Mini-Implants in Orthodontics a Systematic

ONLINE ONLY

Mini-implants in orthodontics: A systematicreview of the literature

Reint Reynders,a Laura Ronchi,a and Shandra Bipatb

Milan, Italy, and Amsterdam, The Netherlands

Introduction: In this article, we systematically reviewed the literature to quantify success and complicationsencountered with the use of mini-implants for orthodontic anchorage, and to analyze factors associated withsuccess or failure. Methods: Computerized and manual searches were conducted up to March 31, 2008, forclinical studies that addressed these objectives. The selection criteria required that these studies (1) reportedthe success rates of mini-implants on samples sizes of 10 implants or more, (2) gave a definition of success, (3)used implants with a diameter smaller than 2.5 mm, and (4) applied forces for a minimum duration of 3 months.Factors associated with implant success were accepted only if potentially influencing variables were con-trolled. The Cochrane Handbook for Systematic Reviews of Interventions was used as the guideline for thisarticle. Results: Nineteen reports met the inclusion criteria, but definitions of success, duration of force appli-cation, and quality of the methodology of these studies varied widely. Rates of primary outcomes ranged from0% to 100%, but most articles reported success rates greater than 80% if mobile and displaced implants wereincluded as successful. Adverse effects of miniscrews included biologic damage, inflammation, and pain anddiscomfort. Only a few articles reported negative outcomes. All proposed correlations between clinical suc-cess and specific variables such as implant, patient, location, surgery, orthodontic, and implant-maintenancefactors were rejected because they did not meet the selection criteria for controlling those variables.Conclusions: Mini-implants can be used as temporary anchorage devices, but research in this field is stillin its infancy. Interpretation of findings was conditioned by lack of clarity and poor methodology of most stud-ies. Questions concerning patient acceptability, rate and severity of adverse effects of miniscrews, and vari-ables that influenced success remain unanswered. This article includes a guideline for future studies of theseissues, based on specific definitions of primary and secondary outcomes correlated with specific operationalvariables. (Am J Orthod Dentofacial Orthop 2009;135:564.e1-564.e19)

Osseointegrated implants are considered reliablesources of anchorage for orthodontists.1-6 How-ever, the large size of these implants limits their

usage. To overcome this problem, mini-implants were de-veloped.7-13 Their advantages, in addition to size, includeminimal anatomic limitations, minor surgery, increasedpatient comfort, immediate loading, and lower costs.11-15

Because these devices are used for specific time pe-riods, mostly rely on mechanical retention, and do notalways osseointegrate, other terms such as miniscrews,miniscrew implants, microscrews, and temporary an-chorage devices have been used.16,17 There is no general

a Private practice, Milan, Italy.b Research associate, Departments of Radiology, Epidemiology and Biostatis-

tics, Academic Medical Center, University of Amsterdam, Amsterdam, The

Netherlands.

The authors report no commercial, proprietary, or financial interest in the prod-

ucts or companies described in this article.

Reprint requests to: Reint Reynders, Via Matteo Bandello 15, 20123, Milan,

Italy; e-mail, [email protected].

Submitted, April 2008; revised and accepted, September 2008.

0889-5406/$36.00

Copyright � 2009 by the American Association of Orthodontists.

doi:10.1016/j.ajodo.2008.09.026

agreement on the nomenclature.18,19 We used the term‘‘mini-implant’’ in the title, because it is currently themost frequently used in the orthodontic literature.

Many mini-implants are now available, and ortho-dontists are trying to incorporate them in various clinicalsituations. However, with the introduction of new tech-niques, questions normally arise. Clinicians desire infor-mation on actual success rates and possible adverseeffects of mini-implants for orthodontic anchorage.They also want to identify variables that could influencesuccess. Although numerous articles on these topics areavailable, confusion arises from differences in their find-ings.20-26 Furthermore, the currently available reviews onmini-implants either were not systematic or asked differ-ent clinical questions.16,17,27-34 Thus, a systematic reviewof the literature was deemed appropriate. The CochraneHandbook for Systematic Reviews of Interventions, theCONSORT guidelines, and the QUOROM statementwere used as the framework for this article. 35,36

The purposes of this review were to record the actualsuccesses and possible negative effects of mini-implantplacement, and to analyze which variables influencesuccess.

564.e1

mailto:[email protected]

564.e2 Reynders, Ronchi, and Bipat American Journal of Orthodontics and Dentofacial Orthopedics

May 2009

CRITERIA FOR CONSIDERING STUDIESFOR THIS REVIEW

Two categories of selection criteria were estab-lished. General measures were applied to find studieson mini-implants and specific selection criteria to im-prove the quality of the articles.

General selection criteria included (1) studies thatanalyzed the success of mini-implants for orthodonticanchorage; (2) only human clinical studies with a mini-mum sample size of 10 miniscrews, with technique ar-ticles, case reports, opinion articles, reviews, andlaboratory, animal, and in-vitro studies excluded; (3)implants with a diameter less than 2.5 mm, becauselarger implants would not be used for specific orthodon-tic indications (eg, interradicular positioning); and (4)no articles on miniplates, because of their different bio-mechanical characteristics.

Specific selection criteria for studies on mini-im-plants included (1) only studies that defined success;(2) only studies that defined the duration of the applica-tion of force; (3) no studies that measured implant suc-cess at less than 120 days of force application,37

arbitrarily chosen because most orthodontic objectivescannot be completed in less than 3 months; and (4) stud-ies that measured success either at a predeterminedtreatment time or at the completion of orthodontic an-chorage objectives.

Patients of both sexes without age restrictions andwith a need of absolute anchorage for orthodontic pur-poses were included.

The outcomes were divided into primary and sec-ondary measures. Primary outcomes were the successor failure of mini-implants as anchorage devices duringorthodontic tooth movement. Secondary outcomes werepossible complications of this treatment.

Immobility, mobility, displacement, and failurewere used as parameters to classify primary outcomes.These measures were examined from the start of the ap-plication of orthodontic forces to 120 days or more37

and were scored as follows.

� Success without mobility (score 0): implants withno clinically detectable mobility that could fulfill allnecessary orthodontic anchorage objectives.� Success with mobility (score 1): implants that had

become mobile but could still fulfill all necessaryorthodontic anchorage objectives.� Success with displacement (score 2): implants that

had become displaced but could still fulfill all neces-sary orthodontic anchorage objectives.� Failure (score 3): implants that were lost or had be-

come unusable, including those that had become in-operative because of excessive tissue proliferation

that could not be reversed by simple excision; im-plants that had caused irreversible biologic damage;implants that could not be used because of the riskof causing irreversible biologic damage; and implantsthat fractured at placement, during orthodontic treat-ment, or at the removal of the screw.� Not specified success (score NSS): the type of success

of implants was not specified and included scores 0, 1,and 2.

Secondary outcomes were divided into 3 categories:biologic damage, inflammation, and pain and discom-fort measures.

Biologic damage was analyzed from the day of im-plant placement until removal. Biologic damage thatoccurred or was detected after removal of the implantwas classified under a separate heading.

� No biologic damage (score 0): no biologic damageand no correcting dental procedures were necessary.� Reversible biologic damage (score 1): biologic dam-

age that is completely reversible with simple dentalprocedures, including removal of hyperplastic tissueand fractured mini-implants that could be removedwithout causing irreversible damage.� Irreversible biologic damage (score 2): biologic dam-

age that is not completely reversible with simple den-tal procedures, including tooth, nerve, sinus, andblood vessel damage; fractured mini-implants thatcould not be removed; and need for orthognathicsurgery caused by uncontrolled biomechanics withmini-implants.� Not specified biologic damage (score NSBD): bio-

logic damage was described, but the type was notidentified.� Postimplant biologic damage (score PIBD):

biologic damage caused by treatment with mini-im-plants, but it occurred or was found after removal ofthe screw.

