Longitudinal and radial microgradients in porosity and canal diameter in femur bone and its implications for bone regeneration and bone repair implants.

Introduction: Bone exhibits hierarchical structural gradients that optimize mechanical performance and regenerative potential. Longitudinal and radial variations in porosity and canal architecture of the femur influence load distribution, vascularization, and remodeling. Understanding these gradients is essential for designing scaffolds and implants that mimic native bone structure and function. This study quantified longitudinal and radial microgradients in porosity and canal diameter along the rabbit femur and explored their implications for bone regeneration and repair implant design. Rabbit femora were divided into proximal, mid-shaft, and distal regions. Methods: High-resolution micro-computed tomography quantified cortical thickness, porosity, and canal diameter along radial and longitudinal axes in micron-scale resolutions. Results and Discussion: Compressive mechanical testing of slices determined local moduli, which were correlated with microstructural parameters to establish structure-function relationships. Cortical thickness peaked at the mid-shaft and decreased toward both ends. Porosity and canal diameter increased radially toward the medullary cavity and longitudinally toward the bone ends. Upto 500 μm bone thickness from the outer surface toward modullary cavity, porosity and canal diameter ranged, respectively, from ∼5% to 40 μm at the mid-shaft to ∼40% and 110 μm at the ends. At 750 μm thickness, porosity and canal diameter ranged, respectively, from ∼5% to 50 ∼m at the mid-shaft to ∼80% and 200 μm at the ends. As expected, compressive moduli declined with increasing porosity and canal size. The mid-shaft, with the lowest porosity and smallest canals, exhibited the highest modulus of around 15 MPa, which decreased to 5 MPa toward the ends. The rabbit femur displays distinct longitudinal and radial microgradients in porosity and canal architecture that govern local stiffness. These gradients define structural benchmarks for designing functionally graded tissue engineering scaffolds and bone implants that replicate native tissue structure and stiffness transitions to promote osteoconduction, osteoinduction, osteogenesis in bone regeneration and improve osseointegration of bone implants.