2.1

Registry protocol

This present systematic review, which was structured based on the preferred reporting items for systematic reviews and meta-analyses (PRISMA) checklist , in accordance with models proposed in the literature . Furthermore, the methods used in this systematic review were registered with PROSPERO (CRD42016036141).

2.2

Search methods

The selection of articles was conducted individually by two of the authors (C.A.A.L. and V.E.S.B.) using the databases PubMed/MEDLINE, Embase and The Cochrane Library, checking articles published until September 2016. The following terms were used in the search strategy: “mini dental implants OR narrow diameter implants OR mini implants overdentures OR mini implants and prosthodontics.”

The same researchers manually searched for articles published until June 2016 in the following specific journals: Clinical Implant Dentistry and Related Research, Clinical Oral Implants Research, International Journal of Oral and Maxillofacial Implants, International Journal of Oral and Maxillofacial Surgery, International Journal of Prosthodontics, Journal of Dental Research, Journal of Dentistry, Journal of Oral Rehabilitation, Journal of Prosthodontics, The International Journal of Prosthodontics and The Journal of Prosthetic Dentistry . A third author (E.P.P.) analyzed all of the differences in choices between C.A.A.L. and V.E.S.B., and a consensus was attained via discussion.

2.3

Eligibility criteria

Eligibility criteria included clinical human studies, randomized controlled trials (RCTs) or prospective studies that evaluated the use of mini implants for rehabilitation with overdenture prosthesis and studies published in English. The exclusion criteria were retrospective studies, in vitro studies, animal studies, biomechanical studies, case reports, and review papers.

2.4

Study selection and risk of bias

Clinical studies were selected based on their titles and abstracts from the electronic searches by two independent researchers. For studies presenting insufficient data in their title and abstract to make a decision about inclusion, the full manuscript was obtained. Studies that did not meet the inclusion criteria after the researchers read their title and abstract were excluded.

A specific question was formulated based on PICO (population, intervention, control, and outcomes) criteria. The focused question was: “Is the use of mini implants feasible for prosthetic rehabilitation with overdentures?” According to these criteria, the population consists of edentulous patients rehabilitated with overdentures; the intervention was edentulous patients rehabilitated with overdentures retained by mini implants; the comparison was edentulous patients rehabilitated with conventional dentures or overdentures retained by standard implants; the primary outcome was the survival rates of the mini implants; and the secondary outcomes included marginal bone loss, satisfaction and quality of life with the mini implants when they were used for retaining overdenture prosthesis.

2.5

Quality assessment

Two investigators (C.A.A.L. and V.E.S.B.) assessed the methodological quality of studies according to their level of evidence as proposed by the National Health and Medical Research Council (NHMRC) levels of evidence and grades for recommendations establish the levels of evidence according to the type of research question, taking into account: Intervention; Diagnostic accuracy; Prognosis; Etiology; Screening Intervention. The hierarchy of the studies are classified into scores (I; II; III-1; III-2; III-3; IV) . In addition, also Newcastle-Ottawa scale (NOS) was used to assess risk bias of the selected studies based on three major components: selection, comparability, and outcome for cohort studies. According to that quality scale, a maximum of nine stars can be given to a study, and this score represents the highest quality. Five or less stars represent a high risk of bias, while six or more stars were considered of low risk of bias .

2.6

Data collection and analysis

The data extracted from the articles were sorted as quantitative or qualitative by one of the researchers (C.A.A.L.) and then checked by two other researchers (J.F.S.J. and F.R.V.). Any disagreements were solved via discussion until a consensus was obtained. The quantitative and qualitative data were tabulated for ease of comparison.