Inflammation was measured either within the firstmonth of implant placement or beyond this time limit.

� No inflammation (score 0): No signs of inflammationduring the entire period of treatment with mini-im-plants.� Temporary inflammation (score 1): inflammation

confined to the first month.� Continuing inflammation (score 2): inflammation

lasted longer than the first month.� Not specified inflammation (score NSI): its duration

was not specified.

Pain and discomfort were measured during the first2 weeks after placement or beyond.38

American Journal of Orthodontics and Dentofacial Orthopedics Reynders, Ronchi, and Bipat 564.e3Volume 135, Number 5

� No pain or discomfort (score 0): no pain or discomfortduring the entire treatment period with mini-implants.� Moderate pain or discomfort (score 1): moderate pain

or discomfort in the first 2 weeks.� Severe pain or discomfort (score 2): Severe pain or

discomfort in the first 2 weeks.� Continuing pain or discomfort (score 3): pain lasting

longer than 2 weeks.� Not specified pain (score NSP): pain and discomfort

were described, but quality or duration were notspecified.

SELECTION CRITERIA FOR VARIABLESINFLUENCING SUCCESS OF MINI-IMPLANTS

The second part of this review addressed variablesthat might influence the success rates of mini-implants.These variables were classified under the following 6headings: implant, patient, location, surgery, orthodon-tics, and implant-maintenance factors. A correlation be-tween mini-implant success and these parameters wastested according to the following criteria: (1) a proposedassociation with success was rejected only when the ar-ticle presented direct proof that at least 1 influencingvariable was not controlled; lack of information aboutthe control of those factors was insufficient to rejecta correlation; and (2) only factors that had been testedfor statistical significance were included in the analysisof variables.

The following electronic data bases were searchedthrough March 31, 2008: Google Scholar Beta,PubMed, Medline, Embase, Science Direct, all 7 Evi-dence Based Medicine Reviews (EBMR), Web of Sci-ence, Ovid, and Bandolier. Librarians specializing incomputerized searches of the health sciences at theAmerican Dental Association assisted us. The mainsubject heading ‘‘orthodontics’’ was combined withthese keywords: implant, screw, mini-implant, minis-crew, microimplant, screw implant, and temporaryanchorage device. For each search engine, the appropri-ate characters were used to truncate or explore searchterms. To avoid inappropriate exclusion, noun, adjec-tive, singular, and plural forms of all keywords wereused (Appendix Table I). Literature in English, French,German, and Italian was considered. To determinewhether the keywords had covered all articles onmini-implants, the following journals were manuallyscreened: The American Journal of Orthodontics &Dentofacial Orthopedics, The Angle Orthodontist, TheEuropean Journal of Orthodontics, The Journal ofOrthodontics, The Journal of Clinical Orthodontics,Seminars in Orthodontics, and The International Jour-nal of Adult Orthodontics and Orthognathic Surgery.

In addition, references from each identified articlewere manually screened for articles that were missedby the electronic search engines. Finally, all manualand electronic searches were solicited for review arti-cles.35 References in the review articles were alsoscreened for pertinent studies. This analysis provideda list of studies on mini-implants with their successrates.

METHODS OF THE REVIEW

All abstracts were read, and the full texts of allrelevant articles were collected and reviewed. Ambig-uous articles were also read to avoid inappropriateexclusion. All procedures were performed indepen-dently by 2 authors (R.R. and L.R.). Differenceswere resolved by rereading and discussion until con-sensus was reached.18,35 Studies were also assessedfor eligibility and methodologic quality without con-sidering the outcomes. For each study, a value wasgiven based on the quality of the following 4 criteria:definition of success, design of the study, descriptionof the methodology, and control of variables. A cleardescription of each criteria accounted for 1 point.Studies were then classified as clear (3 or 4 points),partially clear (2 points), or unclear (0 or 1 point). As-sessment of study validity was not used as a thresholdfor inclusion but only as a possible explanation for dif-ferences in results between studies.35

A decision to perform a meta-analysis was made ifthere were sufficient similarities between studies inthe types of participants, interventions, and outcomes.Although several studies used the same implant system,the significant heterogeneity within and between studiesdid not allow for pooling of data and carrying outa meta-analysis.35

The selection procedures are explained in a flow di-agram (Fig).36 A total of 3364 abstracts without overlapwere found by the search methods and are described inAppendix Table II. Only 52 abstracts met the inclusioncriteria or were retrieved because the abstract did notprovide enough information to justify exclusion (Fig).Twenty-one articles were excluded according to thegeneral selection criteria, and the specific selectioncriteria eliminated another 12, leaving 19 studies. Theexplanations for excluding these articles are given inAppendix Tables III and IV. The assessment of the qual-ity of the 19 studies is given in Table I. Five studies wererated as clear,14,21,22,39,40 8 as partially clear,23,25,37,41-45

and 6 as unclear.12,24,26,38,46,47 No article was a random-ized clinical trial. Another systematic review also didnot find any randomized clinical trials about mini-implants in the literature.29


May 2009

RESULTS

Primary outcomes, defining success and failure ofmini-implants, are presented in Table II. The definitionsof success varied in the studies. Five articles did not spec-ify the type of primary outcomes.12,14,22,38,42 They gavegeneral descriptions of success without specifying thenature of the stability of the implants. Seven articles con-sidered only immobile screws successful,23,26,39-41,43,45

and 5 accepted mobility.24,25,37,44,46 Displacements ofmini-implants were assessed in 3 articles.21,44,47 Moststudies found success rates greater than 80% if usablemobile and displaced implants were included as success-ful. Primary outcomes varied from 0% to 100%. Onearticle compared 2 protocols but did not define successin the second protocol.46 Another article gave a successrate of 86.8%, but our calculations added up to only

* General selection criteria** Specific selection criteria

Excluded abstractsN= 3312Reason: *

Articles retrieved for more detailed analysesN= 52

Excluded articlesN = 21Reason: *

Excluded articlesN= 12Reason: **

Included articles: N=19

Articles retrieved for more detailed analysesN = 31

Abstracts retrieved from all search methods N = 3364

Fig. QUOROM flow diagram.

76.7%.41 The time of assessment of success variedwidely (Table II). Six studies analyzed primary out-comes at specific time periods: 150 days,45 180 days,41

6 months,22,39 8 months,40 and 9 months.21 In the otherstudies, success was measured at the completion of theanchorage objectives, varying from 3 to 37 months.

Among the secondary outcomes, biologic damagewas described in 5 of the 19 articles (TableII).14,24,26,41,46 Three studies found no biologic dam-age,14,41,46 and Park et al24 recorded 8 broken screws,with 3 fractured during placement and 5 during re-moval. Two of 59 screws broke during placement inanother study.26 In both studies, no information was pro-vided about the outcome of the removal of the fracturedimplants. Biologic damage that was caused by treatmentwith mini-implants but occurred after screw removalwas not assessed in any study.

