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Mini-Implants: Where Are We?

Although skeletal anchorage is here to stay inorthodontics, there are still many unansweredquestions.1 This article will describe the developmentof skeletal anchorage and provide anoverview of the current systems and their advantagesand drawbacks.

Evolution of Skeletal Anchorage

Skeletal anchorage systems have evolvedfrom two lines. One category originated as osseointegrateddental implants, which have a solidscientific base of clinical, biomechanical, andhistologic studies. The orthodontic mini-implantswere smaller than the dental implants, but theirsurfaces were treated in the same way. Includedin this category are the retromolar implants describedby Roberts and colleagues2 and the palatalimplant introduced by Wehrbein and Merz.3Both are used for indirect anchorage, meaningthey are connected to teeth that serve as the anchorageunits.

The other category developed from surgicalmini-implants. Creekmore and Eklund insertedone such device below the nasal cavity in 1983,4but it was not until 1997 that Kanomi described amini-implant specifically designed for orthodonticuse.5 Both of these were used as direct anchorage.The following year, Costa and colleaguesdescribed a screw with a special bracket-likehead that could be used for either direct or indirectanchorage.6 In contrast to the osseointegratedimplants, these devices are smaller in diameter,have smooth surfaces, and are designed to beloaded shortly after insertion.

Few of the surgical miniscrews have, to myknowledge, been subjected to systematic studiesanalyzing the tissue reaction to loading. AarhusMini-Implants were placed in monkeys and immediatelyloaded with 25-50cN of force by Melsenand colleagues.7,8 Titanium screws were insertedin dogs and loaded after six weeks with150g coil springs by Ohmae and colleagues.9 Deguchiand colleagues also loaded titanium screwsin dogs after three weeks with 200-300g elastomericchains.10 All three studies confirmed thatmini-implants loaded immediately or shortlyafter placement can be successfully used foranchorage.


Precise indications for skeletal anchorageare not well documented. Most of the publishedarticles have been case reports in which newdevices have been described as alternatives toother anchorage methods--for example, in extractioncases using implants instead of headgear.11,12 Mini-implants have replaced other typesof fixed appliances for the delivery of differentiatedforce systems for posterior tooth movement13or extrusion of impacted canines.14

Miniscrews have also been used as anchoragefor tooth movements that could not otherwisehave been performed. Since 1997, we haveplaced the Aarhus Mini-Implants in many ofthese cases, which fall into the following categories:

  • Patients with insufficient teeth for the applicationof conventional anchorage (Fig. 1).
  • Cases where the forces on the reactive unitwould generate adverse side effects (Fig. 2).
  • Patients with a need for asymmetrical toothmovements in all planes of space (Fig. 3).
  • In some cases, as an alternative to orthognathicsurgery (Fig. 4).
  • Materials and Design

    Although precise specifications are notavailable for many mini-implants, most are madefrom titanium alloys. The alloy used for theAarhus Mini-Implant is Ti6AL-4V ELI accASTM F 136-02a. The Orthodontic Mini Implant(OMI) is made of implant steel 1.4441, whichis still used in traumatology but has been prohibitedfor neurosurgery.

    The diameter of the threaded portion ofminiscrews varies from 1mm to 2mm.5,15,16 Theadvantage of a thin screw such as the AbsoAnchor is the ease of insertion between theroots without the risk of root contact. The drawbackis the potential for fracture, which is closelyrelated to the diameter of the screw17 (Fig. 5A).

    As bone density increases, the resistancecreated by the stress surrounding the screw becomesmore important in removal than in insertionof the screw. At removal, the stress is concentratedin the neck of the screw (Fig. 5B). If anAllen wrench is used for insertion and removal,the hole in the center of the screw will weakenthe neck, which may lead to fracture. A hollowneck facilitates the insertion of a ligature, but alsoweakens the neck. The strength of the screw isoptimized by using a slightly tapered conicalshape and a solid head with a screwdriver slot.

    The head of the mini-implant can be designedfor one-point contact with a hole throughthe neck, as in the Dual-Top Anchor System, theLin/Liou Orthodontic Mini Anchorage Screw(LOMAS), and the Spider Screw. A hook(LOMAS) or a button (AbsoAnchor) can also beused. A bracket-like head design, on the otherhand, offers the advantage of three-dimensionalcontrol and allows the screw to be consolidatedwith a tooth to serve as indirect anchorage. Apatent for this design was granted to the AarhusMini-Implant in 1997 (Fig. 6), but minor variationshave been produced by many companies,including the Dual-Top Anchor System and theTemporary Mini Orthodontic Anchorage System(TOMAS).

