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Miniscrew implants for temporary skeletal anchorage in orthodontic treatment
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Miniscrew implants for temporary skeletal anchorage in orthodontic treatment Moschos A. Papadopoulos and Fadi Tarawneh
INTRODUCTION Anchorage, resistance to unwanted tooth movement, is a prerequisite for the orthodontic treatment of dental and skeletal malocclusions. Absolute or infinite anchorage is defined as no movement of the anchorage unit (zero anchorage loss) as a consequence of the reaction forces applied to move teeth. Such an anchorage can only be obtained by two means, using ankylosed teeth or using implants, both relying on bone for their anchorage. Currently, there are several skeletal anchorage devices that are used for orthodontic purposes, for example orthodontic implants, palatal implants or onplants, zygoma ligatures, zygoma anchors or miniplates, as well as miniscrews, microscrews or miniscrew implants (MIs). Miniscrew implants are small screw-type self-tapping implants, which in contrast to conventional dental or orthodontic implants have smooth, machine-polished surfaces and are designed to be loaded immediately after their insertion and removed easily after treatment (see Fig. 6.1). They carry special heads with or without holes to accommodate auxiliaries or wires for application of orthodontic forces (see Fig. 6.2). The use of MIs to obtain absolute anchorage has become very popular since they provide a non-compliance approach for reinforcing anchorage, ease of use and low cost.1 Currently, there are a large number of commercially available MI systems for orthodontic use in the market (Table 9.1).
TERMINOLOGY Terms such as mini-implant, miniscrew, microimplant and microscrew all refer to devices with smaller dimensions than conventional dental implants and that are removed following treatment; consequently, their use should not be synonymous with conventional dental or orthodontic implants. The terms orthodontic implant and mini-implant refer to systems that require osseointegration in order to be retained in the bone, whereas mini screws to self-tapping devices that do not require osseointegration and are only mechanically retained.2 Other terms used to describe temporary anchorage devices are intraoral or extradental anchorage systems, but the Moyer’s Symposium in 2004 agreed that the term mini-implant should be applied to palatal implants, mini-implants, miniscrews and microscrews.2 Nevertheless, the term mini-implant should be restricted to small prosthetic implants of diameter larger than 2 mm that are osseointegrated and are used to stabilize removable prostheses. The prefixes mini- and micro- are currently used in the literature to describe implants or screws of the same dimension without any differentiation. However, we advocate the use of the term miniscrew implant and this term is used throughout this textbook.
HISTORICAL DEVELOPMENT Skeletal anchorage concepts in orthodontics developed from two areas. The first was from osseointegrated dental implants to give orthodontic implants that were smaller but had similar surfaces and so also osseointe-
58
grated and were difficult to remove after treatment. The second was from surgical miniscrews used to fix osteosynthesis plates and resulted in the development of orthodontic MIs, designed for loading shortly after insertion and easy removal. The first clinical use of miniscrews for absolute anchorage was the treatment of a patient with a deep overbite using a vitallium bone-screw inserted in the anterior nasal spine.3 A mini-implant specifically made for orthodontic use was described in 19974 and a screw with a bracket-like head in 1998.5 Since then, several types of MIs have been introduced, each presenting different designs and features.
COMPOSITION OF MINISCREW IMPLANTS With the exception of the Orthodontic Mini Implant (Leone, Firenze, Italy), which is fabricated from SS, all other systems are manufactured from pure titanium or titanium alloys (medical type IV or type V titanium alloy; see Chapter 5). While both materials are biocompatible, a layer of connective tissue is usually formed around SS implants while direct bone contact and true osteogenesis occurs with pure titanium or titanium alloys.6,7 The rationale for using SS for MIs is that they are less prone to breakage and have less osseointegration than titanium devices. However, a study comparing three different MIs, two SS and one titanium, concluded that overall material performance of SS MIs was inferior to titanium MIs because of their lower yield or tensile strength.7 Removal of MIs is more related to thread design, MI shape and the drill–screw diameter ratio.7 Titanium alloys are favored over pure titanium because of their higher strength, excellent corrosion resistance, biocompatibility and favorable mechanical properties.8,9
OSSEOINTEGRATION Osseointegrated implants are largely used for skeletal anchorage purposes as they remain stable under orthodontic loading.10 However, they do have several disadvantages:11
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they cannot be immediately loaded with orthodontic forces since several months are needed for proper osseointegration they require an invasive technique for both insertion and removal, which should be done by a surgeon there is always need for supraconstruction and laboratory work they can only be inserted in edentulous areas (or in the palate) with sufficient bone quantity and quality they are expensive.
MIs were introduced in an attempt to avoid some of these problems. Their osseointegration is less than half that of conventional dental implants.5,12 A histomorphometric examination of titanium MIs retrieved following clinical use in alveolar bone found randomly organized osseointegration islets on the MI surfaces despite the smooth surface and immediate loading pattern of these implants, although it was associated with an extended period of retention (>6 months) (Fig. 9.1).13
Miniscrew implants for temporary skeletal anchorage in orthodontic treatment
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Table 9.1 Commonly used miniscrew implant systems Product
Company
Address
Aarhus Anchorage System
American Orthodontics
1714 Cambridge Av, Sheboygan, WI, USA (www.americanortho.com)
AbsoAnchor System
Dentos
258 BunJi, Dong-In Dong, Jung-Gu, Taegu, Korea (www.dentos.co.kr)
Ancotek system
Tekka
ZI de Sacuny, BP 82, 118 Ave Marcel Mérieux, 69530 Brignais, France (www.tekka.eu)
Benefit System
PSM Medical Solutions
Moltkestrasse 41, D-78532 Tuttlingen, Germany (distributed by Mondeal-PSM, North America, PO Box 142, 49950 Jefferson St, Indio, CA 92201, USA) (www.psm.ms)
C-Implant
Dentium
3105 Korea Trade Tower 159 Samsung-dong, Gangnam-gu, 135–729 Seoul, Korea (www.implantium.com)
Cizeta Titanium Miniscrew
Cizeta Surgical
Via Caselle, 76 San Lazzaro di Savena, 40068 Bologna, Italy (www.cizetasurgical.it)
Dual-Top Anchor System
Jeil Medical Corporation
775–3 Daesung B/D, Daelim 3 Dong Youngdeungpoku, Seoul, Korea (www.jeilmed.co.kr); distributed by RMO Inc. PO Box 17085, Denver, CO 80217, USA (www.rmortho.com)
IMTEC Mini Ortho Implant
IMTEC Corporation
2401 N. Commerce, Ardmore, OK 73401, USA (www.imtec.com)
Infinitas Mini Implant System
DB Orthodontics
Ryefield Way, Silsden, BD20 0EF, UK (www.dborthodontics.co.uk)
Lin/Liou Orthodontic Mini Anchorage Screw (LOMAS)
PSM Medical Solutions
Moltkestrasse 41, D-78532 Tuttlingen, Germany (www.psm.ms); distributed by Mondeal-PSM, North America, PO Box 142, 49950 Jefferson St, Indio, CA 92201, USA
Micro Implant Universal Skeletal Anchorage System
Stryker Corporation
Stryker Leibinger Micro Implants, 750 Trade Centre Way, Portage, MI 49002, USA (www.stryker.com)
Miniscrew Anchorage System (MAS)
Micerium S.p.a.
Via Marconi 83, 16030 Avegno, Italy (www.micerium.it)
OMI – Orthodontic Ancor System
OsteoMed
3885 Arapaho Rd Addison, TX 75001, USA (www.osteomed.com)
Orlus
Ortholution
207 Dunchon B/D., 416–1, Seongnae-dong, Gangdong-gu, Seoul 134–844, South Korea (www.ortholution.com)
Ortho Easy system
Forestadent
Westliche Karl-Friedrich-Str. 151, 75172 Pforzheim, Germany (www.forestadent.de)
Orthoanchor K1 System
Dentsply Sankin Corporation
Tokyo, Japan (www.dentsply-sankin.com)
Orthodontic Mini-implant
Ortholution
Joongwon-gu, Seongnam-si, 462–725 Kyunggi-do, Korea (www.ortholution.com)
Orthodontic Mini Implant
Leone S.p.A.
Via P. a Quaracchi 50, 50019 Sesto Fiorentino, Florence, Italy (www.leone.it); distributed by Leone America, 501 W. Van Buren, Avondale, AZ 85323, USA
Orthodontic screw
KLS Martin Group
Ludwigstaler Str. 132, 78532 Tuttlingen, Germany www.klsmartin.com)
Spider Screw Anchorage System
HDC
Via dell’Industria 19, 36030 Sarcedo, Italy (www.hdc-italy.com)
Temporary Mini Orthodontic Anchorage System (TOMAS)
Dentaurum
Turnstrasse 31, D-75228 Ispringen, Germany (www.dentaurum.de)
VectorTAS
Ormco
1717 West Collins Ave, Orange, CA 92867, USA (www.ormco.com)
C
A
B
Fig. 9.1 Secondary electron microscopy of miniscrew implants (MIs). (A) Unused MI. (B) Retrieved MI after >6 months inserted in alveolar bone, with integument in the middle third of threaded area. (C) Higher magnification showing organized osseointegration islets on the surface of the retrieved MI. (From Eliades et al., 200913 with permission.)
