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