Becker et al.

International Journal of Implant Dentistry

https://doi.org/10.1186/s40729-018-0144-4
(2018) 4:35
International Journal of
Implant Dentistry

REVIEW Open Access

Efficacy of orthodontic mini implants for en

masse retraction in the maxilla: a
systematic review and meta-analysis
Kathrin Becker1* , Annika Pliska1, Caroline Busch1, Benedict Wilmes1, Michael Wolf2† and Dieter Drescher1†

Abstract
Background/aim: Retraction of the upper incisors/canines requires maximum anchorage. The aim of the present
study was to analyze the efficacy of mini implants in comparison to conventional devices in patients with need for
en masse retraction of the front teeth in the upper jaw.
Material and methods: An electronic search of PubMed, Web of Science, and EMBASE and hand searching were
performed. Relevant articles were assessed, and data were extracted for statistical analysis. A random effects model,
weighted mean differences (WMD), and 95% confidence intervals (CI) were computed for horizontal and vertical
anchorage loss at the first molars in the analyzed patient treatments.
Results: A total of seven RCTs employing direct anchorage through implants in the alveolar ridge were finally
considered for qualitative and quantitative analysis, and further five publications were considered for the qualitative
analysis only (three studies: indirect anchorage through implant in the mid-palate, two studies: direct/indirect
anchorage in the alveolar ridge). In the control groups, anchorage was achieved through transpalatal arches,
headgear, Nance buttons, intrusion arches, and differential moments.
WMD [95% CI, p] in anchorage loss between test and control groups amounted to − 2.79 mm [− 3.56 to − 2.03 mm,
p < 0.001] in the horizontal and − 1.76 mm [− 2.56 to − 0.97, p < 0.001] favoring skeletal anchorage over control
measures. The qualitative analysis revealed that minor anchorage loss can be associated with indirect anchorage,
whereas anchorage gain was commonly associated with direct anchorage. Implant failures were comparable for
both anchorage modalities (direct 9.9%, indirect 8.6%).
Conclusion: Within its limitations, the meta-analysis revealed that maximum anchorage en masse retraction can be
achieved by orthodontic mini implants and direct anchorage; however, the ideal implant location (palate versus
alveolar ridge) and the beneficial effect of direct over indirect anchorage needs to be further evaluated.
Keywords: Bone screws, Orthodontic anchorage procedures, TAD, En masse retraction, Mini implants, Micro
implants, Systematic review, Meta-analysis

Review Nevertheless, anchorage control turned out to be highly

Background demanding as the conventional approaches were com-
Extraction of the permanent teeth for retraction of the monly associated with anchorage loss, i.e., mesial migra-
protruded front teeth is a routine approach in orthodon- tion of the posterior dental anchorage units.
tics. Various techniques such as headgear, Nance button, In order to improve anchorage control, differential mo-
and transpalatal arches (TPA) have been proposed to ments have been described and monitored in clinical studies
achieve sufficient anchorage [5, 8, 9, 12, 28, 31, 45]. [25, 26]. The outcomes were promising, but nevertheless, an-
chorage loss and unexpected space opening most probably
* Correspondence: kathrin.becker@med.uni-duesseldorf.de due to activation failures have also been reported. Some
†
Michael Wolf and Dieter Drescher contributed equally to this work.
1
Department of Orthodontics, Universitätsklinikum Düsseldorf, 40225
authors suggested that consecutive canine and front retrac-
Düsseldorf, Germany tion may be more effective than en masse retraction of the
Full list of author information is available at the end of the article

© The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and
reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to
the Creative Commons license, and indicate if changes were made.
Becker et al. International Journal of Implant Dentistry (2018) 4:35 Page 2 of 12