60 SECTION III: CLINICAL CONSIDERATIONS FOR THE USE OF SKELETAL ANCHORAGE DEVICES IN ORTHODONTICS
Table 9.2 Available head designs of the miniscrew implants Head
Indications
Coupling elements
Advantages
Disadvantages
Crossed-slot (bracket)
Translation, space closure, intrusion, extrusion, uprighting
Elastic chains. coil springs, round/square/rectangular wires
Direct/indirect anchorage easily achieved; more than one coupling element can be used; easy orientation
None
Single slot (bracket)
Translation, space closure, intrusion, extrusion, uprighting
Elastic chains. coil springs, round/square/rectangular wires
Square/rectangular wires can be used; direct and indirect anchorage possible
Orientation of slot important but could be compensated by wire bending
Hook
Translation, space closure, intrusion
Elastic chains, coil springs, round wires
Coupling elements can be easily attached
Orientation of hook is crucial; hook can be bent or broken; indirect anchorage difficult
Button
Translation, space closure, intrusion
Elastic chains, coil springs, round wires
Coupling elements can be easily attached; orientation not needed
Indirect anchorage difficult to achieve
Hole
Translation, space closure, intrusion
Elastic chains, coil springs, round wires
Since reduced osseointegration is advantageous for removal of MIs after clinical use, most are fabricated with smooth surfaces to achieve good bone–implant contact but not osseointegration. The quality and quantity of cortical bone is considered one of the most important factors for achieving mechanical stabilization and retention of MIs. Although a dense trabecular bone is desirable, cortical bone is more important for primary stability of MIs and this is directly related to its thickness: the thicker the cortical plate the better the survival rate.14,15
MINISCREW IMPLANT DESIGNS MIs are designed to provide both direct and indirect anchorage as well as to decrease tissue irritation. Chapter 6 discusses the significance of the various components of a MI – head, neck, endosseous body and thread, tip – in detail.
HEAD The head of the MIs serves two important purposes; to apply torque to the thread during insertion and to act as a point for orthodontic force application. Table 9.2 summarizes the different designs currently available, their indications and coupling methods. The most frequently used shape is the bracket-like design with a single or a double (crossed) slot (e.g. Aarhus Anchorage System, AbsoAnchor System, Ancotek system, Dual-Top Anchor System, Ortho-Easy system, Spider Screw, Temporary Mini Orthodontic Anchorage System) as it can accommodate square and rectangular wires in its slot plus additional auxiliaries such as elastics and ligatures. It can be used for direct and indirect anchorage for uprighting, intrusion, extrusion and mesial and distal translation. The use of rectangular wires also allows for three-dimensional control in contrast to round wires, thus providing an important advantage over other designs. Other MI head shapes, such as the button-like, sphere, double sphere and hook (Aarhus Anchorage System, AbsoAnchor System, Dual-Top Anchor System, IMTEC Mini Ortho Implant, Lin/Liou Orthodontic Mini Anchorage Screw, Miniscrew Anchorage System, Orthoanchor K1 System, Spider Screw) can be used for mesial and distal translation, space closure and intrusion, but attachment of square or rectangular wires is not possible and indirect anchorage is unattainable or very hard to achieve. Furthermore, correctly orientating the hook-shaped MI during insertion is crucial for its use. One design has a hole or eyelet through the head (usually 0.6–0.8 mm in diameter), which can accept tension springs, elastic chains and round
Orientation of hole crucial
wires; this is mostly used for direct anchorage. With this type of head, three-dimensional control is not possible since square and rectangular wires cannot be used. Another disadvantage is the fact that MI heads equipped with holes could be more at risk of fracture during their insertion or removal, since this might make the material around the head less rigid and weaker.
NECK (COLLAR) The transgingival neck (or collar) connecting the head to the body of the MI is smooth and intended to form a non-irritant seal to protect against inflammation and infection of the surrounding soft tissue (see Fig. 6.6). Most common collar shapes are conical (tapering towards the body) or cylindrical but there are also polyangular collars.8,9 Depending on the insertion angle of the MI, pressure areas are formed on the soft tissue in the area of the collar, which appears to be less in conical collars. The diameter of collars is for most MIs identical to the threaded body but having the collar wider than the body, as in the top of a conical collar, is advantageous in terms of providing a good seal between the MI and soft tissue. Ideally, and in order to achieve the best possible seal and hygiene, the diameter of the head should also be smaller or equal to the collar.8,14 However, most MIs have heads that are larger in width than the collar, which can create areas/undercuts that can be more difficult to clean and easier for plaque to accumulate. Consequently, systems with MI heads smaller in diameter than the collars (e.g. Spider Screw, the Ortho-Easy, Temporary Mini Orthodontic Anchorage System, AbsoAnchor systems) are advantageous. As gingiva width varies between 1 and 4 mm in different locations in the oral cavity,8,9,16,17 systems that provide a variety of collars in their inventory are useful. Gingival thickness at the site of insertion should be measured before chosing the MI and this can only be done only after local anesthesia, delaying the selection until just before the insertion procedure. This necessitates the orthodontist having a large range of available MIs, which could be impractical in clinical practice. Fortunately, the most popular sites for MI insertion are covered with 1–2 mm gingiva, and MIs with a collar with 2 mm length are usually sufficient apart from for the palatal gingiva, which could reach 3–4 mm and requires a longer collar.8,9,16,17
THREAD The body of the MI is a threaded part that is inserted into the bone (Fig. 9.1A). The body’s primary function is the retention of the MI in the
Miniscrew implants for temporary skeletal anchorage in orthodontic treatment
bone initially after insertion (primary stability) and afterwards during orthodontic force loading (secondary stability), and its shape and design can influence the stability of the MI as well as the force (torque) needed for insertion and removal. The body can be either conical (e.g. Aarhus Anchorage System, AbsoAnchor System, Miniscrew Anchorage System) or cylindrical (e.g. the Orthodontic Mini Implant) (see Fig. 6.3). The thread design can be a uniform helix rotating either clockwise or counterclockwise with fixed helix angles and pitches or it can be a dual-thread design formed by two different helixes with different angles and pitches in the thread (see Fig. 6.13).18,19 The thread design can influence the bone– implant contact, the stress load of the bone and the insertion and removal torque.8,9,18,19 An ideal MI body shape should allow for a high insertion torque that would sustain the MI, but at the same time not damage bone and cause necrosis, and also allow a high enough removal torque to avoid unwanted pullout but at the same time facilitate MI removal without complications. Conical MI threads have been shown to have an increasing insertion torque compared with cylindrical threads, which require less torque, and a more constant torque, throughout the insertion procedure.7,18,19–23 This makes cylindrical MIs more desirable for insertion, minimizing the risk of bone necrosis, but their higher removal torque makes them more prone to fracture. Dual-thread MIs also have a high insertion torque but with a gentler increase in comparison with conical MIs.18
DIAMETER The diameter of the MIs is a significant factor that may influence (a) the possibility of injuring the neighboring roots when inserted inter-radicularly, (b) the tendency for breakage, and (c) success/failure rates. Chapter 6 discusses in detail the effect of diameter on these issues. Essentially, smaller-diameter MIs have less risk of root contact but a greater risk of fracture, while it is more difficult to achieve an optimal placement torque for good primary stability with narrower MIs. Based on success rates, 1.2 mm can be considered the minimum diameter required to achieve adequate implantation success.2,7,24–26 Diameters of 1.5 mm and 1.6 mm represent a satisfactory compromise. In clinical practice, however, the use of MIs of larger diameter for insertion inter-radicularly can be very difficult to accomplish without the risk of injuring adjacent roots, particularly when the roots are close to each other. In such circumstances, the choice of insertion site should be reconsidered, which may mean adjusting the force application system and applying appropriate orthodontic biomechanics to achieve the required result for the new chosen site of insertion. This is not an issue for palatal insertion, where larger-diameter MIs can be used, eliminating the risk of both MI fracture and root damage. Careful clinical and radiological examination, preparation and measurement to plan the insertion site and the MI diameter before insertion will minimize the risk of breakage failure. Stress distribution within bone is affected by the diameter of MIs. In cortical bone, the thicker the MI diameter, the more favorable the stress distribution.9,27 Three-dimensional finite element analysis has shown that a 1.4 mm diameter implant placed in 1.2 mm thick cortical bone can tolerate 150 g of orthodontic forces, while a 1.8 mm diameter implant can tolerate 350 g of orthodontic forces.9 Therefore, the planned orthodontic forces should also be taken into consideration when selecting the appropriate MI diameter.
LENGTH Length generally refers to the threaded portion of the MI rather than the entire head-to-tip length but care needs to be taken with catalogues to check that total length is not given. Here lengths refer to the threaded
61
part. In general, it is considered that the longer the MI the higher the success rate.28,29 Miniscrew implants are usually provided with lengths ranging from 4.0 to 12.0 mm, although some of them are also available in larger lengths up to 21 mm. The choice of length is determined by the depth and quality of bone, the MI angulation, the soft tissue thickness and the adjacent vital structures.26 Based on hard and soft tissue depths in 20 patients, MIs of 4–6 mm in length were considered safe in most regions, but individual patient variations dictate individual evaluation of bone depth before surgery.5 Although MIs of 6–8 mm have been recommended by most authors,5,8,9,30 MIs shorter than 8 mm are more susceptible to failure26,31 and longer MIs with small diameter can be more prone to breakage and bending. In this regard, longer MIs should also be thicker in diameter, but anatomical limits restrict the use of very long and thick MIs in clinical practice. The length of MIs has less effect than diameter on the distribution of stress, and the majority of the load is borne within the cortical bone.9 The force above which implant length and implant diameter are statistically significant in influencing implant stability was found to be 1 N.21 At low force (0.5 N), no statistically significant difference in displacement according to implant length and implant diameter was observed. However, at high force (2.5 N), MIs of 9 mm length displaced significantly less than the 7 mm MIs, and the 2 mm wide MIs displaced significantly less than their 1.5 mm wide counterparts.21 Further, insertion and removal torques seem to be higher in long MIs.32 The thickness of soft tissue at the insertion site should also be taken into consideration when choosing the appropriate length of MI. Thick gingival tissue sites demand longer MIs. In general, MI lengths of 6–8 mm are recommended for use in the mandible and of 8–10 mm in the maxilla. Nonetheless, individual patient variations in soft tissue and cortical bone thickness should be considered prior to insertion of any MI.8,9
MODE OF INSERTION All types of MI are self-tapping/self-cutting. The term self-tapping relates to the ability of a screw to create a thread as it advances into the bone. This is sometimes achieved by having a cutting flute, a groove in the thread, that drills away the material to make the hole for the screw. Some MIs require a pilot drill before their insertion (non-self-drilling MIs), while others have a fine tip that enables their direct insertion into bone (self-drilling MIs). The diameter of a pilot drill is preferably 0.2–0.3 mm thinner than the diameter of the MI thread, otherwise insertion torque values are significantly decreased and MI primary stability is reduced.8,33,34 Although the length of the pilot drill is determined by the length of the selected MI, deeper holes decrease insertion torque and so it is recommended that drills with depth markings and depth stops should be used.8,34 Occasionally, a pilot drill is needed even for a self-drilling MI; such as in cases where the cortical bone is thicker than 2 mm, as dense bone would eventually bend the fine tip of the MI. Self-drilling MI systems include the Aarhus Anchorage System, the AbsoAnchor System, the Dual-Top Anchor System and the Lin/Liou Orthodontic Mini Anchorage Screw, while Spider Screw offers both nonself-drilling and self-drilling MIs. The advantage of the self-drilling over non-self-drilling MIs is the elimination of the pilot drilling step during insertion, which makes the whole procedure less invasive, less time consuming and more easily accepted by the patient, particularly to young patients and their parents. In addition, since the insertion of the self-drilling MIs is less complicated, they can be easily inserted by orthodontists themselves without requiring referral to a surgeon.
