# Fracture and Deflection of Orthodontic Miniscrews--A Systematic Review
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& - MSs of a2 mm showed higher bending and  
All studies included were in vitro analyses and the materials used comprised artificial (polyurethane foam) bone blocks or pig bone blocks, whereas one article contained an animal study (MSs were introduced in beagle dogs). The total sample size of miniscrews in all studies included was 2097 MSs of different materials, diameters, and lengths. All studies included covered the deformation of orthodontic MSs and their resistance to deflection or fracture depending on specific material characteristics, mini-implant design (shape, length, diameter), or simulated clinical scenarios (e.g., sterilization or reinsertion after root contact). Twenty-one studies include class V titanium mini-implants in the test sample, whereas three studies also include stainless steel mini-implants for comparison, and one study includes only steel mini-implants. The mechanical strength of miniscrews in relation to the alloy was explored in five reviewed articles. [PERSON] et al. concluded that in general, SS-MSs had higher torsional and flexural strength than TA-MSs [16]. However, in the total sample, variation in torsional and flexural strength was more influenced by the diameter (90.3% and 83.5%) than by the type of metal alloy (2.2% and 3.8%). The authors of the study cited have further suggested that the miniscrew's design is of higher importance than the alloy with regard to fracture resistance. Quite similar conclusions could be drawn from the study by [PERSON] et al. [9]. The comparison of SS-MSs and TiA-MSs conceded the advantage of mechanical properties for SS; however, the study had proven that larger-diameter (2.0 mm) MSs performed better than those of a smaller diameter (1.5 mm) regardless of alloy type. Further consistency with these results can also be found in the study by [PERSON] et al. as MSs of \(\omega\)2 mm showed higher bending and fracture resistance than \(\omega\)1.5 mm diameter MSs, but no significant differences between TiA-MSs and SS-MSs of the same diameters were found [4]. The discordance between studies comparing different alloys regarding fracture resistance might be due to different alloy compositions and treatments during the manufacture. A slightly different approach to the mini-implant alloy was adopted in the studies by [PERSON] et al. and [PERSON] et al. [20; 24]. [PERSON] et al. attempted to modify the properties of a titanium alloy and combine the biocompatibility of commercially pure titanium MSs and the mechanical resistance of the Ti-6 Al-4V mini-implants by nanostructured titanium processing. As a result, nTi mini-implants showed torsion resistance compatible with Ti-6 Al-4 V mini-implants and better than cpTi mini-implants. Similar surface finishing and fracture processes among the three types of mini-implants were found [24]. [PERSON] et al. aimed to improve bone-implant contact by roughening the surface of SS mini-implants with a Nd-YAG laser. The laser surface treatment did increase surface roughness without compromising fracture resistance; however, it did not improve bone-implant contact [20]. Studies by [PERSON] et al. proved a positive correlation between forces required to deflect the miniscrew and its diameter--the highest forces required for bending were reported with 1.7, 1.8, 1.9, and 2.0 mm MSs, whereas the lowest values were reported with 1.6, 1.5, and 1.3 mm MSs [25]. In their study, [PERSON] et al. investigated the influence of MS length on the insertion torque and fracture torque values in various cortical bone thicknesses [21]. As a result, an increase in insertion torque was associated with an increase in bone thickness; however, the length of MSs did not influence fracture strength on insertion, flexural strength, or fracture strength on flexion. As for aspects that depend on the operating technique, [PERSON] et al. proved the significance of insertion angle [7].