## 4 Discussion  
Incisor torque values are regarded as crucial to achieve treatment goals and, depending on treatment philosophies, a different torque range has been proposed ranging from the 7\({}^{\circ}\) of the original straight wire appliance to the 22\({}^{\circ}\) of the Bioprogressive prescription [11].  
The full expression of the information integrated in the bracket slot depends on the cohesive contact of a full-size wire with the slot walls. The efficiency of torque expression is reduced if play exists between the wire and the bracket slot [12]. The errors introduced during the fabrication process, in terms of slot height and depth, can increase play and reduce the ability of the appliance to effectively express the inbuilt torque information [5; 8; 11; 13]. It is therefore of paramount importance to assess if the brackets available in the market are respecting the nominal declared values. In addition, aspects related to orthodontic materials have always been of special interest for both researchers and orthodontic clinicians [14; 15].  
All analyzed systems had bodies manufactured by a metal injection molding (MIM) process. MINI TWIN(r) and MINI MASTER(r) were the only systems that combined MIM technology with bracket body fabrication and slot milling. MIM is at present the most popular and cost-effective technology in the bracket industry [14]; it relies on oversized molds to compensate for the contraction that occurs after sintering. Dimensional contraction can change due to different factors (alloy, powder type, debinding method, sintering heat rate, sintering hold time) and has a variable impact on actual dimensions achieved at the end of the manufacturing process [14].  
Dimensional inaccuracy can be related to one of the various steps of the manufacturing process and results in a dimensional difference between the different batches. Dimensional homogeneity within a batch and consistency between batches are the ultimate target of procedure validation protocols. A production procedure, if validated, should be secured from factors that could introduce bias to the production process and its output [15].  
To our best knowledge, this is the first study to assess the torque and tip values of currently marketed stainless steel brackets according to the ISO 27020:2019 norm, which sets tolerance limits for the dimensions of the bracket slot and the inbuilt torque and tip. In a previous report, bracket dimensions and torque were tested according to the DIN 13971-2 norm. The DIN norm describes a similar testing protocol but provides a significantly different tolerance range [13]. Furthermore, the current study is the first to test the variability in different batches of the same bracket system.  
The low method error, as defined by the inter- and intra-operator reliability, the number of brackets systems tested, the number of providers involved, the inter-batch assessment, and the single-blind design employed can be regarded as strengths of our study.  
However, some limitations should also be acknowledged. We did not include stainless steel brackets manufactured by procedures other than MIM or different types of brackets, such as ceramic or self-ligating brackets.