Anatomical implant region – a critical determinant for the osteogenic potency of small extracellular vesicles and rhBMP-2

Abstract Purpose Mechanical stabilization is crucial for bone healing, yet complex fractures, particularly osteoporotic or comminuted, remain challenging due to impaired implant osseointegration, resulting in implant loosening or non-unions. This study investigated whether co-application of small extracellular vesicles (sEVs) derived from human umbilical cord mesenchymal stromal cells (hUC-MSC-sEVs) with a low dose of recombinant human bone morphogenetic protein 2 (rhBMP-2) could enhance screw implant osseointegration. Methods A novel small animal model was established to evaluate the effect of anatomical femur regions on screw integration. Six weeks postoperatively, bone formation and bone–implant contact were assessed by micro-computed tomography and descriptive histology. Biomechanical stability was determined using pull-out tests. Results Outcomes differed significantly between the proximal and distal implant locations, with no improvements in osseointegration observed in the distal region. In the proximal region, application of hUC-MSC-sEVs alone did not significantly improve osseointegration, whereas local application of 1.5 µg rhBMP-2 resulted in measurable biomechanical improvements. No additive or synergistic effects were observed when sEVs were co-administered with rhBMP-2. Descriptive histology supported these findings, demonstrating the most pronounced bone formation at the proximal site following rhBMP-2 treatment. Conclusion Although co-application of sEVs did not confer additional benefit, these results underscore the critical role of implant location in osseointegration. This should be considered in future small animal studies exploring novel strategies to improve screw or implant integration, as well as in the clinical application of osteosynthesis techniques.