Primary Stability of an Expandable Versus a Conventional Short Implant in Low-Density Bone: A Density-Stratified Ex Vivo Comparison

Background: Rehabilitation of atrophic posterior jaws is often challenging due to anatomical limitations, frequently requiring bone augmentation procedures or the use of short implants. Expandable short implants are characterized by an expansion mechanism whose potential contribution to primary stability remains to be fully established. This ex vivo study compared the primary stability of expandable and conventional short implants in porcine rib bone of low and intermediate CBCT-defined density (corresponding approximately to Misch D4 and D3). Methods: A total of 68 implants were placed in 17 fresh porcine ribs, using a randomized complete block design: 34 expandable short implants (ESI; Pyramidion, Ø4.1 × 5 mm) and 34 conventional short implants (CSI; Osstem TS III, Ø4 × 7 mm), equally distributed between two CBCT-defined density strata (D3 and D4, operationally defined). Peak insertion torque (PIT), resonance frequency analysis (ISQ; before and after apical expansion for the ESI group), peak removal torque (PRT), and expansion torque (ESI group only) were recorded. Data were analyzed using a two-way ANOVA (implant group × bone density) with a Bonferroni-corrected significance threshold (α = 0.025) for the two primary outcomes, ISQ and PRT. Results: PIT did not differ significantly between groups in D3 bone, while CSI showed higher PIT in D4 bone (p = 0.0005). ISQ increased significantly following apical expansion in the ESI group (mean + 4.91, 95% CI 3.60 to 6.22; p = 0.0004), more markedly in D4 than D3 bone; final ISQ did not differ significantly between groups at either bone density. PRT was higher in the ESI group in both density strata (p = 0.0004 in D3 and p = 0.0003 in D4), with no significant implant group × bone density interaction. Incomplete apical expansion occurred in 3 of 34 ESI implants, all in D3 bone. Conclusions: In this ex vivo model, no statistically significant difference in final ISQ was detected between the expandable and conventional short implants, and removal torque was higher for the expandable implant across both bone densities; equivalence was not tested. These findings refer to the initial mechanical behaviour under the tested ex vivo conditions, without implying superiority over conventional implant designs; further studies are warranted.