From flat bones to functional healing: redefining rodent calvarial defect models for translational bone regeneration research.

Preclinical models are essential for evaluating bone tissue engineering strategies, but their translational relevance is frequently debated. Rodent calvarial defect models (RCDs) are widely used due to their reproducibility, cost-effectiveness, and compatibility with genetic manipulation. Despite these advantages, their translational relevance remains controversial due to key anatomical and physiological differences from human bones. RCDs heal exclusively via intramembranous ossification and lack biomechanical loading, in contrast to the endochondral ossification and dynamic stress conditions characteristic of long bone healing in humans. Additionally, inconsistencies in defining critical-size defects (CSDs), variation in defect placement relative to cranial sutures, and the influence of age-related skeletal changes hinder cross-study comparisons and clinical extrapolation. These limitations underscore the need for standardized defect parameters, age-matched models, and advanced scaffolds with tunable degradation rates aligned with bone regeneration timelines. Therefore, adopting standardized protocols, integrating advanced biomaterials, and employing clinically relevant testing environments can substantially enhance the predictive power and translational relevance of calvarial defect models. While RCDs serve as a valuable platform for early-stage screening and mechanistic insights, strategic refinements in model design, paired with complementary validation in higher order species, are essential for bridging the gap between preclinical research and clinical application in bone tissue engineering.