Unfortunately, the use of TADs still carries a certain risk of failure distinctly varying in individuals [6, 20, 21]. Patient’s characteristics and local bone quality are often listed as the critical factors [6, 21]. Therefore, TADs stability in the mandible or in the maxilla may differ substantially. Kuroda et al. [20] and Tseng et al. [21] reported that skeletal anchorage remains stable more often in the maxilla than in the mandible. Park et al. [11] and Chen et al. [22] came to similar conclusions placing the TADs distally in the alveolar part of the mandible. Meta-analysis carried out by Hong et al. [14] also confirmed that stability of the TADs placed in the mandible is 2.23 times lower than in the maxilla, which is a statistically significant difference. However, the multiple data demonstrating that achieving stable TAD position in the posterior part of the mandible vary substantially: from 66.7 to 92.8% of cases [1213152021], fully justifying the aim of our study.
Miyawaki et al. [23] tested mini-implants of different sizes in the mandible. The authors showed that the TADs stability improved as the diameter of the screws got larger. Chang et al. [13], quoted in the introduction of this paper, also achieved very high success rate, namely: 92.8% stability of 1680 stainless steel (2 mm ⨯ 12 mm) miniscrews inserted in the mandibular buccal shelf, parallel and distal to the lower first and second molar roots. On the contrary, in the study by Manni et al. [24] the smaller miniscrews (1.3 mm ⨯ 11 mm) showed significantly higher success rate than the larger ones (1.5 mm ⨯ 9 mm and 1.5 mm ⨯ 11 mm). However, the previous mini-implants were placed in both jaws, mainly in the anterior area (intra-alveolar approach), where the cortical bone is thin and the distance between the adjacent teeth is relatively short. Such anatomy somehow forces the use of small diameter screws and immediately favors rejection of the larger ones.
Despite smaller mini-implants are easier to insert between the roots, minor reduction in their size declines the torsional strength significantly and can increase the risk of implant fracture. Therefore, it is advisable to avoid miniscrews smaller than 1.2 or 1.3 mm in diameter when placing into the thick mandibular cortical bone, where miniscrew fracture is more likely to occur [6, 17, 20,21,22,23,24,25]. A meta-analysis performed by Hong et al. [14] also showed that increasing the TAD diameter above 1.4 mm gives a 1.61 times greater chance of stability. It is in accordance with our study, where 91.3% of the SH2018-10 screws were stable compared to 75% of the SH1514-08 screws. It should be emphasized that despite a larger TADs diameter we did not violate biomechanics thanks to appropriate miniscrew location (extra-alveolar approach), that enabled bodily tooth-movement of the mandibular dentition.
Oral mucosa inflammation can result in progressive damage of the cortical bone surrounding the implant’s neck, which endangers its stability [26]. Studies demonstrated that incidence of inflammation statistically contributes to mini-implant loss [27], which is partially in accordance with our study, since it concerned only smaller miniscrews. SH1514-08 screws were nearly 8-times more likely to fail due to inflammation compared with the SH2018-10 ones. Failure of the larger SH2018-10 screws due to inflammation was of no statistical significance (Table 4), nonetheless they caused inflammation in half the cases. Despite the inflammation, the larger TADs were less likely to fail, probably due to their higher bone-miniscrew contact ratio and better mechanical interlocking compared to SH1541-08 ones. In our study, we utilized the optimal position for the TADs according to Chang [13]: lateral to the first and second molar interproximal area, approximately 5 mm from the alveolar crest, and insertion at an angulation of about 30° to the bone surface. According to CBCT measurements of mandibular buccal shelf in Class III patients at a 30° angle for sites 3–7 mm from the alveolar crest, angulating the TAD in comparison to perpendicular approach consistently increased bone contact from 0.56 to 1.23 mm, which was a ~ 25–30% increase at all sites. This was an important consideration, since even a 0.5 mm difference in cortical bone thickness (bone-miniscrew contact ratio) can affect the success rate. The median for inclined cortical bone thickness at the recommended sites ranged from 3.54 to 4.05 mm, which was more than sufficient for primary stability, particularly valuable for the entire arch distalization, which itself requires a stable anchorage [13].