2.7

Additional analysis

Additional analysis was performed using a kappa coefficient calculated to determine the inter-reader agreement in the study-selection process for publication in the PubMed/MEDLINE, Embase and The Cochrane Library databases. The inter-investigator agreement (Kappa) was calculated by evaluating the selected titles and abstracts, and then obtaining a value for selected articles on PubMed/MEDLINE (kappa = 0.89), Embase (kappa = 0.91) and Cochrane Library (kappa = 1.00) presenting a high level of agreement between the reviewers under the Kappa criteria . The survival rates of mini implants were calculated by Kaplan–Meier method in each follow-up interval (0–84 months).

2.1

Registry protocol

This present systematic review, which was structured based on the preferred reporting items for systematic reviews and meta-analyses (PRISMA) checklist , in accordance with models proposed in the literature . Furthermore, the methods used in this systematic review were registered with PROSPERO (CRD42016036141).

2.2

Search methods

The selection of articles was conducted individually by two of the authors (C.A.A.L. and V.E.S.B.) using the databases PubMed/MEDLINE, Embase and The Cochrane Library, checking articles published until September 2016. The following terms were used in the search strategy: “mini dental implants OR narrow diameter implants OR mini implants overdentures OR mini implants and prosthodontics.”

The same researchers manually searched for articles published until June 2016 in the following specific journals: Clinical Implant Dentistry and Related Research, Clinical Oral Implants Research, International Journal of Oral and Maxillofacial Implants, International Journal of Oral and Maxillofacial Surgery, International Journal of Prosthodontics, Journal of Dental Research, Journal of Dentistry, Journal of Oral Rehabilitation, Journal of Prosthodontics, The International Journal of Prosthodontics and The Journal of Prosthetic Dentistry . A third author (E.P.P.) analyzed all of the differences in choices between C.A.A.L. and V.E.S.B., and a consensus was attained via discussion.

2.3

Eligibility criteria

Eligibility criteria included clinical human studies, randomized controlled trials (RCTs) or prospective studies that evaluated the use of mini implants for rehabilitation with overdenture prosthesis and studies published in English. The exclusion criteria were retrospective studies, in vitro studies, animal studies, biomechanical studies, case reports, and review papers.

2.4

Study selection and risk of bias

Clinical studies were selected based on their titles and abstracts from the electronic searches by two independent researchers. For studies presenting insufficient data in their title and abstract to make a decision about inclusion, the full manuscript was obtained. Studies that did not meet the inclusion criteria after the researchers read their title and abstract were excluded.

A specific question was formulated based on PICO (population, intervention, control, and outcomes) criteria. The focused question was: “Is the use of mini implants feasible for prosthetic rehabilitation with overdentures?” According to these criteria, the population consists of edentulous patients rehabilitated with overdentures; the intervention was edentulous patients rehabilitated with overdentures retained by mini implants; the comparison was edentulous patients rehabilitated with conventional dentures or overdentures retained by standard implants; the primary outcome was the survival rates of the mini implants; and the secondary outcomes included marginal bone loss, satisfaction and quality of life with the mini implants when they were used for retaining overdenture prosthesis.

2.5

Quality assessment

Two investigators (C.A.A.L. and V.E.S.B.) assessed the methodological quality of studies according to their level of evidence as proposed by the National Health and Medical Research Council (NHMRC) levels of evidence and grades for recommendations establish the levels of evidence according to the type of research question, taking into account: Intervention; Diagnostic accuracy; Prognosis; Etiology; Screening Intervention. The hierarchy of the studies are classified into scores (I; II; III-1; III-2; III-3; IV) . In addition, also Newcastle-Ottawa scale (NOS) was used to assess risk bias of the selected studies based on three major components: selection, comparability, and outcome for cohort studies. According to that quality scale, a maximum of nine stars can be given to a study, and this score represents the highest quality. Five or less stars represent a high risk of bias, while six or more stars were considered of low risk of bias .

2.6

Data collection and analysis

The data extracted from the articles were sorted as quantitative or qualitative by one of the researchers (C.A.A.L.) and then checked by two other researchers (J.F.S.J. and F.R.V.). Any disagreements were solved via discussion until a consensus was obtained. The quantitative and qualitative data were tabulated for ease of comparison.