62 SECTION III: CLINICAL CONSIDERATIONS FOR THE USE OF SKELETAL ANCHORAGE DEVICES IN ORTHODONTICS
Fig. 9.2 Anchorage. (A) Direct anchorage for the intrusion of a maxillary central incisor. (B) Indirect anchorage for the mesialization of a mandibular second molar after first molar extraction.
A
B
Table 9.3 Advantages and disadvantages of direct and indirect anchorage Type
Advantages
Disadvantages
Direct
Simple mechanics Easy to apply No dental anchorage loss
Load applied directly to implant may cause mobility or loss Should be monitored closely as mechanics used are not fail-safe Placement site for direct anchorage not always achievable or possible
Indirect
Less load applied to implant Fail-safe mechanics Placement site usually easy and preferable
Loss of anchorage in the anchorage teeth group is possible More complicated mechanics and installment
TYPES OF ANCHORAGE Miniscrew implants can provide two different types of anchorage: direct and indirect. When used for direct anchorage, MIs directly receive the reactive forces used to move the teeth by acting themselves as an anchor unit (Fig. 9.2A). In indirect anchorage, MIs are connected through bars or wires to the teeth of the reactive unit supporting their anchorage (Fig. 9.2B). Special considerations should be given to the design of the MI head and the placement site for the type of anchorage to be used. Different head designs are more suitable for one type of anchorage over the other, while placement site could play a major role in determining the type of anchorage. Advantages and disadvantages of direct and indirect anchorage are summarized in Table 9.3.
PROPERTIES OF THE MINISCREW IMPLANTS Currently available MIs vary in their material, diameter and length of threaded portion, design of the head, the existence of a collar and the method of insertion. An ideal MI used for orthodontic anchorage should present some specific properties and characteristics: ■ ■ ■ ■ ■
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fabricated from a biocompatible material provided in different diameter and length sizes available in different designs (i.e. button or bracket head) self-tapping and self-drilling easily and safely inserted in various sites without complicated and invasive approaches immediately loaded removed without the need for complicated accessory equipment or invasive approaches inexpensive (cost beneficial).
LOADING Some authors have suggested a waiting period of 1 to 2 weeks before loading with orthodontic forces but most agree that, since in contrast to dental implants MIs are mechanically retained in bone sufficiently, they can sustain immediate light orthodontic loading without compromising their clinical stability.5,35,36 Only a few studies, mostly on animals, have investigated tissue reaction to immediate loading of MIs; all suggest that immediate loading with orthodontic forces can be performed without any complications.14,37,38 A finite element analysis found that immediate loading should be limited to 50 cN for a 2 mm diameter MI.39 One clinical study of 134 titanium MIs concluded that immediate loading is possible if the applied force is less than 200 cN,14 which is in agreement with a study showing no association between success rates of MIs and the mode of loading.40 This study concluded that vertical forces seem to be responsible for more failures than horizontal forces.40 The ability of MIs to provide absolute anchorage was shown in a study measuring anchorage loss with the use of MIs and conventional molar anchorage for canine retraction.41 A meta-analysis has shown that the use of MIs significantly decreased or negated loss of anchorage, particularly when compared with other conventional orthodontic means such as headgears, transpalatal arches or other biomechanical force systems.42 However, it is questionable if MIs remain stationary throughout their period of loading. In some patients, MIs were shown to move under orthodontic loading,43 and it is, therefore, advised to allow 2 mm of safety clearance between the MI and dental roots of the adjacent teeth.
COMPLICATIONS Complications associated with the use of MIs include inflammation, infection and tissue irritation, injuries to adjacent structures, and failure or fracture of the MI during insertion or removal.44
INFLAMMATION, INFECTION AND TISSUE IRRITATION Inflammation and irritation of the tissues around the MI site might occur, although infection is not generally noted (Fig. 9.3). Meticulous oral hygiene maintained around the MI and the use of 0.2% chlorhexidine mouth rinses or dental floss dipped in 2% chlorhexidine can help to avoid inflammation or infection, or control any that might occur.44 If a patient presents with purulence, palor or inflammation, definitive management with an appropriate antibiotic is indicated. The risk of tissue inflammation is lessened if the MIs are inserted into keratinized attached gingiva avoiding areas of frenum, muscle tissues and mobile mucosa (non-attached gingiva).14,26,45 Hypertrophy of mucosa covering an implant may occur as a complication of placing the MI in
Miniscrew implants for temporary skeletal anchorage in orthodontic treatment
A
B
63
C
Fig. 9.3 Inflammation around a miniscrew implant inserted in the palate for the correction of the crossbite of the maxillary left second molar. (A) Immediately after insertion. (B) Presence of inflammation. (C) After removal of the implant.
non-keratinized gingiva. With such a placement, a healing cap abutment is recommended at the time of insertion. Alternatively, the mucosa could be allowed to cover the MI head if there is an attached wire or an auxiliary that emerges into the oral cavity passing through the mucosa.44
INJURIES TO ADJACENT STRUCTURES The incidence of root injuries caused by trans-alveolar MIs used for temporary intermaxillary fixation of fractured mandibles is very low46 and it could be assumed that the incidence of root damage using MIs for orthodontic anchorage would be even lower as they are inserted after careful planning rather than as an emergency situation. Nonetheless, injuring of the adjacent roots, periodontal ligaments, nerves and blood vessels could occur during MI insertion and is usually indicated by symptoms such as pain on percussion and mastication, or, in cases of root injury, sensitivity to hot and cold.47,48 Under such circumstances, the MI should be removed immediately. The prognosis of an injured tooth will depend on whether there has been injury to the pulp. Histological examination of roots of teeth that had been damaged by MI placement demonstrated an almost complete repair of the periodontal structures 12 weeks after removal of the MIs.49,50 Only in severe injury with displacement of root fragments ankylosis of the lamina dura was noted. External root resorption in response to root injury, as well as evidence of inflammatory infiltrate or necrosis in the pulp tissue or on the injured root surfaces, was not found in any of the injured teeth.49,50
FAILURE OF MINISCREW IMPLANTS Although it is still not clear what precisely affects the success and failure rates of MIs, some factors have been identified:51 ■ ■ ■ ■
■ ■
patient-related factors, e.g. sex, gender, malocclusion, oral hygiene clinician-related factors, e.g. experience, learning curve MI-related factors, e.g. length, diameter, thread design insertion-related factors, e.g. cortical bone thickness, cortical notching, flap surgery, insertion torque, insertion angle treatment-related factors, e.g. loading time, tooth movement outcome-related factors, e.g. inflammation, mobility.
A MI can be lost or become loose through various factors such as inflammation (Fig. 9.3), improper placement, inadequate cortical bone at the chosen site or operator inexperience.47 Based on current evidence, the quality and quantity of the available bone at the insertion site are considered as the most important factors affecting primary stability of MIs. Although most studies have reported success rates of more than 80%,20,28,41,52 or a failure rate of about 16%,53 there is no consensus about how to define
clinical success or failure and many authors define failure in a general way as a state where a MI is not clinically usable. According to Melsen,52 the insertion angle should be kept stable during insertion, and the threaded part should be inserted totally into bone in order to avoid MI failures. Orthodontic loading may also affect the success of MIs. The magnitude and direction of force applied to MIs is one of the most important factors associated with MI failure. Loading with orthodontic forces applied perpendicular to the long axis of the MIs is preferable;54 the strain obtained by loading a MI perpendicular to its long axis with 50 cN was shown to lead to loss of primary stability when the cortex is 0.5 mm or smaller.55 Further, if a MI is used indirectly with a cantilever to a bracket-like head, the clinician should avoid the application of moments around the MI long axis in a counterclockwise direction since it could unscrew the MI, which can lead to loosening and failure.54 Cortical bone thickness is a significant factor in that greater thickness supports better success rates but can have negative effects through greater chance of MI fractures, overheating and necrosis. A clinical cortical bone thickness threshold of 1 mm increased success rate.20 A MI diameter below 1.3 mm should be avoided,14,24,25 and longer lengths tended to give higher success rates (see above).28 Patients with high mandibular plane angles may not be suitable candidates for MIs because they often have thin cortical bone, while statistically significant higher success rates have been found in patients with a deep bite.14,40 Success rates for white patients were found to be higher than for Asian patients40 but there was no effect of gender apart from a general willingness to cooperate with treatment that was better for girls than boys.20 Some studies have also correlated age with success rates, as failure rates were reportedly higher in adolescents than in adults.26,56 Placement in attached/keratinized gingiva has been associated with higher success rates of MIs,14,26,40,45 while root proximity has been reported as a major risk factor for MI failure.57 A recent meta-analysis has found that MI failure was not associated with patient gender, age and insertion site but was significantly associated with jaw of insertion, insertion torque, cortical bone thickness and root contact.51 No definitive conclusion could be drawn for the other factors discussed above. However, more MI failures were observed in the posterior region than the anterior in the maxilla and more failures lingually than buccally in the mandible. Orthodontic MIs overall presented a modest small mean failure rate of 13.5%, indicating their usefulness in clinical practice.51
FRACTURE OF MINISCREW IMPLANTS Fracture of MIs may occur during their insertion or their removal if ones of small diameter are employed (<1.2 mm) or if the collar is too narrow (≤1.3 mm) or is equipped with holes.8,45
64 SECTION III: CLINICAL CONSIDERATIONS FOR THE USE OF SKELETAL ANCHORAGE DEVICES IN ORTHODONTICS
If a MI has to be inserted into high bone density, predrilling should be carried out to minimize the risk of fracture. If a MI fractures during insertion, the shaft might need to be removed with a trephine and a new site should be selected for insertion of a fresh MI.