Inflammation was evaluated in 6 studies and variedfrom 0% to 34% (Table II).12,14,23-25,45 Temporary in-flammation of peri-implant soft tissues was describedin 4 articles.12,14,23,45 Freudenthaler et al14 and Thiru-venkatachari et al23 reported that inflammation was con-trolled by improving oral hygiene. However, Tsenget al25 recorded continuing inflammation in 2 of 45 im-plants. It failed to subside, and the implants were lost orhad to be removed. A similar outcome was described in4 of 32 patients in another study.45 Park et al24 found in-flammation in 34% of the implants but did not specifyits severity or duration. To control peri-implantitis,

Table I. Characteristics of included studies

AuthorsYear of

publication

Assessmentof

validity

Numberof

implants

Designof

study

Freudenthaler et al14 2001 A 12 P

Miyawaki et al12 2003 C 134 R

Liou et al21 2004 A 32 P

Motoyoshi et al22 2006 A 124 P

Thiruvenkatachari et al23 2006 B 18 P

Park et al24 2006 C 227 R

Tseng et al25 2006 B 45 R

Chen et al26 2006 C 59 R

Berens et al46 2006 C 239 ND

Luzi et al37 2007 B 140 P

Wiechmann et al41 2007 B 133 P

Kuroda et al42 2007 B 216 R

Motoyoshi et al39 2007 A 169 P

Kuroda et al38 2007 C 116 R

Motoyoshi et al43 2007 B 87 R

Hedayati et al44 2007 B 27 P

Chaddad et al45 2008 B 32 P

Moon et al40 2008 A 480 R

Kinzinger et al47 2008 C 16 ND

Assessment of validity: A, clear; B, partially clear; C, unclear.

Design of study: P, prospective; R, retrospective; ND, not described.


Table II. Analysis of outcomes of studies on mini-implants

AuthorsTime of successmeasurement Success rate

Rate of biologicdamage

Rate ofinflammation

Rate of painand discomfort

Freudenthaler

et al14ARTT 75% (NSS) 0% (NSBD) 25% (score 1) 37.5% (score 1)

Average: 11 months

Range: 7-20 months

Miyawaki et al12 1 year or ARTT 76.1% (NSS) ND 8.9% (score 1) 15.9% (score 1)

Range: 0%-85%

Liou et al21 9 months 56.25% (score 0) ND ND ND

43.75% (score 2)

Motoyoshi et al22 6 months 85.5% (NSS) ND ND ND

Thiruvenkatachari

et al23ARTT 100% (score 0) ND 10% (score 1) ND

3.5-5.5 months

Park et al24 ARTT 91.6% (scores 0 and 1) 3.5% (NSBD) 34% (NSI) ND

Mean: 15 months Range: 80%-93.6%

SD: 6.16 months

Tseng et al25 ARTT 91.1% (scores 0 and 1) ND 4.4% (score 2) ND

Average: 16 months Range: 80%-100%

Chen et al26 ARTT 84.7% (score 0) 3.4% (NSBD) ND ND

Mean: 19.5 months Range: 72.2%-90.2%

Berens et al46 ARTT Protocol 1: 0% (NSBD) ND ND

Average: 235 days 68.4% (score 0)

Maximum: 733 days 8.3% (score 1)

23.3% (score 3)

Protocol 2:

4.7% (score 3)

Luzi et al37 ARTT 84.3% (score 0) ND ND ND

Minimum:120 days 6.4% (score 1)

Maximum: 37 months

Wiechmann et al41 180 days 76.7% (score 0) 0% (score 2) ND ND

Range: 69.6%-87%

Kuroda et al42 1 year or ARTT 86.4% (NSS) ND ND ND

Range: 35.3%-100%

Motoyoshi et al39 6 months 85.2% (score 0) ND ND ND

Range: 63.8%-97.3%

Kuroda et al38 1 year or ARTT 86.2% (NSS) ND ND Flap group:

Range: 81.1%-88.6% 95%-10%

(scores 1 and 2)

Flapless group:

50%-0% (score 1)

Motoyoshi et al43 ARTT 87.4 (score 0) ND ND ND

6 months or more

Hedayati et al44 ARTT 81.5%(scores 0, 1, and

2)

ND ND ND

Average: 5.4 months

Range: 4-6.5 months

Chaddad et al45 150 days 87.5% (score 0) ND 6.25% (score 1) 80% (score 0)

Range: 82.5%-93.5% 6.25% (score 2) 20% (score 1)

Moon et al40 8 months 83.8 (score 0) ND ND ND

Kinzinger et al47 ARTT 100% (score 2) ND ND ND

6.5 months

ND, Not described; ARTT, anchorage for required treatment time.

Success scores: 0, success without mobility; 1, success with mobility; 2, success with displacement; 3, failure; NSS, unspecified success (includes

scores 0-2).

Biologic damage scores: 0, no damage; 1, reversible damage; 2, irreversible damage; NSBD, unspecified damage (includes scores 1 and 2).

Inflammation scores: 0, no inflammation; 1, temporary inflammation; 2, continuing inflammation; NSI, unspecified inflammation.

Pain and discomfort scores: 0, no pain and discomfort; 1, moderate pain and discomfort; 2, severe pain and discomfort; 3, continuing pain and

discomfort; NSP, unspecified pain and discomfort (includes scores 1-3).


May 2009

Table III. Variables associated with success rates in studies on mini-implants

Association withsuccess suggested

Studies proposingassociation with success

Studies rejectedand reasons for rejection

Implant-related factors

Type: no Miyawaki,12 Park,24 Kuroda,42 Kuroda,38 Chaddad45 Miyawaki12 (a-e), Park24 (a-e), Kuroda42 (a-e),

Kuroda38 (a-e), Chaddad45 (a-e)

Type: yes Miyawaki,12 Wiechmann41 Miyawaki12 (a-e), Wiechmann41 (a-e)

Diameter: yes Miyawaki,12 Wiechmann41 Miyawaki12 (a-e), Wiechmann41 (a-c,e)

Length: no Miyawaki,12 Park,24 Wiechmann,41 Kuroda38 Miyawaki12 (a-e), Park24 (a-e), Wiechmann41

(a-c,e), Kuroda38 (a-e)

Length: yes Chen,26 Tseng25 Chen26 (b,c,e), Tseng25 (b,c,e)

Patient-related factors

Sex: no Motoyoshi,22 Park,24 Kuroda,38 Motoyoshi,43

Moon40Motoyoshi22 (b-d), Park24 (a-e), Kuroda38 (a-e),

Motoyoshi43 (b-d), Moon40 (c-e)

Age: no Miyawaki,12 Motoyoshi,22 Park,24 Kuroda,38 Moon40 Miyawaki12 (a,c-e), Motoyoshi22 (c,d), Park24

(a,c-e), Kuroda38 (a,c-e), Moon40 (c-e)

Age: yes Motoyoshi39 Motoyoshi39 (c,d)

Physical status VNA

Mandibular plane

angle: no

Kuroda38 Kuroda38 (a-e)

Mandibular plane

angle: yes

Miyawaki12 Miyawaki12 (a-e)

Temporomandibular symptoms: no Miyawaki,12 Kuroda38 Miyawaki12 (a-e), Kuroda38 (a-e)

Crowding: no Miyawaki12 Miyawaki12 (a-e)

Anteroposterior jaw relationship: no Miyawaki,12 Kuroda38 Miyawaki12 (a-e), Kuroda38 (a-e)

Location-related factors

Peri-implant bone

quantity: no

Motoyoshi43 Motoyoshi43 (b,d)

Cortical bone

thickness: yes

Motoyoshi43 Motoyoshi43 (b,d)

Keratinized vs oral

mucosa: no

Chaddad45 Chaddad45 (a-e)