2.7

Additional analysis

Additional analysis was performed using a kappa coefficient calculated to determine the inter-reader agreement in the study-selection process for publication in the PubMed/MEDLINE, Embase and The Cochrane Library databases. The inter-investigator agreement (Kappa) was calculated by evaluating the selected titles and abstracts, and then obtaining a value for selected articles on PubMed/MEDLINE (kappa = 0.89), Embase (kappa = 0.91) and Cochrane Library (kappa = 1.00) presenting a high level of agreement between the reviewers under the Kappa criteria . The survival rates of mini implants were calculated by Kaplan–Meier method in each follow-up interval (0–84 months).

3.1

Literature search

The search performed in the databases yielded 1273 references, including 860 references from PubMed/MEDLINE, 351 references from Embase and 62 reference from The Cochrane Library. After duplicate references were removed, 942 studies remained. After detailed reviewed the titles and abstracts of the manuscripts, 40 studies were eligible for analysis. Upon reading the full texts, were excluded 16 studies for the following reasons: they were retrospective studies, case reports/reviews/dental technique, they did not evaluate mini implants in overdentures, there was an absence of clinical parameters, evaluated single unit prosthesis or removable partial denture and there were insufficient data ( Table 1 ). Details about the search strategy are presented in a flow diagram ( Fig. 1 ).

Flow chart showing the steps in the literature search.

3.2

Description of the studies

A total of 24 studies are summarized in Tables 2 and 3 . Of the 24 selected studies, four were RCTs and 20 were prospective studies. A total of 2494 mini implants and 386 standard implants were placed in 896 patients, with mean age of 65.93 years. The mini implant system MDI – O-Ball (3M – ESPE) was the most commonly used in the studies. The mini implants placement in mandibular arch was more prevalent (2330 MI – 93.42%) than the maxillary arch (164 MI – 6.58%).

Table 2

Summary of quantitative characteristics of included studies.