■ ■
The disadvantages are
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CLINICAL APPLICATIONS OF MINISCREW IMPLANTS IN ORTHODONTICS In general, the various MI systems can be used when dental elements lack quantity or quality, such as periodontally involved teeth and partial edentulism, as well as when there is a necessity to undertake tooth movements and minimizing or completely neutralizing undesired side effects of the reactive forces.58 Melsen suggested using MIs as anchorage for tooth movements that could not otherwise be achieved, such as in patients with insufficient number of teeth for the application of conventional anchorage, where the forces on the reactive unit would generate adverse side effects, in patients with a need for asymmetrical tooth movements in all planes of space, and as an alternative to orthognathic surgery.44 In general, MIs can be used to support anchorage of anterior or posterior teeth; for mesialization or distalization of teeth; for intrusion, extrusion and uprighting; and for palatal expansion and interarch coordination.8,9 More specifically, MIs have been used for the correction of deep or open bite, closure of extraction spaces, correction of a canted occlusal plane, alignment of dental midlines, extrusion of impacted canines, extrusion and uprighting of impacted molars, molar intrusion, maxillary molar distalization, distalization of mandibular teeth, en masse retraction of anterior teeth, molar mesialization, maxillary third molar alignment, intermaxillary anchorage for the correction of sagittal discrepancies and for correction of vertical skeletal discrepancies that would otherwise demand orthognathic surgery.8,9,37,59–61
CONCLUSIONS In summary, MIs present several advantages and some disadvantages when used for orthodontic anchorage reinforcement. The advantages are:
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their insertion and removal does not require any particular surgical procedures, in contrast to other means available, such as orthodontic implants, miniplates and onplants, which require flap surgery they can be easily inserted chair-side in one appointment, usually by the orthodontist him or herself there is no need for complicated clinical and laboratory procedures (fabrication of acrylic resin splints through taking imprints with additional implant copying systems to accurately transfer the implant position to the cast models) in order to facilitate safe and precise implant insertion they can be immediate loaded (there is no need for a waiting period for osseointegration, in contrast to orthodontic implants), which reduces the total treatment time they offer a great variety of possible locations for insertion, in contrast to conventional dental implants used for orthodontic anchorage, which require edentulous areas they provide absolute anchorage, which eliminates any undesirable effects on the teeth that otherwise would have been normally used as anchorage patient cooperation is limited to maintaining an immaculate oral hygiene
they can be easily removed they present reasonable cost relative to other conventional methods used for anchorage, while being much cheaper than orthodontic implants.
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if no proper attention is given during insertion, damage of the adjacent tissues or root injuries might occur irritation, inflammation of the peri-implant tissues, and consequent failure of the MI is possible, particularly in patients presenting poor oral hygiene when an oral surgeon is involved for their insertion (mainly when predrilling is required), there is an additional cost and increased stress to the patient.
REFERENCES 1. Papadopoulos MA, Tarawneh F. The use of miniscrew implants for temporary skeletal anchorage in orthodontics: a comprehensive review. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2007;103:e6–15. 2. Carano A, Melsen B. Implants in orthodontics. Prog Orthod 2005;6:62–9. 3. Creekmore TD, Eklund MK. The possibility of skeletal anchorage. J Clin Orthod 1983;17:266–9. 4. Kanomi R. Mini-implant for orthodontic anchorage. J Clin Orthod 1997;31:763–7. 5. Costa A, Raffaini M, Melsen B. Miniscrews as orthodontic anchorage: a preliminary report. Int J Adult Orthodon Orthognath Surg 1998;13:201–9. 6. Christensen FB, Dalstra M, Sejling F, et al. Titanium-alloy enhances bone-pedicle screw fixation: mechanical and histomorphometrical results of titanium-alloy versus stainless steel. Eur Spine J 2000;9:97–103. 7. Carano A, Lonardo P, Velo S, et al. Mechanical properties of three different commercially available miniscrews for skeletal anchorage. Prog Orthod 2005;6:82–97. 8. Ludwig B, Baumgaertel S, Bowman J. Mini-implants in orthodontics: Innovative anchorage concepts. Hanover Park, IL: Quintessence; 2008. 9. Lee J, Kim J, Park Y, et al. Applications of orthodontics mini-implants. Hanover Park, IL: Quintessence; 2008. 10. Bantleon HP, Bernhard T, Crismani AG, et al. Stable orthodontic anchorage with palatal osseointegrated implants. World J Orthod 2002;3:109–16. 11. Fritz U, Ehmer A, Diedrich P. Clinical suitability of titanium microscrews for orthodontic anchorage: preliminary experiences. J Orofac Orthop 2004;65:410–18. 12. Vande Vannet B, Sabzevar MM, Wehrbein H, et al. Osseointegration of miniscrews: a histomorphometric evaluation. Eur J Orthod 2007;29:437–42. 13. Eliades T, Zinelis S, Papadopoulos MA, et al. Characterization of retrieved orthodontic miniscrew implants. Am J Orthod Dentofacial Orthop 2009;135:10. 14. Miyawaki S, Koyama I, Inoue M, et al. Factors associated with the stability of titanium screws placed in the posterior region for orthodontic anchorage. Am J Orthod Dentofacial Orthop 2003;124:373–8. 15. Stahl E, Keilig L, Abdelgader I, et al. Numerical analyses of biomechanical behavior of various orthodontic anchorage implants. J Orofac Orthop 2009;70:115–27. 16. Costa A, Pasta G, Bergamaschi G. Intraoral hard and soft tissue depths for temporary anchorage devices. Semin Orthod 2005;11:10–15. 17. Chaimanee P, Suzuki B, Suzuki EY. “Safe zones” for miniscrew implant placement in different dentoskeletal patterns. Angle Orthod 2011;81:397–403. 18. Hong C, Lee H, Webster R, et al. Stability comparison between commercially available mini-implants and a novel design. Angle Orthod 2011;81:692–9. 19. Lim SA, Cha JY, Hwang CJ. Insertion torque of orthodontic miniscrews according to changes in shape, diameter and length. Angle Orthod 2008;78:234–40. 20. Motoyoshi M, Yoshida T, Ono A, et al. Effect of cortical bone thickness and implant placement torque on stability of orthodontic mini-implants. Int J Oral Maxillofac Implants 2007;22:779–84. 21. Chatzigianni A, Keilig L, Reimann S, et al. Effect of mini-implant length and diameter on primary stability under loading with two force levels. Eur J Orthod 2011;33: 381–7. 22. Siegele D, Soltesz U. Numerical investigation of the influence of implant shape on stress distribution in the jaw bone. Int J Oral Maxillofac Implants 1989;4:333–40. 23. Kim JW, Baek SH, Kim TW, et al. Comparison of stability between cylindrical and conical type mini-implants: mechanical and histological properties. Angle Orthod 2008;78:692–8. 24. Wilmes B, Ottenstreuer S, Su YY, et al. Impact of implant design on primary stability of orthodontic mini-implants. J Orofac Orthop 2008;69:42–50. 25. Wilmes B, Rademacher C, Olthoff G, et al. Parameters affecting primary stability of orthodontic mini-implants. J Orofac Orthop 2006;67:162–74. 26. Reynders R, Ronchi L, Bipat S. Mini-implants in orthodontics: a systematic review of the literature. Am J Orthod Dentofacial Orthop 2009;135:564. 27. Gallas MM, Abeleira MT, Fernandez JR, et al. Three-dimensional numerical simulation of dental implants as orthodontic anchorage. Eur J Orthod 2005;27:12–16.
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28. Chen CH, Chang CS, Hsieh CH, et al. The use of microimplants in orthodontic anchorage. J Oral Maxillofac Surg 2006;64:1209–13. 29. Berens A, Wiechmann D, Dempf R. Mini- and micro-screws for temporary skeletal anchorage in orthodontic therapy. J Orofac Orthop 2006;67:450–8. 30. Poggio P, Incorvati C, Velo S, et al. Safe zones: a guide for miniscrew positioning in the maxillary and mandibular arch. Angle Orthod 2006;76:191–7. 31. Chen Y, Kyung HM, Zhao WT, et al. Critical factors for the success of orthodontic mini-implants: a systematic review. Am J Orthod Dentofacial Orthop 2009;135: 284–91. 32. Kim YK, Kim YJ, Yun PY, et al. Effects of the taper shape, dual-thread, and length on the mechanical properties of mini-implants. Angle Orthod 2009;79:908–14. 33. Uemura M, Motoyoshi M, Yano S, et al. Orthodontic mini-implant stability and the ratio of pilot hole implant diameter. Eur J Orthod 2012;34:52–6. 34. Baumgaertel S. Predrilling of the implant site: Is it necessary for orthodontic miniimplants? Am J Orthod Dentofacial Orthop 2010;137:825–9. 35. Ohashi E, Pecho OE, Moron M, et al. Implant vs. screw loading protocols in orthodontics: a systematic review. Angle Orthod 2006;76:721–7. 36. Buchter A, Wiechmann D, Koerdt S, et al. Load-related implant reaction of miniimplants used for orthodontic anchorage. Clin Oral Implants Res 2005;16:473–9. 37. Ohnishi H, Yagi T, Yasuda Y, et al. A mini-implant for orthodontic anchorage in a deep overbite case. Angle Orthod 2005;75:444–52. 38. Labanauskaite B, Jankauskas G, Vasiliauskas A, et al. Implants for orthodontic anchorage: meta-analysis. Stomatologija 2005;7:128–32. 39. Dalstra M, Cattaneo PM, Melsen B. Load transfer of miniscrews for orthodontic anchorage. Orthodontology 2004;1:53–62. 40. Antoszewska J, Papadopoulos MA, Park HS, et al. Five-year experience with orthodontic miniscrew implants: a retrospective investigation of factors influencing success rates. Am J Orthod Dentofacial Orthop 2009;136:158. 41. Thiruvenkatachari B, Pavithranand A, Rajasigamani K, et al. Comparison and measurement of the amount of anchorage loss of the molars with and without the use of implant anchorage during canine retraction. Am J Orthod Dentofacial Orthop 2006;129:551–4. 42. Papadopoulos MA, Papageorgiou SN, Zogakis IP. Clinical effectiveness of orthodontic miniscrew implants: a meta-analysis. J Dent Res 2011;90:969–76. 43. Liou EJ, Pai BC, Lin JC. Do miniscrews remain stationary under orthodontic forces? Am J Orthod Dentofacial Orthop 2004;126:42–7. 44. Melsen B. Mini-implants: Where are we? J Clin Orthod 2005;39:539–47. 45. Park Y, Lee SY, Kim DH, et al. Intrusion of posterior teeth using miniscrew implants. Am J Orthod Dentofacial Orthop 2003;123:690–4.
