# Impact of Implant Design and Bone Properties on the Primary Stability of Orthodontic Mini-Implants
## 1 Introduction  
The introduction of mini-implants into orthodontics stands out as one of the most significant changes in clinical work [1]. On the market, many types of implants are available, having similar characteristics, made of an alloy titanium and vanadium or aluminium. Most common is the mini-screw system. From a clinical point of view, the screw consists of three parts: the head, which can have a different shape; the transgingival part; and the threaded body, which is mainly in form of a cylinder, cone, or a combination of the two. These implants provide the clinical practitioners with solid control of 3D tooth displacement [2], which is why their application is ever-increasing. In the current practice of using mini-implants, the preference is given to a smaller diameter and lower profile of the head for better hygiene and greater comfort for the patient. In essence, these are self-tapping bone screws with a diameter of 1.5-2.5 mm and a length of 6-10 mm, with different lengths of the transmuosal neck (1.5-2.5 mm) which compensate for the thickness of the mucosa. The head can have classic twist slots, cross-placed slots 0.021 \(\times\) 0.025, or a ball. A chain or loop of a Ni-Ti tension spring is applied to the ball, and a wire is applied to the slot.  
Stability of orthodontic mini-implants is generally defined by two major components [3]: primary stability achieved by mechanical bonding between the bone and the mini-implants; and secondary stability achieved through continuous bone remodeling around the implant, thus leading to osseointegration [4].  
Primary stability of a mini-implant depends on a number of factors, primarily including the place where the implant itself is inserted, its proximity to the root, geometric design of the mini-implant, soft tissue condition, operating techniques, and the strength and the duration of the applied orthodontic forces. [PERSON] et al. [5] were among the pioneers who dealt with this specific group of parameters. Many authors after them have also used these parameters to describe the factor of primary stability [6; 7; 8]. The length of the mini-implant is determined by cortical bone thickness (CBTC) and quality, implant insertion angle, thickness of transmucosal tissues, and by properties of other surrounding anatomical structures [9; 10]. Experimental studies performed on a synthetic bone show that the geometrical design characteristics of mini-implants, featured through the so-called thread shape factor (TSF), play a key role in maintaining primary stability [11; 12].  
Once the implant is inserted into the bone, a number of specific corresponding measurement methods are used in order to evaluate its primary stability. These methods distinguish between non-invasive clinical tests (insertion torque, radiography, periotest, etc.) and invasive research methods (pull-out test and removal torque testing) [13].  
Many in vitro studies have used wood, polyvinyl chloride, and pig bone as the test sample materials [13; 14; 15]. The thickness of the pig bone cortex is similar to the thickness of the human maxillary and mandibular cortexes [16]. The specific purpose of this study was to identify the correlation between the bone properties, the geometrical design of commercial orthodontic mini-implants, pull-out forces, and primary stability.