Exposed vs closed

mucosa: yes

Park24 Park24 (a-e)

Same success

maxilla and mandible

Miyawaki,12 Motoyoshi,22 Motoyoshi,39 Chaddad,45

Motoyoshi,43 Moon40Miyawaki12 (a-e), Motoyoshi22 (b-d),

Motoyoshi39 (c,d), Chaddad45 (a-e),

Motoyoshi43 (b-d), Moon40 (c-e)

Mandible more

failures than maxilla

Park,24 Wiechmann,41 Kuroda42 Park24 (a-e), Wiechmann41 (a-c,e), Kuroda42

(a-e)

Lower success

lingual mandible

Wiechmann41 Wiechmann41 (a-c,e)

Molar area

lower success than

premolar area in mandible

Moon40 Moon40 (d,e)

Molar area

same success as premolar

area in maxilla

Moon40 Moon40 (d,e)

Left side

higher success than

right side

Park24 Park24 (a-e)

No difference

between left and right sides

Motoyoshi,22 Motoyoshi,43 Moon40 Motoyoshi22 (b-d), Motoyoshi43 (b-d), Moon40

(c-e)

Root proximity: yes Kuroda42 Kuroda42 (a-e)

Surgery-related factors

Flapless/flap surgery: no Miyawaki,12 Moon40 Miyawaki12 (a-c,e), Moon40 (c,e)

Direction of placement: no Park24 Park24 (a-c,e)

Placement torque: yes Motoyoshi,22 Motoyoshi,39 Chaddad,45 Motoyoshi43 Motoyoshi22 (b,c), Motoyoshi39 (c), Chaddad45

(a-c,e), Motoyoshi43 (b,c)

Self-drilling vs self-tapping

technique

VNA

Different surgeons VNA


Table III. Continued

Association withsuccess suggested

Studies proposingassociation with success

Studies rejectedand reasons for rejection

Orthodontic-related factors

Magnitude of force: no Kuroda38 Kuroda38 (a-e)

Timing of force

application: no

Miyawaki12 Miyawaki12 (a-e)

Timing of force

application: yes

Motoyoshi39 Motoyoshi39 (c,d)

Duration of force VNA

Type of force: no Park24 Park24 (a-e)

Type of orthodontic

movement: yes

Kuroda38 Kuroda38 (a-e)

Direction of force VNA

Implant-maintenance factors

Antibiotics prescription VNA

Chlorhexidine prescription VNA

Oral hygiene: no Park24 Park24 (a-e)

Control of peri-implant

inflammation: yes

Miyawaki,12 Park24 Miyawaki12 (a-e), Park24 (a-e)

Control of mobility: yes Park24 Park24 (a-e)

Total 70 70

Reasons for rejection: a, implant-related factors were not controlled; b, patient-related factors were not controlled; c, location-related factors were

not controlled; d, surgery-related factors were not controlled; e, orthodontics-related factors were not controlled; f, implant maintenance-related

factors were not controlled.

VNA, Variable was not analyzed.

these authors recommended placing implants in the ker-atinized gingiva or to cover the screws by soft tissue,and to improve oral hygiene.

Pain and discomfort were recorded in 4 of the 19studies (Table II).12,14,38,45 Freudenthaler et al14 re-ported minor pain after placement that lasted only 1day in 3 of 8 patients. Similar findings were reportedby Chaddad et al45 in 2 of 10 patients. Kuroda et al38

analyzed both the quality and the duration of pain dur-ing the first 2 weeks after placement. One hour afterimplantation, 95% of the patients who had screwsplaced after raising a mucoperiosteal flap reportedpain, compared with 50% of those who had undergonea flapless approach. After 2 weeks, the values were10% and 0% for the respective techniques. Patients inthe flap group described significantly more intensepain and for a longer period than those in the flaplessgroup. A similar finding was recorded by Miyawakiet al12 in 7 of 44 patients within a week after implantplacement.

Variables proposed as having possible associationswith success are given in Table III. The studies pre-sented 70 correlations between at least 1 variable andclinical success; many were contrary associations. Allwere rejected because parameters selected as indepen-dent variables were not controlled and therefore didnot meet the inclusion criteria. Explanations for rejec-

tion are listed in Table III and can be verified in TablesIV to IX.

DISCUSSION

Nineteen studies were selected from computerizedand manual searches through March 31, 2008, to pro-vide data regarding the success of mini-implants. Casereports and technique articles describing the specialmerits of a specific miniscrew were those most often ex-cluded by the general selection criteria. Twelve studieswere excluded because of imprecise methodology. Ran-domized clinical trials were not available, and the qual-ity of most included studies was low (Table I). Thesefindings were surprising, considering the wide interestin the clinical applications of mini-implants as ortho-dontic anchorage devices. An analysis of the impactof the quality of the various studies was superfluous be-cause all correlations between proposed variables andsuccess were rejected by the inclusion criteria, and out-comes from both high- and low-quality articles weresimilar (Tables II and III).

For the primary outcomes, most studies found suc-cess rates greater than 80%, with a range of 0%to100%. Five factors are possible explanations for thisvariation. First, the studies used a wide range of defini-tions for primary outcomes. Success was analyzed withvarious benchmarks including with or without mobility,