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46. Fabbroni G, Aabed S, Mizen K, et al. Transalveolar screws and the incidence of dental damage: a prospective study. Int J Oral Maxillofac Surg 2004;33:442–6. 47. Melsen B, Verna C. Miniscrew implants: The Aarhus Anchorage System. Semin Orthod 2005;11:24–31. 48. Maino BG, Mura P, Bednar J. Miniscrew implants: The Spider Screw Anchorage System. Semin Orthod 2005;11:40–6. 49. Renjen R, Maganzini AL, Rohrer MD, et al. Root and pulp response after intentional injury from miniscrew placement. Am J Orthod Dentofacial Orthop 2009;136: 708–14. 50. Asscherickx K, Vannet BV, Wehrbein H, et al. Root repair after injury from miniscrew. Clin Oral Implants Res 2005;16:575–8. 51. Papageorgiou SN, Zogakis IP, Papadopoulos MA. Failure rates and associated risk factors of orthodontic miniscrew implants: a meta-analysis. Am J Orthod Dentofacial Orthop 2012;142:577–95. 52. Schätzle M, Männchen R, Zwahlen M, et al. Survival and failure rates of orthodontic temporary anchorage devices: a systematic review. Clin Oral Implants Res 2009;20:1351–9. 53. Wiechmann D, Meyer U, Buchter A. Success rate of mini- and micro-implants used for orthodontic anchorage: a prospective clinical study. Clin Oral Implants Res 2007; 18:263–7. 54. Melsen B, Graham J, Baccetti T, et al. Factors contributing to the success or failure of skeletal anchorage devices: an informal JCO survey. J Clin Orthod 2010;44: 714–18. 55. Dalstra M, Cattaneo PM, Melsen B. Load transfer of miniscrews for orthodontic anchorage. Orthod 2001;1:53–62. 56. Chen YJ, Chang HH, Huang CY, et al. A retrospective analysis of the failure rate of three different orthodontic skeletal anchorage systems. Clin Oral Implants Res 2007;18:768–75. 57. Kuroda S, Yamada K, Deguchi T, et al. Root proximity is a major factor for screw failure in orthodontic anchorage. Am J Orthod Dentofacial Orthop 2007;131:68–73. 58. Fortini A, Cacciafesta V, Sfondrini MF, et al. Clinical applications and efficacy of miniscrews for extra-dental anchorage. Orthodontology 2004;1:87–98. 59. Papadopoulos MA. Orthodontic treatment of Class II malocclusion with miniscrew implants. Am J Orthod Dentofacial Orthop 2008;134:604. 60. Lee J, Park HS, Kyung H. Micro-implant anchorage for lingual treatment of a skeletal class II malocclusion. J Clin Orthod 2001;35:643–7. 61. Bae S, Park H, Kyung H. Clinical application of micro-implant anchorage. J Clin Orthod 2002;36:298–302.
Miniscrew implants for temporary skeletal anchorage in orthodontic treatment Moschos A. Papadopoulos and Fadi Tarawneh
INTRODUCTION Anchorage, resistance to unwanted tooth movement, is a prerequisite for the orthodontic treatment of dental and skeletal malocclusions. Absolute or infinite anchorage is defined as no movement of the anchorage unit (zero anchorage loss) as a consequence of the reaction forces applied to move teeth. Such an anchorage can only be obtained by two means, using ankylosed teeth or using implants, both relying on bone for their anchorage. Currently, there are several skeletal anchorage devices that are used for orthodontic purposes, for example orthodontic implants, palatal implants or onplants, zygoma ligatures, zygoma anchors or miniplates, as well as miniscrews, microscrews or miniscrew implants (MIs). Miniscrew implants are small screw-type self-tapping implants, which in contrast to conventional dental or orthodontic implants have smooth, machine-polished surfaces and are designed to be loaded immediately after their insertion and removed easily after treatment (see Fig. 6.1). They carry special heads with or without holes to accommodate auxiliaries or wires for application of orthodontic forces (see Fig. 6.2). The use of MIs to obtain absolute anchorage has become very popular since they provide a non-compliance approach for reinforcing anchorage, ease of use and low cost.1 Currently, there are a large number of commercially available MI systems for orthodontic use in the market (Table 9.1).
TERMINOLOGY Terms such as mini-implant, miniscrew, microimplant and microscrew all refer to devices with smaller dimensions than conventional dental implants and that are removed following treatment; consequently, their use should not be synonymous with conventional dental or orthodontic implants. The terms orthodontic implant and mini-implant refer to systems that require osseointegration in order to be retained in the bone, whereas mini screws to self-tapping devices that do not require osseointegration and are only mechanically retained.2 Other terms used to describe temporary anchorage devices are intraoral or extradental anchorage systems, but the Moyer’s Symposium in 2004 agreed that the term mini-implant should be applied to palatal implants, mini-implants, miniscrews and microscrews.2 Nevertheless, the term mini-implant should be restricted to small prosthetic implants of diameter larger than 2 mm that are osseointegrated and are used to stabilize removable prostheses. The prefixes mini- and micro- are currently used in the literature to describe implants or screws of the same dimension without any differentiation. However, we advocate the use of the term miniscrew implant and this term is used throughout this textbook.
HISTORICAL DEVELOPMENT Skeletal anchorage concepts in orthodontics developed from two areas. The first was from osseointegrated dental implants to give orthodontic implants that were smaller but had similar surfaces and so also osseointe-
58
grated and were difficult to remove after treatment. The second was from surgical miniscrews used to fix osteosynthesis plates and resulted in the development of orthodontic MIs, designed for loading shortly after insertion and easy removal. The first clinical use of miniscrews for absolute anchorage was the treatment of a patient with a deep overbite using a vitallium bone-screw inserted in the anterior nasal spine.3 A mini-implant specifically made for orthodontic use was described in 19974 and a screw with a bracket-like head in 1998.5 Since then, several types of MIs have been introduced, each presenting different designs and features.
COMPOSITION OF MINISCREW IMPLANTS With the exception of the Orthodontic Mini Implant (Leone, Firenze, Italy), which is fabricated from SS, all other systems are manufactured from pure titanium or titanium alloys (medical type IV or type V titanium alloy; see Chapter 5). While both materials are biocompatible, a layer of connective tissue is usually formed around SS implants while direct bone contact and true osteogenesis occurs with pure titanium or titanium alloys.6,7 The rationale for using SS for MIs is that they are less prone to breakage and have less osseointegration than titanium devices. However, a study comparing three different MIs, two SS and one titanium, concluded that overall material performance of SS MIs was inferior to titanium MIs because of their lower yield or tensile strength.7 Removal of MIs is more related to thread design, MI shape and the drill–screw diameter ratio.7 Titanium alloys are favored over pure titanium because of their higher strength, excellent corrosion resistance, biocompatibility and favorable mechanical properties.8,9
OSSEOINTEGRATION Osseointegrated implants are largely used for skeletal anchorage purposes as they remain stable under orthodontic loading.10 However, they do have several disadvantages:11
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they cannot be immediately loaded with orthodontic forces since several months are needed for proper osseointegration they require an invasive technique for both insertion and removal, which should be done by a surgeon there is always need for supraconstruction and laboratory work they can only be inserted in edentulous areas (or in the palate) with sufficient bone quantity and quality they are expensive.
MIs were introduced in an attempt to avoid some of these problems. Their osseointegration is less than half that of conventional dental implants.5,12 A histomorphometric examination of titanium MIs retrieved following clinical use in alveolar bone found randomly organized osseointegration islets on the MI surfaces despite the smooth surface and immediate loading pattern of these implants, although it was associated with an extended period of retention (>6 months) (Fig. 9.1).13
Miniscrew implants for temporary skeletal anchorage in orthodontic treatment
59
Table 9.1 Commonly used miniscrew implant systems Product
Company
Address
Aarhus Anchorage System
American Orthodontics
1714 Cambridge Av, Sheboygan, WI, USA (www.americanortho.com)
AbsoAnchor System
Dentos
258 BunJi, Dong-In Dong, Jung-Gu, Taegu, Korea (www.dentos.co.kr)
Ancotek system
Tekka
ZI de Sacuny, BP 82, 118 Ave Marcel Mérieux, 69530 Brignais, France (www.tekka.eu)
Benefit System
PSM Medical Solutions
Moltkestrasse 41, D-78532 Tuttlingen, Germany (distributed by Mondeal-PSM, North America, PO Box 142, 49950 Jefferson St, Indio, CA 92201, USA) (www.psm.ms)
C-Implant
Dentium
3105 Korea Trade Tower 159 Samsung-dong, Gangnam-gu, 135–729 Seoul, Korea (www.implantium.com)
Cizeta Titanium Miniscrew
Cizeta Surgical
Via Caselle, 76 San Lazzaro di Savena, 40068 Bologna, Italy (www.cizetasurgical.it)
Dual-Top Anchor System
Jeil Medical Corporation
775–3 Daesung B/D, Daelim 3 Dong Youngdeungpoku, Seoul, Korea (www.jeilmed.co.kr); distributed by RMO Inc. PO Box 17085, Denver, CO 80217, USA (www.rmortho.com)
IMTEC Mini Ortho Implant
IMTEC Corporation
2401 N. Commerce, Ardmore, OK 73401, USA (www.imtec.com)
Infinitas Mini Implant System
DB Orthodontics
Ryefield Way, Silsden, BD20 0EF, UK (www.dborthodontics.co.uk)
Lin/Liou Orthodontic Mini Anchorage Screw (LOMAS)
PSM Medical Solutions
Moltkestrasse 41, D-78532 Tuttlingen, Germany (www.psm.ms); distributed by Mondeal-PSM, North America, PO Box 142, 49950 Jefferson St, Indio, CA 92201, USA
Micro Implant Universal Skeletal Anchorage System
Stryker Corporation
Stryker Leibinger Micro Implants, 750 Trade Centre Way, Portage, MI 49002, USA (www.stryker.com)
Miniscrew Anchorage System (MAS)
Micerium S.p.a.