front segment to preserve anchorage. Despite this, the effect- Search strategy
iveness of this approach is still discussed, and controversial The PubMed database of the US National Library of
outcomes have been reported [18, 60]. Medicine, EMBASE, and the Web of Knowledge of
In the past two decades, temporary orthodontic an- Thomson Reuters were used as electronic databases to
chorage devices (TADs) including orthodontic mini perform a systematic search for relevant articles published
implants and mini-plates have been introduced to im- in the dental literature between 1992 and Dec 31, 2017.
prove anchorage control [39, 56, 57]. Orthodontic im- Furthermore, the Cochrane Central Register of Controlled
plants can be loaded directly after insertion and are Trials (CENTRAL) was searched manually.
usually removed after treatment completion [17, 36]. A commercially available software program (Endnote
Therefore, orthodontic mini implants most often have X7, Thomson, London, UK) was used for electronic title
a smooth surface to ease removal [32], whereas mini-plates management. Screening was performed independently
are more invasive and require surgical intervention and by two authors (K.B. and M.W.). Disagreement regarding
flap preparation [6]. For this reason, orthodontic mini im- inclusion during the first and second stage of study se-
plants are frequently used, and two concepts are predom- lection was resolved by discussion.
inant: One is to stabilize a dental anchorage unit by The combination of key words (i.e., Medical Subject
connecting it to the implant (indirect anchorage), and the Headings MeSH) and free text terms included:
other is to directly load the orthodontic mini implant with
the reactive forces (direct anchorage) [42]. Search terms PubMed/MEDLINE (including MeSH
Accordingly, there is a need to identify if orthodontic terms)
mini implants are more effective to control anchorage
compared to conventional devices, and to assess if the (“en-masse retraction” OR “incisor retraction” OR
direct or indirect anchorage concept is more beneficial. “front retraction” OR “orthodontic gap closure” OR or
The aim of this systematic review was therefore to ad- “orthodontic space closure” OR “extraction therapy”
dress the following question: “In patients with a need for [mh])AND (“mini implants” OR “micro screws” OR
en masse retraction of the upper front teeth, what is the “micro implants” OR “skeletal anchorage” OR “palatal
efficacy of orthodontic mini implants for anchorage implant” OR “skeletal” OR “skeletal anchorage” OR
quality compared with conventional devices?” “implant” OR “bone screw” OR “temporary anchorage
device” OR “TAD” OR “Bone screws”[mh] OR
“intraosseous screw” OR “dental implants”[mh])
Methods AND (“anchorage loss” OR “anchorage quality” OR
This systematic review was structured and conducted “quality of life” OR “benefit” or “harm” OR “efficacy”
according to the preferred reporting items of the OR “side effects” OR “effect” OR “orthodontic
PRISMA statement [34]. anchorage procedures”[mh] OR “treatment
outcome”[mh])
Search terms EMBASE (including EMTREE terms)
Focused question
The focused question serving for literature search was struc- (“en-masse retraction” OR “incisor retraction” OR
tured according to the PICO (Patients, Intervention, Control, “front retraction” OR “orthodontic gap closure” OR
Outcome) format: “In patients with a need for en masse re- “orthodontic space closure” OR “extraction therapy”
traction of the upper front teeth, what is the efficacy of [EMTREE]) AND (“mini implants” OR “micro screws”
orthodontic mini implants for anchorage control compared OR “micro implants” OR “skeletal anchorage” OR
with conventional devices?” According to the PICO conven- “palatal implant” OR “skeletal” OR “skeletal anchorage”
tion, this question has been formulated as follows: OR “implant”[EMTREE] OR “bone screw”[EMTREE]
OR “tooth implant”[EMTREE] “temporary anchorage
 Patients: for which subgroups of patients with a device” OR “TAD” OR “Bone screws” OR “intraosseous
need for en masse retraction of the upper incisors/ screw” OR “dental implants”) AND (“anchorage loss”
canines OR “anchorage quality” or “quality of life” OR “benefit”
 Intervention: do orthodontic mini implants have a OR “harm” OR “efficacy” OR “side effects” OR “effect”
benefit over conventional devices? OR “orthodontic anchorage”[EMTREE] OR “treatment
 Control: compared to forgoing orthodontic mini outcome”)
implants (compared to conventional treatment)
 Outcome: with regard to treatment efficacy Hand search
(anchorage control), treatment duration, potential The electronic search was complemented by a hand
harms (inflammation, implant loss) search of the following journals: American Journal of
Becker et al. International Journal of Implant Dentistry (2018) 4:35 Page 3 of 12

