From Volume 8, Issue 1, January 2015 | Pages 6-12
Orthodontic mini-implants are able to provide reliable anchorage, but are reported to have varying success (stability) rates. Which factors and issues determine mini-implant success and how can this be maximized? This paper provides an interpretation of the relevant research findings, with a particular focus on the latest published studies, to help the orthodontist both understand and maximize his/her mini-implant successes.
A revolution in orthodontic anchorage began at the start of the 21st century with the introduction of skeletal anchorage fixtures customized for orthodontic purposes. These are commonly known as orthodontic mini-implants (OMIs), mini-screw implants (MSIs) and temporary anchorage devices (TADs). The first two terms are widely regarded as the most accurate ones,1 and are used interchangeably in most peer-reviewed journals. The author prefers the term OMI and this will be used throughout this series of two papers. These papers aim to provide a contemporary update for the orthodontic team, firstly on the principles of stable mini-implant anchorage and secondly on the wide-ranging clinical applications.
Maxillofacial surgical (fracture and osteotomy) fixation screws were used as the original OMIs in the late 1990s. Subsequently, during the first decade of the new millennium, this technology was customized for orthodontic purposes, especially in terms of the head and neck screw design, plus the associated insertion instrumentation. The head portion is used to connect to orthodontic appliances and the neck traverses the mucosa (in contrast to fixation screws which are placed submucosally). Typical OMI endosseous (body) dimensions are 1.3-2 mm in diameter and 5-10 mm in length. The majority of commercially available OMIs have polished and smooth endosseous surfaces, in contrast to dental (tooth) implants. Hence, they rely on mechanical retention within the bone. This is observed at the histological level in the form of bone-implant contact (BIC), but crucially this does not equate to osseointegration, where the latter is defined as an ankylotic union between a metal fixture and the adjacent bone. This is an important distinction since solely physical retention means that OMIs may be immediately loaded and are easily unscrewed during their removal (without the need for bone trephination).
OMIs are inserted using a small amount (eg 0.1-0.2 ml) of local anaesthetic agent and a recent prospective split mouth study demonstrated significantly higher levels of patient comfort and a quicker procedure time with the use of local compared to topical anaesthesia.2 Multiple questionnaire studies have shown that this procedure is well tolerated by patients, similar to orthodontic separators, and is a significantly better experience than dental extractions. Furthermore, the most recent prospective study, comparing OMIs to conventional forms of anchorage, concluded that OMIs are preferable to headgear usage.3 This form of skeletal anchorage may be used for variable time spans ranging, in the author's experience, from a few months to several years. Successful implantation is judged in terms of the OMI remaining stable (static) under normal orthodontic force application (eg 150-200 g) for a minimum of six months. Fortunately, the majority of failures occur within the first few months of insertion.4,5 Conversely, if an OMI appears to be clinically immobile and asymptomatic after two to three months then it is unlikely to develop problems under normal orthodontic loading. Failure typically manifests as either noticeable lateral mobility of the OMI, or excessive peri-implant soft tissue swelling/hyperplasia (Figure 1), although patients rarely present with acute pain related to peri-implantitis. There appears to be a reasonable consensus in the recent literature that success rates for OMIs in the maxilla and mandible are broadly 90% and 80%, respectively. However, what causes such a difference between the jaws and why does it seem counter-intuitive that mandibular insertion sites have a lower success rate? The answers to these questions are the central theme of this paper. Consequently, the factors involved in determining OMI success or failure will be outlined here, specifically using the latest research findings as the evidence base. Those wishing to scrutinize the established (especially pre-2010) reference material are advised to refer to an OMI textbook.6 These factors are typically divided into three categories:
Why do success rates differ between patients and between insertion sites (within the same patient)? Fortunately, multiple research studies have now been published on the variety of patient factors which may affect OMI stability. Consequently, a clear consensus has emerged, especially over the last five years, on which patient factors have most influence on OMI success. These may be subdivided into three categories:
A range of retrospective studies have not shown any significant difference in success rates according to patient sex.6 Similarly, such studies suggested that patient age is irrelevant to OMI success. However, this conclusion may have been reached because the sizes of the age-related subsamples in these retrospective studies were too small for meaningful statistical analysis. In effect, a lack of study power/statistical significance, interpreted as a lack of evidence, led to the conclusion that age is unimportant, even when age-related trends could be identified in the data.7 However, studies which have evaluated age in relation to the primary outcome measure of OMI success suggest otherwise. One particular prospective clinical study measured success rates in 169 OMIs inserted in 57 patients. They were divided into adolescent and adult age groups, where the OMIs were loaded either early (2-3 weeks after insertion) or late (at three months).8 The early-loaded adolescent group had a lower success rate (64%) compared to both the late-loaded adolescent and early-loaded adult groups (97% and 92%, respectively). This was a clear and statistically significant difference for adolescent patients, whereas adults appeared to be less affected by early loading. The authors concluded that age, in terms of the adolescent versus adult stages, is an important factor for OMI stability, and that delayed loading should be considered in adolescent patients. This is best judged in clinical terms if an OMI has a low insertion torque (which will be discussed in detail in the ‘mini’ section) when inserted in a growing patient.
