Purpose Despite the extensive development of antibacterial biomaterials, there are few reports on the effects of materials on the antibacterial ability of the immune system, and in particular of neutrophils. In this study, we observe differences between the in vivo and in vitro anti-infective efficacies of silver nanoparticles (AgNPs). The present study was designed to further explore the mechanism for this inconsistency using ex vivo models and in vitro experiments. Methods AgNPs were synthesized using the polyol process and characterized by transmission electron microscopy and X-ray photoelectron spectroscopy. The antibacterial ability of AgNPs and neutrophils was tested by the spread-plate method. The infected air pouch model was prepared to detect the antimicrobial ability of AgNPs in vivo. Furthermore, blood-AgNPs-bacteria co-culture model and reactive oxygen species (ROS) measurement were used to evaluate the effect of AgNPs to neutrophil-mediated phagocytosis and ROS production. Results The antibacterial experiments in vitro showed that AgNPs had superior antibacterial properties in cell compatible concentration. While, AgNPs had no significant antibacterial effect in vivo, and pathological section in AgNPs group indicated less neutrophil infiltration in inflammatory site than S. aureus group. Furthermore, AgNPs were found to reduce the phagocytosis of neutrophils and inhibit their ability to produce ROS and superoxide during ex vivo and in vitro experiments. Conclusion This study selects AgNPs as the representative of inorganic nano-biomaterials and reveals the phenomenon and the mechanism underlying the significant AgNPs-induced inhibition of the antibacterial ability of neutrophils, and may have a certain enlightening effect on the development of biomaterials in the future. In the fabrication of antibacterial biomaterials, however, attention should be paid to both cell and immune system safety to make the antibacterial properties of the biomaterials and innate immune system complement each other and jointly promote the host’s ability to resist the invasion of pathogenic microorganisms.

INTRODUCTION Intra-articular impacted fragment (IAIF) is commonly found in the Pilon variant posterior malleolar (PVPM) fracture. Poor quality of reduction may result in articular incongruity, leading to ankle function impairment and ultimately traumatic arthritis. However, the debating remains on if the IAIF must be reduced, and how the presence or malreduction of the IAIF influences the prognosis in PVPM fractures. This retrospective study is to evaluate the prognostic effect of the IAIF reduction in the PVPM fractures. METHODS We studied 22 patients with PVPM fractures admitted to our hospital between June 2020 and June 2021, who were treated with open surgery and followed-up for 12 months retrospectively. The patients were divided into two groups: six patients without IAIF (No-IAIF group) and 16 patients with IAIF (IAIF group). Burwell-Charnley scoring system and ankle CT scan were used to evaluate the radiographic outcomes. At postoperative 12 months, American Orthopedic Foot and Ankle Score (AOFAS) was set as primary outcome. Visual Analog Scale (VAS) pain score, the Medical Outcomes Study 36-Item Short-Form Health Survey (SF-36) were secondary outcomes and associated complications were recorded. Postoperative articular stepping ≥2mm or articular incongruity on the sagittal view of CT scans was considered “poor” reduction, otherwise “anatomical”. The study abided by the Declaration of Helsinki and is approved by Human Ethics Committee of our hospital. All persons gave their informed consent prior to their inclusion in the study. RESULTS The articular surface of five of six patients in No-IAIF group and eight of 16 patients in IAIF group were considered “anatomically” reduced. Statistical difference of AOFAS, SF-36 and VAS was found significant between the No-IAIF and IAIF groups at postoperative 12 months. In the fractures considered with “anatomical” reduction, significant difference was only seen in SF-36 between the patients with and without IAIF. Three of 22 patients received syndesmotic fixations. No difference of associated complications was noted. DISCUSSION The results of this study indicated that the short-term clinical effect was dependent of the presence the IAIF. Importantly, the reduction satisfactory was statistically associated with improved clinical outcomes at 12 months postoperatively. SIGNIFICANCE The presence of the IAIF negatively affect the clinical outcome and the anatomical reduction of the articular congruity will positively affect the prognosis of PVPM fracture.