Orthodontics and Dentofacial Oorthopedics, The Angle potentially relevant studies and eligibility was assessed ac-
Orthodontist, European Journal of Orthodontics, Journal cording for the criteria defined in advance. Disagreements
of Orofacial Orthopedics, Orthodontics and Craniofacial were resolved by open discussion occasionally arbitrated
Research, and Seminars in Orthodontics. by an independent assessor (D.D.). A data extraction tem-
Finally, the references of all selected full-text articles plate was generated including the items’ study design,
and related reviews were scanned. If required, the corre- population, type of implants, number of implants, location
sponding authors were contacted and requested to pro- of the implants, time points of observation, treatment dur-
vide missing data or information. ation, control intervention, measurement method, and
primary and secondary outcomes as well as risk of bias
Study selection (Additional file 1). Data extraction was performed inde-
During the first stage of study selection, the titles and pendently by at least two review authors.
abstracts were screened and evaluated according to the For qualitative and quantitative data analysis, the hori-
following inclusion criteria: zontal and vertical anchorage loss values associated with
direct and indirect anchorage against a control measure
1. English language were defined as primary outcomes. For qualitative data
2. Prospective controlled clinical trials (CCT) (for analysis, transversal anchorage loss, treatment duration,
qualitative synthesis) or randomized controlled and implant failures with direct and indirect anchorage
clinical trials (RCT) (for qualitative and quantitative were defined as secondary outcomes.
synthesis, parallel group designs) in humans
comparing mini implant based on conventional Quality assessment of selected studies
anchored treatments A quality assessment of all selected full-text articles was
3. Patients: general population (all ethnicities, performed according to the Cochrane Collaboration’s
community dwelling) tool for assessing risk of bias (low, high, unclear) includ-
4. Measurement of anchorage loss of the first upper ing the following domains: random sequence generation,
molars during retraction allocation concealment, blinding of outcome assessment,
incomplete outcome data, selective reporting, and other
At the second stage of selection, all full-text articles sources of bias.
identified during the first stage were acquired. During Quality assessment was performed in two different
this procedure, the pre-selected publications were evalu- phases. In the first phase, quality assessment was con-
ated according to the following exclusion criteria: ducted independently by at least two authors (A.P., C.B.,
K.B.) based on the published full-text articles. In the sec-
1. Patients younger than 12 years ond phase, disagreements were resolved by discussion. A
2. No bilateral extraction of one upper premolar per risk of bias table was completed for each included study.
site
3. Inclusion of less than five patients Dealing with missing data and zero values
4. Lack of clinical data on anchorage loss When data were not available in the printed report, we
5. Measurement of anchorage loss not by calculated the missing information whenever possible
superimposition of lateral cephalograms or (e.g., by subtracting pre- and post en masse retraction
superimposition of study casts values). In cases where a zero variance (0.00 mm) was
6. Previous orthodontic treatment presented in the summary tables, these values were
7. Treatment in control group not specified changed to 0.01 mm to enable meta-analysis. The corre-
8. Inclusion of diseased patients, e.g., patients with sponding authors of the published studies were con-
systemic diseases, periodontal disease, and tacted when needed.
syndromes
9. Other treatment than en masse retraction and mini Data synthesis
implants Heterogeneity among the clinical trials, meta-analysis
10. Other sources of skeletal anchorage than (i.e., weighted mean differences and 95% confidence
orthodontic mini implants or micro implants intervals, random effects model to account for potential
methodological differences between studies), forest plots,
Data extraction and method of analysis and publication bias (Egger’s regression to quantify the
At least two review authors examined the titles and ab- bias captured by funnel plots) were assessed using a soft-
stracts of the identified studies and reports independently. ware program (Review Manager (RevMan) version 5.2.
Reports which were clearly not relevant were excluded, Copenhagen: The Nordic Cochrane Centre, The Cochrane
whereas full-text documents were retrieved for all Collaboration, 2012).
Becker et al. International Journal of Implant Dentistry (2018) 4:35 Page 4 of 12

Results Table 1 List of excluded studies (with reason)

