

In [Table 2], the results of the quality assessment are summarized. Six studies of the included studies were designed as randomised controlled trials (RCTs). Three studies had a crossover clinical trial design. Five were prospective case–control studies, one was a comparative analysis, and another one was a retrospective case–control study. In addition, follow-up data were provided in all the 16 studies. The full checklist (Cochrane Collaboration's tool for assessing the risk of bias) was applied for RCTs and clinical crossover trials. Six studies were considered as low and three as unclear risk of bias.{Table 2}

Included studies

The 16 publications meeting the inclusion criteria described randomized controlled trials (n = 6), crossover clinical trials (n = 3), prospective case–controls (n = 5), comparative analysis (n = 1), and retrospective case–control (n = 1). Publication dates ranged between 2001 and 2019. Overall, 917 patients with a mean age of 62.15 years had been treated with 2390 dental implants. The dropout rate ranged between 0% and 20.6%, resulting in a total number of 734 patients providing data for the primary outcome. The observation period ranged between 3 months and 10 years. 43 implants were lost, resulting in a cumulative implant survival rate of 98.2%.


The meta-analysis as shown in [Figure 2], [Figure 3], [Figure 4], [Figure 5] was performed by combining the results of 16 studies by using fixed- and random-effect meta-analysis, with an overall acceptable heterogeneity among the studies (I2 = 96%). The result of various attachments used in implant-supported overdenture reported statistically significant heterogeneity (Q = 374.7403, df = 15, and P < 0.0001). The statistics of fixed-effect model reported an MD of − 0.0880 (95% CI = −0.1536; −0.0225). The random-effect model reported an MD of − 0.1440 (95% CI − 0.8126–0.5247) [Figure 2], [Figure 3], [Figure 4], [Figure 5]. The combined results of 16 studies reported to be statistically significant (P < 0.01). The studies related to ball attachments show significant values of retention, survival, and overall performance and the bar attachments produce moderate tissue changes and bone resorption. The locator attachments require constant maintenance and repair. The magnetic attachments reported higher bone resorption under functional force. The evaluation of the funnel plot as per [Graph 1] reported no asymmetry as only a small amount of studies are included, publication bias is unlikely to be present. Summarization of all the studies included is given in [Table 3] and outcome is presented in [Graph 2].{Figure 2}{Figure 3}{Figure 4}{Figure 5}{Table 3}[INLINE:1][INLINE:2]


This systematic review and meta-analysis identified 16 studies that gave maximum comparable outcome and follow-up results, out of which nine were randomized and crossover clinical trials. The most commonly used attachments in all the studies were the ball attachments followed by bar, locator, magnet, conus and equator, and various combinations.

Twelve studies reported the significance of ball attachment. The mean bone loss in the 1st year for ball group (0.51 ± 0.20) was comparable with magnetic attachment.[19] The larger diameter ball attachment reported less replacement than locator attachment.[20] The combined effect of ball and locator attachment results in higher retention that surely provides better quality of life.[21] A randomized crossover clinical trial stated that the overall mean retention was higher for retentive anchors than locator attachments (difference of 5.0 N, 95% CI: 2.5–7.6; P = 0.0005); the patients preferred the attachments that were delivered the last.[22] Patient satisfaction was 64% for the ball and 100% for the conus-retained implant overdentures.[23] The probing depth for ball attachments was quite shallower than that of the bar group and showed excellent peri-implant tissue response as reported by a crossover clinical trial.[24] Eight studies evaluated and compared the significance of bar attachments. The bar attachment has an excellent 5-year survival rate, i.e., 94.2%.[25] However, the bar attachment reported mucosal changes, gingival inflammation, and bone resorption under functional load.[26],[27] Plaque index was significantly high in magnetic group initially, but after 18 months, bar–supported group showed significant increase in gingival inflammation. The follow-up repair and maintenance of the bar attachment was significantly low (0.8/individual).[28] Survival was 89.1% for the bar and 93.5% for locator groups (marginal bone loss: 6 mm).[29] Implant survival was 96.7% in the locator group and 97.9% in the bar group.[30] Thus, the bar group was more vulnerable than the ball group with respect to maintaining peri-implant tissue health as compared to all the attachments. There are seven studies reporting the significance of locator attachment. One year observational study that evaluated the wear pattern of locator attachments using scanning electron microscope reported 16 replacements of locator housing and 34 prosthetic complications.[20],[31] Locator attachments reported a 97.7% success rate and 11% incidence of replacement of locator male among 50 subjects in 2 years of treatment period.[32] Marginal bone loss was estimated at 0.58 ± 0.71 after 1 year and up to 6 mm after 2 years for locators and 0.31 ± 0.47 after 1 year and up to 10 mm after 2 years for bar group.[29],[30] The two studies reported the significance of magnetic attachment. The magnetic attachment reported more plaque retention, high bone resorption under functional force, and high displacement in all directions as compared to bar attachment.[27]

Out of the total nine randomized and crossover clinical trials included in the review, eight studies compared ball attachments with other attachments such as bar, magnet, locators, conus, and equators and reported that ball attachments require 6.7 repairs in 3 years,[28] has high maintenance cost,[19] show shallower probing depths even after 10 years of use [24] and a modified plaque index of the (0.39 ± 0.39) lowest in 3 years.[23] Thus, the ball attachments fair well with the highest retention values and lowest peri-implant inflammation but high maintenance visits.[33],[34] One of the randomized controlled trials compared ball and locator, of which bar group showed lower marginal bone loss and a slightly higher survival rate as compared to locators.[30] Extensive studies should be done with a long-term follow-up and maintenance data for the new emerging locator attachments as they can be a breakthrough treatment of choice.

