Single vs two implant-retained overdentures for edentulous mandibles: a systematic review

Single vs two implant-retained overdentures for edentulous mandibles: a systematic review

MENA Dental Talents

12. August 2019

Ahmed Yaseen Alqutaibi, Amal Kaddah, Marco Esposito, Radwan Algabri, Adnan Alfahad, Mohammed Farouk, Ali Alsourori

Purpose: To compare prosthesis and implant failure, patient satisfaction, prosthetic complications and peri-implant marginal bone loss of mandibular overdentures (IOD) supported by a single or two implants

Materials and methods: Manual and electronic database (PubMed and Cochrane) searches were performed to identify randomised controlled trials, without language restriction, comparing single vs two implant supported mandibular overdentures. Two investigators extracted data independently. The Cochrane tool was used for assessing the quality of included studies. Meta-analyses were performed for the included RCTs.

Results: Six publications corresponding to four RCTs were identified. Three RCTs (corresponding to five publications) were included and one trial was excluded. Follow-ups in function were 1, 3 and 5 years after loading. All included studies were considered to be at a high risk of bias. The pooled result revealed more prosthesis failures at overdentures supported by two implants at 1 year (three trials) (P = 0.02; Risk Difference: -0.12, 95% CI: -0.22, -0.02), however, there were non-significant differences at 3 years (two trials) (P = 0.22; Risk Difference: -0.32, 95% CI: -0.83, 0.19) and at 5 years (one trial) (P = 0.95; Risk Difference: 0.01, 95% CI: -0.22, 0.24). Regarding implant failures, there were more implant losses in overdentures supported by two implants at 1 year (three trials) (P = 0.02; Risk Difference: -0.12, 95% CI: -0.22, -0.02) and at 5 years (one trial) (P = 0.95; Risk Difference: -0.15, 95% CI: -0.28, -0.02), however, there were non-significant difference at 3 years (two trials) (P = 0.2; Risk Difference: -0.33, 95% CI: -0.84, 0.18). After five years in function, meta-analyses revealed that there were non-significant differences regarding overall prosthetic complications when mandibular overdentures supported by a single implant were compared with overdentures supported by two implants.

Conclusion: Mandibular overdentures retained by a single implant have comparable results to those retained by two implants. However, this should be interpreted with caution as all the included studies were considered at a high risk of bias.

Conflict of interest statement: None declared.

Introduction

The rehabilitation of completely edentulous mandibles with conventional dentures is associated with retention and stability problems1. These problems substantially decreased with the advent of implant-supported mandibular overdentures2-4.

Regrettably, implant-retained overdentures have a higher financial cost than conventional complete dentures5. One of the issues when restoring edentulous mandibles with implant-supported overdentures relates to the optimal number of implants needed to provide satisfactory stability and retention. However, this issue remains controversial. Some investigators have claimed that improvements in treatment outcomes can be achieved by increasing the number of implants supporting overdentures, but many studies have revealed that two implants are enough for clinical success6,7.

Although two consensus conferences concluded that a mandibular overdenture supported by two implants was the “minimum standard” of treatment for edentulous patients8, some believe that this standard ignores those who cannot afford two implants and implies that less than two implants is negligent care. However, the use of two implants could be outside the financial capacity of several compromised edentulous patients, especially in developing countries9,10. Indeed, there is no evidence for a single standard of care for edentulous mandibles with or without implants11, or a link between the number of implants and complications or satisfaction with overdentures12.

Geriatric patients wearing conventional dentures often have functional and psychological problems because of poor retention and/or stability. These patients are under constant fear of losing their dentures during different jaw movements. They do not usually seek the replacement of their unstable complete denture by a fixed prosthesis and are pleased when provided with a stable denture at limited financial cost13,14.

Overdentures supported by a single implant were recommended by some investigators15-17 because of their ability to improve patient satisfaction and are considered by some to be more cost-effective17-19. A study by Liddelow and Henry15 showed good results with 25 patients who had mandibular overdentures retained by one immediately loaded single midline implant.

In addition to possible financial savings with a single implant overdenture, there are also potential surgical advantages. For example, midline implant placement allows for simplified imaging and flap design, without concern for the position of the mental foramen or possible postoperative complications related to direct or indirect damage to branches of the inferior alveolar nerve19. In addition, in 2010 Gonda et al reviewed retrospectively the records of 85 subjects enrolled between 2003 and 2008 and followed up for at least 17 months reporting that fracture incidence of mandibular-retained overdentures retained by one implant was not significantly different from that retained by two implants20.

