From Volume 8, Issue 2, April 2015 | Pages 56-61
Since it is well established that orthodontic mini-implants provide stable anchorage in all three dimensions, the focus has progressed to understanding the related biomechanics. This paper describes the key biomechanical advances for mini-implant treatments, especially in terms of optimized movement of the target teeth.
When orthodontic mini-implants (OMIs) became readily available in the early years of the new millennium they were initially ‘competing’ with existing osseointegrated fixtures such as restorative (dental) and orthodontic palatal implants. Therefore, the initial body of research work and publications focused on the biological and clinical factors which appeared to affect OMI success (stability) rates and whether this compared favourably with these other bone fixtures. Subsequently, as both clinicians and researchers began to see clear evidence of OMI stability and low morbidity, these fixtures became more widely utilized for anchorage reinforcement. These aspects have been extensively detailed, using the most recent research evidence, in the first paper in this series.1 However, in recent years the focus has broadened to include new clinical applications, ie how may we use mini-implant anchorage optimally.2 A large number of case reports have been published, often documenting impressive results of different types of malocclusions being successfully treated with OMIs. However, as pointed out by the late Dr Vince Kokich,3 some of these reports have strayed into bone anchorage usage at the expense of other clinical options, and this has perhaps caused a distraction from the quality and innovation of the majority of publications. Fortunately, based on a combination of these case reports, and especially from recent systematic reports (in the form of case series and randomized controlled trials), we can see that OMIs provide reliable anchorage. However, anchorage is only one part of the clinical picture, and these studies did not necessarily analyse the nature and range of biomechanical effects on the dentition.
Over a number of years following the introduction of OMI, anchorage orthodontists began to observe clinical side-effects, such as the development of lateral openbites (Figure 1), and realized that this was due to a combination of uncontrolled canine tipping and an unfavourable vertical component of the traction (which isn't overtly expressed with conventional fixed appliance mechanics where the traction runs alongside the archwire). Therefore, it has become apparent that OMIs provide the benefits of enhanced anchorage control, but with the potential for stronger expression of undesirable tooth movements. Consequently, the second phase of OMI clinical innovation and research included a focus on the reduction of such biomechanical side-effects. Furthermore, the clinical observations resulting from customized OMI biomechanics have crucially altered our understanding and expectations of orthodontic tooth movements and treatment outcomes. This paper aims to highlight these new biomechanical approaches, perhaps best exemplified by the addition of powerarms to the OMI armamentarium.
Powerarms (elongated hooks) are now regarded as a key component in OMI treatments since they have enabled direct traction to be applied much more parallel to the occlusal plane during both incisor retraction and molar protraction treatments (Figure 2). This has eliminated the use of oblique vectors of traction and hence reduced any vertical intrusive side-effects. It has also, perhaps inadvertently, paved the way for the next advance in OMI direct anchorage techniques. This centres on the ‘receiving end’ of the traction and possibilities for enhanced control of the target teeth. This review paper will describe current biomechanical approaches based on this latest awareness and the shift from maximum anchorage-focused goals to include maximized treatment effects on the target teeth. Since it is now possible to control anchorage, and hence tooth movements, in all three dimensions (3D), OMI applications are best discussed in each of the three planes of space: antero-posterior, transverse and vertical. Furthermore, OMI treatments may be subdivided within these 3D categories according to the principal treatment objective for the most common orthodontic treatment plans (Table 1). This paper will describe these subgroup topics within the context of these 3D clinical applications.
| Dimension | Clinical applications | ||
|---|---|---|---|
| Antero-posterior | Anterior tooth retraction and torque | Molar distalization | Molar protraction |
| Transverse | Centreline correction | Occlusal plane levelling | Bone anchored palatal expansion |
| Vertical | Incisor intrusion | Molar intrusion | Tooth extrusion |
Orthodontists will be most familiar with the classical use of orthodontic anchorage reinforcement to prevent mesial movement (anchorage loss) of the maxillary molar teeth. Such reliable anchorage is readily accomplished with OMIs, using either direct or indirect anchorage. Indirect anchorage involves using OMI(s) to stabilize the anchor tooth/teeth, such as when the maxillary first molar is anchored by a wire connection to mid-palatal OMI(s) (Figure 3). Orthodontic traction, eg a closing coil spring, is then attached between the molar hook (on the anchor unit) and the anterior (target) teeth or archwire hook. However, there is a risk of anchorage loss due to wire distortion or failure of the molar attachment. Indirect anchorage also relies on conventional fixed appliance traction mechanics and therefore entails its limitations, such as the risk of incisor retroclination (controlled tipping), rather than providing scope for true bodily incisor retraction. This is only feasible with direct traction between an OMI and powerarm.