The biological explanation for this age effect is that adolescent patients have relatively lower levels of cortical thickness and density, as demonstrated by CT studies of alveolar bone, and higher rates of bone remodelling.9,10,11 Experimental evidence for this has been provided by a recent study of OMIs in young and adult rats involving a combination of immediate and delayed (six week) loading regimens.12 Mobility was evaluated using the periotest and it showed that the immediately loaded OMIs in young rats had the worst periotest values. The authors consequently recommended a latency period in adolescent patients to assist with the development of secondary stability, in concordance with the conclusions of the aforementioned clinical study.
Is OMI success affected by the patient's dentofacial morphology? There is no evidence that variations in the antero-posterior plane affect OMI success, ie it appears to be irrelevant whether the patient has a Class I, II or III skeletal pattern. However, variation in the vertical dimension does influence OMI success, as revealed by several studies comparing patients with increased and reduced maxillo-mandibular planes angles (MMPA). The high MMPA (long face) patients had a significantly higher failure rate for OMIs inserted in maxillary buccal alveolar sites.5,13 The biological explanation, according to a CBCT study of adult jaws, is that high MMPA (dolichofacial) patients have thinner cortical plates than those with reduced vertical facial parameters.14,15 This information is most relevant to anterior openbite (AOB) cases where maxillary OMIs are used to provide anchorage for molar intrusion. As explained in the following section, for a variety of reasons it is preferable to use palatal rather than buccal alveolar sites. This is in addition to the need to consider the effects of the patient's age, ie an adolescent with a long face pattern may be at particular risk of poor primary stability.
The two key considerations in terms of insertion site anatomy are:
While the latter has not been the specific topic of a prospective study, there is a consensus in the literature, based on retrospective case series, that insertion sites should ideally be located in areas of attached gingiva.6 This avoids the potential destabilizing effects of mobile tissue movements and oral hygiene interferences. The latter is likely to cause chronic peri-implant gingival inflammation, which in turn risks the OMI healing process. More recently, some authors, such as Dr Sebastian Baumgaertel, have sought to define the soft tissue boundaries better and proposed that the mucogingival junction (MGJ) should also be considered as a ‘safe zone’.16 This is important when there is a limited height of attached gingiva available at the insertion site (Figure 2a), or the OMI needs to be sited at a relatively apical level (Figure 2b). However, in practical terms, it is important to ensure that the adjacent loose mucosa is pulled clear of the MGJ in order to avoid it wrapping around the OMI threads during insertion.
For many orthodontists, the principal issue concerning interproximal anatomy is whether the OMI is likely to contact an adjacent dental root and cause iatrogenic damage. Given the anxiety that this risk has aroused, multiple research studies have been undertaken in recent years, primarily in animal models but also in human subjects, where the principal aim has been to traumatize tooth roots and then observe the healing response. The methodologies have also featured both acute trauma, such as the direct drilling of pilot drills and self-drilling OMIs into dental roots,17 and the movement of teeth into contact with adjacent OMIs. Fortunately, the consistent observation in these various studies has been that the cementum repairs after the trauma, with this process being evident histologically within four weeks.