With regard to patient satisfaction, treatment and maintenance times as well as component costs, Walton et al19 recorded that the single implant overdenture group required significantly less financial costs when compared with that of a two implant overdenture group. After 1 year most participants (43 in each group) were satisfied, and there were no statistical differences in satisfaction or prosthetic maintenance, whereas the single-implant group required significantly less time and financial costs for the initial treatment.

On the other hand, it is necessary to evaluate single-IODs limitations and shortcomings. Many authors reported some problems associated with single implant overdenture treatment options, including a risk of potential vascular damage21. Another disadvantage of single implant overdentures is their increased risk of midline denture fracture due to the limited amount of acrylic resin and increased space occupied by the attachment housings22.

Nevertheless, more evidence is needed to evaluate whether a single implant could satisfactorily retain a mandibular overdenture over time and that clinicians could reliably offer this treatment, which is less expensive than two implants, as an alternative to a conventional mandibular denture19.

The aim of this systematic review was to evaluate whether single-IODs show similar outcomes in terms of prosthetic and implant failures, complications, patient satisfaction, quality of life and peri-implant marginal bone loss when compared to two-IODs in patients with completely edentulous mandibles.

Material and methods

This systematic review was conducted and reported in strict accordance with the preferred reporting items for systematic reviews and meta-analyses (PRISMA) guidelines23.

Focused question

The population, intervention, comparisons, outcomes (PICO) format was used to define a focused clinical question.

Population or participants: Patients needing mandibular implant overdentures Intervention: single implant mandibular overdentures.

Comparison: two implant mandibular overdentures.

Outcome: patient satisfaction, oral health related quality of life, prosthesis failures, implant failure, marginal bone loss and complications.

Protocol and registration

The protocol of this systematic review was registered at PROSPERO International prospective register of systematic reviews.

Eligibility criteria

Types of studies

Randomised controlled trials (RCTs) of parallel group-design.

Types of participants

Patients with completely edentulous mandibles

Types of interventions

Mandibular overdentures supported by one versus two un-splinted implants. For trials to be considered in this review the post-loading follow-up has to be of at least one year after initial loading of the implant-supported prostheses.

Outcome measures

Primary outcomes measures were:

Prosthesis failure: planned prosthesis that could not be placed due to implant failure(s), loss of the prosthesis secondary to implant failure(s), and any replacement of the prosthesis.

Implant failure: implant mobility and removal of stable implants dictated by progressive marginal bone loss or infection (biological failures) and any mechanical complication such us implant fractures or platform deformations rendering the implant not usable (mechanical failures). Biological failures were grouped as early (failure to establish osseointegration) and late failures (failure to maintain the established osseointegration).

Secondary outcome measures were:

Patient satisfaction measured by ordinal scale or visual analogue scale (VAS).

Oral health-related quality of life measured by oral health impact profile.

Complications: biological (e.g. peri-implantitis, fistula, nerve damages, hyperplasia of the soft tissues) and mechanical (e.g. overdenture fracture, overdenture relining, relining of opposing maxillary complete denture, occlusal adjustments, loss of attachments retention that require O-ring replacement and metal housing de-attachment).

Peri-implant marginal level changes measured on periapical radiographs taken after implant placement and at the 1-, 3-, 5- and 10-year follow-up.

Information sources:

Electronic databases (PubMed and the Cochrane library) were searched to identify randomised controlled trials without time or language restrictions, reporting on mandibular IODs. In addition a manual search of the following implant-related journals was done: Clinical Implant Dentistry and Related Research, Clinical Oral Implants Research, European Journal of Oral Implantology, Implant Dentistry, Journal of Clinical Periodontology, Journal of Dental Research, Journal of Oral and Maxillofacial Implants, Journal of Periodontology, and The International Journal of Periodontics & Restorative Dentistry. Moreover, online databases providing information about clinical trials in progress were checked (clinicaltrials.gov; www.centerwatch.com/clinicaltrials; www.clinicalconnection.com). The last performed search was on 20 January 2017.

Unpublished studies:

We wrote to all the authors of the identified RCTs, scanned the bibliographies of all identified RCTs and relevant review articles and used personal contacts in an attempt to identify unpublished or ongoing RCTs.

