Are implant threads important for implant stability? an in vitro study using low-density polyurethane sheets

Different strategies are known to improve implant primary stability (PS) and the bone-to-implant contact in post-extractive conditions, such as the macro-geometry of screws and apical threads, which can enhance the mechanical characteristics. In any case, the role of the apical area design in maintaining or improving the PS, especially in low-quality bone, still remains unclear. Thus, the present study aimed at evaluating in vitro the Insertion Torque (IT), Removal Torque (RT), and Resonance Frequency Analysis (RFA) of different implant apical threads in a cylindrical (EE) and in three conical implants (T3, TAC, Intra-lock) inserted in simulated post-extraction conditions on low-density polyurethane foams of 10 and 20 pounds per cubic foot (PCF), with and without the addition of a cortical sheet of 30 PCF in density. The IT, RT, and RFA values of all the implants tested were directly proportional to the polyurethane density and to the presence of the cortical sheet, but TAC and Intra-lock implants, which had the latest-generation thread design, always showed significantly higher values (e.g., IT: 18.6 and 18.6 Ncm, RT: 10.8 and 13.7 Ncm, RFA mean: 46 and 43 ISQ, in the 20 PCF density with the cortical sheet for TAC and Intra-lock, respectively). In particular, TAC implants also reached the highest RFA values in the lowest-density foam (19 ISQ). In conclusion, the present in vitro study demonstrated that TAC and Intra-lock apical designs and macro-geometries could play a key role in determining the PS and the polyurethane-implant contact in simulated post-extraction conditions in low-density artificial bone.