The Orthodontic Mini-implant Clinical Handbook. Richard Cousley
is relatively thick and dense, leading to pressure necrosis of peri‐implant bone and subsequent secondary (delayed) failure [24,42–46]. This risk is reduced by pilot drilling, especially within the first 2 mm of drilling depth [43,46]. However, pilot drilling is a nuisance if it requires a low‐speed surgical handpiece and saline irrigation (to avoid heat necrosis). Therefore, it is ideal to perforate thick and dense cortical bone in order to avoid excess torque, but avoid drilling deep into the cancellous bone, and to simplify the insertion process by avoiding conventional pilot drilling. The Infinitas system achieves this balance with a customised cortical bone punch (Figure 3.6) which easily perforates dense cortical bone (and the mucosa), with simple slow manual clockwise rotations, up to a maximum depth of 2 mm. Its use is recommended for all posterior mandibular and palatal (alveolar and midpalate) insertion sites in adults, and it is fully compatible with the Infinitas guidance stent. Some orthodontists also prefer to use the punch in buccal sites to initially indent the cortex, without necessarily perforating it. This creates a ‘purchase’ point and avoids slippage of the mini‐implant tip at the start of an oblique insertion.
Figure 3.6 Diagram of the Infinitas cortical bone punch traversing the mucosa and inserting into a thick cortical layer.
3.2.2 Infinitas Guidance System
Mini‐implant planning involves determination of the optimum mesiodistal and vertical position, and also ideally the vertical and horizontal angles of insertion; that is, 3D planning is required. However, it may still be difficult to accurately position a mini‐implant, typically in posterior or palatal locations, because of restricted visual and instrument access. Further discrepancies may arise when the planning and insertion processes are carried out by an inexperienced orthodontist, or especially by different clinicians (e.g. an orthodontist and surgeon). For example, a recent retrospective study of 50 orthognathic cases (where mini‐implants were inserted for intermaxillary fixation) showed considerable variation in the surgical mini‐implant insertion angles. Perhaps more worryingly for secondary stability was the cone beam computed tomography (CBCT) observation that 41% of these mini‐implants had some degree of root contact, even after allowing for the overestimating effect of this on panoramic radiographs [47].
Research has shown that clinical inexperience is associated with greater risks of mini‐implant–root proximity and hence increased failure rates [17,29,48,49]. In the situation where the insertion is done by a different clinician, the orthodontist should ideally avoid the need to fully describe the insertion site, angles and clinical plan, and then hope that this plan is both understood and followed. Instead, the 3D positional information should be clearly dictated by a stent, removing the need for educated guesswork or improvisation by the surgeon. Unfortunately, placement of an interproximal wire or custom‐made wire guide at the approximate insertion site or on an adjacent fixed appliance usually only indicates the superficial insertion position and not the 3D angles of insertion. This approach is also prone to radiographic parallax errors. Even techniques using a circular guide tube allow excessive lateral play of the insertion instrument [50], and partly rely on visual alignment of the mini‐implant with the tube's adjacent edges unless the screwdriver is physically steered by engagement within the guide tube.
These problems can be minimised by the use of a stent capable of reliably transferring the 3D prescription from the planning to the insertion stages, and then physically guiding the insertion process. It is also ideal if relatively little laboratory expertise and cost are required. The Infinitas guidance system meets these requirements by using three simple components: a mini‐implant analogue, abutment and guide cylinder which work with a plastic baseplate to form an insertion stent for precise control of insertion instruments (Figure 3.7). There are six simple steps involved in the fabrication of this stent, and these may be carried out by an orthodontist (with access to a vacuum or pressure forming machine) with or without the assistance of an orthodontic technician (following the orthodontist's prescription) as described here.
A stent is capable of reliably transferring the 3D prescription from the planning to the insertion stages.
1 Plan the insertion details using a dental model and radiographsFor manual stent fabrication, the optimum position and angulations for each mini‐implant are determined by combining radiographic information (e.g. a periapical or CBCT view of the interproximal space) with the topographical features of a plaster model of the dental arch. In effect, it is much easier to visualise the insertion angles on a model than in the patient's mouth, and the surface contour of the model typically highlights the appropriate insertion space as a concave indentation between tooth roots. In order to preserve as much detail as possible, it is important to avoid waxing/blocking out any fixed appliance brackets near the insertion site when a dental impression is recorded for stent fabrication.Figure 3.7 Handpiece insertion of an Infinitas mini‐implant on the palatal aspect of the alveolus, where the end of the screwdriver insert fits within the 3D stent. The mini‐implant's body can be seen through the plastic guidance cylinder.
2 Drill a pilot hole in the modelA pilot hole is drilled in the dental model at the planned vertical and mesiodistal insertion point and angles, using a plaster drill and a contra‐angle dental handpiece (Figure 3.8). It is recommended that this hole is initially drilled to only half of the analogue length. This gives the option of drilling the remaining depth of the hole at a new insertion angle, should this be appropriate on judging a half‐inserted analogue.
3 Insert the mini‐implant analogue into the modelThe analogue is manually screwed into position in a clockwise direction, using the guidance kit screwdriver. If in doubt, the analogue should only be partially seated until its 3D position has been checked, so that an alteration of the insertion site/directions is easily made by further drilling.
4 Fit the abutment onto the analogueThe abutment is manually fitted onto the head of the analogue and usefully amplifies the analogue's insertion angles (Figure 3.9). If the 3D position of the analogue appears to be suboptimal then it should be removed from the model and the insertion process repeated with different location/angulation details (after filling in the plaster hole as necessary). In particular, it is much easier to refine the position in a dental model at this stage than in the patient's mouth at the insertion visit!
5 Slide the guide cylinder over the abutmentThe internal diameter of the cylinder is similar to the external diameter of the abutment, ensuring a close fit between their surfaces.
6 Form the stent baseplateThe assembled combination of dental model, analogue, abutment and guide cylinder components is placed into a pressure or vacuum forming machine such as that used for orthodontic retainer fabrication. The stent baseplate is then formed using a thick (ideally 1.5 mm) thermoplastic blank. This is much thicker than retainer blanks (which are typically 1 mm), in order to reduce flexing of the stent (if the screwdriver is leant upon). The baseplate, incorporating the guide cylinder, is then trimmed to the desired size, usually incorporating four or more crowns and the insertion site (Figure 3.10). If there are already brackets on the teeth then the baseplate edge should be trimmed clear of them.
Figure 3.8 Drilling of the palatal aspect