Description of studies Reference Reason for exclusion
Study selection Barros et al. (2017) [3] Anchorage loss at first molar not
The search for the review was undertaken at December specified
31, 2017. A total of 2046 potentially relevant titles and Borsos et al. (2012) [7] No en masse retraction (two step
abstracts were found during the electronic and manual canine and front retraction)
search (676 after duplicate removal) of which 99 titles Dai et al. (2009) [10] Chinese language
were considered relevant for abstract screening. During Durrani et al. (2017) [13] Anchorage loss a first molar not
the first stage of study selection, 58 publications were specified
excluded based on the abstract. For the second phase, El-Beialy et al. (2009) [14] Anchorage loss a first molar not
the complete full-text articles of the remaining 41 publi- specified
cations were thoroughly evaluated. A total of 29 papers Garfinkle et al. (2008) [16] Anchorage loss at first molar not
had to be excluded at this stage because they did not specified
comply with the inclusion or exclusion criteria of the Heo et al. (2007) [18] No mini implants used for
present systematic review (Table 1). anchorage
Finally, a total of 12 publications (reporting on 12 Herman et al. (2006) [19] Anchorage loss a first molar not
specified
studies) were considered for the qualitative and a total
of 9 publications for the quantitative assessment (Fig. 1). Janson et al. (2013) [22] Anchorage loss a first molar not
specified
However, only two RCTs were found comparing indirect
anchorage with conventional anchorage devices, whereas Jee et al. (2014) [23] Use of mini implants and
mini-plates
7 studies compared direct anchorage with the control
Kuroda et al. (2009) [27] T0 ceph before leveling (anchorage
intervention. At this stage, it was decided to perform the loss not specified during en masse
quantitative analysis only for the direct anchorage retraction only)
groups. The two studies comparing indirect anchorage Liu et al. (2011) [29] Anchorage loss at first molar not
with a control intervention were included in the qualita- specified
tive analysis only. Summary details of the included stud- Ma et al. (2015) [30] Full-text unavailable (requested but
ies are given in Table 2. no response from authors)
Miyazawa et al. (2010) [33] Anchorage loss at first molar not
specified
Risk of bias in the included studies
Monga et al. (2016) [35] Retrospective study
The review author’s judgment about each risk of bias
item for each included RCT is presented in Table 3 and Park et al. (2004) [38] Case report
Fig. 2. From the studies included in the meta-analysis, Park et al. (2007) [40] Case report
two studies were assessed at low risk of bias [11, 49], Park et al. (2008) [41] Retrospective study
three studies at moderate risk [1, 28, 50], and two at Santiago et al. (2009) [43] No en masse retraction, anchorage
high risk of bias [4, 52]. Risk of bias was not judged for loss at first molar not specified
the studies included in the qualitative synthesis that ei- Shi et al. (2008) [44] Extraction of premolars or
ther had no control group, employed indirect anchorage molars
(see above, the “Study selection” section), had more than Thiruvenkatachari et al. (2006) [46] Canine retraction only
one test group, or lacked a non-implant control group Turkoz et al. (2011) [47] No premolars extracted
[5, 9, 48, 54, 57]. Upadhyay et al. (2012) [51] No premolar extraction
Gollner et al. (2009) [17] No premolar extraction
Characteristics of the patients Wehrbein et al. (1996a) [55] Case report
The study samples consisted of patients exhibiting an Wehrbein et al. (1996b) [56] Case report
Angle Class II,1 malocclusion [1, 5], patients with an
Xu et al. (2008) [59] Language not meeting inclusion
Angle Class I requiring front retraction with maximum criteria
anchorage [4], patients exhibiting dental bi-maxillary pro-
Xun et al. (2004) [61] Language not meeting inclusion
trusion with Angle Class I [9], patients with a need for ex- criteria
traction of four premolars (one in each quadrant) and Yao et al. (2008) [62] Retrospective study, mini-plates, and
maximum anchorage for front retraction [28], patients in mini implants used
need of extraction of the first upper premolars and front
retraction [52], Angle Class I [49], patients with Angle
Class II,1 with dental protrusion [50], or either Angle
Class I or Class II with dental protrusion [11].
Becker et al. International Journal of Implant Dentistry (2018) 4:35 Page 5 of 12