Search strategy:

Two reviewers independently performed the search (Alqutaibi and Algabri). Combinations of controlled terms (MeSH) and keywords were used whenever possible. The search terms used for the search in PubMed were as follows (((((((single implant overdentures) OR one implant overdenture) OR 1 implant overdenture) OR double implant overdenture) OR two implant overdenture) OR 2 implant overdenture)) AND ((((((Overdentures) OR Implant supported overdentures)) OR ((((("Dental Prosthesis, Implant-Supported"[Mesh]) OR "Denture, Overlay/therapeutic use"[Mesh]) OR "Jaw, Edentulous"[Mesh]) OR "Mandible"[Mesh]) OR "Mouth, Edentulous"[Mesh])) OR (((Dental prosthesis) OR Removable dental prostheses) OR Implant assisted Overdentures))) AND (randomized controlled trial[Publication Type]) OR (controlled clinical trial[Publication Type]) OR (clinical trial as topic[MeSH Terms:noexp]) OR (randomized[Title/Abstract]) OR (randomly[Title/Abstract]) OR (trial[Title])) NOT ((animals[MeSH Terms]) NOT humans[MeSH Terms])))

The search terms used for the search in Cochrane were as follows ("overdentures":ti,ab,kw or Removable dental prostheses:ti,ab,kw or Implant supported overdentures:ti,ab,kw or edentuolus Mandible:ti,ab,kw or Implant assisted Overdenture:ti,ab,kw (Word variations have been searched) AND (single implant overdenture:ti,ab,kw or one implant overdenture:ti,ab,kw or 1 implant overdenture:ti,ab,kw or double implant overdenture:ti,ab,kw or two implant overdenture:ti,ab,kw or 2 implant overdenture:ti,ab,kw. Word variations have been searched.

Data Collection and Analyses

Selection of studies:

Two investigators (Alqutaibi and Algabri) independently shortlisted the searched articles by performing an initial thorough title and abstract screening. Inclusion of articles for the full text analyses was performed only after a mutual agreement between the two. Where there were disagreements, it was resolved by means of a consensus discussion presided over by the third reviewer (Kaddah).

Data extraction and management:

Data extraction was performed after a mutual agreement on the final list of included publications. Data were extracted independently by the two investigators (Alqutaibi and Algabri) and were reciprocally blinded to each other’s extraction. The following information was extracted: name of author(s), publication year, country where study carried out, study design, intervention type, loading protocol, implant system, attachment type, observation period, number of patients, number of implants placed and failed, dropouts, implant survival rate, patient satisfaction score and complications.

Risk of bias in studies and quality assessment:

The risk of bias assessment of the included trials was done by two reviewers (Alqutaibi and Algabri) independently using the Cochrane collaboration’s tool28, six specific domains – namely sequence generation, allocation concealment, blinding, incomplete outcome data, selective outcome reporting, and other bias. An RCT was assigned “Low risk of bias” if all domains were at low risk of bias, “Unclear risk of bias” if there was an unclear risk of bias of at least one domain, and “High risk of bias” if at least one domain was scored as being at a high risk of bias. In case of disagreement, agreement was achieved between the two assessors.

Statistical analyses

Measures of treatment effect:

For dichotomous outcomes, the estimate of effect of an intervention was expressed as risk differences (RDs) together with 95% confidence intervals (CIs). For continuous outcomes, mean differences (MDs) and standard deviations were used to summarise the data for each group with 95% CIs.

Unit of analysis issues:

The statistical unit was the patient and not the implants.

Missing data:

If the included articles had any missing relevant information, the corresponding authors were contacted by email. In case of no responses, reminder emails were sent.

Data synthesis:

Alqutaibi performed all statistical tests using the REVMAN software release version 5.329. The meta-analyses were performed, based on the Mantel-Haenszel method, for the included studies. Meta-analyses were undertaken where studies of similar comparisons reported the same outcome measures. Mean difference (MD) for the patient satisfaction and quality of life scores and marginal bone loss were calculated and compared between the two studied interventions (single-IODs and two-IODs). Risk differences (RD) for prosthesis and implant failures and complications were calculated and compared between the two studied interventions (single-IODs and two-IODs). Confidence intervals were set at 95% (95% CI).

Weighted means across the studies were calculated using a fixed-effects model. Where statistically significant (P < 0.05) heterogeneity was detected, a random-effects model was used to assess the significance of treatment effects.

Assessment of heterogeneity:

The significance of any variations in the estimates of the treatment effects from the different trials was to be assessed by means of Cochran’s test for heterogeneity, and heterogeneity would have been considered significant if P value < 0.1. Heterogeneity between the studies was assessed using the I-squared statistic (I2-statistic), which describes the variation percentage due to heterogeneity rather than chance30. I2 over 50% was considered as moderate to high heterogeneity.

