The Orthodontic Mini-implant Clinical Handbook. Richard Cousley
Orthodontic Mini‐implant Principles and Potential Complications
This chapter could alternatively have been titled ‘the advantages and disadvantages’ or, more trendily, ‘the pros and cons’ of mini‐implants, since it describes what we have to gain and possibly lose from their use. However, before we embark on these details, it's important to summarise what is meant by orthodontic mini‐implants and how we’ve arrived at current clinical applications.
1.1 The Origins of Orthodontic Bone Anchorage
Orthodontic‐specific skeletal fixtures were developed from two distinct sources:
restorative implants
maxillofacial surgical plating kits [1].
Orthodontic implants were first produced in the 1990s by modification of dental implant designs, making them shorter (e.g. 4–6 mm length) and wider (e.g. 3 mm diameter). However, they retained the crucial requirement for osseointegration, which is a direct structural and functional union of bone with the implant surface causing clinical ankylosis of the fixture. In contrast, orthodontic miniplates and mini‐implants (miniscrews) are derived from bone fixation technology, and primarily rely on mechanical retention rather than osseointegration. In effect, modification of the maxillofacial bone plate design, adding a transmucosal neck and intraoral head, resulted in the miniplate, whilst adaption of the fixation screw design produced the mini‐implant. Since the start of this millennium, a wide variety of customised orthodontic mini‐implants have been produced and these are now used in the vast majority of orthodontic bone anchorage applications. Orthodontic implants are no longer in standard use and the invasive nature of miniplates tends to limit their use to orthopaedic traction (e.g. Class III) cases or occasionally where the alveolar and palatal sites are too limited for mini‐implant usage (as exemplified in Chapter 8).
1.2 The Evolution of Mini‐implant Biomechanics
Hindsight is a wonderful tool, especially with new treatment modalities such as orthodontic mini‐implants. Much has changed in my clinical practice since I first used mini‐implants, back in 2003. And when one looks at the early texts on mini‐implants (including the first edition of this textbook), the evolution of techniques is also very apparent. The mini‐implant evolution in the 10 years from circa 2005 to 2015 may be best summarised as shown in Table 1.1.
1.3 3D Anchorage Indications
This gradual refinement of mini‐implant techniques has been accompanied by a substantial increase in the range of clinical applications for mini‐implants. The proportion of these uses will vary between orthodontists, depending on their individual caseloads, and even on financial and cultural influences. Overall, it's best to subdivide modern anchorage control according to each of the three dimensions and ‘other’ applications, with common examples for each category listed below.
| Anchorage dimension | |
| Anteroposterior | Incisor retraction and torqueMolar distalisationMolar advancement |
| Vertical | Single/multiple teeth intrusionTooth extrusion |
| Transverse | Centreline correctionsAltering occlusal planeRapid maxillary expansion (RME) |
| Other | Intermaxillary fixation (IMF) and tractionTemporary dental restorative abutment |
Table 1.1 The clinical evolution of orthodontic mini‐implant anchorage is subdivided into three chronological stages, with a description of the main focus at each stage along with representative clinical examples and associated side‐effects
| Stage of evolution | Clinical focus | Technique examples | Side‐effects |
| 1 | Reliable anchorage | Direct anchorage, from alveolar sites Indirect anchorage, especially palatal sites | Vertical effects of oblique traction, e.g. lateral openbites and uncontrolled incisor movements Hidden anchorage loss due to failings of connecting anchorage components |
| 2 | Minimised side‐effects | Traction powerarms Rigid transpalatal auxiliaries | Prevention of incisor extrusion/retroclination Prevention of molar buccal/palatal tipping movements during intrusion |
| 3 | Optimised target tooth movements (in addition to anchorage control) | Controlled 3D tooth movements during:incisor retractionmolar distalisationmolar intrusion | Bodily movement of target teeth, e.g.torque control during incisor retraction, bodily distalisation of molars, vertical molar intrusion movements |
1.4 Using the Right Terminology
Unfortunately, a misleading array of terms has been used for bone anchorage devices (BADs) and their applications in both journals and the commercial literature. Essentially, it is best to encompass all types of fixtures which provide skeletal anchorage under the umbrella terms BADs or temporary anchorage devices (TADs), although the latter term does not indicate the essential role of bone in this anchorage. This book covers only one of the three types of BADs: mini‐implants. Whilst the terms mini‐implant and miniscrew are used interchangeably in the literature, it is erroneous to use the terms microscrews or microimplants since these fixtures are small (mini) and not microscopic. I prefer the term mini‐implant since it conveys the small size and implantable nature of these temporary fixtures.
Second, there appears to be much misunderstanding over whether mini‐implants osseointegrate. Most mini‐implants are made from either titanium or titanium alloy and histological studies show variable levels of bone–implant contact (BIC) [2,3]. However, it is misleading to refer to this as osseointegration. Rather, clinical usage and percussion indicate that mini‐implants are mechanically retained (like bone fixation screws) rather than forming a clinically discernible ankylotic union with the bone (which occurs with restorative implants secondary to the initial BIC phase). Hence, mini‐implants can be immediately loaded and easily unscrew, usually without anaesthetic, at any time after insertion. This may be because of their relatively smooth surface and possibly because the surface contact is more a physical phenomenon than a biochemical one.
Mini‐implants are mechanically retained (like bone fixation screws) rather than forming a clinically discernible ankylotic union with the bone.
1.5 Principal Design Features
Most mini‐implants have three constituent parts: the head, neck and body (Figure 1.1), and are fabricated from a titanium alloy such as surgical grade 5 (Ti‐6Al‐4V). Grade 5 machined (smooth) titanium alloy is a good choice for mini‐implants because it supports rapid cell proliferation and has good cytocompatibility (which is better than stainless steel) and cell adhesion [4,5]. The head is the platform which