Direct anchorage for incisor movements is typically achieved using an OMI inserted transmucosally on the buccal side of the alveolus, between the first molar and second premolar roots. Direct anchorage provides both stable anchorage (the avoidance of mesial movement of the adjacent molars since no traction is applied to these teeth) and enhanced control of incisor teeth movements. Interestingly, this additional torque control is classically more anchorage demanding than conventional traction, which tends to retrocline and tip teeth. The improved biomechanical control occurs because the powerarm's long length applies the traction closer to the centre of resistance of the target teeth (than with a conventional short hook). Powerarms are usually crimped onto a rigid (eg 0.019” x 0.025”) stainless steel archwire (Figure 2), but they may also be bonded directly onto either the labial or palatal surface of an individual target tooth (Figure 4). Several designs of crimpable powerarms are currently available: the original ‘shepherd’s crook' (Figure 2) and the more recently introduced sigmoid version (Figure 5). The author has found that the latter one is more adaptable in terms of the level of traction and the ease with which it may be contoured between the lip and gingival soft tissue zones. Both of these powerarm designs provides better torque control as the incisors are less prone to retroclination (lingual tipping of their crowns) during their retraction.4,5,6,7 In turn, this ensures that the combination of the planned antero-posterior tooth movements and an optimal aesthetic result may be achieved, even in adult ‘camouflage’ patients (Figure 6). However, given the amount of palatal movement of the upper incisor roots which may now be achieved, it's particularly important to confirm the anterior palatal alveolar width on a lateral cephalogram prior to treatment. If this is limited, eg where the anterior palate has been moulded by a digit habit, then the planned amount of incisor root retraction should be reduced to avoid an increased risk of incisor root resorption, due to contact with the palatal cortical plate.
Conventional fixed appliance biomechanics for molar distalization is typically applied at the coronal level. However, this tends to tip the molar teeth because the force is applied at some distance coronal to the molar's centre of resistance (furcation point). This tipping effect on the first molar is exacerbated if it is not counteracted by contact with an adjacent fully erupted second molar crown, although in turn the second molar then tends to tip distally.8 All of the so-called ‘non-compliance’ distalizers also inevitably suffer anchorage loss, both during molar distalization and then during retraction of the anterior teeth, unless they have been bone anchored.9,10 Fortunately, the use of mini-implants now provides the required combination of effective anchorage and better control of bodily tooth movements when molar teeth are being distalized or protracted (mesialized).
Alveolar insertion sites provide limited scope for such antero-posterior molar movements, especially when molar distalization needs to be followed by a second phase of premolar and anterior tooth retraction. Therefore, maxillary molar distalization, especially involving half a unit or more of molar movement, is best achieved with OMIs inserted in the mid-palate area, since there is no potential root interference (Figure 7). This also facilitates application of the distalizing force at the furcation level and hence better bodily molar movement.2,11 Whilst this indirect anchorage is more technically demanding, in terms of distalizer appliance design, fabrication and insertion, such distalizers tend to be easy to re-activate during molar distalization. This typically involves re-compression of the pushcoil (active component) to ensure continuous force application.
Similarly, mid-palatal anchorage may be used for molar protraction, where the posterior teeth need to be moved mesially to close spaces due to either hypodontia or premature tooth losses. This involves a similar combination of midpalatal anchorage and a U-shaped base wire, but with traction applied from the frame to the molar(s). Alternatively, direct anchorage may be utilized when premolar mesialization is not required in the maxillary arch and for any form of molar protraction in the mandibular arch. This involves the application of traction from an OMI, ideally inserted mesial or distal to the first premolar, to a molar powerarm (Figure 8). Such molar protraction can obviate the need for long-term restorative pontic provision and it should be considered as a treatment option where there is sufficient alveolar bone for orthodontic space closure.12 However, in the author's experience, the greater the alveolar deficiency then the greater the potential for side-effects such as mesial tipping of the molar and subsequent binding of the archwire in the molar tube. This leads to mesial shunting of the entire arch and manifests as advancement and intrusion of the anterior teeth, although these side-effects can be counteracted with the use of conventional intra-arch traction (Figure 8).