Assessment of reporting biases:

If there had been a sufficient number of trials (more than 10) in any meta-analyses, publication bias would have been assessed according to funnel plot asymmetry. If asymmetry were identified we would have examined possible causes.

Results

Study selection

The database search resulted in a total of 425 articles (PubMed = 310; The Cochrane Library = 115), of which 315 potentially relevant articles were selected after removing duplicates. Two more relevant articles were identified from the reference crosschecks. After the initial screening, six publications19, 24-27, 31 were selected for additional evaluation of the full-text version for being directly related with the aim of the present study. Of these, five publications19, 24-27 corresponding to three RCTs fulfilled the inclusion criteria and were subsequently analysed in this systematic review and one31 was excluded. Reason of exclusion was problems with study design and deficient data. The authors did not reply to our emails (Fig 1). Details of all included studies are summarised in Tables 1 and 2.

Fig 1 Flow diagram showing the details of the data search, identification and selection process.

Table 1 Summary of findings.

Study ID Country Follow-up month Age (Mean) Gender Loading time (days) Implant system Implant type Type of attachment No. of subjects Subjects per group Implant per subject Implant placed Subject analysed Patient satisfaction(VAS[1]) Mean ± SD[2] OHIP[3]Mean ± SD MBL[4] Mean ± SD Failed implant Failed prosthesis
At 1 year At 3 year At 5 year At 1 year At 3 year At 5 year
Bryant 201424 Walton 200919 Canada 60 67 43M[5] 43F[6] Early (42) Straumann Solid Screw, SLA surface Ball 86 42 1 42 29 68± 49 NR NR[7] 0 0 0 0 3 9
44 2 88* 33 75± 30.37 NR NR 5 5 5 5 7 10
Kronstrom 2014 25 Kronstrom 200926 Canada 36 53.3 16M 20F Immediate (0) Nobel Biocare TiUnite Groovy implant plasma oxidised Ball 36 17 1 17 11 NR 83.2 ± 11.4 NR 3 3 NA 3 3 NA[8]
19 2 38 8 NR 78.6 ± 15.5 NR 7 7 NA 7 7 NA
Tavakolizadeh 201527 Iran 12 59 11M 9F Early (42) Implantium Simple line II Solid Screw, SLA surface Ball 20 10 1 10 10 70± 20.3 NR 0.6 ± 0.67 0 NA NA 0 NA NA
10 2 0 0 70±15 NR 51 0 NA NA 0 NA NA

VA S = Visual analogue scale; SD = Standard deviation; OHIP = Oral health impact profile; MBL = Marginal bone loss; M= Male; F = Female; NR = Not reported; NA = Not available

Table 1 Summary of findings.

Study ID Country Follow-up month Age (Mean) Gender Loading time (days) Implant system Implant type Type of attachment No. of subjects Subjects per group Implant per subject Implant placed Subject analysed Patient satisfaction(VAS[1]) Mean ± SD[2] OHIP[3]Mean ± SD MBL[4] Mean ± SD Failed implant Failed prosthesis
At 1 year At 3 year At 5 year At 1 year At 3 year At 5 year
Bryant 201424 Walton 200919 Canada 60 67 43M[5] 43F[6] Early (42) Straumann Solid Screw, SLA surface Ball 86 42 1 42 29 68± 49 NR NR[7] 0 0 0 0 3 9
44 2 88* 33 75± 30.37 NR NR 5 5 5 5 7 10
Kronstrom 2014 25 Kronstrom 200926 Canada 36 53.3 16M 20F Immediate (0) Nobel Biocare TiUnite Groovy implant plasma oxidised Ball 36 17 1 17 11 NR 83.2 ± 11.4 NR 3 3 NA 3 3 NA[8]
19 2 38 8 NR 78.6 ± 15.5 NR 7 7 NA 7 7 NA
Tavakolizadeh 201527 Iran 12 59 11M 9F Early (42) Implantium Simple line II Solid Screw, SLA surface Ball 20 10 1 10 10 70± 20.3 NR 0.6 ± 0.67 0 NA NA 0 NA NA
10 2 0 0 70±15 NR 51 0 NA NA 0 NA NA

VA S = Visual analogue scale; SD = Standard deviation; OHIP = Oral health impact profile; MBL = Marginal bone loss; M= Male; F = Female; NR = Not reported; NA = Not available

Characteristics of included studies

The included trials compared a single implant (in the mandibular midline) vs two implants placed between the mental foramina supporting mandibular overdentures.

