Risk factors for implant-supported overdenture failures: A systematic review

Abstract

Aim:

Implant-supported overdenture presents failures. To minimize and avoid the occurrence of implant and prosthetic complications, a number of considerations must be taken into account. This systematic review aimed to point out variables associated with failure rates of implant-supported overdenture.

Settings and Design:

The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines were implemented during carrying out this systematic review.

Materials and Method:

An automatic electronic literature search of peer-review articles was conducted on the MEDLINE database by querying its PubMed portal, the Cochrane Library and LILACS in addition to a complementary manual search. Clinical trial (CT), Randomized controlled trial (RCT) and Observational study (OS) from 2018 to 2023 were included. Two examiners, independently, selected studies based on the inclusion criteria and recorded the extracted data. The risk of bias of the included studies was evaluated via the Cochrane Collaboration's tool.

Results:

After the screening and eligibility assessment stages, the inclusion criteria allowed to retain 20 articles: 13 RCT, one prospective CT, three prospective studies, two retrospective studies and one cross-sectional OS. The synthesis of the articles allowed to deduct that the failures of the implant-supported overdenture are variable and related to several factors such as the general condition of the patient, the edentulous arch, the number of implants and the loading protocol, which can be considered to minimize them.

Conclusion:

Aptitude of the dentist and regular follow-up as well as intense patient care can minimize implant-retained overdenture failure rate. In spite of this, this denture remains the best solution suitable for most patients and providing them with comfort, satisfaction and improved quality of life.

INTRODUCTION

Implant therapy is a safe and long-lasting solution for replacing missing teeth in edentulous patients using either fixed or removable implant-supported prostheses. Other than patient expectations, anatomic, esthetic, and economic factors guide therapeutic decisions.[1,2]

As per the consensus reached by McGill, implant-supported overdenture is considered an increasingly important treatment approach for edentulous patients.[3] It has emerged as the preferred and highly effective choice for full arch rehabilitation. Securing the removable prosthesis with an attachment system ensures the retention and improves the masticatory ability and general functioning and effectiveness of the complete prosthesis. Hence, it has been clearly proven that patients’ quality of life improves after implant treatment.[1,4,5] In fact, this alternative has demonstrated its effectiveness with a high implant and prosthetic survival rate and increased patient satisfaction.

Numerous clinical trials (CTs) have extensively documented the long-term prognosis and treatment invariability of implant-supported dentures. However, complications and failures can still be faced despite progressed strategies of applications, and researchers do not yet fully understand the process of their occurrence.[1]

Over the last two decades, one of the main concerns in implant investigation has been favorable and/or unfavorable outcomes of implants from a biological perspective, since that the success of implant treatment primarily depends on the bone stability surrounding the implant.[1,6] However, the assessment of implant treatment outcomes takes into account not only the long-term viability of the implant but also the esthetic and functional factors of the dentures.[7]

Latterly, there has been a shift in implant studies toward examining factors that impact prosthetic results and contribute to patient satisfaction. It shows that implant-retained prostheses often experience mechanical issues, affecting the balance and performance of the prosthesis, which proves that this type of treatment presents failures also.[1]

To minimize and avoid the occurrence of implant and prosthetic failures and complications, implant-supported removable prosthesis treatment requires careful consideration of several factors.

The objectives of this systematic review of the literature are to study the aspects of failure of implant-supported overdentures and its risk factors, and to address the approaches to prevention and management of these complications.

METHODS

The conduct of this study adhered to the standards outlined in the Preferred Reporting Items for Systematic Reviews and Meta-Analyses statement.[8,9]

Study protocol

The research question was addressed by following the PICO design recommended by the Centre for Evidence-Based Medicine for conducting a systematic literature review as listed below:

• P – Population: Maxillary and/or mandibular edentulous patients

• I – Intervention: Implant-supported complete removable prostheses

• C – Comparison: Failure rate of implant-supported overdentures compared to conventional complete removable prostheses

• O – Outcome: identifying factors leading to failure.

Thus, we propose addressing the research question: What are the aspects and the etiologies of implant-supported complete overdenture failures?

