## 4 Discussion  
Measuring insertion torque and primary implant stability, this experiment attempted to compare a novel implant design characterized by a change in core diameter and thread geometry (AlfaGate) to an existing, well established [33] tapered implant design (NobelActive) when used for immediate placement in simulated extraction sites and sinuslift situations respectively. The methodology applied in this experiment followed comparable previous studies on round [34, 37, 38] and wedge-shaped [39] dental implants. In order to benefit from standardized conditions, polyurethane foam was used as bone surrogate material instead of using cadaver bone [40], which may be seen as the major limitation of this experiment. Compared to a recently introduced implant with an \"inverted body shift\" [25], which is characterized by an increase in its outer diameter for engaging socket walls [3, 19, 21, 26], the experimental implant used here gains stability from an increased core diameter in combination with a condensing thread design in the middle portion of the implant. The outer diameter of the implant of the novel implant does not change in the middle and cervical parts, which allows it to be also placed in healed sites. An unsolved problem in implant dentistry is the evaluation of alveolar bone quality based on which different drill protocols are recommended [41]. Many conventional screw-type implants tend to induce high levels of stress in the cervical peri-implant bone area, leading to bone loss [42]. In order to reduce this phenomenon, greater drill diameters or counters driftls are recommended thereby removing precious bone quantity. The novel implant design is aimed at simplifying the surgical procedure as osteotomy preparation is mainly governed by the core diameter of the implant (Figure 1b) to be used instead of bone quality judgement. In a previous experiment [34], the novel implant was tested in homogeneous bone surrogates while the situations simulated here are more critical as primary stability has been shown to be lower in immediate implant cases as compared to healed sites [22, 23, 24]. Questioning two clinical studies on immediate implant placement [14, 15] where no difference between implant design has been described, greater primary stability was observed here with the novel implant design. The general notion that implant design in fact impacts stability in extraction sites is supported by two previous reports [38, 43]. Deviating from clinical practice where immediate implants in the anterior maxilla are placed such way that the implant leaves the alveolus palatally [10, 11] and engages bone areas apical to the socket, socket depth and implants had a length of 10 mm for creating a worst case scenario. In contrast to the sinuslifting procedure, 5.0 mm implants had to be chosen here in order to engage lateral walls which seems to be in line with clinical practice [14, 15]. Similar to extraction sites, a wide variety of clinical situations exists in sinuslifting which can not be standardized [28, 30, 31] and it may be argued whether or not a residual bone height of 3 mm underneath the sinus floor is sufficient for placing implants. However,  
Figure 4: Torque development over time during implant insertion for both implant types investigated in simulated sinuslift (**a**) and extraction site (**b**). different clinical studies reported residual bone heights in the range of 3.1 to 9.6 mm [30], 3.86 mm [31] and even reaching a minimum of 2 mm of residual bone height [28]. As such the situation simulated here with a layer of residual bone 3 mm in thickness may not even be considered a worst-case scenario. Besides bone quantity, bone quality has also been reported to be compromised in sinuslift procedures with soft bone being present [29] as compared to the rather compact foam material used here. In this in vitro study standardized bone models made from polyurethane foam material have been used as a bone surrogate [36; 44; 45; 46].