Background: Inter-implant distance (IID) plays a crucial role in maintaining peri-implant bone stability and osseointegration. Narrower IIDs (< 3 mm) have been associated with increased bone loss, but the threshold varies with implant connection type. The undersized preparation protocol, which induces static strain in the bone, enhances primary stability but may lead to microstructural damage, remodelling, or necrosis. The effect of this technique on osseointegration in adjacent implants with varying IIDs is not well documented. This study investigates the impact of undersized and non-undersized preparation protocols on osseointegration at different IIDs.

Methods: The study utilized eight sheep, with 64 implants placed in the mandibles following two surgical protocols: undersized preparation (UP) and non-undersized preparation (NUP). Implants were positioned at two IIDs (2 mm and 4 mm). Biomechanical, histomorphometric, and micromorphometric analyses were performed five weeks post-surgery. Bone-to-implant contact (BIC), bone volume fraction (BVTV), and bone mineral density (BMD) were measured in the regions of interest (inner and outer portions relative to the IID). A linear mixed model approach was applied to analyse the data with statistical significance set at p < 0.05.

Results: The analyses showed no statistically significant differences between the surgical protocols or IIDs for the evaluated parameters ( p >0.05). Nevertheless, trends were observed, with higher BIC and increased bone remodelling in the inner regions at 2 mm IID, particularly when using the undersized preparation protocol. Additionally, BVTV values were higher in the inner portions at 4 mm IID, suggesting reduced bone remodelling compared to 2 mm IID. These results indicate that while mechanical stress influenced trends in bone response, the overall resilience of peri-implant bone healing was evident.

Conclusions: No significant differences in osseointegration were observed between surgical protocols or IIDs. However, the trend of increased remodelling and higher BIC at 2 mm IID highlights the mechanical impact of undersized preparation in close implant spacing. These findings emphasize the complexity of peri-implant bone response to mechanical forces, necessitating further clinical studies to validate these results in human models.