## 4 Discussion
In low-density polyurethane blocks, it might be proposed to increase the under-preparation of the site up to 1.5-1.8 mm, since a 2.2 mm size seemed to show higher results mainly with 4 \(\times\) 13 mm TAC implants in the 10 PCF density block without exhibiting a statistical significance. Thus, the initial study hypothesis considering the RFA, RT, and IT increase proportionally to the increase in artificial bone density and site under-preparation was accepted, demonstrating no significant differences in ISQ values between the two implant diameters and under-sizing protocols. Regarding the IT, significant higher values have always been reported by 4 \(\times\) 13 mm TAC implants and the 2.2 mm protocol (23.34 \(\pm\) 0.79 Ncm and 11.88 \(\pm\) 0.86 Ncm in 10 PCF density blocks with and without the cortical sheet, respectively, and 46.21 \(\pm\) 0.79 Ncm and 39.28 \(\pm\) 0.68 Ncm in 20 PCF density blocks with and without the cortical sheet, respectively), except when compared with 3.3 \(\times\) 13 mm TAC implants and the 2.2 mm protocol in the 20 PCF density block without the cortical sheet. RT, instead, is clinically defined as the force needed to remove an implant from the bone and it is considered a predictable way to prove implant stability, indirectly providing information on the degree of bone-to-impact contact (BIC) [46]. Considering a maximum of 80 Ncm as a limit to prevent mechanical damage to implants and 30 Ncm as a valid level for RT assessment, it may be used for comparative tests on implant surfaces, macromorphologies, and materials [47; 48]. Here, the RT values, as well as IT ones, were proportional to the increase in the polyurethane block density and to the decrease in the under-preparation size for both the implants tested. In particular, the values reached 39.09 \(\pm\) 0.72 Ncm with 4 \(\times\) 13 mm implants and 2.2 mm burs and 32.38 \(\pm\) 0.68 Ncm with 3.3 \(\times\) 13 mm implants and 2.2 mm burs in the 20 PCF density block with the cortical sheet, without statistically significant differences. In contrast, in the 10 PCF density block without the cortical sheet, the lowest RT values were registered for 4 \(\times\) 13 mm TAC implants (6.81 \(\pm\) 0.82 Ncm) and for 3.3 \(\times\) 13 mm TAC implants (7.39 \(\pm\) 0.34 Ncm) in the sites prepared with the 3.2 mm bur and the 2.8 mm bur, respectively, always not reporting significant differences. However, in the same bone density, 4 \(\times\) 13 mm implants showed the highest RT values in 2.8 mm and 2.2 mm prepared sites (9.3 \(\pm\) 0.43 and 10.41 \(\pm\) 0.43 Ncm, respectively). In addition, the increase in RT values, in concomitance with the increase in the under-preparation, was about 4-14 Ncm lower than the corresponding IT values in 20 PCF density blocks with and without the cortical sheet and about 2-6 Ncm lower for 10 PCF density blocks with and without the cortical sheet. Based on the aforementioned results, this indicates a higher stability and contact with the material for both implants in the higher-density blocks and mainly with the 2.2 mm under-sizing protocol. Concerning the limitations of the present study, the absence of human individual variability, bone response, and natural health or pathological bone microenvironment, as well as variables regarding the surgical technique, should be considered in order to better discuss these data. Although polyurethane foams have been often used in implant research because of their consistent and reproducible testing nature, they will never be able to fully replicate the complex properties of a real bone. Therefore, the results obtained in this study may not fully reflect the in vivo performance of TAC implants. Other limitations could be related to the choice of only low polyurethane block densities to test the effectiveness of under-sizing and the assessment of a limited type of forces tested on implants.