Contributions of Alveolar Bone Density and Habitual Chewing Side to the Unilateral Failure of Orthodontic Mini-Screws: A Cross-Sectional Study  
[PERSON]  
1 Department of Orthodontic Science, Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University (TMDU), 1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8549, Japan;  
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2 Department of Oral Health Promotion, Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University (TMDU), 1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8549, Japan;  
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[PERSON]  
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Footnote 1: email: [PERSON], [PERSON]; [PERSON], [PERSON].  
There have been many reports on the factors affecting mini-screw loss. The factors reported thus far can be broadly categorized into external and internal (i.e., host) factors. The external factors include the screw diameter, length, shape, and some surgical factors such as insertion torque, site, and angle [14]. On the other hand, the internal factors include age, gender, jaw condition [15; 16], surrounding periodontal inflammation, cortical bone thickness, and alveolar bone density [8; 11; 12]. Multiple factors are expected to affect mini-screw loss; however, no study adjusting for the influence of these confounding factors has been reported. Furthermore, it has been reported that the secondary success rate of mini-screw insertion at the same site of its dislodgement showed lower stability rather than primary insertion, and it should be better to choose a different site for reimplantation on the buccal side of the maxillary alveolar bone [17]. Therefore, evaluating and clarifying the risk of host factors after eliminating the influence of confounding factors may contribute to the selection of insertion sites and even more stability, especially the second time.  
The occlusal force effects on cortical bone thickness [18], alveolar bone density [19; 20], and mandibular bone morphology [21] have been extensively studied. Masticatory movements on both sides are not equal [22]. According to [PERSON] et al. [22] and [PERSON] and [PERSON] [23], approximately 70% of people habitually use one side during mastication, which is called the habitual masticatory side. Habitual chewing induces laterality in masticatory functions, including occlusal force, occlusal contacting area, tongue movement, and masticatory efficiency [24; 25; 26]. Habitual chewing side is thought to be controlled centrally [27], but chewing side can be managed consciously as behavior modification therapy and the preferred side can change through improvement in the occlusal condition with orthodontic treatment [28]. However, there are no reports on the relationship between masticatory functions and the stability of orthodontic mini-screws. This study focused on the habitual masticatory side as a parameter that can be objectively evaluated to compare the left-right difference in masticatory function within individuals and investigated its relationship with the stability of orthodontic mini-screws.  
This study aimed to determine whether the intra-individual factors of the alveolar bone density and habitual masticatory side are involved in the mini-screw loss by retrospectively evaluating cases of unilateral mini-screw loss. The null hypothesis was that alveolar bone density and habitual masticatory side were not involved in the mini-screw loss.