    Another design factor is the cut of thethreads. With self-drilling miniscrews (AarhusMini-Implant, Dual-Top Anchor System, andLOMAS), the apex of the screw is extremely fineand sharp, so that pilot drilling is unnecessary inmost cases.

    The transmucosal portion of the neckshould be smooth. It is also important, however,that screws be available with different necklengths for various implant sites (Aarhus Mini-Implant, AbsoAnchor, and OMI).

    Selection of Mini-Implant Size and Location

    The diameter of the miniscrew will dependon the site and space available. In the maxilla, anarrower implant can be selected if it is to beplaced between the roots. If stability depends oninsertion into trabecular bone, a longer screw isneeded, but if cortical bone will provide enoughstability, a shorter screw can be chosen. Thelength of the transmucosal part of the neckshould be selected after assessing the mucosalthickness of the implant site.

    Possible insertion sites include, in the maxilla:the area below the nasal spine, the palate, thealveolar process, the infrazygomatic crest, andthe retromolar area (Fig. 7); in the mandible: thealveolar process, the retromolar area, and thesymphysis (Fig. 8). An intraoral radiograph isrequired to determine the correct location. Asmall, ellipsoid template made of rectangularorthodontic wire can be attached to the teeth inthe region with light-cured composite to facilitatethis evaluation (Fig. 9).

    Whenever possible, the mini-implantshould be inserted through attached gingiva. Ifthis is impossible, the screw can be buried beneaththe mucosa so that only a wire, a coilspring, or a ligature passes through the mucosa.In the maxilla, the insertion should be at an obliqueangle, in an apical direction; in the mandible,the screw should be inserted as parallel to theroots as possible if teeth are present (Fig. 8). Atranscortical screw can be used for added stabilityin edentulous areas, where trabecular bone isusually scarce. We do not use surgical guides18 orspecial stents19 for screw placement.


    If no pilot drilling is necessary, I recommendthat the orthodontist insert the mini-implant.Infection control is similar to that for anextraction. The doctor should wear a face maskand a surgical cap and, after a surgical handwash, a pair of sterile gloves. After the localanesthetic is applied, the assistant washes theimplant area with .02% chlorhexidine. The sterilekit is opened, and the correct screw is selectedand inserted while the assistant keeps the lipsapart and the mucosa tight (Fig. 9C).

    Even when self-drilling screws are used,pilot drilling may be required where the cortex isthicker than 2mm, as in the retromolar area or thesymphysis, because dense bone can bend the finetip of the screw. The pilot drill should be .2-.3mm thinner than the screw and should beinserted to a depth of no more than 2-3mm. Pilotdrilling should be done in a surgical environment,as with placement of a dental implant. Ifthis is not feasible in the orthodontic office, theinsertion should be performed by an oral surgeon.

    If a manual screwdriver is used for insertion,it is immediately evident when a root hasbeen contacted, and any damage will be minimal.In tests where notches were intentionally created,histological analysis showed spontaneous repairby the formation of cellular cementum. On theother hand, if the screw is inserted with a low-speeddrill, there is a greater chance of notdetecting a root due to the lack of tactile sensation.

    Antibiotics have been recommended byseveral authors, but should not be routinely prescribed.The risk of infection is obviously greaterwhen drilling is performed, especially when thesame insertion site is entered repeatedly. As longas strict sterility is maintained, however, no infectionwill occur after placement of a mini-implant.

    Force Loading

    The timing of orthodontic force applicationcan vary from minutes to eight weeks. Whenmoderate force is used, there seems to be no reasonnot to load immediately. Dalstra and colleaguesused finite element analysis to calculatethe strain developed in various cortical thicknessesand densities of trabecular bone when aload of 50cN was placed perpendicular to thelong axis of a 2mm-diameter mini-implant17(Fig. 10). They found that with thin cortical boneand low-density trabecular bone, the strain valuesmay exceed the level of microfractures andthus lead to screw loosening.20 Therefore, immediateloading should be limited to about 50cN offorce.

    Mini-Implant Problems

    In five years of experience with skeletalanchorage, I have noticed several common problems,which can be classified as follows:

    Screw-Related Problems

  • A screw can fracture if it is too narrow or theneck area is not strong enough to withstand thestress of removal. The solution is to choose aconical screw with a solid neck and a diameterappropriate to the quality of bone.
  • Infection can develop around the screw if thetransmucosal portion is not entirely smooth. If ascrew system with variable neck lengths is used,the clinician can select one that suits the particularimplant site.
  • Operator-Related Problems

