The Orthodontic Mini-implant Clinical Handbook. Richard Cousley
is the endosseous section with threads around a core and a tapered tip. Mini‐implants were initially available only in self‐tapping (non‐drilling) forms whereby a full‐depth pilot hole had to be drilled before mini‐implant insertion. However, many self‐drilling screws are now available. These have a tapered body shape with sharp tips and threads, and are inserted in a corkscrew‐like manner. Full‐depth predrilling is avoided, although shallow perforation of the cortex is still advantageous where the cortex is thick or dense, for example the posterior mandible and palate.
1.6 Clinical Indications for Mini‐implants
Mini‐implant usage may be broadly divided according to the case application and form of anchorage.
1.6.1 Routine Cases
Cases with high anchorage demands, such as retraction of prominent upper incisors or centreline correction (especially where unilateral anchorage only is required). Orthodontists new to mini‐implant use may find it easiest to introduce them into their clinical practice in such cases since the other aspects of the treatment are usually uncomplicated, enabling the orthodontist to readily recognise the anchorage effects and gain experience.Figure 1.1 The three principal sections of a mini‐implant: the head superficial to the tissues, the neck traversing the mucosa, and the threaded body within the cortical and cancellous bone.
Adults and older adolescents who wouldn’t comply well with other anchorage options, especially headgear.
Where extrusive tooth movements would be unfavourable (risking an anterior openbite or vertical excess).
1.6.2 Complex Cases
Where conventional biomechanics would be limited, for example molar intrusion to correct an anterior openbite.
Where conventional dental anchorage is limited by an inadequate number of anchor teeth (due to tooth loss or hypodontia) or periodontal support.
1.6.3 Direct and Indirect Anchorage
Direct loading is when traction is applied from the mini‐implant's head to an appliance, typically with elastic chain or nickel titanium (NiTi) coil springs (Figure 1.2a). Indirect loading involves using the mini‐implant to reinforce anchor teeth, from which traction is applied (Figure 1.2b,c). The most commonly shown example of this involves midpalatal mini‐implant(s) anchorage of the first molars. This approach is advocated because of the high success rates of parasagittal mini‐implants, even in adolescents. For example, a retrospective study of 384 parasagittal mini‐implants inserted by Dr Björn Ludwig (in Germany) gave a 98% success rate [6]. Whilst indirect anchorage also has a potential advantage of avoiding some potential biomechanical side‐effects (discussed later in this chapter), it carries the risk of insidious anchorage loss through flexing of the intermediary wire connection and undetected tipping or bodily translation of the mini‐implant. This has been reported to cause up to 0.5 mm of movement (anchorage loss) of the anchor tooth, in any one plane, when it is connected by a short piece of 0.019 × 0.025 in steel wire to a mini‐implant [7], and 1–1.4 mm mean anteroposterior mesial drift of indirect molars in cases treated with bimaxillary incisor retraction [8].
Becker et al. [9] recently published a meta‐analysis of en masse retraction treatments involving direct (buccal mini‐implants) and indirect (palatal) anchorage [9]. Their results show that direct traction techniques provided better anchorage control than indirect anchorage approaches in both anteroposterior and vertical planes. This appears to have been due to occult mesial migration of some palatal mini‐implants and bending of the transpalatal arch (TPA) component. Therefore, it was recognised that direct anchorage provides better outcomes in terms of anchorage control despite the possibility of higher mini‐implant stability in midpalate sites. Interestingly, the favourable biomechanical effects of direct anchorage, such as controlled bodily movement of target teeth, were not studied in this meta‐analysis, but represent an additional clinical benefit of direct anchorage usage [10–14]. This aspect will be discussed in detail at the end of this chapter.
In summary, I prefer to use direct anchorage wherever possible and this will be elucidated in the clinical scenario chapters. A key exception to this rule occurs in young patients whose bone immaturity means that the higher success rates of midpalate sites negates the biomechanical limitations of indirect anchorage. In effect, direct anchorage prioritises biomechanical considerations and indirect anchorage focuses on anatomical factors.
Direct anchorage provides better outcomes in terms of anchorage control despite the possibility of higher mini‐implants stability in midpalate sites.
1.7 Benefits and Potential Mini‐implant Complications
Mini‐implants have been shown to provide maximum anchorage along with the following benefits.
No need for additional patient compliance (over and above the compliance required for fixed appliance treatment).
Flexible timing for anchorage control, such that mini‐implant anchorage may be ‘switched’ on and off at virtually any stage in treatment. This differs from conventional options where the anchorage, such as headgear, needs to be applied at the outset and is very difficult to add later in treatment.Figure 1.2 (a) Direct anchorage where this grey elastomeric attachment provides traction from the mini‐implant head to a powerarm on the fixed appliance for en masse retraction of the anterior teeth. (b) The maxillary mini‐implant provides indirect anchorage for molar protraction in this hypodontia case. Horizontal traction is applied, using elastomeric chain connected to a vertical auxiliary wire via a ligature wire. The auxiliary wire is joined to both the main archwire, using a cross‐tube attachment, and the mini‐implant head (where its position is secured by composite resin). (c) Indirect anchorage of the upper incisors during unilateral molar protraction, using an elastomeric chain on the fixed appliance. This involves a 0.019 × 0.025 stainless steel auxiliary wire from the midpalatal mini‐implant's head to the central incisors' palatal surfaces, secured to both with composite resin.
Greater predictability of both the treatment mechanics and clinical outcomes. For example, one can now confidently retract the labial segment without anchorage or torque loss in a controlled manner (as described in Chapter 7).
Reduced treatment time, especially where it's more efficient to move groups of teeth rather than subdivide movements in an attempt to spare anchorage demands. This is exemplified by en masse retraction of the canine and incisor teeth in a single phase, rather than two‐phase retraction of the canines then incisors. A randomised trial showed a four‐month time saving in this respect [15].
3D anchorage control. Traditionally, orthodontists think of anchorage reinforcement in the anteroposterior dimension, with much less emphasis on vertical and transverse anchorage. However, now that it's feasible to control anchorage in all three dimensions, orthodontics can truly aim to correct 3D malocclusion traits.
However, a number of risks and side‐effects have been observed over the years with mini‐implant clinical usage and in the research literature. Fortunately, these are reversible in most clinical situations, but it is important to consider them in an effort to maximise mini‐implant treatment success and to provide informed patient consent. The main risks are described in the following sections.
1.8 Mini‐implant Success and Failure
Failure of a mini‐implant is the ‘risk’ that one ought to focus