ridge. After leveling, alignment, and placement of a pas-

sive stainless-steel arch (varying from 0.019″ × 0.025″ to
0.016″ × 0.0022″), the implants were placed between
the tooth roots. Retraction was achieved through sliding
mechanics using either power chains or nickel titanium
coil springs of usually 100–200 g. Implant lengths varied
from 7 to 9 mm, and the diameter varied from 1.2 to
2.0 mm (Table 2). All implants were loaded within 3 days
[1, 11, 28, 48–50, 52].
In the majority of the indirect anchorage groups, a sin-
gle mini implant was placed in the anterior palate and
connected to the first molars through an individually
fabricated transpalatal arch [5, 54, 57]. Whereas three
studies used the Straumann® Ortho (Basel, Switzerland)
system and employed loading after 3 months of healing
[5, 54], one study used either a 2 × 10 mm Dual Top™
(Jeil Medical Corporation, Seoul, South Korea) or a
2.0 × 11 mm BENEFIT® (Mondeal Medical Systems,
Mühlheim, Germany) implant and employed immediate
loading. One study employed indirect anchorage
through a mini implant located in the alveolar ridge [9]
(Table 2).
In the control groups, the majority of studies
employed transpalatal arches. Interventions such as
headgear, Nance button, intrusion arches, and differen-
tial moments were also employed (Table 2).

Effects of intervention
Anchorage loss
Anchorage loss was a common finding for all control inter-
ventions. In the test groups, anchorage loss was also associ-
ated with indirect anchorage using mid-palatal implants.
Mesial tooth migration was always lower in indirect an-
chorage mode compared to conventional anchorage groups
(if evaluated) [5, 54, 57].
In detail, anchorage loss associated with indirect anchor-
age and a mid-palatal implant amounted to 1.5 ± 2.6 mm
versus 3 ± 3.4 mm [5], 0.7 ± 0.4 (right molar) and 1.1 ±
0.3 mm (left molar) [54], 1.73 ± 0.39 mm (horseshoe), and
Fig. 1 PRISMA study flow diagram
0.36 ± 0.11 mm (posterior reinforcement) versus 4.21 ±
1.17 mm [57]. An anchorage loss of 0.2 ± 0.35 mm versus
2.0 mm ± 0.65 mm was also observed in one study
The study samples considered for the qualitative syn- employing indirect anchorage using two implants in the
thesis consisted of females exhibiting Angle Class II,1 alveolar ridge [9].
malocclusion with upper dental protrusion and an over- In contrast, no anchorage loss [4] or anchorage gain/re-
jet of at least 7 mm [48], patients with a dental Class II, verse anchorage loss (distal movement) was observed in
a need for extraction of the first upper premolars and the groups employing direct anchorage through implants
front retraction [54], or Class III patients with a need for located interdentally in the alveolar ridge [1, 11, 28, 48–
pre-surgical decompensation through premolar extrac- 50, 52].
tion and front retraction [57]. Vertical anchorage loss with molar extrusion was an-
other common observation for the control interventions
Interventions [1, 11, 28, 49, 50, 52]. In the majority of the studies,
The majority of studies employed mini implants in dir- molar intrusion was commonly associated with direct
ect anchorage mode placed bilaterally in the alveolar skeletal anchorage [11, 28, 48–50, 52], but one study
Table 2 Characteristics of the included studies (TPA transpalatal arch, RCT randomized controlled clinical trial, CCT controlled clinical trial)
Reference Number of patients Type of study Control intervention Type of implant (length, material) Number of Location of implant Mode of anchorage
(RCT/CCT/other) implants (direct/indirect)
Al-Sibaie and Hajeer 56 (28 implant, 28 non RCT TPA Self-drilling titanium mini implants 2 Between the maxillary second Direct
[1] implant) (1.6 mm diameter and 7 mm premolar and first molar
length; Tuttlingen, Germany)
Basha et al. [4] 14 (7 implant, 7 non RCT TPA Surgical steel mini implants 2 Placed between the roots of Direct
implant) (1.3 mm diameter, 8 mm length; second premolar and first
SK Surgical, Pune, India.) molar in the maxilla
Benson et al. [5] 51 (23 implant; 24 non RCT Headgear Ortho implant, (6 mm length, 1 Midpalatal Indirect
implant) Straumann, Waldenburg,
Switzerland)
Chopra et al. [9] 50 (25 implant; 25 non RCT Nance button; lingual Self-drilling titanium ortho 4 Buccal alveolar bone between Indirect
implant) arch implants the second premolars and first
molars in all the four quadrants
Davoody et al. [11] 46 (23 implant, 23 RCT Intrusion arch and 1.8–2 mm in width, 8–9 mm in 4 Placed between maxillary Direct
Becker et al. International Journal of Implant Dentistry

non-implant group) mushroom loops length second premolars and first

molars in all four quadrants
Liu et al. [28] 34 RCT TPA Self-tapping titanium mini-screw 2 Between the roots of the first Direct
implants (8 mm length, 1.2 mm molar and the second premolar
diameter, Cibei, Ningbo, China)
(2018) 4:35