  • Application of excessive pressure during insertionof a self-drilling screw can fracture the tip ofthe screw.
  • Overtightening a screw can cause it to loosen.It is crucial to stop turning the screw as soon asthe smooth part of the neck has reached theperiosteum.
  • With a bracket-like screw head, the ligatureshould be placed on top of the screw in the slotperpendicular to the wire (Fig. 11). Turning theligature around the screw will make it impossiblefor the patient to keep the area free of inflammation.
  • It is important not to wiggle the screwdriverwhen removing it from the screw head. Thescrewdriver will not stick if the long extension isremoved before the part surrounding the screw.
  • Patient-Related Problems

  • The prognosis for primary stability of a mini-implantis poor in cases where the cortex is thinnerthan .5mm and the density of the trabecularbone is low.
  • In patients with thick mucosa, the distancebetween the point of force application and thecenter of resistance of the screw will be greaterthan usual, thus generating a large moment whena force is applied.
  • Loosening can occur, even after primary stabilityhas been achieved, if a screw is inserted inan area with considerable bone remodelingbecause of either the resorption of a deciduoustooth or post-extraction healing.
  • Mini-implants are contraindicated in patientswith systemic alterations in the bone metabolismdue to disease, medication, or heavy smoking.
  • Conclusion

    The present article was intended to answersome of the questions raised by an editorial inthis journal.1 In my opinion, skeletal anchorageis clearly not a replacement for other provenanchorage systems. Skeletal anchorage shouldserve merely to expand the orthodontic serviceswe can offer our patients.

    Fig. 1 A. Patient requiring mesial movement of upper molars without distalization of anterior segment. B. Two mini-implants inserted in upper premolar regions; buccal and lingual nickel titanium coil springs used to move molars mesially. C. With no molars present to anchor distally directed forces in lower right quadrant, miniimplant placed in molar area. D. Patient after orthodontic treatment, ready for placement of prosthodontic implants in upper premolar and lower molar regions. (Treated by Jörg Thormann.)
    Fig. 2 A. Patient with agenesis of four lower premolars. B. Mini-implant serving as anchorage for mesial molar movement, avoiding adverse distal forces on anterior teeth. C. Patient after orthodontic treatment, awaiting sufficient maturity for implant placement.
    Fig. 3 A. Patient with asymmetrical occlusal plane due to overerupted premolars in scissor bite. B. Upper acrylic splint worn during fixed appliance treatment of lower arch, with palatal mini-implant used for simultaneous intrusion and lingual tipping of upper left premolars. C. Splint reduced after correction of scissor bite, with palatal implant remaining as anchorage for intrusion of premolars. D. Occlusal plane corrected after first phase of treatment. (Treated by Guillaume Guilbert.)
    Fig. 4 A. 50-year-old female patient with extreme overjet, distal occlusion, and missing lower left canine due to ankylosis. Because problem was dentoalveolar, two mini-implants were placed in mandibular symphysis as direct anchorage for mesial displacement of lower dentition. B. Space opened for implant in place of lower left canine.
    Fig. 5 A. Relationship between internal stress and diameter of mini-implant. (Reprinted by permission.[Ref. 17]) B. Stress distribution at implant removal, concentrated at neck of screw.
    Fig. 6 A. Aarhus Mini-Implant has two different heads, one bracket-type (top) and one with button for coil springs (bottom). Cut of threads makes screws self-drilling. B. Various lengths of threaded portions and smooth transmucosal necks.
    Fig. 7 Maxillary mini-implant locations. A. Below nasal spine. B. In the palate. C. Infrazygomatic crest.
    Fig. 8 Mandibular mini-implant locations. A. Retromolar area and molar region. B. Alveolar process. C. Symphysis.
    Fig. 9 A. Template bent from rectangular wire and affixed with light-cured acrylic. B. Periapical radiograph of template. C. Mini-implant insertion.
    Fig. 10 A. Monkey with two mini-implants buried below mucosa in symphysis and loaded with force against lower canines. B. When force is loaded perpendicular to implants, stress is concentrated in cortex. (Reprinted by permission.[Ref. 17]) C. Deformation is concentrated in trabecular bone because center of rotation is located near or within cortex. (Reprinted by permission.[Ref. 17]) D. Histological section after three months of loading. Note bone density and high percentage of bone-to-implant contact.
    Fig. 11 A. Ligature wire for maxillary mini-implant is not wound around bracket-like screw head, but placed in perpendicular slot. B. Screw head and bracket covered with light-cured composite for comfort.


    Dr. Melsen is Professor and Head, Department of Orthodontics, Royal Dental College, Aarhus University, Vennelyst Boulevard 9, DK-8000 Aarhus C, Denmark, and an Associate Editor of the Journal of Clinical Orthodontics. She has a financial interest in the Aarhus Mini-Implant. E-mail: orthodpt@odont.au.dk.

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