Scaffold-induced endochondral ossification in segmental bone defects is critically dependent on biomaterial degradation kinetics

Biomaterials for tissue regeneration classically serve as delivery systems for growth factors, progenitor cells or aim to mimic the complexity of healthy tissue to support healing - with related limitations in effectiveness or costs. Recently, the possibility to control endogenous tissue regeneration by biomaterial architecture alone was recognized. Biomaterial degradation is needed to achieve full tissue regeneration, but the degradation process and the related immune response might impair regenerative processes. Here, we alter the degradation kinetics of a biomaterial scaffold with channel-like pores to study consequences on endochondral bone healing and to understand details of the scaffold-guided healing process. We show that the scaffold wall degradation driven by multinucleated giant cells does not impede tissue formation and maturation within the pores suggesting a non-inflammatory degradation process. We further demonstrate that the pronounced remodeling processes inherent to endochondral ossification critically affect the integrity of the scaffold walls. Degradation-driven fragmentation of scaffold walls leads to impaired ECM structuring and blood vessel ingrowth resulting in a switch from endochondral to intramembranous ossification. This highlights the necessity to fine-tune material degradability and immunogenicity as key players regarding the targeted process of tissue regeneration and demonstrates the need to consider endogenous tissue remodeling processes for biomaterial design. Adjusting the degradation kinetics in respect to the specific healing scenario will help to maximize the regenerative potential of architectured biomaterials in future. STATEMENT OF SIGNIFICANCE: This study demonstrates that scaffold wall integrity critically determines the healing trajectory in collagen-based biomaterials for bone regeneration. By selectively modulating degradation while keeping scaffold architecture and mechanics constant, we reveal that premature wall fragmentation disrupts extracellular matrix organization, alters vascular and immune cell invasion, and provokes a switch from endochondral to intramembranous ossification, thereby impairing bone defect healing. These findings highlight that scaffold degradation is not merely a clearance process but can be used as an active regulator of the tissue healing mode. Beyond bone healing, our results suggest to use biomaterial degradation more targeted to temporally or spatially steer regenerative processes, offering broad relevance for the design of future biomaterials.