Evaluation of Implant Primary Stability Using a Standard Versus an Underpreparing Drilling Protocol with Different Measurement Methods: An In Vitro Study

Background/Objectives: The primary aim of this study is to evaluate the impact of two different drilling methods on the primary stability of implants inserted into polyurethane blocks simulating different bone densities. The secondary aim is to analyse the indication provided by three different methods used to assess primary stability. Methods: The study was performed on two artificial bone blocks of solid rigid polyurethane without cortical layer and one laminated one. A total of 48 implants were placed following either a Regular Primary Stability (RPS) or an Enhanced Primary Stability (EPS) protocol based on site underpreparation. Immediately after implant placement, artificial intelligence tool (AIT) indication, peak insertion torque (pIT) and Resonance Frequency Analysis (RFA) values were recorded. The values were then categorised as suitable or not suitable for single-crown immediate loading by bone density and drilling protocol. Results: EPS drilling protocol produced significantly higher primary stability values than those in all bone densities, regardless of the primary stability measurement method used, except for the implants placed in D4 and evaluated by AIT. The grouping of values shows that implants placed in D3 and D4 never reached the immediate loading threshold for a single crown. In D2, EPS and RPS protocols both reach the threshold for all implants when evaluated with pIT or RFA; when AIT is used, RPS protocol was not able to reach the single-crown threshold in 50% of cases. Conclusions: Within the limitations of this in vitro study, underpreparation appears a viable technique to improve primary stability, nevertheless it might be insufficient to reach the threshold of immediate loading suitability in low density bone. All systems used are able to report the primary stability increase obtained by underpreparation, but AIT showed the potential to detect a loss of primary stability related to torque drop during insertion. These findings require further confirmation.