Some patients present with a large asymmetrical displacement of the dental centreline, due to either the unilateral absence of teeth or an underlying transverse skeletal asymmetry. Correction of this centreline discrepancy requires anchorage reinforcement on the contralateral side to the centreline displacement. Unfortunately, conventional anchorage, such as a transpalatal arch or headgear, connects the anchor teeth on both sides of the arch even when this anchorage on the side of the centreline shift is at best unwarranted and may even be contra-indicated for tooth movements on this side (such as molar protraction). In contrast, OMIs provide the benefit of effective unilateral anchorage specifically applied to the target side (Figure 9). Unilateral anchorage is also beneficial for correction of vertical asymmetry where the patient has an occlusal plane cant, ie the occlusal plane is tilted (relative to the face) with one side at a lower vertical level than the other side. Fortunately, it is now possible to correct both centreline displacements and many vertical occlusal plane cants using mini-implant anchorage. Such vertical anchorage is best exemplified in the context of molar intrusion, as described in the next section.
It is now recognized that skeletal anchorage has caused a paradigm shift in the management of patients with excessive vertical growth discrepancies, especially those with anterior open-bite (AOB).13,14 This is especially the case for patients with Class I and Class II malocclusions, where closure of the AOB is typically accompanied by Class II improvements such as an overjet reduction (Figure 10).13 Conversely, this may worsen the antero-posterior and facial profile discrepancies in patients with Class III and bimaxillary incisor proclination features. These cases are best treated using conventional options, involving either premolar extractions (with subsequent incisor teeth retraction during space closure), or orthognathic surgery (typically a maxillary osteotomy with vertical impaction and possibly advancement movements). Patients with a significant anterior vertical facial excess, presenting as a ‘gummy smile’, and gross lip incompetence, are also still more appropriately treated with a maxillary impaction osteotomy since the surgical approach is currently more effective at achieving a large reduction in the anterior mid-facial length.
Whilst OMI-assisted intrusion of the maxillary molars may represent more ‘effort’ for the orthodontist, from the patient's perspective it's a minimally invasive treatment compared to dental extractions and orthognathic surgery. Although some authors advocate OMI placement on the buccal side of the maxillary alveolus14 or in mid-palatal sites, this author has found that palatal alveolar sites are ideal for posterior vertical anchorage purposes. In particular, the posterior palatal alveolus offers sufficient interproximal root clearance, a suitable soft tissue environment, reliable cortical bone support, and adequate distance between the OMI and intruding teeth.13 In addition, the insertion of OMIs distal (rather than mesial) to the upper first molar teeth enables direct intrusion of the first molars and makes it simple to apply direct traction to the second molar teeth. The latter is beneficial when the terminal molars require additional intrusion (especially of the palatal cusps) or torque control (Figure 11). The potential negative side-effect is that palatal traction risks causing arch constriction because the anchorage point is medial to the molar crown and its fulcrum point. However, this is readily countered by a customized rigid TPA or quadhelix expander (Figure 10).2,13 These are all technique findings based on clinical observations. More formal research findings are becoming available, initially in the form of retrospective research. For example, a recent study of 31 maxillary molar intrusion cases has indicated that adolescent patients have greater scope for favourable mandibular skeletal improvements following molar intrusion than adults,15 but ideally this requires a prospective study as future evidence. In addition, there is still a need for long-term research results, since AOB correction is classically one of the most prone malocclusion traits to relapse.
Orthodontic mini-implants provide several advantages over and above those available with conventional anchorage reinforcement. On the one hand the resultant anchorage is reliable, minimally invasive, largely independent of patient compliance, and feasible in all three dimensions, while at the same time, it's now possible to produce more consistent bodily movements of the target teeth than with conventional biomechanics, because correct biomechanical designs mean that the forces are applied close to the centres of resistance of the target teeth. This is achieved with the use of powerarms in direct anchorage scenarios, and the positioning of palatal appliances near the level of the molar furcations in indirect anchorage cases.
