The Orthodontic Mini-implant Clinical Handbook. Richard Cousley

3 Maximising Mini‐implant Success: Design Factors

      3.1 Mini‐implant Design Factors

      The endosseous bodies of mini‐implants differ within and between different manufacturers' systems in terms of their:

      It is reasonable to assume that different design characteristics will influence mini‐implant stability and success rates. In particular, an increase in mini‐implant dimensions (i.e. a larger body) leads to greater contact with the surrounding bone surface area. Crucially, both in vivo and in vitro research projects have demonstrated that diameter is the most important factor in terms of primary stability because an increase in diameter leads to increased insertion torque [1–6].

      An increase in body length has a much less pronounced effect (than an increase in diameter) with only a subtle increase in insertion torque and success rates occurring [7,8]. This is because an increase in length provides greater engagement in the cancellous bone, not the cortex, which is concordant with the fundamental influence of cortical bone on mini‐implant stability (as described in Chapter 2). However, increased body length is still likely to be favourable in sites with thin cortical bone (less than 1 mm) where cancellous bone appears to supplement the cortical support. This results in both increased success rates for longer mini‐implants with a longer body [9], and especially in reducing the potential long‐term movement (tilting/migration) of the mini‐implant where the loading force causes excessive peri‐implant bone remodelling but not outright failure [10]. Similarly, relatively large‐diameter mini‐implants are also less likely to be deflected by prolonged loading [11] and, importantly, they are more fracture resistant [12–15].

      This begs the question: why are large (e.g. 2 mm) diameter mini‐implants not universally used for increased fracture resistance and stability? The answer is simple: 2 mm diameter mini‐implants are not easily accommodated in alveolar interproximal spaces so most mini‐implants have midbody diameters of around 1.5 mm for these sites. However, 2 mm diameter mini‐implants may be used in edentulous alveolar sites and the midpalate (where, conversely, length is limited by anatomical parameters).

      Diameter is the most important factor in terms of primary stability because an increase in diameter leads to increased insertion torque.

      The original mini‐implant designs had cylindrical body shapes with self‐tapping threads, and required predrilling of a full‐depth pilot hole. Subsequent designs then favoured tapered (conical) body shapes and these are coincidentally also capable of self‐drilling insertion. Both animal and clinical research studies have shown that the latter is more favourable for primary stability since tapered designs have a higher insertion torque than cylindrical ones, and also a higher removal torque during the bone healing phases [1,2,15–19]. This is because self‐drilling causes less disruption of the peri‐implant bone's original histological architecture and avoids the risk of thermal tissue necrosis (associated with the heat generated by pilot drilling) [6,20–26]. However, predrilling (perforation) of the cortical plate is still valuable in avoiding the generation of excessive torque in thick, dense cortical bone sites, such as in the posterior mandible and midpalate. This is discussed in more detail with regard to insertion technique in Chapter 4.

Finally, the extent of projection of the mini‐implant head into the oral cavity is important since the greater the distance between the position of the loading force and the bone surface, then the higher the risk of an unfavourable force (moment) at the mini‐implant and bone interface [27–32]. Consequently, it is advisable to use a low‐profile mini‐implant design in order to avoid an excessive head and neck length combination relative to the body length, and to fully insert mini‐implants. This is also favourable from the patient's perspective since excessive prominence of a mini‐implant may irritate opposing mucosal and tongue tissues. If the mini‐implant's point of force application does project out from the surface, then lower forces should be used to avoid excessive cortical bone stress [31].

3.2 The Infinitas™ Mini‐implant System

      It may be a bewildering task for orthodontists to select a mini‐implant kit since a great variety of systems are now available worldwide. These differ from one another in terms of their:

      It is unlikely that one system provides dramatically better mini‐implant stability rates than all of the others and key principles are widely applicable. However, since the treatments and illustrations in this book mainly involve use of the Infinitas mini‐implant system (DB Orthodontics Ltd, UK, www.infinitas‐miniimplant.com), it is worth describing both its generic and distinctive features for clarity and reference purposes. The Infinitas mini‐implant is unique in several aspects, especially in its head design and customised three‐dimensional (3D) guidance stent facility [33]. This chapter describes the key design and clinical features of the Infinitas system's dual clinical and guidance kits.

      3.2.1 Infinitas Mini‐implant Design Features

Many mini‐implant head designs have two separate tiers represented by [1] a channel or ‘X’‐shaped cross‐slots at the top of the head for wire engagement and [2] an external circumferential undercut at a more apical level for the application of traction auxiliaries. In contrast, the Infinitas design has a unique, multipurpose head which combines cross‐slots with both external and internal undercuts, all on one vertical level (Figure 3.1). This gives the head a very low profile (intraoral prominence) whilst enabling the direct attachment of all forms of traction auxiliaries and wires (up to 0.021 × 0.025 in. dimensions). In particular, standard nickel titanium (NiTi) coil springs may be directly engaged, within the internal undercut, on one corner of the bracket‐like Infinitas head (Figure 3.2). Aside from patient comfort, this low profile is biomechanically favourable since it reduces the ratio of the head and neck (extra‐bony section) to body length, and hence the risk of adverse tipping moments [34].