Engineering composites inspired cortical-trabecular interlocking effect underlies bone strength in mouse vertebrae

Bone strength is a major determinant of osteoporotic fracture risk. Current clinical assessments, however, rely primarily on imaging-based surrogates such as bone mineral density, offering limited insight into the integrated mechanical behavior of vertebral bone. Inspired by engineered composite structures, in which interfacial interactions substantially enhance mechanical properties, this study introduces the concept of the cortical-trabecular interlocking effect to characterize the mechanical integration between these two compartments and investigate its contribution to vertebral bone strength. Micro-finite element models of mouse L4 vertebrae (control, mild, and severe osteoporosis) and idealized vertebral models were employed to quantify the independent contributions of isolated cortical bone, isolated trabecular bone, and the cortical-trabecular interlocking effect to whole-bone mechanical properties under axial load. To better characterize the cortical-trabecular interlocking effect, we propose a novel microstructural parameter, Lr.BV/TV, defined as the bone volume fraction within the interlocking ring, an interface region where cortical and trabecular bone are structurally integrated. The cortical-trabecular interlocking effect contributed 12.2-26.1% (mean 19.5%) to vertebral yield strength and its magnitude varied systematically with osteoporosis severity. Notably, asynchronous deterioration of cortical and trabecular bone in osteoporosis constrained the effective utilization of the interlocking effect, whereas well-matched components maximized this effect. Lr.BV/TV was identified as the dominant predictor of vertebral strength and outperformed both cortical thickness and trabecular bone volume fraction, accounting for 45.83% of the total explained variance, with its predictive relevance varying across stages of osteoporosis. These findings advance our understanding of vertebral fragility.