ncient mariners used the constellations and theirindividual component stars as landmarks from

which to navigate, often in uncharted waters, as they dis-covered new horizons during their journeys. The advent ofminiscrew anchorage provides a new reference, a kind ofPolaris or “North Star,” that allows today’s orthodontist toplot courses of treatment that may have been previously un-predictable or even impossible with traditional mechanics.

In view of the plethora of publications, continuingeducation courses, and marketing materials available toour specialty on the subject of skeletal anchorage, itwould seem that the use of miniscrews has become ubiq-uitous. The reality is much different. In actuality, very feworthodontists have yet to embrace miniscrew use in dailyclinical practice. Those who have set sail into this unfa-miliar territory have employed them sparingly, and manyhave elected to have someone else insert them for theirpatients. It is the authors’ intended purpose to encouragehesitant practitioners to come aboard and incorporateminiscrews on a more routine basis.

Anchorage ConceptsIn order to move an object, some type of anchorage

or countersupport is required to stabilize a force appliedto that object. In his Third Law, Sir Isaac Newton specifiedthat every action has an equal and opposite reaction. If we

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MiniScrewsA new North Star for orthodontics

are discussing support for orthodontic tooth movement,then forces that are applied between or across teeth willact on all of the dental units involved, perhaps resulting inunintended tooth movement of the so-called “anchor”teeth. The extent of this movement and also the counter-movement depends upon the anchorage support derivedfrom the individual teeth. In other words, dental anchor-age is based on the number of teeth, the length of theirroots, the root surface areas, and the structure of the sup-porting bone surrounding the teeth.

Anchorage quality in orthodontics can be dividedinto three categories: minimum anchorage, medium an-chorage, and maximum anchorage. These three cate-gories may be easily described with the example ofconventional canine retraction after removal of a firstpremolar (Figure 1). If only the individual teeth involvedin the mechanism provide support, you have minimalanchorage. Figure 1A demonstrates that a single premo-lar will not provide sufficient anchorage to retract the ca-nine; rather, the premolar will likely be moved mesially,and there will be anchorage loss. Figure 1B shows twoequal anchorage segments where action and reaction arecomparable, and the result is medium anchorage and re-ciprocal tooth movement. In Figure 1C, the posteriorgroup of teeth is secured and stabilized with a miniscrew(indirect anchorage). The canine can then be retracted

without mesial movement of the posteriorteeth as the reactive force is completely ab-sorbed by the anchorage block. This is max-imum anchorage.

Apart from the quality of the anchorage,the location of the anchorage support is alsoan important factor. A multitude of differentanchorage devices have been employedthroughout the first century of our specialty.

Methods of dental or desmodontal sup-port have included the following:• intraoral, intra-arch devices (such as

By Björn Ludwig, DMD, MSD;Bettina Glasl, DMD, MSD;Thomas Lietz, DMD;Joerg Lisson, DDS, PhD; andS. Jay Bowman, DMD, MSD

Figures 1A–1C: Anchorage quality demonstrated by canine retraction mechanics.Expected results for a) minimum; b) medium (reciprocal); and c) maximum anchorage.

1A 1B 1C

the Nance holding arch, transpalatal arch, lingual arch,and lip bumper);

• modification of fixed appliances (buccal root torque tomove roots into the cortical plate, blocking groups ofteeth together, tip-backs or “setting anchorage,” up-righting springs and auxiliaries); and

• interarch mechanics (Class II or III elastics, fixed func-tional appliances).Extraoral support has been provided by headgear and

face masks.The tools of skeletal anchorage support include im-

plants, on-plants, and miniscrews.This article describes anchorage only within bony

structures; therefore, the terms “skeletal” and “cortical” an-chorage are used interchangeably.

History and Overview of Skeletal AnchorageThe era of skeletal anchorage began in 1945 when

Gainsforth and Higley1 inserted screws into jawbone. Many ex-periments were unsuccessful, and the method had become vir-tually obsolete by the late 1970s. Starting again in 1980, variousresearch groups (such as Creekmore and Eklund,2 Roberts etal,3-5 and Turley et al6,7) took up the subject once more.Creekmore and Eklund2 published the first clinically successfulpatient treatment case in which a metal post served as anchor-age to intrude maxillary incisors and reduce an overbite.