It is known from the literature that irritation around TADs placed in the posterior part of the mandible can be triggered by chewing [11], therefore the attached gingiva is recommended for TADs location in order to avoid interference with the functional movements of the soft tissues and—subsequently—their inflammation [28]. However, larger TADs cannot be inserted into the inter-radicular spaces. For this reason the other option proposed by Chang et al. [13], namely placing miniscrews in elevated position with the screw head at least 5 mm above the soft tissue level, is promising in terms of preventing peri-screw inflammation. Nevertheless, it does not exempt from providing instruction of oral hygiene and monitoring the condition of soft tissues at each appointment in order to reduce the risk of inflammation. Since the stability of the smaller miniscrews was significantly impaired by inflammation, even minor precautions should be taken into consideration. In our study, all patients were instructed to use chlorhexidine gel for two weeks after TADs insertion due to its antibacterial properties, that minimize risk of tissue inflammation, and its ability to slow down epithelialization, reducing the likelihood of soft-tissue overgrowth [29]. Regarding details, better hygiene is often achieved on the left side in right-handed patients, who constitute most of the population [30]. Park et al. [11] stated that TADs placed on the left side exhibited higher success rates than those placed on the right side. However, in our study no statistical difference was found between both sides.
Nevertheless, even when perfect hygiene is maintained and implantation properly performed, small fraction of patients may have a genetic predisposition to TADs failure, especially when they fail bilaterally [13]. Andrucioli et al. [31] evaluated the gene expression of proinflammatory cytokines and osteoclastogenesis mediators in peri-miniscrew gingival tissue samples using real-time polymerase chain reaction, to verify if gingival inflammation and bone resorption could be associated with implant failure. They concluded that the higher IL-6 expression could be associated with miniscrew failure, since a prolonged excessive release of IL-6 was translated into persistent oral inflammation and tissue destruction via proteases, osteoclasts and methylation changes. There are several polymorphisms in the promoter region of IL-6, and among them the IL-6 174 GG genotype plays as a risk factor of chronic periodontitis in Brazilian and Caucasian population [32]. Patients genetically predisposed to periodontitis may have the higher risk of miniscrew failure. Excessive and sustained production of IL-6 is also associated with a variety of inflammatory diseases like rheumatoid arthritis, Castleman disease, systemic-onset juvenile idiopathic arthritis or cytokine releasing syndrome [33]. Potential genetic complications constitute crucial considerations for informed consent since in case of TADs failure alternate treatment methods may be desirable: extractions, headgear or orthognathic surgery.
Pain lasting longer than 48 h is an unfavorable phenomenon, which was also observed by Miyawaki et al. [23] and Kuroda et al. [20]. In the latter study, Kuroda et al. [20] used two types of miniscrews and one type of miniplate. They observed that over 60% of the patients with the larger screws reported pain in the third day after the implantation. This result is similar to ours: smaller miniscrews were significantly better tolerated than the larger ones. Kuroda et al. [20] believed that the muco-periosteal flap reflection was the pain-causing factor. Our results seem to decline such concept: the patients experienced pain regardless our flap-less protocol. Furthermore, despite the reports that the level of prolonged pain after TAD insertion is comparable with the one associated with tooth extraction [34], or even with discomfort related to the initial tooth alignment [35], the issue of postoperative pain threshold is very individual and, thus can’t be disregarded.
In our study, the Kaplan-Meier survival analysis demonstrated that the SH1514-08 miniscrews failed significantly sooner compared to the SH2018-10 ones (p = .002). In the study by Wiechmann et al. [36], majority of the miniscrews with two different diameters (1.1 and 1.6 mm), placed buccally in the mandible failed within 50 days after placement, which is in accordance with the findings of Garfinkle et al. [37], who used 1.6 × 6 mm miniscrews. On the other hand, Chang et al. [13], who evaluated larger 2 × 12 mm miniscrews, reported average failure time of 3.3 months, which was similar to 3.4 months reported by Park et al. [11], evaluating smaller mini-implants with a diameter of 1.2 mm, 5–10 mm long. In our study majority of the miniscrew failures occurred within one to three months from their insertion, after which time success levels remained constant throughout the rest of the treatment. The survival curve was more steep and erratic for smaller TADs than for the larger ones. This observation lends credence to the theories suggesting that by increasing the TAD size one can achieve improved stability.
