Maximising Mini-implant Success: Clinical Factors
A large number of research papers have been published in the orthodontic (and to a lesser extent in the surgical and dental implant) literature at an ever-increasing rate since the start of this millennium. This collective evidence provides a sound basis for mini-implant usage, although it may be difficult for orthodontists and dental colleagues to keep track of all this new information. Consequently, this chapter aims to collate and summarise the essential findings of the most relevant scientific and clinical research papers, in order that orthodontists may both understand and maximise their clinical usage of mini-implants.
Overall success rates
Mini-implant success is generally defined as the fixture remaining stable under continuous orthodontic loading for a minimum of 6 months, although many papers use a year as the minimum term. There is a consensus in the present literature that the success rate varies according to anatomical sites, e.g. 80 and 90% for the mandible and maxilla respectively.1,2,3,4,5,6,7,8,9,10,11,12 This seems counter-intuitive since the mandible is generally regarded as the stronger jaw bone, but the reasons for this paradox will be explained here. Interestingly, mini-implants with minor mobility may still be graded as successful. This is evident clinically by slight rotational or lateral movement of the mini-implant on manipulation. This is painless and consequently asymptomatic for the patient. It is easily resolved by tightening the mini-implant, usually by one half to a full clockwise rotational (insertional) turn, provided that this does not submerge the head, and without the need for anaesthesia. When the mini-implant displays obvious lateral mobility with light digital pressure then this indicates failure and the mini-implant should be removed. Fortunately, most mini-implant failures become clinically evident within the first few months of insertion,4,8,9 enabling early replacement or a modification to the treatment plan. Conversely, when a mini-implant feels firm after two months in situ then normal orthodontic forces may be applied with confidence.
Factors affecting mini-implant success
These are generally subdivided into three categories: patient, mini-implant and technique factors, and will be discussed accordingly.
Whilst mini-implant success appears to be unaffected by patient gender, the antero-posterior skeletal relationship, dental crowding, periodontal and temporomandibular status, several factors clearly affect stability. Their basis and clinical consequences are summarised below.
Cortical bone thickness and density
A combination of clinical, animal and artificial bone studies has demonstrated that the most important patient determinants of primary stability are the density and thickness of the maxillary and mandibular cortical plates. This helps to explain the variations seen in clinical studies of mini-implant success rates where both anatomical sites and individuals differ in terms of the cortical bone layer’s quantity and quality.13 The key facts to consider are:
- Cortical depth typically ranges from 1 to 2 mm and generally increases towards the apical aspect of the alveolus. In the maxillary alveolus cortical depth peaks both mesial and distal to the canines (the canine eminence) and the first molars, which partly accounts for the frequent use of these sites for anterior and posterior anchorage points, respectively. The maxillary alveolar cortex is thicker on the palatal than the buccal side, which contributes to the value of palatal alveolar insertions in anterior openbite correction (discussed in Chapter Nine), and the highest alveolar values for both jaws occur in mandibular molar sites.14,15,16,17,18,19,20,21
- An increase in either the cortical thickness or density leads to an increase in insertion torque (the resistance to rotational insertion).22,23,24,25,26,27,28 Thickness and density are co-dependent factors with density appearing to be the more influential factor in terms of mini-implant primary stability.24,25 The density of the underlying cancellous bone is much less relevant, except where the cortex is less than 1 mm thick, as occurs in some patients’ maxillary sites, and provides inadequate stability on its own.25
- The ideal range of maximum insertion torque appears to be 5–15 Ncm for alveolar sites.11,23,29,30,31,32,33 Maximum torque occurs during final seating of the mini-implant and is felt as an increase in resistance on turning a manual screwdriver, such that difficulty in digital rotation typically equates to the top of this torque range. This is clinically valid without it being necessary to measure this in individual patients. In effect, low torque equates to poor primary stability (inadequate cortical support) and excessive torque results in secondary failure because microscopic bone stress leads to subclinical ischaemic necrosis around the mini-implant threads. This manifests clinically as the mini-implant screwing in with little resistance at the low end of the scale, and it being difficult to manually turn the screwdriver at the high end. Such excessive torque, especially in posterior mandibular sites, may be avoided by initial perforation of the cortical plate, as described later.
- Cortex depth and density are greater in the mandible than the maxilla.34 In theory the mandible may provide greater primary stability, but the reported mandibular success rates are less than those for the maxilla because excessive insertion torque appears to cause high levels of peri-implant bone stress, resulting in secondary microscopic bone necrosis around the threads and hence mini-implant failure.29
- Cancellous bone, which has a similar density in both jaws,34,35,36 has little effect on primary stability, except when the cortex is less than 1 mm (as seen in some maxillary sites). In the long-term cancellous bone may influence secondary stability in terms of stabilising the mini-implant body against migration and tipping.26,28,37
The literature provides data on the average amount of interproximal space available for mini-implant insertion, but it is crucial to recognise that there is wide individual variation depending on the adjacent teeth’s root size, shape (degree of root taper and curvature) and alignment (root proximity/divergence). In addition, there is more space available on the palatal than the buccal aspect of the posterior maxillary alveolus (e.g. 5 and 3 mm, respectively), due to the differential number and shape of the molar roots, specifically the single palatal versus the two buccal roots of the molars. Therefore, assuming reasonable tooth alignment, the typical buccal alveolar insertion sites for the maxilla are: mesial to the first molar, and adjacent to the canines and central incisors; and for the mandible: adjacent to the molars and premolars.38 Crucially, interproximal space is not an absolute barrier and clinically it may be increased by both oblique insertion and pre-insertion root divergence, as described in Chapter Four.