Two trials were conducted in Canada24, 25 and one in Iran27. All trials were conducted in university dental clinics.

One trial27 presented data up to 1 year post-loading, one trial25 to 3 years and one trial24 with 5 years. Regarding loading protocol, two24,27 trials followed early loading at 6 weeks25 and one followed immediate loading protocol.

A prior calculation for the sample size was undertaken in only one trial24.

No major baseline differences were apparent in any of the included trials.

The outcomes were reported as follow:

  • Prosthesis failures: all trials reported this outcome.
  • Implant failures: all trials reported this outcome.
  • Patient satisfaction measured by visual analogue scale (VAS): reported in two trials24,28.
  • Oral heath quality of life measured by oral health impact profile (OHIP): were reported in one trial25.
  • Complications: no biological complications were reported. The following mechanical complications were reported; wearing of plastic O-ring, overdenture fracture, metal housing de-attachment, relining mandibular OD, occlusal adjustment and relining of opposing maxillary dentures.
  • Peri-implant marginal level changes were reported in one trial27.

Quality assessment

The final risk of bias assessment of the included trials is summarised in Table 3. For each trial we assessed whether it was at low, unclear or high risk of bias (see appendices for more details). All trials were judged to be at high risk of bias (Table 3 and Figs 2 and 3).

Table 3 Risk of bias assessment of the included RCTs.

Study ID Random sequence generation Allocation concealment Blinding of outcome assessment Incomplete outcome data Selective reporting Other bias Overall risk
Bryant 201424 Low risk Low risk High risk High risk Low risk Low risk High
Kronstrom 201425 Low risk Unclear risk High risk High risk Low risk Low risk High
Tavakolizadeh 201527 Unclear risk Unclear risk High risk Low risk Low risk Low risk High

Fig 2 Risk of bias summary: review authors’ judgments about each risk of bias item for each included study.
Fig 3 Risk of bias graph: review authors’ judgments about each risk of bias item presented as percentages across all included studies.

Meta-analysis

A meta-analysis was performed for studies with similar comparisons that reported the same outcome measures. The comparisons and number of studies, implants and participants summarised in Table 4.

Table 4 Comparison: single versus two-implant overdentures.

Outcome or Subgroup Studies Participants Statistical Method Effect Estimate
1.1 prosthesis failure 3 142 Risk Difference (M-H, 95% CI) Subtotals only
1.1.1 at one year 3 142 Risk Difference (M-H, Fixed, 95% CI) -0.12 [-0.22, -0.02]
1.1.2 at 3 years 2 105 Risk Difference (M-H, Random 95% CI) -0.32 [-0.83, 0.19]
1.1.3 at 5 years 1 62 Risk Difference (M-H, Fixed, 95% CI) 0.01 [-0.22, 0.24]
1.2 implant failure 3 142 Risk Difference (M-H, 95% CI) Subtotals only
1.2.1 at one year 3 142 Risk Difference (M-H, Fixed, 95% CI) -0.12 [-0.22, -0.02]
1.2.2 at 3 years 2 105 Risk Difference (M-H, Random, 95% CI) -0.33 [-0.84, 0.18]
1.2.3 at 5 years 1 62 Risk Difference (M-H, Fixed, 95% CI) -0.15 [-0.28, -0.02]
1.3 patient satisfaction 2 106 Mean Difference (IV, Fixed, 95% CI) -3.14 [-14.75, 8.47]
1.4 overall complications and maintenance needs 3 142 Risk Difference (M-H, 95% CI) Subtotals only
1.4.1 at one year 3 142 Risk Difference (M-H, Random, 95% CI) 0.15 [-0.13, 0.44]
1.4.2 at 3 years 2 122 Risk Difference (M-H, Random, 95% CI) 0.06 [-0.26, 0.39]
1.4.3 at 5 years 1 86 Risk Difference (M-H, Fixed, 95% CI) 0.04 [-0.06, 0.15]
1.5 prosthetic complications at one year follow-up 3 122 Risk Difference (M-H, 95% CI) Subtotals only
1.5.1 Overdenture fracture 2 122 Risk Difference (M-H, Fixed, 95% CI) 0.05 [-0.04, 0.14]
1.5.2 Overdenture relining 2 122 Risk Difference (M-H, Fixed, 95% CI) 0.02 [-0.05, 0.08]
1.5.3 CUD relining 1 36 Risk Difference (M-H, Fixed, 95% CI) 0.12 [-0.08, 0.33]
1.5.4 Occlusal adjustment 2 122 Risk Difference (M-H, Fixed, 95% CI) 0.06 [-0.09, 0.21]
1.5.5 Metal housing de-attachment 2 122 Risk Difference (M-H, Fixed, 95% CI) 0.00 [-0.09, 0.10]
1.5.6 O-ring replacement 3 142 Risk Difference (M-H, Random, 95% CI) 0.04 [-0.16, 0.24]
1.6 prosthetic complications at 3 years follow-up 2 122 Risk Difference (M-H, Fixed, 95% CI) Subtotals only
1.6.1 Overdenture fracture 2 122 Risk Difference (M-H, Fixed, 95% CI) 0.04 [-0.08, 0.15]
1.6.2 Overdenture relining 2 122 Risk Difference (M-H, Fixed, 95% CI) 0.01 [-0.10, 0.11]
1.6.3 CUD relining 1 36 Risk Difference (M-H, Fixed, 95% CI) 0.13 [-0.11, 0.37]
1.6.4 Occlusal adjustment 2 122 Risk Difference (M-H, Fixed, 95% CI) 0.02 [-0.14, 0.17]
1.6.5 O-ring replacement 2 122 Risk Difference (M-H, Random, 95% CI) -0.13 [-0.40, 0.14]
1.6.6 Metal housing de-attachment 2 122 Risk Difference (M-H, Random, 95% CI) 0.04 [-0.14, 0.22[NW1] ]

