# Experimental & FEM Analysis of Orthodontic Mini-Inplant Design on Primary Stability
## 1 Introduction  
The main objective of successful orthodontic treatment is achieving stable anchorage [1, 2, 3]. In the last decade, great effort was put into achieving skeletal or so-called absolute anchorage, with the use of small titanium screws. Different experimental research led to the conclusion that screws, plates and similar mini-constructions enable absolute anchorage, which, compared with classic dental implants, offers insignificant anatomical limitations, easy application and lower prices. Still, one of their biggest advantages is the possibility of usage immediately after implant application [4, 5, 6]. Primary stability is the basic precondition of absolute anchorage establishment in orthodontic treatment, and it is achieved with a mechanical bond between the mini-implant and the bone [7].  
The function of an orthodontic mini-implant is to provide a stable anchorage. They are temporary; -they usually remain in place for some months of treatment, and then they are removed. From the orthodontic treatment point of view, the use of classical osseointegrating dental implants has many more negative aspects. One of the main disadvantages is the long period of osseointegration of the implant before it can be loaded with force (even from 4 to6 months). Additionally, it would require a rather painful and invasive surgical protocol, which is expensive and uncomfortable for the patient, and in the end, their use with patients under 16 years of age was excluded. During the last decade, a great effort was made in order to achieve a skeletal or absolute anchorage, using various types of small titanium screws and plates. The conclusions of many studies conducted are that screws, plates and similar mini-constructions can provide an absolute anchorage, compared with conventional dental implants, they are much less invasive, have an anatomical limitation of implantation, are simple and easy to put in and remove, less expensive than conventional implants, can be used with children and, in general, significantly improve orthodontic treatment with minimal disturbances to the patient. One of their most important advantages is that they can be loaded with force immediately after placement. Nevertheless, despite all the possibilities that orthodontic mini-implants offer, there are certain problems that occur. It is primarily about loosening and prematurely removing the screw or miniplate. Unlike dental implants, which achieve stability through osseointegration, the stability of mini-implants is achieved through mechanical retention. Compared to dental implants, the degree of stability of mini-implants is relatively low. Since the use of a temporary skeletal anchorage does not require osseointegration, primary stability is of great significance for clinical success.  
The effects of mini-implant design and load distribution on the implant-bone system are subjects of very advanced scientific researches these days. Many factors, such as load type, material properties and nature of established bond, have an impact on primary stability and load distribution on the implant-bone system [8; 9; 10].  
The purpose of the pull-out test is to present mechanical characteristics of primary stability for the better understanding of the behaviour of a mini-implant inserted into the bone along with the effects of geometric parameters of a mini-implant (main thread diameter, smallest core thread diameter, thread length, thread shape factor, etc.) on primary stability [11; 12; 13; 14; 15].  
FEM analysis has become an increasingly used tool in the field of medicine, where the behaviour of any structure can be predicted with geometry modelling, complex material properties, boundary conditions and loads [16]. Analysis results give insight about stress distribution and biomechanical changes in the implant-bone system. This method gives favourable degree of reliability and accuracy \"without the risks and expenses of implantation\" [17].