Soft tissue and oral hygiene
Poor oral hygiene and peri-implant soft tissue inflammation are risk factors for secondary failure.1,10,12,39,40,41 Since these problems are more likely in loose (non-keratinised) mucosa, it is almost always recommended that mini-implants are inserted through attached mucosa. This should minimise soft tissue disruption and the destabilising effects of mobile peri-implant tissue.
Maxillo-mandibular planes angle (MMPA)
Patients with a high MMPA have an increased risk of failure for maxillary buccal mini-implants because of their relatively thin maxillary buccal cortical plates.1,9,40 However, these patients also typically present with an anterior openbite which may benefit from maxillary molar intrusion. Fortunately this problem of poor stability of maxillary buccal mini-implants may be avoided by palatal alveolar mini-implant insertion.22,42
Whilst primary stability is readily achieved in adults, adolescents have a significantly higher mini-implant failure rate.43 This is due to their reduced cortical thickness and density,16 and higher bone remodelling levels, which may compromise a mini-implant both in terms of primary then secondary stability (less able to withstand loading during the most vulnerable phase of bone resorption within the first month). Therefore, whilst mini-implants are still successful in adolescents it is advisable be cautious and keep the loading force low (e.g. 50 g) for the initial six weeks after insertion.
Heavy tobacco consumption is associated with a significantly higher failure rate.44 Therefore, whilst cigarette smoking is not an absolute contra-indication to mini-implant usage, smokers should be warned of the risk and advised to stop before mini-implant insertion.
Mini-implant bodies differ in terms of their dimensions (diameter and length), shape (cylindrical or tapered) and thread design. Notably, an increase in dimensions leads to greater bone surface engagement. Diameter is the most important factor in terms of primary stability because an increase in diameter leads to increased insertion torque.23,24,45,46,47,48 The effect of an increase in body length is much less pronounced than that due to an increase in diameter, with only a subtle increase in insertion torque occurring, because length increases involve the cancellous, not cortical, bone. However, increased body length may be favourable in areas of thin cortical bone by supplementing the cortical support in providing primary stability and especially in reducing the potential long-term drift of mini-implants.49 Relatively large diameter mini-implants are also less likely to be deflected by prolonged loading,50 and importantly they are more fracture resistant.10,51,52,53 However, 2 mm diameter mini-implants are not easily accommodated in many interproximal spaces so most mini-implants have mid-body diameters of around 1.5 mm.
Original mini-implant designs had cylindrical body shapes with self-tapping threads, and required pre-drilling of a full depth pilot hole. More recent designs have used tapered (conical) body shapes and are capable of self-drilling insertion. This is favourable for primary stability since animal and clinical research has shown that tapered designs have a higher insertion torque than cylindrical ones, and also a higher removal torque during the bone healing phases.12,23,24,28,53,54,55 This is because self-drilling avoids the risk of thermal tissue necrosis (associated with the heat generated by pilot drilling) and causes less disruption of the peri-implant bone’s original histological architecture.48,56,57,58,59,60,61,62 However, pre-drilling (perforation) of the cortical plate may still be valuable in avoiding the generation of excessive torque in dense/thick cortex sites, e.g. the posterior mandible, as discussed in the following section on insertion technique.
Finally, the projection of the head into the oral cavity is important since the further away the loading force is applied from the bone surface, the higher the risk of an unfavourable force (moment) at the mini-implant and bone interface.63,64,65 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.
Clinical technique factors
As with all clinical techniques, it is an unfortunate truth that mini-implant success rates increase with clinical experience.2,4,12,66,67 Therefore, novices should carefully plan their mini-implant biomechanics and insertion steps, and consider the use of a guidance stent to reduce root proximity risks.66,68 Mini-implants may be inserted manually with a customised screwdriver, or by using a contra-angle handpiece. The technique choice is a practical one depending on the ease of intra-oral access and hence screwdriver directional control and handling. However, it is also easier to appreciate and control the insertion torque with a manual screwdriver, whereas tactile sensation is lost with handpiece usage.
The key technique variations which have been studied are whether a pilot hole is drilled prior to mini-implant insertion and whether the insertion should be made perpendicular to the cortex or angled obliquely. Pre-drilling reduces the insertion torque with the greatest effect occurring within the fir/>