The meta-analysis of the four studies reported the survival rate of maxillary implant-supported overdenture.[25],[26],[30],[33] Mericske Stern et al. reported that there is minimum requirement of 4–6 implants to achieve 94.2% of cumulative survival rate with 5-year follow-up period.[25] In regards of maxillary arch, there is a definite requirement of 4–6 implants along with bar attachment [26],[30],[33] in order to distributes the stresses more on multiple attachment, thereby it improves the cross-arch stabilization for maxillary overdenture.[25] However, use of fewer implant (< 4 implants) increases the peri-implant mucosal changes and patient compliance in terms of oral hygiene maintenance are potent risk factors for failure of maxillary implant-supported overdentures.[33],[35],[36] The fixed restoration is more logical treatment of choice than overdenture when the situation dictates the use of 4–6 implants for maxillary overdenture,[26] as addition of two or more implant or use of zygomatic implant or angular implant favors fixed restorations than overdenture. The current scenario of zygomatic implant. The current scenario of zygomatic implant with anterior two implants or zygomatic Quad implant reported a survival rate of 97.4% over 7 years for fixed restorations.[37] Similarly, combinations of axial and angular implant showed comparable survival rates. The angled implants had an overall success rate of 96.5% and the straight implants had a 97% success rate for 1–12 years with fixed restorations.[38] The current concept of 6–8 short implant (4–6 mm length, depends on the availability of space) reported similar survival rate (pooled cumulative survival rate of 99.060% for 1845 implants after a mean follow-up of 3.2 ± 1.7 in posterior maxilla)[39] to long conventional implant with augmentation of bone along with lateral sinus floor elevation. The advantage of short implant is that it completely eliminates the donor site morbidity as no grafting procedure is required, which is used for the traditional conventional implant. The use of short implant reduces the treatment time as grafting procedure of autogenous bone graft needs minimum 6 months to 1-year healing time before the placement of conventional long implant, and there is no contact of removable prosthesis to grafting site during healing time.[39] Hence, it can be concluded that fixed restoration is the logical choice instead of overdenture for the rehabilitation of maxillary arch. However, the cost of the zygomatic implant placement or conventional implant with bone grafting is higher than that of the implant-supported overdenture, but with the use of short or angular implant, the cost of the fixed restorations is almost equal to the combined cost of implant, attachment, and the denture.[37],[40],[41] In the similar situation of mandibular arch, only two to four implants with adequate attachment are sufficient to provide support, stability, and retention for mandibular denture provided the support of the residual ridge is sufficient in terms of height and width. Bhargava et al. proposed the classification for mandibular implant-supported overdenture on the basis of inter-foramina distance (IFD) and available vertical restorative space (AVRS).[42] The practical advantage of the classification is the safe distance for implant placement and attachment selection. The classification is divided into three classes, i.e., Class I, Class II, and Class III. Class I: the Class I has two subtypes of class Ia (IFD >40 mm and AVRS >14 mm) and Class Ib (IFD 30–40 mm and AVRS >14 mm). The class Ia advises five or fewer implants with bar and clip attachment or bar and locator attachment and Class Ib advises four or fewer implants with bar and clip or ball and locator attachment. Class II: the Class II has similar subtypes such as Class II: Class IIa (IFD >30 mm and AVRS 8–14 mm) and Class IIb (IFD <30 mm and AVRS 8–14 mm). The Class IIa advises four implants with ball and locators/Dalla-Bona attachment's and Class IIb advises two implants with ball and stud type of attachment. Class III: the Class III has again two subtypes. Class IIIa (IFD >30 mm and AVRS 6–8 mm) and Class IIIb (IFD <30 mm and AVRS 6–8 mm). Class IIIa advises four implants with ball or locator attachment's and Class IIIb advises two implants with locator attachments.[42] Cost-wise mandibular implant-supported overdenture is more economical than fixed restorations as compared to maxillary overdenture.[30],[40],[41] However, there are few listed prosthetic complications associated with implant-supported mandibular overdenture, which are: overdenture loss of retention or adjustment (30%), overdenture rebasing or relining (19%), clip or attachment fracture (17%), overdenture fracture (12%), opposing prosthesis fracture (12%), acrylic resin base fracture (7%), abutment screw loosening (4%), abutment screw fracture (2%), and implant fracture (1%).[43] Recently, there are various emerging attachment systems like Kerators provide higher degree retention and patient satisfaction [44] but with more mucosal changes.[45] The rate of retention loss in attachments was higher in attachments with plastic parts within their components, rather than those totally made up of noble metals.[46] Further Randomised Controlled trials are required for evaluation of newer attachment systems in terms of retention loss and mucosal changes.[47]


The implant-supported overdenture is one of the economic options as compared to fixed implant prosthodontics as it not only secures the retention and stability, but also significantly increases the masticatory efficiency, survival rate, and patient satisfaction. The meta-analysis evaluated the various attachments by combining the results of 16 studies and suggests that ball attachments give excellent outcome when the number of implants is less and if more than two implants are placed, bar or locator attachments give better outcome but require extensive follow-ups. Similarly, in maxillary arch splinted bar attachments [26] are favorable and in mandibular arch, un-splinted ball or locators are favorable. Thus, according to the clinical situation, AVRS, and the quality of hard and soft tissues, the attachments and their combinations have to be selected.[42] Ball and locator attachments show excellent survival, favorable tissue response, and efficient patient satisfaction for mandibular overdentures. Excluding the cost factor, implant-supported fixed prosthesis is the ideal treatment alternative as compared to implant-supported overdentures in maxillary arch.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.


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