Upadhyay et al. [49] 30 (15 implant, 15 RCT Treatment in control Custom made at our institute by 2 Placed between the maxillary Direct
non-implant) group not specified: modifying conventional surgical second premolar and first molar,
Nance holding arch, screws, measuring 1.3 mm in preferably between the attached
extraoral traction, diameter and 8 mm in length and movable mucosae
banding of the
second molars, and
differential moments
Upadhyay et al. [48] 23 Other (cohort No control group Titanium mini implants (1.3 mm 2 Placed between the roots of the Direct
study) in diameter and 8 mm in length) first molar and the second
premolar in both upper quadrants
Upadhyay et al. [50] 40 (20 implant, 20 non RCT Conventional methods Titanium mini implants (1.3 mm 4 Between the roots of the first molar Direct
implant) such as headgears, diameter, 8 mm length) and second premolar in all four
transpalatal arches, quadrants
banding of second
molars, application of
differential moments
Victor et al. [52] 20 (10 implant, 10 RCT NiTi closed coil spring Absoanchor—SH 1312-08; 4 Placed between the roots of second Direct
non-implant) (1.3 mm diameter, 8 mm premolar and first molar in the upper
length) arch, the screw insertion was
angulated at 40° and 8 mm gingival
to the archwire
Wehrbein et al. [54] 9 Other (cohort No control group Orthosystem (diameter 3.3 mm, 1 Midpalatal Indirect
study) lengths are 4 and 6 mm)
Wilmes et al. [57] 20 (10 in implant group CCT TPA 2.0 × 10 mm, Dual Top™, Jeil 1 Placed in the anterior palate Indirect
of which 5 patients had Medical Corporation, Seoul, South
Page 6 of 12
Table 2 Characteristics of the included studies (TPA transpalatal arch, RCT randomized controlled clinical trial, CCT controlled clinical trial) (Continued)
Reference Number of patients Type of study Control intervention Type of implant (length, material) Number of Location of implant Mode of anchorage
(RCT/CCT/other) implants (direct/indirect)
additional transversal Korea, or 2.0 × 11 mm, BENEFIT,
reinforcement and 5 did Mondeal Medical Systems,
not, 10 in non-implant Mühlheim a.d. Donau, Germany
group)
Becker et al. International Journal of Implant Dentistry
(2018) 4:35
Page 7 of 12
Becker et al. International Journal of Implant Dentistry (2018) 4:35 Page 8 of 12

Table 3 Risk of bias judgment according to the Cochrane observed a minor extrusion tendency of 0.02 ± 0.93 mm
Collaboration associated with direct anchorage [1]. Vertical anchorage
loss associated with indirect anchorage has not been
evaluated.
Transversal anchorage loss with a mean expansion of
1.73 ± 0.39 mm following retraction was observed in one
study employing indirect anchorage through a mid-palatal
mini implant coupled with a horseshoe arch [57]. This ten-
dency of transversal expansion could be reduced to 0.36 ±
0.11 mm by integration of a posterior reinforcement elem-
ent. In contrast, a significant decrease in inter-molar width
was observed in two studies employing direct anchorage
through mini implants in the alveolar ridge [48, 50]. The
inter-molar width reduction amounted to − 1.83 ± 1.29 mm
[50] and may be counterbalanced by a transpalatal arch or
by applying buccal crown torque on the molars [48]. The
remaining studies, which analyzed lateral cephalograms
only, did not report on anchorage loss in the transversal di-
mension. None of the studies compared transversal changes
following skeletal anchorage with conventional control
measures.

Retraction velocity and treatment duration

In the test groups, the monthly rate of posterior move-
ment from the incisors amounted to 0.35 mm with a
mean retraction duration of 12.9 months [1], 0.85 mm
with a mean retraction duration of 6.0 months [4],
0.11 mm with a mean retraction duration of 21.76 months
[9], 0.28 mm with a mean retraction duration of 26 months
[28], 0.85 mm with a mean retraction duration of
8.61 months [49], and 0.44 mm with a mean retraction
duration of 9.4 months [48].