Search strategy

Two reviewers independently performed an electronic search, over 6 months, via MEDLINE (PubMed), the Cochrane central register of controlled trials (central) and Lilacs databases for articles in French or English, published in journals of dentistry through Boolean equations formulated with the Boolean operators “AND” and “OR” and keywords as follow: (“Mouth, Edentulous” OR “Denture, Complete”) AND (“Dental Prosthesis, Implant-Supported” OR “Denture, Overlay”) AND (“Prosthesis Failure” OR “Dental Restoration Failure”); (“Mouth, Edentulous” OR “Denture, Complete”) AND (“Dental Prosthesis, Implant-Supported” OR “Denture, Overlay” [Mesh]) AND “Osseointegration;” (“Mouth, Edentulous” OR “Denture, Complete”) AND (“Dental Prosthesis, Implant-Supported” OR “Denture, Overlay”) AND “Dental Prosthesis Retention;” (edentulous mouth OR complete denture) AND (implant supported prosthesis OR denture overlay) AND (complaints OR dissatisfaction OR (“complications” OR “complications”)); (“denture, overlay” OR (“denture” AND “overlay”) OR “overlay denture” OR: overdenture”) AND (failure OR complaints OR dissatisfaction OR (“complications” OR “complications”)); (ball AND “attachment”) AND (failure OR complaints OR dissatisfaction OR (“complications” OR “complications”)); (bar AND “attachment”) AND (failure OR complaints OR dissatisfaction OR (“complications” OR “complications”)); (locator AND “attachment”) AND (failure OR complaints OR dissatisfaction OR (“complications” OR “complications”)).

Study selection and intervention

Inclusion criteria

All peer-reviewed articles answering the research question, published in French or English, between January 2018 and March 2023, and corresponded to the study designs CT, randomized controlled trial (RCT), or observational study (OS), were included in this systematic review.

The current study defines implant failure as any:

• Osseointegration defect (immediate or early loading by provisional prosthesis of a nonembedded implant) leading to implant loss

• Implant loss/fracture after functional loading

• Advanced peri-implantitis leading to implant loss.

Peri-implant tissue parameters such as bone loss (BL), probing depth (PD), bleeding on probing, plaque index (PI), gingival index (GI), and peri-implant mucositis were also analyzed, as complications that could, in the long term and/or severe form, compromise implant survival.

In addition, the following events are considered prosthetic failures:

• Fracture or loss of screw or attachment system

• Overload of the attachment system due to a faulty insertion or an inadequate implant axis

• Unexplained wear or fracture of prosthetic teeth

• Unexplained fracture of denture base

• Patient dissatisfaction.

A distinction must be made between prosthetic failures and events resolved by maintenance and follow-up, such as matrix activation or change.

Exclusion criteria

• Publication before 2018

• Languages other than French or English

• Study design: case report/series, reviews and meta-analysis, letter to editors, technical notes

• Papers without available abstract or full-text

• Implant-retained dentures in partially edentulous patients

• Edentulous patients treated with mini-implants

• Fixed implant dentures in edentulous patients

• Studies with no implant or/and prosthetic failure.

Quality assessment

The risk of bias was independently assessed by the reviewers using the Cochrane Collaboration tool for the randomized studies, cohort studies, and case–control studies. The researches were, further, classified into categories of low, uncertain, or high risk of bias.

Data collection

Articles resulting from the electronic search were exported to the bibliographic software “Zotero” in order to remove duplicates and sort them.

The investigators conducted the data extraction process and, independently, examined the titles of the articles to eliminate irrelevant studies and those that did not fulfill the inclusion criteria. If titles did not contain sufficient information, the abstracts were examined and then the complete text article of all the remaining studies was analyzed. Any difference of opinion was discussed to reach a conclusion by consensus. A standardized reading grid (Format) containing the data to be extracted was drawn up by the working group to produce a critical analysis of the articles selected following the search.

RESULTS

Results search

Search was carried out by an electronic component through three search engines and a complementary manual search, and was stopped in March 2023.

Initially, 188 records were identified, of which 94 duplicates, one in Spanish, one in Chinese and 18 unavailable full texts were eliminated. Consequently, 74 titles, 36 abstracts and 30 full texts were, respectively, reviewed to finally identify 20 articles included to be analyzed: One prospective CT,[10] 13 RCT,[11,12,13,14,15,16,17,18,19,20,21,22,23] three prospective studies,[24,25,26] two retrospective studies[27,28] and one cross-sectional OS.[29] The overall selection process is illustrated in the flow chart [Figure 1].

Figure 1.

Study flow diagram according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses statement

Quality assessment

Concerning RCTs, the risk of bias analyzing was conducted following the complete checklist of Cochrane Collaboration’s tool. Ten studies had an unclear risk of bias, and three were classified as having a low risk [Table 1].

Table 1.