Author	Study Design	Patients, n	Mini- implant, n	Mean age, years	Diameter/Length	Implant system	Arch (maxilla/mandible)	Range of follow-up	Level of evidence		Groups evaluated
									NHMRC	NOS
Temizel et al.	Prospective	32	99 MI 35 SI	70.25	MI: 1.8–2.4 × 13–15 mm SI: 3.3–3.7 × 11–13 mm	MDI – O Ball /3M ESPE tioLogic-ST Implants / Dentaurum Implants GmbH	Mandible	2 years	III-2	9	G1: 4 or 5 MI G2: 2 or 4 SI
Zygogiannis et al.	Prospective	10	40 MI 70 MI	70.6 68	1.8–2.1 × 10–15 mm MI: 1.8–2.4 × 13–15 mm	MDI – O Ball /3M ESPE	Mandible	1.5 years	III-2	6	G: 4 MI immediate loading
Hasan et al.	Prospective	26	33 SI		SI: 3.3–3.7 × 11–13 mm	MDI – O Ball /3M ESPE tioLogic-ST Implants / Dentaurum Implants GmbH	Mandible	NR	III-2	6	G1: 4 or 5 MI G2: 2 or 4 SI
Batisse et al.	Prospective	11	44 MI	72	2.7 × 9–15 mm	Eurotecknica	Mandible	NR	III-2	7	G1: Conventional denture G2: 4MI
Enkling et al.	Prospective	20	80 MI	65	1.8 × 13–15 mm	MDI – O Ball /3M ESPE	Mandible	1 year	III-2	6	G: 4MI
Elsyad	Prospective	28	112 MI	62.9	1.8 × 12–18 mm	MDI – O Ball /3M ESPE	Mandible	5 years	III-2	6	G: 4 MI
Peršic´ et al.	Prospective	122	200 MI 144 SI	63.13	NR	MDI – O Ball /3M ESPE	Mandible	3 years	III-2	7	G1: 200 MI G2: 112 SI (Locator) G3: 32 SI (Bar)
Catalan et al.	Prospective	7	14 MI	NR	1.8 × 13–15 mm	MDI – O Ball /3M ESPE	Mandible	7 years	III-2	6	G: 2 MI
Souza et al.	RCT	120	236 MI 80 SI	59.5	MI: 2.0 ×10 mm SI: 4.0 × 10 mm	MDL – Intra-Lock Morse-Lock Straight Intra-Lock	Mandible	1 year	II	9	G1: 4 MI G2: 2 MI G3: 2 SI
Mangano et al.	Prospective	NR	57 MI *	71.1	2.7 × 10–13 mm	Tixos Nano – Leader Implants	Mandible	4 years	III-2	6	G: 3 or 4 MI
Ribeiro et al.	RCT	120	236 MI 80 SI	59.5	MI: 2.0 ×10 mm SI: 4.0 ×10 mm	MDL – Intra-Lock Morse-Lock Straight Intra-Lock	Mandible	NR	II	6	G1: 4 MI G2: 2 MI G3: 2 SI
Šćepanović et al.	Prospective	30	120 MI	NR	1.8 ×13 mm	MDI – O Ball /3M ESPE	Mandible	1 year	III-2	6	G: 4 MI
Preoteasa et al.	Prospective	23	110 MI	62	1.8–2.4 × 10–18 mm	MDI – O Ball /3M ESPE	Maxilla and Mandible	3 years	III-2	6	G1: 5 or 6 MI (maxilla) G2: 4 MI (mandible)
Maryod et al.	Prospective	36	144 MI	64.1	1.8 × 15 mm	MDI – O Ball /3M ESPE	Mandible	3 years	III-1	9	G1: 4 MI with immediate loading G2: 4 MI with early loading
Ashmawy et al.	Prospective	12	48 MI	NR	1.8 × 15 mm	MDI – O Ball /3M ESPE	Mandible	NR	III-2	8	G1: Conventional denture G2: 4 MI
Tomasi et al.	Prospective	21	80 MI	71	1.8–2.4 × 7–14 mm	Dentatus AB – Stockholm	Maxilla and Mandible	1 year	III-2	6	G: 4 MI (except for two patients that used 3 MI, and one used 2 MI)
Elsyad et al.	Prospective	19	114 MI	63.8	1.8–2.4 × 15 mm	MDI – MAX Thread / Sendaxs	Maxilla	2 years	III-1	9	G1: 6 MI with FPC G2: 6 MI with PPC
Omran et al.	Prospective	14	28 MI 14 SI	55	MDI: 1.8 ×15 mm SI: NR	MDI: Sendax MAX– IMTEC SI: Tapered Internal, Biohorizons	Mandible	1 year	III-1	9	G1: 4 MI G2: 2 SI
Šćepanović et al.	Prospective	30	120 MI	NR	1.8 × 13 mm	MDI – O Ball /3M ESPE	Mandible	1 year	III-2	6	G: 4 MI
Elsyad et al.	Prospective	28	112 MI	62.9	1.8 × 12–18 mm	MDI – O Ball /3M ESPE	Mandible	3 years	III-2	6	G: 4 MI
Jofre et al.	RCT	45	90 MI	71	1.8 × 15 mm	Sendax MDI – IMTEC	Mandible	2 years	II	9	G1: 2 MI BG2: 2 MI PB
Jofre et al.	RCT	45	90 MI	71	1.8 × 15 mm	Sendax MDI – IMTEC	Mandible	1.25 years	II	8	G1: 2 MI B G2: 2 MI PB
Morneburg and Pröschel,	Prospective	67	134 MI	69	2.5 × 9–15 mm	Komet – Dental Lemgo	Mandible	6 years	III-2	6	G: 2 MI
Griffitts et al.	Prospective	30	116 MI	67	1.8 × 10–18	Sendax MDI – IMTEC	Mandible	1 year	III-2	5	G: 4 MI

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