Via Marconi 83, 16030 Avegno, Italy (www.micerium.it)
OMI – Orthodontic Ancor System
OsteoMed
3885 Arapaho Rd Addison, TX 75001, USA (www.osteomed.com)
Orlus
Ortholution
207 Dunchon B/D., 416–1, Seongnae-dong, Gangdong-gu, Seoul 134–844, South Korea (www.ortholution.com)
Ortho Easy system
Forestadent
Westliche Karl-Friedrich-Str. 151, 75172 Pforzheim, Germany (www.forestadent.de)
Orthoanchor K1 System
Dentsply Sankin Corporation
Tokyo, Japan (www.dentsply-sankin.com)
Orthodontic Mini-implant
Ortholution
Joongwon-gu, Seongnam-si, 462–725 Kyunggi-do, Korea (www.ortholution.com)
Orthodontic Mini Implant
Leone S.p.A.
Via P. a Quaracchi 50, 50019 Sesto Fiorentino, Florence, Italy (www.leone.it); distributed by Leone America, 501 W. Van Buren, Avondale, AZ 85323, USA
Orthodontic screw
KLS Martin Group
Ludwigstaler Str. 132, 78532 Tuttlingen, Germany www.klsmartin.com)
Spider Screw Anchorage System
HDC
Via dell’Industria 19, 36030 Sarcedo, Italy (www.hdc-italy.com)
Temporary Mini Orthodontic Anchorage System (TOMAS)
Dentaurum
Turnstrasse 31, D-75228 Ispringen, Germany (www.dentaurum.de)
VectorTAS
Ormco
1717 West Collins Ave, Orange, CA 92867, USA (www.ormco.com)
C
A
B
Fig. 9.1 Secondary electron microscopy of miniscrew implants (MIs). (A) Unused MI. (B) Retrieved MI after >6 months inserted in alveolar bone, with integument in the middle third of threaded area. (C) Higher magnification showing organized osseointegration islets on the surface of the retrieved MI. (From Eliades et al., 200913 with permission.)
60 SECTION III: CLINICAL CONSIDERATIONS FOR THE USE OF SKELETAL ANCHORAGE DEVICES IN ORTHODONTICS
Table 9.2 Available head designs of the miniscrew implants Head
Indications
Coupling elements
Advantages
Disadvantages
Crossed-slot (bracket)
Translation, space closure, intrusion, extrusion, uprighting
Elastic chains. coil springs, round/square/rectangular wires
Direct/indirect anchorage easily achieved; more than one coupling element can be used; easy orientation
None
Single slot (bracket)
Translation, space closure, intrusion, extrusion, uprighting
Elastic chains. coil springs, round/square/rectangular wires
Square/rectangular wires can be used; direct and indirect anchorage possible
Orientation of slot important but could be compensated by wire bending
Hook
Translation, space closure, intrusion
Elastic chains, coil springs, round wires
Coupling elements can be easily attached
Orientation of hook is crucial; hook can be bent or broken; indirect anchorage difficult
Button
Translation, space closure, intrusion
Elastic chains, coil springs, round wires
Coupling elements can be easily attached; orientation not needed
Indirect anchorage difficult to achieve
Hole
Translation, space closure, intrusion
Elastic chains, coil springs, round wires
Since reduced osseointegration is advantageous for removal of MIs after clinical use, most are fabricated with smooth surfaces to achieve good bone–implant contact but not osseointegration. The quality and quantity of cortical bone is considered one of the most important factors for achieving mechanical stabilization and retention of MIs. Although a dense trabecular bone is desirable, cortical bone is more important for primary stability of MIs and this is directly related to its thickness: the thicker the cortical plate the better the survival rate.14,15
MINISCREW IMPLANT DESIGNS MIs are designed to provide both direct and indirect anchorage as well as to decrease tissue irritation. Chapter 6 discusses the significance of the various components of a MI – head, neck, endosseous body and thread, tip – in detail.
HEAD The head of the MIs serves two important purposes; to apply torque to the thread during insertion and to act as a point for orthodontic force application. Table 9.2 summarizes the different designs currently available, their indications and coupling methods. The most frequently used shape is the bracket-like design with a single or a double (crossed) slot (e.g. Aarhus Anchorage System, AbsoAnchor System, Ancotek system, Dual-Top Anchor System, Ortho-Easy system, Spider Screw, Temporary Mini Orthodontic Anchorage System) as it can accommodate square and rectangular wires in its slot plus additional auxiliaries such as elastics and ligatures. It can be used for direct and indirect anchorage for uprighting, intrusion, extrusion and mesial and distal translation. The use of rectangular wires also allows for three-dimensional control in contrast to round wires, thus providing an important advantage over other designs. Other MI head shapes, such as the button-like, sphere, double sphere and hook (Aarhus Anchorage System, AbsoAnchor System, Dual-Top Anchor System, IMTEC Mini Ortho Implant, Lin/Liou Orthodontic Mini Anchorage Screw, Miniscrew Anchorage System, Orthoanchor K1 System, Spider Screw) can be used for mesial and distal translation, space closure and intrusion, but attachment of square or rectangular wires is not possible and indirect anchorage is unattainable or very hard to achieve. Furthermore, correctly orientating the hook-shaped MI during insertion is crucial for its use. One design has a hole or eyelet through the head (usually 0.6–0.8 mm in diameter), which can accept tension springs, elastic chains and round
Orientation of hole crucial
wires; this is mostly used for direct anchorage. With this type of head, three-dimensional control is not possible since square and rectangular wires cannot be used. Another disadvantage is the fact that MI heads equipped with holes could be more at risk of fracture during their insertion or removal, since this might make the material around the head less rigid and weaker.
NECK (COLLAR) The transgingival neck (or collar) connecting the head to the body of the MI is smooth and intended to form a non-irritant seal to protect against inflammation and infection of the surrounding soft tissue (see Fig. 6.6). Most common collar shapes are conical (tapering towards the body) or cylindrical but there are also polyangular collars.8,9 Depending on the insertion angle of the MI, pressure areas are formed on the soft tissue in the area of the collar, which appears to be less in conical collars. The diameter of collars is for most MIs identical to the threaded body but having the collar wider than the body, as in the top of a conical collar, is advantageous in terms of providing a good seal between the MI and soft tissue. Ideally, and in order to achieve the best possible seal and hygiene, the diameter of the head should also be smaller or equal to the collar.8,14 However, most MIs have heads that are larger in width than the collar, which can create areas/undercuts that can be more difficult to clean and easier for plaque to accumulate. Consequently, systems with MI heads smaller in diameter than the collars (e.g. Spider Screw, the Ortho-Easy, Temporary Mini Orthodontic Anchorage System, AbsoAnchor systems) are advantageous. As gingiva width varies between 1 and 4 mm in different locations in the oral cavity,8,9,16,17 systems that provide a variety of collars in their inventory are useful. Gingival thickness at the site of insertion should be measured before chosing the MI and this can only be done only after local anesthesia, delaying the selection until just before the insertion procedure. This necessitates the orthodontist having a large range of available MIs, which could be impractical in clinical practice. Fortunately, the most popular sites for MI insertion are covered with 1–2 mm gingiva, and MIs with a collar with 2 mm length are usually sufficient apart from for the palatal gingiva, which could reach 3–4 mm and requires a longer collar.8,9,16,17
THREAD The body of the MI is a threaded part that is inserted into the bone (Fig. 9.1A). The body’s primary function is the retention of the MI in the
Miniscrew implants for temporary skeletal anchorage in orthodontic treatment
bone initially after insertion (primary stability) and afterwards during orthodontic force loading (secondary stability), and its shape and design can influence the stability of the MI as well as the force (torque) needed for insertion and removal. The body can be either conical (e.g. Aarhus Anchorage System, AbsoAnchor System, Miniscrew Anchorage System) or cylindrical (e.g. the Orthodontic Mini Implant) (see Fig. 6.3). The thread design can be a uniform helix rotating either clockwise or counterclockwise with fixed helix angles and pitches or it can be a dual-thread design formed by two different helixes with different angles and pitches in the thread (see Fig. 6.13).18,19 The thread design can influence the bone– implant contact, the stress load of the bone and the insertion and removal torque.8,9,18,19 An ideal MI body shape should allow for a high insertion torque that would sustain the MI, but at the same time not damage bone and cause necrosis, and also allow a high enough removal torque to avoid unwanted pullout but at the same time facilitate MI removal without complications. Conical MI threads have been shown to have an increasing insertion torque compared with cylindrical threads, which require less torque, and a more constant torque, throughout the insertion procedure.7,18,19–23 This makes cylindrical MIs more desirable for insertion, minimizing the risk of bone necrosis, but their higher removal torque makes them more prone to fracture. Dual-thread MIs also have a high insertion torque but with a gentler increase in comparison with conical MIs.18
DIAMETER The diameter of the MIs is a significant factor that may influence (a) the possibility of injuring the neighboring roots when inserted inter-radicularly, (b) the tendency for breakage, and (c) success/failure rates. Chapter 6 discusses in detail the effect of diameter on these issues. Essentially, smaller-diameter MIs have less risk of root contact but a greater risk of fracture, while it is more difficult to achieve an optimal placement torque for good primary stability with narrower MIs. Based on success rates, 1.2 mm can be considered the minimum diameter required to achieve adequate implantation success.2,7,24–26 Diameters of 1.5 mm and 1.6 mm represent a satisfactory compromise. In clinical practice, however, the use of MIs of larger diameter for insertion inter-radicularly can be very difficult to accomplish without the risk of injuring adjacent roots, particularly when the roots are close to each other. In such circumstances, the choice of insertion site should be reconsidered, which may mean adjusting the force application system and applying appropriate orthodontic biomechanics to achieve the required result for the new chosen site of insertion. This is not an issue for palatal insertion, where larger-diameter MIs can be used, eliminating the risk of both MI fracture and root damage. Careful clinical and radiological examination, preparation and measurement to plan the insertion site and the MI diameter before insertion will minimize the risk of breakage failure. Stress distribution within bone is affected by the diameter of MIs. In cortical bone, the thicker the MI diameter, the more favorable the stress distribution.9,27 Three-dimensional finite element analysis has shown that a 1.4 mm diameter implant placed in 1.2 mm thick cortical bone can tolerate 150 g of orthodontic forces, while a 1.8 mm diameter implant can tolerate 350 g of orthodontic forces.9 Therefore, the planned orthodontic forces should also be taken into consideration when selecting the appropriate MI diameter.
