## 4 Discussion  
Several surgical techniques have been proposed to improve implant PS in cancellous low-quality and low-density bone, among which there is a surgical technique, called under-preparation of the bone site, which consists of preparing the bone site with a smaller  
Figure 10: RFA measurements (RFA-BL on the top and RFA-MD on the bottom) derived from the two types of implants inserted in all the artificial bone densities. Data are expressed as means \(\pm\) SD, following the Kolmogorov–Smirnov test. * \(p\) < 0.05, **_p_ < 0.01, ***_p_ < 0.001, ***_p_ < 0.0001.  
size than the diameter of the implant used [5]. [PERSON] et al. [35] showed a correlation between the implant PS and the thickness of a cortical artificial bone, with statistically significant higher IT and RT values for implants inserted in under-sized sites compared to press-fit sites in vitro. For this reason, the under-preparation of the bone sites seemed to improve the implant PS. In another in vitro study performed on fresh humid poor-quality bovine bone [36], it was demonstrated that a 10% under-preparation of the site was enough to increase the implant PS. Later, an animal study on sheep [37] also showed improved biomechanics of implants placed into under-drilled sites. However, during the early healing period, a major remodeling of the cortical bone was observed. Similar results were found by [PERSON] et al. [38] in a systematic literature review with meta-regression, in which it was reported that marginal bone loss had a relationship with the drilling preparation technique and with the bone density, besides a greater mismatch between the implant and the osteotomy site, related to a higher marginal bone loss in higher bone densities. Even though the use of under-sized drilling may lead to higher marginal bone loss, it was used to increase the PS [39]. Moreover, in a human cadaver study, other researchers [23] reported an improved PS with under-sized implant site preparations and demonstrated significant correlations among the bone density, the IT, and the RFA. In addition, self-tapping implants with self-cutting flutes were also found to influence PS [9]. In fact, osseedensification determines a higher IT also in low-quality bone [40], as well as a significant increase in bone density, PS, bone-to-implant contact (BIC), and bone area frequency occupancy [41; 42; 43]. Thus, according to some in vitro and cadaver studies [5; 6; 22; 26; 44], the implant site under-preparation is able to significantly improve not only the IT and RFA, but also the implant PS in low-quality bone. Moreover, in preparing dental implant osteectomies, it was revealed that a low-speed drilling without irrigation has been comparable to conventional drilling [45]. In this work, IT values increased according to the site under-preparation, and so to the decreased size of the site, showing always a statistically significance among the different artificial bone densities. The ISQ score results were high in all the experimental conditions, especially in the highest artificial bone densities, where the reported RFA values were significantly higher in respect to those of implants inserted in the 10 PCF density block without the cortical sheet, except for 4 \(\times\) 13 mm TAC implants and the \"short\" protocol in the 20 PCF density block without the cortical sheet. Moreover, 3.3 \(\times\) 13 mm TAC implants inserted in the 20 PCF density block without the cortical sheet exhibited significantly higher ISQ also in respect to the corresponding implants inserted in the 10 PCF density block with the cortical sheet. Nonetheless, no significant differences related to the amount of site under-preparation and to the implant diameter among the implants tested on the same polyurethane block have been reported in the present study. In other words, all these implants have shown a good stability using all the under-preparation protocols, mostly in higher-density polyurethane blocks.