Risk of bias assessment of randomized controlled trials according to the Cochrane collaboration tool

Author	Random sequence generation (selection bias)	Allocation concealment (selection bias)	Blinding of participant (performance bias)	Blinding of personnel (personal bias)	Blinding outcome assessment (detection bias)	Incomplete outcome data (attrition bias)	Other bias
Glibert M (2018)
ElSyad MA (2018)
Salman A (2019)
ELsyad MA (2019)
Park JH (2019)
Nischal K (2020)
Kern M (2021)
Kappel S (2021)
ELsyad MA (2022)
De Souza RF (2022)
Onclin P (2022)
Slot W (2022)
Slot W (2023)

: Low risk of bias. : Unclear risk of bias. : High risk of bias

Included studies characteristics

Sample sizes in all articles ranged from 10[20] to 158[17] for a total of 782 patients. The study population comprised 346 women and 397 men. Considering their mean age, the average age of the study population ranged from 57.4 to 72. The general state of health was not specified for all study populations in the selected articles. Yet, the disorders identified were diabetes, hypertension and cardiovascular disease, as well as smoking and alcohol habits.

Regarding the arch treated with an implant-prosthetic rehabilitation, analysis identified (n = 10) articles dealing with mandibular implant-retained complete prostheses, including (n = 5) describing prostheses stabilized on a single implant, (n = 4) on 2 implants and (n = 1) on 3 implants; and (n = 11) studies dealing with maxillary implant-supported overdentures, with (n = 3) studies reporting results with four to six implants, while the rest presented only four implant supports. Variables in relation to prosthetic design are summarized in Table 2.

Table 2.

Variables studied in relation to implant-supported overdenture design

Author	Implant(s)					Attachment system	Overdenture
	Number/patient	Site	Diameter (mm)	Length (mm)	Surface
Nogueira TE (2018)	1	Medial zone	3.75	9 11 13 15	NS	Type I: Nitrite-coated titanium ball.A + nylon matrix Type II: Titanium ball.A + rubber matrix	Nonmetal-reinforced resin
Glibert M (2018)	4	Canine 1st molar	4	9–11	± Rough	Titanium milled bar + 8 mm extension	Reinforced with metal framework Partial palatal coverage
ElSyad MA (2018)	2	Canine	3.7	11	NS	Bar Telescopic locator	NS
Passia N (2019)	1	Medial zone	3.8	11 13	Rough	Ball.A	No metal framework
Lian M (2019)	4	NS	3.3 4.1	10 12	NS	Bar Locator	Reinforced with metal framework Partial palatal coverage
Salman A (2019)	2	Canine lateral incisor	4	8	Micro-rough	Locator	NS
ELsyad MA (2019)	2	Canine	3	12	NS	Ball.A	NS
Park JH (2019)	4	Anterior maxillary zone	3.1–5	7–13	Rough	Bar Ball.A	Reinforced with metal framework Total palatal coverage
Mañes Ferrer JF (2020)	4	NS	4	10	NS	Locator Bar	No metal framework
Bouhy A (2020)	4	2 anterior 2 posterior	3.3 4.1 4.8	6–12	NS	Locator	No metal framework Total palatal coverage
Onclin P (2020)	4–6	NS	4.1	10 12 14	Sand-blasted and acid-etched	Bar	NS
Nischal K (2020)	1	Medial zone	3.75	11.5	NS	Dalla Bona (Ball.A) Locator	NS
Kern M (2021)	1	Medial zone	3.8	11	Rough	Ball.A	Nonreinforced resin
Kappel S (2021)	2 anteriors 2 posteriors	Canine 2nd molar	3.3 4.1	10	NS	Locator	Reinforced with metal framework Total palatal coverage
García-Minguillán G (2021)	3/4	NS	NS	NS	NS	Bar Ball.A Locator Magnet	Reinforced with metal framework
	2/3
ELsyad MA (2022)	2	Canine region	3.7	11	NS	Bar Telescopic crown locator	NS
De Souza RF (2022)	1	Medial zone	3.3	10 12	NS	Novaloc Locator	NS
Onclin P (2022)	4	Central/lateral incisor Canine/1st premolar	3.5	NS	NS	Milled bar Locator	With reinforcement structure Without palatal coverage
Slot W (2022)	4 6	Canine + 1st molar Canine + middle of premolar + 1st molar	4.1	12	Rough	Milled bar	Reinforced with metal framework Partial palatal coverage
Slot W (2023)	4 6	11/13/21/23 11/13/15/21/23/25	4	11	Rough	Milled bar with distal extension	Nonmetal-reinforced resin

NS: No specification, A: Attachment. : Mandible. : Maxilla

Failures of implant-supported complete removable prostheses were divided into implant failure and prosthetic failure [Table 3].