LENGTH Length generally refers to the threaded portion of the MI rather than the entire head-to-tip length but care needs to be taken with catalogues to check that total length is not given. Here lengths refer to the threaded
61
part. In general, it is considered that the longer the MI the higher the success rate.28,29 Miniscrew implants are usually provided with lengths ranging from 4.0 to 12.0 mm, although some of them are also available in larger lengths up to 21 mm. The choice of length is determined by the depth and quality of bone, the MI angulation, the soft tissue thickness and the adjacent vital structures.26 Based on hard and soft tissue depths in 20 patients, MIs of 4–6 mm in length were considered safe in most regions, but individual patient variations dictate individual evaluation of bone depth before surgery.5 Although MIs of 6–8 mm have been recommended by most authors,5,8,9,30 MIs shorter than 8 mm are more susceptible to failure26,31 and longer MIs with small diameter can be more prone to breakage and bending. In this regard, longer MIs should also be thicker in diameter, but anatomical limits restrict the use of very long and thick MIs in clinical practice. The length of MIs has less effect than diameter on the distribution of stress, and the majority of the load is borne within the cortical bone.9 The force above which implant length and implant diameter are statistically significant in influencing implant stability was found to be 1 N.21 At low force (0.5 N), no statistically significant difference in displacement according to implant length and implant diameter was observed. However, at high force (2.5 N), MIs of 9 mm length displaced significantly less than the 7 mm MIs, and the 2 mm wide MIs displaced significantly less than their 1.5 mm wide counterparts.21 Further, insertion and removal torques seem to be higher in long MIs.32 The thickness of soft tissue at the insertion site should also be taken into consideration when choosing the appropriate length of MI. Thick gingival tissue sites demand longer MIs. In general, MI lengths of 6–8 mm are recommended for use in the mandible and of 8–10 mm in the maxilla. Nonetheless, individual patient variations in soft tissue and cortical bone thickness should be considered prior to insertion of any MI.8,9
MODE OF INSERTION All types of MI are self-tapping/self-cutting. The term self-tapping relates to the ability of a screw to create a thread as it advances into the bone. This is sometimes achieved by having a cutting flute, a groove in the thread, that drills away the material to make the hole for the screw. Some MIs require a pilot drill before their insertion (non-self-drilling MIs), while others have a fine tip that enables their direct insertion into bone (self-drilling MIs). The diameter of a pilot drill is preferably 0.2–0.3 mm thinner than the diameter of the MI thread, otherwise insertion torque values are significantly decreased and MI primary stability is reduced.8,33,34 Although the length of the pilot drill is determined by the length of the selected MI, deeper holes decrease insertion torque and so it is recommended that drills with depth markings and depth stops should be used.8,34 Occasionally, a pilot drill is needed even for a self-drilling MI; such as in cases where the cortical bone is thicker than 2 mm, as dense bone would eventually bend the fine tip of the MI. Self-drilling MI systems include the Aarhus Anchorage System, the AbsoAnchor System, the Dual-Top Anchor System and the Lin/Liou Orthodontic Mini Anchorage Screw, while Spider Screw offers both nonself-drilling and self-drilling MIs. The advantage of the self-drilling over non-self-drilling MIs is the elimination of the pilot drilling step during insertion, which makes the whole procedure less invasive, less time consuming and more easily accepted by the patient, particularly to young patients and their parents. In addition, since the insertion of the self-drilling MIs is less complicated, they can be easily inserted by orthodontists themselves without requiring referral to a surgeon.
62 SECTION III: CLINICAL CONSIDERATIONS FOR THE USE OF SKELETAL ANCHORAGE DEVICES IN ORTHODONTICS
Fig. 9.2 Anchorage. (A) Direct anchorage for the intrusion of a maxillary central incisor. (B) Indirect anchorage for the mesialization of a mandibular second molar after first molar extraction.
A
B
Table 9.3 Advantages and disadvantages of direct and indirect anchorage Type
Advantages
Disadvantages
Direct
Simple mechanics Easy to apply No dental anchorage loss
Load applied directly to implant may cause mobility or loss Should be monitored closely as mechanics used are not fail-safe Placement site for direct anchorage not always achievable or possible
Indirect
Less load applied to implant Fail-safe mechanics Placement site usually easy and preferable
Loss of anchorage in the anchorage teeth group is possible More complicated mechanics and installment
TYPES OF ANCHORAGE Miniscrew implants can provide two different types of anchorage: direct and indirect. When used for direct anchorage, MIs directly receive the reactive forces used to move the teeth by acting themselves as an anchor unit (Fig. 9.2A). In indirect anchorage, MIs are connected through bars or wires to the teeth of the reactive unit supporting their anchorage (Fig. 9.2B). Special considerations should be given to the design of the MI head and the placement site for the type of anchorage to be used. Different head designs are more suitable for one type of anchorage over the other, while placement site could play a major role in determining the type of anchorage. Advantages and disadvantages of direct and indirect anchorage are summarized in Table 9.3.
PROPERTIES OF THE MINISCREW IMPLANTS Currently available MIs vary in their material, diameter and length of threaded portion, design of the head, the existence of a collar and the method of insertion. An ideal MI used for orthodontic anchorage should present some specific properties and characteristics: ■ ■ ■ ■ ■
■ ■
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fabricated from a biocompatible material provided in different diameter and length sizes available in different designs (i.e. button or bracket head) self-tapping and self-drilling easily and safely inserted in various sites without complicated and invasive approaches immediately loaded removed without the need for complicated accessory equipment or invasive approaches inexpensive (cost beneficial).
LOADING Some authors have suggested a waiting period of 1 to 2 weeks before loading with orthodontic forces but most agree that, since in contrast to dental implants MIs are mechanically retained in bone sufficiently, they can sustain immediate light orthodontic loading without compromising their clinical stability.5,35,36 Only a few studies, mostly on animals, have investigated tissue reaction to immediate loading of MIs; all suggest that immediate loading with orthodontic forces can be performed without any complications.14,37,38 A finite element analysis found that immediate loading should be limited to 50 cN for a 2 mm diameter MI.39 One clinical study of 134 titanium MIs concluded that immediate loading is possible if the applied force is less than 200 cN,14 which is in agreement with a study showing no association between success rates of MIs and the mode of loading.40 This study concluded that vertical forces seem to be responsible for more failures than horizontal forces.40 The ability of MIs to provide absolute anchorage was shown in a study measuring anchorage loss with the use of MIs and conventional molar anchorage for canine retraction.41 A meta-analysis has shown that the use of MIs significantly decreased or negated loss of anchorage, particularly when compared with other conventional orthodontic means such as headgears, transpalatal arches or other biomechanical force systems.42 However, it is questionable if MIs remain stationary throughout their period of loading. In some patients, MIs were shown to move under orthodontic loading,43 and it is, therefore, advised to allow 2 mm of safety clearance between the MI and dental roots of the adjacent teeth.
COMPLICATIONS Complications associated with the use of MIs include inflammation, infection and tissue irritation, injuries to adjacent structures, and failure or fracture of the MI during insertion or removal.44
INFLAMMATION, INFECTION AND TISSUE IRRITATION Inflammation and irritation of the tissues around the MI site might occur, although infection is not generally noted (Fig. 9.3). Meticulous oral hygiene maintained around the MI and the use of 0.2% chlorhexidine mouth rinses or dental floss dipped in 2% chlorhexidine can help to avoid inflammation or infection, or control any that might occur.44 If a patient presents with purulence, palor or inflammation, definitive management with an appropriate antibiotic is indicated. The risk of tissue inflammation is lessened if the MIs are inserted into keratinized attached gingiva avoiding areas of frenum, muscle tissues and mobile mucosa (non-attached gingiva).14,26,45 Hypertrophy of mucosa covering an implant may occur as a complication of placing the MI in
Miniscrew implants for temporary skeletal anchorage in orthodontic treatment
A
B
63
C
Fig. 9.3 Inflammation around a miniscrew implant inserted in the palate for the correction of the crossbite of the maxillary left second molar. (A) Immediately after insertion. (B) Presence of inflammation. (C) After removal of the implant.
non-keratinized gingiva. With such a placement, a healing cap abutment is recommended at the time of insertion. Alternatively, the mucosa could be allowed to cover the MI head if there is an attached wire or an auxiliary that emerges into the oral cavity passing through the mucosa.44
INJURIES TO ADJACENT STRUCTURES The incidence of root injuries caused by trans-alveolar MIs used for temporary intermaxillary fixation of fractured mandibles is very low46 and it could be assumed that the incidence of root damage using MIs for orthodontic anchorage would be even lower as they are inserted after careful planning rather than as an emergency situation. Nonetheless, injuring of the adjacent roots, periodontal ligaments, nerves and blood vessels could occur during MI insertion and is usually indicated by symptoms such as pain on percussion and mastication, or, in cases of root injury, sensitivity to hot and cold.47,48 Under such circumstances, the MI should be removed immediately. The prognosis of an injured tooth will depend on whether there has been injury to the pulp. Histological examination of roots of teeth that had been damaged by MI placement demonstrated an almost complete repair of the periodontal structures 12 weeks after removal of the MIs.49,50 Only in severe injury with displacement of root fragments ankylosis of the lamina dura was noted. External root resorption in response to root injury, as well as evidence of inflammatory infiltrate or necrosis in the pulp tissue or on the injured root surfaces, was not found in any of the injured teeth.49,50
FAILURE OF MINISCREW IMPLANTS Although it is still not clear what precisely affects the success and failure rates of MIs, some factors have been identified:51 ■ ■ ■ ■
■ ■
patient-related factors, e.g. sex, gender, malocclusion, oral hygiene clinician-related factors, e.g. experience, learning curve MI-related factors, e.g. length, diameter, thread design insertion-related factors, e.g. cortical bone thickness, cortical notching, flap surgery, insertion torque, insertion angle treatment-related factors, e.g. loading time, tooth movement outcome-related factors, e.g. inflammation, mobility.