There are now numerous options for cortical anchorage,ranging from the use of (artificial or pathologically) anky-losed teeth to miniplates (adapted from orthognathic sur-gery) and even prosthetic osseointegrated implants (Figure2). Wehrbein and co-workers8-10 were the first to present animplant system specially designed for orthodontics (theOrthosystem from Straumann). Another example, theMidplant (HDC), was designed, like the Orthosystem,to be inserted into the palate. Both methods have beendemonstrated to be safe and successful.

In recent years, the requirements for cortical an-chorage techniques have been carefully defined in theliterature.11-14 Under closer inspection, miniscrews ap-pear to have the edge compared to other methods dueto numerous advantages:

• biocompatibility;• small size;• simplicity of insertion and use;• favorable primary stability;• capacity for immediate loading;• adequate resistance against orthodontic forces;• ability to enhance typical orthodontic mechan-

ics, often independent from patient cooperation;• improved treatment effectiveness and efficiency;• ease of removal; and• cost-effectiveness.

Miniscrew PropertiesAll types of skeletal anchorage (including minis-

crews) are, by definition, implants. It is important,

however, to differentiate miniscrews from typical dentalimplants.

More than 30 different terms for skeletal temporaryanchorage screws are in use in the international literature.The most common of these are mini-implant, mini-pin,miniscrew, or TAD (temporary anchorage device). Theterm “miniscrew” appears to provide the most accurate(and most palatable) description of these “miniaturescrews,” especially when discussing their use with prospec-tive patients (Figure 3). There are also more than 30 manu-facturers of miniscrew systems (Figure 4, page 40), with thenumber of different screws offered per system ranging fromtwo to 154. It can be an overwhelming and perplexingprocess to sort through all of these options and select those

April/May 2008 OrthodonticProductsOnline.com 39

Figure 2: The range of cortical anchorage options.

Figures 3A–3D: Examples ofminiscrew-supported mechanicsfor predictable unilateral spaceclosure. Miniscrews preventedthe typical reactive side effect ofmidline shift or anterioranchorage loss that would haveoccurred with traditionalorthodontic mechanics.

3A 3B

3C

3D

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devices that are needed fordaily practice. The follow-ing is an overview of themost important decision-making criteria for choos-ing a miniscrew system.

MaterialsMiniscrews are typical-

ly fabricated from pure tita-nium or from an alloy oftitanium with aluminum or vanadium. The safe biocom-patibility of these materials when in direct contact withbone has been clearly proven in the literature.

OsseointegrationBranemark15 was the first to define the concept of os-

seointegration. He described it as “a direct functional andstructural link between living bone tissue and the surfaceof a force-absorbing implant.”15-17 Several authors have in-dicated that there is no intention to anchor miniscrews byosseointegration; rather, primary retention is described as

a “skeletal resistance block.”18,19 In the opinion of Cope20

and Bumann and co-workers,21 miniscrews are specificallyanchored by mechanical stabilization and not by osseoin-tegration. Since typical dental analogue implants andminiscrews are fabricated from the same materials, thereappears to be nothing to preclude osseointegration, exceptthe lack of surface coating and the smaller surface area ofthe miniscrew.

Miniscrew DiametersThe stability of a miniscrew in the

bone depends primarily on its diame-ter, and not on its length (Figure 5).22-23

The diameter of available miniscrewsvaries between 1.2 mm and 2.3 mm. Inthis case, “diameter” is the outside di-ameter of the threads. For safe and se-cure primary mechanical stabilization,a minimum amount of bone is re-quired around the shank of the minis-crew (Figure 6). Although there is no

definitive answer as to theamount of bone required,it appears that from be-tween 0.5 mm to 2 mm isnecessary for stabilizationto reduce premature loss.The amount of bone be-tween the roots of teeth,therefore, defines the max-imum diameter of screwthat can be used in a par-

ticular site. In short, the total distance between rootsshould be at least 1 mm greater than the diameter of thechosen miniscrew to provide adequate bone support.

