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

## TL;DR

This study examines how porosity and canal size in rabbit femurs vary along different directions and how these variations affect bone stiffness and could inform better implant design.

## Contribution

The study quantifies longitudinal and radial microgradients in porosity and canal diameter in rabbit femurs and links them to mechanical properties for implant design.

## Key findings

- Porosity and canal diameter increase radially toward the medullary cavity and longitudinally toward the bone ends.
- Compressive modulus decreases with increasing porosity and canal size, with the mid-shaft showing the highest stiffness.
- The findings suggest structural benchmarks for designing functionally graded bone implants that mimic native bone structure.

## Abstract

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.

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 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.