Table 3.

Variables studied in relation to implant and prosthetic treatment results

Author	Follow-up	Implant loss/failure rate		Peri-implant tissues complications	Attachment		Prothesis
					Failure rate	Complication	Failure rate	Complication
Nogueira TE (2018)	2 years	G_IL: 5.3%	8.9%	NS	A.Loss: 16.3% A.change: 14%	Matrix Change: 153.5% Matrix/housing reincorporation: 28%	Failure rate: 58.2% Fracture: 23 Unsatisfactory retention: 2 patients	Relining: 7% Hyperplasia covering ball.A surgical removal: 3
		G_DL: 28.6%		NS
Glibert M (2018)	21 months	3.6%		Bone loss ICMT: 0.26±0.32 ICWMT: 0.24±0.36 ECMT: 0.22±0.33 ECMT: 0.19±0.23 PD: 3.26 mm BP: 23.4% PI: 39.5%	This article does not deal with prosthetic results
ElSyad MA (2018)	1 year	G_Bar: 3.33%	1.11%	Bone loss: 0.44±0.13 PD: 1.25±0.68 Highest PI Highest GI	This article is not concerned with prosthetic results
		G_Telescopic: 0%		Bone loss: 0.49±0.1 PD: 0.56±0.5 Lowest PI Lowest GI
		G_Locator: 0%		Bone loss: 0.30±0.07 PD: 1.12±0.61
Passia N (2019)	10 years	0%		PD: 2–4 mm BP: 28 sites	Ball.A loss: 5 Barr change: 4	Matrix activation: 29 Die change: 23	Failure rate: 72% Prosthesis fracture: 8	Relining: 14
Lian M (2019)	3–9 years	G_Bar: 1.7%	3.8%	Peri-implantitis: 3.6% Bone loss: 0.6–1.7 mm (>3 mm in 3 implants) PD: 2.1–3.9 mm (>5 mm in 4 implants) PI: 0.99±0.93 GI: 0.88	Failure rate: 7.1% Bar fracture: 1	A.screws loss: 2 Clip reactivation: 21	Failure rate: 14.3% Tooth fracture: 2	Relining: 6
		G_Locator: 5.6%		Peri-implantitis: 4.4% Bone loss: 0.6–1.8 mm (>3 mm in 4 implants) PD: 2–3.8 mm (>5 mm in 5 implants) PI: 0.66±0.8 GI: 0.65	Failure rate: 5.9% A.Loss: 4	A.screws loss: 4 Patrice change: 17	Failure rate: 11.8% Tooth fracture: 2	Relining: 9
Salman A (2019)	60 months	G_IL: 0%		Peri-implant mucositis: 54% Peri-implantitis: 0 Bone loss: 0.18±0.41 Plaque score: 79.55% BP: 23.86% PD: 2.27 mm	0%	Insert change: 4	Failure rate: 36.3% Prosthesis Fracture: 2 Tooth wear: 1 Insufficient number of implants: 1	Trimming: 1 Relining: 5
		G_DL: 0%		Peri-implant mucositis: 58% peri-implantitis: 1 Bone loss: 0.89±0.74 Plaque score: 65.63% BP: 35.42% PD: 2.45 mm	0%	Insert change: 5 Locator pin loss: 1	Failure rate: 50% Prosthesis fracture: 2 Tooth wear: 4	Trimming: 2 Relining: 8
ELsyad MA (2019)	12 months	Control G (nonirradiated): 5%	2.5%	Implant stability: −1.48±0.51 Bone loss: 0.6±0.34	This article does not deal with prosthetic results
		Study G (irradiated): 0%		Implant stability: −2.36±0.78 Bone loss: 0.4±0.197
Park JH (2019)	1 year	G_Bar.A: 0%		Bone loss: 0.34±1.03 PI; GI; BP: Highest level	Failure rate: 1.5% Screw fracture: 1	Matrix change: 7 Die reconnection: 2 Screw loss: 2	Failure rate: 37.5% Tooth fracture: 3 Prosthetic base fracture: 3	0%
		G_Ball.A: 0%		Bone loss: 0.34±0.7	0%	Matrix change: 12 Screw loss: 1	Failure rate: 25% Tooth fracture: 3 Prosthetic base fracture: 1	0%
Mañes Ferrer JF (2020)	5–17 years	G_Barr: 20%	23.75%	NS	Clip fracture: 30% 3 Screw fracture: 7.5% A.loss: 1	0%	Failure rate: 60% Tooth fracture: 20% Tooth wear: 40%	0%
		G_Locator: 27.5%		NS	A.fracture: 5% Patrice loss: 15%	Insert change/A.wear: 7.5%	Failure rate: 70% Prosthetic base fracture: 4 Tooth fracture: 3	Relining: 40% (4 in 3 patients)
Bouhy A (2020)	1 year	13.8%		Bone remodeling: 1.01±0.77 acceptable PI and GI Hyperplasia + pain and loss of healing abutment	0%	Inserts change: 6.9%	Failure rate: 37.9% Prosthesis fracture: 24.1% Tooth fracture: 13.8%	Relining after implants loss: 34.4%
Onclin P (2020)	1–8.6 years	4.3%		Bone loss: 0.32±0.46 PD: 4.55±1.59 mm	This article is not concerned with prosthetic results
Nischal K (2020)	1 year	Dalla Bona: 0%		Bone loss: 1.54±0.16	0%	Sleeve/spring problem: 1	0%	Relining: 1
		Locator: 0%		Bone loss: 1.71±0.22	0%	Sleeve/spring problem: 2	Failure rate: 10% Prosthesis fracture	Relining: 3
