Microgradients in porosity and canal diameter in femur bone

Abstract

Bone exhibits hierarchical structural gradients that optimize mechanical performance and regenerative potential. Longitudinal and radial variations in cortical porosity and canal architecture 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 cortical 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. High-resolution micro-computed tomography quantified cortical thickness, porosity, and canal diameter along radial and longitudinal axes in micron-scale resolutions. Compressive mechanical testing of cortical 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 micron cortical thickness from the outer surface toward modullary cavity, porosity and canal diameter ranged, respectively, from ~5% and 40 {micro}m at the mid-shaft to ~40% and 110 {micro}m at the ends. At 750 micron cortical thickness, porosity and canal diameter ranged, respectively, from ~5% and 50 {micro}m at the mid-shaft to ~80% and 200 {micro}m at the ends. As expected, compressive moduli declined linearly with increasing porosity and canal size. The mid-shaft, with the lowest porosity and smallest canals, exhibited the highest modulus of around 15{square}MPa, which decreased to 5{square}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.