# Impact of Implant Design and Bone Properties on the Primary Stability of Orthodontic Mini-Implants
## 4 Discussion
We can see from Table 4 that the existence of a statistically significant difference of the average values of the pullout force was confirmed between the CBTC <0.62 mm and the CBTC 0.62-0.67 mm (_p_ < 0.029), and between the CBTC <0.62 mm and the CBTC \(\geq\)0.68 mm (_p_ < 0.014), whereas no statistically significant difference was confirmed between the CBTC samples \(\geq\)0.68 mm and the CBTC samples 0.62-0.67 mm (_p_ < 0.972). In our research, we have paid special attention to the design of the mini-implants tested. Through a great number of studies, various authors have examined the effects that the length of mini-implants can have on their long-term stability. Some authors have found that their stability is greater the deeper the implant is inserted into the bone, which at the same time requires the use of longer implants, but on the other hand these authors also indicate that the greater length requires a higher insertion torque, which can result in the occurrence of micro-cracks in the bone, which will eventually result in implant fall-out or failure [1; 16]. On the other hand, some studies [21; 22] indicate that the length of implants is not of relevance for their stability. Additionally, in one study [20] a group of authors compared mechanical stability and clinical applications between two different mini-implant designs--a newly invented design of orthodontic mini-implants (3 mm diameter and 2 mm length) and a commercially available design of mini-implants (1.5 mm diameter and 6 mm length), where they found that primary stability of the newly invented design of implants was considerably greater compared to the one found in the commercially available design of mini-implants. The use of the newly invented implant design, due to their small length, was supported by the fact that the vast majority of studies have shown that primary stability depends not so much on the length of the implant, but depends more on the surface to which the implant is applied, and on the implant-bone interface itself, which is in turn largely influenced by the implant design. The implant design includes the following features: body shape (cylindrical, or tapered), individual implant lengths and diameters, thread depths, thread and taper angles, and thread pitch (Table 1). Prior to comparing the differences detected in the geometry of the tested implants (tapers k, pitch p, and inner diameters d1), it is important to mention that we have come across a rather unexpected piece of information specified in the user manual supplied by the manufacturer. Specifically, after analyzing the geometry of the Korean implants (Perfect Anchor), we found that the length and diameter of the implants do not match the information presented in the user manual supplied by the manufacturer. Instead of the specified 8 mm, the length of the threaded section of the Korean screw was actually 7.7 mm. Additionally, the diameter of all Korean screws was 1.6 mm, although the diameter of ten of them was supposed be 1.4 mm, as it was specified on the original product packaging. This information explains the fact that no significant statistical differences were observed in terms of the pull-out test results in the group of the Korean implants. As far as the Dentaurum Tomas-pin implants were concerned, the length and diameter corresponded to the information presented in the user manual supplied by the manufacturer. A comparative analysis of the thread profile bearing depth and thread pitch of the mini-implants showed an observable difference between all three types of implants. For a better understanding of this relationship and its impact on primary stability, the so-called thread shape factor (TSF) plays an important role used for evaluation of the load-bearing capacity of the implant-bone interface [12]. Simply put, TSF represents the ratio between the mean thread bearing depth and the pitch.