The Orthodontic Mini-implant Clinical Handbook. Richard Cousley
and tip."/>
Figure 2.2 A panoramic radiograph which illustrates the typical variations in interproximal spaces, as affected by dental crowding, root length, morphology and tip. For example, the left mandibular first premolar's distal tip has resulted in more than average space between it and the second premolar, but less between it and the canine root.
2.3 Soft Tissue and Oral Hygiene
Clinical experience indicates that there are distinct differences in soft tissue thickness according to anatomical location. This has been borne out in a ultrasound study of soft tissue thickness at mini‐implant insertion sites, where the midpalate mucosa was the thinnest (mean 0.8 mm), followed by buccal sites (mean 1.3 mm) and palatal alveolar sites (mean 3.1 mm) [33]. In practical terms, these findings support the use of short‐neck mini‐implants in buccal sites and long‐neck versions in palatal alveolar sites. In theory, short‐neck mini‐implants could be used in the midpalate, but longer neck versions may be helpful here to provide a means of attachment by elevating the head position. However, there is no evidence relating attached tissue thickness and mini‐implant success rates per se.
Figure 2.3 Photographs showing typical peri‐implant soft tissue inflammation at (a) time of insertion in the right mandibular area and (b) after seven months of elastomeric traction. General problems with oral hygiene and resultant gingival hyperplasia are also evident.
What is known is that poor oral hygiene and peri‐implant soft tissue inflammation (Figure 2.3) are risk factors for secondary failure [34–39]. Since these problems are more likely to occur 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. The zone of mucosa which lies immediately around (coronal or just apical to) the mucogingival junction (MGJ) has been popularised by Sebastian Baumgaertel as a potential site, but there are no published data on success rates in this specific area [40,41]. It is likely that the key ‘take home’ message is that insertions should be sited in attached gingiva sites, with the option of ‘straying’ into the loose mucosa adjacent to the MGJ, provided that the sulcular tissues are stretched taut. This avoids interference during the insertion stage, by preventing the loose mucosa from wrapping around the mini‐implant threads.
It is almost always recommended that mini‐implants are inserted through attached mucosa.
2.4 Maxillomandibular Planes Angle
Patients with a dolichofacial (long face) pattern and a high maxillomandibular planes angle (MMPA) (Figure 2.4) have an increased risk of failure for maxillary buccal mini‐implants. This is an anatomical factor, due to their relatively thin maxillary buccal cortical plates (compared with ‘short’ face, brachyfacial, patients) [34,38,42–45]. However, these dolichofacial patients also typically present with an anterior openbite which may benefit from maxillary molar intrusion. This could affect mini‐implant stability in those patients who need mini‐implant intrusion for correction of an anterior openbite, and this factor will be discussed further in Chapter 10. Fortunately, this problem of poor stability of maxillary buccal mini‐implants may be avoided by mini‐implant insertion in palatal alveolar sites [11,46].
Figure 2.4 Lateral cephalograms of mini‐implant anchorage patients exhibiting (a) long and (b) short face patterns. However, note that these images cannot illustrate the cortical bone characteristics.
2.5 Age
While primary stability is readily achieved in adults, adolescent patients have a significantly higher mini‐implant failure rate in alveolar sites [47]. This is due to their reduced levels of cortical thickness and density [5,48,49] and higher rates of bone remodelling, which may compromise a mini‐implant in terms of both primary and secondary stability. In effect, immature cortical bone is less able to withstand force applications, especially during the first month after insertion. The risk of secondary failure, occurring during the healing phase when bone resorption processes predominate, appears to be much more critical in adolescents than adults because they have lower primary stability to begin with. It then only takes a small drop in bone support to reduce the total mini‐implant stability below the threshold for success. Therefore, whilst mini‐implants are still successful in adolescents, it is advisable to be cautious and use light loading forces (e.g. 50 g) for the first six weeks after insertion.
Alternatively, the midpalate may be a preferable insertion site in young patients, even if this means accepting the limitations of indirect anchorage, because of its high reported success rates in children [24].
Whilst mini‐implants are still successful in adolescents, it is advisable to be cautious and keep the loading force low (e.g. 50 g) for the initial six weeks.
2.6 Cigarette Smoking
Heavy tobacco consumption is associated with a significantly higher failure rate [50]. Therefore, whilst cigarette smoking is not an absolute contraindication to mini‐implant usage, smokers should be warned of the risk and advised to stop before mini‐implant insertion.
2.7 Body Mass Index
Ironically, as cigarette smoking appears to be on the decline, we are treating more patients at the upper and lower ends of the body mass spectrum. Is this relevant? Well, my clinical observation has been that adolescent patients with a low Body Mass Index (BMI) have higher failure rates. This has recently been evidenced by a CT study of maxillary cortical bone thickness and density in relation to BMI and chronological age [49]. There was a statistically significant increase in both bone parameters with an increase in both BMI and patient age. Therefore, it is reasonable to assume that young patients with a low BMI will have higher failure rates for mini‐implants. This may be partly negated by the use of midpalate sites in adolescents, assuming that indirect palate anchorage is feasible,
References
1 1 Lemieux, G., Hart, A., Cheretakis, C. et al. (2011). Computed tomographic characterization of mini‐implant placement pattern and maximum anchorage force in human cadavers. Am. J. Orthod. Dentofac. Orthop. 140: 356–365.
2 2 Laursen, M.G., Melsen, B., and Cattaneo, P.M. (2013). An evaluation of insertion sites for mini‐implants. A micro‐CT study of human autopsy material. Angle Orthod. 83: 222–229.
3 3 Baumgaertel, S. and Hans, M.G. (2009). Buccal cortical bone thickness for mini‐implant placement. Am. J. Orthod. Dentofac. Orthop. 136: 230–235.
4 4 Deguchi, T., Nasu, M., Murakami, K. et al. (2006). Quantitative evaluation of cortical bone thickness with computed tomographic scanning for orthodontic implants. Am. J. Orthod. Dentofac. Orthop. 129: 721.e7–e12.
5 5 Farnsworth, D., Rossouw, P.E., Ceen, F., and Buschang,