Читать книгу The Orthodontic Mini-implant Clinical Handbook - Richard Cousley - Страница 44

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].

Figure 3.1 Diagram of the Infinitas mini‐implant head showing its four bracket‐like wings, divided by perpendicular cross‐slots, and the external and internal undercuts. A rectangular wire is illustrated within the transverse cross‐slot.

Figure 3.2 Diagram of an obliquely inserted Infinitas mini‐implant with a coil spring engaged within the internal undercut of a single wing of its head.

The Infinitas design has a unique, multipurpose head which combines cross‐slots with both external and internal undercuts, all on one vertical level.

The Infinitas head's precise engineering means that it is a relatively delicate structure. Therefore, the insertion screwdriver achieves a secure connection by engaging the neck section, and hence avoids large and potentially deformational forces on the mini‐implant head. Consequently, the internal shape of the screwdriver closely matches the pentagonal shape of the coronal part of the neck (Figure 3.1). The more apical section of the neck is tapered to enable mini‐implant insertion at both perpendicular and oblique angles to the cortical plate, maximising cortical engagement while minimising compression of the adjacent mucosa (Figure 3.3). There are short (1.5 mm) and long (2.5 mm) neck length versions, to allow for the mucosal depth at typical buccal and palatal insertion sites, respectively [35]. Whilst buccal insertions are routinely performed with a direct transmucosal technique, a reusable circular mucotome (soft tissue punch) is used to remove thick (palatal) gingiva or loose mucosa prior to insertion of a long neck mini‐implant. If this is orientated perpendicular to the bone surface then it is easy to achieve a ‘clean’ cut around the circular edge of the soft tissue punch, for ease of removal of the corresponding piece of tissue.

The Infinitas body has only four size variables: 1.5 and 2.0 mm diameters (as measured in the midbody region) and 6 and 9 mm lengths. In combination with the two neck variables and the multipurpose head design, this means that a range of only five mini‐implants provides for all alveolar and palatal insertion options (Table 3.1). This helps to simplify both the clinical decision‐making process and inventory. Each mini‐implant version is available in both non‐sterile and sterile packaging. The latter has the advantage that the mini‐implant is ready to use, straight out of the packet (Figure 3.4), whereas the non‐sterile versions need to be sterilised in the Infinitas kit prior to use.

Figure 3.3 Diagram of an obliquely inserted mini‐implant where the body traverses the cortical layer at an angle, facilitated by the tapered shape of the neck. Notably, one side of the head is closer to the mucosa than the opposite side.

Figure 3.4 Packaging of presterilised Infinitas mini‐implants. The mini‐implant body is inside a rubber sheath, within the sterile envelope. The operator may pick up the mini‐implant by holding this sheath, without the need for sterile gloves, whilst the (sterile) screwdriver is engaged on the mini‐implant neck.

All the Infinitas bodies feature a self‐drilling tip and threads (Figure 3.5), since self‐drilling insertion preserves more original bone than a predrilled technique [20,24]. Engagement of the cortical bone plate, in order to maximise primary stability, is also enhanced by two specific Infinitas design features. First, the thread continues to the coronal end of the body such that it may be fully seated in bone. Second, the 1.5 mm (narrow) Infinitas body version has an additional tapered feature at its coronal end, such that the thread diameter gradually widens from 1.5 to 2 mm before the junction with the neck (Figure 3.4). This creates a clinically noticeable increase in torque during the final stage of insertion as the body part engaging the cortex widens from the cylindrical to tapered form, with the effect of increasing primary stability [2,36,37]. Another benefit of this additional body taper is that it greatly increases strength in this critical coronal area since a 0.2 mm increase in diameter can increase strength by 50% [37,38]. This helps to minimise the risk of fracture of the body during insertion and removal [39,40].

Table 3.1 The Infinitas mini‐implant range and typical insertion sites

Colour code	Diameter	Body length	Neck length	Typical insertion sites and inclinations
	1.5	9	Short	Maxilla: buccal alveolus Maxilla: oblique insertions in anterior alveolus Mandible: oblique insertions
	1.5	6	Short	Maxilla: perpendicular insertions in anterior alveolus Mandible: perpendicular insertions
	1.5	9	Long	Palatal alveolus (maxilla)
	2.0	6	Long	Midpalate
	2.0	9	Long	Edentulous areas

Figure 3.5 Diagram showing how the body's coronal section tapers out from a 1.5 to 2.0 mm thread diameter.

So why not have a 2 mm diameter over most of the body length? Unfortunately, this would greatly increase the risk of close root proximity in narrow interproximal sites (compared to the 1.5 mm midbody size). Hence, this tapered body design provides a 2 mm diameter superficially where it is most beneficial for strength and stability, but avoids root proximity problems since the narrowest interroot space is frequently 4 mm in from the external bone surface [41].

This tapered body design provides a 2 mm diameter superficially where it is most beneficial for strength and stability, but avoids root proximity problems.

Excessive insertion torque is most likely to occur in the mandible and palate where the cortex 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.