# Fracture and Deflection of Orthodontic Miniscrews--A Systematic Review
## 4 Discussion  
Many terms have been used for orthodontic temporary anchorage devices in the included studies. Although 'temporary anchorage device' (TAD) is a broad term that could also cover appliances other than screws, such as mini plates, the present study opts for the use of the term'miniscrew' (MS) as it properly describes the proprieties of the auxiliaries concerned. The main factors influencing MS plastic deformation and fracture include specific material characteristics, mini-implant design (shape), and miniscrew length and diameter. The two main metal alloys used for the production of MSs are stainless steel (SS) and medical titanium alloy (Ti-6 Al-4V) [2]. Considering the properties of the compared miniscrew materials, it is not possible to unequivocally state the mechanical superiority of a particular alloy [16]. Taking into account the higher torsional and flexural strength of stainless steel MSs, it could be an indication for clinical application in more demanding sites such as the mandibular buccal shelf and infrazygomatic crest or when orthodontic screws are used to anchor high orthopedic forces as in miniscrew-assisted rapid palatal expansion (MARPE) [27]. However, currently, the majority of temporary anchorage devices are manufactured from either commercially pure titanium or grade V titanium alloy (Ti-6 Al-4V) [16]. The latter is favored due to its mechanical resistance surpassing pure titanium [28]. The reason behind the widespread use of titanium alloy and its being the preferred choice over stainless steel arises from the exceptional biocompatibility of this material. It has been proven that grade V titanium alloy is neither cytotoxic nor genotoxic [29], while featuring high strength, corrosion resistance, and low weight. Still, in consideration of MS fracture and deflection, the micro and macro designs of MSs are factors of higher importance. The most extensively researched variable with a well-proven impact on MS mechanical strength is screw diameter. All reviewed studies remain consistent in results: fracture torque value increases with increasing MS diameter [1; 3; 4; 9; 11; 22; 26; 27]. This relation might lead to the conclusion that MSs of 2 mm diameter should be predominantly recommended for clinical use. However, the application of such considerable-diameter MSs may not be possible in many cases due to insertion site requirements. Most commonly, MSs are inserted into the inter-radicular space. The often-limited inter-radicular site creates an additional risk of root contact during insertion for thicker MSs. As such, attempts to avoid periodontal (root) contact by changing the insertion angle may hinder the stability of the MSs [7]. In some cases, the extra-radicular areas can be recommended as an alternative location for MSs, such as the buccal shelf in the mandible or the infrazygomatic crest in the maxilla [9; 27]. As for the MS length, the studies did not confirm its influence on fracture or flexural strength [21]. Thus, the choice of longer MSs should be a matter of insertion site requirements rather than an aim to increase the mechanical strength of the implant, especially since length's overestimation, clinically resulting in the partial insertion of MSs, was proven to increase fracture risk upon MS loading [6]. The mechanical properties of the mini-implant are also attributed to the screw's shape. Higher values of fracture torque, and thus better mechanical resistance, can be attained \begin{table}
\begin{tabular}{c c} \hline
**Author and Year** & **[PERSON], 2016 [20]** \\ \hline Selection bias & Not present. \\ \hline Performance bias & Not present. \\ \hline Detection bias & There was no sample size calculation or blinding. \\ \hline Attrition bias & There was no description of who performed the measurements and whether they were repeated. \\ \hline Reporting bias & Not present. \\ \hline Other biases & Not present. \\ \hline \end{tabular}
\end{table} 
Table 3: The risk of bias in the animal study. by cylindrical or mixed designs as compared to tapered screws [5; 6].