May 2009

Table IV. Implant-related factors in studies on mini-implants

AuthorsNumber ofimplants Implant type

Diameter (D)and length (L) Success rate*

Freudenthaler et al14 12 Leibinger D 2 mm 75%

L 13 mm

Miyawaki et al12 10 Photo but not described D 1.0 mm 0%

L 6 mm

Miyawaki et al12 101 Photo but not described D 1.5 mm 83.9%

L 11 mm

Miyawaki et al12 23 Photo but not described D 2.3 mm 85%

L 14 mm

Liou et al21 32 Leibinger D 2 mm 100%

L 17 mm

Motoyoshi et al22 124 Biodent D 1.6 mm 85.5%

L 8 mm

Thiruvenkatachari et al23 18 Not described D 1.3 mm 100%

L 9 mm

Park et al24 19 Leibinger D 1.2 mm 84.2%

L 5 mm

Park et al24 157 Osteomed D 1.2 mm 93.6%

L 6, 8,10 mm

Park et al24 46 Absoanchor D 1.2 mm 89.1%

L 4, 6, 7, 8, 10 mm

Tseng et al25 15 Leibinger D 2 mm 80%

L 8 mm


L 10 mm


L 12 mm

Chen et al26 18 Abosanchor D 1.2 mm 72.2%

L 6 mm

Chen et al26 41 Absoanchor D 1.2 mm 90.2%

L 8 mm

Berens et al,46 protocol 1 133 Absoanchor D 1.3-2 mm 76.7%

Dual Top L not described

Berens et al,46 protocol 2 106 Absoanchor D 1.3-2 mm 95.3%

Dual Top L not described

Luzi et al37 140 Aarhus D 1.5, 2 mm 90.7%

L 9.6, 11.6 mm

Wiechmann et al41 79 Absoanchor D 1.1 mm 69.6%

L 5, 6, 7, 8, 10 mm

Wiechmann et al41 54 Dual Top D 1.6 mm 87%

L 5, 6, 7, 8, 10 mm

Kuroda et al42 195 Absoanchor D 1.3 mm

L 6, 7, 8, 10, 12 mm

Maxilla, 77.1%-95.8%

Mandible, 35.3%-83.9%

Kuroda et al42 21 Martin D 1.5 mm Maxilla

L 9 mm 50%-100%

Motoyoshi et al,39 early

load adolescent group

47 Biodent D 1.6 mm

L 8 mm

63.8%

Motoyoshi et al,39 late

load adolescent group

36 Biodent D 1.6 mm

L 8 mm

97.2%

Motoyoshi et al,39 early load

adult group

86 Biodent D 1.6 mm

L 8 mm

91.9%

Kuroda et al38 37 Keisei D 2.0, 2.3 mm 81.1%

L 7, 11 mm

Kuroda et al38 79 Absoanchor D 1.3 mm 88.6%

L 6, 7, 8, 10, 12 mm

Motoyoshi et al43 87 Biodent D 1.6 mm 87.4%

L 8 mm

Hedayati et al44 27 O&M Medical D 2.0 mm 81.5%

L 9, 11 mm


Table IV. Continued

AuthorsNumber ofimplants Implant type

Diameter (D)and length (L) Success rate*

Chaddad et al45 17 Dual Top D 1.4, 1.6, 2.0 mm 82.5%

L 6, 8, 10 mm

Chaddad et al45 15 C-Implant D 1.8 mm 93.5%

L 8.5 mm

Moon et al40 480 Dual Top D 1.6 mm 83.8%

L 8 mm

Kinzinger et al47 16 T.I.T.A.N. Pin-System D 1.6 mm 100%

Dual Top L 8-9 mm

*Definition of success as established by the authors of the respective studies.

with or without displacement, or not specified (Table II).Second, the timing of assessment of the primary out-comes differred among the studies—from 3 to 27months after the application of orthodontic forces. Animplant lost after 4 months could then be defined eitheras a failure or a success depending on the time of its as-sessment. Some articles measured primary outcomesfrom the day of placement, not from the start of appliedorthodontic forces.23,26 Third, the interpretation of pri-mary outcomes was hampered by differences in studydesign and methodology. Fourth, variables were fre-quently not controlled and could have easily skewedthe findings. Fifth, removal and replacement of implantsin the same patient could have introduced underreporting.

Secondary outcomes caused by the placement ofminiscrews were only sporadically mentioned in thestudies (Table II). Studies showing an intervention tobe effective are more likely to be published and may re-sult in overestimate of effectiveness due to publicationbias.35,48-50 Various forms of biologic damage havebeen presented in the orthodontic literature including:root trauma, soft-tissue irritation, nerve injury, traumato blood vessels, and sinus perforation.13,21,25,27,51

Furthermore, mini-implants have been proposed as analternative for certain orthognathic surgical procedures,but could also be its cause when uncontrolled biome-chanics are applied. Displacements of apparently stablemini-implants were recorded in 3 studies,21,44,47 andthis finding was confirmed in the literature.52 To avoidtrauma to adjacent structures, a safety clearance of 2mm was recommended in interdental areas.21,52 How-ever, root damage caused by screws was shown toheal in animal studies53,54 and in a report of 2 patients.55

Daimaruya et al56 reported no harm to the nerve after in-trusion of molars into the neurovascular bundle of dogs.Information on damage caused by miniscrews shouldbe collected up to a year after their removal, becauseconsequences of fractured implants, and root, nerve,

and other forms of trauma, can appear later. Long-term screening was not part of the protocol in any ofthe 19 studies.

Information about the character and the duration ofinflammation of the peri-implant tissues was rarelygiven; these parameters require further investigation.Similar conclusions can be drawn with regard to painand discomfort.

The assessment of variables influencing the successof mini-implants was complicated because of the smallnumber of failures, the lack of clarity of the selectedstudies, the wide variety in designs, and the many vari-ables involved. Tables IV through IX show the many un-controlled variables. Rejection measures were relativelylenient, because proposed associations were rejectedonly when the article presented direct proof that a poten-tial influencing factor was not controlled. Elimination ofcorrelations would have been instant if not describingcontrollable variables had been an exclusion criterion.Notwithstanding these tolerant measures, all proposedassociations were rejected (Table III). The large numberof contrary correlations were probably the testimony ofthe poor control of variables. The proposed associationsshould therefore be interpreted as strictly hypotheticalvariables that could influence success; they are dis-cussed below. To facilitate this discussion, these vari-ables were divided into 6 categories: implant, patient,location, surgery, orthodontic, and implant-mainte-nance factors.

Implant-related factors are summarized in TableIV. Implants are made of various materials and differin design and surface treatment. Implant types variedbetween and within the 19 studies (Table III) or werenot specified.12,23,46 Eight studies did not controlfor implant type, diameter, and or length when com-paring outcomes.12,24,38,41,42,45-47 Both confirmingand refuting associations were found between implanttype and primary outcomes (Table IV). However, the


May 2009

Table V. Patient-related factors in studies on mini-implants

Authors Sex and number of implants Age (y) Physical status Dental status

Freudenthaler et al14 4 females: NIND 22.1 (mean) ND D

4 males: NIND Range: 13-46

Miyawaki et al12 42 females: NIND 21.8 (mean) ND D

9 males: NIND SD: 7.8

Liou et al21 16 females: 32 implants Range: 22-29 ND ND

Motoyoshi et al22 37 females: 114 implants 24.9 (average) ND ND

4 males:10 implants SD: 6.5

Range: 13.3-42.8

Thiruvenkatachari et al23 7 females: 12 implants 19.6 (mean) D D

3 males: 6 implants Range: 16-21

Park et al24 52 females: 138 implants 15.5 (mean) ND ND

35 males: 89 implants SD: 8.3

Tseng et al25 14 females: NIND 29.9 (mean) ND ND


Chen et al26 20 females: NIND 29.8 (mean) ND ND


Berens et al46 61 females: NIND 28 (mean) ND ND


Luzi et al37 60 females: NIND 34.3 (mean) ND D


Wiechmann et al41 36 females: NIND 26.9 (mean) ND ND


Range: 13.5-46.2

Kuroda et al42 92 females: NIND 22.5 (mean) ND ND


Motoyoshi et al,39 adolescent group 24 females: NIND 15.9 (mean) ND ND


Range: 11.7-18.9

Motoyoshi et al,39 adult group 24 females: NIND 26.2 (mean) ND ND


Range: 20.4-36.1

Kuroda et al38 63 females: NIND 21.8 (mean) ND ND


Motoyoshi et al43 28 females: 76 implants 24.4 (average) ND D

4 males: 11 implants SD: 6.5

Range:14.6-42.8

Hedayati et al44 ND 17.4 (mean) ND D

Range 15.5-19

Chaddad et al45 ND Range: 13-65 D ND

Moon et al40 131 females: 323 implants Young: 14.4 (mean) Range: 10-18 ND ND

78 males: 157 implants Adult: 26.2 (mean)

Range: 19-64

Kinzinger et al47 6 females: 12 implants 12.2 (average) ND ND

2 males: 4 implants

D, Described; ND, not described; NIND, number of implants by sex not described.

orthopedic literature as well as laboratory and animalstudies have demonstrated the importance of the archi-tecture of implants on success.57-60 Furthermore, theimplant material has an impact on the placement tech-nique. Compared with pure titanium, titanium alloysare stronger, and drilling a pilot hole is frequently un-necessary.28

Implant diameters ranged from 1.0 to 2.3 mm, withsuccess rates varying from 0% to 100% (Table IV). All10 mini-implants with a diameter of 1.0 mm were lost in1 study, but variables were not sufficiently controlled for

a correlation with failure.12 Studies on porcine iliacbone segments confirmed an association between im-plant diameter and success.61,62 Implants with a smallerdiameter are easier to place between the roots, buta small decrease in this dimension significantly in-creases the torsional strength and therefore the risk offracture.7,26,63-65 It has been suggested that implantssmaller than 1.3 mm should be avoided, especially inthe thick cortical bone of the mandible.7,64 Fractureswere also reported in 2 studies with implants of thesedimensions.24,26