This leaves the question of whether root proximity (as distinct from actual root contact) affects OMI success? This association was first suggested by a retrospective qualitative study by Kuroda et al, which principally utilized two-dimensional imaging.18 Subsequent studies have investigated three-dimensional (3D) root proximity in relation to OMI success rates, using cone beam computerized tomography (CBCT).19,20,21,22,23,24 In particular, Jung et al19 performed a CBCT study of 228 OMIs inserted in the (clinically typical) buccal site mesial to the maxillary first molar tooth. They found a statistically significant link between root proximity and success, and indicated that a minimum of 0.5 mm OMI-root separation is required. This recent study supports the findings of Min et al, who reported that a 0.1 mm increase in OMI-root distance resulted in a 70-fold increase in success.20 The likely explanation for this phenomenon is that a reduced thickness of peri-implant bone results in limited depth for buffering of the unfavourable ‘jiggling’ forces from the periodontal ligament to the adjacent OMI and favourable bone remodelling (secondary stability).
Hence, a consensus has emerged that there is both a clinically and statistically significant link between root proximity and success, such that the closer the OMI is to a root, the higher the chances of failure. This is especially the case for mandibular inssertion sites where success rates appear to be most affected by close root proximity.18,24 Furthermore, root proximity appears to be a more important factor than cortical bone properties (which will be discussed in the following section).20 On the other hand, a minority of successful OMIs show signs of close root proximity, yet are still successfully loaded. For example, two recent studies both reported high (eg 95%) success rates, despite the observation of root contact for 17-20% of their self-drilled and pre-drilled OMIs.21,22 One of these studies also demonstrated a strong tendency for freehand (non-stent guided) OMI insertions to be angled towards the molar tooth root (on the distal side of the insertion site).21 Consequently, in clinical practice, a good ‘rule of thumb’ is to consider re-insertion, especially in mandibular sites, if a patient reports pain during insertion and the adjacent tooth has an altered percussion sound and/or radiographic overlap of the OMI and root. However, whilst best avoided, it's reassuring that OMI-root contact does not inevitably result in OMI failure, and that the dental tissues heal without clinically detectable sequelae. In the case of self-drilling OMIs, this may be because their sharp tip tends to blunt easily rather than penetrate the root surface. Finally, clinical techniques often now involve divergence of the adjacent roots in order to increase the interproximal space prior to OMI insertion. I now thoroughly recommend this simple yet effective clinical step as a prospective means of reducing root proximity risks (Figure 3), and also possibly to reduce the pressure pain experienced by patients immediately following insertion.
The main focus of research at a microscopic level has been to determine how stability is affected by the quantity and quality of the peri-implant bone. Finite element analysis (FEA) has been used in multiple studies to examine the OMI-bone interaction and consistently demonstrates that most of the OMI loading ‘pressure’ is concentrated on the cortical bone layer immediately adjacent to the OMI threads.6,25 This issue has been explored in further detail with both in vitro and in vivo research techniques involving animal, artificial and human bone material, and clinical observations. These studies have been focused on the relative contribution of the peri-implant cortical and trabecular (cancellous) bone layers, and especially on the influence of the cortex properties (its thickness and density). As with FEA research, artificial bone studies have the benefit of enabling standardization of the cortical bone properties, whereas animal and clinical studies tend to provide information on prolonged/secondary OMI stability. Therefore, a range of seminal research approaches and findings will be summarized here to help elucidate these details.
OMI stability changes over time and is classified in terms of primary and secondary stages. The former occurs at the time of insertion, whilst the latter is a longer term attribute which develops during the first three months after insertion as a result of bone remodelling. This was recently demonstrated in a clinical study by Nienkemper et al where the stability of midpalatal OMIs was measured by resonance frequency analysis.26 These values decreased between two and four weeks following insertion, then increased and plateaued by six weeks. Primary stability is typically gauged, both experimentally and clinically, by measurement of the maximum insertion torque (MIT). In essence, a high torque value (rotational resistance to screw insertion) recorded during the final stage of OMI insertion indicates that the primary stability is likely to be good, whereas low torque indicates poor mechanical retention and an increased risk of failure. However, it's not quite that simple since excessive torque can also have a negative effect by reducing secondary stability (due to bone microfractures and pressure necrosis). Torque is measured in Newton centimetres (Ncm) and several studies have indicated that the ideal clinical torque range for OMI success is 5-15 Ncm.27,28 This may be difficult to measure in the clinical situation using the motorized screwdriver and torque measuring devices currently available, but one study of torque delivery using a manual screwdriver technique indicated that this torque range is unlikely to be exceeded with digital rotations alone.29 However, this measurement of the rotational forces applied, by operators to the screwdriver, indicated that forearm/wrist rotation should be minimized since this is likely to raise the applied force above the ideal torque range. So what bone factors influence this insertion torque?