Implant failures
The overall success rates of the orthodontic mini implants
varied among the studies. A success rate of 95.7% with a
loss of 2 from 46 implants was reported by Upadhyay et
al. [48], and the implants could be replaced immediately.
Two patients developed a peri-implant inflammation

Fig. 2 Graphic visualization of the risk of bias judgements

Becker et al. International Journal of Implant Dentistry (2018) 4:35 Page 9 of 12

which was resolved through improved oral hygiene. A loss Based on six studies [1, 11, 28, 48–50, 52], the WMD [95%
of 5 of 72 implants was reported by Upadhyay et al. [49], CI, p] in vertical anchorage loss between test and control
and in 2 patients, treatment was discontinued due to in- groups amounted to − 1.76 [−.56 to − 0.97 mm, p < 0.0001]
flammation, which was resolved through improved oral favoring skeletal anchorage over control measures. The het-
hygiene. Davoody et al. [11] observed a success rate of erogeneity among the studies was high (τ2 = 0.82, I2 = 92%)
84% (5 of 30 implants), and Basha et al. [4] reported a suc- (Fig. 4).
cess rate of 71.4%. In their study, 4 of 14 implants became Funnel plots of the intervention effect estimates (pre-
loose during treatment but could be replaced subse- sented as mean differences) plotted against standard er-
quently. In further 4 patients, treatment was discontinued rors are presented in Figs. 5 and 6. Their symmetricity
due to inflammation, which was resolved through im- suggests the absence of publication bias.
provement of oral hygiene. A success rate of 96% with a
loss of 2 from 50 implants in the upper alveolar ridge due Discussion
to peri-implant inflammation was observed by Chopra et The present systematic review was conducted to address
al. [9], who employed indirect anchorage in the alveolar the following focused question: “In patients with a need
ridge. Similar values were reported by Benson et al. [5], for en masse retraction of the upper front teeth, what is
who employed indirect anchorage through a mini im- the efficacy of orthodontic mini implants for anchorage
plant in the mid-palate. In their study, in 6 of 24 patients, control compared with conventional anchorage devices?”
the implant failed to reach primary stability. In 4 patients, The literature search revealed that efficacy of anchor-
the implant had to be replaced during treatment, and age control of orthodontic mini implants in comparison
in 2 patients, treatment was compromised due to im- to conventional devices was evaluated in nine random-
plant failure. All implant failures occurred among the ized clinical trials (RCTs) [1, 4, 5, 9, 11, 28, 48–50, 52].
first implants placed by the surgeon, and no implant Seven of these studies employed direct anchorage in the
loss was observed for implants with sufficient primary alveolar ridge, whereas one study employed indirect an-
stability. chorage together with a buccal implant [9], and one
A success rate of 100% with no signs of implant mobil- study used a mid-palatal implant and indirect anchorage
ity, inflammation, or loss were observed in two studies [5]. Each of these studies reported on anchorage loss in
[54, 57] in which indirect anchorage through mid-palatal the horizontal dimension, whereas vertical and transver-
implants was employed. sal anchorage loss was only addressed in six and one of
Summarizing these findings, implant loss was observed these studies, respectively. One cohort study also evalu-
at 8 of 93 implants (8.6%) in the indirect anchorage ated vertical anchorage loss associated with mini im-
group. In the direct anchorage groups, implant loss was plants [48], whereas transversal changes have also been
reported for 16 of 162 implants (9.9%). addressed in one controlled clinical trial and in one co-
hort study [54, 57].
Meta-analysis Data syntheses of respective RCTs revealed a gain of
Meta-analysis was performed on RCTs reporting on an- anchorage for direct anchorage in the horizontal and
chorage loss at the first molar. vertical dimension, whereas indirect anchorage was asso-
Based on seven studies [1, 4, 11, 28, 49, 50, 52], the ciated with minor amounts of anchorage loss. Conven-
weighted mean differences (WMD) [95% CI, p] in hori- tional treatments were commonly associated with a
zontal anchorage loss between test and control groups mesial migration and extrusion of the first upper molars.
amounted up to − 2.79 mm [− 3.56 to − 2.03 mm, p < Even though all studies favored orthodontic mini im-
0.0001] favoring skeletal anchorage over conventional an- plants over conventional devices, distal migration and
chorage devices (Fig. 3). The heterogeneity among the an- slight molar intrusion were only observed in groups
alyzed studies was high (τ2 = 0.89, I2 = 89%). employing direct anchorage through mini implants in