Kern M (2021)	5 years	IL: 11.1%	3.6%	PD: 2.3 à 2.8 mm S.S: 14.5% (29 sites)	0%	Ball.A change: 12% Matrix change: 136% Matrix activation: 186%	Failure rate: 64% Fracture of the Ball.A zone: 50% Tooth fracture: 14%	Relining: 86% Ulceration: 44%
		DL: 1.9%		PD: 2.3 à 2.8 mm S.S: 22.6% (47 sites)	0%	Change of Ball.A: 13% Matrix change: 105% Matrix activation: 156%	Failure rate: 46.5% Ball.A fracture zone: 28.8% Canine zone fracture: 15.4% Distal zone fracture: 2.6% Unsatisfactory retention: 1 patient	Relining: 76% Ulceration: 38%
Kappel S (2021)	3–5 years	18.9%		Peri-implantitis: 3.1% PI: 63%	Abutment loss: 4	Die change: 8 times	Failure rate: 8.33% Framework fracture: 2	Prosthetic microcracks: 6 Relining after implant loss: 5 Relining: 2
García-Minguillán G (2021)	NS	Does not mention implant failures or complications			0%	0%	Failure rate: 60.5% Metal framework or base fracture: 11.6% Poor retention: 14% Unsatisfactory occlusion: 14% Poor stability: 20.9%	Prosthetic stomatitis: 2.3% Ulcers: 14% Relining: 16.3%
ELsyad MA (2022)	5 years	Barr IL: 3.3% DL: 0%	1.11%	NS	Failure rate: 8.6% A.fracture: 2.1% A.screw fracture: 1.3% Retentive components separation: 5.2%	Abutment screws Loss: 3.2% Activation of matrices: 67.5% Wear/deformation of retention elements: 9.1% Replacement of clips: 3.9%	Failure rate: 48.1% Fracture of prosthesis: 22.1% Tooth wear: 15.6% Tooth separation/fracture: 10.4%	Rebasing: 14.3% Mucositis: 10.4% Pain: 14.3% Ulceration: 14.3% Hyperplasia: 18.2%
		Telescopic: 0%		NS	Failure rate: 11.5% A. screw fracture: 5.1% Retentive prosthetic component separation: 6.4%	Abutment screws loss: 12.2% Retentive components wear/deformation: 41.7% Retentive components replacement of: 25.6%	Failure rate: 47.4% Fracture of prosthesis: 11.5% Tooth wear: 15.4% Tooth separation/fracture: 20.5%	Rebasing: 5.1% Mucositis: 7.7% Pain: 7.7% Ulcers: 6.4% Hyperplasia: 12.8%
		Locator: 0%		NS	Failure rate: 6.9% A. screw fracture: 2.5% Prosthesis retentive component separation: 4.4%	Abutment screw Loss: 7.6% Retentive component wear/deformation: 77.8% Retentive components replacement: 36.7%	Failure rate: 12.7% Prosthesis fracture: 2.5% Tooth wear: 5.1% Tooth separation/fracture: 5.1%	Relining: 5.1% Mucositis: 8.9% Pain: 6.3% Ulcers: 10.1% Hyperplasia: 5%
De Souza RF (2022)	NS	Does not deal with implant failures and complications			NL Die deformation: 20%		0%	Pain/adjustment: 5
					LC Die deformation: 40%		Failure rate: 40% Prosthesis fracture: 2	Pain/adjustment: 2
Onclin P (2022)	5 years	Barr: 0%	4.7%	Peri-implant mucositis: 45% Peri-implantitis: 22.8% Bone loss: −0.99±0.96 PD: 0.7 mm	0%	Remplacement matrice	Failure rate: 5% Prosthesis wear and fracture	0%
		Locator: 7.8%		Peri-implant mucositis: 69.6% Peri-implantitis: 44.9% Bone loss: −1.41±1.38 PD: 0.6 mm	0%	Remplacement matrice Abutment screws loss: 1	Failure rate: 8.7% Tooth fracture: 2	0%
Slot W (2022)	10 years	G_4 implants: 0%	1.7%	Peri-implant mucositis: 34.6% Peri-implantitis: 38.5% Bone loss: 0.66±0.58 BP: 42.3% PD: 4.8 mm	Failure rate: 15.3% New bar: 4	0%	Failure rate: 91.9% New prosthesis: 7 Tooth/prosthesis repair: 17	Hyperplasia removal: 2 Relining: 0 Bite adjustment: 4
		G_6 implants IL: 0% DL: 3.3%		Peri-implant mucositis: 65% Peri-implantitis: 20% Bone loss: 0.73±0.9 BP: 40% PD: 4.2 mm	Failure rate: 35% New bar: 7	0%	Failure rate: 240% New prosthesis: 12 Tooth/prosthesis repair: 36	Hyperplasia removal: 3 Relining: 1 Bite adjustment: 2
Slot W (2023)	10 years	G_4 implants: 0%	1.7%	Peri-implant mucositis: 52.6% Peri-implantitis: 10.5% Bone loss: 0.41±0.37 PI: 0.6±0.8 BP: 0.3 PD: 0±1	Failure rate: 26% New bar: 5	0%	Failure rate: 135.5% Base/tooth repair: 21 New prosthesis: 8	Hyperplasia removal: 2 Relining: 0 Bite adjustment: 2
		G_6 implants IL: 0% DL: 2.9%		Peri-implant mucositis: 52.9% Peri-implantitis: 23.5% Bone loss: 0.7±1.07 PI: 0.2±0.7 BP: 0.3 PD: 0.7±1.1	Failure rate: 35% New bar: 6	0%	Failure rate: 171% Base/tooth repair: 20 New prosthesis: 12	Relining: 2 Bite adjustment: 3

