Influence of Implant Macrogeometry on Primary Stability in Different Polyurethane Bone-Density Models: An In Vitro Study

Background: Primary stability (PS) is a crucial determinant of long-term dental implant success, particularly in compromised or low-density bone sites. While macrogeometry is known to influence initial mechanical anchorage, the specific contribution of thread dimensions across varying bone densities remains incompletely understood. This in vitro study evaluated the influence of two different implant macrogeometries characterized by distinct thread configurations on primary implant stability within standardized polyurethane foam models representing different bone qualities. Methods: Two implant designs featuring distinct thread profiles were tested: the Evolution KRI (larger threads, pitch 1.8–2.2 mm, depth 0.55–1.025 mm) and the Evolution PRI (smaller threads, pitch 1.0 mm, depth 0.5 mm), across three fixture diameters (4.0, 4.5, and 5.0 mm). Implants were inserted into three standardized polyurethane block configurations: Group I (25 PCF uniform block, simulating D2 dense trabecular bone), Group II (10 PCF trabecular base with a 3 mm 40 PCF cortical layer, simulating D3 bone), and Group III (12.5 PCF trabecular base with a 2 mm 40 PCF cortical layer, simulating D4 bone). Each experimental group included 10 implants (n = 10). Two RFA measurements were collected for each implant and averaged prior to statistical analysis. Group comparisons were performed using one-way ANOVA followed by Tukey’s post hoc test. Primary stability was quantitatively assessed using Resonance Frequency Analysis (RFA) to record Implant Stability Quotient (ISQ) scores. Results: Statistically significant differences in ISQ values were observed among the experimental groups (F = 9.909,p < 0.0001,R2 = 0.4785). Implants featuring larger threads (KRI) demonstrated consistently higher mean ISQ scores compared to smaller-thread implants (PRI) across all bone densities. In Group I (D2 model), KRI implants achieved a significantly higher mean ISQ than PRI implants (67.1 ± 1.91 vs. 65.1 ± 1.43; p = 0.0472). In Group II (D3 model with 3 mm cortical layer), this difference was most pronounced, with KRI reaching 68.5 ± 0.58 compared to 64.8 ± 2.15 for PRI (p < 0.0001). In Group III (D4 model with 2 mm cortical layer), KRI maintained a higher stability trend (68.0 ± 0.93 vs. 66.5 ± 1.42), though the inter-system difference did not reach statistical significance (p = 0.2401). Inter-group comparisons confirmed that the highest overall primary stability was obtained by large-thread implants in the presence of a 3 mm cortical layer. Conclusions: Thread macrogeometry significantly influences primary implant stability; the tested KRI macrogeometry demonstrated higher ISQ values than the PRI macrogeometry under specific polyurethane configurations, particularly in intermediate trabecular–cortical bone structures.