Three-dimensional mapping of cortical bone thickness in subjects with different vertical facial dimensions
This study compared cortical bone thickness among subjects with different vertical facial dimensions in the entire tooth-bearing region of both jaws, using CBCT. This aimed to provide reference data for clinicians that will aid in mini-implant placement in subjects with varying facial types.
Cortical bone thickness is the key determinant of initial stability of mini-implants, and thin cortical bone was reported to increase the risk of mini-implants failure [19]. On the other hand, areas with thick cortical bone can increase the risk of mini-implant breakage and bone micro-fractures. This can pose two important questions: Do subjects with different vertical facial dimensions have different cortical bone thickness? And if yes, what are the clinical implications of such differences on mini-implant stability?
Masumoto et al. through measurements of cortical bone thickness on 31 dry skulls of modern Japanese males found that cortical bone thickness of the mandibular first and second molar sections was thicker in short-faced subjects than in average- and long-faced subjects [5]. Swasty et al. investigated differences in CBT in patients with different vertical facial dimensions using CBCT [20]. It was reported that the long-faced group had thinner cortical bone in almost all sites. Unfortunately, this study measured CBT at one third and two thirds the distance from the base of the mandible to the alveolar crest, rather than using a standard site for the measurements.
Similar to our study, Ozdemir et al. measured CBT at 4 mm from the alveolar crest, which appears to correspond to the attached gingiva [17]. This was reported to be a favorable area for mini-implant placement, considering the lower probability of inflammation. They found a close relation between facial type and cortical bone thickness, at the inter-dental sites from the distal aspect of the canine to the mesial aspect of the second molar. Cortical bone thickness in the low-angle group was significantly higher than in the high-angle group in all four measured sites.
In our study, significant group differences were detected with high-angle subjects having significantly narrower inter-radicular CBT at some sites as compared to average- and low-angle subjects. These were in the posterior region of the maxilla and mandible on the buccal side and palatally in the maxilla mesial and distal to the lateral incisor, at the vertical height in which mini-implants are commonly inserted for orthodontic anchorage.
Horner et al. reported that the cortical bone was 0.08 to 0.64 mm thicker in the hypodivergent than in the hyperdivergent subjects [16]. This is similar to the findings of our study in which the cortical bone was 0.16 to 0.62 mm thicker in the hypodivergent than in the hyperdivergent subjects.
Furthermore, the results of this study may be correlated with the findings of previous studies [20, 21], in which a significant relationship was found between facial type and alveolar thickness. High-angle subjects were found to have thinner alveolus at almost all sites in the mandible. This could be associated with the finding that these are the same subjects that showed a thinner cortical plate in the posterior region of the mandible when compared to normal- or low-angle cases.
Such differences in CBT among subjects with different vertical facial dimensions can have significant clinical implications. An association between higher risk of failure of mini-implants and subjects with high mandibular plane angle has been previously reported [15]. On the other hand, Kuroda et al. insisted that there was no correlation between the success rate of mini-implants and the mandibular plane angle [22]. From the results of our study, it appears that although there is a correlation between the vertical facial dimensions and inter-radicular cortical bone thickness at the vertical height in which mini-implants are commonly inserted, this is evident in only few sites: primarily located in the posterior region of the maxilla and mandible on the buccal side and palatally in the anterior region of the maxilla. More studies are thus needed to determine the exact relationship between the vertical skeletal pattern and the success rate of mini-implants.
Another clinically related important fact is that high-angle subjects when compared to the other two groups tended to have more sites with cortical bone thickness less than 1 mm, which according to Motoyoshi et al. can increase the risk of failure of mini-implants placed at these sites [9].
This should merit our attention to take precautions to increase the success rate of mini-implants placed in high-angle patients, especially if they are an important part of our treatment plan. This can include to monitor and emphasize oral hygiene more strictly or to use partially osseointegrated mini-implants that may offer higher stability. Another option would be to use mini-plates which were reported to be associated with a lower failure rate than mini-implants [23]. Furthermore, extensive research is needed exploring different ways to promote mini-implant stability as they are being used more and more in our practice.
Perhaps most importantly, the results give clinicians reference data for measurements of cortical bone thickness for subjects with different vertical skeletal patterns. However, several factors must be taken into consideration when evaluating the results. First, this study did not investigate the difference between male and female subjects. However, in a recent study by Farnsworth et al., no sex differences in cortical thickness in either the maxilla or the mandible were found between males and females [10]. Since maximum bite force is not a regular or habitual function, like mastication, for example, it might not be expected to produce sex differences in cortical thickness. Second, inherent limitations of CBCT imaging should also be considered [24]. Partial volume averaging can influence the spatial resolution. Thin bone is especially susceptible to partial volume averaging. Third, the differences in the density of cortical bone were not evaluated in this study. In future research, we should also evaluate bone mineral density of cortical bone. As well as bone quantity (bone thickness), bone quality (mineralization) can affect initial stability values for orthodontic mini-implants [6].