**3. Macro-design of dental implants**

The macro-design of dental implants determines their stability and their capacity to withstand the functional loads. The length, diameter, shape, and design of the screw are influential factors in the bone–implant interface. In the long term, these features may even determine the implant's survival.

### **3.1. Implant length**

Implant length is the distance from the prosthetic platform to the apex of the implant. Some authors have reported a lower survival rate for short implants, especially those <7 mm long [26, 27]. Eckert et al. [28], however, noted that the relation between implant length and survival was limited and was only noticeable when implants were <13 mm in length.

The type of bone and the cortical bone anchoring are probably more important factors than the implant length. Nonetheless, the indications of implants with extra short lengths (5–6 mm) should be carefully studied and considered, especially in areas with poor bone quality. Manufacturers are making great efforts to improve these implant surfaces so as to increase the area in contact with the bone and thus improve their prognosis [29, 30].

#### **3.2. Implant diameter**

The implant diameter is the distance from the outermost point of the screw to the opposite side. It measures the external dimension of the implant screw and should not be confused with the size of the implant platform.

Implant diameters usually range from 3 to 7 mm to make them compatible with the most sizes of alveolar processes. The choice of diameter depends on both surgical and prosthetic factors. In order to achieve maximum primary stability, the implant should be lodged between the vestibular-lingual/palatal cortical bones. From a biomechanical point of view, wider implants are able to join a larger amount of bone to the implant surface and obtain a higher bicortical anchorage, thus achieving a better distribution of stress in the surrounding bone. Another advantage of large diameter implants is that they can be inserted immediately in failure sites [31–33]. Some authors have found that increasing implant diameter by 1 mm increases the surface of bone–implant contact by 35% [34]. However, another parameter to consider is the crestal bone around the implant. According to Misch [35], this bone has a strong influence on the occlusal load; this author hypothesizes that it may be even more important than the length and diameter of the implant itself.

The primary stability of dental implants at the time of surgery has been considered an important factor for integration [36]. Langer et al. recommended large diameter implants to improve primary stability in low-density bones. The authors argue that increasing the diameter increases the bone–implant contact, thereby reducing initial implant mobility [37].

Small diameter implants have been introduced for narrow residual alveolar ridges and for edentulous spaces with small interdental distances. These implants do not include miniimplants, which are used to hold temporary dentures and have diameters <2.7 mm [38]. The main indications for narrow implants are the lower incisors, upper lateral incisors, and the restoration of teeth with residual spaces smaller than 5 mm without any possibility of space recovery or bone regeneration [39]. The main limitation of these implants is their reduced resistance to occlusal loads [40].

#### **3.3. Implant shape**

Shape has been one of the most thoroughly studied aspects of implant design. The most current implant systems are solid cylinders with thread; hollow implants are rare today. As for the design of the thread, attempts have been made to increase their self-threading capacity and to reduce heat generation during implantation. These design variations are most often applied in the crestal and apical areas. Some designs have attempted to imitate the natural root with a stepped cylindrical shape in the apical and crestal third of the implant. Some authors note that stepped cylindrical implants achieve better stress distribution and crestal bone load than conical and cylindrical implants [41].

Kan et al. [42] reported that threaded implants provide the best immediate retention. Other studies show that the use of a serrated thread can increase primary stability and that thread geometry plays an important role in the biomechanical properties of the implants [43, 44].