Table VI. Location-related factors in studies on mini-implants

Authors Implant siteBone

conditionKeratinized or

nonkeratinized mucosaExposedor closed

Freudenthaler et al14 Buccally in mandible D ND ND

Miyawaki et al12 Posterior buccal alveolar bone in maxilla and mandible D Keratinized ND

Liou et al21 Zygomatic buttresses of maxilla D Nonkeratinized Exposed

Motoyoshi et al22 Posterior buccal alveolar bone in maxilla and mandible ND ND ND

Thiruvenkatachari et al23 Buccal alveolar bone in maxilla and mandible ND ND ND

Park et al24 Various locations in maxilla and mandible D Both Both

Tseng et al25 Various locations in maxilla and mandible D ND ND

Chen et al26 Various locations in maxilla and mandible ND ND ND

Berens et al46 Various locations in maxilla and mandible ND Both ND

Luzi et al37 Various locations in maxilla and mandible D Both Exposed

Wiechmann et al41 Various locations in maxilla and mandible ND Keratinized ND

Kuroda et al42 Various locations in maxilla and mandible ND Keratinized ND

Motoyoshi et al39 Posterior buccal alveolar bone in maxilla and mandible D Keratinized ND

Kuroda et al38 Various locations in maxilla and mandible D Both Exposed

Motoyoshi et al43 Posterior buccal alveolar bone in maxilla and mandible D ND ND

Hedayati et al44 Palate and posterior buccal alveolar bone in mandible ND ND ND

Chaddad et al45 Posterior buccal alveolar bone in maxilla and mandible ND Both ND

Moon et al40 Posterior buccal alveolar bone in maxilla and mandible D Keratinized ND

Kinzinger et al47 Anterior palate D Keratinized Exposed

D, Described; ND, not described.

Table VII. Surgery-related factors in studies on mini-implants

Authors Flap or flapless surgery Placement technique Direction of placement

Freudenthaler et al14 Flap ST and same-size pilot hole Perpendicular to bone

Miyawaki et al12 Both ND ND

Liou et al21 Flap ST and smaller pilot hole ND

Motoyoshi et al22 Flapless ST and smaller pilot hole ND

Thiruvenkatachari et al23 ND ND ND

Park et al24 Flap ST and smaller pilot hole Various angulations

Tseng et al25 Flapless SD and smaller pilot hole in cortex ND

Chen et al26 Flapless SD and smaller pilot hole in cortex ND

Berens et al46 Flapless SD and smaller pilot hole in cortex ND

Luzi et al37 Flapless SD and no pilot hole ND

Wiechmann et al41 Flapless SD and smaller pilot hole in cortex ND

Kuroda et al42 Flapless ST and smaller pilot hole 20�-40� to long axis of tooth

Motoyoshi et al39 Flapless ST and smaller pilot hole 30� to long axis of tooth

Kuroda et al38 Both ST and smaller pilot hole ND

Motoyoshi et al43 Flapless ST and smaller pilot hole ND

Hedayati et al44 Flapless ST and same-size pilot hole 30� to midsagittal plane

Chaddad et al45 Flapless SD and pilot hole in cortex only (size ND) ND

Moon et al40 Both SD and no pilot hole 70�-80� to long axis of tooth

Kinzinger et al47 Flapless SD and ST, no pilot hole ND

ND, Not described; SD, self-drilling screw; ST, self-tapping screw.

The length of a mini-implant is determined by depthand quality of the bone, screw angulation, transmucosalthickness, and adjacent vital structures.25,38,63,66 Shortscrews in regions with thick soft tissues, such as the pal-atal mucosa, can easily become dislodged.25,67,68 Lon-ger screws are recommended in these sites.46,68

The minimal depth of placement of a mini-implant isat least 5 to 6 mm.13,25,38 but deeper placements have

been recommended when bone quality is low.45,46

Screw length was correlated with success in 2 studies,but this association did not pass the inclusion criteriaand requires further analysis (Table III).25,26

Of the patient-related factors (Table V), most stud-ies found a disproportionate division of the sexes withan excess of females (Table V). Ten studies did not de-fine the numbers of implants for each sex, and 2 articles


May 2009

Table VIII. Orthodontics-related factors in studies on mini-implants

AuthorsType of orthodontic

movementTiming of force

applicationForce

magnitude Type of forceDuration of force

applicationDirectionof force

Freudenthaler

et al14Mandibular molar

protraction

Immediate (at time of

surgery)

150 g Continuous and

intermittent

Average: 11 months D

Range: 7-20 months

Miyawaki et al12 Various movements

in both jaws

Immediate (\1 month) \200 g Continuous 1 year or ARTT ND


in both jaws

Delayed (1-3 months) \200 g Continuous 1 year or ARTT ND


in both jaws

Delayed (.3 months) \200 g Continuous 1 year or ARTT ND

Liou et al21 En-masse retraction of

maxillary anteriors

Immediate (after 2

weeks)

400 g Continuous 9 months D

Motoyoshi et al22 Retraction of anterior

teeth in both jaws


surgery)

\200 g ND 6 months ND

Thiruvenkatachari

et al23Retraction of canines

in both jaws

Immediate (after 15

days)

100 g Continuous 3.5-5.5 months or

ARTT

D

Park et al24 Various movements

in both jaws

Immediate and delayed

(no time definition)

\200 g Continuous and

Intermittent

Mean: 15 months or

ARTT

ND

SD: 6.16 months

Tseng et al25 Various movements

in both jaws

Immediate (after 2

weeks)

100-200 g Continuous and

intermittent

Average:16 months ND

ARTT

Chen et al26 Various movements

in both jaws

Immediate (after 2

weeks)


intermittent

Mean: 19.5 months ND

ARTT

Berens et al46 Various movements

in both jaws


surgery)

Maximum

of 150 g

Intermittent Average: 235 days ND

Maximum: 733

days

ARTT

Luzi et al37 Various movements

in both jaws


surgery)

50 g Continuous Minimum:120 days

or ARTT

D

Maximum: 37

months

Wiechmann et al41 Various movements

in both jaws


surgery)


intermittent

180 days D

Kuroda et al42 ND Immediate and delayed

(after 0-12 weeks)


intermittent

1 year or ARTT ND


teeth in both jaws

Immediate (after 2-4

weeks) and delayed

(more than 3

months)

Approximately

200 g

Continuous 6 months ND

Kuroda et al38 Various movements

in both jaws

Immediate and delayed

(0-12 weeks)


intermittent

1 year or ARTT ND


teeth in both jaws

Immediate (at surgery) ND ND 6 months or more ND

Hedayati et al44 Canine retraction in

both jaws

Immediate (after 7-11

days)

180 g Continuous Average: 5.4

months

ND

Range: 4-6.5

months

Chaddad et al45 Various movements

in both jaws

Immediate (at surgery) 50-250 g Continuous and

intermittent

150 days ND

Moon et al40 ND Immediate (after 2-3

weeks)

\200 g Continuous and

intermittent

8 months ND

Kinzinger et al47 Distalization of

maxillary molars

Immediate (after 1

week)

200-240 g Continuous Average: 6.5

months or ARTT

D

ND, Not described; D, described; ARTT, anchorage for required treatment time.