Single vs two implant-supported overdentures (prosthesis failure)

The meta-analyses of three trials24,25,27 regarding prosthesis failures comparing single-IODs and two-IODs showed more patients having prosthesis failures at two-IODs implants at the first year (three RCTs) (I2 =1%, P = 0.02; RD: -0.12, 95% CI: -0.22, -0.02). However, there were no differences between the two interventions at third year (two RCTs) (I2 = 87%, P = 0.22; RD: -0.32, 95% CI: -0.83, 0.19) and fifth year (one RCT) (P = 0.95; RD: 0.01, 95% CI: -0.22, 0.24) (Fig 4)

Fig 4 Forest plot comparison: prosthesis failures at different follow-up periods.

Single vs two implant-supported overdentures (implant failure)

The meta-analyses of three trials24,25,27 regarding implant failures comparing single-IODs and two-IODs showed significantly more patients having implant failures at two-IODs at one year (I2 = 1%, P = 0.02; RD: -0.12, 95% CI: -0.22, -0.02).

Similarly, at 5 years, the meta-analyses of one trial showed significantly more patients having implant failures at two-IODs (P = 0.02; RD: -0.15, 95% CI: -0.28, -0.02).

However, at 3 years, the meta-analyses of two trials showed non-significantly more patients having implant failures at two-IODs (I2 = 87%, P = 0.2; RD: -0.33, 95% CI: -0.84, 0.18) (Fig 5).

Fig 5 Forest plot comparison: implant failures at different follow-up periods.

Single vs two implant-supported overdentures (patient satisfaction)

The meta-analyses of the two trials24,27 comparing single-IODs and two-IODs showed no significant MD for the VAS between the two modalities. The overall MD for patient satisfaction for the two interventions was not significant (Fig 4: I2 = 0%, P = 0.60; MD: -3.14, 95% CI: -14.75, 8.47) at 1-year follow-up (Fig 6)

Fig 6 Forest plot comparison: patient satisfaction at 1-year follow-up.

Single vs two implant-supported overdentures (complications and maintenance needs)

Overall complications and maintenance needs:

The meta-analyses of three trials24,25,27 regarding overall complications and maintenance needs comparing single-IODs and two-IODs showed not significant RDs between the two modalities. The forest plot (Fig 7) revealed that the RD for overall complications and maintenance needs for the two interventions at the 1-year follow-up was not significant favouring two-IODs (I2 = 76%, P = 0.30; RD: 0.15, 95% CI: -0.13, 0.44). The same effect was revealed at the 3-year follow-up (I2 = 71%, P = 0.71; RD: 0.06, 95% CI: -0.26, 0.39) and the 5-year follow-up (P = 0.43; RD: 0.04, 95% CI: -0.06, 0.15).

Fig 7 Forest plot of comparison: overall complications and maintenance needs at different follow-up periods.