G: Group, NS: No specification, DL: Delayed loading, IL: Immediate loading, ICMT: Internal connection with micro-threading, ICWMT: Internal connection without micro-threading, ECMT: External connection with micro-threading, PD: Probing depth, BP: Bleeding on probing, PI: Plaque index, GI: Gingival index, NL: Novaloc, LC: Locator. : Implant results. : Prosthetic results

According to Onclin et al.,[21] Slot et al.,[23] and Lian et al.,[27] bone defects were restored by the guided bone regeneration technique, giving a reduced overall implant failure rate, as respectively, 4.7%, 1.7%, and 0%. On the other hand, in the randomized trial by Kappel et al., a minimally invasive augmentation procedure applied by manual osteome (bone spreading or internal sinus floor augmentation), had no effect on long-term implant survival when bone grafting was absent (failure rate 18.9%).[18] From another consideration, in Kern’s trial, the implant and prosthetic failure rate was increased in the immediate loading group than in the delayed one, considering that all failures occurred within the first 12 weeks.[17]

In situations with a single mandibular implant,[13,20] where a removable prosthesis was retained through a single Locator attachment, except simple peri-implant tissue manifestations (with mean values for peri-implant mucositis: 54% ± 2%, peri-implantitis: 0.5 ± 0.5, BL: 0.53 ± 0.5, PB: 29.64, DP: 2.36), no implant failure was reported, independently of the type of functional loading. The prosthetic failure rate ranged from 36.3% to 50%, manifested mainly by fractures of the prosthetic base. Comparing the result of Elsyad et al.[19] of complete implant-supported overdenture with two mandibular implants, with that of Kern et al.[17] of a single-implant prosthesis with two-stage surgery and delayed loading, for an identical follow-up time, a comparable implant failure rate was noted (respectively 1.1% and 1.9%). Overall, implant failure of a single-implant prosthesis ranged from 0% to 8.9%, and from 0% to 2.5% for a two-implant prosthesis.