Table IX. Implant maintenance-related factors in studies on mini-implants

Authors Antibiotic protocol Chlorhexidine protocol Oral-hygiene protocol Peri-implantitis protocol Mobility protocol

Freudenthaler et al14 ND ND ND OH reinforcement ND

Miyawaki et al12 3 days PI ND ND ND ND

Liou et al21 1 week PI 1 week PI OH instruction ND ND

Motoyoshi et al22 3 days PI ND ND ND ND

Thiruvenkatachari et al23 ND ND ND OH reinforcement ND

Park et al24 ND ND OH instruction OH reinforcement Monitor force levels

Tseng et al25 ND ND OH instruction Local cleaning

and antibiotics

ND

Chen et al26 ND ND ND ND ND

Berens et al46 ND ND ND ND ND

Luzi et al37 ND 1 week PI OH instruction OH reinforcement ND

Wiechmann et al41 ND ND OH instruction Analgesics and antibiotics ND

Kuroda et al42 ND ND ND ND ND


Kuroda et al38 ND ND ND ND ND


Hedayati et al44 1 hour

before placement

1 week PI ND ND ND

Chaddad et al45 ND 1 week PI ND ND ND

Moon et al40 ND ND ND ND ND

Kinzinger et al47 ND ND ND ND ND

ND, Not described; PI, postimplant placement; OH, oral hygiene.

failed to show how the sexes were divided in their sub-jects.44,45 Sex and success were not correlated accord-ing to 5 articles,22,24,38,40,43 but a study that usedcomputed tomography measured thinner cortical bonethickness in females in the attached gingiva mesial tothe maxillary first molar.69 Most studies had a widerange of the age variable. Immediate loading of mini-implants showed significantly higher success rates inadults compared with adolescents in a study by Mo-toyoshi et al.39 This finding probably indicates that thebone density of adolescents is insufficient to support im-mediate loading with orthodontic forces. However, softtissue and bone thickness around the first molars varysignificantly in the vertical and horizontal dimensionsand could have skewed this proposed association.69,70

Physical and dental statuses were described in only2 and 6 of the 19 articles, respectively, and their impacton success rates needs additional clarification (Table V).Osteoporosis, uncontrolled diabetes, periodontal dis-ease, smoking, and pharmacologic prescriptions suchas bisphosphonates are considered risk factors for clas-sic dental implants.24,71-74 It is probably wise to avoidthe use of mini-implants in these patients or to monitorthem carefully, allowing longer healing periods andapplying specific loading protocols.71,72

Location-related factors (Table VI) include hard-tis-sue parameters. Animal studies showed that the holdingpower of mini-implants is determined by the quality andquantity of the bone into which they are placed.61,62,75

Initial stability after placement was facilitated by

greater cortical bone thickness.62 However, several re-ports warned about the risk of overheating during im-plant placement in areas with a dense cortex.20,24 Onestudy found higher success rates when the corticalbone was at least 1.0 mm thick.43 In that article, peri-im-plant bone quantity was not correlated with success andseemed therefore a less important factor for implant sta-bility than cortical bone thickness. However, the widerange in the age variable could have distorted these pro-posed associations (Table V).

A relationship between success and the character ofthe soft tissues has been proposed.20,24,45 It was recom-mended to position implants in keratinized gingivarather than nonkeratinized mucosa.19,20,63 Keratinizedgingiva is thought to reduce the development of hyper-trophic tissues and inflammation.66,76 To avoid thesesecondary outcomes, it also was suggested to cover im-plant heads with mucosa.24,76

Proper implant site selection was proposed as a keyfactor for the success of mini-implants.7,13,20,37,41,46,77

Therefore, any correlation with success was rejectedwhen the position of the implant was not precisely indi-cated (Table III). Primary outcomes varied betweenplacement sites.24,40-43 Differences in success were re-corded between the premolar and molar areas in themandible,40 and a study noted that root proximity wasa major risk factor for screw failure.42 This latter findingwas confirmed in an animal study.54 Furthermore, grow-ing third molars, exfoliating tooth buds, periodontal dis-eases, and edentulous areas are thought to change bone


May 2009

quality.44,76,78 To obtain better insight into these param-eters, controlled studies are necessary and might lead tosite-specific protocols for implant placement.

Surgery-related factors (Table VII) include experi-ence of the surgeon, sterilization, flap or flapless sur-gery, self-tapping or self-drilling technique, pilothole preparation in the cortex only or for the entirescrew depth, diameter of the pilot hole, cooling tech-nique, drill speed and pressure, direction of placement,steady or wiggling placement procedures, monocorti-cal vs bicortical anchorage, and placement tor-que.9,12,19,22,24,37,39,40,43,45,63,76,77 Studies of dentalimplants indicate that gentle surgical placement isa key element for success.72 Excessive surgical traumaand thermal injury can lead to osteonecrosis and fi-brous encapsulation of the implant.79-81 Failure ratescan probably be reduced with increasing clinical expe-rience.15,37,40 In most studies, screw implants wereplaced according to a specific protocol (Table VII).Similar success rates were found for both flap and flap-less procedures, but the influence of cortical pilot holepreparation or self-tapping or self-drilling techniqueswas not analyzed in any study.12,40 Four studies corre-lated the amount of placement torque with success(Table III).22,39,43,45 Because of the suggestion thatexcessively high seating forces could cause necrosisand local ischemia,82 specific torque levels wererecommended for the maxilla and the mandible.22,39

One study analyzed the impact of placement directionon success.24 Various angulations were chosen to avoidbiologic damage and to increase contact with corticalbone.24,39 However, no surgery-related factor met theselection criterion for variables (Table III).

Orthodontics-related factors were divided in 6 cate-gories: timing, magnitude, type, duration, and directionof force, and type of orthodontic movement. Each isdiscussed separately, but there are probably delicaterelationships between them.

There is controversy about the proper timing of or-thodontic force application.9,12,24,38,39,71,77,83 Compari-son of outcomes was complicated because of the wideinterpretation of ‘‘immediate,’’ varying from the timeof surgery to 4 weeks later (Table VIII). Immediateloading might promote the mechanical stability ofscrews,83 especially in sites with poor bone quality,84

but the opposite also was reported.71 Research on den-tal implants showed that placement into soft, spongybone with poor initial stability often risks the formationof connective tissue encapsulation, similar to pseu-doarthrosis.85-87 Animal studies reported that immedi-ate loading of mini-implants can be successful,88,89

but stresses generated by functional and orthodonticforces should not be neglected.90 Motoyoshi et al39

found significantly lower failure rates for immediatecompared with delayed loading in adolescents, butnot for the early load group in adults (Table IV).This could indicate that immediate loading is possibleif bone is denser and more mature.

Force levels varied from 50 to 400 g, but most stud-ies used forces of 200 g or less. Because excessive strainlevels might lead to screw loosening in areas with thincortical bone and low-density trabecular bone,76,91 itwas recommended to start with forces of 50 g and in-crease them after initial healing.45,76 Liou et al21 foundsignificant screw displacements after applying immedi-ate forces of 400 g. However, a study using the sameprotocol correlated screw displacement to the durationand not to the direction or magnitude of forces of 200to 425 g.52 These latter authors suggested, however,that loading beyond this force range could cause differ-ent outcomes and required further investigation. Re-gardless, displacements were also seen at lower forcelevels.44,47

Both light continuous and the more extreme initialforces of intermittent loading have been used for ortho-dontic tooth movement. However, the type of force andits relationship to implant stability were analyzed inonly 1 study.24 Four methods of force applicationwere compared, but no correlation was found with pri-mary outcomes.

