Abstract
BackgroundThe mechanical strength of mini-implants is a critical factor due to their small diameters. Currently, it is not possible to state whether there is a relevant difference between the mechanical properties of stainless steel (SS-MIs) and titanium alloy mini-implants (TA-MIs). The objective of this study was to test the null hypothesis that there is no difference in the mechanical strength of SS-MIs and TA-MIs, and to analyze, by scanning electron microscopy (SEM), the SS-MI, and TA-MI threads resistance to morphological damage after insertion.MethodsA standardized sample of 504 SS-MIs and TA-MIs with diameters ranging from 1.2 mm to 1.8 mm was used. Torsional fracture was performed in 154 MIs. Flexural strength of 280 MIs was evaluated at 1 mm and 2 mm-deflection. The threads of 70 MIs were morphologically analyzed by scanning electron microscopy (SEM), before and after their insertion in high-density artificial bone blocks. Comparisons between SS-MIs and TA-MIs were performed with t tests or Mann-Whitney U tests. A multiple linear regression analysis was used to evaluate the influence of variables on the ranging of MI mechanical strength.ResultsSS-MIs had higher fracture torque. The mean difference between the SS-MIs and TA-MIs fracture torque was of 4.09 Ncm. The MI diameter explained 90.3% of the total variation in fracture torque, while only 2.2% was explained by the metallic alloy. The SS-MI group presented a higher deformation force during the 1mm and 2mm-deflection. The mean difference between the flexural strength of SS and TA-MIs at 1 mm and 2 mm-deflection was of 18.21 N and 17.55 N, respectively. There was no noticeable morphological damage to the threads of SS-MIs and TA-MIs.ConclusionsThe null hypothesis was rejected. SS-MIs were 13.2% and 20.2% more resistant to torsional fracture and deflection, respectively. The threads of the SS-MIs and TA-MIs were not damaged during the insertion and removal process. Thus, the use of SS-MI can reduce the fracture risk without increasing the MI diameter.
Highlights
The mechanical strength of mini-implants is a critical factor due to their small diameters
To our knowledge, this is the first study comparing the mechanical strength of several diameters of Stainless steel (SS)-MIs and Titanium alloy (TA)-MIs using a MI sample with standardized morphological features made by the same manufacturer, which could contribute with more reliable results
The results showed that MIs with the same diameter and different brands had a high variation in the fracture torque [2]
Summary
The mechanical strength of mini-implants is a critical factor due to their small diameters. It is not possible to state whether there is a relevant difference between the mechanical properties of stainless steel (SS-MIs) and titanium alloy mini-implants (TA-MIs). Titanium alloy mini-implants (TA-MIs; Ti-6Al-4V) are the most widely used because it has higher mechanical strength than commercially pure titanium and is best suited to the small diameter of MIs, reducing the fracture risk during insertion and removal [8,9,10]. Two studies compared SS-MIs and TA-MIs, but they evaluated different MI brands and designs, compromising MI sample standardization [12, 13] The results of these studies did not point in the same direction, perhaps due to a large variation in the MI samples [2, 14]. One of these studies showed that SS-MIs and TA-MIs had similar mechanical strength [12], while the other two studies found that SS-MIs were more resistant to mechanical failure than TA-MIs [11, 13]
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