Failure was more common with maxillary than mandibular rehabilitations. The average of maxillary implant failure rate was about 5%, higher than the mandibular rate measuring 3.3%. The highest implant failure rates were noted with maxillary prostheses retained by four implants and locator attachments, measuring 27.5%,[25] 18.9%,[18] and 13.8%.[26] These values were matched by respective prosthetic failure rates of 60%–70% if the prosthesis with partial palatal coverage did not feature a metal framework,[25] and 37.9% for the prosthesis with total coverage and without metal framework,[26] while reinforced prostheses with metal framework and total palatal coverage showed only 8.33% prosthetic failure rate.[18]

Slot et al.[22,23] noted a higher prosthetic maintenance rate for maxillary prostheses on six implants. Relining has been the most widely used solution to these problems.

Peri-implant clinical complications also depended on the type of attachment used. The incidence of peri-implantitis in the bar group aligned with the findings reported by Slot et al. and remained lower than the Locator group that suffered a higher incidence of infection.[22,23]

DISCUSSION

The implant-supported overdenture is associated with prosthetic deficit, attachment component loss or dysfunction, as well as soft and hard tissue complications around the implants.

Single implant placed in the mandibular medial region showed low implant failure while, with two-implant-supported overdenture, did not exceed 2.5%.[12] The highest implant failure rates were noted with maxillary four-implants-supported overdentures, with values of 13.8%,[26] 18.9%[18] and 23.75%.[25]

Depending on the attachment, the locator attachment with single-implant-supported overdenture described the highest value of prosthetic failure (40%).[20] Concerning the maxillary implant-supported overdenture, the use of a conjunction bar on four implants caused a prosthetic failure rate of 30%,[25] whereas with six implants the value became 15.3%.[22] The distal extension of the bar increased the latter value and for the same conditions to 26%–35%.[23] Prosthetic base failures have affected the maxilla rather than the mandible. It varied from 40%[20] to 72%[24] for mandibular prostheses without reinforcement and on a single ball attachment, and measured 12.7%[19] with two implants and a bar. Maxillary base failure was inversely proportional to implant failure and was increased by the design of partial palatal coverage and the absence of metal reinforcement. The combination with the most reasonable failure values was described with four implants surmounted by a conjunction bar with total palatal coverage and metal reinforcement.[15] However, failures vary according to the factors considered in the analysis.

Edentulous arch to be rehabilitated

Bone quality may be considered the fundamental cause of the implant failure difference between maxilla and mandible noted in the included studies.[30] The maxillary edentulous bone is often of reduced quantity and quality and corresponds to types 3 and 4, while the mandibular bone is of types 2 and 3.[31,32]

Sailer et al. found fracture rates three times higher for maxillary implant-supported prostheses than for lower prostheses. A possible explanation for these issues could be the higher flexion moments of the terminal (posterior) implants in the maxilla.[2]

Preimplant preparation

It has been proposed that the use of membranes may promote bone formation and positively influence implant survival.[33,34] Onclin et al.[28] reported the employ of a bone graft from the iliac crest, retro-molar region, and calvaria; while iliac crest autogenous blocks were used to raise the sinus floor in another randomized trial.[22] In both studies, the failure rate was low and BL was comparable to other studies for the same follow-up period. Despite advances in mandibular preimplant preparation techniques, selected articles did not involve bone augmentation procedures for the mandibular arch.

Implant characteristics

Based on the McGill consensus, the preferred treatment option for edentulous mandibles is the placement of two-implant-supported complete prostheses. In this perspective, increasing substantiation supports employing single-implant prostheses as a viable treatment option. However, none of the studies in the selected articles compared the two options of using one or two implants to stabilize a mandibular prosthesis. In the randomized trial by Nogueira et al., the high implant failure rate at 2 years for a delayed loading mandibular prosthesis supported by a single implant was due to poor bone quality in osteoporotic patients and not to the prosthetic design.[10] While a 10-year follow-up of a mandibular prosthesis retained on a one-only implant located at the medial region showed no implant failure.[24]

A meta-analysis comparing mandibular removable prostheses with one or two implants, indicated that, in comparison to prostheses retained by two implants located in the canine region, there were no significant differences in prosthetic problems in prostheses supported by one implant, generally located in the middle zone, after 5 years of functioning. The incorporation of a metal framework to the acrylic resin can potentially reduce the incidence of resin fracture.[35]

For the maxilla, Di Francesco et al.[36] carried out a systematic review and a meta-analysis concerning the number of implants required, and found that implant survival may be compromised when fewer than four implants are placed, and that a minimum of four implants, whether placed in the anterior or posterior sector, had an excellent long-term prognosis and was considered necessary to achieve a high implant survival rate. With a larger number of implants, there was no significant difference in failure rates between prosthesis stabilized with four implants and with six implants, for both implant and prosthetic results. Samely, Di Francesco concluded that, beyond four implants, the number of implants had no influence on bar-supported overdenture when considering patient satisfaction, implant and prosthesis viability, and prosthetic issues.[37]