The duration of force application varied widely,from 3 to 37 months (Table VIII). Little is known aboutthe long-term stability of miniscrews. Implant displace-ment was found in 3 studies after 9, 5.4, and 6.5 months,respectively.21,44,47 Wang and Liou52 found a correlationbetween the duration of force application and theamount of displacement, but they suggested that therelatively high forces could have influenced thisassociation.

A potential relationship between implant successand the direction of force application has been hypoth-esized.13,14,20,63,66 Costa et al13 suggested that minis-crews could loosen when a moment was generated inthe unscrewing direction. Cheng et al20 recommendedavoiding lateral, torsional, and extrusive forces, andFreudenthaler et al14 suggested that the placement siteof the mini-implant should be the same as the centerof resistance of the teeth.

Mini-implants were prescribed for a wide variety oforthodontic indications (Table VIII). The influence ofthe type of orthodontic movement on primary outcomeswas analyzed in only 1 study.38 Significantly higher fail-ure rates were found for intrusive movements comparedwith retraction and protraction of teeth, but this correla-tion was rejected by the inclusion criteria for variables(Table III).


Implant maintenance-related factors (Table IX) in-cluded control of peri-implantitis. Prophylactic antibi-otics, chlorhexidine rinses, oral-hygiene instructions,and reinforcements are important factors of implantmaintenance.39,43,63,66,76,92 The possible relationshipbetween success and antibiotics or chlorhexidine wasnot analyzed in any study (Table IIII). Park et al24 asso-ciated control of peri-implantitis with success but foundno correlation between oral-hygiene measures and pri-mary outcomes (Table III). They also reported highersuccess rates on the left side of the mouth; this findingwas considered a consequence of better oral hygieneby right-handed patients.93

Research on dental implants has demonstrated thatmicro-movements of more than 100 mm are sufficientto jeopardize healing and can cause fibrous encapsula-tion.87,94 Park et al24 recommended monitoring implantmobility and orthodontic forces regularly and reportedthat mobile screws could be successful if the forceswere less than 200 g. Beyond orthodontic-force factors,other force variables including occlusion and tonguejiggling might also influence outcomes.19,37,90 Monitor-ing these factors should become a part of the implant-maintenance protocol.

Future research should apply a standardized meth-odology to analyze primary and secondary outcomesof using mini-implants in orthodontic treatmentprotocols. Our definitions of outcome measures areproposed as initial guidelines for this purpose. Fur-thermore, our classification system of the variablesthat could influence success rates is suggested asa starting framework for research on mini-implants.Studies should focus on implant systems for specificorthodontic indications by testing 1 hypothesis ata time. Further randomized clinical trials are neededto analyze the differences in outcomes betweenmini-implants and other forms of anchorage. Becausethis research is generally costly, implant manufac-turers should be solicited to fund high-quality inde-pendently conducted trials.29

CONCLUSIONS

1. The analysis of success rates was complicated be-cause of various definitions of primary outcomes,different timings of success assessment, poor meth-odologies, and lack of clarity in most studies.

2. Rates of primary outcomes of mini-implants withdiameters of 1.0 to 2.3 mm ranged from 0% to100%. Most studies reported success rates greaterthan 80% if mobile and displaced implants wereincluded as successful.

3. Adverse effects of mini-implants included biologicdamage, inflammation, and pain and discomfort.Few articles reported on these outcomes.

4. Variables suggested as having an association withthe success of mini-implants were divided into 6categories: implant, patient, location, surgery,orthodontic, and implant-maintenance factors. Allproposed correlations were rejected by the selectioncriteria for this review, because the parameters se-lected as independent variables were not controlled.

5. This systematic review has shown that clinical stud-ies on mini-implant placement are still in their in-fancy. A proposal for a standardized methodologyfor future studies was presented with our classifica-tion system for variables and specific definitions ofprimary and secondary outcomes.

We thank Charles Greene, University of Illinois, andLouis Keith, Northwestern University, for reviewingthis manuscript; Mary Kreinbring, American Dental As-sociation library, for assistance with the computerizedsearches; and Rossella Bassi, Elisabetta Bello, andAlice Marino for preparing the tables.

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May 2009

APPENDIX: SELECTION PROCEDURES

Appendix Table I. Keywords for the search engines

Keywords Abbreviations used*

Orthodontics Orthodon, Orthodontics, Orthodontic

Implants Implant, Implants

Screw Screw, Screws

Mini-implant Mini-implant, Mini-implants

Miniscrew Mini-screw, Mini-screws, Miniscrew,

Miniscrews

Microimplant Micro-impant, Micro-implants,

Microimplant, Microimplants

Screw implant Screw implant, Screw

implants

Temporary anchorage

device

Temporary anchorage device,

Temporary anchorage

devices

*For each search engine, the appropriate characters (*, $, and so on)

were used to truncate or explore search terms.

Appendix Table II. Abstracts retrieved by electronic,hand, and reference searching

Search methodNumber of abstracts

without overlap

Google Scholar 3309

PubMed (AA) 0

Embase (AA) 16

Science direct (AA) 24

Other search

engines (AA)

0

Hand searching (AA) 1

References review

articles (AA)

12

References selected

articles (AA)

2

Total 3364

AA, Additional abstracts that were not retrieved by any other search

method.

Google Scholar was used as the basis, because it had the largest num-

ber of abstracts and therefore the most overlap.

Appendix Table III. Articles excluded by general selec-tion criteria (n 5 21)

AuthorsYear of

publicationReasons

for exclusion

Wehrbein et al1 1998 1D

Wehrbein et al2 1999 1D

Bernhart et al3 2001 1D

Lee et al4 2004 1B (case)

Philippart and

Philippart-Rochaix52004 1B (technique)

Sung et al6 2004 1B (case)

Park et al7 2004 1B (case)

Travess et al8 2004 1B (review)

Yao et al9 2005 1E

Park et al10 2005 1B (technique)

Melsen and Verna11 2005 1B (technique)

Herman and Cope12 2005 1B (technique)

Crismani et al13 2005 1D

Maino et al14 2005 1B (technique)

Maino et al15 2005 1 B (technique)

Chen et al16 2006 1A

Cho17 2006 1B (case)

Kyung18 2006 1B (review)

Arcuri et al19 2007 1D

Wang and Liou20 2008 1A

Lim et al21 2008 1B (laboratory)

General exclusion criteria: 1A, study did not analyze success of mini-

implants; 1B, not a clinical study on humans but a technique article,

case report, opinion article, review article, or laboratory, animal,

or in-vitro study; 1C, sample size smaller than 10 mini-implants;

1D, implant diameter .2.5 mm; 1E, miniplates.

Appendix Table IV. Articles excluded by specific selec-tion criteria (n 5 12)

AuthorsYear of

publicationReasons

for exclusion

Costa et al22 1998 2A, 2B

Fritz et al23 2004 2A, 2C

Cheng et al24 2004 2B

Gelgor et al25 2004 2A

Park et al26 2005 2C

Berens et al27 2005 2A,2C

Kircelli et al28 2006 2A

Herman et al29 2006 2C

Wu et al30 2006 2A, 2B

Xun et al31 2007 2A

Chen et al32 2007 2B

Gelgor et al33 2007 2A

Specific exclusion criteria: 2A, no definition of success; 2B, no dura-

tion of force application; 2C, duration of force application\3 months.


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