## 2 Introduction to the Concept with Milestone Questions
### Is There a Mechanic Coupling of Different Cortical Bones during or after Osseointegration?  
The existing theories about \"osseointegration\" focus exclusively on the border between the implant's surface and the bone [16; 17; 18]. We suggest, however, focusing on later developments which take place after the bone-implant force system comes under regular function, and during the period while and after the endosseous peri-implant bone has matured.  
Natural teeth are surrounded by fibers, and the fibers insert into the \"lamina cribrosa\", a perforated cortical (IC) bone, which transmits some of the masticatory forces into the surrounding spongious bone (SB), while most of the forces are transmitted to the outer cortical (OC) layer of the jaw bone. The mechanical coupling of these outer and inner cortical layers develops over years during the formation of the teeth. [PERSON] et al. [24] defined a new systematic terminology for osseofixated implants and suggested that working with the so-called \"first cortical\" layer is identical with the outer cortical (OC) layer. Since an endosseous vertical cortical (IC) layer seems to be missing in osseofixated implants, the latter cortical layer has not yet been defined.  
If a classic two-stage implant is inserted into the jaw bone, the surgeon's goal is to achieve primary stability by pressing it into the drill hole. Part of this stability is achieved by compressing the trabecular bone and by pressing the first cortical layer against the implant body [25]. In many implants, specific threads are designed together with additional notches in the apical part of the implant for blood clot and subsequential callus formation [26]. As a result of the implant design (for example, reverse conical neck implants) and the use of a countersink drill, this connection between the implant and the outer cortical cannot be expected to be fully tight immediately following implant placement [27; 28], Hence, such implants are covered by a flap or healing screw which creates a sterile compartment for the healing phase.  
Inside the bone, points of direct contact between some of the spongious struts and the implant are created during insertion [11]. The more the design of the implant and the drilling procedure leads to the compression of the spongious bone, the more stability is created during the insertion of the implant. Since typical two-stage implants have to provide space for an internal connection (to the abutment), the implant provides almost parallel walls in the top part and conicity only in the apex area. Thereby, the chances of achieving a lot of stability through compression are low.  
After \"osseointegration\" has occurred, the collar zone of the implant towards the first cortical layer is sealed with the new bone, and inside the spongious bone, new struts find contact with the implant's surface. The next step is the formation of a full cortical bone around the endosseous implant surface until a state shown in Figure 3 has been reached [29].  
After the implant is axially loaded on intrusion (vertical arrows show the direction of loading), the thickness of the cortical layer around the implant's surface increases, and also, the first cortical layer gains thickness (Figure 3). The angle between the surface of the implant and the first cortical layer is about 90 degrees, as presented in Figure 3.  
From the \"bone's point of view\", such an angle resembles a very strong convexity. The fact that the bone is in direct contact with the implant does not improve the situation. As mentioned earlier, the bone is not glued to the implant surface.