Fig. 3 Forest plot for anchorage loss in the horizontal dimension

Becker et al. International Journal of Implant Dentistry (2018) 4:35 Page 10 of 12

Fig. 4 Forest plot for anchorage loss in the vertical dimension

the alveolar ridge. It has been suggested that the distal have been reported in orthodontic mini implant groups
and intrusive forces result from the direction of the re- [48, 54, 57]. Whereas an expansion tendency was ob-
traction forces causing some binding (or increase in fric- served in conjunction with palatal implants and indirect
tion) of the archwire to the brackets or tubes. Friction anchorage [57], inter-molar width reduction and palatal
may have prevented sliding thus causing the force to be tipping of the molar crowns were observed in a study
transmitted through the archwire to the dentition [11, employing direct anchorage and implants in the alveolar
48, 50]. Whether this effect will be more pronounced if ridge [48]. Hence, posterior reinforcement and applica-
a coil spring is left in place for a couple of months after tion of differential moments have been suggested to
completion of front retraction as suggested by Upadhyay avoid these side effects in the respective studies.
et al. [48] has not been analyzed so far. Notably, the ob- Implant loosening or complete failures have been re-
served effects varied from absolute anchorage with no ported in some studies, whereas others observed a 100%
tooth migration [4] to varying amounts of distal migra- success rate. Discontinuation of treatment owing to in-
tion up to − 0.88 mm ± 1.13 mm. Hence, the underlying flammation was reported for implants placed in the al-
biomechanical causes need to be further analyzed. veolar ridge only. However, in several cases, resolution
Indirect anchorage through implants in the alveolar was successfully achieved through improved oral hygiene
ridge was associated with mesial molar migration in all [4, 48, 49]. Whereas adverse effects including root dam-
studies included in the present review [5, 9, 54, 57]. age, or loss of tooth sensibility have been reported in lit-
Nonetheless, anchorage loss with indirect anchorage was erature [15, 21], none of these complications were
significantly lower compared to the conventional devices reported in the included studies. Also, no failures due to
[5, 9, 57]. It has been suggested that the anchorage loss root contact have been reported in the included studies,
at indirectly anchored mid-palatal implants may be even though root proximity is considered to be a major
caused by a slight bending of the transpalatal bars which risk factor for implant loosening [53].
pass from the implant to the anchor teeth [54]. Add- The implant failure rates of 9.9% and 8.6% were com-
itionally, implant migration, which describes a displace- parable between direct and indirect anchorage groups
ment of an implant while maintaining stability, may have and also lower compared to the failure rate of 13.5% re-
contributed to the findings [5]. ported by two systematic reviews [2, 37]. Interestingly,
Transversal changes have not been compared to con- two of three studies reporting on implant failures in the
ventional devices, and controversial transversal effects alveolar palate observed a 100% success rate that relates

Fig. 5 Funnel plot for anchorage loss in the horizontal dimension Fig. 6 Funnel plot for anchorage loss in the vertical dimension (MD
(MD mean difference, SE standard error) mean difference, SE standard error)
Becker et al. International Journal of Implant Dentistry (2018) 4:35 Page 11 of 12

to the achievement of the respective treatment goal [54, Competing interests

57]. In the other study evaluating mid-palatal implants, Kathrin Becker, Annika Pliska, Caroline Busch, Benedict Wilmes, Michael Wolf,
and Dieter Drescher declare that they have no competing interests.
implant failure was observed only among the first series
of implants placed by an unexperienced surgeon, and no
Publisher’s Note
implant losses were noted for implants that had reached Springer Nature remains neutral with regard to jurisdictional claims in
primary stability [5]. This finding is in line with other published maps and institutional affiliations.
studies reporting on high success rates for orthodontic
Author details
implants in the alveolar palate [20, 24, 36, 58]. 1
Department of Orthodontics, Universitätsklinikum Düsseldorf, 40225
Düsseldorf, Germany. 2Department of Orthodontics, Universitätsklinikum
RWTH Aachen, Aachen, Germany.
Conclusions
The present systematic review and meta-analysis re- Received: 4 May 2018 Accepted: 27 August 2018
vealed that orthodontic mini implants are associated
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