Overdenture fracture:

The meta-analyses of the two trials24,25 regarding overdenture fracture comparing the single-IODs and two-IODs showed non-significant RDs for the overdenture fracture between the two modalities. The forest plots (Figs 8 to 10) revealed that the overall RDs for overdenture fracture for the two interventions were non-significant at one year (I2 = 0%, P = 0.25; RD: 0.05 95% CI: -0.04 , 0.14), at 3 years (I2 = 0%, P = 0.51; RD: 0.04 95% CI: -0.08, 0.15) and at 5 years (P = 0.15; RD: 0.13 95% CI: -0.05, 0.30).

Relining of mandibular overdenture (MOD):

The meta-analyses of the two trials24,25 regarding relining of MOD comparing the single-IODs and two-IODs showed no significant RDs between the two modalities. The forest plots (Figs 8 to 10) revealed that these studies were non-significantly favoured two-IODs. Overall RDs were (I2 = 0%, P = 0.59; RD: 0.02, 95% CI: -0.05, 0.08) at 1-year follow-up, (I2 = 0%, P = 0.91; RD: 0.01, 95% CI: -0.10, 0.11) at 3-year follow-up and (P = 0.70; RD: 0.03, 95% CI: -0.13, 0.19) at 5-year follow-up.

Fig 8 Forest plot of comparison: prosthetic complications at 1-year follow-up.
Fig 9 Forest plot of comparison: prosthetic complications at 3-year follow-up.
Fig 10 Forest plot of comparison: prosthetic complications at 5-year follow-up.

Relining of complete upper denture (CUD):

Only one trial25 reported on relining of CUD comparing the single-IODs and two-IODs showed no significant RDs between the two modalities. The forest plot (Figs 8 and 9) revealed that this study slightly favoured two-IODs at one year (P = 0.24; RD: 0.12, 95% CI: -0.08, 0.33) and at three year (P = 0.30; RD: 0.13, 95% CI: -0.11, 0.37).

Occlusal adjustments:

The meta-analyses of the two trials24,25 regarding occlusal adjustments comparing the single-IODs and two-IODs showed no significant RDs between the two modalities. The forest plot revealed that these studies non-significantly favoured two-IODs (I2 = 0%, P = 0.42; RD: 0.06, 95% CI: -0.09, 0.21) at 1-year follow-up and (I2 = 0%, P = 0.83; RD: 0.02, 95% CI: -0.14 , 0.17) at 3-year follow-up. At the 5-year follow-up the forest plot of one trial showed also no difference between the two interventions (P = 0.85; RD: -0.02, 95% CI: -0.23, 0.19 (Figs 8 to 10).

O-ring replacements:

The meta-analyses of the three trials24,25 27 regarding O-ring replacements comparing the single-IODs and two-IODs showed no significant RDs between the two modalities. The forest plots (Figs 8 to 10) revealed that two studies25, 27 favoured single-IODs. Overall RDs for O-ring replacements for the two interventions were not significant (I2 = 58%, P = 0.70; RD: 0.04, 95% CI: -0.16, 0.24) at one-year follow-up and (I2 = 65%, P = 0.36; RD: -0.13, 95% CI: -0.40, 0.14) at three-year follow-up. However, there were statistically significant more O-ring replacement in the two implants group at five-year follow-up (P = 0.02; RD: -0.20, 95% CI: -0.37, -0.03).

Metal housing de-attachment:

The meta-analyses of the two trials24, 25 regarding metal housing de-attachment comparing the single-IODs and two-IODs showed non-significant RDs between the two modalities. The forest plots (Figs 8 to 10) revealed that one24 study favoured two-IODs and other one25 slightly favoured single IODs at the 3-year follow-up.

Single vs two implant-supported overdentures (oral health quality of life)

The only trial25 reporting oral health quality of life (OHIP) comparing the single-IODs and two-IODs showed no significant MD between the two modalities (P = 0.48; MD: 4.60, 95% CI: -8.08, 17.28) at the 3-year follow-up (Fig 11)

Fig 11 Forest plot comparison: oral health quality of life at first year follow-up.

Single vs two implant-supported overdentures (marginal bone loss)

The only trail reporting on marginal bone loss measured on periapical radiographs comparing single-IODs and two-IODs showed no significant MD between the two modalities (Fig 10; P = 1; MD: 0.00, 95% CI: -0.52, 0.52) at one-year follow-up (Fig 12)

Fig 12 Forest plot comparison: marginal bone loss.