While it appears to be the main source of skeletal anchorage, multiple anatomical studies have indicated that the cortical layer of alveolar bone is relatively thin, ie 1-2 mm.6 This has been substantiated by a recent human study using micro-computerized tomography (CT) to measure the cortical layer with a high degree of accuracy.30 It showed that the buccal alveolar cortex in both maxillary and anterior mandibular insertion sites was often less than 1 mm in thickness. The corresponding palatal alveolar and posterior mandible cortical depths were 1.3 mm and 2 mm, respectively. So, if the cortex is often thin in some sites (eg the maxilla), is there a maximum limit of cortical depth? Interestingly, a recent FEA study of cortical thickness has indicated that values greater than 2.0 mm are not beneficial since thicker cortical bone appears unable either to absorb additional orthodontic force or cause a decrease in the peak peri-implant bone stress. This is because the loading stress appears to be concentrated in a small fulcrum (leverage) zone within the cortex, not throughout the layer.31 Hence 1-2 mm depth of cortex appears to be the ideal for primary stability, although this could only be evaluated in individual circumstances using CBCT. Fortunately, the abovementioned micro CT study also demonstrated that the effective depth of cortical engagement can be increased by oblique insertion of the OMI.30 This will be discussed further in the technique section below.
At first glance, the emphasis placed on the cortical layer may suggest that cancellous bone provides little if any primary stability benefit, except perhaps where the cortex is thin (less than 1 mm) or virtually absent, as indicated by a recent canine study.32 However, a micro CT study of OMIs in a canine model showed that OMI loading resulted in a 3% loss of bone volume in the cortical layer (compared to controls), yet a volume increase in the cancellous bone.33 The authors postulated that this was due to a combination of insertion damage (microfractures and compression necrosis) and excessive loading force exceeding the cortex's physiological limit. At the same time, bone formation higher on the compression side of the OMIs suggests that both the cortical and cancellous bone tissues respond and contribute to OMI secondary stability.
There is a consensus in the literature that increases in cortical thickness result in higher insertion torque values.6 Furthermore, an artificial bone study indicated that torque is more influenced by quantitative than qualitative cortical changes.34 In other words, it is the cortical thickness not density which has most influence. However, changes in density still show a reasonable correlation with primary stability, as demonstrated by Marquezan et al, where the mineral density of a 1 mm thick layer of animal cortical bone had a 0.52 and 0.71 correlation with insertion and removal torques, respectively.35 A similar correlation effect was not demonstrated for variations in cancellous bone density. In practical terms, sites such as the posterior mandible in adults have high values for both cortical depth and density, resulting in the risk of excessive insertion torque and the need for a pre-drilling step.
Most commercially available OMIs are manufactured from grade 5 machined (smooth) titanium alloy. Given the established safety record of titanium implants and fixation screws, it was naturally assumed that this alloy is a good choice for orthodontic mini-implants. This has now been confirmed by a cytotoxicity study where this titanium alloy was shown to support rapid cell proliferation, good cytocompatibility and cell adhesion.36 Research has also shown that stainless steel is an acceptable material. For example, a recent animal study found no significant differences between stainless steel and titanium alloy OMIs in terms of insertion and removal torque, and also in the histological extent of bone-implant contact (after the human equivalent time of 18 weeks).37
OMIs are more frequently differentiated according to their mode of insertion (self-tapping versus self-drilling), endosseous shape (cylindrical versus tapered body) and dimensions (diameter and length). Several of these properties may coincide within the same body design, eg tapered OMIs often have a pronounced thread depth and are inserted with a self-drilling technique. Initial studies comparing pre-drilled (self-tapping) cylindrical and self-drilling tapered OMIs indicated that the latter group had higher insertion torque values and primary stability.6 What about in the longer term? A recent prospective clinical study has provided the first evidence on this issue. Yoo et al analysed 227 tapered and cylindrical self-drilling OMIs after a mean loading period of 15 months.38 They found that the mean insertion torque was 2 Ncm higher for the tapered OMIs, but that there was no statistically significant difference between tapered and cylindrical designs in terms of long-term success rates and removal torque. This finding appears to contradict earlier publications, and it may be that primary stability was more affected by root proximity problems than any design differences. In addition, the tapered OMIs may not have been as deeply inserted as the cylindrical ones (resulting in less insertion torque). Furthermore, it is arguable that such design-related differences in technique may only significantly affect OMI insertions where the primary stability is borderline (eg in thin cortical bone sites), and not where the bone support is sufficient to overcome design limitations.