A MI can be lost or become loose through various factors such as inflammation (Fig. 9.3), improper placement, inadequate cortical bone at the chosen site or operator inexperience.47 Based on current evidence, the quality and quantity of the available bone at the insertion site are considered as the most important factors affecting primary stability of MIs. Although most studies have reported success rates of more than 80%,20,28,41,52 or a failure rate of about 16%,53 there is no consensus about how to define
clinical success or failure and many authors define failure in a general way as a state where a MI is not clinically usable. According to Melsen,52 the insertion angle should be kept stable during insertion, and the threaded part should be inserted totally into bone in order to avoid MI failures. Orthodontic loading may also affect the success of MIs. The magnitude and direction of force applied to MIs is one of the most important factors associated with MI failure. Loading with orthodontic forces applied perpendicular to the long axis of the MIs is preferable;54 the strain obtained by loading a MI perpendicular to its long axis with 50 cN was shown to lead to loss of primary stability when the cortex is 0.5 mm or smaller.55 Further, if a MI is used indirectly with a cantilever to a bracket-like head, the clinician should avoid the application of moments around the MI long axis in a counterclockwise direction since it could unscrew the MI, which can lead to loosening and failure.54 Cortical bone thickness is a significant factor in that greater thickness supports better success rates but can have negative effects through greater chance of MI fractures, overheating and necrosis. A clinical cortical bone thickness threshold of 1 mm increased success rate.20 A MI diameter below 1.3 mm should be avoided,14,24,25 and longer lengths tended to give higher success rates (see above).28 Patients with high mandibular plane angles may not be suitable candidates for MIs because they often have thin cortical bone, while statistically significant higher success rates have been found in patients with a deep bite.14,40 Success rates for white patients were found to be higher than for Asian patients40 but there was no effect of gender apart from a general willingness to cooperate with treatment that was better for girls than boys.20 Some studies have also correlated age with success rates, as failure rates were reportedly higher in adolescents than in adults.26,56 Placement in attached/keratinized gingiva has been associated with higher success rates of MIs,14,26,40,45 while root proximity has been reported as a major risk factor for MI failure.57 A recent meta-analysis has found that MI failure was not associated with patient gender, age and insertion site but was significantly associated with jaw of insertion, insertion torque, cortical bone thickness and root contact.51 No definitive conclusion could be drawn for the other factors discussed above. However, more MI failures were observed in the posterior region than the anterior in the maxilla and more failures lingually than buccally in the mandible. Orthodontic MIs overall presented a modest small mean failure rate of 13.5%, indicating their usefulness in clinical practice.51
FRACTURE OF MINISCREW IMPLANTS Fracture of MIs may occur during their insertion or their removal if ones of small diameter are employed (<1.2 mm) or if the collar is too narrow (≤1.3 mm) or is equipped with holes.8,45
64 SECTION III: CLINICAL CONSIDERATIONS FOR THE USE OF SKELETAL ANCHORAGE DEVICES IN ORTHODONTICS
If a MI has to be inserted into high bone density, predrilling should be carried out to minimize the risk of fracture. If a MI fractures during insertion, the shaft might need to be removed with a trephine and a new site should be selected for insertion of a fresh MI.
■ ■
The disadvantages are
■
CLINICAL APPLICATIONS OF MINISCREW IMPLANTS IN ORTHODONTICS In general, the various MI systems can be used when dental elements lack quantity or quality, such as periodontally involved teeth and partial edentulism, as well as when there is a necessity to undertake tooth movements and minimizing or completely neutralizing undesired side effects of the reactive forces.58 Melsen suggested using MIs as anchorage for tooth movements that could not otherwise be achieved, such as in patients with insufficient number of teeth for the application of conventional anchorage, where the forces on the reactive unit would generate adverse side effects, in patients with a need for asymmetrical tooth movements in all planes of space, and as an alternative to orthognathic surgery.44 In general, MIs can be used to support anchorage of anterior or posterior teeth; for mesialization or distalization of teeth; for intrusion, extrusion and uprighting; and for palatal expansion and interarch coordination.8,9 More specifically, MIs have been used for the correction of deep or open bite, closure of extraction spaces, correction of a canted occlusal plane, alignment of dental midlines, extrusion of impacted canines, extrusion and uprighting of impacted molars, molar intrusion, maxillary molar distalization, distalization of mandibular teeth, en masse retraction of anterior teeth, molar mesialization, maxillary third molar alignment, intermaxillary anchorage for the correction of sagittal discrepancies and for correction of vertical skeletal discrepancies that would otherwise demand orthognathic surgery.8,9,37,59–61
CONCLUSIONS In summary, MIs present several advantages and some disadvantages when used for orthodontic anchorage reinforcement. The advantages are:
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■
■
■
■
■
■
their insertion and removal does not require any particular surgical procedures, in contrast to other means available, such as orthodontic implants, miniplates and onplants, which require flap surgery they can be easily inserted chair-side in one appointment, usually by the orthodontist him or herself there is no need for complicated clinical and laboratory procedures (fabrication of acrylic resin splints through taking imprints with additional implant copying systems to accurately transfer the implant position to the cast models) in order to facilitate safe and precise implant insertion they can be immediate loaded (there is no need for a waiting period for osseointegration, in contrast to orthodontic implants), which reduces the total treatment time they offer a great variety of possible locations for insertion, in contrast to conventional dental implants used for orthodontic anchorage, which require edentulous areas they provide absolute anchorage, which eliminates any undesirable effects on the teeth that otherwise would have been normally used as anchorage patient cooperation is limited to maintaining an immaculate oral hygiene
they can be easily removed they present reasonable cost relative to other conventional methods used for anchorage, while being much cheaper than orthodontic implants.
■
■
if no proper attention is given during insertion, damage of the adjacent tissues or root injuries might occur irritation, inflammation of the peri-implant tissues, and consequent failure of the MI is possible, particularly in patients presenting poor oral hygiene when an oral surgeon is involved for their insertion (mainly when predrilling is required), there is an additional cost and increased stress to the patient.
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46. Fabbroni G, Aabed S, Mizen K, et al. Transalveolar screws and the incidence of dental damage: a prospective study. Int J Oral Maxillofac Surg 2004;33:442–6. 47. Melsen B, Verna C. Miniscrew implants: The Aarhus Anchorage System. Semin Orthod 2005;11:24–31. 48. Maino BG, Mura P, Bednar J. Miniscrew implants: The Spider Screw Anchorage System. Semin Orthod 2005;11:40–6. 49. Renjen R, Maganzini AL, Rohrer MD, et al. Root and pulp response after intentional injury from miniscrew placement. Am J Orthod Dentofacial Orthop 2009;136: 708–14. 50. Asscherickx K, Vannet BV, Wehrbein H, et al. Root repair after injury from miniscrew. Clin Oral Implants Res 2005;16:575–8. 51. Papageorgiou SN, Zogakis IP, Papadopoulos MA. Failure rates and associated risk factors of orthodontic miniscrew implants: a meta-analysis. Am J Orthod Dentofacial Orthop 2012;142:577–95. 52. Schätzle M, Männchen R, Zwahlen M, et al. Survival and failure rates of orthodontic temporary anchorage devices: a systematic review. Clin Oral Implants Res 2009;20:1351–9. 53. Wiechmann D, Meyer U, Buchter A. Success rate of mini- and micro-implants used for orthodontic anchorage: a prospective clinical study. Clin Oral Implants Res 2007; 18:263–7. 54. Melsen B, Graham J, Baccetti T, et al. Factors contributing to the success or failure of skeletal anchorage devices: an informal JCO survey. J Clin Orthod 2010;44: 714–18. 55. Dalstra M, Cattaneo PM, Melsen B. Load transfer of miniscrews for orthodontic anchorage. Orthod 2001;1:53–62. 56. Chen YJ, Chang HH, Huang CY, et al. A retrospective analysis of the failure rate of three different orthodontic skeletal anchorage systems. Clin Oral Implants Res 2007;18:768–75. 57. Kuroda S, Yamada K, Deguchi T, et al. Root proximity is a major factor for screw failure in orthodontic anchorage. Am J Orthod Dentofacial Orthop 2007;131:68–73. 58. Fortini A, Cacciafesta V, Sfondrini MF, et al. Clinical applications and efficacy of miniscrews for extra-dental anchorage. Orthodontology 2004;1:87–98. 59. Papadopoulos MA. Orthodontic treatment of Class II malocclusion with miniscrew implants. Am J Orthod Dentofacial Orthop 2008;134:604. 60. Lee J, Park HS, Kyung H. Micro-implant anchorage for lingual treatment of a skeletal class II malocclusion. J Clin Orthod 2001;35:643–7. 61. Bae S, Park H, Kyung H. Clinical application of micro-implant anchorage. J Clin Orthod 2002;36:298–302.