A publication bias assessment was not possible due to the low number of available RCTs.

Discussion

The meta-analyses revealed that there are significant differences between the RD of the two investigated interventions (single vs two-IODs) regarding the implant failure and prosthesis failure in favour of the single-IODs. However, there was non-significant increased risk of overdenture fracture of single-IODs compared to that of the two-IODs.

There was no significant difference between the RD of the two investigated interventions (single vs two-IODs) with respect to overall complications and maintenance needs, patient satisfaction, oral health related quality of life, and marginal bone loss.

The conclusions drawn from this review should be interpreted with caution because all included RCTs were considered at a high risk of bias and there were just three. The number of patients followed up to 5 years was also too small a number to draw a reliable conclusion.

Recently, single implant retained-overdentures have gained popularity and have been reported as a successful treatment concept as an alternative to the two-implant overdenture due to their lower costs and minimal tissue trauma32-34. This treatment modality may have the potential to eventually become the new minimum standard recommended for the compromised elderly edentulous mandible, with the accumulation of more robust evidence for the mandibular single-IODs particularly with long-term outcomes beyond 10 years.

The treatment of the compromised elders requires a minimally invasive and an effective approach at the same time. The single-IODs may just provide such a solution in elderly edentulous patients. On a different note, this protocol may show an economical advantage for the healthcare policy makers. Walton et al19 reported that, after the 1-year follow up the treatment and maintenance costs associated with the single-IOD modality was less than that of the two-IODs. Therefore, this protocol would reduce the healthcare costs compared with the conventional implant two-IODs, so more people could benefit with an effective therapy at a lower cost.

There is a paucity of literature regarding randomised controlled trials on single implant overdentures. From a clinical point of view, the placement of implants in the midline should be approached with a considerable amount of caution, as it has been reported that especially in women there is a risk of injury to the midline lingual canal vessels35.

Another potential limit would be that single-midline IODs present an additional degree of freedom as denture kinetics is not limited to a rotation of the denture during occlusal load, clinically evident as sinking in of the posterior denture saddles. Single-IODs may be associated with excessive lateral movements especially when the occlusion has with premature contacts. As the occlusion in complete dentures is dynamic and changes during the wearing period, regular remounting and relining of the dentures is therefore recommended.

Another factor to consider is the bone loss in the posterior area of the mandible. A large area of support, as present in a 4-implant overdenture, seems to prevent posterior bone loss compared with two-implant overdentures36. Nonetheless, due to age-related atrophy less muscle force is exerted by elderly patients37, and consequently, there is little mechanical load to accelerate the posterior bone atrophy. Of course, the alveolar resorption is multifactorial and not yet fully understood; however, mechanical load may be just one factor to be considered.

Conclusions

Implants and prosthesis failures were less common at single-IOD whereas patient satisfaction, peri-implant marginal bone loss and overall complications and maintenance needs of single IODs were not significantly different from two IODs. However this should be interpreted with caution as all the included studies were at a high risk of bias. Well-designed randomised controlled trials, comparing single IODs and two IODs using different attachment systems, different loading protocol and reporting on patient-related outcomes with follow-ups of at least 5 years, are highly recommended to evaluate the effectiveness of single IODs.

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Authors

 

Ahmed Yaseen Alqutaibi

Lecturer, Department of Prosthodontics, Faculty of Oral and Dental Medicine, Ibb University, Ibb, Yemen

Marco Esposito DDS, PhD

Freelance researcher and Associated Professor, Department of Biomaterials, The Sahlgrenska Academy at Göteborg University, Göteborg, Sweden

Radwan S Algabri

PhD student, Department of Prosthodontics, Faculty of Oral and Dental Medicine, Cairo University, Cairo, Egypt

Adnan Alfahad

PhD student, Department of Prosthodontics, Faculty of Oral and Dental Medicine, Cairo University, Cairo, Egypt

Amal F Kaddah

Professor, Department of Prosthodontics, Faculty of Oral and Dental Medicine. Cairo University, Cairo, Egypt

Mohammed Farouk

Lecturer in Prosthodontics, Department of Prosthodontics, Faculty of Oral and Dental Medicine. Cairo University, Cairo, Egypt

Ali Alsourori

PhD student, Department of Prosthodontics, Faculty of Oral and Dental Medicine, Cairo University, Cairo, Egypt

Correspondence to:

Ahmed Yaseen Alqutaibi

10 Ali Sherif

Almanial, Cairo, Egypt

Email : am01012002@gmail.com