It is preferable to place miniscrews through attachedmucosa. Not only is the insertion easier, but there is also lessrisk of tissue damage and subsequent tissue irritation orovergrowth of the implant. Publications by Poggio et al,24

Schnelle et al,25 and Costa et al26-27 provide some clues as to thevertical distance that is typically available between the ce-mentoenamel junction (CEJ) and the mucogingival junction(MGJ). These investigations clearly show that the diameter ofan interradicular miniscrew should not exceed 1.6 mm. Anexception may be miniscrews inserted into the infrazygomat-ic ridge or on the lingual alveolus between the maxillary firstmolar and the second premolar. This, in fact, is the site withthe widest interradicular space28 (Figure 7, page 42).

Miniscrew LengthsThe length of the various miniscrews on the market

ranges from 5 mm to 14 mm. Typically, the length of theminiscrew refers only to the shaft or shank (the threadedsection). As with the diameter, the selection of the length ofa miniscrew is dependent upon the amount of bone avail-able. Depending on the region, the total thickness of thealveolus is between 4 mm and 16 mm.29

The length of a screw, however, is of secondary impor-tance when it comes to secure anchorage—the diameter ismuch more critical. Various investigations have shown thatthe thickness of the cortical plate plays the most importantrole in miniscrew stability.30-32 For example, FEM analyseshave demonstrated that the typical orthodontic load is ap-plied only in the region of the cortical bone.33,34

When selecting the length of a miniscrew, the depthof the gingiva must also be consid-ered. The average depth of the gin-giva is about 1.25 mm except in afew locations such as the retromolarregion. The ratio between the lengthof the head (the part of the minis-crew outside the bone) and thelength of the threaded shaft (thepart of the screw inside the bone)should be at least 1:1. Consequently,Poggio et al24 recommended minis-crews lengths of 6 mm to 8 mm, andCosta26,27 suggested that miniscrews

Figure 4: Eight examples of the more than 30 miniscrews that arecurrently available. From left to right: Ortho Easy from Forestadent,Aarhus from American Orthodontics, AbsoAnchor from Dentos, DualTop from Jell Medical, LomasMondeal, Osas from Dewimed, SpiderScrew from HDC, and tomas from Dentaurum.

Figure 5: Loads to miniscrews as determined by diameter.

Figure 6: A radiograph demonstrating therange of available interradicular space forplacement of a miniscrew.

October 2007 OrthodonticProductsOnline.com 41OrthodonticProductsOnline.com April/May 200841

should be from 6 mm to 10 mm. On the basis of these in-vestigations, it would appear that longer screws are unneces-sary except in unusual circumstances. This has also beenconfirmed by numerous, anecdotal clinical experiences.

Color-coding of miniscrews for different lengths or di-ameters helps to facilitate the selection process. This color-coding is accomplished by an anodized surface coating(such as that used by Forestadent’s OrthoEasy). This oxide layer also appears to en-hance retention of the miniscrew.35

Miniscrew Head DesignSome suppliers have a special head

variation for every potential orthodontic application in-cluding:

• hook tops;• ball-shaped heads;• eyelets;• simple slots;• cross-shaped slots; and• universal heads.

The screw head must be very small and compact (lowprofile) to ensure that the patient experiencesminimal discomfort and tissue irritation (Figure8). On the other hand, the head must also belarge enough so that the coupling elements (suchas coil springs, wire segments, auxiliaries, andelastic chains) can be easily and securely fastenedto it (Figure 9, page 44).

Transgingival PortionThe transgingival portion, also known as

the gingival neck, is a most important part of aminiscrew. Perforation of the gingiva occurswhen a miniscrew is inserted. This provides a

potential access route for microorgan-isms, thereby posing a risk of perimu-cositis or peri-implantitis, one of themain causes of premature loss ofminiscrews.36-37 During the immediatepostoperative phase, the mucosashould adapt as closely as possible tothe screw to seal the area.38 The mostadvantageous shape for that to occurappears to that of a cone, as this shapenaturally results in a safe seal withouta pressure zone. A cone-shaped trans-gingival collar appears to seal the mu-cosal perforation wound, much as acork seals a bottle, and it may help toreduce bleeding. Some miniscrewsfeature no transgingival collar so thatthe screw resembles a pan-head screw.These types of miniscrews may resultin a void between the screw and thesoft tissue that could provide a site for

plaque or debris accumulation.