While the trial of Park et al.[15] stated that there was no significant association between implant length and diameter and implant failure, Di Francesco’s review reported that implant length was a factor influencing implant failure rates,[36] and several systematic reviews had noted an increase in failure rates when implants were shorter than 10 mm.[2,23,38]

According to Kern et al.,[17] Salman et al.,[13] and Slot et al.,[22,23] the use of implants with sandblasted and acid-etched rough surfaces made a low long-term failure rate. The evidence that implant surface is a factor influencing implant failure had also been confirmed by the results of Worni et al., who conducted an analysis to examine the potential outcomes of titanium surface topography on osseointegration.[39] As well, according to Liddelow and Henry[40] smooth surface resulted a high implant failure rate (37.5%): With machined surface implants, osseointegration was less successful than with a micro-rough. Surface fluoride modification improved bone-implant contact and led to the minimal bone remodeling observed in the Salman et al.’s study.[13]

Attachment system

When using implant–retained overdentures, the implants offer a firm and stable foundation for the attachment. Situation, choice, angulation, and placing are not major concerns due to the wide range of available attachments that can accommodate different situations.[5]

No significant difference between the ball attachment and the Locator concerning implant failure. However, the first showed a less BL: the inflexible behavior of the Locator attachment restricts the movement of the prosthesis and can potentially result increase pressure on the bone bordering the implant, whereas the ball attachment dissipates stress on the bone more effectively. For the same reasons, Nischal et al.[16] reported a higher prosthetic care requirement and failure rate for Locator systems compared with ball attachments.

Park et al.[15] showed that, compared to the axial attachment group, the bar group had significant implant failure and more serious clinical indicators such as PI, GI, and PD, but allowed better stress distribution and therefore better implant prognosis.

At prosthetic level, Elsyad et al.[19] found that bar exhibited the highest occurrence of prosthetic failure, and the Locator exhibited the highest frequency of maintenance requirements associated with attachment constituents.[21,25]

Kern et al.[17] suggested a higher implant survival rate if attachment components were incorporated into the prosthesis base using the chair-side technique. Moreover, this procedure is sensitive, the slightest prosthesis mispositioning can lead to occlusal and/or implant overload.

Functional loading of the prosthesis

It should be considered that tissues are slightly swollen while implant placement, and that prosthesis instability may occur after tissues healing, if immediately loaded. The use of an elastic attachment or spacer can enhance the result.

From a prosthetic perspective, if attachments are incorporated into resin bases during the same session as implant placement when soft tissues are swollen, dentures need to be adjusted quite widely in the anterior zone to ensure sufficient spacing for the matrix. This can conduct to thin residual resin bases and can increase the risk of fracture, compared with delayed loading. To reduce this risk, it has been suggested that increasing the volume of the prosthesis base, using thermo-polymerizable resin to incorporate retention caps, in addition to metal reinforcement, can enhance the hardness of the base if immediate loading is scheduled.[40,41]

Prosthesis design

The majority of authors agreed on the importance of integrating a metal framework into the base of the prosthesis to ensure better distribution of forces and to reduce the incidence of fractures and complications. However, in the included studies, there were no trials comparing the influence of palatal coverage on implant results. In fact, implants placed under nonpalatal maxillary prostheses recorded higher stresses than those placed under prostheses with palatal coverage. Furthermore, partial palatal coverage had little effect on implants connected by bar, and had no influence on a maxillary prosthesis stabilized by six implants.[42]

In terms of prosthetic failures, according to Osman et al.’s systematic review, maxillary implant prostheses have a higher fracture rate when designed without palatal coverage.[43]

Antagonist arch

Bouhy et al.[26] explained the high rate of implant failure obtained because of the nature of the mandibular antagonist arch. A systematic review by Ohkubo and Baek, confirmed that antagonistic teeth did not pose a risk factor for the survival of mandibular implant prostheses, whereas for maxillary ones.[44]

CONCLUSION

It would remain difficult to claim a zero implant or prosthetic failure rate in complete implant-supported overdenture. In order to prevent this risk of failures, it seems important to resort to clinical and radiological preimplant planning, enabling rationalization of the implant and prosthetic decision, as well as opting for guided surgical procedures that have evolved thanks to the digitization from planning to operative intervention. Maintenance is an integral part of treatment since it enables to anticipate or resolve the biological or mechanical complications occurrence, often predisposing and predictive of failure.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.