Most of the initial research on OMI dimensions focused on body (endosseous) diameter and length effects such that diameter appears to influence primary stability more than length.6 This is because diameter and length changes tend primarily to affect the cortical and cancellous bone, respectively. More recent research studies have also included neck dimensions in their analyses. For example, three FEA studies have highlighted that excessive head and neck length (emergence profile) are more unfavourable features, in terms of adjacent bone stress, than a short body length.39,40,41 It is helpful to consider this both when selecting an OMI head design and also during the insertion stage, when the OMI should be fully inserted to minimize the distance from the force application to bone surface (and resultant lever effect). However, remember that FEA methods examine the OMI interaction only at the time of insertion. Hence, further research is required on the effects of body length on secondary stability (over time) since longer body length OMIs tend to have higher success rates, especially in thin cortical bone sites.6
There is a general consensus in implantology literature that 700 revolutions per minute (rpm) is the maximum speed for pilot drilling. However, the optimum speed for self-drilling OMI insertion has not yet been defined. The best currently available evidence on this issue comes from a study by Matsuoka et al which measured temperature increases in the peri-implant cortex during the insertion process.42 Since it was shown that, in the absence of irrigation, an insertion speed of 250 revolutions per minute caused over ten degrees of temperature rise, the authors recommended that the clinical insertion speed should be limited to 150 rpm. This is considerably faster than the typical manual insertion speed, but is relevant to contra-angle (motorized) handpiece use.
Aside from excessive heat generation, the peri-implant bone may also be negatively affected by excessive insertion pressure and the consensus is that a pre-drilling step should be utilized in areas with thick cortical bone, such as mandibular molar sites. However, clinical experience had indicated that the additional clinical step of pre-drilling is not advantageous for maxillary buccal alveolar sites. This has now been confirmed in a recent prospective clinical study of 140 OMIs where identical self-drilling OMIs were inserted in the maxilla either with or without a pilot drilling step.43 There was a high overall success rate (96%) irrespective of pre-drilling or no drilling. Notably, the mean maximum insertion torque was relatively low (7 Ncm) and it is therefore likely that pre-drilling effects would be different for mandibular sites. This site difference has been observed in a canine study where pre-drilling reduced the occurrence of bone damage in the posterior mandible (with a mean cortical thickness of 2.2 mm) but not the maxilla (0.9 mm mean thickness).44 This is provided that the drill hole (for cortical perforation) has a sufficient diameter to prevent excessive torque and hence cortical micro-fractures.45
There continues to be some debate over the optimum angle (inclination) for OMI insertion, despite artificial and animal bone block studies indicating that a 20-30° apically directed inclination (Figure 4) is favourable for primary stability.6 The evidence in favour of this relatively subtle angle of oblique insertion has recently been confirmed in a canine study, where 20-40° insertion angles (to the bone surface) were more favourable for stability than either perpendicular or more oblique angles.46 This is probably because such oblique insertion angles maximize the BIC within the thin cortical layer.
Over the years, most orthodontists have used immediate loading, although often erring on the use of a low force, eg 50 g within the first six weeks.6 The author uses a lightly stretched powerchain for low force application and also because it suffers force decay after several weeks. In theory, this is helpful since this decay time corresponds to the transition stage between primary and secondary stability when the bone support is lowest.26 A recent research study has supported this clinical practice since it demonstrated that 50 g immediate loading of self-drilling OMIs resulted in the histological observation of enhanced BIC and bone remodelling.47 Furthermore, continuous normal orthodontic loading has been shown to result in a gradual increase in the trabecular (cancellous) bone's hardness on the compression side of the OMI.48
Studies have demonstrated that OMIs have a high stability (success) rate, especially when used in a standardized evidence-based manner, and with the benefit of clinical training and experience. Research studies have also now identified a number of patient, anatomical, design and technique parameters that influence this success. In particular, root proximity and insufficient cortical thickness are clear risk factors. Orthodontists can utilize this information in order to maximize the chances of OMI stability in individual cases by appropriate anatomical site and OMI selection, and especially by avoiding close root proximity. The latter involves a simple technique step, as discussed in this paper.