Simplification of InventoryIt seems readily apparent that choosing a specific

miniscrew that could be used for the majority of clinical ap-plications would simplify the process of incorporatingthem into daily practice. Miniscrews with a head design fea-

turing either some type of cross-slot ortube can be used for both direct and in-direct anchorage mechanics. In addition,screws of 1.4 mm to 1.6 mm in diameterprovide for adequate strength and suffi-cient primary stability (dependent upon

available bone), and only upon occasion will larger-diame-ter screws be needed (for placement in the infrazygomaticridge or lingual alveolus between the upper first molars andsecond premolars). Miniscrews 8 mm in lengths are suit-able for the majority of applications; however, a few screwsof 6 mm and 10 mm length could be kept on hand for spe-cific locations.28 These guidelines should help you tostreamline your selection process and maintain a cost-ef-fective inventory.

Figure 7: Larger-diameter (2-mm) miniscrews may be inserted between the maxillary firstmolars and second premolars to provide more stable anchorage support for molar distalization(without anchorage loss that can result from standard dental anchorage) for the Horseshoe Jet(a modification of the Distal Jet from AOA Laboratories, Racine, Wis). Since the roots of thesecond premolars are angled toward the buccal, they will bypass 8-mm miniscrews that areinserted at an angle to the cortical bone. This precludes the need to reposition miniscrews afterthe molars have been distalized (as is required when they are inserted into the buccal alveolus).The distal set-screw is then locked to convert the Horseshoe Jet to a miniscrew supportedholding arch, and retraction of the remaining teeth is initiated.

Figures 8A and 8B: Height or profile difference for two clinical situations.

For more information on miniscrews,please see Mini-implants in Orthodontics:Innovative Anchorage Concepts, edited byLudwig, Baumgaertel, and Bowman, andpublished by Quintessence.

8A 8B

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ConclusionsAlthough the application of skeletal anchorage concepts

to traditional orthodontic mechanics has opened new vistasof treatment predictability, effectiveness, and efficiency, uni-versal adoption of miniscrews throughout our specialty willendure some growing pains. Although miniscrews are notvital for the majority of patient care, they can certainly im-prove many aspects of specif-ic treatment mechanics andhelp to reduce reliance uponpatient compliance.

It has been more than 75 years since the concept ofskeletal anchorage was conceived, but now the imaginationand inventiveness of today’s orthodontists are making itsapplication feasible, safe, and reliable for daily clinical use.Certainly, significant due diligence and careful continuingeducation are a necessity prior to adopting miniscrew an-chorage. Perhaps miniscrew anchorage will soon be seen asa new reference point or “North Star” on which a constella-tion of predictable orthodontic mechanics may be based.z

Björn Ludwig, DMD, MSD, is in private practice in Traben-Trarbach, Germany. He can be reached at bludwig@kieferorthopaedie-mosel.de.

Bettina Glasl, DMD, MSD, is in private practice in Traben-Trarbach, Germany. She can be reached at bglasl@kieferorthopaedie-mosel.de.

Thomas Lietz, DMD, is in private practice in Neulingen,Germany.

Joerg Lisson, DDS, PhD, is the head of the orthodontic de-partment at the University of Homburg/Saar, Germany.

S. Jay Bowman, DMD, MSD, isin private practice in Portage,Mich. He can be reached at drjwyred@aol.com.

Figures 9A–9D: For practical reasons, it may be advisable to useminiscrew systems that offer a single, universally applicable headdesign. It would seem feasible to connect all types of forceelements (elastic threads, elastic chains, round wires, squarewires, coil springs, auxiliaries, etc) to one ideal head design,facilitating the application of direct and/or indirect anchoragesupport to the same miniscrew design.

References for this article areavailable with the online version atOOrrtthhooddoonnttiiccPPrroodduuccttssOOnnlliinnee..ccoomm.

9A 9B

9C 9D

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