Anterior orthodontic mini-implants have been well validated for the effective correction of deep bite.5 However, due to limited inter-radicular space in mandibular anterior region, especially among patients with anterior crowding, the clinical application of mini-implants for the intrusion of mandibular incisors was impeded.3 Fortunately, it is suggested that extra-alveolar mini-implants that are inserted outside dental roots may expand the clinical applications of mini-implants.8 Mandibular symphysis is a complex articulation formed by the fusion of left and right halves of mandible.9 As fusion progresses, mandibular symphysis grows anteriorly and laterally, resulting in an adequate bony projection anterior to incisor roots.10 This renders mandibular symphysis an excellent candidate for extra-alveolar orthodontic mini-implants placement that could be used for the intrusion of mandibular anterior teeth.
Our recent study (not yet published) found that mandible is more susceptible to asymmetry as compared to cranial bone and maxillae. Thus, we tested whether bone thickness was similar between the left and the right sides. Our results revealed that both OBT and CBT did not differ between two sides. This suggests that the insertion techniques are similar for left and right sides, yet practitioners’ laterality should be considered.
It has been well documented that the thickness of alveolar bone was significantly influenced by different facial types11, 12. Consistently, we found that both OBT and CBT were significantly thicker in the low-angle cases than in average- and high-angle cases. This finding is in line with Hoang et al.13 but disagrees with Sadek et al.12, which could be attributed to different insertion heights were measured. Specifically, Hoang et al. measured the OBT at the apical level while Sadek et al. measured CBT at the height of 4 mm and 7 mm that were not apical enough. Thus, we suggest that OBT and CBT differed among different facial types only at the apical level.
Sex and age factors play important roles in the development of bone thickness and a previous study revealed that bone thickness was influenced by both sex and age factors.4 However, we found that both OBT and CBT were similar between males and females, which could be attributed to different regions that were investigated. Interestingly, although OBT was similar between adults and adolescents, CBT was significantly greater among adults than adolescents, which is consistent with the study by Cassetta et al.14 These findings suggest that overall bone thickness in the mandibular symphysis region may not while cortex may grow among adolescents.
Our results revealed that both OBT and CBT were thickest at location A while thinnest at location D. As is mentioned above, mandibular symphysis is formed by the fusion of two halves of a mandible and grows laterally as fusion progresses.10 This median-to-lateral growth pattern explains why OBT and CBT were thickest at location A while thinnest at location D. Interestingly, we found that the OBT was different among the four locations (A > C > B > D) below 10 mm while similar beyond 12 mm, which could be attributed to the influence of dental roots. Specifically, location A and C were inter-dental areas while dental roots were in location B and D, rendering OBT greater at location A and C than at location B and D. In contrast, dental roots were not in the way of mini-implant insertion beyond 12 mm for all the four sites, thus OBT was similar among the four locations beyond 12 mm. Likewise, similar results were found for CBT. CBT was thinner at root areas (B and D) than at inter-dental areas (A and C). This could be attributed to the fact that alveolar bone was expanded and cortex thinned while teeth erupt, resulting in a thinner or even no cortex around dental roots, which is supported by the phenomenon that alveolar bone defects labial to dental roots were highly prevalent among general population.15, 16
In clinical practice, in case of inadequate bone quality and/or quantity, it is prudent to place orthodontic mini-implants more apically and with certain angulations.17, 18 For example, it is advised to place a mini-implant with an angle of 60–70 degrees to the occlusal plane at the infrazygomatic region to avoid root damage.19 Due to limited inter-radicular space of the mandibular anterior areas, it is very likely that root damage is encountered during the insertion of mini-implants at this area.20 Thus, to avoid potential root damage at this area, inserting mini-implants more apically with angulations is recommended. This could be explained by the following two factors. Firstly, dental roots become smaller and inter-radicular space is larger at a more apical level. Secondly, alveolar bone buccal to dental roots becomes thicker with larger insertion angulation. This is supported by our results that both OBT and CBT increased with an increase in insertion height and insertion angle. Thus, we suggest that orthodontic mini-implants could be placed apically with certain angulation at the mandibular anterior area, in order to avoid root damage.
The stability of orthodontic mini-implants is influenced by several factors21–23. Of particular, insertion depth and cortical thickness are of vital importance24, 25. A great body of evidence reveals that mini-implants are stable with adequate insertion depth and appropriate cortical thickness26, 27. It is well-documented that mini-implants are stable if the insertion depth is greater than 5 mm 28. Interestingly, theoretically speaking, mini-implants could be more stable with greater cortical thickness. However, cortical fracture may happen during the insertion of mini-implant if cortical thickness is greater than 2 mm, rendering a cortical thickness of 1–2 mm to be appropriate for the insertion of mini-implants29. Thus, we mapped the yellow areas with the following requirements: OBT was greater than 5 mm and CBT was 1–2 mm (Fig. 8A). In the yellow area, there is a trade-off relationship between insertion height and insertion angle. Specifically, insertion angle should be larger if the insertion height is lower, e.g., insertion angulation was 45–60 degrees at the height of 4–6 mm while 0–60 degrees at the height of 12 mm. However, soft tissue inflammation is very likely if the insertion is too apical. On the other hand, mini-implant slippage is more likely if the insertion angle is too large. Thus, clinicians should choose not-too-apical insertion site in the yellow area where soft tissue inflammation and slippage of mini-implants are less likely to happen.
As mentioned above, the two halves of mandible that fuse at the median plane form a bony projection, rendering the location A the best insertion site at the mandibular symphysis region. As depicted in Fig. 8A and B, we suggest that the optimal insertion site is 6–10 mm apical to CEJ between the two central incisors with an insertion angulation of 0–60 degrees.